The Effect of Alloying Elements on the Metastable Zr Solid Solubility and Precipitation of L1 2 Al 3 Zr Precipitates in Aluminium Alloys

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1 roceedngs of the 12th Internatonal Conference on Alumnum Alloys, September 5-9, 2010, Yokohama, Japan 2010 The Japan Insttute of Lght Metals pp The Effect of Alloyng Elements on the Metastable Zr Sold Solublty and recptaton of L1 2 Al 3 Zr recptates n Alumnum Alloys Zhhong Ja 1, Jesper Frs 1,2 Børge Forbord 2,3, Jan Ketl Solberg 1, and Knut Marthnsen 2 1 Department of Materals Scence and Engneerng, Norwegan Unersty of Scence and Technology (NTNU), N-7491 Trondhem, Norway 2 SINTEF Materals and Chemstry, N-7465 Trondhem, Norway 3 Scandpower AS, N-7462 Trondhem, Norway Metastable L1 2 Al 3 Zr precptates hae been found to be ery effcent n resstng the recrystallsaton of alumnum alloys. recptaton of Al 3 Zr precptates can be modfed by other exstent alloyng elements n mult-component alumnum alloys through arous mechansms. One of them s that addton of alloyng elements could change Zr solublty and thus, result n araton of amount of Al 3 Zr precptates. In ths work, three alloyng elements, Mn, S, Fe, often used or found n alumnum alloys and ther effects on Zr solublty and L1 2 Al 3 Zr precptaton hae been nestgated. A model for calculatng zrconum sold solublty has been deeloped and compared wth Thermo-Calc calculatons. The present model s based on the CALHAD method usng the COST507 database and s sutable for estmatons of metastable zrconum solublty. Applcaton of the model to an Al-Zr alloy contanng Mn, S and/or Fe predcts that S decreases and Mn ncreases the metastable zrconum solublty, whereas Fe appears to hae no nfluence. Expermental nestgatons by TEM are n agreement wth the model predcton. Keywords: Alumnum alloy, Zrconum, recptaton, Modellng, Transmsson Electron Mcroscopy 1. Introducton Addtons of zrconum and subsequent formaton of Al 3 Zr-precptaton hnder recrystallsaton of alumnum alloys durng thermomechancal processng by exertng a drag force (Zener-drag) on mong subgran-/gran boundares. Al 3 Zr can crystallze n the stable tetragonal DO 23 or the metastable cubc L1 2 crystal structure [1]. The latter crystal structure s coherent wth the alumnum matrx and ery effcently promotes a hgh recrystallzaton resstance through the formaton of a hgh densty of small precptates. Other alloyng elements lke lthum, magnesum, copper, znc etc. may, howeer, sgnfcantly affect the precptaton knetcs of Al 3 Zr by alterng the leel and dstrbuton of Al 3 Zr precptates compared to that of a bnary Al-Zr alloy [2-6]. Calculatons of phase equlbrum n Al-L-Zr alloys hae for nstance shown that the solublty of zrconum s drastcally reduced when lthum s present [7]. Sgl [8] has proposed a model for the nteractons between elements n sold soluton. The metastable zrconum solublty was ealuated by neglectng the nfluence of other possble phases n mult-elements alloys, and the effcency of the alloyng elements n decreasng the zrconum solublty was found to be n the order Zn<Cu<Mg<L. The effect on Al 3 Zr precptaton of addng other alloyng elements, lke S, Fe, Mn and Sc, to Zr-contanng alumnum alloys has also been studed expermentally [5, 9-10], and these elements hae been found to enhance Al 3 Zr-precptaton. Especally dstnct mproements of the number densty and dstrbuton of Al 3 Zr precptates hae been obsered by TEM when scandum s added. Westengen et al. [5] hae dscussed possble reasons for the poste effects of S and Fe. The elements may act as nucleaton stes lowerng the Al 3 Zr nucleaton barrer, and/or be ncorporated n the nucleus, whch lowers the nterface boundary energy or the olume free energy of the precptates. Forbord et al. [9] proposed another possble explanaton, that addtons of Mn, S and Fe may decrease the sold solublty of zrconum n alumnum and thus ncrease the drng force for

