Precipitation and Hardening in Magnesium Alloys

Transcription

1 Precipittion nd Hrdening in Mgnesium Alloys JIAN-FENG NIE Mgnesium lloys hve received n incresing interest in the pst 12 yers for potentil pplictions in the utomotive, ircrft, erospce, nd electronic industries. Mny of these lloys re strong becuse of solid-stte precipittes tht re produced by n ge-hrdening process. Although some strength improvements of existing mgnesium lloys hve been mde nd some novel lloys with improved strength hve been developed, the strength level tht hs been chieved so fr is still substntilly lower thn tht obtined in counterprt luminum lloys. Further improvements in the lloy strength require better understnding of the structure, morphology, orienttion of precipittes, effects of precipitte morphology, nd orienttion on the strengthening nd microstructurl fctors tht re importnt in controlling the nucletion nd growth of these precipittes. In this review, precipittion in most precipittion-hrdenble mgnesium lloys is reviewed, nd its reltionship with strengthening is exmined. It is demonstrted tht the precipittion phenomen in these lloys, especilly in the very erly stge of the precipittion process, re still fr from being well understood, nd mny fundmentl issues remin unsolved even fter some extensive nd concerted efforts mde in the pst 12 yers. The chllenges ssocited with precipittion hrdening nd ge hrdening re identified nd discussed, nd guidelines re outlined for the rtionl design nd development of higher strength, nd ultimtely ultrhigh strength, mgnesium lloys vi precipittion hrdening. DOI:.07/s Ó The Minerls, Metls & Mterils Society nd ASM Interntionl 2012 I. INTRODUCTION MAGNESIUM is the lightest of ll commonly used structurl metls, with density pproximtely two thirds tht of luminum nd one qurter tht of steels. Mgnesium is n bundnt element, comprising 2.7 pct of the Erth s crust, nd it is vilble commercilly with purity exceeding 99.8 pct. Mgnesium hs reltively low melting temperture nd high specific het. Hence, mgnesium nd its lloys my, thus, be redily cst to ner-net shpe by conventionl csting methods. Becuse of such ttrctive fetures, mgnesium lloys hve received considerble reserch over the lst decde for potentilly wider nd lrger pplictions in the utomotive, ircrft, erospce, nd 3C (computer, communiction, nd consumer electronic product) industries. The nnul production rte of mgnesium metl ws pproximtely 450,000 tons in 2001 nd reched ~720,000 tons in Despite the considerble efforts mde thus fr, the doption of mgnesium lloys in engineering pplictions remins limited compred with tht chieved for luminum lloys. One importnt technicl reson is tht there re limited mgnesium lloys for designers to select from for specific pplictions, nd within these limited choices, the most cost-effective mgnesium lloys hve indequte properties such s yield strength, creep-resistnce, formbility, nd corrosion resistnce. The ccumulted JIAN-FENG NIE, Professor, is with the Deprtment of Mterils Engineering, Monsh University, Clyton, VIC 3800, Austrli. Contct e-mil: Jinfeng.nie@monsh.edu Mnuscript submitted Februry 1, Article published online July 21, 2012 empiricl experience, rther thn bsic understnding, provides the tools for prcticl design nd development of mgnesium lloys with better mechnicl nd chemicl properties. Mny mgnesium csting nd wrought lloys chieve their useful mechnicl properties vi ge hrdening, which involves (1) solution tretment t reltively high temperture within the -Mg single-phse region, (2) wter quenching to obtin supersturted solid solution of lloying elements in mgnesium, nd (3) subsequent ging t reltively low temperture to chieve controlled decomposition of the supersturted solid solution into fine distribution of precipittes in the mgnesium mtrix. The decomposition of the supersturted solid solution often involves the formtion of series of metstble or equilibrium precipitte phses tht hve different resistnce to disloction shering. Therefore, the control of the precipittion is importnt if the mximum precipittion strengthening effect is to be chieved. Attempts to improve the ge-hrdening response of mgnesium lloys inevitbly requires n in-depth understnding of precipittion, precipittion hrdening, nd microstructurl fctors tht re most importnt in controlling the precipittion of strengthening phses nd the strength of precipittion-hrdenble lloys. For precipittion-hrdened mgnesium lloys, their microstructures often contin distribution of plte-shped or lth-rodshped precipittes of intermedite or equilibrium phses formed prllel or norml to the bsl plne of the mgnesium mtrix phse. In the lst century, the crystl structure, composition, nd orienttion reltionship of these precipittes hve been chrcterized primrily using conventionl trnsmission electron microscopy (TEM) METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

2 nd electron diffrction. As consequence of the resolution nd limittion of these techniques, the chrcteristic fetures of some precipitte phses nd the precipittion sequence in mny lloys were not clerly estblished. In the first decde of this century, with the ssistnce of highresolution trnsmission electron microscopy, prticulrly tomic-resolution high-ngle nnulr drk-field scnning trnsmission electron microscopy (HAADF-STEM), nd three-dimensionl tom probe (3DAP), some puzzles on the structure nd composition of precipitte phses in some existing mgnesium lloys hve been solved. These modern chrcteriztion fcilities lso gretly fcilitte the identifiction of precipittes in mgnesium lloys tht re developed in recent yers. Such knowledge on the crystllogrphy of precipitte phses provides the bsis for the understnding of the formtion nd strengthening mechnisms of the precipitte phses nd, more importntly, for the rtionl lloy design in prctice. The purpose of this rticle is to provide comprehensive review of the literture on precipittion nd hrdening in most, if not ll, ge-hrdenble mgnesium lloys. Becuse few books on mgnesium lloys [1 4] nd some review rticles on precipittion in mgnesium lloys [5 8] nd prticle hrdening [9 12] re lredy in the literture, the emphsis of this rticle will be focused on (1) the structure, morphology, nd orienttion of precipittes, precipittion sequence nd hrdening response in ech of the mjor lloy systems; (2) the effects of precipitte shpes on strengthening; nd (3) the rtionl design of microstructures for lrger gehrdening response nd therefore higher strength. Some unsolved issues tht require dditionl reserch re lso highlighted nd discussed. II. PRECIPITATION AND AGE-HARDENING RESPONSE A. Mg-Al-Bsed Alloys 1. Precipittion The mgnesium-rich side of the Mg-Al binry phse digrm includes equilibrium solid phses -Mg nd b- Mg 17 Al 12, s well s eutectic temperture of 7 K (437 C). The b phse hs body-centered cubic structure (spce group I 43m) with the lttice prmeter ~ 1.06 nm. [13] The equilibrium solid solubility of Al in -Mg is 11.8 t. pct (12.9 wt pct) t the eutectic temperture, nd it decreses to pproximtely 3.3 t. pct t 473 K (200 C). [14] The equilibrium volume frction of precipittes chievble in the Mg-Al lloys ged t 473 K (200 C) cn rech substntilly lrge vlue of 11.4 pct. This thermodynmic feture provides unique opportunity for generting lrge volume frction of precipittes by using conventionl ging tretments, i.e., solution tretment t pproximtely 692 K (420 C), followed by wter quench nd subsequent ging t temperture in the rnge of 373 K to 573 K (0 C to 300 C). Unfortuntely, during the isotherml ging tretment in the temperture rnge 373 K to 573 K (0 C to 300 C), the precipittion process seems to involve solely the formtion of the equilibrium b phse (Tble I). Although the b precipittes re resistnt to disloction shering, [15,16] their distribution is reltively corse, presumbly becuse of the reltively high diffusion rte of Al toms in the solid mtrix of mgnesium nd possibly high concentrtion of vcncies in the - Mg mtrix. Consequently, the ge-hrdening response of Mg-Al lloys [15,17 20] is not s pprecible s expected (Figure 1()). [19,20] Previous studies [16,21] reveled tht the precipittion of the equilibrium b phse occurs both discontinuously nd continuously. The discontinuous precipittion is lso known s cellulr precipittion, nd in this rection, the supersturted solid solution phse decomposes into the b phse nd n phse tht is structurlly identicl to the phse but hs less sturted concentrtion of luminum. The discontinuous precipittion initites in grin boundries nd expnds towrd the grin center in cellulr form. [22] The cell comprises lmellr structure of b nd phses, nd the cell interfce seprting nd is high ngle boundry. The continuous precipittion occurs inside the grins. The continuous nd discontinuous precipittions occur simultneously nd compete with ech other during isotherml ging of Mg-Al lloys. Duly et l. [23 25] reported tht the continuous precipittion is fvored t both high nd low ging tempertures nd tht discontinuous precipittion domintes the microstructure t intermedite tempertures. They proposed tht the disppernce of discontinuous precipittion t high ging tempertures is cused by the volume diffusion of solute tht prevents the nucletion nd growth of the cellulr colonies nd tht the bsence of discontinuous precipittion t low ging tempertures is the result of lower driving force, which is cused by the occurrence of continuous precipittion in the erly stge of ging tretment. A more recent study of binry Mg-9 wt pct Al lloy nd lloy AZ91 [26] indictes tht in the binry Mg-9 wt pct Al lloy smples ged t 423 K (150 C) or cooled from the solution temperture to room temperture, only discontinuous precipittes re observed, wheres tht only continuous precipittes form when the binry nd the AZ91 lloys re ged t 623 K (350 C). It is lso found tht both discontinuous nd continuous precipittes form when the lloys re ged t intermedite tempertures of 473 K or 523 K (200 C or 250 C). It ws proposed [26] tht whether discontinuous precipittion occurs lso depends on the concentrtion of vcncies in ddition to the ging temperture. For most continuous precipittes nd discontinuous precipittes of the b phse in the lmellr structure, they were initilly reported to dopt the exct Burgers orienttion reltionship, i.e., ð011þ b == ð0001þ ; 1 11 b == 2 1.[27] Subsequent trnsmission electron microscopy studies [28,29] indicte tht the orienttion reltionship is ctully ner the Burgers. The b precipittes in this orienttion reltionship hve plte morphology, with their brod surfce prllel to (0001) (Figures 1(b) nd (c)). Although the b pltes in this orienttion reltionship re often described s incoherent, [7,8,30] mple experimentl evidence demonstrtes tht the equilibrium b phse is in fct not incoherent. Aprt from the pprent lttice mtching between the b phse nd surrounding mtrix phse in 3892 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

3 Tble I. Prt of the Whole Precipittion Sequence in Individul Mgnesium Alloy Systems Precipittion process is not well studied; *d is seprtion distnce of columns of RE toms; # low Gd:Zn weight rtio nd low Gd content. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

4 the plte brod surfce, or hbit plne, the lttice mtching is lso found in interfces defining the mjor nd minor side fcets of individul b pltes. [28,29,31] Despite the irrtionl orienttion of these side fcets with respect to both precipitte nd mtrix lttices, the mjor nd minor side fcets (Figure 2()) re invribly prllel to the moire fringes defined by the intersection of 1 0 nd 0 33 b, nd of nd 4 11 b, respectively. Figure 2() shows the mjor side fcet of thin b plte tht is embedded in the mtrix phse. This mjor interfce is prllel to the moire fringes resulting Fig. 1 () Isotherml ging curves of mgnesium lloy AZ91 t 373 K nd 473 K (0 C nd 200 C) (dpted nd reproduced from Refs. [19] nd [20]). (b nd c) Trnsmission electron microgrphs showing the distribution nd morphology of b precipittes in smples ged for 8 h t 473 K (200 C). Electron bem is prllel to 2 1 in (b) nd [0001] in (c). (b) is reproduced from Refs. [29] nd [20], nd (c) is from Ref. [20]. Fig. 2 () [0001] Trnsmission electron microgrph showing the prllelogrm shpe of b precipittes in AZ91 (dpted nd reproduced from Refs. [29] nd [20]). (b) High-resolution trnsmission electron microgrph showing the mjor plnr interfce of b precipitte such s tht shown in (); the plnr interfce is indicted by moiré plnes (reproduced from Ref. [33]). (c) Schemtic digrm showing the orienttion reltionship between two sets of lttice plnes, e.g., 1 0 nd 0 33 ; nd their resultnt moire plne, which corresponds to shown in (b). This digrm demonstrtes the coherent b mtching of the two sets of lttice plnes within the moire plne. [34] 3894 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

5 from the overlpping of the 1 0 nd 0 33 b plnes (Figure 2(c)), nd it contins some ledges whose unit height is defined by the interplnr spcing of the moiré fringes. The migrtion of this interfce in its norml direction seems to involve the formtion nd lter gliding of moire ledges within the interfce plne. [32,33] These observtions suggest the existence of commensurte mtching of 1 0 nd 0 33 b plnes[29,33,34] in the mjor fcet interfce, nd of nd 4 11 b in the minor fcet interfce. This commensurte mtching, together with the fct tht 0 33 b nd 4 11 re the b closest-pcked plnes in the b lttice nd 1 0 is the ner closest-pcked plne in the mgnesium lttice, suggests tht the mjor nd minor side fcets of ech b plte hve reltively low interfcil energies. Aprt from the ner Burgers orienttion reltionship, two other orienttion reltionships hve lso been reported for the b phse, [19,27,29,35,36] nmely 1 b == 1 0, ½111Š b == ½ 0001 Š nd 1 b == 1 0, ½115 Š b == ½0001Š. For the former orienttion reltionship, the b precipittes hve rod shpe with their long xes prllel to ½0001Š. The ½0001Š rods hve hexgonl cross section, with the bounding fcets prllel to 1 0 == 330. For the ltter orienttion b reltionship, the b precipittes develop rod shpe with their long xes inclined with respect to the ½0001Š direction. Although these rods re more effective thn the (0001) in impeding disloction gliding on the bsl plne, only smll frction of them exists in the microstructure. It is currently uncler how to promote the rod-shpe precipittes t the expenses of the (0001) pltes. Such n effort inevitbly requires n in-depth understnding of the trnsformtion strins ssocited with ech of the orienttion reltionships nd the ctivtion energy brrier to nucletion. 2. Effects of cold work nd lloying dditions Cold work fter solution tretment nd prior to ging, microlloying, nd mcrolloying dditions to Mg-Al lloys do not seem to produce significnt enhncement in the ge-hrdening response. Clrk reported [15] tht cold work fter solution tretment nd prior to ging of Mg-9 wt pct Al lloy could increse the ge-hrdening response. The enhnced ging kinetics nd the mximum hrdness is ttributble to high number density of disloctions nd twins in the cold-worked smples before ging nd heterogeneous nucletion of b precipittes on such lttice defects. It ws noted tht the density of precipittes formed t the twin interfce nd within twins is higher thn tht in the untwinned mtrix regions. The preferentil precipittion within twins my be ttributed to the presence of stcking fults [37] nd higher density of disloctions within the twins. [38] The effects of microlloying dditions on precipittion nd ge hrdening of mgnesium lloy AZ91 were studied by Bettles et l. [18] They dded, seprtely, 0.1 t. pct Li, B, C, Ti, Sr, Ag, Mo, B, Pb, or 0.05 t. pct Si to AZ91. Even though the dditions of such microlloying elements could influence the kinetics of precipittion nd hrdening, they did not led to ny pprecible increse in the mximum hrdness vlues of the lloys (Figure 3). Therefore, it ws speculted tht the individul dditions of these elements did not increse the nucletion rte of the b phse or result in the formtion of ny new precipitte phses tht cn further enhnce the ge-hrdening response. The mcrolloying ddition of C to Mg-Al lloys hs been shown to improve the creep resistnce. [39,40] It is commonly reported in the erly studies tht the dded C toms rect with Al toms to form Al 2 C tht hs C15 structure (spce group Fd3m; = nm), [39] (Mg,Al) 2 C of C14 structure, [40] or mixture of the two Lves phses during csting. In subsequent study, [41] it ws reported tht the intermetllic phse formed in the Mg-Al-C lloys hs in fct C36 structure ( = nm, c = nm). This C36 Lves phse hs now been confirmed in mny recent studies to exist s n equilibrium phse in the Mg-Al-C system (Figure 4). [42] For long time, it hs been generlly ccepted tht the Mg-Al-C lloys re not ge hrdenble. However, pper published in 2005 [43] reported the precipittion of C15 when high-pressure die-cst Mg-4.5Al-3.0C-0.14Sr-0.25Mn (wt pct) lloy, Fig. 3 Effects of microlloying dditions on the ge-hrdening response of AZ91 t 473 K (200 C). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

6 designted AXJ530, is ged t 573 K (300 C). The C15 precipittes form s pltes on (0001), with the following orienttion reltionship with the mtrix phse: (111) C15 == (0001) nd 1 C15 ==. In subsequent study, [44] it ws reported tht the sme lloy exhibits n ge-hrdening phenomenon when it is ged t 448 K to 523 K (175 C to 250 C) nd tht this ge-hrdening response is cused by the precipittion of the C15 pltes. In very recent study [45] of Mg-2Al-2C nd Mg-2Al- 2C-0.3Mn (wt pct) lloys, which were produced by permnent mold csting nd creep tested t 448 K nd 473 K (175 C nd 200 C) under 50 MP, the C15 pltes were observed to form in the s-cst microstructures of the two lloys, nd ordered G.P. zones form on the bsl plne of the mtrix phse in the crept smples. These ordered G.P. zones were inferred to be similr to those observed in Mg-RE-Zn nd Mg-C-Al lloys, which will be described in Section II D, nd their number density in the Mn-contining lloy is higher thn tht in the Mn-free lloy. B. Mg-Zn Bsed Alloys 1. Phse equilibri nd precipittion The mgnesium-rich side of the Mg-Zn binry phse digrm is more complex thn tht of the Mg-Al binry phse digrm. The eutectic temperture is 613 K (340 C), nd the mximum solid solubility of Zn in mgnesium is 6.2 wt pct (or 2.4 t. pct) t the eutectic temperture. [14] The eutectic rection is such tht the liquid phse solidifies into mixture of -Mg nd Mg 7 Zn 3 phses. The Mg 7 Zn 3 phse hs n orthorhombic structure (spce group Immm, = nm, b = nm, nd c = nm [46] ), nd it is thermodynmiclly stble only t tempertures bove 598 K (325 C). At tempertures t nd below 598 K (325 C), the Mg 7 Zn 3 phse decomposes, vi n eutectoid Fig. 4 Isotherml section of the Mg-Al-C ternry phse digrm t 673 K (400 C) (reproduced from Ref. [42]). The c phse in the digrm is the b-mg 17 Al 12 phse in the present pper. rection, into -Mg nd MgZn. The intermetllic phse tht is in equilibrium with -Mg t tempertures below 598 K (325 C) is MgZn. The structure of this intermetllic phse ws not unmbiguously estblished before 2006, even though the Mg-Zn bsed lloys hve received considerble ttention in the pst yers. Bsed on the X-ry diffrction results, Khn [47] proposed rhombohedrl structure for the MgZn phse nd expressed the lttice prmeters in hexgonl version, with = nm nd c = 1.84 nm. However, this rhombohedrl structure hs so fr not been confirmed by the others. A recent study mde by trnsmission electron microscopy nd electron microdiffrction [48] suggests tht the MgZn phse hs bse-centered monoclinic structure ( = 1.6 nm, b = nm, c = nm, b = deg). During het tretments of Mg-8 wt pct Zn lloy t tempertures below 598 K (325 C), it ws observed [48] tht the primry intermetllic prticles of the Mg 7 Zn 3 phse tht hd formed during the solidifiction process decompose into divorced lmellr structure of -Mg nd Mg 4 Zn 7. The Mg 4 Zn 7 phse is metstble, nd it grdully replced by the equilibrium phse MgZn fter prolonged het tretment. The Mg 4 Zn 7 phse hs bse-centered monoclinic structure nd the following orienttion reltionship: ½001Š Mg4 Zn 7 == ½0001Š nd ð630þ Mg4 Zn 7 == 01.[48] The structure of this metstble Mg 4 Zn 7 phse is identicl to tht of the equilibrium Mg 4 Zn 7 phse (spce group B/2m, = nm, b =1.428nm,c = nm, c = 2.5 deg) in the Mg-Zn system [49,50] rther thn the triclinic structure ( = nm, b = nm, c = nm, = 96 deg, b = 89 deg, c = 138 deg) ssumed for the Mg 2 Zn 3 phse by Gllot nd Grf [51] nd dopted by the others. [6,8,31] Even though it is still uncler whether the monoclinic phse Mg 4 Zn 7 is ctully identicl to the Mg 2 Zn 3 phse, it is commonly ccepted [52] tht these two phses re the sme, s only one intermetllic phse exists t compositions close to Mg-( ) t. pct Zn in the Mg-Zn binry phse digrm. The equilibrium solid solubility of Zn in mgnesium decreses substntilly with temperture, nd the controlled decomposition of the supersturted solid solution of Zn in mgnesium cn produce n ge-hrdening effect. [52 56] The ging curves of two binry Mg-Zn lloys re provided in Figure 5(). Depending on the lloy composition nd ging tempertures, it is commonly ccepted in the literture [6,8,30] tht the decomposition of the supersturted solid-solution mtrix phse reportedly involves the formtion of G.P. zones, b 0 1 (MgZn 2), b 0 2 (MgZn 2), nd b (Mg 2 Zn 3 ). The formtion of G.P. zones, which re described s coherent disks formed on (0001), hs not been supported by direct experimentl evidence so fr. Although G.P. zones, together with Zn clusters nd G.P.1 zones, were reported in recent studies [57,58] to form in Mg- 2.8 t. pct Zn lloy ged t tempertures 295 K, 343 K, 371 K, nd 433 K (22 C, 70 C, 98 C, nd 160 C), no compelling experimentl evidence ws provided to support the existence of such Zn clusters nd G.P. zones. The hrdness vlues reported in this 3896 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

