The development of new iron based shape memory alloys

Transcription

1 University f Wllngng Research Online University f Wllngng Thesis Cllectin University f Wllngng Thesis Cllectins 1996 The develpment f new irn based shape memry allys Li Huijun University f Wllngng Recmmended Citatin Huijun, Li, The develpment f new irn based shape memry allys, Dctr f Philsphy thesis, Department f Materials Engineering, University f Wllngng, Research Online is the pen access institutinal repsitry fr the University f Wllngng. Fr further infrmatin cntact the UOW Library: research-pubs@uw.edu.au

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3 THE DEVELOPMENT OF NEW IRON BASED SHAPE MEMORY ALLOYS A thesis submitted in fulfilment f the requirements fr the award f the degree f - - Dctr f Philsphy frm THE UNIVERSITY OF WOLLONGONG by HUUUN LI (M. ENG.) Department f Materials Engineering 1996

4 CANDIDATE'S CERTIFICATE This is t certify that the wrk presented in this thesis was carried ut by the candidate in the labratries f the Department f Materials Engineering, the University f Wllngng, and has nt been submitted t any ther university r institutin fr a hi degree. Huijun Li

5 1 Abstract The bjective f this research wrk was t develp new Fe-Mn-Si based shape memry allys with prperty and cst advantages ver existing cmmercial Fe-Mn-Si and Fe- Mn-Si-Cr-Ni allys. The wrk invlved the investigatin f the effect f allying elements, heat treatment and precipitatin n the shape memry effect and crrsin resistance f candidate allys. The mechanism f stress induced y»e martensitic transfrmatin and its reverse transfrmatin, and the effect f thermmechanical training n the shape memry effect were systematically studied. TEM bservatins supprted the cncept that the regular verlapping f stacking faul can result in the frmatin f bulk e martensite plates. Stacking faults were als fund t exist in e martensite plates, and it is inferred that these faults can act as embrys fr e yy reverse transfrmatin. It was fund that the shape memry capacity f Fe-Mn-Si based shape memry allys varies with annealing temperature, and this effect can be explained in terms f the effect f annealing n y<-» transfrmatin. Annealing at abut 873K was fund t be ptimal t frm the dislcatin structures which are favurable fr stress induced martensitic transfrmatin, thus resulting in the best shape memry behaviur. Strengthening f the austenite matrix is generally cnsidered t be an effective met fr imprving the shape memry effect f ferrus shape memry allys, and althugh ausaging has been fund t imprve transfrmatin reversibility and shape memry effect in Fe-Ni based allys, precipitatin f cherent NisTi particles in austenite was fund in the present wrk t degrade the shape memry effect f Fe-Mn-Si based shape memry allys. TEM results shwed that precipitates can pin Shckley partial dislcatins, suppressing stress induced y >e transfrmatin. Optical micrscpy and

6 ii X-ray diffractin results cnfirmed that the amunt f stress induced e martensite decreased when precipitatin ccurred. It was fund that allying Fe-Mn-Si ternary allys with Al and Cu significantly enhanced their resistance t hydrchlric acid attack. Fr example, Fe-28Mn-6Si-lAllCu and Fe-20Mn-6Si-7Cr-lCu (wt%) allys develped in the wrk exhibited bth gd shape memry effect and crrsin resistance t hydrchlric acid. Furthermre, Fe-Mn-Si-Cr-Ni-Cu crrsin resistant allys develped n the basis f cnventinal Fe-Mn-Si-Cr-Ni stainless steel allys exhibited imprved and increasing resistance t hydrchlric acid crrsin with increasing Cu cntent. The new Fe-Mn-Si-Al-Cu and Fe-Mn-Si-Cr-Ni-Cu allys als shwed better crrsin resistance than their cnventinal cunterparts in sulphuric acid. Immersin tests in 3.5% NaCl slutin shwed that Cu did nt significantly imprve the crrsin resistance. Amng the allys examined, the reference Fe-Mn-Si-Cr-Ni ally gave the best crrsin resistance in 3.5% NaCl slutin. Hwever, the crrsin resistance f Fe-Mn-Si-Cr- Cu allys in 3.5% NaCl was relatively high and increased with increasing Cr cncentratin. Ptentistatic tests n Fe-Mn-Si-Cr-Ni and Fe-Mn-Si-Cr-Ni-Cu allys indicated that the additin f Cu is beneficial in facilitating passivatin. The shape memry effect f the allys investigated was significantly imprved by thermmechanical training, with the efficiency f training being a functin f the number f training cycles, the training strain, and the recvery annealing temperature Generally, the shape memry effect increased with increasing training cycle number, and reached a saturatin value after 4 t 6 training cycles. The ptimal training stra was fund t depend n the ally cmpsitin. An excessive training strain induces sl defrmatin which des nt cntribute t the recvery strain. The recvery annealing temperature was fund t be crucial t the thermmechanical training, and the current study indicated that annealing at abut 873K was mst effective fr imprving the shape memry effect.

7 iii Thermmechanical training makes stress induced martensitic transfrmatin easier, because the training prcess creates dislcatin structures which prmte the nucleatin f martensite. Training resulted in a finer martensite plate size, reducing the lcal transfrmatin shear and vlume strains which need t be accmmdated in the austenite. Thermmechanical training als influenced e»y reverse transfrmatin, with the Af temperature decreasing with number f training cycles. In cntrast, the Ms and As temperatures remained nearly cnstant. Therefre, the imprvement f shape memry effect n training is due t the facilitatin f bth stress induced martensitic transfrmatin and its reverse transfrmatin. Several f the new allys, and particularly thse based n Fe-Mn-Si-Cr-Cu, shwed excellent strain recvery after training. Recvery strains f up t 5.4% were btained, significantly higher than thse btained fr the Fe-Mn-Si and Fe-Mn-Si-Cr-Ni reference allys and higher than strains previusly reprted in the literature fr Fe-Mn- Si based allys. The crrsin resistance f the Fe-Mn-Si-Cr-Cu allys was als similar t that f the cmmercial stainless ally (Fe-Mn-Si-Cr-Ni) and because f the replacement f up t 10wt% Ni with lwt% Cu, the new allys are much less expensive.

8 IV Acknwledgements Firstly, I wuld like t express my deepest gratitude t my supervisr, Prfessr Dr Dunne, fr his wise expert guidance, patient assistance, and cnstant encuragement during the entire prject. I wuld nt be able t present this wrk withut his acad help and financial supprt. I wish t express my thanks t the supprting staff in the Department f Materials Engineering, in particular G. Tillman, G. Hamiltn, N. Mackie, R. Kinnell and R. De Jng, fr their technical assistance. I wuld like als thank Assciate Prfessr N. Kennn fr sharing his thughts and expertise, and Dr. J. Zhu fr his help in prepar new allys. Special thanks are due t my parents in law fr their financial supprt during the f year f my study. Finally, I wuld like t express my thanks t my wife and my parents, fr their lve and supprt, which have helped me t reach my present psitin.

9 V Cntents Abstract i Acknwledgements iv Intrductin xii List f Symbls and Abbreviatins xiv PART A LITERATURE REVIEW 1 Chapter 1 Irn based shape memry allys Intrductin FeMnSi based shape memry allys The effect f Mn and Si cntents n shape memry effect The effect f allying n shape memry effect Mechanical prperties Crrsin resistance Pitting crrsin resistance Stress crrsin cracking resistance General crrsin resistance Phase transfrmatin and shape memry mechanism Ple Mechanism Stacking fault mechanism Factrs influencing shape memry effect in FeMnSi and FeMnSiCrNi Amunt f strain Defrmatin temperature Annealing temperature The effect f annealing temperature n transfrmatin behaviur 15

10 vi The effect f annealing temperature n shape memry effect Thermal cycling Prestrain abve Md Thermmechanical training Grain refinement Objectives f the research 19 PART B EXPERIMENTAL INVESTIGATION 21 Chapter 2 Experimental Details Systems investigated Fe-Mn-Si system Fe-Mn-Si-Cr-Ni system Fe-Mn-Si-Cr-Ni-Cu system Fe-Mn-Si-Al-Cu system Fe-Mn-Si-Ni-Cu system Fe-Mn-Si-C-Cu system Fe-Mn-Si-Cr-Ni-C system Fe-Mn-Si-Cr-Cu system Ni3Ti precipitatin strengthened Fe-Mn-Si based allys Experimental prcedure Phase transfrmatin characteristics Shape memry effect Mechanical prperties Crrsin resistance Thermmechanical training Micrstructure Experimental techniques 30

11 vii Ally preparatin Heat treatment Ht rlling Measurements f phase transfrmatin temperatures Bending tests Tensile tests Hardness tests X-ray diffractin Crrsin tests Immersin crrsin tests Ptentistatic andic plarizatin measurements Micrscpy Optical micrscpy Transmissin electrn micrscpy Scanning electrn micrscpy 37 Chapter 3 Shape memry effect Results Phase transfrmatin temperatures The effect f annealing temperature n transfrmatin temperatures The effect f pre-strain n transfrmatin temperature Bending tests FeMnSi and FeMnSiCrNi shape memry allys The new shape memry allys Metallgraphic bservatins Optical micrscpy Electrn micrscpy f parent phase Electrn micrscpy f martensite 44

12 v i i i 3.2 Discussin Transfrmatin behaviur and mechanism f y<-> e transfrmatin The effect f annealing n shape memry effect 46 Chapter 4 The effect f precipitatin n shape memry effect Results Intrductin Shape memry effect Transfrmatin Behaviur The effect f precipitatin n phase transfrmatin temperature The measurement f stress induced martensite by X-ray diffractin The measurement f stress induced martensite by ptical micrscpy Discussin The effect f precipitatin n the amunt f stress induced martensite The effect f precipitatin n shape memry effect 52 Chapter 5 Mechanical behaviur Results The strength f allys The effect f annealing n the strengths f allys The effect f defrmatin temperature n the strengths f allys Discussin Annealing temperature effect Defrmatin temperature effect 55 Chapter 6 Crrsin testing 5 7

13 ix 6.1 Results Immersin crrsin testing Crrsin resistance in hydrchlric acid Crrsin resistance in sulphuric acid Crrsin resistance in 3.5%NaCl slutin Ptentistat tests Scanning electrn micrscpy Discussin The effect f allying elements n crrsin resistance The effect f Cu cntent The effect f Cr cntent The effect f Ni cntent 62 Chapter 7 Thermmechanical training Results The effect f thermmechanical training n shape memry effect FeMnSi ternary shape memry ally FeMnSiCrNi shape memry ally FeMnSiCrNiCu shape memry allys FeMnSiAlCu shape memry allys FeMnSiCrCu shape memry allys The effect f training n phase transfrmatin temperatures The effect f training n mechanical behaviur Metallgraphic features Optical micrscpy Electrn micrscpy Discussin...7 5

14 X Factrs influencing training The effect f recvery annealing temperature The effect f training strain The effect f training cycles Mechanism f thermmechanical training The effect f matrix yield stress and the critical stress t martensite Martensite nuclei The effect f training n phase transfrmatin temperatures The size f the martensite plates 79 Chapter 8 General Discussin Intrductin Ally design The mechanism f martensitic transfrmatin Shape memry effect Crrsin resistance Thermmechanical training Effect f cpper n shape memry effect Ally cst 86 Chapter 9 Cnclusins 8 7 Chapter 10 Suggestins fr future research Develpment f new allys Applicatins Pipe jints Fasteners 90 References 9 2 Publicatins 9 7