2 2097 formng Al 3 Zr precptates. In concluson, the nature of the stmulatng effect of these elements on Al 3 Zr precptaton s not yet really clear. In the present work, we hae calculated the metastable zrconum solublty n Al-Zr alloys, wth and wthout addtons of Mn, S and/or Fe n order to better understand the effect of these elements. Scandum s, howeer, not ncluded n the calculatons because Zr atoms segregate to Sc-rch clusters and form Al 3 (Sc,Zr)-precptates [11], and ths behaour do not ft nto the present model. 2. The model and expermental methods 2.1 The model The alloyng system s modelled as a sold soluton (-Al phase) n thermodynamcal equlbrum wth Al 3 Zr precptates (named as the -phase). The molar Gbbs free energy of the sold soluton s wrtten as ( ) G = x G + RT x ln x + G 0,xs, (1) where x 0 s the mole fracton of element n sold soluton, G s the Gbbs free energy for the pure,xs element, G s the excess Gbbs energy, and R and T are the gas constant and temperature, respectely. Only bnary nteractons are ncluded n the excess energy, whch s expressed n terms of Redlch-Kster polynomals as G = x x L ( x x ), (2), xs, j j j j> 0 where L j are (temperature dependent) Redlch-Kster coeffcents, obtaned from the COST 507 database [12]. The chemcal potentals for the sold soluton are calculated as G G μ = + = + +μ 0,xs G x j G RT ln( x ) x j xj, (3) where μ = Q x ( x x ) + x ( x x ) x x Q ( + 1)( x x, xs 1 j j j j j k jk j k j j k> j s the excess energy contrbuton, and Q j Q j ), are modfed Redlch-Kster coeffcents, defned as The molar Gbbs free energy of Al 3 Zr wth L1 2 structure s gen by G Lj, < j = 0, = j. (5) ( 1) Lj, > j = 3μ +μ = 3μ +μ Al Zr Al Zr (4), (6) where the last relaton follows from the assumpton of thermodynamcal equlbrum. The formaton enthalpy Δ H = kj/mol and entropy Δ S = J/(mol K) for Al 3 Zr are obtaned from the FTLte FactSage database [12]. We can also wrte the Gbbs free energy for Al 3 Zr as G = 3G + G +ΔH TΔS. (7) 0 0 Al Zr Combnng Eq. (3) wth Eqs. (6-7) leads to the solublty product

3 2098, xs H T S n n ( x ) exp Δ Δ μ =. (8) RT In order to sole Eq. (8) wth respect to x Zr, we need to express x n terms of x Zr for Zr. Ths s done by consderng the nomnal content of element x = fx + (1 f ) x, (9) 0 where f s the mole fracton of Al 3 Zr. Hence, the mole fracton of element n sold soluton s 0 0 x fx xzr xz r x =, wth f =. (10) 1 f x x The formalsm presented here s an extenson of the regular soluton model, consstent wth the CALHAD method. Een though all ths can be accomplshed wth fewer approxmatons n commercal software lke Thermo-Calc and FactSage, not eeryone has access to these. Therefore smple models, lke the aboe formalsm hae a alue by ts own. Fe expermental alloys [3, 9, 14-15], for whch the chemcal compostons are lsted n Table 1, hae been consdered. Calculatons of the metastable zrconum solublty hae been performed by usng the present model. Only the matrx phase and Al 3 Zr were consdered and only bnary nteractons between elements were ncluded. Table 1 Chemcal composton of the alloys, n wt% No. Alloy Zr Mn S Fe 1 AlZr (H) AlZrMn (H) AlZrS (H) AlZrSFe (C) AlZrMnSFe (C) H - hgh purty, C - commercal purty. 