7 Fig. 5 () Age-hrdening response of Mg-8 wt pct Zn nd Mg-5 wt pct Zn lloys t 423 K nd 473 K (150 C nd 200 C). (b nd c) Trnsmission electron microgrphs showing [0001] rods nd (0001) pltes in Mg-8Zn lloy ged for 00 h t 473 K (200 C). (d) The cross section of [0001] rod of Mg 4 Zn 7 nd the substructure within this precipitte. Electron bem is prllel to 2 1 in (b) nd [0001] in (c nd d). (c) is dpted from Ref. [65]. work re lso unrelisticlly high compred with those reported by the others. Further creful chrcteriztion in the future using 3DAP nd tomic-resolution HAADF-STEM, s well s n in-depth nlysis of these chrcteriztion results, re necessry before the notion of G.P. zones nd clusters is formlly ccepted in the precipittion sequence of Mg-Zn lloys. The metstble phse b 0 1, which is lso described s MgZn, [52,59] forms s ½0001Š rods, wheres the metstble phse b 0 2 forms s (0001) pltes. Bsed on observtions from X-ry diffrction nd selected-re electron diffrction (SAED) ptterns, both b 0 1 nd b0 2 phses were suggested to hve hexgonl structure ( = nm, c = nm) [56,60 63] tht is identicl to tht of MgZn 2 (spce group P6 3 /mmc, = nm, c = nm). [64] Another hexgonl structure ( = nm, c = nm) ws lso reported for b 0 1,[59] but it hs not been confirmed so fr. The orienttion reltionships for these two precipitte phses re tht ½0001Š b 0 == nd == b 0 1 (0001) between b 0 1 nd -Mg,[60 62] nd ð0001þ b 0 == 2 (0001) nd == between b 0 b nd -Mg. [56,60] A recent electron microscopy study of precipitte phses in Mg-8 wt pct Zn lloy ged t 473 K (200 C) [65] indictes tht the precipitte structures nd orienttion reltionships re more complicted thn those reported in erly studies. Figures 5(b) through (d) show precipittes typicl of Mg-8 wt pct Zn smples ged for 00 hours t 473 K (200 C). Most precipittes re b 0 1 rods/lths, wheres frction of b0 2 pltes is lso visible in the microstructure. Electron microdiffrction ptterns obtined from the b 0 1 rods indicte tht, contrry to the trditionl view, they hve bsecentered monoclinic structure ( = nm, b = nm, c = nm, c = 2.5 deg) tht is similr to tht of Mg 4 Zn 7, nd tht the orienttion reltionship is such tht ½001Š b 0 == ½ 0001 Š 1 nd ð630þ b 0 1 ~== 01. This orienttion reltionship is identicl to tht observed between Mg 4 Zn 7 nd -Mg phses in METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

8 the eutectoid rection within primry prticles of the Mg 7 Zn 3 phse. The proposed crystl structure nd the orienttion reltionship re subsequently confirmed in seprte studies on precipittes in Mg-Zn-Y lloys. [66 68] In these ltest electron microscopy studies, it is reveled tht the Mg 4 Zn 7 phse hs complex substructure nd plnr defects elongted long the long xis of b 0 1 rods (Figure 5(d)). In very recent study,[69] it ws reported tht the b 0 1 rods contin mixture of Mg 4 Zn 7 nd MgZn 2 phses tht hve the following orienttion reltionship: ½0Š Mg4 Zn 7 == ½0001Š MgZn2 nd 20 1 Mg 4 Zn 7 == 0 1. Some domins of fcecentered cubic structure (C15) were lso proposed to MgZn 2 exist inside the b 0 1 rods. It is uncler currently whether ny in situ trnsformtion from one structure to the other occurs within the b 0 1 rods. It seems necessry to employ tomic-resolution HAADF-STEM to resolve the complex substructure of the b 0 1 rods. A smll frction of the b 0 1 phse ws lso found[65] to dopt rrely reported blocky shpe nd different orienttion reltionship with the -Mg phse, e.g., ½001Š b 0 == 1 nd ð250þ b 0 ~ == (0001). 1 All recent studies hve confirmed tht the b 0 2 phse hs the MgZn 2 structure ( = nm, c = nm) nd the orienttion reltionship reported in erly studies. Similr to b 0 1 rods, the b0 2 pltes lso hve complex substructure of domins nd plnr defects. [65] Most prticles of the b 0 2 phse dopt plte morphology, but smll frction of the b 0 2 phse lso exists s lths with their long xis prllel to ½0001Š. These b 0 2 lths cn be distinguished from the b 0 1 rods from their morphology becuse their cross section hs lrger spect rtio nd pper s ner prllelogrm shpe with the brod surfce prllel to. The orienttion reltionship between these b 0 2 lths nd -Mg is tht == [0001] b 0 nd ð0001þ 2 b 0 == This orienttion reltionship is clerly different from tht ssocited b 0 2 pltes but identicl to tht reported for b0 1 rods in erly studies. [60 62] A few b 0 2 lths with rrely reported orienttion reltionship == [0001] b 0 nd == were lso found. [65] The brod b 0 2 surfce of these lths is ~ 6 deg from the nerest plne insted of being prllel to. Surprisingly, the structure nd composition of the equilibrium b phse hve long been ccepted s those of the Mg 2 Zn 3 phse, i.e., the triclinic structure ( = nm, b = nm, c = nm, = 96 deg, b =89deg, c = 138 deg) nd the Mg 2 Zn 3 composition, even though the lloy composition lies in the (-Mg + MgZn) two-phse field. Bsed on the Mg- Zn binry phse digrm nd more recent electron microscopy study, it seems pproprite to suggest tht the equilibrium b phse hs MgZn composition nd bse-centered monoclinic structure ( = 1.6 nm, b = nm, c = nm, b = deg). Recent studies indicte clerly tht the precipittion sequence in Mg-Zn lloys contining 4 to 9 wt pct Zn nd ged isothermlly t 393 K to 533 K (120 C to 260 C) is different from tht ccepted trditionlly. [6 8] The probble precipittion sequence in the Mg-Zn lloys is provided in Tble I. Although some detiled informtion hs been gined in recent yers, with the help of dvnced chrcteriztion fcilities, on the structure nd morphology of precipittes in Mg-Zn bsed lloys, there still is lck of reports in the literture tht elucidte the detils of the full precipittion process nd provide some insightful understnding of the nucletion nd growth behviors of the precipitte phses in this group of lloys. For exmple, why do the structures of the metstble b 0 1 nd b 0 2 phses resemble closely those of the Mg 4Zn 7 nd MgZn 2 phses tht exist s equilibrium phses in the Mg- Zn binry phse digrm? If n in situ structurl trnsformtion from Mg 4 Zn 7 to MgZn 2 exists within the b 0 1 rods, then why does the MgZn 2 phse lso form prior to or simultneously with the Mg 4 Zn 7 phse? 2. Effects of lloying dditions Becuse the ge-hrdening response of binry Mg-Zn lloys is limited, efforts hve been mde in the pst to improve the ge-hrdening response of Mg-Zn lloys vi mcrolloying nd microlloying dditions. Exmples of mcrolloying dditions include Cu, [5,70] Co, [71] nd B [72] (Figure 6()). In these lloys, the solution tretment temperture cn be incresed from 593 K to 608 K (320 C to 335 C), typiclly used for Mg-Zn binry lloys, to 703 K to 713 K (430 C to 440 C) for Mg-Zn- X (X = Cu, B, nd Co) lloys without cusing ny locl melting of the csting lloys (Figures 6(b) nd (c)). This might be the result of substntilly incresed eutectic temperture in the Mg-Zn-X lloys. The use of much higher temperture for the solution tretment llows more Zn toms to be dissolved into the mgnesium mtrix fter the solution tretment nd possibly more vcncies to be chieved fter the wter quench. The higher concentrtions of Zn toms nd vcncies cn result in n enhnced ge-hrdening response during the isotherml ging tretment. A comprison of the ging curves of Mg-8Zn nd Mg-8Zn-1Co (wt pct) lloys (Figures 6() nd 5()) indictes tht the mximum hrdness vlue chievble t 473 K (200 C) is incresed by pproximtely 18 pct nd tht the ging time needed to chieve the mximum hrdness is reduced from ~24 hours to ~3 hours. Another comprison of microstructures of pek-ged smples of these two lloys (Figures 6(d) through (g)) revels the incresed mximum hrdness is ssocited with denser distribution of precipittes in the Mg-8Zn-1Co (wt pct) lloy. In contrst to mcrolloying dditions, microlloying dditions to Mg-Zn lloys generlly cnnot rise the eutectic temperture, nd thus, they do not permit higher tempertures to be used for the solution tretment. [73 77] However, the dditions of pproprite lloying elements cn eqully result in substntil enhncement in ge-hrdening response, s demonstrted in Figure 7(). [75,77] The dditions of 0.1 to 0.35 wt pct C to Mg-(4-6)Zn lloys, [78 80] the ddition of Ag, or the combined ddition of Ag nd C to Mg-6 wt pct Zn lloy [75,76,81 85] cn result in significnt enhncement in ge-hrdening response nd tensile yield strength (Tble II). The improved ge-hrdening 3898 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

9 Fig. 6 () Effects of mcrolloying dditions of Cu nd Co on ge-hrdening response of Mg-Zn lloys t 473 K (200 C). (b nd c) Reflected light microgrphs showing retined intermetllic prticles in solution treted ZC63 nd ZO81 lloys respectively. (d through g) Trnsmission electron microgrphs showing distribution of precipittes in (d nd e) Mg-8Zn lloy, nd (f nd g) Mg-8Zn-1Co lloy. (c through g) re dpted from Ref. [71]. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

10 response is ssocited with refined distribution of rod-shped precipittes (Figures 7(b) nd (c) [76,77] nd Figure 8). [78] The nlysis of 3DAP dt suggests the cosegregtion of C nd Zn toms in the preprecipittion stge. The Ag toms do not ssocite with the C/ Zn clusters nd re uniformly distributed in the mgnesium mtrix phse before they segregte to precipittes tht form in the pek-ged condition. Becuse the combined ddition of C nd Ag leds to the lrgest increment in the mximum hrdness, it is uncler how the C nd Ag toms fcilitte the nucletion rte of the precipittes. A more thorough chrcteriztion of the distribution of the C nd Ag toms nd of the structure nd orienttion reltionships of the precipittes, s well s considertions of solute-vcncy binding energies in mgnesium, [86,87] re ll required if the precise role of C nd Ag in the nucletion is to be reveled. Some other studies indicte tht mcrodditions of RE elements [56,88] nd microlloying dditions of Sn nd In [89] to Mg-Zn lloys hve little effect on the gehrdening response. A commercil mgnesium lloy developed from the Mg-Zn-RE system is ZE41, Mg-4.2 wt pct Zn-1.3 wt pct RE-0.7 wt pct Zr (where RE represents rre-erth misch-metl). This lloy is often fbricted in the T5, insted of T6, condition for pplictions in helicopter trnsmission housings. The most commonly studied lloying ddition in recent yers seems to be Y. The dditions of Y to Mg-Zn lloys led to the formtion of reltively lrge prticles of qusicrystlline phse. [90,91] The formtion of such qusicrystlline prticles does not contribute much to the lloy strength, nd therefore, the Mg-Zn-Y lloys re generlly extruded to chieve finer mgnesium grins for strengthening purpose [92 94] (Tble II). C. Mg-Zn-Al-Bsed Alloys Mg-Zn-Al bsed lloys, with the Zn:Al weight rtio in the rnge 1:1 to 3:1, hve received some interest in the lst 15 yers for developing csting lloys for elevted temperture pplictions. Although the Mg-Zn-Al ternry phse digrm is reltively well estblished compred with other Mg-bsed ternry phse digrms, [95 98] the identities of the equilibrium intermetllic phses in the Mg-Zn-Al lloys re still controversil. Bsed on X-ry diffrction observtions, the equilibrium intermetllic phse in the Mg-Zn-Al lloys hs been determined [99 1] to be the T phse tht hs n tomic composition of Mg 32 (Al,Zn) 49 nd body-centered cubic structure (spce group Im3, ~ 1.4 nm [2] ). In contrst, the equilibrium intermetllic phse in the Mg-8 wt pct Zn-(4 8) wt pct Al lloys hs been found [3] to the / phse. For the Mg-8 wt pct Zn-(4 8) wt pct Al lloys, the ltest version of the Mg-Zn-Al isotherml section t 593 K (320 C) [97,98] (Figure 9) indictes tht the equilibrium intermetllic phse is / insted of T (or s in the Figure 9). The / phse ws originlly reported to hve composition of Mg 5 Al 2 Zn [4] 2 nd primitive orthorhombic structure ( = nm, b = nm, nd c = nm). [5] A subsequent study using trnsmission electron microscopy nd convergent-bem electron diffrction [6] Fig. 7 () Age-hrdening response of microlloyed Mg- 2.4 t. pct Zn during isotherml ging t 433 K (160 C) (reproduced from Refs. [75] nd [77]). (b nd c) Trnsmission electron microgrphs showing distribution of precipittes in extruded nd pek-ged smples of Mg-2.4Zn-0.1Ag-0.1C-0.16Zr (t. pct) lloy (reproduced from Refs. [76] nd [77]) VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

11 Tble II. Tensile Properties t Room Temperture of Mgnesium Csting nd Wrought Alloys Tensile Properties Alloy Composition (wt pct) Testing Condition UTS (MP) YS (MP) E (pct) Csting Alloys AZ91 Mg-8.7Al-0.7Zn-0.1Mn T ZK61 Mg-6Zn-0.7Zr T ZC63 Mg-6Zn-2.7Cu-0.3Mn T ZA85 Mg-7.7Zn-4.9Al-0.3Mn T QE22 Mg-2.5Ag-2.1Di-0.7Zr T WE54 Mg-5.2Y-3RE-0.7Zr T WE43 Mg-4Y-3.4RE-0.7Zr T MRI201S T MEZ Mg-2.5RE-0.5Zn T NEZ Mg-3Nd-0.5Zn T EV31 Mg-3Nd-2Gd-0.6Zn-0.5Zr T VZ61 Mg-6Gd-1Zn-0.6Zr T XZE111 Mg-1C-1Zn-1Nd-0.6Zr T VW114 Mg-11.3Gd-3.8Y-0.7Zr T VWZ1142 Mg-11.1Gd-4.1Y-1.7Zn-0.5Zr T VE112 Mg-11Gd-2Nd-0.5Zr T VQ182 Mg-18.2Gd-1.9Ag-0.3Zr T VW2 Mg-Gd-2Y-0.5Zr T Extrusion Alloys AX43 Mg-3.5Al-3.3C-0.4Mn F ZK60 Mg-6Zn-0.6Zr F ZX51 Mg-4.7Zn-0.5C F ZXK601 Mg-5.7Zn-0.2C-0.8Zr F ZQX6 Mg-6Zn-0.4Ag-0.2C-0.6Zr F T XZ11 Mg-1C-1Zn-0.6Zr F WE43 Mg-4Y-3Nd-0.5Zr T WE54 Mg-5.3Y-3.5Nd-0.5Zr T Elektron 675 Mg-6Y-7Gd-0.5Zr T T VK14 Mg-14Gd-0.5Zr F T T VZ142 Mg-14Gd-2.3Zn F WZ75 Mg-6.7Y-4.9Zn F VWZ62 Mg-Gd-5.7Y-1.6Zn-0.7Zr F T ZW51 Mg-5Zn-0.9Y-0.2Zr F Sheet/Plte Alloys ZM61 Mg-6Zn-1Mn T ZAM631 Mg-6Zn-3Al-1Mn T ZQX6 Mg-6.3Zn-0.5Ag-0.2C-0.1Zr T VWZ1142 Mg-11Gd-4Y-1.7Zn-0.5Zr T UTS: ultimte tensile strength; YS: yield strength; E: elongtion. confirms the primitive orthorhombic unit cell nd lttice prmeters proposed in the erly studies, nd it indictes tht the / phse hs spce group of Pbcm nd composition of Mg 21 (Zn,Al) 17. For Mg-Zn-Al lloys with compositions lying in the (-Mg + /) two-phse field nd produced by high-pressure die csting or permnent mold csting, recent studies using TEM nd convergent-bem electron diffrction ptterns [7,8] indicte tht primry intermetllic prticles in the s-cst condition hve qusi-crystlline structure (point group of m 3 5, qusi-lttice prmeter ~0.515 nm) nd composition of pproximtely Mg 55 Al 19 Zn 26. The qusi-lttice prmeter is very close to those of icoshedrl phses formed in rpidly [9] [1] solidified Mg 32 Al 17 Zn 32 nd Mg 32 (Al,Zn,Cu) 49 intermetllic lloys, but it is smller thn the tht (0.528 nm) reported for the icoshedrl Mg 38.5 Al 52 Zn 9.5 phse in Al-Mg-Zn lloys. [111] Even though the equilibrium T phse is often regrded s the crystlline pproximnt of the icoshedrl phse, [112] the metstble qusi-crystlline phse does not trnsform to the T phse fter prolonged heting t elevted tempertures such s 598 K (325 C). Insted, the qusi-crystlline prticles re grdully replced by the / phse without ny intermedite phses formed between them. Given tht the composition of the Mg-8 wt pct Zn-4 wt pct METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

12 Fig. 8 Trnsmission electron microgrphs showing distribution of precipittes in ( nd c) Mg-4 wt pct Zn, nd (b nd d) Mg-4 wt pct Zn-0.35 wt pct C lloys. Electron bem is prllel to [0001] in ( nd b) nd 2 1 in (c nd d). The smples re solution treted t 618 K (345 C) for 2 h, rmped to 803 K (530 C) in 2 h, nd then t 803 K (530 C) for 12 h, wter quenched nd ged t 450 K (177 C) for 28.9 h. Reproduced from Ref. [78]. Al lloy is in the (-Mg + /) two-phse field, the replcement of the qusi-crystlline phse by the equilibrium / phse is not surprising t ll. The icoshedrl phse hs composition closer to tht of the / phse thn to the T phse, nd the / unit cell hs icoshedrl clusters. [5,6] The trnsformtion from metstble icoshedrl phse to nonpproximnt crystlline phse hs been observed in Mg 65 Zn 25 Y intermetllic lloy. [113] The ging curves of Mg-8Zn, Mg-8Zn-4Al, nd Mg- 8Zn-8Al (wt pct) lloys, which re solution treted for 4 hours t 598 K (325 C) (Mg-8Zn nd Mg-8Zn-4Al) nd 623 K (350 C) (Mg-8Zn-8Al), wter quenched, nd ged t 473 K nd 423 K (200 C nd 150 C), re provided in Figures () nd (b). [114] The ge-hrdening response is significntly enhnced by ternry ddition of 4 to 8 wt pct Al to the Mg-8Zn lloy. The mximum hrdness vlues chieved in the Mg-8Zn- 8Al lloy re considerbly higher thn those obtined in Mg-6 wt pct Zn-3 wt pct Cu (ZC63), Mg-8 wt pct Zn- 1 wt pct Co (ZO81) (Figure 6()), nd Mg-8 wt pct Zn- 1.5 wt pct RE [56] lloys. The quternry ddition of 0.5 wt pct C to Mg-8 wt pct Zn-4 wt pct Al lloy does not led to ny enhncement in ge-hrdening response (Figure (c)), nd n increse in the C content from 0.5 wt pct to 1.0 wt pct cuses reduced ge-hrdening response (Figures (c) nd (d)), even though the C ddition retrds the overging of the lloys. [115] The effects of preging on the ge-hrdening response of Mg-6Zn-3Al-1Mn (wt pct) lloy were studied by 3902 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

13 Fig. 9 Isotherml section of the Mg-Zn-Al ternry phse digrm t 593 K (320 C) (reproduced from Ref. [98]). The c phse in the digrm is the b-mg 17 Al 12 phse in the present pper. Oh-ishi et l. [116] Their lloy ws ged initilly t 343 K (70 C) for 48 hours nd followed by subsequent ging t 423 K (150 C). It ws found tht the preging enhnces the ge-hrdening response t 423 K (150 C) nd tht the double-ged microstructure contins finer distribution of precipittes. The sheet of this lloy, produced by twin-roll csting nd hot rolling, exhibits tensile yield strength of 319 MP (Tble II) fter it is preged t 343 K (70 C) for 24 hours nd then ged t 423 K (150 C) for 24 hours. [117] In seprte study of Mg-2.4 t. pct Zn-2 t. pct Al (Mg-6.2 wt pct Zn-2.1 wt pct Al) lloy, it ws reported [84] tht the combined ddition of 0.1 t. pct Ag nd 0.1 t. pct C to this lloy could led to significnt enhncement of the ging response t 433 K (160 C) nd tht the mximum hrdness chievble ws rised by pproximtely 27 pct, from ~75 VHN in the Mg- 2.4Zn-2Al lloy to ~95 VHN in the Mg-2.4Zn-2Al- 0.1Ag-0.1C lloy. The mximum hrdness vlue does not chnge when the Al content in the lloy is incresed or decresed by 1 t. pct, nd it remins unchnged even when the Al content is reduced to zero. These observtions suggest tht the ge-hrdening response of the Fig. Age-hrdening response of Mg-8Zn, Mg-8Zn-4Al, nd Mg-8Zn-8Al (wt pct) lloys during isotherml ging t () 473 K (200 C) nd (b) 423 K (150 C). Reproduced from Ref. [114]. Effects of C dditions on the ge-hrdening response of Mg-8Al-4Zn (wt pct) lloy t (c) 473 K (200 C) nd (d) 423 K (150 C). Reproduced from Ref. [115]. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