15 XI Intrductin 1. Backgrund Shape memry allys are functinal materials which can "remember" and return t a preexisting size and shape with the input f a relatively small amunt f thermal energy. This shape memry effect ccurs in allys exhibiting a reversible martensitic phase change, as they can accmmdate strain by the mtin f transfrmatin prduct interface rather than by irreversible plastic strain r slip defrmatin. Tw typical industrial shape memry ally systems, Ni-Ti and Cu based allys have been develped. Ni-Ti shape memry ally is capable f recverable strains f up t 8%. Hwever, the high cst f Ni-Ti has stimulated the develpment f alternatives. Cu based shape memry allys are cheap cmpared with Ni-Ti allys, and can shw a maximum 5% recverable strain, but they have several disadvantages. Cu based allys are prne t decmpsitin int the equilibrium phases a and y during verheating (t 200 C r higher), prne t a stabilisatin f the martensite (the As temperature will increase slightly during lng term ageing in the martensitic phase, even at rm temperature). Mrever, the rapid grain grwth during annealing ften results a very large grain size in the rder f mm, which limits the fracture stress and the fracture strain t a few percent. Fe-Mn-Si based shape memry allys were develped in the 80s. The reprted maximum recverable strain s far is nly abut 3%, and higher recverable strains wuld facilitate the applicatin f these allys as heat shrinkable cuplings. The crrsin resistance f Fe- Mn-Si ternary allys is relatively pr being similar t mild steels. In rder t prevent crrsin, Fe-Mn-Si-Cr-Ni shape memry allys (als called stainless steel based shape memry allys) have been develped by the cmpanies Ugine-Savie (France), NKK

16 Xll (Japan) and Raychem (USA), but they are mre cstly because f the additin f Cr and Ni, and particularly, a maximum nickel cntent f abut 10%. Therefre, the develpment f new crrsin resistant Fe-Mn-Si based shape memry allys with relatively lw cst (cmpared with Fe-Mn-Si-Cr-Ni allys), gd crrsin resistance and imprved shape memry effect is a wrthwhile gal. 2. Objectives f present research The bjective f this research wrk was t develp new, lw cst, crrsin resistant Fe Mn-Si based shape memry allys with a recvery strain higher than reprted value f abut 3%. The research wrk invlved ally design and selectin, characterisatin f martensitic transfrmatin and the crrelatin with the shape memry effect, crrsin behaviur, and ptimisatin f thermmechanical training.

17 List f symbls and abbreviatins 1. Symbls A area in m 2 As temperature at which austenitic transfrmatin starts n heating Af temperature at which austenitic transfrmatin finishes n heating fe vlume fractin f 8 martensite h thickness f the sample fr bending test i current density L length f specimen befre tensile defrmatin Li length f specimen after tensile defrmatin L2 length f defrmed sample after recvery heating Ms temperature at which martensitic transfrmatin starts n cling Mf temperature at which martensitic transfrmatin finishes n cling Md maximum temperature at which martensitic transfrmatin can be stress-induced N number f thermmechanical training cycles R bend radius in bending test Rl bend radius (r radius f curvature) after applicatin f pre-strain

18 XV R2 bend radius (r radius f curvature) after recvery heat treatment T temperature (K) T n Neel temperature f austenite T temperature at which the critical stress fr martensite frmatin equals the critical stress fr slip defrmatin T Temperature at which the free energy vs temperature curves f y and e phase f the same cmpsitin t intersect each ther Vc'i pitting ptential (pitting crrsin ccurs abve this ptential) W mass lss in grams during crrsin test y austenite phase 8 epsiln martensite phase a' alpha prime martensite phase 2 Abbreviatins ASTM the American Sciety fr Testing and Materials bcc bdy centred cubic bet bdy centred tetragnal DSC differential scanning calrimetry fee face centred cubic

19 xvi fct face centred tetragnal hep hexagnal clse packed SME shape memry effect TEM transmissin electrn micrscpy TMA thermal mechanical analyser

20 PART A LITERATURE REVIEW

21 2 ****************************************************************** Chapter 1 Irn based shape memry allys 5ft 5ft 5ft 5f? 5ft 5ft 5ft 5ft 5ft 5jt 5ft 5ft 5(1 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5Jt 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5)t 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5ft 5f 1.1 Intrductin There are many allys which exhibit a shape memry effect. Althugh mst f them are nn-ferrus allys such as Ni-Ti and cpper based allys, it has been fund that sme kinds f irn-based allys d exhibit the shape memry effect. Basically, knwn ferru shape memry allys can be divided int three types accrding t the crystallgraphy the austenite - martensite transfrmatin: (1) fee (y) - bec r bet (a' martensite), (2 (y) - hep (8 martensite), and (3) fee (y) - fct martensite. Shape memry behaviur in ferrus allys has been identified fr bth thermelastic and nn-thermelastic martensitic transfrmatins, in cntrast t nn-ferrus shape memry allys which are typically assciated with thermelastic transfrmatin (which is a reversible, lw hysteresis, martensitic transfrmatin in which temperature and stress are interchangeable in prviding the driving frce fr martensitic transfrmatin, ie. a martensite plate r dmain grws r shrinks as the temperature r stress is lwered r raised.)

22 The shape memry effect in a' martensitic allys has been reprted fr Fe-Ni-C [1], Fe- Ni-C-Ti [2, 3] and Fe-Ni-C-Al [4], in which the martensite has an abnrmally high tetragnality, the a'/ y interface remains relatively mbile and the reverse transfrmatin ccurs by reverse interface mvement. Ordered Fe-Pt allys in which bet a' is frmed, als shw thermelastic transfrmatin and shape memry behaviur [5]. Furthermre, shape memry has been bserved in assciatin with martensite in Fe-Ni [6] and weakly rdered Fe-Pt [7] in which bec a' martensite is prduced. The shape memry effect assciated with 8 martensitic allys has been reprted in Fe-Mn-Si [8-9], Fe-Cr-Ni [10] and Fe-Mn-Si-Cr-Ni [11], in which the austenite has a lw stacking fault energy and 8 martensite can frm by verlapping f stacking faults. Shape memry behaviur assciated with fct martensite has nly been investigated in the less-cmmn allys, such asfe-pt[12]andfe-pd[13]. Because the cst f Fe-Pt and Fe-Pd allys is very high, the research wrk n these systems is nly f academic interest. Irn-nickel based allys such as Fe-Ni-C, Fe-Ni- C-Ti and Fe-Ni-C-Al exhibit prblems such as tempering f martensite and the transfrmatin temperature Ms being t lw (<200K). A pre-strain applied just abve r near the Ms temperature is cnvenient t prduce defrmatin by the frmatin f stress induced martensite, which can be recvered by heating t a higher temperature. The tempering f martensite impedes reverse transfrmatin by precipitatin at interfaces, leading t the degradatin f shape memry capacity. These cnsideratins have spurred the develpment f Fe-Mn-Si based shape memry allys, which are inexpensive, have gd wrkability and pssess a suitable transfrmatin temperature range (near rm temperature). The pre-strain can be given at rm temperature rather than at very lw temperature.

23 4 1.2 Fe-Mn-Si based shape memry allys The effect f Mn and Si cntents n shape memry effect The cmpsitin f Fe-Mn-Si allys is critical t shape memry effect. It is well knwn that a weak shape memry effect can be btained in Fe-Mn binary allys [14] which underg austenite t epsiln martensite transfrmatin. The degree f shape memry effect rises with increasing Mn cntent up t abut 20wt%, but a further additin f Mn degrades the shape memry effect. This behaviur can be explained by the magnetic cntributin t phase stability[15]. The Neel temperature (Tn), which is a magnetic transfrmatin temperature frm a paramagnetic phase t an antiferrmagnetic phase, rises with increasing Mn cntent. Belw this temperature antiferrmagnetic rdering takes place in austenite, leading t the stabilizatin f austenite. As a result martensitic transfrmatin is suppressed, with the degradatin f the shape memry effect. It is therefre desirable t decrease Tn belw the martensitic transfrmatin temperature. Sat et al.[16] added Si t Fe-Mn ally in rder t decrease Tn and fund that nearly cmplete shape memry effect is attainable in a single crystal f Fe-30Mn-lSi ally if the memry inducing defrmatin is perfrmed in the directin which activates a particular Shckley partial dislcatin [17,18]. Frm a practical view pint, it is desirable t btain a cmplete shape memry effect in the plycrystalline state. Recent research! 19] has revealed that a 100% shape memry effect (ie. cmplete strain recvery) can be btained in plycrystalline Fe-Mn-Si allys. The effect f Mn cntent n phase and magnetic transfrmatin temperatures is indicated in Figure 1.1 [20]: Ms decreases as Mn cntent increases, while Tn increases. Si can evidently reduce Tn and enhance the strength f austenite, thus inhibiting the intrusin f permanent slip during reversible defrmatin by frmatin f stress induced martensite. In additin, Si plays an imprtant rle in the reversible mvement f Shckley partial dislcatins, Si can make thse partial dislcatins reversible, which is beneficial t

24 Mn Cnlenl (v/l%) Figure 1.1 Effect f Mn cntent n the Neel temperature and Ms f Fe-Mn-Si allys[20].

25 shape memry effect, the mechanism still remains unclear, but mre than 6.5% will cause the ally t becme brittle. Figure 1.2 shws the effect f Si cntent n Ne temperature f Fe-30Mn ally [21]. In Figure 1.3[21], the magnitude f shape memry effect f plycrystalline Fe-Mn-S allys is presented as functins f Mn and Si cntents. Nearly perfect shape memr attained in the cmpsitin range f 28 t 34wt% Mn and 4 t 6.5wt% Si The effect f allying n shape memry effect f Fe-Mn-Si allys Recently, cnsiderable research wrk has been carried ut n the effect f additi allying elements n shape memry effect f Fe-Mn-Si allys [22-26]. This wrk can summarised as fllws. Carbn - markedly imprves shape memry effect f Fe-Mn-Si ally by strengthening austenitic matrix t restrict plastic strain accmmdatin, but preferably shul 1% t avid the frmatin f carbide. Nickel - cntributes t strength withut impairing shape memry effect and inhib frmatin f ferrite. Increasing Ni cntent will depress Ms temperature. Ni impai ht wrkability f allys at ver 10%. Chrmium - imprves shape memry effect, crrsin resistance, high temperature xidatin resistance and yield stress. The Neel temperature, Ms temperature and s fault energy decrease with increasing Cr cntent. But Cr may create a lw melting intermetallic cmpund and cause ferrite frmatin at ver 10%. Cbalt - imprves shape memry effect (the reasn is unclear) and ht wrkabilit expensive. Mlybdenum - imprves shape memry effect and heat resistance but impairs ht wrkability and shape memry effect at ver 2%.