2.2 Expermental procedures The alloys (Al-0.15 wt.%zr and Al-0.15 wt.%zr-1.0 wt.%mn) nestgated n ths work were drect chll cast at a temperature of ~700 o C and subsequently water-quenched. The bllet has column shape wth dameter 100 mm and heght 180 mm. The samples cut from the bulk materal were homogenzed at 630 o C for 240 h and/or precptaton annealed at 450 o C for 100 h n an ar-crculatng furnace, followed by mmedate water-quenchng. A JEOL 2010 transmsson electron mcroscopy (TEM) operated at 200 kev was used for nestgatons of precptates. An electron energy loss spectrometer was appled for measurng thckness of the samples where the mages were taken, n order to calculate the olume fracton of precptates. The TEM samples were prepared by electro-thnnng n 75% methanol and 25% ntrc acd n a Struers Tenupol at a temperature of -30 o C. 3. Results and dscusson Fgure 1 shows the zrconum solublty n the bnary Al-Zr alloy as calculated by both the present model and by Thermo-Calc. Two databases, COST507 and TTAL3 hae been used n the Thermo-Calc calculaton, and only COST507 n the calculaton based on the present model. (TTAL s produced by N. Saunders, and the authors hae used t wth permsson from Hydro Alumnum.) The two methods ge qute dfferent alues for the zrconum solublty n alumnum sold soluton. The reason s analysed as follows: the present model explctly defnes the unt cell of the precptates to Zr Zr

4 2099 contan 3 Al and 1 Zr atom, so the calculaton ges the solublty n equlbrum wth the metastable L1 2 phase, whch s n good agreement wth that from frst-prncple calculatons [1]. The Thermo-Calc calculatons usng the COST507 database, howeer, nclude the small dsplacements that make the unt cell 4 tmes larger and ge the solublty n equlbrum wth the stable DO 23 phase [16]. In addton, the Thermo-Calc calculatons usng the TTAL3 database ge an abnormal rght shft of the solublty cure whch mples hgher solublty. FactSage calculatons usng ether of the databases ge smlar results as Thermo-Calc. The reason why the TTAL3 database ges ths odd result s not yet clear. Fg. 1 Zrconum solublty n the bnary Al-Zr alloy calculated by both the present model and the Thermo-Calc method usng COST 507 and TTAL3 databases. Lterature data for Zr solublty n the presence of stable DO 23 and metastable L1 2 are gen for comparson. Fg. 2 Zrconum solublty cures n equlbrum wth metastable Al 3 Zr precptates n fe alloys, calculated by the present model. The zrconum solublty at 450 o C s ndcated by the dotted lne. In the present work, we manly focus on the Zr solublty n equlbrum wth the metastable L1 2 phase. From the aboe comparson, the present model ges a reasonable predcton of the metastable solublty of zrconum. Therefore, the model has been appled to ternary alloy systems to study the effect of alloyng elements on the solublty of zrconum n equlbrum wth metastable zrconum precptates. Fgure 2 shows the zrconum solus n equlbrum wth metastable Al 3 Zr for fe selected alloys calculated by the model. Addtons of alloyng elements ge small changes of the Zr solublty. In the present smplfed model t s assumed that the alloyng elements are present n sold soluton, although some mult-component phases hae been obsered n ery low amounts. The addton of Mn leads to an ncrease n the zrconum solublty, whereas S-addtons lead to a slght decrease n the zrconum solublty. An almost complete oerlap between the Zr solus of the ternary