14 Mg-Zn-Al-Ag-C lloys is similr to tht of the Mg-Zn- Ag-C lloys tht is shown in Figure 7(). Wheres the Mg-Zn-Al lloys exhibit substntil ge-hrdening response during isotherml ging in the temperture rnge 373 K to 473 K (0 C to 200 C), the solid-stte precipittes formed in these lloys hve not been chrcterized in detil. Figure 11 shows microstructure typicl of Mg-8 wt pct Zn-8 wt pct Al lloy, homogenized nd solution treted for 122 hours t 598 K (325 C), wter quenched, nd then ged for 120 hours t 473 K (200 C). The microstructure contins predominntly dispersion of rhombic precipittes. These precipittes seem to distribute heterogeneously throughout the mgnesium mtrix phse, distributing long lines tht re pproximtely prllel to h i directions when the microstructure is viewed in the [0001] direction. Electron microdiffrction ptterns obtined from such precipittes re indictive of icoshedrl structure with qusi-lttice prmeter of pproximtely 0.52 nm, insted of the T phse tht hs been reported for most solid-stte precipittes formed in high-pressure die cst Mg-8 wt pct Zn-5 wt pct Al lloy. [118] Currently, there is lck of convergent bem electron diffrction ptterns to estblish unmbiguously whether these precipittes hve perfect icoshedrl symmetry or re crystlline pproximnt with very lrge unit cell prmeter, but tomic-resolution trnsmission electron microscopy does not revel ny periodic crystl structure or nnoscle twins existing within such prticles. The orienttion reltionship between the rhombic precipittes nd the mtrix phse is tht ð8=13; 5=8; 3=5Þ i == 21 nd [5-fold] i == [0001]. The fcets of the rhombic precipitte re not prllel, or close to prllel, to ny of the close-pcked plnes of the two phses (Figure 11(d)). However, it ws found [119] tht the two rhombic interfces re exctly prllel to the moire plnes defined by the intersection of 3=5; 8=13; 5= 8 nd 1 0 plnes, nd the i 1 0= 6; 0=0; 0=0 nd 01 plnes, respectively. This i observtion indictes tht the 3=5; 8=13; 5= 8 nd i 10 plnes re fully coherent within one of the Fig. 11 Trnsmission electron microgrphs showing distribution nd morphology of precipittes in Mg-8 wt pct Zn-8 wt pct Al lloy ged t 473 K (200 C) for 120 h. Electron bem is prllel to 21 in () nd [0001] in (b through d) VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

15 precipitte mtrix interfces, wheres tht the 1 0= 6; 0=0; 0=0 nd 01 plnes re lso fully i coherent within nother precipitte mtrix interfce. Note tht 3=5; 8=13; 5= 8 i nd 1 0= 6; 0=0; 0=0 re the i closest-pcked plnes in the icoshedrl phse, nd 01 is ner closest-pcked plne in the mtrix phse. Some of the icoshedrl prticles lso hve truncted rectngulr or squre shpe when viewed in the ½0001Š orienttion, nd these icoshedrl prticles hve nother orienttion reltionship with the mtrix phse: 8=13; 5=8; 3=5 i == 1 0 nd [2-fold] i == ½0001Š. Inspection of imges of the icoshedrl prticles projected long the ½0001Š direction lso revel tht some of them contin high density of plnr defects or polycrystlline ggregtes. But the crystllogrphy of such defects is uncler. The microstructure of Mg-8 wt pct Zn-8 wt pct Al lloy smples ged for 120 hours t 473 K (200 C) lso hs smll frction of reltively corse, lth-shped / precipittes tht hve the following orienttion reltionship with the mtrix: ð002þ / == ð0002þ ; ½0Š / ==. The brod surfce of these / lths is prllel to (0001). In ddition to the precipittes of the equilibrium / phse, some precipittes of the equilibrium phse b-mg 17 Al 12 re lso found in smples ged for 120 hours t 473 K (200 C). The orienttion reltionship nd morphology of these b precipittes re similr to those observed in Mg-Al binry lloys. Although there is no doubt tht the precipittion sequence in the Mg-8 wt pct Zn-8 wt pct Al lloys nd lloys of similr compositions involves the formtion of metstble icoshedrl phse nd the equilibrium / nd b phse (Tble I), the ctul precipittion sequence in such lloys is more complex nd, therefore, requires detiled chrcteriztion using dvnced imging nd diffrction techniques in the future. For exmple, some reltively corse prticles tht hve point group perfectly consistent with tht of the T phse (spce group I3m, = 1.42 nm) re occsionlly observed in the smple ged for 120 hours t 473 K (200 C). The orienttion reltionship between these cubic precipittes nd the mtrix phse is such tht ð002þ T == ½0Š T == ½0001Š. The T phse precipittes were lso reported to form during isotherml ging t 443 K (170 C) of high-pressure die cst Mg-8 wt pct Zn-4.8 wt pct Al-0.3 wt pct Mn (designted ZA85). [118] Wheres the T phse is n equilibrium phse in the ternry Mg-Zn-Al phse digrm, it is difficult to ssess whether this phse is n equilibrium phse in the Mg-8 wt pct Zn-8 wt pct Al ternry lloy becuse three equilibrium phses, -Mg, /, nd b-mg 17 Al 12, hve lredy been detected in this lloy. In Mg-6.2Zn-2.1Al (wt pct) lloy nd those of similr compositions, it hs been reported tht the precipittion sequence is similr to tht in Mg-Zn binry lloys. [84] However, it ws reported [116] in seprte study tht the ddition of 3 wt pct Al to Mg-6 wt pct Zn- 1 wt pct Mn lloy leds to chnge in precipitte morphology. The bsl pltes formed in the Al-free lloy re replced by cuboidl precipittes in the Al-contining lloy. In ddition, sphericl G.P. zones enriched in Zn lso reportedly form in the Al-contining lloy fter the lloy is ged t 343 K (70 C). These G.P. zones reportedly ct s heterogeneous nucletion sites for the metstble phses tht precipitte during subsequent ging t 423 K (150 C), resulting in finer distribution of precipittes. D. Mg-C-Bsed Alloys The Mg-C system hs some potentil for developing precipittion-hrdenble lloys. The equilibrium solid solubility of C in mgnesium is 0.82 t. pct (1.35 wt pct) t the eutectic temperture of K (516.5 C), nd it is pproximtely zero t 473 K (200 C). [14] The equilibrium intermetllic phse t the Mg-rich end of the Mg-C phse digrm is Mg 2 C tht hs crystl structure (spce group P6 3 /mmc, = nm, c = nm) similr to tht of the mgnesium mtrix phse (P6 3 /mmc, = nm, c = nm). [13] This similrity in crystl structure my result in higher nucletion rte nd, hence, higher number density of precipittes in Mg-C lloys. Assuming tht the precipittes formed during isotherml ging t 473 K (200 C) hve Mg 2 C composition, the mximum volume frction of precipittes chievble t 473 K (200 C) is clculted to be pproximtely 2.2 pct for Mg-1 wt pct C lloy, which is dequte to yield required strength. An exmple is the conventionl precipittion-hrdenble Al-0.6 wt pct Si-1.0 wt pct Mg (6061) wrought lloy. The volume frction of solid-stte precipittes is pproximtely 2 pct in this lloy. However, tensile yield strength of 275 MP is chieved in the T6 condition. [8] Becuse C hs low density (1.55 g/cm 3 ), Mg-C lloys hve the dded dvntge of preserving the low density of mgnesium, nd the ddition of C cn lso reduce the flmmbility of molten mgnesium nd improve the oxidtion nd corrosion resistnce of mgnesium. [1] Therefore, efforts hve been mde in the pst 15 yers to develop precipittion-hrdenble lloys bsed on the Mg-C system. [76, ] A study mde by Nie nd Muddle [120] indictes tht the Mg-1 wt pct C lloy exhibits only moderte gehrdening response during isotherml ging t 473 K (200 C). However, they noticed tht the ddition of 1 wt pct of Zn to the binry lloy led to substntil increse in pek hrdness nd n ccelerted rte of ging. A subsequent study [121] indictes tht the quternry ddition of 1 wt pct Nd to the Mg-1C-1Zn-0.6Zr (wt pct) lloy cn led to subsequent increse in the mximum hrdness nd strength (Figure 12()). The resultnt Mg-1C-1Zn-1Nd-0.6Zr (wt pct) lloy exhibits tensile yield strength of 153 MP t room temperture (Tble II) nd 135 MP t 423 K (150 C). [121] A comprison of the microstructures of these lloys indictes tht the significnt increse in mximum hrdness nd strength is ssocited with refined distribution nd improved therml stbility of bsl precipitte pltes (Figure 12(b)). The crystllogrphic fetures of these bsl pltes nd their electron diffrction ptterns resemble closely those formed in Mg-Nd/ Ce-Zn lloys. However, it remins to be unmbiguously estblished whether the precipittes in the Mg-C-Zn METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

16 Fig. 12 () Aging curves of Mg-C bsed lloys t 473 K (200 C). (b) Trnsmission electron microgrph showing the distribution of bsl precipitte pltes in Mg-1C-1Zn-1Nd-0.6Zr (wt pct) lloy ged for 00 h t 473 K (200 C). (c) Atomic-resolution HAADF-STEM imge showing ordered G.P. zones in Mg-0.3C-0.6Zn (t. pct) lloy. () nd (b) re reproduced from Ref. [121], nd (c) is from Ref. [76]. nd Mg-Nd/Ce-Zn lloys re structurlly identicl to ech other. An improved understnding on this spect cn fcilitte the optimiztion of lloy composition for improved therml stbility nd creep resistnce. In the s-cst microstructure of the Mg-1C-1Zn (wt pct) lloy, the primry intermetllic phse hs composition of Mg 69.4 C 27 Zn 3.6 nd hexgonl crystl structure (point group 6/mmm, ~ 0.61 nm, c ~ 1.02 nm) tht seems isomorphous with Mg 2 C. Two types of solidstte precipitte phses re observed in the s-cst microstructure: hexgonl phse with = nm, c = nm, nd hexgonl phse with = nm, c = nm. [120] Both precipitte phses form s thin pltes on (0001), nd they hve identicl orienttion reltionships with respect to the mtrix phse: (0001) p == (0001), 2 1 p ==. However, the ltter hexgonl phse, i.e., the one with = nm, c = nm, hs much more uniform distribution thn the former hexgonl phse. It ws rgued [120] tht for the observed orienttion reltionships between precipitte nd mtrix phses, lttice prmeter of = nm permits perfect lttice mtching between precipitte nd mtrix phses in the hbit plne, nd this improved lttice mtching gives rise to higher nucletion rte nd consequently n enhnced ge-hrdening response in the Mg-C-Zn lloy. It ws suggested [120] lso tht n incorportion of Zn toms into the Mg 2 C unit cell cn chnge the lttice prmeters nd, therefore, reduce the lttice misfit between the precipitte nd mgnesium mtrix phses within the (0001) hbit plne. A subsequent study of microstructures of Mg- 0.3 t. pct C-0.3 t. pct Zn lloy ged for different times t 473 K (200 C) [124] indicte the following precipittion sequence: monolyers of G.P. zones on (0001), lrger (0001) pltes of n unidentified phse, nd rectngulr Mg 2 C(Zn) phse. Ech G.P. zone ws reported to contin pproximtely 18 t. pct C nd 8 t. pct Zn. Two orienttion reltionships were reported for the Mg 2 (C,Zn) phse: (0001) p == (0001), 12 p == 01, nd 0001 ð Þ p == 01 ; 12 p == 2 1. In seprte but more recent study of precipittion- nd ge-hrdening response t 473 K (200 C) of Mg-0.3 t. pct C-Zn lloys, [76] it ws reported tht the mximum ge-hrdening response is obtined when the Zn content is 0.6 t. pct. Any increse or decrese in the Zn content in the lloy diminishes the ge-hrdening response. In this recent study, [76] HAADF-STEM ws employed to chrcterize the precipittes in the Mg-0.3 t. pct C-0.6 t. pct Zn lloy ged for different times t 473 K (200 C). [76] It ws found tht monolyer G.P. zones (Figure 12(c)) were responsible for the ge hrdening nd tht these G.P. zones hd n ordered structure tht is identicl to tht in Mg-RE-Zn lloys. [125] The totl tomic concentrtion of C nd Zn toms in ech ordered G.P. zone is ~33 pct. These ordered G.P. zones re thermlly stble nd still dominte the microstructure fter overging (16 hours) t 473 K (200 C). An increse in the Zn content from 0.6 t. pct to 1.6 t. pct in the Mg-0.3 t. pct C lloy leds to chnge in the precipittion sequence. It ws reported [76] tht the precipittion rection in the Mg-0.3 t. pct C-1.6 t. pct Zn lloy t 473 K (200 C) included the formtion of [0001] rods of metstble b 0 1 -Mg 4Zn 7 phse, (0001) pltes of the equilibrium Mg 6 C 2 Zn 3 phse (spce group P31c, = 0.97 nm, c = 1.0 nm), nd lths of n unknown phse. The brod surfce of these lths is prllel to (0001) nd their long xis is prllel to 2 1 nd. The orienttion reltionship between the Mg 6 C 2 Zn 3 phse nd the mgne- sium mtrix is such tht ð0001þ p == ð0001þ ; p ==. The Mg 6C 2 Zn 3 phse ws originlly reported to form s cuboidl prticles in melt-spun ribbons of Mg-6 wt pct Zn-1.5 wt pct C lloy. [126] These cuboidl precipittes reportedly dopt two 3906 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

17 orienttion reltionships: == 0001 p ð Þ ; ½0001Š p == 11 20, nd == 0001 p ð Þ ; ½0001Š p == [127] Although it hs been demonstrted tht dditions of Zn nd Nd cn enhnce the ge-hrdening response of Mg-C lloys, the effects of other lloying elements hve received little ttention. As discussed in the Section II A, the high-pressure die-cst Mg-Al-C-Sr (AXJ530) lloy exhibits some ge hrdening when they re ged t 448 K to 523 K (175 C to 250 C), [44] nd this ge hrdening is ssocited with the formtion of (0001) precipitte pltes of Al 2 C phse (spce group Fd3m, = nm). The totl concentrtion of ternry nd quternry lloying elements such s C nd Sr in this AXJ530 lloy is high for the purpose of cstbility nd creep resistnce, wheres the potentil of developing dilute precipittion-hrdenble lloys bsed on the Mg-Al-C system ws not explored. In very recent study, [123] it ws demonstrted tht the ternry ddition of n pproprite mount of Al (0.3 wt pct) to Mg-0.5 wt pct C lloy cn remrkbly enhnce the ge-hrdening response t 473 K (200 C) (Figure 13()). The mximum ge-hrdening response chievble t the ging temperture is reduced if the Al content in the Mg-C-Al lloy is higher or lower thn 0.3 wt pct. The pek-ged microstructure contins dense distribution of nnoscle precipitte pltes on (0001). Bsed on the monolyer thickness of these precipitte pltes, the enrichment of C in these prticles nd the selected-re electron diffrction ptterns recorded mtrix regions contining such precipittes. These precipitte pltes re inferred [123] to be ordered G.P. zones such s those observed in the Mg-C-Zn nd Mg-RE-Zn lloys. The concentrtions of C nd Al toms in the G.P. zone, mesured from 3DAP, re pproximtely 6 t. pct nd 7 t. pct, respectively. The ordered G.P. zones re grdully replced by (0001) pltes of the equilibrium phse Al 2 C with continued ging t 473 K (200 C). The formtion of the reltively lrger bsl pltes of the Al 2 C phse (Figure 11(b)) ws reported to cuse the overging of the lloy. The orienttion reltionship between the Al 2 C pltes nd the mgnesium mtrix phse is such tht (111) p == (0001) nd 01 1 p == 0 1, which is identicl to tht ssocited with the Al 2 C pltes formed in AXJ530 lloy. [43] The occurrence of the ge-hrdening phenomenon in the Mg-C-Al system offers the potentil for developing precipittion-hrdenble wrought lloy. [123] In 2011, n unusul Mg-3.5Al-3.3C-0.4Mn (wt pct) extrusion lloy ws developed. In the s-extruded condition, this lloy exhibits tensile yield strength of 4 MP, together with n elongtion to frcture of 5.6 pct [128] (Tble II). Among the RE-free mgnesium lloys, the strength level chieved in this lloy is exceptionlly impressive, nd this is ttributed to the formtion of plte-shped nd sphericl-shped precipittes, bsl texture, nd refined grin size of mgnesium. The identities of the two types of precipittes were not verified, but they were ssumed [128] to be identicl to those formed in Mg-0.5C-0.3Al (wt pct) [123] nd Mg-6Al-3.2C-0.5Mn (wt pct) [129] lloys. E. Mg-Sn-Bsed Alloys Mg-Sn-bsed lloys hve received some ttention in recent yers for developing csting nd wrought lloy products. [77, ] The Mg-Sn binry system itself is idel for developing precipittion-hrdenble lloys. [133] The mximum equilibrium solid solubility of Sn in -Mg is pproximtely 3.35 t. pct (or 14.5 wt pct) t the eutectic temperture 834 K (561 C), nd it decreses to pproximtely 0.1 t. pct t 473 K (200 C). [14] The equilibrium volume frction of precipittes obtinble t 473 K (200 C) is pproximtely 4.7 pct for Mg-7 wt pct Sn lloy. The equilibrium intermetllic phse in the Mg-Sn binry lloys is b-mg 2 Sn (spce group Fm3m, = 0.68 nm). It is rther unfortunte tht during isotherml ging tretments in the temperture rnge of 433 K to 573 K (160 C to 300 C), the precipittion process in the Mg-Sn binry lloys does not involve the formtion of ny metstble precipitte phses. The precipittes formed during the ging process re generlly much corser thn the precipittes in other precipittion-hrdenble mgnesium lloys. Therefore, Fig. 13 () Aging curves of Mg-C-Al lloys t 473 K (200 C). (b) Trnsmission electron microgrph showing distribution of bsl pltes in Mg-0.5C-0.3Al lloy ged for 00 h t 473 K (200 C). The lloy compositions re in weight percentge. Reproduced from Ref. [123]. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

18 ttempts hve been mde in recent yers [77, ] to refine the distribution of precipittes in Mg-Sn lloys by microlloying dditions. As shown in Figures 14() nd (b), [77,134] ternry dditions of Zn to Mg-Sn lloys cn remrkbly improve the ge-hrdening response, nd subsequent microlloying dditions of Cu, N, Ag, nd C, nd mcrolloying dditions of Al, to the resultnt ternry Mg-Sn-Zn lloys cn led to n even greter ge-hrdening response. A comprison of microstructures indictes tht the Zn dditions cn increse the precipitte number density, even thorough the refined precipittes re still the b phse, nd tht quternry dditions of N cn led to much finer distribution of b phse (Figures 14(c) through (e)). [134,136] Although the microlloying elements such s Zn nd N cn pprently enhnce the nucletion rtes nd, thus, the number density of b precipittes, s well s chnge the morphology of the precipittes, it remins to be estblished whether toms of such microlloying elements segregte into the precipitte or the precipitte mtrix interfce. It is uncler how microlloying elements cn enhnce the precipitte nucletion rte nd chnge the precipitte morphology, nd whether the precipitte morphology chnge is ssocited with ny chnge in orienttion reltionship. Additionl work is needed to gin n in-depth understnding of such fundmentl issues. Bsed on X-ry diffrction nd crystllogrphic nlysis in n erly study, Derge et l. [137] reported tht three orienttion reltionships (OR) exist between b nd mgnesium phses, nmely ð111þ b == ð0001þ nd 1 ð111þ b == ð0001þ nd ð1þ b == ð0001þ nd 1 11 b == 2 1 b == 2 1 b == 2 1 ðor-1þ ðor-2þ ðor-3þ Twenty-five yers lter, these three orienttion reltionships were confirmed by Henes nd Gerold [138] in their X-ry diffrction observtions. They lso reported n dditionl orienttion reltionship: ð1þ b == ð0001þ nd ½001Š b == 2 1 ðor-4þ Fig. 14 ( nd b) Age-hrdening response t 473 K (200 C) of Mg-Sn lloys without nd with dditions of microlloying elements. Trnsmission electron microgrphs showing distribution of precipittes in (c) Mg-2.2Sn, (d) Mg-2.2Sn-0.1Zn, nd (e) Mg-1.3Sn-1.2Zn-0.12N lloys. All lloy compositions re in tomic percentge. () nd (b) re reproduced from Refs. [77] nd [134] respectively, (c) nd (d) re from Ref. [136], nd (e) re from Ref. [134] VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