26 O 2 Si cntent (wt%) Figure 1.2 Effect f Si cntent n the Neel temperature f Fe-30Mn aily[21].

27 Mn cntent (wt%) 38 Figure. 1.3 Effect f Mn and Si cntent n the magnitude f shape memry fr Fe-Mn-Si ally[21].

28 Aluminium - is used as a dexidant and imprves shape memry effect but has n added effect at ver 1%. Cpper - is a y frming element and imprves the crrsin resistance withut impair shape memry effect. Cu increases the stacking fault energy, and when the additin is ver 3%, e martensitic transfrmatin is suppressed, and ht wrkability is impaired. Calcium - imprves shape memry effect thrugh MnS shape cntrl but excessive additin impairs strength and fatigue prperties. A pssible explanatin is that the additin f Ca can mdify the inclusin shape frm strings t spheres, reducing mechanical barriers t stress induced martensitic transfrmatin[24,25]. Rare earths - appears t have same effect as Ca. Nitrgen - imprves the crrsin resistance and yield strength f austenite. If the additin is ver 0.4%, it will cause the frmatin f nitrides f Cr and Si Mechanical prperties Research wrk n mechanical prperties f Fe-Mn-Si based allys, which has been reprted recently[8,11,28,29], shws that the fracture strength and yield strength f allys are largely dependent n the defrmatin temperature. Figure 1.4[8] presents the stress-strain curves f plycrystalline Fe-31Mn-6.5Si fr varius temperatures. Characteristic features nted are as fllws: (1) the wrk hardening rate at small strain is much greater in plycrystalline than in a single crystal with the [413] tensile axis. (2) Ductility decreases while ultimate strength increases, ntably as temperature becmes lwer. (3) Yield stress f plycrystals increases with decrease in temperature but it shws cmplex behaviur at lw temperatures depending n the strain at which the yield stress is defined. The temperature dependence f the yield stress, defined as the 0.2% prf stress fr plycrystals and single crystals, is shwn in Figure 1.5[8]. If the tensile directin fr the single crystal is [414] in rder t btain a single shear a/6[121] n a (111) plane, the

29 " I 1 ^-*293K 800 /,338 K V) / y^^^~~\386k Id ^V^ CE / - Single ~~\4I8K 333K A498K ~ STRAIN/% Figure 1.4 Stress strain curves f Fe-31Mn-6.5Si ally btained by simple tensile tests[8].

30 TEMPERATURE / K 500 Figure 1.5 Temperature dependence f yield stress f Fe-31Mn-6.5Si [8].

31 maximum Schmid factr will be btained. The Schmid factr gives the rati f shear stress t tensile stress, and is determined by the angles f the directin f the applied stress t the slip directin and t the nrmal t the slip plane. Figure 1.5 shws that fr bth the single crystal and the plycrystal the slpe f the temperature dependence shws a clear transitin at the same temperature T 0 (380K). At this temperature, the free energy vs temperature curves f y and 8 phases f the same cmpsitin are cnsidered t intersect each ther. The decrease f yield stress belw T indicates that the stress required t induce martensitic transfrmatin decrease. The higher yield stress in the plycrystals than that in the single crystals may be due partly t the smaller Schmid factr in the defrmatin cntrlling grains and partly t the higher wrk hardening rate. Similar results are als reprted by Otsuka[29]. Figure 1.6 shws the temperature dependence f yield stress in Fe-30Mn-6Si ally. In rder t btain a gd shape memry effect, it is essential t generate martensite withut inducing slip defrmatin, because slip defrmatin is irreversible. The critical stress fr martensite frmatin and that fr slip defrmatin crrespnd t the measured yield stress at T<T' and T>T', respectively. This bservatin leads t the cnclusin that memry defrmatin must be induced belw T\ Cyclic defrmatin is als markedly affected by temperature [8]. Figure 1.7 shws the cyclic stress-strain lps btained at r clse t saturatin in stress r strain cntrlled tests. Bth the saturatin stress and the saturatin strain amplitude exhibited a clear transitin in the vicinity f Md, at abut 400K. Figure 1.8 shws the variatin f the strain amplitude in stress cntrlled tests at Aa/2=392 MPa. The slwer saturatin in the strain amplitude at a lwer temperature indicates the slwer saturatin in the internal structure. It shuld be nted that the amplitude did nt saturate t fracture at 293 K. Such a slw saturatin belw the Md is apparently attributable t the ccurrence f y-e transfrmatin.

32 -340CI w300 w >- critical stress fr martensite frmatin T AL 1 1 critical stress fr slip defrmatin Fe-32Mn-6Si J _J Temperature (K) Figure 1.6 Temperature dependence f yield stress which crrespnds t the critical stress fr martensite frmatin (T<T) and that fr slip defrmatin (T>T')[29].

33 E93K Strss Cntrl 343K /( 423K/I 500 K K / 347 K Ttal Strain Cntrl 200 a. 5 tn tn LU rr h Strain 3*/ L Strain 3 V. U) (b) Figure 1.7 Stress strain lps at saturatin in stress (a) and strain (b) cntrl tests[8].

34 NUMBER OF CYCLES Figure 1.8 Plastic strain amplitude pltted against number f cycles in stress cntrl tests[8].

35 Crrsin resistance Because the crrsin resistance f Fe-Mn-Si ally is relatively pr[ll], a series f Cr, Ni crrsin resistant allys have been develped by sme cmpanies, such as Ugine- Savie (France) and NKK (Japan). Up until the present, nly a few research reprts n crrsin resistance f Fe-Mn-Si allys have been reprted[l 1,30], but a systematic investigatin has been carried ut by NKK n Fe-Mn-Si-Cr-Ni allys (stainless steel based shape memry allys)[30]. The cmpsitins f recently develped allys are shwn in Table 1.1. There are tw types f allys, a "crrsin resistant type" and a "highly crrsin resistant type". The "crrsin resistant type" is based n the Fe-Mn-Si-Cr-Ni system but cntains less than 13% Cr. The "highly crrsin resistance type" is based n the Fe-Mn-Si-Cr-Ni-C system and cntains 13% Cr r mre. The cntents f allying elements ther than Cr are adjusted s that the Ms and Af temperature will sit in the apprpriate range (Ms near rm temperature, Af lwer than 673K). Table 1.1. Cmpsitin ranges f allys (NKK) Ally type Cr Ni Mn Si C Fe Crrsin resistant type 7-13% 10% Max 15% Max 7% Max Balance Highly crrsin 13-15% 10% 15% 7% 15% Balance resistant type Max Max Max Max The crrsin experiments by NKK were cncerned with: pitting crrsin resistance tests, stress crrsin cracking resistance tests and general crrsin resistance tests. The test cnditins are shwn in Table 1.2.

36 9 Table 1.2 Test methd Item Cnditins Objective Remarks Ande plarizatin curve measurement Measurement in 5% H 2 S0 4, 30 C, General crrsin resistance In accrdance with US G0579 (ASTM G5-87)* Ferric test Chlride Immersin fr 24 hurs in the aqueus slutin, 10% Pitting crrsin resistance In accrdance with JIS G0578 FeCl 3-6H 2 O-0.05N HC1, 20 C (ASTM G48-76)* Pitting crrsin Measurement f pitting Pitting crrsin In accrdance ptential crrsin ptential in the resistance with JIS G0577 measurement aqueus slutin, 3.5% NaCl, 30 C 42% MgCl 2 test U-bend test piece, Stress crrsin In accrdance immersin fr up t 120 cracking resistance with JIS G0576 hrs in the aqueus slutin, biling 42% MgCl 2 (ASTM G36-73)* ()*: cmparable ASTM Methd Pitting crrsin resistance The NKK results f test accrding t JIS G0578 are shwn in Figure 1.9(a), and indicate that the crrsin rate f the 9Cr type ally and 13Cr type ally are nea same as 304 stainless steel, and much smaller than that f 420 and 430 stainless st Figure 1.9(b) shws the results f the pitting ptential (V c '100) measurements, wi 13Cr type ally shwing a slightly higher pitting ptential than 304 stainless stee

37 100' 20 C, 10%FeCl 3 6H /20 N HC1 24hrs immersin test 10- C3 VI i i- fc U 0.1 >. a a. >> *-» L. C\ u r^ O C3 U a. * - * >* i- U m (N I) Q. >, E- en u Q. >, H m. (a) 600> 30 C, 3.5% NaCl 400- u c/3 > > 0- D " : > et O a. >^ u. U CT\ 1 ^ rt u D. >. t- u m 1 Tf CN U D. >. H (b) i ^f m u & >> H 1 rn u a. >, H Figure 1.9 Results f pitting crrsin test, (a) Ferric chlride test, (b) Pitting ptential measurement[30]

38 has excellent pitting crrsin resistance. The pitting ptential f 9Cr ally is h that f 430 and 420 stainless steel, but is lwer than that f 304 stainless steel. 10 These results indicate that the 13Cr ally has almst the same pitting crrsin resistance as 304 stainless steel and the 9Cr type ally has a slightly higher resis than 430 stainless steel Stress crrsin cracking resistance U-bend test pieces (surface strain: abut 11%) were immersed in an aqueus slutin biling 42% MgCl 2, and were checked every 24 hurs[30]. The results are listed in Table 1.3. Cracks initiated within 24 hurs in bth the 9Cr and 13Cr allys. In 304 stainless steel, accrding t the reprted data, cracks were bserved within 24 hur when immersed under the same cnditins. In 420 steel, general crrsin ccurred an n stress crrsin cracking was bserved. Table 1.3 Results f stress crrsin cracking test Test methd 9Cr type 13Cr type ally 420 steel 304 steel Remarks ally 42% MgCl 2 test Crack initiatin Crack initiatin (within 24 hrs) N crack ( 120 hrs) Crack initiatin 120 hrs immersin (JIS G0576) (within 24 hrs) (within 24 hrs) General crrsin resistance The general crrsin resistance f allys is characterized by the andic plarizati The results f the active peak current density (i critical, an index f general crr resistance) and the passive current density (i passive, an index f passive film f

39 are shwn in Figure 1.10[30], the active peak current density values f the 9Cr and 13Cr allys are almst equal t thse f 420 and 430 stainless steel, which means that the allys have nearly the same general crrsin resistance. Furthermre, the passive current density values measured als suggest that the passive films frmed n 9Cr and 13Cr allys are as stable as that f 18Cr type stainless steel. 1.3 Phase transfrmatin and shape memry mechanism In Fe-Mn-Si based shape memry allys, the shape memry effect is gverned b phase transfrmatin, and in cntrast t nn-ferrus shape memry allys, martensitic transfrmatin in Fe-Mn-Si based shape memry allys des nt shw thermelastic transfrmatin. Therefre, the mechanism f ye transfrmatin becmes a critical pint, which has been extensively studied[27,31-36]. Fe-Mn binary austenitic allys have a lw stacking fault energy (<40mJ/m2)[3 the additin f Si decreases the stacking fault energy even further[38]. Therefre, the stacking fault energy f Fe-Mn-Si based shape memry allys is very lw, and perfect dislcatins are easily split int tw Shckley partial dislcatins. The martensite is frmed by the mvement f Shckley partial dislcatins in the directin f a/6<121> n every secnd {111 }y plane, such that a change in the stacking fault sequence ABCABC t ABABAB ccurs thrugh {111} Y <121> shear. A bulk hep crystal ( e martensite ) with a {111 }y//{0001 } e, <110> Y //<ll20> e rientatin relatinship t austenite is prduced. Tw mechanisms f e martensitic transfrmatin have been prpsed: the ple mechanism and the stacking fault mechanism Ple mechanism The ple mechanism was prpsed by Seeger[34] in dealing with a partial disl multiplicatin mechanism. It has been applied by Hshin et al[31,33] t explain y-e martensitic transfrmatin in an Fe-31Mn-6Si single crystal. The multiplicatin f Shckley partial dislcatins ccurs n every secnd {111 }y plane, and thus

40 1 passive (a) a 4 1 &, c -4 I u E3 i critical Current density 3 zl CO C uuuuu (b) O i critical i passive c U H U. 1 ' 1 O. U 1 5-, u D. U 1 CN D- E- 1 v D- E- 1 O C- E- Figure 1.10 Ande plarizatin characteristics f 9Cr and 13Cr type stainless steel based shape memry allys, (a) schematic drawing f ande plarizatin curve, (b) results[30]