AlZrS alloy and the quaternary AlZrSFe alloy ndcates that the effect of Fe-addtons on the zrconum solublty s neglgble. The zrconum solublty at 450 o C s about 0.11 wt% accordng to the present model (Fgure 2) and about 0.19 wt% usng Thermo-Calc and the TTAL3 database (Fgure 1). Snce the concentraton of zrconum n the bnary Al-Zr alloy s 0.15 wt%, there should be metastable Al 3 Zr precptates n the alloy after annealng at 450 o C accordng to the present model. Ths result s, howeer, not n agreement wth the Thermo-Calc calculaton. TEM nestgatons of the hgh-purty Al-0.15 wt% Zr alloy, homogenzed at 630 o C for 240 h and precptaton annealed at 450 o C for 100 h, hae been carred out. The purpose of the homogenzaton treatment of the alloy before the precptaton annealng s to elmnate the zrconum segregaton from the castng, whch may lead to an early precptaton n zrconum-enrched regons. The results of the homogenzaton treatment showed that the zrconum segregaton was greatly remoed [14]. Fgure 3a s a dffracton pattern from the Al-Zr alloy sample, for whch the projecton drecton s some degrees off [100] n order to obtan hgher ntenstes n some of the superreflectons. The

5 2100 superreflectons located mdway between 000 and 022, or equalent pars of reflectons, ndcate that there s a coherent relatonshp between the metastable L1 2 phase of the Al 3 Zr precptates and the Al matrx. By selectng the superreflecton that s marked n Fgure 3a, the TEM dark feld mage of Fgure 3b was obtaned. A dense dstrbuton of metastable Al 3 Zr precptates wth a dameter n the range of nm was obsered. Ths obseraton supports the present model predcton that zrconum s n a supersaturated condton at 450 o C, and ndcates that the Thermo-Calc calculaton based on the TTAL database oerestmates the metastable zrconum solublty. Fg. 3 (a) Dffracton pattern from the Al-0.15 wt% Zr alloy showng the man dffractonand superreflectons. (b) TEM dark feld mage usng the dffracton spot ndcated by the arrow n (a) shows metastable Al 3 Zr precptates. The specmen was homogenzed at 630 o C for 240 h followed by water quenchng, and subsequently precptaton annealed at 450 o C for 100 h. As far as the Mn-addton s concerned, a negate effect on Al 3 Zr precptaton s predcted by the present model, whle expermental work by some of the present authors [9] ndcates that the effect should be poste. In order to clarfy ths nconsstency, TEM nestgatons of a hgh purty bnary Al-Zr alloy and a hgh purty ternary Al-Zr-Mn alloy, only annealed at 450 o C for 100h, were carred out, Fgure 4. It s edent from the results that there are more Al 3 Zr precptates n the bnary alloy than n the ternary alloy. The olume fracton of Al 3 Zr precptates aeraged from ten randomly taken mages was for the Al-Zr alloy and for the Al-Zr-Mn alloy. Fg. 4 TEM dark feld mages from (a) Al-Zr alloy and (b) Al-Zr-Mn alloy. Both alloys were precptaton annealed at 450 o C for 100 h. The expermental result n ref. [9] s therefore assumed to be due to the combned presence of S and Mn (and possbly also Fe) and not from Mn alone. Ths assumpton s supported by both experments and the present model [5, 9] whch ndcate that S-addtons promote the formaton of Al 3 Zr precptates. A large number of Al 3 Zr precptates were obsered n the Al-Zr-S alloy annealed at 450 o C for 12h, but no Al 3 Zr precptates were obsered n the bnary Al-Zr alloy n the same condton. reously, t has been assumed that S-atom clusters may act as nucleaton stes and lower the nucleaton barrer for the Al 3 Zr phase, an explanaton that s supported by the fact that Al 3 Zr precptates nucleate both from homogeneous and heterogeneous stes n the Al-S-Zr alloy [3]. The