19 nd tht these four orienttion reltionships were ssocited with b precipittes tht were generted during ging t 433 K to 573 K (160 C to 300 C). In these erly studies, it ws not mentioned which one of the four orienttion reltionships is most populr nd wht the precise precipitte morphology is for ech of the four orienttion reltionships. An exmintion of the orienttion reltionships reported in recent yers [132,134,135, ] indictes tht even now, this informtion still remins uncler. Although the most commonly reported orienttion reltionship in recent yers seems to be OR-1, vriety of morphologies hs been reported for this OR-1, including the following: () Long lths, which were formed with the brod surfce prllel to (0001) nd the long xis prllel to 2 1, in Mg-1.9 t. pct Sn lloy ged for 240 hours t 473 K (200 C). [135] (b) Short lths, which were formed with the brod surfceprllel to (0001) nd the long xis prllel to 2 1, in Mg-1.3Sn-1.2Zn (t. pct) lloy ged for 211 hours t 473 K (200 C) [134] nd ged smples of Mg-2.2Sn-0.5Zn (t. pct) lloy. [136] (c) Short [0001] lths in Mg-1.3Sn-1.2Zn (t. pct) lloy ged 211 hours t 473 K (200 C). [134] (d) Short rods in Mg-7.8Sn-2.7Al-0.7Si-0.7Zn-0.2Mn (wt pct) lloy produced by die csting. [132] (e) Polygons in Mg-1.3Sn-1.2Zn-0.12N (t. pct) lloy ged 6.7 hours t 473 K (200 C), [134] Mg-2.2Sn- 0.5Zn (t. pct) lloy pek ged t 473 K (200 C), [139] nd Mg-5.3Sn-0.3Mn-0.2Si (wt pct) lloy ged for hours t 523 K (250 C). [140] In contrst, the long lths, which were formed on (0001) nd elongted long 2 1 in Mg-5.3Sn 0.3Mn-0.2Si (wt pct) lloy ged for hours t 523 K (250 C), [140] nd the short lths, formed on (0001) in Mg-2.1Sn-1Zn-0.1Mn (t. pct) lloy pek-ged t 473 K (200 C), [142] hve lso been reported to hve the OR-4. The polygons in Mg-2.2Sn-0.5Zn (t. pct) lloy pek ged t 473 K (200 C), [139] the ged smples of Mg- 2.2Sn-0.5Zn (t. pct) lloy [136] nd the short lths formed with brod surfce prllel to the pyrmidl plne of the mtrix phse in Mg-1.3Sn-1.2Zn (t. pct) lloy ged for 211 hours t 473 K (200 C) [135] hve been reported to hve the OR-3. Furthermore, Zhng et l. [140] reported tht the (0001) pltes in their lloy dopt the following orienttion reltionship: ð111þ b == ð0001þ nd 2 1 is 9 deg wy from 1 ðor-5þ The OR-5 cn be relted to the OR-1 by rottion of 9 deg bout the ½111Š b == ½0001Š xis. However, Sski et l. [139] reported tht the bsl pltes in their lloy hve the OR-2. Two other different orienttion reltionships hve lso been reported, nmely: ð1þ b == ð0001þ nd 31 1 b == 1 0 ðor-6þ ð111þ b == 01 nd 1 b == 2 1 ðor-7þ b Theprecipitte morphology is short lth elongted long for the OR-6[140] nd rod elongted in the pyrmidl plne of the mtrix phse. [136] It is currently uncler why severl orienttion reltionships exist nd wht microstructurl fctor dicttes the formtion of these orienttion reltionships. It remins to be exmined whether the ppernce of different orienttion reltionships nd morphologies ssocited with these orienttion reltionships is cused by the vrition of concentrtion of Sn toms in the solid-solution mtrix. Any locl segregtion of Sn toms my cuse chnge in the lttice prmeter nd, in turn, cn promote the formtion of prticulr orienttion reltionship. F. Mg-Nd/Ce-Bsed Alloys 1. Mg-Nd nd Mg-Ce binry lloys The identity of the equilibrium intermetllic phse t the Mg-rich side of the Mg-Nd binry phse digrm hs been controversil. The erly version of the phse digrm [14] indictes tht the equilibrium intermetllic phse is Mg 12 Nd tht hs tetrgonl structure (spce group I4/mmm, = nm, c = nm). However, more recent studies [ ] indicte tht the equilibrium intermetllic phse is Mg 41 Nd 5 (spce group I4/m, = nm, c = nm) insted of Mg 12 Nd (Figure 15). The mximum solid solubility of Nd in mgnesium vries significntly in the literture, rnging from 0. t. pct (0.59 wt pct) t 821 K (548 C) to 0.63 t. pct (3.62 wt pct) t 825 K (552 C). [14,146] A recent study using tom probe tomogrphy [147] indictes tht the equilibrium solid solubility of Nd in mgnesium is pproximtely 0.32 t. pct (1.87 wt pct) t 793 K (520 C), nd 0.11 t. pct (0.65 wt pct) t 673 K (400 C). The equilibrium solid solubility of Nd decreses with temperture, down to nerly zero t 473 K (200 C), implying potentil for precipittion hrdening. The ging curve of Mg-3 wt pct Nd lloy t 473 K (200 C) is shown in Figure 16(). [148] The decomposition of supersturted solid solution of mgnesium in the temperture rnge of 333 K to 623 K Fig. 15 Mg-Nd binry phse digrm (Courtesy R. Schmid-Fetzer). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

20 (60 C to 350 C) is currently ccepted s involving the formtion of G.P. zones, b, b, nd b phses. [5,8,149,150] Wheres the G.P. zones hve been reported to form s (1) needles long the [0001] direction [149] in Mg-2.9 wt pct Nd (Mg-0.5 t. pct Nd) lloy or s (2) pltelets on in Mg-2.5 wt pct Di (80 pct Nd-20 pct Pr)-0.6 wt pct Zr lloy, [151] there ws lck of direct experimentl evidence, or trnsmission electron microscopy imges, to support the formtion of such G.P. zones in these lloys t the erly stge of ging. It ws not cler whether G.P. zones did form in the Mg-Nd binry lloys nd, if so, wht morphology they might dopt. In very recent study, using HAADF-STEM, of precipitte phses formed in Mg-0.5 t. pct Nd smples ged t 443 K (170 C), [152] some peculir zigzg rrys of Nd toms ligned pproximtely long 2 1 re observed nd regrded s G.P. zones. Ech zigzg rry comprises severl V-shped, or N-shped, units tht re seprted by n lmost regulr spcing. Ech V-shped nd N-shped unit is mde of three nd four columns of Nd toms, respectively. The seprtion distnce between two neighboring columns of Nd toms within ech unit is invribly ~0.37 nm, nd the plne defined by the two neighboring columns of Nd toms is invribly prllel to. Some hexgonl rings, defined by six columns of Nd toms, re lso observed in this ltest work. [152] The seprtion distnce between two neighboring columns of Nd toms within the hexgonl ring is gin pproximtely 0.37 nm, nd the prism plne of the hexgonl ring is prllel to. Ech hexgonl ring cn be regrded s being constructed by linking three vrints of the V-shped unit mentioned in the preceding prgrph. This rrngement of Nd toms resembles tht in the D0 19 structure (spce group P6 3 / mmc, = nm, c = nm, Mg 3 Nd composition), [151] even though Sito nd Hirg did not mention it. Moreover, the re defined by the hexgonl ring is precisely tht of the bsl plne of the D0 19 unit cell (Figure 16(b)). The b phse ws originlly reported to hve D0 19 structure nd n orienttion reltionship tht is in the form (0001) b == (0001) nd 2 1 b 00== 2 1. Therefore, these hexgonl rings my be regrded s the metstble b phse. The ltest experimentl observtions mde by HAADF-STEM indicte clerly tht the b precipittes re not 2 1 pltes. Fig. 16 () Age-hrdening response t 473 K (200 C) of Mg-3 wt pct Nd lloys without nd with Zn dditions (reproduced from Ref. [148]). (b) Schemtic digrm showing D0 19 structure. The perspective viewing direction is [0001]. Lrge purple circles nd smll gry circles represent Nd nd Mg toms, respectively. (c nd d) Trnsmission electron microgrphs showing distribution of b 1 precipittes in Mg-3 wt pct Nd lloy ged for h t 523 K (250 C). [153] (e) Trnsmission electron microgrph showing the distribution of precipittes in Mg-3 wt pct Nd wt pct Zn, ged for 52 g t 473 K (200 C) (reproduced from Ref. [148]). 39 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

21 The HAADF-STEM work of Sito nd Hirg [152] lso reveled the existence of precipitte phse tht hs never been reported for the binry Mg-Nd lloys. This phse, designted b, hs n orthorhombic structure ( = 0.64 nm, b = 1.14 nm, c = 0.52 nm), Mg 7 Nd composition, nd lenticulr morphology. Although it is not mentioned by the uthors, the following orienttion reltionship cn be deduced from Figure 7 in their work [152] : (0) b == 1 2 nd [001] b == [0001]. Most of these fetures re similr to those of the b phse in Mg-Gd, Mg-Y, Mg-Gd-Y, nd WE54 lloys, s will be discussed in Section II G. The b phse mentioned in previous studies ws reported to form s pltes on. Pike nd Noble [149] suggested tht the b phse hd hexgonl structure ( = 0.52 nm, c = 1.30 nm) nd the following orienttion reltionship with respect to the mtrix phses: 1 2 b == 0, 14 b == (0001). In contrst, 0 Krimzdeh [150] nd Grdwell [151] indicted tht b hd fce-centered cubic structure ( = nm) nd composition close to Mg 20 Nd 17. Krimzdeh [150] lso suggested tht the orienttion reltionship between b nd the mtrix ws such tht 112 b == 0, [1] b == [0001]. The structure, orienttion reltionship, nd morphology proposed by Krimzdeh [150] nd Grdwell [151] re essentilly the sme s those of b 1 phse (spce group Fm 3m, = 0.74 nm) in Mg-Gd, Mg-Gd-Y, nd WE54 lloys. To void ny confusion nd to be consistent with those symbols used for similr structures, it is pproprite to use b 1 to replce b used in the previous studies. The b 1 precipittes often form heterogeneously on preexisting disloctions, leding to nonuniform distribution of these precipittes (Figure 16(c)). [153] The b phse, which ws originlly regrded s the equilibrium phse but is in fct metstble phse, hs body-centered tetrgonl structure ( = nm, c = nm) nd composition of Mg 12 Nd. [8,149] It forms s rods with their long xis prllel to [0001]. The cross section of the b rods hs hexgonl shpe, with the prism fcets prllel to. The orienttion reltionship between b nd the mtrix ws reported [154] to be the following: (002) b == nd [0] b == [0001]. Bsed on the preceding discussion of individul precipitte phses nd the fct tht the equilibrium precipitte phse is Mg 41 Nd 5, insted of Mg 12 Nd, the prt of the whole precipittion sequence in Mg-Nd binry lloys is provided in Tble I. To preserve the symbols commonly used in the literture for similr structures nd to void ny confusion, in this newly proposed sequence the b phse represents the newly reported orthorhombic phse, b 1 phse represents the b phse reported in the previous studies, nd b e is used s the finl precipitte phse, i.e., Mg 41 Nd 5. It is to be noted tht the whole precipittion sequence is fr from well estblished; the erly stge precipittion still remins to be unmbiguously estblished, nd even the lter stge of the precipittion requires more chrcteriztion nd nlysis. For exmple, the gry contrst regions connecting b 1 precipittes or segments shown in Figure 16(d) [153] cnnot be ttributble to ny of the precipitte phses known in the Mg-Nd system. The temperture-time-trnsformtion digrms for vrious precipitte phses formed in the Mg-0.5 t. pct Nd lloy were studied nd reported by Pike nd Noble. [149] Becuse the identities of the precipitte phses were not unmbiguously estblished in tht work, some cution should be tken if such temperture-timetrnsformtion digrms re to be used. The ge-hrdening response nd precipittion sequence of Mg-Ce binry nd Mg-MM (MM represents Ce- or Nd-rich misch-metl) lloys were lso studied in the pst. [154,155] Becuse the equilibrium solid solubility of Ce in mgnesium is lower thn tht of Nd, lower ge-hrdening response is expected for the Mg-Ce nd Mg-MM lloys. Wei et l. [154] nd His et l. [155] investigted the precipittion sequence of Mg- 1.3 wt pct MM (Ce-rich MM) nd Mg-1.3 wt pct Ce lloys. The identities nd formtion mechnisms of the intermedite precipitte phses re controversil in these two studies. Nevertheless, the precipittion in the Mg- Ce nd Mg-MM lloys is expected to be similr to tht in the Mg-Nd lloys. 2. Mg-Nd-Zn nd Mg-Ce-Zn lloys The intermetllic prticles formed in the s-cst microstructures of the Mg-3Nd-0.5Zn nd Mg-3Nd- 1.35Zn (wt pct) lloys re similr to those in the Mg-Nd binry lloys. They hve the tetrgonl structure of the Mg 12 Nd phse, even though they contin some Zn. The intermetllic prticles in the interdendritic regions of snd cst Mg-2.5 wt pct RE-0.5 wt pct Zn lloy, designted MEZ, hve Mg 12 (L 0.43 Ce 0.57 ) composition nd high density of unidentified plnr defects. [156] These intermetllic prticles re more difficult to dissolve into the solid-solution mtrix phse during the solution tretment, nd the volume frction of the retined such prticles increses with the Zn content in the lloy. The ddition of Zn seems to led to reduction in eutectic temperture nd, therefore, the use of lower temperture for solution tretment. For exmple, loclized melting in grin boundries ws noted fter the Mg- 3Nd-1.35Zn lloy ws solution treted for 24 hours t 803 K (530 C), [157] nd therefore, 783 K (5 C) ws used for the solution tretment. Wheres the retin intermetllic prticles in the Mg-3Nd-1.35Zn lloy still hve the structure of the Mg 12 Nd phse fter 24 hours solution tretment t 783 K (5 C), they grdully decompose nd re replced by the c phse (fcecentered cubic, = 0.72 nm) fter prolonged het tretments t 473 K nd 598 K (200 C nd 325 C). [157] The structure of this c phse is similr to tht of the Mg 3 Nd phse, except the lttice prmeter is little smller. The structures of the equilibrium intermetllic phses t the Mg-rich side of the Mg-Nd-Zn or Mg-Ce- Zn ternry phse digrm hve so fr not been unmbiguously estblished. [ ] Therefore, it is difficult to know whether the Mg 3 Nd is n equilibrium phse t the Mg-rich side of the Mg-Nd-Zn lloys nd, if so, whether c is the Mg 3 Nd phse. Ternry dditions of Zn to Mg-Nd or Mg-Ce lloys do not led to ny pprecible chnge in the gehrdening response (Figures 16() nd 17()), but they METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

22 led to significnt improvement in creep resistnce. [157] The ge-hrdening response of the Mg-Nd-Zn lloys is greter thn tht of the Mg-Ce-Zn lloys becuse of higher sturtion of solutes in the s-quenched condition nd higher volume frction of precipittes fter ging. As shown in Tble II, the tensile yield strength of Mg-Nd-Zn bsed lloy, designted NEZ, is much higher thn tht of Mg-Ce-Zn bsed lloy designted MEZ. Quternry dditions of Gd to the resultnt Mg- Nd-Zn lloys cn result in improvement in the gehrdening response, [162] nd this discovery leds to the development of commercil lloy designted EV31, which is lso known s Elektron 21. The EV31 lloy is ge hrdenble, nd it possesses dequte strength (Tble II) nd excellent creep resistnce t tempertures up to 473 K (200 C). The ddition of more thn 1.35 wt pct Zn to the Mg-3 wt pct Nd lloy results in decresed hrdening response. The commercil lloy EZ33 (Mg-3.2 wt pct MM-2.7 wt pct Zn-0.7 wt pct Zr) hs lrge mount of insoluble intermetllic prticles t grin boundries fter csting. This lloy does not hve much ge-hrdening response nd therefore is often used in the T5 condition. The precipittion sequence in Mg-2.8 wt pct Nd- 1.3 wt pct Zn lloy ws reported to include lowtemperture rection nd the formtion of c nd c phses in n erly study. [163] The low-temperture rection ws noted during isotherml resistivity mesurements t tempertures between 323 K nd 423 K (50 C nd 150 C). The nture of this low-temperture rection ws not fully chrcterized in tht study, nd it ws speculted [163] tht it ws relted to the formtion of G.P. zones. In 2003, Ping et l. [125] reported the first 3DAP result on mgnesium lloys. In their study, the trnsmission electron microscopy nd 3DAP were combined to chrcterize the structure, morphology, nd composition of nnoscle precipittes formed in Mg-2.4RE-0.4Zn-0.6Zr (wt pct, where 2.4 wt pct RE includes 1.3 wt pct Ce, 0.6 wt pct L, 0.4 wt pct Nd, nd 0.1 wt pct Pr) csting lloy, solution treted for 16 hours t 798 K (525 C), cold-wter quenched, nd then ged for 48 hours t 473 K (200 C). Hitherto unreported ordered G.P. zones, which form s (0001) disks of single tomic plne thickness, were observed in this lloy. The RE/Zn toms hve n ordered hexgonl distribution within individul G.P. zone plnes, with = nm. The orienttion reltionship between the two-dimensionllyordered G.P. zones nd -Mg mtrix phse is tht GP == : The 3DAP results indicte tht the G.P. zones contin pproximtely 3.2 t. pct Nd, 1.0 t. pct Ce, nd 1.2 t. pct Zn (Figure 17(b)). The presence of both RE nd Zn toms in the G.P. zones is to reduce the elstic strin ssocited with individul toms of RE nd Zn becuse RE toms re lrger thn Mg wheres Zn toms re smller thn Mg. In the study mde by Nuttll et l., [163] the c phse ws reported to hve hexgonl structure ( = nm, c = nm), nd it formed s pltes on the bsl plne of -Mg. The orienttion reltionship between c nd the mtrix phse ws reported to be such tht (0001) c == (0001),2 1 ==. In more c VOLUME 43A, NOVEMBER 2012 recent studies [125,148] of pek-ged smples of Mg-Nd- Zn nd Mg-RE-Zn lloys, most precipittes form s (0001) pltes (Figure 16(e)). They re similr to the c phse but were designted c. [125] In contrry to the erly work of Nuttll et l., [163] the bsl pltes of c were proposed to hve hexgonl structure ( = nm, c = nm) in these recent studies. This structure is similr to the Mg 5 (Ce,Zn) phse (spce group P 62m, = nm, c = nm). [164] The precipittes formed in ged smples of Mg-2.5RE-0.5Zn (wt pct) lloy (MEZ) (Figures 17(c) nd (d)) re similr to those in the Mg-Nd-Zn lloys. [156] The equilibrium phse c ws reported [163] to hve fce-centered cubic structure ( = 0.72 nm) nd n orienttion reltionship (011) c == (0001) nd 1 11 c == 2 1 ; nd to form s rods with their long xes prllel to nd 2 1 directions. A more recent study [157] confirmed the structure nd orienttion reltionship proposed in the erly study, but the morphology of the c phse ws proposed to be plte. The hbit plne of the c plte vries from plte to plte nd is, therefore, irrtionl, but it is lwys prllel to the moire plne defined by the intersection of {220} c nd plnes, implying coherent mtching of these two sets of lttice plnes within the hbit plne. Note tht the structure nd the orienttion reltionship of the c phse re similr to those of the b 1 phse in the Zn-free lloys, i.e., Mg-Nd binry lloys. The incorportion of Zn into the precipittes leds to the reduction in the lttice prmeter from ~0.74 nm to ~0.72 nm, nd the reduction in the lttice prmeter leds to chnge of the orienttion of the precipitte pltes to preserve the mtching between the precipitte nd mtrix lttices. 3. Mg-Nd-Ag lloys The ternry phse digrm of the Mg-Nd-Ag system is rther incomplete, [165] nd therefore, the equilibrium solid solubility of Ag in Mg nd the equilibrium intermetllic phses in the Mg-rich end of the Mg-Nd- Ag system re both uncler. According to the Mg-Ag binry phse digrm, the mximum equilibrium solid solubility of Ag in Mg is ~15 wt pct t the eutectic temperture of 745 K (472 C), nd it flls to pproximtely 2 wt pct t room temperture. The intermetllic phse t the Mg-rich side of the phse digrm is Mg 3 Ag (spce group P6 3 /mmc, = nm, c = nm). More thn 50 yers go, Pyne nd Biley [166] found tht the reltively low tensile strength of Mg-Nd lloys could be considerbly incresed by Ag dditions. This discovery subsequently led to the development of commercil lloy designted QE22, Mg- 2Nd-2.5Ag-0.7Zr (wt pct). The lloy QE22 is ge hrdenble, nd its ging curves t 423 K to 573 K (150 C to 300 C) re shown in Figure 18. [167] Within the temperture nd time selected, mximum hrdness vlue of ~85 VHN is obtinble when the lloy is ged t 423 K (150 C). After the pek hrdness is obtined t ech temperture, the prolonged ging leds to only slight reduction in hrdness. In the temperture rnge 473 K to 573 K (200 C to 300 C), the decomposition of the supersturted solidsolution phse of -Mg in QE22 lloy ws reported [5,151] METALLURGICAL AND MATERIALS TRANSACTIONS A