41 martensite frms directly. Figure 1.11 shws a schematic drawing f the ple mechanism. Frm Burgers vectr analysis, it was fund that a small angle grain bundary was cmpsed f three types f perfect dislcatin, ±(a/2)[0tl], ±(a/2)[011] and ±(a/2)[ll0]. A dislcatin pair is generated frm the three dislcatin nde by the fllwing plausible dislcatin reactin: (a/2)[l TO] + (a/2)[011] + (a/2)[0ll] -^ (2a/3)[lTl] + (a/6)[tl2] where (2a/3)[l 11] is the ple dislcatin. If the dislcatin energy is taken as prprtinal t the square f the Burgers vect ttal dislcatin energy remains the same in the abve reactin. Based n this analysis, Hshin et al[46,48] cnsidered that the ple mechanism in y- transfrmatin is energetically feasible Stacking fault mechanism The stacking fault energy f Fe-Mn-Si based shape memry allys is very lw. Stacking faults are induced n {111 }y planes by the mtin f Shckley partial dislcatins. The further extensin and verlapping f stacking faults results in the frmatin f bulk hep structure - martensite[36,32]. This suggests that stacking faults can act as nuclei fr martensite frmatin. Fujita et al[35] divided this mechanism int the fllwing three stages, based n the study f y- transfrmatin in 18/8 stainless steel. (1) The frmatin f wide stacking faults (2) The irregular verlapping f stacking faults (3) The regular verlapping f stacking faults Since the ple mechanism usually dminates in high stacking fault energy allys, it is mre than likely that the stacking fault verlap mechanism is the mre pssible

42 P: b = ^ (itl) S : b = (121) " 2_d t Figure 1.11 Schematic drawing f the ple mechanism prpsed by Seeger[34]. plane.) ( martensite will increase in thickness bth abve and belw the starting

43 peratinal mechanism in Fe-Mn-Si based shape memry allys which have very lw stacking fault energy. 1.4 Factrs influencing shape memry effect f Fe-Mn-Si based shape memry allys Amunt f strain It has been fund that the amunt f strain (smetimes called pre-strain) has a str influence n the shape memry effect f Fe-Mn-Si based allys[36,39,40,41]. The results shw that shape memry effect decreases when the amunt f strain is abve a certain level. Figure 1.12[36], which plts the amunt f strain against shape memry effect, indicates that the shape memry effect is higher than 75%, if the amunt f strain is smaller than 2%. With an increasing amunt f strain, hwever, the shape memry effect decreases appreciably. It is evident that a significant amunt f unrecverable permanent strain is accumulated with an increasing amunt f strain. During defrmatin belw the Md temperature, defrmatin generally ccurs by bth stress induced martensite transfrmatin and slip. At higher strain, the cntributin f the slip prcess seems t be much larger, and therefre an increasing prprtin f the strain is unrecverable. Rbinsn et al[39] fund that Af temperature increased with increasing strain, indi that the dislcatins assciated with plastic strain bstruct the mvement f Shckley partials by which the reverse transfrmatin ccurs, thus requiring a larger driving frce (higher temperature) fr the reverse transfrmatin t ccur n heating Defrmatin temperature The shape memry effect f Fe-Mn-Si based allys is largely dependent n the defrmatin temperature. Figure 1.13 [3 6] shws tensile test (4% strain) results between -196 C (77K) and 150 C (423K). It is evident that the shape memry effect is maximised between -20 C and 20 C, and decreases significantly with increasing temperature abve

44 < 6 Strain (7.) Figure 1.12 Effect f the amunt f strain n the shape memry effect f Fe-14Mn-6Si- 9Cr-6Ni[36] Temperature CO Figure 1.13 Effect f the prestraining temperature n the shape memry effect f Fe- 14Mn-6Si-9Cr-6Ni (4% prestrain)[36].

45 40 C. In the specimen prestrained at 150 C, shape memry behavir was almst absent. Specimens prestrained belw -80 C shwed a shape memry effect which was relatively cnstant with respect t the temperature f prestraining. Thus, a large shape memry effect culd nly be btained in specimens strained at temperatures belw 40 C. These results supprt the view that the defrmatin temperature must be between Ms and TQ (equilibrium temperature f y<-» transfrmatin). Similar results were als reprted by Sat et al[18]. The Ms and T temperatures f a Fe-27Mn-3Si ally were fund t be 339K and 414K, respectively, and the best shape memry effect was btained after defrmatin in the temperature range f 360k t 380K. Recently it is reprted by Yang et al[42] that a better shape memry effect can be btained by defrming Fe-Mn-Si based allys at very lw temperature (77K). There is a certain amunt f thermally induced martensite existing in allys at 77K. This suggests that the thermally induced martensite cannt be simply treated as an bstacle t stress induced martensitic transfrmatin and shape memry effect. The implicatin is that the frmatin prcess and distributin f thermal -martensite may be different fr defrmatin at temperature well belw rm temperature and fr defrmatin at rm temperature (Ms>rm temperature). Althugh bth structures will have pre-existing martensite befre defrmatin, in the latter case, the already frmed spntaneus thermal martensite suppresses the frmatin f stress induced martensite. In the frmer case, due t the large undercling and applied stress, the pre-existing martensite may start a re-rientatin prcess. The pre-existing martensite with a preferred rientatin will grw. In additin, it is knwn that the stacking fault energy decreases with decreasing temperature, and s stacking faults are mre easily frmed and extended, and can act as nucleatin sites fr y > transfrmatin Annealing temperature Annealing (slutin treatment) at high temperature will affect the phase transfrmat behaviur and shape memry effect f Fe-Mn-Si based allys thrugh its influence n micrstructures.

46 The effect f annealing temperature n transfrmatin behaviur It is reprted[43] that Ms temperature increases with increasing annealing temperatu but As temperature changes very little. Nishiyama[44] suggested that the reasn fr that the change f transfrmatin temperature with annealing temperature is that a higher "quenching" temperature prduces mre frzen-in vacancies and in turn mre nucleatin sites fr martensitic transfrmatin The effect f annealing temperature n the shape memry effect Figure 1.14 [45] shws the effect f annealing temperature n the shape memry effect f tw Fe-Mn-Si allys. It shuld be nted that the shape recvery first increases and then decreases with increasing annealing temperature. The as-ht rlled samples usually cntains sme martensite, which is prbably due t finish rlling temperature belw the nn-recrystallizatin temperature and the frmatin f sme strain induced martensite n cling. Annealing under apprpriate cnditins after ht rlling will nt nly remve this martensite but als remve the wrk hardened structure. Mre cmplete remval f the wrk hardened structure by annealing at higher temperatures will make the austenitic matrix sfter, and mre prne t defrmatin by cnventinal slip rather than by transfrmatin. On the ther hand, very lw annealing temperature can be assciated with incmplete remval f residual strain frm rlling. A maximum shape recvery is thus expected after annealing at an intermediate temperature Thermal cycling The shape memry effect and martensitic transfrmatin behaviur f Fe-Mn-Si based allys are largely influenced by thermal cycling[45,46,47]. Figure 1.15a[46] shws the changes in Ms, As, Af temperature and the amunt f martensite f Fe-24Mn-6Si ally at 305K by thermal cycling between 305K and 573K. It is seen that the Ms temperature slightly decreases with increasing number f thermal cycles, while the As temperature is

47 DALLOY NO. (. ALLOY NQ ANNEALING TEMPERATURE, K Figure 1.14 Shape memry effect as a functin f annealing temperature. Ally N.3 Fe-26.72Mn-3.58Si Ally N.4 Fe-32.05Mn-5.48Si[45]

48 3 A 5 6 Number f v: 1^420 ai ^400 a g.380 E i f i i r i i i i r (b) At L>-DHDHD-4D-{>0-n-Q-n-n As Ms fc 4> I H Numb er f.1 11 Ka Cycles Figure 1.15 Changes in Ms, As and Af temperature and the amunt f martensite at 305K by thermal cycling[46]. IX) CO 50 u D 40 e 30 c d 20 c 10 LL (a) thermal cycling between 305K and 573K (b) thermal cycling between 305K and 873K

49 almst cnstant and Af significantly increases with thermal cycling. Similar results were als reprted by Gsh et al[45]. Figure 1.15b[46] shws the changes in Ms, As, Af temperature and the amunt f martensite f Fe-24Mn-6Si ally at 305K by thermal cycling between 305K and 873K. The As and Af temperature are almst cnstant, whereas Ms temperature decreases smewhat with increasing the number f thermal cycles. When the thermal cycling was perfrmed between 305K and 1173K, the results were essentially the same as that between 305K and 873K. Accrding t the results f micrstructural analysis, the enhancement f the amunt f martensite by thermal cycling between 305k and 573K can be explained by micrstructural memry. On the ther hand, the micrstructural memry is lst after cycling between 305K and 873K (r between 305K and 1173K), because f the recvery f y by dislcatin climb and rearrangement. Sade et al[47] reprted anther situatin in the study f Fe-(24-35)Mn-(2-5)Si ally by thermal cycling between 77K and 598K. The Ms was lwered abut 10K, As was raised remarkably, and hysteresis was increased. The results suggest that sessile lattice defects (dislcatins and stacking faults) are prduced, which impede the mvement f Shckley partials in the transfrmatin interface. The effect f thermal cycling n shape memry effect was studied by Gsh et al[45], wh cncluded that shape recvery deterirates with thermal cycling. The results are summarised in Table 1.4.

50 Table 1.4 The effect f thermal cycling n shape memry effect f Fe-32.02Mn-5.48Si ally [45] 17 Heat treatment Shape recvery (%) 1. Annealed at 1373K and cled t 77K heated and held at 573K fr 5 mins and 42 cled t 77K 3. 2+heated and held at 573K fr 5 mins and cled t 77K, repeated 3 times Prestrain abve Md temperature (Ausfrming) The strength f the austenite has an imprtant bearing n the shape memry effect. If the shape change is effected by stress-induced martensitic transfrmatin withut accmpanying plastic defrmatin f the austenite, cmplete shape recvery can be btained by reverse martensitic transfrmatin. Strengthening f the parent phase raises the yield stress, increasing the stress necessary fr slip. Rbinsn and McCrmick [40] have recently reprted that pre-straining at elevated temperature (ausfrming) can imprve the shape memry effect in Fe-30%Mn-6.5%Si. The purpse f pre-straining is t harden the parent phase, thus reducing the extent f slip accmpanying the stress-induced y - transfrmatin and imprving the reversibility f the - y transfrmatin n heating. The results indicate that the stress required fr slip with increasing pre-strain, up t 4%, increases mre strngly than the stress required t induce the transfrmatin. Larger values f pre-strain increase bth the transfrmatin and yield stresses, thus reducing the transfrmatin strain fr cnditins f cnstant stress. Federzni et al [48] als reprted that the shape memry effect f a Fe-Mn-Si-Ci-Ni ally can be largely imprved by defrmatin at high temperature, by which austenite is hardened.