6 2101 present calculatons predct that the effect of S-addtons on the precptaton of Al 3 Zr precptates can also be due to decreased zrconum solublty n alumnum sold soluton. As judged from Fgure 2, Fe-addtons are not expected to hae any nfluence on the Zr solublty n alumnum, and thus, on the olume fracton of Al 3 Zr precptates. Howeer, a dstnct ncrease n the number densty and olume fracton of Al 3 Zr precptates s found upon addng Fe [9]. Ths obseraton could be due to a smlar mechansm as for S,.e. that Fe atoms promote Al 3 Zr nucleaton by formng clusters actng as nucleaton stes by lowerng the Al 3 Zr nucleaton barrer. Moreoer, Fe seems to be more effcent than S n ths respect. 4. Conclusons The model presented n ths work s sutable for calculatng the metastable L1 2 zrconum solublty. Thermo-Calc usng the COST507 database ges the zrconum solublty n equlbrum wth stable DO 23. The model has been appled on fe selected alumnum alloys and reeals dfferent nfluences of S, Fe and Mn on the metastable Zr solublty. S and Mn decreases and ncreases the zrconum solublty, respectely, whereas Fe appears to hae no nfluence on the solublty. The large number of Al 3 Zr precptates obsered n S-contanng Al-Zr alloys may be attrbuted to both the ncrease of the drng force for Al 3 Zr nucleaton and the lowerng of the Al 3 Zr nucleaton barrer by S-atom clusters actng as nucleatng stes. Fe does not lower the Zr-solublty, but s een more effecte than S n enhancng the nucleaton rate of the Al 3 Zr precptates. Mn, howeer, weakens the precptaton capablty of Al 3 Zr precptates by ncreasng the sold solublty of zrconum n alumnum. The authors would lke to thank Dr. Ka Tang (SINTEF, Materals and Chemstry) for the FactSage calculaton. Fnancal support from the Norwegan Research Councl s gratefully acknowledged. References [1] E. Clouet, J.M. Sanchez, C. Sgl, hyscal Reew B, 65 (2002) [2] M. Kanno, B.L. Ou, Scrpta Metall., 24 (1990) [3] K. Ranganathan, H.R. Last, T.H. Sanders, n: L. Arnberg, O. Lohne, E. Nes, N. Ryum (Eds.) The 3rd Internatonal Conference on Alumnum Alloys, Trondhem, Norway, 1992, pp [4] D. Srnasan, K. Chattopadhyay, Metallurgcal and Materals Transactons, 36A (2005) [5] H. Westengen, O. Reso, L. Auran, Alumnum, 12 (1980) [6] J.D. Robson,.B. rangnell, Materals Scence and Engneerng, 352A (2003) [7] N. Saunders, Zetschrft Fur Metallkunde, 80 (1989) [8] C. Sgl, n: J.F. Ne, A.J. Morton, B.C. Muddle (Eds.) The 9th Internatonal Conference on Alumnum Alloys (Insttute of Materals Engneerng Australasa Ltd, Brsbane, Australa, 2004, pp [9] B. Forbord, H. Hallem, K. Marthnsen, n: J. Ne, A. Morton, B. Muddle (Eds.) Materals Forum, Insttute of Materals Engneerng Australasa Ltd, Brsbane, Australa, 2004, pp [10] Y.W. Rddle, H. Hallem, N. Ryum, Mater Sc Forum, (2002) [11] B. Forbord, W. Lefebre, F. Danox, H. Hallem, K. Marthnsen, Scrpta mater., 51 (2004) [12] [FTlte database, 2009, [13] I. Ansara, A.T. Dnsdale, M.H. Rand, n, Cost 507, European Commsson, [14] B. Forbord, H. Hallem, K. Marthnsen, n: J. Ne, A. Morton, B. Muddle (Eds.) Materals Forum, (Insttute of Materals Engneerng Australasa Ltd, Brsbane, Australa, 2004), pp [15] Z.H. Ja, G.Q. Hu, B. Forbord, J.K. Solberg, Materals Scence and Engneerng, 444A (2007) [16] H.W.L. hllpps, Annotated equlbrum dagrams of some alumnum alloy systems (Insttute of Metals, 1959).