23 Fig. 17 () Age-hrdening response of Mg-2.5 wt pct RE-0.5 wt pct Zn (MEZ) lloy t 423 K, 450 K, nd 498 K (150 C, 177 C, nd 225 C). (b) 3DAP composition profile of ordered G.P. zones in Mg-2.4 wt pct RE-0.4 wt pct Zn-0.6 wt pct Zr lloy ged for 48 h t 473 K (200 C). (c nd d) Trnsmission electron microgrphs showing distribution nd morphology of precipittes in the s-cst microstructure of MEZ lloy produced by snd-csting. ( nd c) re reproduced from Ref. [156], nd (b) is from Ref. [125]. to occur vi two independent precipittion sequences. One sequence involves the formtion of G.P. zones, in the form of [0001] rods, metstble c phse tht lso forms s [0001] rods, nd the equilibrium phse (Mg 12 Nd 2 Ag) tht hs lth morphology nd yet to be determined hexgonl structure. The other sequence hs the formtion of G.P. zones of n ellipsoid shpe, the metstble b phse of n equixed morphology, nd the equilibrium phse Mg 12 Nd 2 Ag. Grdwell [151] proposed tht both types of G.P. zones formed simultneously during ging t tempertures up to 523 K (250 C). Without providing the trnsformtion mechnisms, he further proposed tht the rod-like G.P. zones trnsformed into the rod-shped c phse, wheres tht the ellipsoidl G.P. zones trnsformed into the equixed b phse. The metstble c phse reportedly hs hexgonl structure ( = nm, c = nm), but the orienttion reltionship between c nd the mgnesium mtrix phse hs not been reported. The metstble b phse lso hs hexgonl structure ( = nm, c = nm), nd its orienttion reltionship with -Mg phse is such tht (0001) b == (0001) nd b == The equilibrium phse Mg 12Nd 2 Ag ws originlly reported to hve complex hexgonl structure. [5,151] But in two seprte studies of lloy QE22, [168,169] this equilibrium phse ws inferred, without ny strong supportive evidence, to be (Mg,Ag) 12 Nd tht hs tetrgonl structure ( = 1.03 nm nd c = 0.59 nm), i.e., isomorphous to tht of Mg 12 Nd. To estblish the structures of ll precipitte phses unmbiguously, including tht of the equilibrium precipitte phse, in the lloy QE22, it seems necessry to employ modern fcilities such s tomic-resolution HAADF-STEM nd electron microdiffrction in ny efforts to be mde in the future studies. Grdwell [151] studied the precipittion-hrdening mechnism in QE22 by exmining foils tken from specimens tht hd been strined 2 pct in tension fter ging for vrious times t 473 K (200 C). He concluded tht pek hrdness coincided with the trnsition from precipitte cutting to Orown looping nd tht METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

24 mximum ge hrdening ws ssocited with the presence of the c nd b precipittes. The lloy QE22 in the pek-ged condition hs superior tensile properties nd creep resistnce over mny other mgnesium lloys. QE22 in its pek-ged condition exhibits 0.2 pct proof strength of 205 MP t room temperture, 195 MP t 373 K (0 C), nd 165 MP t 473 K (200 C) (Tble II). The creep strength, the stress required to produce 0.2 pct creep strin in 500 hours, of this lloy is 135 MP t 423 K (150 C) nd 65 MP t 473 K (200 C). [170] However, this lloy is reltively expensive nd, thus, hs limited pplictions only in the ircrft nd erospce industries. G. Mg-Gd-Bsed nd Mg-Y-Bsed Alloys 1. Mg-Gd binry lloys The equilibrium solid solubility of Gd in mgnesium is reltively high (4.53 t. pct or wt pct) t the eutectic temperture of 821 K (548 C) nd decreses exponentilly with temperture to pproximtely Fig. 18 Age-hrdening response of Mg-2.1 wt pct Nd-2.5 wt pct Ag-0.7 wt pct Zr lloy (QE22) lloy t 373 K to 573 K (0 C to 300 C) [167] t. pct (5.0 wt pct) t 523 K (250 C) nd to 0.61 t. pct (3.82 wt pct) t 473 K (200 C), [14] forming n idel system for precipittion hrdening. [171] However, binry Mg-Gd lloys contining less thn wt pct Gd show little or no precipittion-hrdening response during isotherml or isochronl ging of supersturted solid solutions of these lloys. [172,173] It is often necessry to increse the Gd concentrtion to the rnge to 20 wt pct to enhnce the precipittionhrdening response. [ ] The ging curve of Mg- 15Gd-0.5Zr (wt pct) lloy is shown in Figure 19(). [177] The hrdness vlue in the s-quenched condition is over 70 VHN, which is substntilly higher thn tht of the mgnesium lloys described in previous sections. This reltively high vlue of hrdness is the result of solidsolution strengthening in supersturted mgnesium mtrix. It ws demonstrted recently [176] tht n pprecibly high 0.2 pct proof strength of 445 MP cn be chieved in Mg-14 wt pct Gd-0.5 wt pct Zr lloy when this lloy is produced by the combined processes of hot extrusion, cold work, nd ging (Tble II). The precipittes in Mg-Gd binry lloys contining more thn wt pct Gd were studied in recent yers by conventionl trnsmission electron microscopy [177] nd HAADF-STEM. [178,179] The precipittion sequence is now generlly ccepted [177] to involve the formtion of b, b, b 1, nd b phses, which is similr to tht originlly proposed for Mg-Y-Nd bsed lloys. [180] The metstble b phse hs n ordered D0 19 structure ( = nm, c = nm). The orienttion reltionship between b nd -Mg phses is such tht [0001] b == [0001] nd 2 1 b 00== 2 1. The b precipittes were originlly reported to hve plte-like morphology, with their hbit plne lmost prllel to 2 1. However, this morphology is not consistent with experimentl observtions mde by HAADF-STEM imges obtined in recent yers (Figure 19(b)). [178] The metstble b phse usully forms s lenticulr prticles with their brod surfce prllel to 2 1 (Figure 19(c)).[178] It hs bse-centered orthorhombic Brvis lttice ( = nm, b = nm, nd c = nm) nd n orienttion reltionship with respect to the mtrix phses: (0) b == 1 2 nd Fig. 19 () Age-hrdening response of Mg-15Gd-0.5Zr (wt pct) lloy t 523 K (250 C) (dpted from Ref. [177]). (b nd c) HAADF-STEM imges showing distribution nd morphology of precipittes in Mg-5 t. pct Gd lloy ged t 473 K (200 C) for 5 h nd h, respectively (reproduced from Ref. [178]). Electron bem is prllel to [0001] in (b nd c) VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

25 [001] b == [0001]. Bsed on the nlysis mde from tomic-resolution HAADF-STEM imges (Figure 20()), [179] n tomic structure of the b phse ws proposed, (Figures 20(b) nd (c)). [179] The zigzg rrys of Gd toms in the unit cell, indicted by dotted line, re consistent with those of bright dots in tomicresolution HAADF-STEM imges. The composition of the b phse inferred from this model is Mg 7 Gd. The b 1 phse [177] hs fce-centered cubic structure tht is isomorphous to tht of the b 1 phse in WE54 lloy, [180] which hs the Mg 3 Nd or Mg 3 Gd structure (spce group Fm 3m; = 0.74 nm). The orienttion reltionship is such tht 112 == b 1 nd ½1Š b 1 == [0001], nd its hbit plne is prllel to. Precipittes of the equilibrium b phse hve fce-centered cubic structure (spce group F 43m; = 2.2 nm) nd Mg 5 Gd composition. The orienttion reltionship of the b phse with -Mg is (1) b == (0001) nd ½1 11Š b == ½2 1 Š. The b phse forms s pltes prllel to.[177] 2. Mg-Y binry lloys The Mg-rich prt of the Mg-Y binry phse digrm indictes tht the mximum solid solubility of Y in mgnesium is 3.75 t. pct (12.5 wt pct) t 839 K (566 C) (the eutectic temperture), nd it decreses to 0.75 t. pct (2.69 wt pct) Y t 473 K (200 C). The equilibrium intermetllic phse b hs composition of Mg 24 Y 5 nd body-centered cubic structure (spce group I 43m, = nm). The equilibrium volume frction of precipittes chievble t 473 K (200 C) is pproximtely 4.5 pct for Mg-5 wt pct Y lloy nd.6 pct for Mg-8 wt pct Y composition. The ge-hrdening response of binry Mg-Y lloys is remrkble when the Y concentrtion in the lloy is t or bove 8 wt pct nd the ging tretment is crried out t temperture close to 473 K (200 C) [181,182] (Figure 21()), even though the hrdening kinetics re sluggish during the first 0 hours ging t 473 K (200 C). The precipitte phses formed during isotherml ging of Mg-Y binry lloys were reported [5] to include b, b, nd b. The b phse hs bse-centered orthorhombic lttice ( = 0.64 nm, b = nm, nd c = nm) [150] nd n orienttion reltionship (001) b == (0001), [0] b == 21. Becuse the structures of b nd b phses were reported to be the sme, [5] it is pproprite, t lest currently, to merge the b nd b phses into b in the precipittion sequence. The morphology of the b in Mg-2 t. pct Y lloy (Figure 21(b)) [183] is remrkbly different from tht in Mg-5 t. pct Gd lloy (Figure 19()) even though the structure of b (Mg 7 Y) in the Mg-2 t. pct Y lloy is reportedly the sme s tht in the Mg-5 t. pct Gd lloy (Figures 19(b) nd (c)). The difference ws ttributed to the difference between lttice prmeters of Mg 7 Gd ( = nm, b = nm, nd c = nm) nd Mg 7 Y structures, which results in different lttice misfits with Mg mtrix. [183] Precipittes of the equilibrium phse b hve plte shpe. The orienttion reltionship between b nd mgnesium mtrix phse is exct Burgers with hbit plne of the b pltes prllel to or ner Burgers Fig. 20 () [001] Atomic-resolution HAADF-STEM imge nd (b nd c) unit cell of b phse in Mg-5 t. pct Gd lloy (reproduced from Ref. [179]). with the plte hbit plne prllel to (Figure 21(c)). [184] 3. Mg-Y-Nd nd Mg-Gd-Nd lloys The ge-hrdening response of Mg-Y-Nd lloys is much higher thn tht of counterprt Mg-Y nd Mg-Nd binry lloys. [185,186] The most successful commercil mgnesium lloys developed to dte vi precipittion hrdening, in terms of strength nd creep resistnce, hve been WE54 (Mg-5 wt pct Y-2 wt pct Nd-2 wt pct HRE) nd WE43 lloys bsed on the Mg-Y-Nd system [6,187,188] (Tble II). The ging curves of mgnesium lloy WE54 t 523 K nd 473 K (250 Cnd200 C) re shown in Figure 22. [189] The hrdness vlue in the s-quenched condition is 68 VHN, which is close to tht of the Mg-15Gd-0.5Zr (wt pct) lloy shown in Figure 19(), even though the totl concentrtion of METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

26 Fig. 21 () Aging curve of Mg-8 wt pct Y lloy t 473 K nd 523 K (200 C nd 250 C). (b) HAADF-STEM imge of b precipittes in Mg-2 t. pct Y lloy (reproduced from Ref. [183]). (c) TEM imge showing b precipitte pltes in Mg-.2 wt pct Y lloy, solution treted for 24 h t 823 K (550 C), wter quenched, nd then ged for 24 hours t 623 K (350 C) (reproduced from Ref. [184]) VOLUME 43A, NOVEMBER 2012 lloying dditions is pproximtely 9 wt pct. Cold work of this lloy by 6 pct, fter solution tretment nd prior to ging, cn rise the hrdness of s-quenched smples to 75 VHN. Only slight increse in hrdness is obtined when the level of plstic deformtion is incresed from 6 pct to 12 pct. During isotherml ging t 523 K (250 C), the hrdness of the undeformed smple increses grdully to plteu hrdness of 87 VHN fter 24 hours. In contrst, the smple with 6 pct deformtion reches mximum hrdness of ~97 VHN in 8 hours nd tht with 12 pct deformtion chieves mximum hrdness of ~94 VHN in 4 hours. Therefore, it is possible to obtin higher hrdness vlues in much shorter ging time by cold work. With prolonged ging t 523 K (250 C), the difference in hrdness between undeformed nd deformed smples diminishes grdully. Higher hrdness vlues cn be obtined for both undeformed nd deformed smples t 473 K (200 C) (Figure 22(b)). Although the precipittion-hrdening response of deformed smples is ccelerted, the mximum hrdness chievble t 473 K (200 C) is not ffected significntly by cold work. A mximum hrdness between 6 nd 1 VHN is chieved t 473 K (200 C) for both deformed nd undeformed smples. The microstructure of the undeformed smple ged for 24 hours t 523 K (250 C), which is close to the pek-ged condition, is shown in Figure 22(c). It hs reltive corse distribution of 1 0 pltes of b 1 phse, with globulr b prticles ttched to the two ends of ech of the prismtic pltes. In contrst, the 1 0 precipitte pltes formed smples deformed by 6 pct prior to 4 hours ging t 523 K (250 C) hve much lrger dimeter nd higher number density, nd the number density of globulr prticles of b is significntly reduced (Figure 22(d)). Apprently, the disloctions introduced by the cold work promote the nucletion the growth of the 1 0 precipitte pltes t the expense of b phse. Observtions mde by trnsmission electron microscopy [189] indicte tht b 1 precipittes form heterogeneously on preexisting disloctions, similr to the intermedite precipitte phse Mg 3 X (X = Nd, Ce, or MM) in Mg-Nd, Mg-Ce, nd Mg-MM lloys tht lso nuclete preferentilly on disloctions. The comprison of the microstructures nd ging curves of the undeformed nd deformed smples indictes tht the remrkble increse in hrdness, from 75 VHN in undeformed smples to 93 VHN in deformed smples, is ssocited with the formtion of 1 0 precipitte pltes of lrger dimeter nd higher number density. As discussed in Section III, n increse in the prismtic plte dimeter, number density, or combintion of both, cn effectively reduce interprticle spcing nd, therefore, increse hrdness or strength. The T6 condition of lloy WE54 typiclly involves solution tretment of 8 hours t 798 K (525 C), hot wter quench, nd subsequent ging tretment of 16 hours t 523 K (250 C). [188,190] The ged microstructure t mximum hrdness ws initilly reported [5,188] to contin metstble b nd equilibrium b phses s dispersed precipittes, nd both phses were described to form s pltes on 1 0 plnes of the METALLURGICAL AND MATERIALS TRANSACTIONS A

27 Fig. 22 Age-hrdening response of WE54 lloy t () 523 K (250 C) nd (b) 473 K (200 C). (c nd d) Trnsmission electron microgrphs showing distribution of precipittes in (c) smples pek-ged t 523 K (250 C), nd (d) smples cold-worked 6 pct fter wter quench nd then ged to pek hrdness t 523 K (250 C). Reproduced from Ref. [189]. mgnesium mtrix phse. In erly studies, the b phse ws reported to hve Mg 12 NdY composition [190,191] nd bse-centered orthorhombic structure ( = nm, b = nm, c = nm). [5,150] The orienttion reltionship of b ws reported to hve the form ð0þ b 0 == 1 2, ½001Š b 0 == [000l]. The proposed structure nd orienttion of the b phse re similr to those of the b formed in binry Mg-Y lloys. [5] The b phse ws reported to hve Mg 14 Nd 2 Y composition [191] nd fce-centered cubic structure ( = nm). [5,150] The orienttion reltionship between b nd -Mg mtrix is such tht 112 b == 1 0, [1] b == [000l], which is identicl to tht observed between b nd -Mg in binry Mg-Nd lloys. [150] In the following yers, severl studies [180, ] hve been mde to use modern fcilities to chrcterize the precipittes in ged smples of WE54 nd WE43. It ws reported [193,194] tht during the erly stge of ging of WE43 lloy, monolyer precipittes with D0 19 ordering form with two possible hbit plnes: nd 1 0, nd tht the precursor monolyers form the b (D0 19 ) phse wheres the precursor 1 0 monolyers led to the formtion of the b phse. However, the similrity between these Mg-Y-Nd-bsed METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

28 lloys nd Mg-Gd lloys suggests tht the precipittion in the erly stge of the WE lloys is likely to be similr to tht in Mg-Gd lloys (Tble I). Figure 23() shows the microstructure typicl of smples ged to mximum hrdness (48 hours t 523 K [250 C]). The microstructure contins dispersion of plte-shped precipittes in contct with irregulr, globulr prticles. An exmintion of electron microdiffrction ptterns obtined from individul globulr precipitte indictes tht these prticles re b phse nd tht the structure nd orienttion reltionships proposed for the b phse in erly studies re correct. However, the observed morphology of the b phse is clerly different from tht from previous suggestions, [5,150] i.e., b forms s pltes prllel to 1 0. In contrst to the erly studies, Nie nd Muddle [180] reported tht the 1 0 pltes hve fce-centered cubic structure, with ~ 0.74 nm, nd n orienttion reltionship tht is of the form 112 == 1 0 b 1 ; ½1Š b1 === [0001]. This phse ws designted b 1 becuse it hd not been reported previously in WE lloys. The structure b 1 is similr to tht of Mg 3 X phse (X = Nd, L, Ce, Gd, Pr, Dy, nd Sm), which hs fce-centered cubic structure (spce group Fm 3m, = 0.74 ± 0.01 nm) nd form exclusively s 1 0 pltes. [149,153,154] The b 1 phse invribly forms jointly with other prticles, either with b precipittes such s tht shown in Figure 23() or with themselves in the form of trids (Figure 23(b)). In the ltter cse, the mgnesium mtrix region isolted by the three b 1 Fig. 23 () Conventionl TEM nd (b) HAADF-STEM imges showing the rrngement of b nd b 1 precipittes in pek-ged smples of WE54 lloy. (c) HAADF-STEM imge showing n enlrgement of (b). () is reproduced from Ref. [180], nd (b nd c) is from Ref. [196] VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

29 vrints is rotted by pproximtely.5 deg (Figure 23(c)). [196] These observtions indicte tht the formtion of b 1 phse involves lrge sher strin, [180] which will be discussed in Section IV. The 1 0 pltes of b 1 trnsform in situ to the equilibrium phse b during prolonged ging t 523 K (250 C). Exmintion of pttern symmetries in the higher order Lue zone nd zero order Lue zone ptterns recorded from the b pltes, together with recognition of bsent reflections in the observed electron microdiffrction ptterns, indictes spce group of F 43m ( = 2.2 ± 0.1 nm). This structure is isomorphous with Mg 5 Gd. The orienttion reltionship is similr to tht observed between b 1 nd the mtrix phse. The ge-hrdening response nd precipittion in Mg- 7Gd-2.3Nd-0.6Zr nd Mg-7Dy-2.3Nd-0.6Zr (wt pct) lloys hve been studied nd compred with those in Mg-4Y-2.3Nd-0.6Zr (wt pct) lloy. [197] Among the three lloys exmined, the gretest ge-hrdening response is found in the Gd-contining lloy. The b 1 phse is lso confirmed to occur in the Mg-Gd-Nd-Zr nd Mg-Dy- Nd-Zr lloys, nd the structures nd morphologies of precipittes nd the precipittion sequences in the three lloys re identicl. 4. Mg-Gd-Y-bsed lloys The Mg-Gd-Y lloys hve received considerble interest in recent yers becuse of their potentil in chieving higher strength nd better creep resistnce. The Gd:Y tomic rtio in these lloys is importnt. When the Gd:Y tomic rtio is in the rnge 3:1 to 1:1 nd the totl concentrtion of the lloying dditions is 2.75 t. pct, the tensile yield strengths of the Mg-Gd-Y lloys re lower thn tht of the counterprt Mg-Gd binry lloy nd higher thn tht of the counterprt Mg-Y binry lloy. [198] The T5 smples of the hot-rolled Mg-Gd-Y lloys hve superior tensile strengths to commercil WE54 lloy. The ging curves of two representtive lloys in this ctegory re shown in Figures 24() nd (b). [199,200] The effects of Zn dditions re lso shown in Figure 24(b). In generl, the dditions of Zn reduce the ge-hrdening response of these lloys. When the Zn content is over 1.5 to 2.0 t. pct, the gehrdening response of the resultnt Mg-Gd-Y-Zn lloys diminishes completely. [201] However, n impressively high vlue of 473 MP hs been chieved for the 0.2 pct proof strength in n extruded nd ged Mg-1.8Gd-1.8Y- 0.7Zn-0.2Zr (t. pct) or Mg-Gd-5.7Y-1.6Zn-0.7Zr (wt pct) lloy [202] (Tble II). The remrkble ge-hrdening response chieved in the Mg-Gd-Y lloys is ttributble to dense distribution of precipittes in the microstructure. The precipittion sequence in this group of lloys seems to be similr to Mg-Gd, Mg-Gd-Nd, nd Mg-Y-Nd lloys. The b precipittes formed in the pek-ged smples hve lenticulr shpe (Figure 24(c)), which is more effective in impeding disloction slip thn the globulr b precipittes formed in the WE54 lloy. 5. Mg-Gd-Zn lloys Becuse the equilibrium solid solubility of Gd in mgnesium is pproximtely 0.8 t. pct t 523 K Fig. 24 Aging curves of () Mg- wt pct Gd-3 wt pct Y- 0.4 wt pct Zr lloy t 523 K (250 C), nd (b) Mg-2 t. pct Gd- 1.2 t. pct Y-(0-1) t. pct Zn-0.2 t. pct Zr lloys t 498 K (225 C). (c) TEM imge showing b precipittes in pek-ged smples (16 h t 523 K [250 C]) of Mg- wt pct Gd-3 wt pct Y-0.4 wt pct Zr lloy. ( nd c) re reproduced from Ref. [199], nd (b) is from Ref. [200]. (250 C) nd 0.6 t. pct t 473 K (200 C), Mg-Gd lloys contining less thn 1.0 t. pct (6.1 wt pct) Gd hve poor ge-hrdening response t 473 K to 523 K (200 C to 250 C) becuse of the low volume frctions of precipittes (Figure 25()). For exmple, the mximum METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