51 1.4.6 Thermmechanical training It has been reprted[8,27,28,29,36,39,41,49,50,51] that defrming and annealing cycles (training) can imprve the shape memry effect f Fe-Mn-Si based allys. Figure 1.16[29] shws that the recverable strain is imprved by thermmechanical training which cnsists f the repetitin f defrmatin by 2.5% and annealing at 873K fr 10 minutes. After 5 cycles f training, cmplete shape memry effect is achieved. The mechanism f thermmechanical training can be interpreted as fllws, based n the reprted results. (1) Training suppresses slip defrmatin and generates martensite at lw stress. Figure 1.17 shws the temperature dependence f yield stress f initial samples and trained samples, and it demnstrates that the critical stress fr martensite frmatin is lwered and the critical stress fr slip defrmatin is raised by thermmechanical training. The shaded prtin becmes larger after training leading t favurable cnditins fr shape memry effect. This result suggests that training strengthens the austenitic matrix, s that slip defrmatin is suppressed. Furthermre, training induces many stacking faults, which can act as nucleatin sites fr martensite frmatin, leading t lw critical stress fr martensitic transfrmatin. (2) The amunt f martensite is increased and the stress-induced martensite gradually tends t frm in a dmain manner (ie. the particular riented martensite bands becme dminated in sme dmains f the grain) with increasing training cycles[41]. The stacking faults induced by thermmechanical training can act as martensite nucleatin sites and therefre prmte the kinetics f stress induced y-» transfrmatin. The frmatin f stress-induced martensite in the dmain manner can be related t the internal stress field develped gradually with increasing thermmechanical training cycles. Since such internal stress des nt exist in the specimen defrmed in the first cycle, the stress induced martensite plates are hmgeneusly dispersed in the whle f the grain because they are induced nly by the external applied stress. The residual strain

52 ih ' «' ' UA H ~50- CO ^ r Fe-32Mn-6Si nl I I I I I L The number f times f "training" Figure 1.16 Effect f thermmechanical training n shape memry effect f a Fe-32Mn 6Si ally (training strain 2.5%)[29]

53 CO $300 +-> CO.200 *>- - after training (4 times Fe-32Mn-6Si J i i Temperature (K) Figure 1.17 Effect f thermmechanical training n temperature dependence f yield stress f a Fe-32Mni-6Si ally. Annealing temperature is 873K[29].

54 induced by training will lead t frmatin f an internal stress field which will affect martensitic transfrmatin. Since the internal stress is nt hmgeneus in ne grain, the E martensite plates will frm in dmains, which can prevent intersectin f martensite plates and reduce the extent f stabilizatin f the martensite Grain refinement There are tw different views abut the influence f grain size n the shape memry effect f Fe-Mn-Si based allys. Murakami et al[20] reprted that there is n distinguishable influence f grain size frm im n the shape memry effect f a Fe-32Mn-6Si ally. Hwever, Tan et al[52] fund that the shape memry effect f a Fe- 29.4Mn-6.2Si ally decreases with increasing grain size. The recvery rati decreased frm abut 75% t 20% when grain size increased frm abut 10u\m t 60 Lim. The reprted explanatin is that since the martensite cannt pass thrugh grain bundaries during defrmatin, the carser the grains, the higher the stress cncentratin that will be built up. When the lcal stress near grain bundaries due t the piled up partial dislcatins exceeds the critical reslved shear stress fr slip, a/2<110> perfect dislcatins will be frmed in the adjacent austenite grain. The experimental results shwed that perfect dislcatins are generated in austenite with 40(xm grain size defrmed 3% in tensin, whereas n perfect dislcatins are prduced in austenite with 10 im grain size defrmed 3% in tensin. As perfect dislcatins are easily induced in carse grained austenite, the shape memry effect decreases with increasing grain size. 1.5 Objectives f the research Althugh cnsiderable wrk has been reprted n Fe-Mn-Si based shape memry allys in areas such as phase transfrmatin mechanism, the rigins f the shape memry effect, the thermmechanical training effect and ther practical and fundamental issues still remain t be clarified. Frm the practical view pint, the shape memry effect f Fe- Mn-Si based shape memry allys is presently inferir t NiTi and cpper-based shape memry allys. Furthermre, the crrsin resistance f Fe-Mn-Si ternary allys is very

55 pr, and althugh "stainless" allys have been develped, the cst f Fe-Mn-Si-Cr-Ni allys is quite high, and the develpment f alternative lw cst allys with gd crrsin resistance is still a wrthwhile bjective. Frm the fundamental view pint mechanisms f bth phase transfrmatin and thermmechanical training are still nt clear, particularly the rle f austenite strengthening. Therefre, the main bjectiv this research are as fllws: (1) T develp new Fe-Mn-Si based shape memry allys, which have gd shape memry effect, high crrsin resistance and lw cst. (2) T study the effect f precipitatin strengthening n shape memry effect f Fe- Mn-Si based allys. (3) T study the mechanism f y- transfrmatin. (4) T clarify the mechanism f thermmechanical training.

56 PART B EXPERIMENTAL INVESTIGATION 21

57 22 ************************************************************** Chapter 2 Experimental Details ***************************^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^;j:;j:;). ;(:j(:;j, 2.1 Systems Investigated The crrsin resistance f Fe-Mn-Si ternary shape memry allys is pr, and the recverable strain f these allys is less than 2%. In rder t imprve the crrsin resistance and shape memry effect f Fe-Mn-Si based allys, the fllwing ally systems were selected fr investigatin Fe-Mn-Si system Tw Fe-Mn-Si ternary allys were chsen as reference allys, and the cmpsitins listed in Table 2.1. Table 2.1 The cmpsitins f Fe-Mn-Si allvs Ally Mn(%) Si (%) Fe 1# 28 6 balance 2# 31 6 balance

58 Fe-Mn-Si-Cr-Ni system Recently, Fe-Mn-Si-Cr-Ni shape memry allys (stainless steel based allys) have successfully develped by the cmpanies Ugine-Savie (France), NKK (Japan) and Raychem (USA). Based n the reprted results[24,25], fur Fe-Mn-Si-Cr-Ni allys were prepared in the present research wrk. Table 2.2 gives the ally cmpsitins. Table 2.2 The cmpsitins f Fe-Mn-Si-Cr-Ni allys Ally Mn (%) Si (%) Cr (%) Ni (%) Fe 3# balance 4# balance 5# balance 6# balance Fe-Mn-Si-Cr-Ni-Cu system The crrsin resistance can be imprved by the additin f Cu. Three ally; allys were designed in rder t study the effect f Cu. (Table 2.3) Table 2.3 The cmpsitins f Fe-Mn-Si-Cr-Ni-Cu allys Ally Mn(%) Si (%) Cr (%) Ni (%) Cu (%) Fe 7# balance 8# balance 9# balance

59 Fe-Mn-Si-Al-Cu system The additin f Al has been reprted t imprve shape memry effect allys[24,25], and Cu imprves crrsin resistance[53,54]. Based n Mn-Si-Al-Cu allys have been selected in the prject in rder t dev crrsin resistant shape memry allys. The cmpsitins are listed Table 2.4 The cmpsitins < f Fe-Mn-Si-Al-Cu allys Ally Mn (%) Si (%) Al(%) Cu (%) Fe 10# Balance 11# Balance 12# Balance 13# Balance Fe-Mn-Si-Ni-Cu system Althugh Cu imprves the crrsin resistance f steels, it can impa wrkability[55]. Ni nt nly imprves the shape memry effect f all als vercmes the influence f Cu n the ht wrkability f allys [ Mn-Si-Ni-Cu allys, which are listed in Table 2.5, were designed bas philsphy. Table 2.5 The Dmpsitins f Fe-Mn-Si-Ni-Cu allys Ally Mn (%) Si (%) Ni (%) Cu (%) Fe 14# Balance 15# Balance 16# Balance

60 Fe-Mn-Si-C-Cu system It has been fund that C can imprve the shape memry effect and ht wrkability f Fe-Mn-Si based allys[24,25]. In an attempt t btain an ally with effect, gd ht wrkability and crrsin resistance, three Fe-Mn-S (Table 2.6) were prepared. Table 2.6 The cmpsitins f Fe-Mn-Si-C-Cu allys Ally Mn(%) Si (%) C (%) Cu (%) Fe 17# Balance 18# Balance 19# Balance Fe-Mn-Si-Cr-Ni-C system Fr the purpse f investigating the effect f C n the shape memr Si-Cr-Ni ally, the allys listed in Table 2.7 were prepared. Table 2.7 The cmpsitins f Fe-Mn-Si-Cr-Ni-C system Ally Mn(%) Si (%) Cr (%) Ni (%) C (%) Fe 20# Balance 21# Balance 22# Balance Fe-Mn-Si-Cr-Cu system Bth Cr and Cu are beneficial fr the crrsin resistance f Fe-Mndecreases the stacking fault energy f the austenitic matrix, which

61 26 favurable fr imprving shape memry effect[22]. On the ther hand, Cu is a austenite frming element, which can act like Ni t sme extent, and is a cheaper additin. Therefre, eleven Fe-Mn-Si-Cr-Cu allys (Table 2.8) were prepared. Table 2.8 The cmpsitins f Fe-Mn-Si-Cr-Cu allys Ally Mn(%) Si (%) Cr (%) Cu (%) Fe 23# Balance 24# Balance 25# Balance 26# Balance 27# Balance 28# Balance 29# Balance 30# Balance 31# Balance 32# Balance 33# Balance Ni 3 Ti precipitatin strengthened Fe-Mn-Si based allys It has been bserved that the extent f shape memry in ferrus allys is sensitive t the fllwing types f treatments f the parent phase: ausfrming, cld wrking and annealing, rdering, ausaging, slid slutin strengthening and grain refinement. It is therefre reasnable t cnclude that the strength f austenite has a marked effect n

62 27 the shape memry effect in ferrus allys. The effectiveness f austenite strengthening in imprving transfrmatin reversibility has als been demnstrated fr ther ferrus allys, ntably Fe-C-Ni-Ti allys, which can be "ausaged" t frm cherent Ni3Ti precipitates and thus prduce a stiffened austenitic phase in which martensitic transfrmatin ccurs with reduced hysteresis. In rder t study the effect f precipitatin n the shape memry effect f Fe-Mn-Si based allys, the fllwing allys were prepared. Table 2.9 The < cmpsitins f Fe-Mn-Si-Ni-Ti allys Ally Mn(%) Si (%) Ni (%) Ti (%) Fe 34# Balance 35# Balance Table 2.10 The cmpsitins f Fe-Mn-Si-Cr-Ni-Ti allys Ally Mn(%) Si (%) Cr (%) Ni (%) Ti (%) Fe 36# Balance 37# Balance 2.2 Experimental Prcedure Phase transfrmatin characteristics Phase transfrmatin characteristics f Fe-Mn-Si based shape memry allys which underg y<-> transfrmatin are usually represented by the phase transfrmatin temperatures, Ms, As and Af. These temperatures are highly crrelated t the

63 micrstructure, especially dislcatin structures which directly affect the shape memry effect f allys. Phase transfrmatin temperatures can change with grain size, thermal cycling, thermmechanical training and quenching rate. Therefre, the thermal r thermmechanical histry f an ally is expected t influence the micrstructure and the shape memry prperties. The measurement f phase transfrmatin temperatures, temperature intervals and temperature hysteresis was carried ut using differential scanning calrimetry (DSC) and thermmechanical analysis (TMA) Shape memry effect Shape memry effect is the mst imprtant prperty f shape memry allys. In rder t assess the shape memry, a bending test was adpted because it prvided a simple and effective methd fr quantifying the shape memry effect. In the thermmechanical training prcess, tensile tests were emplyed in measuring the shape m e m r y effect, as this methd als allws mre accurate cntrl f the prestrain Mechanical prperties T btain gd shape memry effect, it is imprtant t increase the stress fr slip defrmatin and lwer the stress fr frming martensite. Bth critical stresses are markedly affected by thermmechanical training, pre-annealing, thermal cycling, etc. Therefre, the effect f these prcesses n the critical stress was investigated. In these experiments, the temperature dependence f yield stress was determined fr different cnditins Crrsin resistance The crrsin resistance f Fe-Mn-Si ternary allys is pr. As mentined previusly, Fe-Mn-Si-Cr-Cu, Fe-Mn-Si-Al-Cu and Fe-Mn-Si-Cr-Ni-Cu allys have been designed

64 fr btaining gd crrsin resistance. The crrsin testing was based n cmparisn with 304 stainless steel and an Fe-Mn-Si-Cr-Ni ally Thermmechanical training It has been reprted[8,27,28,29,36,39,41,49,50,51] that thermmechanical training can significantiy imprve the shape memry effect f Fe-Mn-Si based allys. In rder t fin ptimum training cnditins and t investigate the training mechanism, a series f training tests were designed t study the effect f: (1) training cycles n shape memry effect, (2) recvery annealing temperature n shape memry effect, (3) amunt f pre-strain n shape memry effect, and (4) strain rate n shape memry effect. Detailed and extensive infrmatin, such as recvered strain, permanent strain, transfrmatin temperature and yield stress can be btained frm training prcedures Micrstructure The micrstructure (dislcatin structure, martensite mrphlgy and distributin, etc f Fe-Mn-Si based shape memry allys is critical t shape memry effect. Sme methds fr imprving shape memry effect can be explained using micrstructural analysis. Therefre, bservatins f micrstructure were cnducted t elucidate: (1) the general structure f allys, (2) the effect f thermmechanical training n micrstructure, (3) the structure after annealing at different temperatures, and (4) the surface mrphlgy after crrsin testing.

65 30 Optical and electrn micrscpy were emplyed fr micrstructural examinatin. 2.3 Experimental techniques Ally preparatin During the first step f selecting allys with gd shape memry, allys were prep by arc melting using high purity materials. Ingts f log mass were hmgenized at 1373K fr 48 hurs in evacuated quartz capsules. Ht rlling was carried ut at 132 t btain 1 mm thick strips fr bending and crrsin tests. The allys selected fr mre detailed research were prepared by vacuum inductin melting and casting f 6 kg ingts. In rder t reduce dendritic segregatin, a sla f cast irn was used, Figure 2.1. Slidificatin ccurred very rapidly with this The cmpsitins f selected allys are listed in Table Table 2.13 The cmpsitins f se ected allys Ally C Mn Si Cr Ni Cu Al P S Fe 1# Bal. 6# Bal. 7# Bal. 10# Bal. 25# Bal. 28# Bal.

66 100 mm 20 mm L. Z. y 200 mm Figure 2.1 The schematic drawing f slab casting muld

67 Heat treatment All f the specimens after ht rlling and machining were slutin treated at temperatures frm 673K t 1273K in rder t eliminate the influence f machining and t prduce single phase y. High purity argn gas was purged int the furnace t prevent xidatin. In sme cases ( high temperature r lng duratin ), the specimens were sealed in evacuated quartz tubes befre heat treatment Ht rlling Frm previus experience, the ht defrmatin resistance f Fe-Mn-Si based shape memry allys is very high cmpared with general carbn steels. On the ther hand, the crack tendency increased with increasing rlling temperature abve 1323K. Therefre, the slab ingts with 20 mm thickness were rlled at 1323K, with a reductin f abut 5-7% per pass t btain 6 mm thick plate Measurement f phase transfrmatin temperatures The investigatin f the As, Af and Ms temperatures was carried ut using differenti scanning calrimetry (Mettler DSC 30) and a thermmechanical analyser (Mettler TMA 40). DSC samples f abut 20 mg mass were sealed in a standard aluminium pan with a lid prir t being placed int the furnace. An empty reference aluminium pan was placed in an adjacent psitin in the furnace. Bth pans were placed n the furnace base which was cnnected with several pairs f micrthermcuples. The data n heat flw relative t the reference were autmatically recrded by the central cmputer (Mettler TA3000) as a functin f temperature. A dt matrix printer pltted the DSC curve either n-line r subsequent t the test. In this study, the heating and cling rates fr DSC calibratin and measurement were 10 K/min. High purity lead, zinc and indium were used in calibratin f the DSC temperature measurement. The precisin f temperature cntrl was C and the cling medium was liquid nitrgen.

68 A typical DSC curve is shwn in Figure 2.2. The peaks crrespnd t the frward and reverse phase transfrmatins. In the case f martensitic transfrmatin, the heat peaks are relatively brad. Cnventinally, the start and end pints f the deviati the base line f the DSC curve are respectively regarded as the Ms and Mf temperatur when cling, r as the As and Af temperature when heating. This methd was emplyed in the present study. The TMA samples were abut 1 mm thick, and the tw surfaces f the sample that cme int cntact with the quartz sample supprt and measuring prbe were as nearly para as pssible. The heating rate was 10 K/min, and the cnstant prbe frce was 0.2 N, which is recmmended fr hard samples. Argn gas was purged int the furnace t avid xidatin f the sample. A typical TMA curve fr austenitic transfrmatin in Fe-Mn-Si based allys is shwn Figure 2.3, in which the length extensin is pltted against temperature. The start end pints f the deviatin frm the simple extensin line (linear line) are regard the As and Af temperatures Bending tests Bending tests were used as an effective and relatively simple way f characterizing memry behavir. In the present study, 1 mm thick strips were bent at rm temperature, and heated up t 873K fr 10 minutes t allw the riginal shape, r pa theref, t be regained. By cmparing the amunt f initial defrmatin with the amunt f strain remaining (residual strain) after reverse transfrmatin, a measure f the degree f shape me can be btained, as illustrated in Figure 2.4. The frmula used t link bend radius strain was: strain = [2(R/h)+\y l

69 ! heating >.9 CO «X Sfl w cling y/- Temperature (~C) Figure 2.2 Schematic differential scanning calrimeter prfiles fr martensitic and austenitic transfrmatins.9 M w Temperature ("C) Fgiure 2.3 Typical TMA curve fr reverse transfrmatin t austenite

70 sample i bending frce Figure 2.4 Schematic drawing f the equipment cnstructed fr bend testing.

71 where h is the sample thickness and R the bend radius f the sample. This equatin was used t prvide an accurate determinatin f the strains prduced by bending. Fr the set-up in Figure 2.4, the pre-strain is: Pre-strain = [2(Ri/h)+l]- 1 where Ri is the radius f curvature f the sample after the pre-strain. and the residual strain is: Residual strain = [2(R 2 /h)+l]-l where Ri is the radius f curvature f the sample after recvery heating (Figure 2.5). Thus the shape recvery rati can be evaluated by means f the fllwing frmula: Recvery rati (%) = [1-(residual strain/pre-strain)]x100% Tensile tests The samples fr tensile testing were machined accrding t the specificatin given in Figure 2.6. Tensile tests were carried ut using an Instrn 4302 universal test machi The machine was equipped with a data acquisitin system which cnsisted f a data access cable, IBM cmpatible PC, a X-Y chart recrder and a dt matrix printer. Installed sftware recrded test data and parameters, such as lading frce, pltting range and extensin. A furnace with a range f 200K t 523K was used fr tensile testing at different temperature. A lw crss-head speed f 0.2mm/min was applied fr general testing. The shape memry effect was measured with the prcedure shwn in Figure 2.7. Tw indentatins were made n the surface f the specimen by using a Vickers hardness tester befre the tensile test. The distance between the tw indentatins (Ln) was precisely measured using a hrizntal travelling micrscpe. L 0 was usually set t 40

72 Figure 2.5 Schematic drawing shwing the methd used fr measuring the bend radius f a sample after recvery heating.

73 L = 40mm l T d = 3mm Figure 2.6 The specificatin f rund bar tensile test sample Li "w^ l r 0 (a) 0 L2 0 0 f (b) 4 '. Figure 2.7 The evaluatin f shape memry effect, (a) after straining, (b) after recvery annealing. Shape memry effect is given by (Li-L2)/(Li-L)X100 in %.

74 mm. Li and L 2 are the gauge lengths after pre-straining and recvery annealing, respectively. The pre-strain is represented by the fllwing frmula: Pre-strain = (Li-L)/Ln The residual strain (unrecvered strain) is: Residual strain = (L2-L)/Ln Therefre, the recvery rati is: Recvery rati (%) = [1-(residual strain/pre-strain)]x100% = (Li-L 2 )/(Li-L 0 )X100% Hardness tests Hardness measurements were perfrmed using a Vickers hardness tester with a test lad f 5 kg. The mean hardness value was btained by averaging five hardness measurements fr each sample X-ray diffractin X-ray diffractin was perfrmed with an X-ray diffractmeter ( Philips ), using flat rectangular specimens. The surfaces f specimens were electrlytically plished. The target material and P filter were selected as C and Fe, respectively. The tube vltage and lading current were 30 kv and 11.7 ma, respectively. The scanning speed was 17min. The integrated intensities f (200)y and (1011) were used t characterize the vlume fractin f y and phase, respectively Crrsin tests Immersin crrsin test The samples fr the immersin crrsin test were prepared accrding t ASTM Gl- 90 [57]. The samples were grund using silicn carbide paper t prduce a 1200 grit

75 finish. All crners and edges were runded ff and any burrs were remved during the grinding prcess. The samples were decntaminated with methanl and dried with ht air, then weighed t the nearest 0.1 mg immediately prir t testing. The test tem was 25 C. The crrsin media were chsen as fllws: (1) hydrchlric acid: 5%, 10%, 15%, 20% (2) sulfuric acid: 5%, 10%, 15%, 20% (3) NaCl water slutin: 3.5% (4) tap water Reagent grade chemicals and distilled water were used in all tests except the test water. In this study, the samples were immersed in the medium fr 4 hurs, then remved frm the test slutin, rinsed and scrubbed with a nyln bristle brush unde running water and finally immersed in methanl and dried thrughly. When dry, the samples were re-weighed t O.lmg. The crrsin rate was btained as fllws: Crrsin rate (g/m 2 hr) = W/(4A) where: W = mass lss in grams, and A = area in m 2 The crrsin test in tap water was assessed by the extent f rusting Ptentistatic andic plarizatin measurements The ptentistat used in this study was an AMEL Mdel 553. The ptentistat had the functin f incrementing the vltage whilst measuring the current respnse f the cell. The test was perfrmed accrding t ASTM G5-87[58]. The ptential was incremented 50 mv every 5 minutes, recrding the current at the end f each 5 minut

76 perid at ptential. A saturated calmel electrde (SCE) was used as the reference. The slutin was 1.0 N H 2 S0 4 frm A.C.S. reagent grade acid and distilled water. The test temperature was cntrlled at 30 C. The edges f the sample were cated with Amercat 90 befre the test, as shwn in Figure 2.8. Amercat 90 is a phenlic resin with a cnstant rati f resin/curing agent. It has gd adherence t the test sample and has gd resistance t crevice crrsin Micrscpy Optical micrscpy The micrstructure was examined and phtgraphed using a Nikn Optipht metallurgical micrscpe. The samples were cld-munted using epxy resin. Because the Ms temperatures f the mst allys are arund rm temperature, martensite is easily induced by grinding and mechanical plishing. In rder t eliminate this effect, samples were electrlytically plished after gently grinding with silicn carbide paper f grade The electrlyte had the fllwing cmpsitin: 10% perchlric acid 90% 2-butxyethanl The slutin is relatively viscus, and in sme cases, the additin f 0.5% distilled imprved the plishing. The plishing vltage was 30-40V. After plishing, samples were etched in a slutin f 1.2% K 2 S 2 0s and 0.5% NH 4 HF 2 in distilled water Transmissin electrn micrscpy The detailed micrstructure, such as dislcatin structure and the internal structur martensite, was examined by transmissin electrn micrscpy. The samples were mechanically thinned using silicn carbide paper f grade 1200 t reduce the thickness t looum. Then they were electrplished using a Struers twin jet plisher (TENUPOL-2 ) in a slutin f 10% perchlric acid and 90% acetic acid. The electrplishing vltage

77 Amercat 90 sample surface Figure 2.8 The schematic drawing f the sample fr the ptentistatic test.

78 was 50V, at rm temperature. The bservatins were carried ut n a JEOL 2000FX transmissin electrn micrscpe perating at 200kV Scanning electrn micrscpy The surfaces f the samples after crrsin testing were examined using scanning electrn micrscpy, perating at 20kV. A Hitachi S450 scanning electrn micrscpe was emplyed in the study. In sme cases, the samples fr ptical micrscpy were bserved by scanning electrn micrscpy in rder t study martensite in mre detail.