30 volume frction of precipittes chievble t 523 K (250 C) is pproximtely 1.1 pct, ccording to the level rule clcultion, if the precipittes re ssumed to hve composition close to tht of the equilibrium phse Mg 5 Gd. Therefore, it hs been of commercil interest to explore the possibility of enhncing the precipittionhrdening response of the diluted Mg-Gd lloys vi the use of microlloying dditions of low-cost elements. A recent study [173] reported tht dditions of 1 to 2 wt pct Zn to Mg-6 wt pct Gd lloy could considerbly enhnce the solid-solution strengthening effect nd generte reltively strong precipittion-hrdening response (Figure 25()). The 3DAP work [203] did not revel ny detectble clusters of Gd nd/or Zn toms in the s-quenched smples of Mg-6Gd-1Zn-0.6Zr (wt pct) lloy. But the sttisticl nlysis of the 3DAP dt suggests tht cosegregtion of Gd nd Zn toms occurs in the s-quenched smples. The co-segregtion phenomenon of Gd nd Zn toms is ttributed to the tomic size difference between the solute nd the solvent. The tomic rdius is nm for Gd, for Zn, nd for Mg. Therefore, substituting Mg tom by Gd tom leds to compression strin, wheres replcing Mg tom by Zn tom cuses n extension strin. It is therefore energeticlly fvorble for Gd nd Zn toms to segregte to ech other to minimize the elstic strin ssocited with individul Gd or Zn toms. It ws further speculted [203] tht the Gd nd Zn toms form Gd-Zn dimers in the -Mg solid solution nd tht these dimers re more effective brriers (thn those from individul toms of Gd or Zn) to the motion of disloctions nd therefore contribute to the lrge hrdness increse in the s-quenched condition. The experimentl observtions mde by TEM indicte tht the precipittion process during isotherml ging t 473 K nd 523 K (200 C nd 250 C) of the Mg-6Gd-1Zn-0.6Zr (wt pct) lloy involves the Fig. 25 () 523 K (250 C) ging curves of Mg-6 wt pct Gd-Zn lloys. (b) Conventionl TEM imge showing the distribution of precipittes in pek-ged smples of Mg-6Gd-1Zn-0.6Zr lloy. (c nd d) Atomic-resolution HAADF-STEM imges showing structures of c nd c phses in Mg-6Gd-1Zn-0.6Zr lloy, ged for 2 h t 523 K (250 C). (e) HAADF-STEM imge showing precipittes in Mg-6Gd-1Zn-0.6Zr lloy, ged for 00 h t 523 K (250 C). Note tht the precipitte pltes form in pirs or clusters. (f) Schemtic digrm showing probble solvus lines of metstble nd equilibrium phses. ( nd b) re reproduced from Ref. [173], (c through e) re from Ref. [203] VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

31 formtion of metstble c nd c phses nd tht the pek hrdness is ssocited with the c precipittes (Figure 25(b)). Both precipitte phses form s plteshped prticles on (0001). Most precipittes in the pek-ged condition re c phse tht hs n ordered hexgonl structure (spce group P 62m; = nm nd c = nm) with n ABA stcking sequence of the close-pcked plnes (Figure 25(c)) nd composition of Mg 70 Gd 15 Zn 15. [203] This structure is subtly different from tht reported for the bsl pltes in Mg-2Gd-1Zn (t. pct) lloy [204] in terms of the distribution of Zn toms nd the c vlue of the unit cell. The orienttion reltionship between c nd -Mg phses is such tht (0001) c == (0001) nd == 2 1 c 00 : The thickness of the c pltes is often of single unit cell height. This phse is fully coherent with the mtrix in its hbit plne, but with reltively lrge misfit strin ( 0.16) in the direction norml to the hbit plne. It is to be noted tht G.P. zones hve been reported [204,205] to form in Mg-2Gd-1Zn (t. pct) nd Mg-1.5Gd-1Zn (t. pct) lloys. However, n inspection of the imges of nd electron diffrction ptterns from these G.P. zones indictes tht they re ctully c phse. With continued ging t 523 K (250 C), the c precipittes re grdully replced by c precipittes. The c phse hs disordered hexgonl structure (spce group P 3m1; = nm nd c = nm) nd MgGdZn composition. The orienttion reltionship is such tht ð0001þ c 0 == (0001) nd 2 1 == 2 1 c 0 : The c pltes re perfectly coherent with the mtrix phse in their hbit plne nd in the direction norml to their hbit plne. Their thickness is gin invribly of single unit cell height, nd their spect rtio is considerbly lrger thn tht of c. In contrst to the c pltes, the c pltes hve n ABCA stcking order of their closepcked plnes (Figure 25(d)). Therefore, the formtion of c precipitte of single-unit cell height genertes sher strin of pproximtely This lrge sher strin cn impose lrge brrier to the nucletion of c pltes. Although c is metstble phse, it is remrkbly resistnt to thickening during isotherml ging t 523 K (250 C). It is still less thn 1 nm in thickness nd remins the dominnt precipitte phse in the microstructure of the Mg-6Gd-1Zn-0.6Zr (wt pct) lloy fter long-term ging (00 hours t 523 K [250 C]) (Figure 25(e)). Around the sme time, Ymski et l. [206] studied the precipittion process in Mg-2Gd-1Zn (t. pct) lloy. The Gd nd Zn concentrtions in their lloy re lmost twice of those in the Mg-6 wt pct Gd-1 wt pct Zn-0.6 wt pct Zr (Mg-1 t. pct Gd-0.4 t. pct Zn- 0.2 t. pct Zr) lloy. The precipittion in the Mg-2Gd- 1Zn (t. pct) lloy involves the formtion of stcking fults nd 14H phse on (0001) nd b, b 1, nd b phses. The stcking fults nd 14H precipittes reportedly form t intermedite nd high tempertures (573 K to 773 K [300 C to 500 C]), wheres the b, b 1, nd b phses form t low tempertures (~473 K [200 C]). It ws suggested [206] tht the 1 t. pct Zn ddition to the Mg-2 t. pct Gd lloy reduced the stcking fult energy nd, therefore, promoted the formtion of stcking fults nd the 14H phse. The stcking fults in the Mg-2 t. pct Gd-1 t. pct Zn lloy were reported to be I 1 nd I 2 intrinsic stcking fults, nd segregtion of Gd nd Zn toms into the two tomic plnes round ech single stcking fult ws observed. The ppernce of these so-clled stcking fults resembles tht of the c phse in the sme lloy system, nd to void confusion, it is pproprite to cll them c precipittes. Otherwise, it would be difficult to understnd why these so-clled stcking fults form preferentilly only t intermedite nd high tempertures rther thn t both high nd low tempertures, i.e., in the temperture rnge 298 K to 773 K (25 C to 500 C). Furthermore, the stcking fult energy would be unrelisticlly low when the extrordinrily long length (the seprtion distnce between the two Shockley prtils binding the stcking fult) is tken into ccount. The work by Nie et l. [203] suggests tht the I 1 nd I 2 intrinsic stcking fults reported by Ymski et l. [206] re in fct two twin-relted vrints of c precipitte phse. Although some prticles of the equilibrium intermetllic phse b-mg5gd strt to form in well overged smples of the Mg-6Gd-1Zn-0.6Zr (wt pct) lloy, the metstble phses b, b, nd b 1, which usully form in Mg-Gd nd Mg-Gd-Y lloys, nd c phse (14H) re not observed in the Mg-6Gd-1Zn-0.6Zr (wt pct) or Mg- 1Gd-0.4Zn-0.2Zr (t. pct) lloy. In the Mg-2 t. pct Gd- 1 t. pct Zn lloy, the 14H phse cn form from the supersturted solid-solution phse of -Mg nd from the decomposed primry intermetllic prticles Mg 3 Gd (Fm 3m, = 0.72 nm) tht hve lredy formed in the s-cst microstructure. The 14H seems to be n equilibrium phse in this lloy nd other lloys of similr compositions. [200,201,206,207] Although Mg-1Gd-0.4Zn- 0.2Zr, Mg-2Gd-1Zn, nd Mg-2.5Gd-1Zn (t. pct) lloys hve similr Gd:Zn tomic rtio, the precipitte process in these lloys is quite different. It is currently difficult to rtionlize the observtions in different lloys becuse of lck of isotherm sections of the ternry Mg- Gd-Zn phse digrm. Nevertheless, one plusible explntion is tht the 473 K to 523 K (200 C to 250 C) ging temperture rnge is bove the solvus lines of b, b, nd b 1 for the Mg-1Gd-0.4Zn-0.2Zr (t. pct) lloy tht hs lower contents of Gd nd Zn, but it is below the solvus lines of these metstble phse for the Mg-2Gd-1Zn (t. pct) nd Mg-2.5Gd-1Zn (t. pct) compositions (Figure 25(f)). 6. Mg-Y-Zn lloys The experimentl evidence ccumulted thus fr indictes tht Zn dditions to Mg-Y lloys significntly reduce the equilibrium solid solubility of Y in mgnesium. Therefore, the volume frction of solid-stte precipittes is quite low in the resultnt Mg-Y-Zn lloys, nd the Mg- Y-Zn lloys exhibit little ge-hrdening response, s shown in Figure 24(c). Consequently, the Mg-Y-Zn lloys, for exmple Mg-2 t. pct Y-2 t. pct Zn (Mg- 6.7 wt pct Y-4.9 wt pct Zn), [208] re usully hot extruded to chieve useful tensile properties (Tble II). It is perhps for the lck of ge hrdening tht the precipittion sequence in the Mg-Y-Zn lloys hs not been well chrcterized in the pst. In the erly studies METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

32 of Mg-Y-Zn lloys, [209,2] some plnr defects were observed to form on the bsl plne of the mgnesium mtrix phse. Bsed on the invisibility nlysis of the two-bem TEM imges obtined from these plnr defects, these defects were reported to be I 1 intrinsic stcking fult bounded by Frnk prtil disloctions ðb ¼ 1= ; þ c typeþ. This fult cn be generted vi the condenstion of vcncies onto single tomic plne, which leds to disloction loop bounded by Frnk prtils with b ¼1=2h0001i, nd subse- formtion of Shockley prtil loop quent b ¼1=3 within this tomic plne. Similr plnr defects re lso observed in n erly study of Mg-Th-Zn lloys ged t 573 K to 673 K (300 C to 400 C), [211] but they were reported to be intrinsic stcking fults I 2 bounded by Shockley prtil disloctions b ¼1=3. It is pprent tht wide rnge of plnr fetures exists in the Mg-Y-Zn, Mg-Gd- Zn, nd Mg-Th-Zn lloys. Some of them hve mny chrcteristic fetures of the stcking fults, but the nlysis mde did not drw ny cler conclusions concerning their identity nd reltionships. The stcking fults in mgnesium lloys include intrinsic fults I 1 nd I 2 nd extrinsic fult E. [212] An I 1 fult cn be produced by removing the B plne, usully vi the condenstion of ggregtes of vcncies, bove n A plne nd then shering the remining plnes bove the A plne by displcement of 1=3. In this cse, the stcking order of the closely pcked plnes is chnged from ABABABAB to ABABACAC ði 1 Þ: An I 2 fult is generted directly by the pssge of Shockley prtil disloction on (0001) or by directly shering the hexgonl lttice by displcement of 1=3. The pssge of the Shockley prtil, or shering, chnges the stcking sequence of closely pcked plnes from ABABABAB to ABABCACA ði 2 Þ: These two types of intrinsic stcking fults cn be distinguished by tht fct tht the I 1 fult is bound by pir of Frnk prtils b ¼ 1= ; wheres tht the I 2 fult is bound by pir of Shockley prtil disloctions b ¼ 1=3 : The extrinsic fult E cn be generted by inserting C plne into the ABAB stcking sequence. The stcking of the closely pcked plnes is then chnged from ABABABAB to ABABCABAB ðeþ: Note tht the I 2 fult, rther thn I 1, yields n ABCA stcking tht is chrcteristic of the c structure in Mg-Gd-Zn nd Mg-Y-Zn lloys. Although the E fult cn lso generte the ABCA stcking, the pcking order of closely pcked plnes outside the ABCA segment is distinguishbly different from tht ssocited with the I 2 fult. If the ABCA segment is tken s precipitte plte, then the closely pcked plnes of the mgnesium lttice t both sides of the plte re symmetriclly rrnged for the E fult, nd symmetriclly stcked for the I 2 fult. In more recent yers, the plnr defects in the microstructure of Mg-8Y-2Zn-0.6Zr (wt. pct) lloy solution treted for 1 hour t 773 K (500 C) nd wter quenched were chrcterized in detil using conventionl trnsmission electron microscopy imging techniques nd modern Z-contrst imging technique, i.e., tomic-resolution HAADF-STEM. [213] These defects were nlyzed using both trditionl gæb nd gær invisibility criteri nd computer imge simultion. The lloy microstructure contins three types of plnr fetures on (0001) (Figure 26()). The first type is smll ribbons. They re intrinsic stcking fults I 2, bounded by two Shockley prtil disloctions, rther thn I 1. The second type is precipitte c phse tht hs single unit cell height nd is lwys ssocited with Shockley prtil disloctions (Figure 26(b)). [214] The structure of the c phse seems similr to tht in the Mg-Gd-Zn lloys (Figure 25(d)). The third type is the 14H precipitte phse tht is gin lwys ssocited with Shockley prtil disloctions. This type of plnr fetures is often inferred to be n intrinsic stcking fult I 2 bounded by Frnk prtil disloction in the literture. [209] The 14H phse is n equilibrium phse in the Mg-Y-Zn system. The probble precipittion sequence in the Mg-Y-Zn lloys is given in Tble I. The 14H phse ws originlly reported [215] to hve disordered hexgonl structure ( = nm nd c = nm). The close-pcked plnes of this structure hve long-period stcking order nd re rrnged in n ACBCBABABABCBCA stcking sequence. But in more recent study, [216] it ws proposed tht 14H hs in fct n ordered hexgonl structure ( = nm, c = nm), nd Mg 12 YZn composition tht is identicl to tht of the equilibrium X phse in the Mg-Y- Zn system. [217,218] The stcking sequence of the closepcked plnes is ABABCACACACBABA in the 14H lttice, [203,216] nd the orienttion reltionship between 14H nd -Mg is tht (0001) 14H == (0001) nd H == The 14H unit cell is mde up of two structurl units, or building blocks, tht re seprted by three (0001) plnes of mgnesium (Figure 26(c)). Ech structurl unit hs n ABCA-type stcking sequence, nd Y nd Zn toms hve n ordered rrngement in the B nd C lyers, i.e., the two middle lyers, of ech structurl unit. The stcking sequence of the close-pcked plnes in the structurl unit is such tht ech structurl unit hs sher component with respect to the mtrix phse. The two structurl units within the 14H unit cell re twin-relted. Therefore, the joint formtion of these two blocks genertes zero net sher reltive to the -Mg mtrix. In the s-cst microstructures of Mg-Y-Zn lloys, produced by either conventionl ingot csting or rpid 3922 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

33 Fig. 26 () Conventionl TEM imge showing the distribution of precipittes in Mg-8Y-2Zn-0.6Zr (wt pct) lloy solution treted for 1 g t 773 K (500 C) nd wter quenched. (b nd c) Atomic-resolution HAADF-STEM imges showing c, 18R, nd 14H phses in Mg-8Y-2Zn-0.6Zr (wt pct) lloy solution treted for 1 h or 16 h t 773 K (500 C). Reproduced from Refs. [213] nd [214]. solidifiction processing, it is often to observe primry intermetllic prticles of long-period stcking ordered (LPSO) structure. It seems tht this LPSO structure cn redily form from the melt, irrespective of the solidifiction rtes in the csting. This LPSO phse cn be produced in nnoscles nd lrger volume frctions when Mg-Y-Zn lloys re produced by the rpid solidifiction process. For exmple, the size of the intermetllic prticles of the LPSO phse rnges from 50 to 250 nm in Mg-2 t. pct Y-1 t. pct Zn lloy produced by gs tomiztion, compction, nd hot extrusion. [219] This lloy cn hve 0.2 pct proof strength exceeding 600 MP nd n elongtion to frcture of 5pct. [220] In the erly studies, [219,221] the intermetllic prticles in the Mg-Y-Zn lloys ws reported to hve 6H structure, which hs monoclinic unit cell ( = 0.56 nm, b = 0.32 nm, c = 1.56 nm nd b = 88 deg) nd n ABCBCB stcking sequence of the close-pcked plnes. In the proposed 6H structure, the A nd B lyers re significntly enriched by Zn nd Y, [221] with the Y nd Zn content of pproximtely t. pct nd 3 t. pct, respectively, in ech of these two lyers. [219] In subsequent studies, [215,222] the 6H structure ws regrded s not correct nd ws superseded by n 18R structure (hexgonl unit cell, = nm, c = 4.86 nm) with n ACBCBCBACACACBABABA stcking sequence of the close-pcked plnes. This structure is identicl to tht of the X-Mg 12 YZn phse proposed by Luo nd Zhng. [223] In ll such studies, the term order refers to the ordered stcking of the close-pcked plnes rther thn ordered rrngement of Y nd Zn toms in the close-pcked plnes. In very recent study, [216] the 18R unit cell ws reported to be ordered monoclinic ( = nm, b = nm, c = nm, nd b = deg), with Y nd Zn toms occupying some specific positions of the unit cell. The orienttion reltionship between the 18R nd -Mg phses is such tht (001) 18R == (0001) nd [0] 18R == ½1 2Š. The 18R unit cell hs n ACACBABABACBCBCBACA stcking sequence of its close-pcked plnes nd is mde up of three structurl units, with two djcent units seprted by two (0001) plnes of mgnesium. Similr to tht in the 14H phse, ech structurl unit of the 18R phse hs n ABCA-type stcking sequence, nd Y nd Zn toms hve n ordered rrngement in the B nd C lyers, i.e., the two middle lyers of ech structurl unit. The experimentl mesurements nd this ltter model of 18R indicte tht its stoichiometric composition is Mg YZn rther thn Mg 12 YZn, which hs long been ssumed nd commonly ccepted in previous studies. This error occurred presumbly becuse the 18R structure ws mistkenly tken s the structure of the equilibrium X-Mg 12 YZn phse in the work of Luo nd Zhng. [223] Luo nd Zhng [223] did not provide ny informtion on their smple preprtion conditions, nd it is very likely tht the intermetllic prticles tht they studied re the 18R phse, rther thn the equilibrium X-Mg 12 YZn phse. The 18R phse is observed predominntly in the s-cst microstructure of Mg-Y-Zn lloys. Both the ccumulted experimentl evidence nd the clculted Mg-Y- Zn phse digrm shown in Figure 27 [224] indicte tht 18R is not thermodynmiclly stble t tempertures below 773 K (500 C); it is grdully replced by 14H fter prolonged het tretment t 623 K to 773 K (350 C to 500 C). It is to be noted tht the stcking sequence of the close-pcked plnes of the structurl units of the 14H nd 18R phses is the sme s tht of the c phse (Figures 25(d) nd 26(b)), even though the c phse is disordered. In two very recent studies of LPSO structures in Mg-5 t. pct Gd-3.5 t. pct Al [225] nd Mg- 2 t. pct Y-1 t. pct Zn, Mg-9 t. pct Y-6 t. pct Zn nd Mg-2 t. pct Er-1 t. pct Zn [226] lloys, the structurl unit of the LPSO structures ws proposed to hve n ordered enrichment of Y nd Zn toms in ll four lyers, insted of the middle two lyers, of the ABCA units. In the Mg-5 t. pct Gd-3.5 t. pct Al lloy, the METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