79 ********************************************************************** Chapter 3 Shape memry effect ********************************************************************** 3.1 Results Phase transfrmatin temperatures The phase transfrmatin temperatures, Ms, As and Af are imprtant parameters fr F Mn-Si based shape memry allys, particularly the Ms temperature, which shuld be arund rm temperature. Phase transfrmatin temperatures are largely influenced b the micrstmcture, and since annealing at high temperature and pre-strain will chan austenite structure, the effects f annealing temperature and amunt f prestrain transfrmatin temperatures have been studied in the present wrk The effect f annealing temperature n transfrmatin temperatures The samples were annealed after ht rlling at temperatures frm 673K t 1273K fr 30 minutes. The Ms temperature was measured in the as annealed cnditin, while the and Af temperatures were measured after 4% pre-strain at rm temperature. Figure 3 shws the changes in Ms, As and Af with annealing cnditins. It can be seen that t

80 a" i I H I H m en t- r- 200 T3 U 1 it m r~ vo 1 en r- r- 1 it en t- 1 it m OS r- 1 m r- 1 it m r- ~- 1 it en r-- CN Annealing cnditin Annealing cnditin 700 W P a u I - m r- SO m r- r- 1 it m r- 00 en c- s m r- 1 it m r- ^- m r- (N Annealing cnditin Figure 3.1 The effect f annealing n the phase transfrmatin temperatures f allys 1#,6# and 28#. Ally 1# Fe-28Mn-6.2Si Ally 6# Fe-13Mn-4.9Si-10Cr-5.6Ni Ally 28# Fe-20.4Mn-5.6Si-7.3Cr-0.97Cu

81 39 results fr the three selected allys (1#, 6# and 28#) are similar. With increasing annealing temperature, Ms temperatures slightly increased, while the As and Af temperatures decreased by abut 10K t 30K. In the ther wrds, the thermal hysteresis AT(Af-Ms) decreased with increasing annealing temperature The effect f pre-strain n transfrmatin temperatures In Fe-Mn-Si based shape memry allys, y-> transfrmatin will ccur during prestraining under the applied stress, and the amunt f stress induced martensite is related t the pre-strain[36]. The amunt the stress induced martensite increases with increasing pre-strain, and the stress required fr the frmatin f new martensitic plates als increases during pre-straining. Figure 3.2 shws the changes in As and Af temperatures with increasing the amunt f pre-strain fr samples annealed at 873K fr 30 minutes. It can be seen that the As temperatures f the three allys (1#, 6# and 28#) remained nearly cnstant with increasing pre-strain, whereas the Af temperatures markedly increased. By raising pre-strain frm 2% t 15%, the Af temperature increased by mre than 70K. When the pre-strain was less than 8%, the increase in Af temperature is very bvius; and when the pre-strain was larger than 8%, the Af temperature apprached a saturatin value. The gap between As and Af (Af-As) became wider with increasing pre-strain, which suggests that the driving frce needed fr the reverse transfrmatin is significantly increased Bending tests Bend testing was cnsidered t be an effective and relatively simple methd t characterize the shape memry effect. In the present study, bending tests were used t select allys with gd shape memry effect, as well as the ptimal annealing temperature.

82 Strain (%) Strain (%) 800 W <u (U I H Strain (%) Figure 3.2 The effect f pre-strain n the reverse transfrmatin temperatures determined by TMA.

83 Fe-Mn-Si and Fe-Mn-Si-Cr-Ni shape memry allys Fe-Mn-Si and Fe-Mn-Si-Cr-Ni allys are tw f the traditinal irn based shape memry allys based n the y <-> transfrmatin. Six different allys (tw Fe-Mn-Si allys and fur Fe-Mn-Si-Cr-Ni allys) were selected as reference allys. It has been reprted that variatin in the annealing cnditins has a strng influenc the shape memry effect by the reversing existing martensite (frm rlling r machining) and changing the micrstructure f the parent phase, especially the dislcatin and stacking fault structure[36,45]. Figure 3.3 shws the effect f changing the annealing temperature (annealing duratin f 30 minutes) and bending strain n the recvery rati fr tw Fe-Mn-Si allys (1# and 2#). The annealing temperature has nly a relatively minr influence n the shape memry effect ver the selected temperature range (673 tl273k). Hwever, it can be cncluded frm these curves that the best shape memry effect can be btained when the annealing temperature is abut 873K. Figure 3.4 shws similar results fr Fe-Mn-Si-Cr-Ni allys, except that the shape memry effect f the selected Fe-Mn-Si-Cr-Ni allys is much higher than that f Fe-Mn- Si ternary allys. The ptimum annealing temperature is als abut 873K. This result suggests that the micrstructure f parent phase created by annealing at 873K is beneficial fr y<-> transfrmatin The new shape memry allys The effect f annealing temperature n shape memry effect f allys develped in the curse f this wrk is similar t the results fr the Fe-Mn-Si and Fe-Mn-Si-Cr-Ni allys. Therefre, the results fr bending tests at 873K fr 30min were used t cmpare the shape memry capacity t the new allys. Figure 3.5 shws the changes f shape memry effect with increasing bending strain in Fe-Mn-Si-Cr-Ni-Cu allys (7#-9#) which were develped based n ally 6#. Ally 9#, which cntains 3% Cu, shwed significant cracking after ht rlling, because the Cu

84 100 l#(bending) e > <D I 4.0 Bending strain (%) r fr 50- > Bending strain (%) gure 3.3 The shape memry effect at different annealing temperature fr allys 1# and 2#. Ally 1# Fe-28Mn-6Si Ally 2# Fe-31Mn-6Si

85 > O Bending strain (%) 100 C3 Li CD > O 673K ^ 873K» 1073K A 1273K Bending strain (%) Figure 3.4 The shape memry effect at different annealing temperature fr allys 3#-6#. Ally 3# Fe-20Mn-5Si-8Cr-5Ni Ally 4# Fe-17Mn-5Si-9Cr-6Ni Ally 5# Fe-16Mn-5Si-12Cr-5Ni Ally 6# Fe-13Mn-5Si-10Cr-6Ni

86 100 2 u > u <u Bending strain (%) 100 # 2 > <u 04 Bending strain (%) Figure 3.4 cntinued.

87 100 Ui > 1) 04 -ffl- 7# 8# 6# Bending strain (%) Figure 3.5 The shape memry effect f allys 7# and 8# (6# is the reference ally) Ally 7# Fe-13Mn-5Si-10Cr-6Ni-1 Cu Ally 8# Fe-13Mn-5Si-10Cr-6Ni-2Cu

88 41 prmtes ht-shrt cracking. It is clear frm Figure 3.5 that the shape memry effect f ally 7# (1% Cu cntent), which shwed gd ht wrkability, is very clse t that f allys 6#. Hwever, the shape memry effect f ally 8# (2% Cu cntent) is reduced by a small amunt. Therefre, an apprpriate Cu cntent fr Fe-Mn-Si-Cr-Ni allys is abut 1%. Fe-Mn-Si-Al-Cu allys were als develped frm Fe-Mn-Si ternary allys, and the sh memry effect measurements are given in Figure 3.6, which shws that ally 10# ( with 1% Al and 1% Cu) has the best shape memry effect. The results fr allys 1# and 2# are pltted as references. It can be cncluded that allying with Al (<2%) and Cu (-1%) can markedly imprve the shape memry effect f Fe-Mn-Si ternary allys. The ingt f ally 12# ( with 3% Cu cntent ) fractured during ht rlling, because the high Cu cntent impairs the ht wrkability f the ally. Because Cu can cause ht shrt cracking, Ni and C were added t imprve the ht wrkability. Tw allys system were develped based n this idea: Fe-Mn-Si-Ni-Cu and Fe-Mn-Si-C-Cu, respectively. Hwever, ht shrt cracking still ccurred during ht rlling when Cu cntent was increased up t 3% (allys 16# and 19#). Figure 3.7 shws the shape memry effect f the new allys. It is seen that the shape memry effect f the allys with 1% Cu (allys 14# and 17#) has been imprved t sme extent, whereas the shape memry capacities f the allys with 2% Cu (15# and 18#) are clse t thse f Fe-Mn-Si ternary allys. Figure 3.8 gives the shape memry effect f anther newly develped ally system, Mn-Si-Cr-Ni-C allys (allys 20#, 21# and 22#). The purpse f this experiment was t investigate the effect f C cntent (up t 4%) n the shape memry effect f Fe-Mn- Si-Cr-Ni allys. It can be seen that the additin f C des nt prduce any imprvement in shape memry effect. In rder t imprve the crrsin resistance f Fe-Mn-Si ternary allys, Fe-Mn-Siallys have been develped. The new allys can be divided in t three types, 5Cr type,

89 100 t$ S-i > O -ffl- ---*-- 10# n# 13# 1# 2# 2 4 Bending strain (%) Figure 3.6 The shape memry effect f allys 10#, 11# and 13# (1# and 2# are reference allys). Ally 10# Fe-28Mn-6Si-lAl-lCu Ally 11# Fe-28Mn-6Si-l Al-2Cu Ally 13# Fe-28Mn-6Si-2 Al-1 Cu

90 80- **l *^v \ ** X_ Fe-Mn-Si-Ni-Cu * * x ^. *x ^ * X x. *jl x * ^^^^. -ffl- * 14# 15# 1# 2# 20- i i i Bending strain (%) 100 > O 04 -H- H- 17# 18# 1# 2# Bending strain (%) Figure 3.7 The shape memry effect f Fe-Mn-Si-Ni-Cu and Fe-Mn-Si-C-Cu allys. Ally 14# Fe-28Mn-6Si-0.5Ni-lCu Ally 14# Fe-28Mn-6Si-lNi-2Cu Ally 17# Fe-28Mn-6Si-2C-lCu Ally 18# Fe-28Mn-6Si-2C-2Cu