34 HAADF-STEM imges obtined indicte tht Gd toms re rrnged with n order in ll four lyers of the structurl unit, with greter enrichment in the inner two lyers of the unit. However, it should be pointed tht the SAED ptterns recorded from the LPSO phse in this lloy (Figure 2 in Reference 225) re not fully consistent with those obtined from the LPSO phse in the Mg-Y-Zn lloys (Figure 11 in Reference 225), nd therefore, more efforts re needed in the future to estblish whether the structure reported for the LPSO phse in the Mg-Gd-Al lloy is representtive of tht of the Mg-Y-Zn lloys. In the Mg-2 t. pct Y-1 t. pct Zn, Mg-9 t. pct Y-6 t. pct Zn nd Mg-2 t. pct Er-1 t. pct Zn lloys, [226] the HAADF-STEM imges of the 18R nd 14H phses do not show ny strong evidence of systemtic ordering of Y nd Zn toms in the two outer lyers. The SAED ptterns obtined from the LPSO phses in these three lloys re not fully self-consistent in terms of the intensity of some reflections. Agin, nother systemtic study is needed in the future to reconcile the HAADF-STEM imges nd SAED ptterns obtined from different lloys nd lloys prepred under different processing conditions. Although 18R nd 14H re the most frequently observed LPSO structures in Mg-Y-Zn lloys nd in mny other mgnesium lloys such s Mg-Gd-Zn, Mg- Gd-Y-Zn, Mg-Dy-Zn, Mg-Ho-Zn, Mg-Er-Zn, Mg-Tm- Zn, Mg-Tb-Zn, Mg-Y-Cu(-Zn), nd possibly Mg-Gd- Al, [200,201,203,206,225, ] few other long-period structures such s H nd 24R hve lso been reported. [215] However, whether these LPSO structures re lso ordered s well s their reltionships with the ABCAtype building block nd 18R nd 14H structures, remins to be unmbiguously estblished. 7. Mg-Gd-Ag lloys A recent study [232,233] indictes tht the ddition of 2 wt pct Ag to the Mg-6Gd-0.6Zr (wt pct) lloy cn led to significnt ccelertion nd increse in the gehrdening response in the resultnt Mg-6Gd-2Ag-0.6Zr lloy (Figure 28()). When combined with 1 wt pct Zn, Fig. 27 Verticl section of the Mg-Y-Zn ternry phse digrm from Mg-rich corner to Mg 80 Y Zn through 18R nd 14H phses (Courtesy R. Schmid-Fetzer). this ddition cn result in nother substntil increse in the pek hrdness vlue, with mximum hrdness vlue of ~92 VHN obtinble t 473 K (200 C) (Figure 28()). This improved ge-hrdening response is ssocited with the formtion of dense distribution of nnoscle bsl precipitte pltes tht re not vilble in the Mg-6Gd-0.6Zr lloy. The selected-re electron diffrction ptterns recorded from the nnoscle pltes indicte they my hve hexgonl structure with lttice prmeters = nm, c = 0, 1, or two times of c (i.e., c = 0, 0.521, or nm) nd n orienttion reltionship of (0001) h == (0001), h == : Bsed on wht hs been reported for similr pltes formed in Mg-Nd-Ag [5,151] nd Mg- Y(-Zn)-Ag lloys, [234] the lttice prmeters of the structure of these pltes re likely to be = nm nd c = nm. An pprecible mximum hrdness vlue of over 130 VHN is obtined when the concentrtion of Gd is incresed from 6 wt pct to 15 wt pct (Figure 28(b)). [235] When tested t room temperture in the pek-ged condition, the Mg-15Gd-2Ag-0.6Zr lloy hs 0.2 pct proof strength of 320 MP nd n ultimte strength of 347 MP. [235] 8. Mg-Y(-Zn)-Ag lloys Figure 28(c) shows the ging curves t 473 K (200 C) of Mg-6Y-1Zn-0.6Zr lloys with systemtic Ag dditions. The Ag-free lloy hs rther poor ge-hrdening response t 473 K (200 C), which is ttributble to (1) little precipittion of fine-scle precipittes inside individul mgnesium grins, (2) the preferentil formtion of corse precipittes of 14H in grin boundries, nd (3) reltively high frction of retined intermetllic prticles tht formed during solidifiction. Systemtic dditions of Ag to the Mg-6Y-1Zn-0.6Zr (wt pct) lloy hve been reported [236] to reduce the volume frction of retined intermetllic prticles in the microstructure nd to promote the formtion of fine-scle precipittes t the expense of corse precipittes of 14H. The lloy contining 2 wt pct Ag exhibits remrkble gehrdening response during isotherml ging t 473 K (200 C). An increse in the Ag content to 3 wt pct leds to nother increse in the mximum hrdness vlue chievble. In the pek-ged condition, the Mg-6Y-3Ag- 1Zn-0.6Zr (wt pct) lloy hs hrdness vlue of pproximtely 0 VHN. It should be emphsized tht similr enhncement effect on the ge-hrdening response is lso expected in the counterprt Mg-Y-Ag lloys, i.e., lloys without ny Zn dditions. These observtions re similr to those observed in Mg-Gd (-Zn) lloys contining Ag dditions. [232] The enhnced ge-hrdening response is ssocited with dense distribution of fine-scle bsl precipitte pltes of c (Figure 28(d)), which were not observed in the Ag-free lloy, nd the number density of these fine-scle precipittes increses with n increse in the Ag content in the lloy. It is bit puzzling tht the c precipitte pltelets re invribly of single unit cell thickness, comprising three tomic plnes, irrespective of the ging period tht is used (Figures 28(e) nd (f)). [234] After prolonged ging (5800 hours) t 473 K (200 C), the c pltelets remin very thin, with thickness of single 3924 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

35 Fig. 28 Aging curves t 473 K (200 C) of () Mg-6Gd-2Ag-bsed lloys (reproduced from Ref. [232]), (b) Mg-15Gd-2Ag-bsed lloys, [235] nd (c) Mg-6Y-(1-3)Ag-1Zn-bsed lloys (reproduced from Ref. [236]). HAADF-STEM imges showing the distribution of precipittes in (d nd f) pek-ged (224 hours t 473 K [200 C]), nd (g) over-ged (5800 h t 473 K [200 C]) smples of Mg-6Y-2Ag-1Zn-0.6Zr lloy (reproduced from Ref. [234]). All lloy compositions re in weight percentge. unit cell. Although they re thermlly stble nd resistnt to thickening, they tend to form clusters in the direction norml to the pltelet brod surfce, insted of thickening, which re often seprted by few tomic plnes (Figure 28(g)). It is currently uncler wht fctors cuse the cluster distribution of the c pltelets. Bsed on selected-re electron diffrction ptterns recorded from regions contining c pltelets nd tomic-resolution HAADF-STEM imges of the c pltelets, the structure of the c pltelets is proposed to be hexgonl ( = nm, c = nm). The precise symmetry of the structure, i.e., the spce group nd rrngement of individul toms in the unit cell, remins to be unmbiguously estblished. The orienttion reltionship between c nd -Mg phses is tht ð0001þ c 00 == (0001) nd ½ Š c 00 ==½2 1 Š. The observed structure, orienttion reltionship, nd morphology of the c phse re similr to those of pltelets in Mg-Gd-Zn, Mg-Nd-Zn, Mg-Ce-Zn, nd Mg-C-Zn lloys. The ging tretments of Mg-6Y-2Ag-1Zn-0.6Zr (wt pct) lloy in the temperture rnge 473 K to 623 K (200 C to 350 C) indicte tht precipittion in this lloy lso involves the formtion of G.P. zones, c, nd 14H phses. The G.P. zones hve monolyer tomic structure on (0001), nd they form in the erly stge of ging t 473 K (200 C). These G.P. zones re replced by c precipitte pltes during continued ging t 473 K (200 C). The precipittion in the lloy seems to involve the formtion of G.P. zone, c, c, 14H, nd d (Tble I), where d is n equilibrium phse (spce group Fd 3m; = 1.59 nm) tht forms in grin boundries. [234,237] H. Other Mgnesium Alloy Systems 1. Mg-In-C lloys The equilibrium solid solubility of indium in Mg is pproximtely 19.4 t. pct t the peritectic temperture of 757 K (484 C), nd it decreses only to t. pct t 600 K (327 C) nd t. pct t 473 K (200 C). [14] Given the lrge solid solubility of indium in mgnesium t tempertures close to 473 K (200 C) nd the price of indium, the Mg-In system is by ny mens not for developing lloys for engineering ppliction. For this reson, the Mg-In lloys hve so fr received little ttention. However, in recent study, [238] Mendis et l. [238] reported tht the ddition of 0.3 t. pct C to mgnesium lloys contining very dilute mount (0.6 to 1.0 t. pct) of In could led to remrkble ge-hrdening response (Figure 29()). This ge-hrdening response is severl folds lrger thn tht of the binry Mg-0.3 t. pct C lloy, nd therefore it is not relted with formtion of precipittes intrinsic of binry Mg-In lloys or Mg-C lloys. An inspection of pek-ged smples of Mg-1In- 0.3C (t. pct) lloy using HAADF-STEM nd 3DAP revels tht the this ge hrdening is ssocited with the formtion of dense distribution of precipitte pltelets forming on plnes of the mgnesium mtrix phse (Figure 29(b)). [238] These prismtic pltelets re typiclly three plnes thick nd 20 nm in dimeter, nd they re fully coherent with the mtrix phse. They re comprised of pproximtely 7.0 t. pct C nd 3.5 t. pct In (Figure 29(c)). [238] The structure of these prismtic pltelets is yet to be METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

36 determined. The phse equilibri nd the precipittion sequence in this lloy re lso both unknown. Nevertheless, the formtion of prismtic pltes in this lloy system is exciting, nd it will be described nd discussed in the next section. The prismtic precipitte pltes re the most effective brriers to bsl disloctions nd twins tht re operting in the plstic deformtion process. Any detiled study in the future of the structure nd formtion mechnism of the precipitte pltes will shed light on the explortion for the principles for generting prismtic pre- cipitte pltes in mgnesium lloys. 2. Mg-Bi-Zn lloys The mximum equilibrium solid solubility of Bi in Mg is 1.12 t. pct (or 8.87 wt pct) t the eutectic temperture (826 K [553 C]), nd it decreses to pproximtely zero t 473 K (200 C). The equilibrium intermetllic phse t the Mg-rich side of the phse digrm is -Mg 3 Bi 2 nd hs hexgonl structure (P 3ml, = nm, c = nm). The mximum volume frction of the equilibrium precipitte phse -Mg 3 Bi 2 is pproximtely 3.38 pct if the lloy composition is Mg-8.85 wt pct Bi nd if 473 K (200 C) is selected s the ging temperture. Although the Mg-Bi system is idel for designing precipittion-hrdenble lloys, the ge-hrdening response of binry Mg-Bi lloys is too low to be considered for lloy development. It hs been reported recently [239] tht the ternry ddition of 0.5 t. pct Zn to Mg-0.8 t. pct Bi lloy cn enhnce the mximum hrdness chievble by pproximtely 40 pct nd tht n even higher vlue of the mximum hrdness cn be obtined when the Zn concentrtion is incresed from 0.5 t. pct to 1.0 t. pct (Figure 30()). The microstructures of pek-ged smples of Mg-0.8Bi nd Mg-0.8Bi-1.0Zn lloys re compred in Figures 30(b) nd (c). [239] It is pprent tht precipittes in the Zn-contining lloy hve denser distribution. These precipittes were reported to be the equilibrium phse -Mg 3 Bi 2, nd they form s smll pltelets on plnes of the mgnesium mtrix phse. The orienttion reltionship between the -Mg 3 Bi 2 pltelets nd the surrounding mtrix phse is tht ð0001þ Mg3 Bi 2 == ; Mg 3 Bi 2 == ½0001Š. A frction of [0001] rods lso forms in the Mg-0.8Bi-1.0Zn lloy, nd they re often in contct with the -Mg 3 Bi 2 pltelets. Without mking ny distinction with the [0001] rods of Mg 4 Bi 7 in binry Mg-Zn lloys, these rods re designted MgZn 2 with the following orienttion reltionship: ð11 20Þ b 0 ==ð0001þ 1 nd ½0001Š b 0 == ½11 20Š 1. It ws speculted [239] tht the [0001] rods form first during the ging process nd then ct s heterogeneous nucletion site for the Mg 3 Bi 2 precipittes. If this specultion is ccepted s representtive, then ny subsequent increse in the nucletion rte of -Mg 3 Bi 2 precipittes would require denser distribution of [0001] rods. Any detiled chrcteriztion in the future of precipittes in the erly stges of ging using tomic-resolution HA- ADF-STEM is expected to provide some insightful results on heterogeneous nucletion nd the role of Zn in the precipittion of -Mg 3 Bi 2 in Mg-Bi-Zn lloys. III. EFFECTS OF PRECIPITATE SHAPES ON STRENGTHENING A. Plstic Deformtion by Bsl Slip 1. Mg lloys contining sher-resistnt prticles The increment in criticl resolved sher stress (CRSS) produced by the need for bsl slip disloctions to bypss point obstcles is given by [,240] Gb Ds p ¼ p 2pk ffiffiffiffiffiffiffiffiffiffi ln d p ½1Š 1 m r 0 where Ds p is the increment in CRSS cused by dispersion hrdening, G is the sher modulus of the mgnesium mtrix phse, b is the mgnitude of the Burgers vector of the gliding disloctions in mgnesium, m is Poisson s rtio, k is the effective plnr interprticle spcing, d p is the men plnr dimeter of the prticles, nd r 0 is the core rdius of the disloctions. Within Eq. [1], the k vries with the shpe nd orienttion of the prticles. For sphericl prticles of uniform Fig. 29 () Age-hrdening response of Mg-In-C lloys during isotherml ging t 473 K (200 C), (b) HAADF-STEM imge showing (c) 3DAP compositionl profile of precipitte pltes in Mg-1In-0.3C (t. pct) lloy pek-ged t 473 K (200 C). Reproduced from Ref. [238] VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

37 dimeter d t nd distribution in tringulr rry on the slip plne, the prticle spcing is given by k ¼ L p d p ¼ p 1:075 ffiffiffiffiffiffiffiffiffi pd t N v d t 4 ¼ 0:779 pffiffi 0:785 d t ½2Š f where L p is the men plnr center-to-center distnce between prticles, N v is the number of prticles per unit volume of the mtrix phse, nd f is the volume frction of prticles. If the prticle shpe is chnged from sphere to plte, then both the effective plnr interprticle spcing (Figures 31() nd (b)) nd the disloction line tension will be chnged. For mgnesium lloys contining tringulr rry of rtionlly oriented precipitte pltes of uniform size, the strengthening effect produced by such prticles cn be estimted using the equtions in Reference 241. These equtions re derived under the ssumption of uniform periodic tringulr rry of prticles intersecting the slip plne of the mgnesium mtrix phse. For fixed volume frctions nd number densities of precipittes, the vritions in the rtio Ds(plte)/Ds(sphere) with plte spect rtio re shown in Figure 32() for precipitte volume frction of For simplified comprison, the bse microstructure is ssumed to comprise precipittes in regulr tringulr rry distribution on the slip plne of the mtrix phse. As shown in Figure 32(), the CRSS increments produced by prismtic pltes re invribly lrger thn those produced by equivlent volume frctions of bsl pltes or sphericl prticles, nd the difference increses substntilly with incresing plte spect rtio. For given distribution of precipitte pltes, the effective plnr interplte spcing k decreses with n increse in Fig. 30 () Aging curves of Mg-Bi-Zn lloys t 433 K (160 C). (b nd c) TEM imges showing distribution of precipittes in (b) Mg-0.8 t. Pct Bi nd (c) Mg-0.8 t. pct Bi-1 t. pct Zn lloys, both pek ged t 433 K (160 C). Reproduced from Ref. [239]. Fig. 31 Schemtic digrms showing ( nd b) effects of the orienttion of precipitte pltes on interprticle spcing for disloction slip on bsl plne of the mgnesium mtrix phse (dpted from Refs. [20] nd [241]), nd (c) prismtic plte shered by disloctions gliding in bsl plne of the mgnesium mtrix phse (reproduced from Ref. [20]). METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

38 the plte spect rtio. There is criticl vlue of spect rtio for which the effective interprticle spcing becomes zero, nd the prismtic pltes form closed prismtic volumes, which re defined by prismtic pltes. If the pltes re ssumed to remin resistnt to disloction shering, then the disloctions generted within the prismtic volumes cnnot escpe nd the theoreticl Orown increment becomes infinitely lrge. In prctice, the ccumultion of disloctions my led to locl stress concentrtions exceeding the yield strength of precipittes nd precipitte shering. The effect of number density of precipittes on the vrition in the effective interprticle spcing with n increse in the number density of precipittes is shown in Figure 32(b) for volume frction of For identicl volume frctions nd number densities of precipittes, the prismtic precipitte pltes re much Fig. 32 Vrition of Ds p (plte)/ds p (sphere) with () plte spect rtio nd (b) number density for prismtic nd bsl precipitte pltes clculted ssuming Orown strengthening of sher-resistnt prticles with volume frction of (c) Interfcil strengthening from shered prticles. more effective in reducing interprticle spcing thn bsl pltes nd sphericl prticles. For n spect rtio of either 50:1 or 25:1, the smllest interprticle spcing is lwys ssocited with prismtic pltes. 2. Mgnesium lloys contining sherble prticles Sherble precipittes cn impede the movement of gliding disloctions through vriety of disloction/ prticle interction mechnisms, including those described s interfcil or chemicl strengthening, coherency strengthening, stcking-fult strengthening, modulus strengthening, nd order strengthening. [,240] For ech such proposed mechnism of strengthening, the contribution of the sherble precipittes to the CRSS of n lloy cn be represented by n eqution of the following form: 2 Ds p ¼ p L p b ffiffiffi F 3=2 ½3Š C 2 where C is the disloction line tension in the mgnesium mtrix nd force F is mesure of the resistnce of the precipittes to disloction shering. Assuming tringulr rry distribution of sphericl prticles of uniform dimeter d t, Eq. [3] my be rewritten in the form Ds i ¼ 1:788 d t bf C c 3=2 for interfcil strengthening, [242] where c i is the specific interfcil energy between the precipitte nd mgnesium mtrix phses. For precipittes of morphologies other thn sphere, it is currently difficult to ssess their quntittive contributions to the CRSS increment becuse there is lck of pproprite version of the strengthening equtions for such precipitte shpes nd orienttions. Becuse it hs been reported [9,240,242,243] tht interfcil strengthening cn become mjor strengthening mechnism in lloys contining plte-shped precipittes, it is convenient to restrict the current review to interfcil strengthening. The quntittive contribution of the interfcil strengthening to the CRSS for mgnesium lloys contining rtionlly oriented precipitte pltes re given by [244] Ds i ¼ 1:461 d t Ds i ¼ 1:359d t t 2 t bf C ½4Š c 3=2 for ð0001þ plte ½5Š bf c 3=2 for C plte These equtions re derived bsed on n idel regulr tringulr rry of prticles in the slip plne. It is lso ssumed tht there is continuity of the slip plne between mgnesium mtrix nd precipitte, nd tht the mgnitude of the Burgers vector of the disloctions in the mgnesium mtrix is identicl to tht in the precipitte phse (Figure 31(c)). For given vlue of disloction line tension, the vritions of Ds(plte)/Ds(sphere) with plte spect rtio s function of plte spect rtio re shown in Figure 32(c). It is evident tht the contribution of ½6Š 3928 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

39 interfcil strengthening cn become significnt when prticles tke plte shpe on prismtic plnes of the mgnesium mtrix phse. For identicl volume frctions nd number densities of precipittes per unit volume, the yield stress increments produced by prismtic precipitte pltes re orders of mgnitude lrger thn those produced by bsl pltes nd sphericl prticles. The increments in CRSS produced by prismtic pltes increse substntilly with n increse in plte spect rtio nd re up to three orders of mgnitude lrger thn tht produced by spheres, when the plte spect rtio is in the rnge of :1 to 70:1. Mny existing mgnesium lloys re strengthened by (0001) precipitte pltes. The precipitte pltes formed in these lloys re often extrordinrily thin, less thn 1 nm, nd their spect rtio is remrkbly lrge. Improvement in lloy strength might be chieved by incresing the number density nd/or thickness of the (0001) pltes. Such pproches seem, however, to hve limittion becuse simultneous increse in the number density nd thickness of precipitte pltes or incresing one fctor while mintining the other constnt is prcticlly difficult to chieve. Although much higher number density of (0001) pltes hs been produced in mny of the lloys discussed in Section II, the ge-hrdening response nd the mximum hrdness nd strength vlues of these lloys re still much lower thn those obtined in counterprt luminum lloys. [245] A substntil improvement in strength in these lloys would be difficult to chieve unless prismtic pltes of lrge spect rtio re introduced to replce, or coexist with, the (0001) precipitte pltes. For prismtic pltes, with d t >> t t, outlined in Figure 33(b), if we ssume tht ech of the three pltes is idelly distributed t the center of ech side of the isosceles tringle, then the increment in CRSS for 12 twinning disloctions to bypss precipitte pltes cn be pproximted by [249] : 8 9 Gb >< Ds p ¼ p 2p ffiffiffiffiffiffiffiffiffiffi 1 >= qffiffiffi 1 v >: A 0:825t t f 0:305At t 0:981t t >; ln 0:914t pffiffiffiffi t A ½8Š b Similrly, the increment in CRSS for 12 twinning disloctions to bypss precipitte pltes cn be estimted by [249] : B. Plstic Deformtion by Twinning 1. Mg lloys contining sher-resistnt prticles It ws reported by Clrk [15] tht twinning becomes difficult to occur when the size nd number density of Mg 17 Al 12 precipittes increse. The precipitte-twin interctions in the Mg-Al lloys nd effects of precipitte shpe on twinning hve been studied. [ ] In the current review nd for simplicity, the propgtion of twins is considered to involve the motion of twinning disloctions, nd only 12 twins re considered in the plstic deformtion process. In ddition, the lttice prmeters of Mg re tken s = nm, c = nm (c/ = ). For 12 twin to bypss sphericl prticles of uniform dimeter d t nd of tringulr rry distribution, the increment in CRSS cn be pproximted by Eqs. [1] nd[2]. For (0001) pltes, of uniform dimeter d t nd thickness t t, which re idelly distributed in tringulr rry in the 12 twin plne (Figure 33()), the increment in CRSS for 12 twinning disloctions to bypss (0001) precipitte pltes cn be pproximted by [249] 8 9 Gb < Ds p ¼ p 2p ffiffiffiffiffiffiffiffiffiffi 1 = 1:327At 1 m : pffiffiffiffi t 1:462t t ; 1:152 Af þ0:785af 1:462f ln 1:072t pffiffiffiffi t A ½7Š b Fig. 33 Schemtic digrms showing ( nd b) the effects of the orienttion of precipitte pltes on interprticle spcing for twinning on 12 plne of the mgnesium mtrix phse, nd (c) the quntittive effects of the orienttion nd spect rtio of precipitte pltes on Orown strengthening. The precipitte volume frction is [249] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

40 8 9 Gb >< Ds p ¼ p 2p ffiffiffiffiffiffiffiffiffiffi 1 >= : rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 m q >: 0:752 A f Að0:579A þ 1:727Þþ0:117A 2 þ 0:690A þ 1:021 >; : ln 0:886t pffiffiffiffi t A b t t f ½9Š If the logrithmic terms in Eqs. [8] nd [9] re ssumed to be identicl nd in the cse of sphericl precipittes nd bsl pltes, the bse microstructure is ssumed to comprise precipittes in regulr tringulr rry distribution in the 12 twin plne. For identicl volume frctions nd number densities of precipittes per unit volume of the mtrix phse, the rtio Ds p (plte)/ds p (sphere) is invribly lrger thn unity, when the plte spect rtio is in the rnge 5:1 to 70:1, which mens tht precipitte pltes re more effective in impeding twinning disloction propgtion (Figure 33(c)). Furthermore, the rtio Ds p (plte)/ Ds p (sphere) increses substntilly with incresing plte spect rtio. For given distribution of prismtic pltes, n increse in the plte spect rtio cn led to significnt reduction in effective interprticle spcing. For either or pltes, there is gin criticl vlue of plte spect rtio for which the effective interprticle spcing becomes zero. This implies tht, t such criticl plte spect rtio, the pltes will divide single mgnesium grin into mny enclosed prismtic volumes. If the pltes re ssumed to remin sher resistnt, then the 12 twins generted within prismtic volume re constrined in this prismtic volume. IV. MICROSTRUCTURAL DESIGN FOR HIGHER STRENGTH For precipittion-hrdened mgnesium lloys, whether prticles re shered or re sher resistnt, nd whether the deformtion mode is bsl slip or twinning, the prticle strengthening models indicte tht precipitte pltes formed on prismtic plnes of the mgnesium mtrix phse provide the most effective brrier to gliding disloctions nd propgting twins in the mgnesium mtrix. The models further suggest tht higher strength cn be chieved if high density of intrinsiclly strong, plte-shped precipittes with prismtic nd bsl hbit plnes nd of lrge spect rtio cn be developed in the microstructure (Figure 34). [249] Wheres it is now possible to generte microstructure contining both prismtic nd bsl pltes in some mgnesium lloys such s those bsed on the Mg-Gd-Y- Zn system, nd the bsl pltes in such microstructure hve lrge spect rtio, the spect rtio nd number density of the prismtic pltes re much lower thn those typicl of precipitte pltes formed in highstrength luminum lloys. In high-strength precipittion-hrdened luminum lloys, the mximum hrdness nd yield strength is commonly ssocited with microstructures contining high density of {0} nd {111} precipitte pltes of lrge spect rtio (typiclly bove 40:1) [245,250] (Figure 35), nd these strengthening precipittes re often intermedite or equilibrium phses tht re intrinsiclly stronger thn G.P. zones nd metstble precipittes formed in the erly stge of ging. [251] Improvements in the lloy strength thus require n increse in the number density nd/or spect rtio of prismtic pltes of intermedite or equilibrium precipitte phses. [252] It is to be noted tht the trditionl pproch to the strength improvement rrely considers the precipitte plte spect rtio nd, insted, puts n emphsis on precipitte number density. The prticle-strengthening models suggest tht the precipitte plte spect rtio is n importnt strengthening fctor, s lest s importnt s the precipitte number density, nd tht remrkble enhncement in lloy strength could be chieved if the spect rtio of the prismtic precipitte pltes is substntilly incresed. How to introduce intrinsiclly strong, prismtic precipitte pltes into the mgnesium mtrix is currently difficult question to nswer. However, one possible pproch my involve considertion of phse equilibri nd lttice mtching of mgnesium nd precipitte structures. [33,253,254] The precipitte phse my be selected from the equilibrium or ner equilibrium phses tht form vi rections mong mgnesium nd/or dded lloying elements. These phses re expected to be intrinsiclly strong nd, therefore, more resistnt to shering nd plstic deformtion. Inspections of the structures of these equilibrium phses nd tht of mgnesium cn revel whether it is possible to hve the prismtic plne s n invrint strin plne (hbit Fig. 34 Microstructurl design for precipittion-hrdenble mgnesium lloys with higher strength. A ner-continuous network of prismtic nd bsl precipitte pltes in () cn divide single mgnesium grin into mny ner-isolted blocks in (b) nd therefore effectively impede propgtion of disloctions nd twins. [249] 3930 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

41 plne) between the precipitte nd mgnesium structures. One good exmple is the b 1 phse (spce group Fm 3m, = 0.74 nm) in Mg-Nd, Mg-Y-Nd, Mg-Gd, Mg-Gd-Y, nd Mg-Gd-Nd lloys. The structure of this phse cn be generted from the mgnesium lttice by n invrint plne strin trnsformtion. [180] Therefore, it is fully predictble tht the Mg 3 Nd precipittes form s pltes on, even without exmining the microstructures using TEM. It is perhps lso constructive to compre precipittes of Mg 17 Al 12 in Mg-Al lloys nd Mg 24 Y 5 in Mg-Y lloys. [254] Although the Mg 17 Al 12 nd Mg 24 Y 5 phses Fig. 35 () Development of precipittion-hrdenble Al-Cu bsed lloy vi microlloying nd mcrolloying dditions. The ging temperture is 473 K (200 C). Trnsmission electron microgrphs showing {0} precipitte pltes of h in () Al-4Cu lloy ged 24 h t 473 K (200 C), (b) Al-4Cu-0.05Sn lloy ged 3 h t 473 K (200 C), nd (c) Al-4Cu-0.3Mg-0.5Si lloy ged 3 h t 473 K (200 C), nd {111} precipitte pltes of (d) X phse in Al-4Cu-0.3Mg-0.4Ag lloy ged 0 h t 473 K (200 C), nd (e) T 1 phse in Al-4Cu-1Li-0.3Mg-0.4Ag lloy ged 5 h t 473 K (200 C). Electron bem is prllel to h001i in ( through c) nd h011i in (d nd e). The lloy compositions re in weight percentge. Reproduced from Ref. [245]. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

42 hve sme crystl structure (spce group I 43m; 58 toms per unit cell) nd similr lttice prmeters nd orienttion reltionships, they hve drsticlly different hbit plnes. The Mg 17 Al 12 phse hs lttice prmeter of ~1.064 nm. For the observed orienttion reltionship between the Mg 17 Al 12 phse nd the mtrix 1 == 0001 p ð Þ ; ½111Š p == ; the lttice misfit between 1 p nd (0001) m is 2.7 pct if the lttice prmeters of the Mg-9 wt pct Al lloy re tken s the following: = nm, c = nm. [255] The Mg 17 Al 12 phse dopts plte shpe, with its hbit plne prllel to 1 p nd (0001) m. The equilibrium Mg 24 Y 5 phse hs lttice prmeter of ~1.125 nm, [14] which is slightly lrger thn tht of Mg 17 Al 12. Given tht its orienttion reltionships is lso 1 p == (0001), [111] p == ; the lttice misfit between 1 p nd (0001) m is 2.9 pct. The Mg 24 Y 5 phse lso dopts plte shpe, but its hbit plne is prllel to insted of (0001). Wheres both (0001) nd hbit plnes re geometriclly possible for the Burger s orienttion reltionship [253] nd becuse smll chnge in the lttice prmeter of the precipitte phse, or the mtrix phse, cn cuse this to hppen, it is currently uncler why the Mg 17 Al 12 precipittes form predominntly on (0001). If Al toms in the Mg 17 Al 12 unit cell could be prtilly replced by Y toms, then the lttice prmeter of the Mg 17 Al 12 phse would be modified to reduce the lttice misfit between precipittes nd the mtrix phse, nd it becomes energeticlly possible to chieve higher nucletion rtes nd number density of precipittes. Unfortuntely, Y nd Al toms rect strongly in the molten metl to form Al 2 Y prticles before lloy csting, leving few Y toms in the mgnesium solid solution mtrix to form the Mg 17 (Al 1-x Y x ) 12 phse during the ging process. It is lso to be noted tht Mg-Th lloys re lso strengthened by precipitte pltes[ ] even though these lloys were bndoned mny yers go becuse of the toxicity of Th. The mximum solid solubility of Th in mgnesium is only pproximtely 0.52 t. pct or 4.75 wt pct t the eutectic temperture of 855 K (582 C). [14] The mximum volume frction of the equilibrium precipitte phse Mg 23 Th 6 in Mg- 3.2 wt pct Th-0.7 wt pct Zr lloy (HK31) ged t 473 K (200 C) is estimted to be ~1.8 pct. The prismtic precipitte pltes formed in this lloy re quite thin. It is worth exmining the crystl structure nd formtion mechnism of the key strengthening precipitte phse in pek-ged Mg-Th lloys. The knowledge to be gined from this lloy cn be used to select precipitte phses tht cn form from nontoxic lloying elements but led to similr or even better ge-hrdening effects. Eqully, n in-depth knowledge on the structure nd formtion mechnisms of prismtic precipitte pltes formed in Mg-In-C lloys (Figure 29(b)) cn lso provide useful guidelines to the formtion nd mnipultion of prismtic pltes in mgnesium lloys. In mgnesium lloys tht re lredy strengthened by prismtic pltes of intrinsiclly strong precipitte phses, ny subsequent enhncement in yield strength requires n increse in the precipitte plte spect rtio nd/or precipitte number density. Approches to chieving n increse in plte spect rtio nd number density my lie in the use of (1) microlloying dditions, which my prtition to either mtrix or precipitte phse to improve mtching between the precipitte nd mtrix phse; (2) cold work fter solution tretment nd prior to ging, which introduces disloctions to provide heterogeneous nucletion sites for intermedite nd/or equilibrium precipitte phses; (3) duplex or multiple isotherml ging; nd (4) single or multiple nonisotherml ging. Such pproches inevitbly involve the considertion of the crystl structure, the orienttion reltionship, the formtion mechnism of strengthening precipitte phses, nd the elstic strins ssocited their nucletion. Currently, there is lck of selection rule for microlloying elements. The estblishment of this selection rule inevitbly requires n in-depth understnding of the formtion mechnisms of the plte-shped precipittes. It ws proposed [180] tht the b 1 structure cn be generted phenomenologiclly from the -Mg lttice by sher on the 1 0 plne in the direction, combined with n expnsion in the direction norml to the sher plne, which ws confirmed in recent study using tomic-resolution HAADF-STEM (Figure 23(c)). [196] The trnsformtion strin itself is n invrint plne strin, with the invrint plne prllel to the sher plne 1 0. This formtion mechnism is similr to tht proposed for c pltes in Mg-Gd-Zn [203] nd Mg-Y-Zn [214] lloys, proeutectoid pltes in Ti- Cr lloy, [ ] nd for precipitte pltes of c in Al-Ag lloys, h in Al-Cu lloys, T 1 in Al-Cu-Li(-Mg-Ag) lloys, nd X in Al-Cu-Mg-Ag lloys. [265,266] Figure 36 shows schemticlly n initil b 1 pltelet tht hs thickness of one unit cell nd is constrined within the -Mg mtrix phse. If the precipitte phse is much stronger thn the mtrix, then the formtion of this pltelet will result in regions of expnsion (E) nd contrction (C) in the mtrix surrounding the pltelet nd to the ssocited ccumultion of significnt sher strin energy. [180] A mechnism is thus required to operte to minimize the sher strin energy during nucletion (nd growth) of the precipitte phse. One potentil pproch to ccommodting the sher strin energy involves n extrinsic supply of reltively lrger solute toms to the E regions nd of corresponding concentrtions of vcncies to the C regions. Such n environment might be found in the vicinity of clusters combining lrger solute toms with locl excess of vcncies. Although currently there is no evidence for such solute-rich clusters in Mg-Nd, Mg-Gd, Mg-Gd-Y, Mg-Gd-Nd, nd Mg-Y-Nd lloys, it is to be noted tht Nd, Gd, nd Y toms hve reltively high binding energies with vcncies [180] nd re ll lrger thn Mg toms. The sher strin ssocited with n embryo precipitte plte might be reduced or removed if Nd, Gd, or Y toms segregte to the extended regions nd ssocited vcncies to the compressed regions during nucletion. If such mechnism were to operte, then it is plusible tht ny prticles evolved from such soluterich clusters would be expected to form t the end fcets rther thn the brod surfces of the b 1 plte. This is in perfect greement with experimentl observtions tht 3932 VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

43 Fig. 36 Schemtic digrm showing b 1 pltelet constrined in mtrix mgnesium lttice. Open nd filled circles represent toms in b 1 nd Mg lttices respectively. Reproduced from Ref. [180]. b 1 pltes form invribly in ssocition with b prticles tht re in contct with the end-fcets of b 1 pltes (Figure 23()). It is to be noted tht similr phenomen hve been documented in Al-Cu-Sn lloys, where the formtion of h pltes involves lrge sher strin [265,266] nd Sn prticles re invribly locted t the end fcet of h pltes when they form in ssocition with the h pltes. [267,268] The ccumulted experimentl evidence seems to suggest tht the most effective microlloying elements in enhncing nucletion rtes of plte-shped precipittes re those hving lrger tomic sizes nd strong binding energies with vcncies. Although it is strightforwrd exercise for the selection of elements with lrger tomic sizes, it is currently difficult to judge which elements hve lrge binding energies with vcncies in the mtrix of mgnesium. There is n urgent need for more theoreticl [86,87] nd experimentl [269] work on this spect. An lterntive wy to ccommodte the sher strin energy involved in the precipitte formtion nd, therefore, to fcilitte the nucletion of b 1 precipittes is to introduce disloctions of pproprite Burgers vectors. As shown in Figure 22, the cold work prior to ging cn promote the formtion of b 1 phse nd the mximum hrdness. Although the interction between preexisting disloctions nd the b 1 phse hs not been exmined in detil using trnsmission electron microscopy, it is to be noted tht in Al-Cu lloys, prtil disloction with Burgers vector of =2h001i on {0} cn cncel the sher strin ssocited with the formtion of plte of h of single unit cell thickness nd thus fcilitte nucletion of h [270] ; nd tht in Al-Cu-Li lloys, the sher strin involved in the formtion of T 1 precipitte pltes of unit cell thickness my be eliminted if nucletion occurs on Shockley prtil disloction. [271] It is interesting to note tht some prismtic pltes of b 1 phse cn form hexgonl-shped, continuous network in loclized regions of the mgnesium mtrix (Figure 37). [196,272] The formtion of such honeycomb structure of the b 1 pltes is likely to be ssocited with preexisting hexgonl network of disloctions inside mgnesium grins. It is of significnce to strengthening if this honeycomb structure could be extensively generted inside individul mgnesium grins. Although the yield strength chieved thus fr in mgnesium lloys is still much lower thn tht obtined in counterprt luminum lloys (Figure 38 nd Tble II), it is to be noted tht significnt progress hs been mde over the pst 12 yers. The yield strengths of precipittion-hrdenble mgnesium lloys were generlly less thn 200 MP, which re lower thn those of luminum lloys, before yer But round yers 2007 to 2009, the yield strength of mgnesium lloys pproched 320 MP for csting lloys [235] nd 475 MP for extrusion lloys. [202] A similr or even fster pce is expected in the development of highstrength mgnesium lloys if the level of mgnesium reserch ctivities enjoyed over the pst 12 yers cn be mintined in the future. The precipittion hrdening phenomenon in luminum lloys ws discovered ccidentlly by Alfred Wilm in 1906, [ ] nd the strength of luminum lloys mde t tht time ws comptible to tht of precipittion-hrdenble mgnesium lloys in yer In the following yers, the gehrdening response nd the strength of the lloys were grdully improved with the dditions of microlloying elements, either individully or in combintion (Figure 35). In 1989, it ws discovered tht dditions of smll mounts of lithium to Al-Cu-Mg-Ag lloys could stimulte n even greter ge-hrdening response, [276,277] nd n Al-Cu-Li-Mg-Ag lloy designted Weldlite 049 (McCook Metls, LLC, McCook, IL) ws developed by Pickens et l. [277] Although the composition of this lloy is not too much different from tht of the Al-Cu binry bse lloy, this lloy hs n extrordinry ge-hrdening response nd hs n ultrhigh strength exceeding 700 MP when tested t room temperture. V. UNSOLVED ISSUES AND SUMMARY As consequence of the tremendous efforts mde over the pst decde, considerble dvnces hve been mde in the understnding of precipittion nd hrdening in mgnesium lloys nd the development of highstrength mgnesium lloys vi precipittion hrdening. However, s clss of engineering mterils, mgnesium lloys re still inferior to luminum lloys in terms of ge-hrdening response nd strength. Further ttempts to improve the strength of existing lloys nd the development of new high-strength lloys require n insightful understnding of the fctors importnt in controlling the nucletion nd growth of precipittes nd the strength of the lloys, which inevitbly involves considertion of the structure, orienttion reltionship, morphology, nd formtion mechnisms of precipittes nd the interction between these precipittes nd disloctions nd twins during the deformtion process. Such understndings form the pltform of ny rtionl design nd development of mgnesium lloys with improved strength. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, NOVEMBER

44 Fig. 38 Development of precipittion-hrdenble csting nd wrought mgnesium lloys. Representtive precipittion-hrdenble luminum csting (356 nd W319) nd wrought (2195) lloys re lso provided for the purpose of comprison of tensile yield strength. Fig. 37 () Trnsmission electron microgrph, nd (b) scnning electron microgrph showing the formtion of continuous network of prismtic pltes of b 1 phse, i.e., honeycomb structure, in Mg-0.99 t. pct Sm nd WE54 lloys, respectively. () is reproduced from Ref. [272] nd (b) is from Ref. [196]. In precipittion-hrdened mgnesium lloys with multiple lloying elements, the metstble or equilibrium phse equilibri tht determine the formtion of key strengthening phses hve not been exmined systemticlly. Future improvements in the design of mgnesium lloys for higher strength will require n improved understnding of the metstble nd equilibrium phse equilibri leding to those intermedite or equilibrium phses forming s plte-shped precipittes on prismtic plnes. In prticulr, there needs to be concerted systemtic effort to understnd more completely the role of microlloying dditions in determining metstble or equilibrium phse equilibri, especilly in promoting the number density nd/or spect rtio of the plte-shped precipittes outlined in Figure 34. There re still mny unsolved nd chllenging issues on precipittion nd hrdening in mgnesium lloys, which provide mple opportunities for further reserch in the forthcoming yers. These issues include the following: 1. The structure nd composition of precipitte phses formed in the erly stge of isotherml ging re poorly understood in mny, if not ll, commercil nd lbortory lloys. The full spectrum of the precipittion sequence nd especilly the reltionship between djcent precipitte phses in the whole precipittion sequence remins cler. For exmple, how does n lredy formed precipitte phse influence the nucletion of nother precipitte phse? It hs been demonstrted in recent yers tht the ppliction of tomic-resolution HAADF-STEM, i.e., Z-contrst imging technique, cn revel more structurl nd compositionl informtion t the tomic scle. The use of this imging technique, combined with 3DAP nd other chrcteriztion techniques, is expected to resolve this issue. 2. Mny of the precipitte phses in mgnesium lloys form s bsl pltes, nd often their thickness is of single unit cell height nd remins so even fter prolonged ging t elevted temperture. These pltes lso commonly form in pirs or clusters. It is currently not cler why such precipitte pltes resist thickening nd why they form in pirs or clusters. These phenomen do not seem to be solely relted to the elstic strin effects tht re imposed on the precipittes nd the surrounding mtrix. 3. The prticle strengthening models indicte tht combined distribution of intrinsiclly strong prismtic nd bsl pltes of lrge spect rtio is needed for developing higher strength mgnesium lloys. Although rich knowledge hs been ccumulted for generting bsl pltes of lrge spect rtio, it is currently difficult to know how to generte prismtic pltes of the required spect rtio. The fctors determining the hbit plne of plteshped precipittes nd the phse equilibri in multielement system remin lrgely unknown. The precise role of microlloying elements in the nucletion nd growth of strengthening precipitte phses is lso lrgely unknown, nd the selection rules of microlloying elements remin empiricl VOLUME 43A, NOVEMBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A