91 42 7Cr type and locr type, meaning that the Cr cntents are 5%, 7% and 10%. The Cu cntent is 1%. The shape memry capacities f allys were again measured by bending tests. Figure 3.9 gives the changes f recvery rati f 5Cr type allys as a functin f b strain. It is evident that the shape memry effect f 5Cr type Fe-Mn-Si-Cr-Cu allys des nt change significantly when Mn cntent varies frm 18% t 26%. Allys 25# and 32# have relatively gd shape memry effect. When Cr cntent increases t 7%, the effect f changing the Mn cntent becmes mre marked. Figure 3.10 shws the shape memry effect f 7Cr type allys, which indicates that ally 28# exhibits the best shape memry effect. Fr locr type Fe-Mn-Si-Cr-Cu allys, with Mn cntents dwn t 20%, a large amunt f ferrite frms in the allys, which becme very brittle. A pssible explanatin is that the slubility f Cu in ferrite is very lw, and precipitatin f Cu causes embrittlement. Amng the selected locr type allys, ally 30# shws the best shape memry effect, as illustrated in Figure During the ally selectin phase f the wrk, many allys with gd shape memry capacities were fund. On this basis, allys 7#, 10#, 25# and 28# were selected fr further investigatin with allys 1# and 6# being used as reference allys Metallgraphic bservatin Because the transfrmatin behaviurs and micrstructures f Fe-Mn-Si based shape memry allys are similar, allys 6# and 28# were selected fr detailed study f the micrstmcture Optical micrscpy The micrstructures f ally 28# specimens after annealing at different temperatures checked by ptical micrscpy, and phtmicrgraphs are shwn in Figure With

92 100 Ui <D > O Bending strain (%) Figure 3.10 The shape memry effect f 7Cr type Fe-Mn-Si-Cr-Cu allys. Ally 26# Fe-26Mn-6Si-7Cr-lCu Ally 27# Fe-23Mn-6Si-7Cr-1 Cu Ally 28# Fe-20Mn-6Si-7Cr-1 Cu Ally 33# Fe-18Mn-6Si-7Cr-1 Cu 100 ^ 29# > 04 -ffl- 30# Bending strain (%) Figure 3.11 The shape memry effect f locr type Fe-Mn-Si-Cr-Cu allys. Ally 29# Fe-26Mn-6Si-1 OCr-1 Cu Ally 30# Fe-23Mn-6Si-1 OCr-1 Cu

93 100 20# -ffl- 21# 22# -- 6# Bending strain (%) Figure 3.8 The shape memry effect f Fe-Mn-Si-Cr-Ni-C allys. Ally 20# Fe-13Mn-5Si-10Cr-6Ni-2C Ally 21# Fe-13Mn-5Si-10Cr-6Ni-3C Ally 22# Fe-13Mn-5Si-10Cr-6Ni-4C 100 s- & > 04 Bending strain (%) Figure 3.9 The shape memry effect f 5Cr type Fe-Mn-Si-Cr-Cu allys Ally 23# Fe-26Mn-6Si-5Cr-1 Cu Ally 24# Fe-23Mn-6Si-5Cr-lCu Ally 25# Fe-20Mn-6Si-5Cr-lCu Ally 32# Fe-18Mn-6Si-5Cr-1 Cu

94 Figure 3.12 The effect f annealing temperature n the micrstructures f ally 28#. (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

95 43 increasing annealing temperature up t 1273K, the austenite grain size remained nearly cnstant. Hwever, the amunt f thermal martensite increased, as the Ms temperature was increased by raising the annealing temperature. The shape memry effect f Fe-Mn-Si based shape memry allys varies with annealing temperature. The annealing prcess must affect stress induced y-> transfrmatin, as the shape memry effect is assciated with this transfrmatin. A tensile strain f 4% was emplyed t investigate the effect f annealing temperature the mrphlgy f stress induced martensite. The shape recvery rati is illustrated in Figure The shape memry effect slightly increased with increasing annealing temperature up t 873K, then decreased dramatically. The best shape memry effect was btained by annealing at 873K. Micrgraphs f allys 6# and 8# after annealing at different temperatures fllwing 4% tensile defrmatin at rm temperature are shwn in Figure 3.14 and Figure 3.15, respectively. Tw effects can be identified by checking the micrstructure carefully. First, the size f the martensite plates increased with increasing annealing temperature, suggesting that the density f the dislcatin structures which are invlved in martensite nucleatin was reduced thrugh dislcatin reactins and annihilatin at high temperature. Stress induced y > transfrmatin prceeded by the grwth f the existing martensite plates rather than the frmatin f new martensite plates. Secndly, the distrtin f the martensite plate decreased with increasing annealing temperature. This may be explained n the basis that the relief f residual internal stress is greater with increasing annealing temperature, with a subsequent effect n stress induced y > transfrmatin Electrn micrscpy f parent phase In rder t study the effect f annealing temperature n shape memry effect, the micrstructure f the parent phase and the stress induced martensite were examined in detail by transmissin micrscpy.

96 100 m ally 6# ^ c3 t-i b 5C > a i I.I it» f 11 -i 'r,,, i' ^ w & it ^ w en en en en en en r- t-- r- r- t r-- v c-~ O Annealing temperature T t- 100' jx] ally 28# > T3 J3 en c- T en t- r- en I en r- Annealing temperature T en en t- r> Figure 3.13 The shape recvery rati f 4% tensile strain fr different annealing temperatures. (6# and 28#)

97 20 urn Figure 3.14 The effect f annealing temperature n the mrphlgy f stress indu martensite (4% strain) in ally 6# (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

98 20 urn Figure 3.15 The effect f annealing temperature n the mrphlgy f stress induced martensite (4% strain) in ally 28# (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

99 44 Defect density (such as dislcatin, vacancies) and cnfiguratin (such as intersectin f stacking faults, dislcatin tangles) vary with changing annealing cnditins. The micrstructures f allys 6# and 28# annealed at different temperature are shwn in Figure 3.16 and Figure 3.17, respectively. Qualitative bservatins indicated that the dislcatin density was lw when the annealing temperature was high, and that stacking faults were usually in a single variant. On the cntrary, the dislcatin density was higher when the annealing temperature was lw, and the intersectin f stacking faults became a dminant feature. Based n the analysis f electrn diffractin patterns and the mrphlgy f the stacking faults, it was fund that stacking faults can be riented in ne f the 12 {111} <112>FCC system. The stacking fault energy f Fe-Mn-Si based shape memry allys is very lw (see sectin 1.3), and perfect dislcatins can easily split int tw 1/6<112> partial edge dislcatins, which frm stacking faults by extending n the (Hl)fcc plane alng the <110>f cc directin. In rder t reduce interface energy, the stacking fault usually extends in ne directin, hwever, it can extend in ther <110>f cc directins when barriers ccur Electrn micrscpy f martensite Figure 3.18 and Figure 3.19 shw the martensite mrphlgies fr different annealing cnditins fr allys 6# and 28# after 4% strain. It was als fund that martensite is riented in {111 }<112>f cc, cnfirming that y >e transfrmatin is highly related t the stacking faults. The scale f the stress induced martensite plates became larger when the annealing temperature was raised abve 1073K. Fr reverse transfrmatin f a large martensite plate, the resistance f transfrmatin is likely t be greater because f plastic accmmdatin surrunding the plate, leading t the degradatin f shape memry effect. The reasn fr the develpment f large size martensite plates culd be that the density f nucleatin sites is reduced n high temperature annealing and, therefre, y >e martensitic transfrmatin tends t ccur with the grwth f existing martensite plates rather than the frmatin f new martensite plates, resulting in the carser martensite structure.

100 loonm Figure 3.16 The effect f annealing temperature n the micrstructure f the parent phase in ally 6# (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

101 (a) (b) loonm Figure 3.17 The effect f annealing temperature n the micrstructure f the par phase in ally 28# (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

102 j 1 1 ^H 1 Ammm rnrnm -1 m mmmm 1 (b) loonm Figure 3.18 The effect f annealing temperature n the mrphlgy f stress ind martensite in 6# ally (4% strain, TEM bservatin) (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

103 (c) (d) loonm Figure 3.19 The effect f annealing temperature n the mrphlgy f stress induced martensite in 28# ally (4% strain, TEM bservatin) (a) ht rlled, (b) 873K, 30min, (c) 1073K, 30min, and (d) 1273K, 30min

104 3.2 Discussin Transfrmatin behaviur and mechanism f y transfrmatin The shape memry effect f Fe-Mn-Si based shape memry allys is assciated with Y(fcc) <^e(hcp) phase transfrmatin. The stacking sequences f clse packed plane fr fee and hep phases are ABCABCABCABC and ABABABABAB, respectively. Therefre, the transfrmatin invlves a rearrangement f the clse packed plane (lll)fcc- The stacking fault energy is very lw in this kind f ally, and the stacking fault is a majr defect in the fee parent phase, and can prvide the embrys fr phase, fr example: ABCABCABCABCABC (fee) ABCABABCABCABCA (stacking fault) hep embry The verlapping f this kind f structure can frm bulk hep phase - martensit the investigatin, it was fund that the verlapping f stacking faults was a cmmn feature (Figures 3.16 and 3.17). This is a simple and easy way t frm martensite minr structural and energy changes. Because bth fee and hep are clse packed structures with different stacking sequences, with free energy differences f a similar level t stacking fault energy, which is very lw. Therefre, the rientatin relatinship f parent phase and martensite shuld be {111 }f cc //{0001 }hcp, <110>fcc//<l 120>hcp, which is schematically shwn in Figure This result was cnfirmed by electrn diffractin. By bserving martensite plates in detail, it was fund there was a "band" structure in the martensite plates, which suggests that the verlapping f stacking faults is nt hmgeneus, i.e. there are sme stacking faults within the martensite. These stacking faults may act as the y embrys during the ->y reverse transfrmatin. Figure 3.21 clearly shws an irregular sequence f stacking faults. S, the nature f y >

105 Figure 3.20 Schematic drawing shwing the rientatin relatinship between martensite and austenite. loonm Figure 3.21 Edge-n bservatin f verlapping f stacking faults

106 46 transfrmatin is likely t be that martensite nucleates frm stacking faults, and grws by verlapping f stacking faults. The reverse transfrmatin ( ->y) is assciated with reverse mvement f Shckley partial dislcatins. Because the Ms temperatures f selected allys are near rm temperature, atms in the parent phase are in the activated state at rm temperature, a small energy fluctuatin culd result atmic displacements. The mvement f Shckley partial dislcatins was bserved by transmissin electrn micrscpy due t the extra thermal energy induced by the electrn beam (Figure 3.22). The results f the current research supprt the hypthesis that y-» transfrmatin in Fe- Mn-Si based shape memry allys ccurs by the verlapping stacking fault mechanism. Reverse mtin f stacking faults n heating leads t reversin t the parent phase, allwing shape memry t take place The effect f annealing n shape memry effect The shape memry effect f Fe-Mn-Si based shape memry allys varies with changing annealing cnditins. This means that annealing must affect y<-> transfrmatin. Frm ptical micrscpy, it was fund that althugh there was n bvius change in grain size, the size f the stress induced martensite increased with increasing annealing temperature. In cntrast, electrn micrscpy shwed that the dislcatin density decreased with increasing annealing temperature. Based n these results, the effect f annealing temperature n shape memry effect can be explained as fllws: (1) the effect f annealing temperature n stress induced y» transfrmatin The frmatin f martensite is highly related t stacking faults. The regular arrang f dislcatins and partial dislcatins is beneficial fr the nucleatin and grwth f martensite. Hwever, sme cmplex defects, such as dislcatin jgs, kinks, vacancy clusters, etc., may retard the splitting f perfect dislcatins and the mvement f partial dislcatins, reducing the amunt f stress-induced martensite. After apprpriate annealing, the numbers f cmplex defects decrease thrugh "nn-cnservative mtin" (such as climb) caused by thermal activatin, making stress-induced y-> transfrmatin

107 (a) (b) loonm Figure 3.22 The TEM bservatin f the mvement f Shckley partials (a) riginal structure, (b) after illuminating under electrn beam fr 5 min.

108 47 easier. When the annealing temperature is t high, the numbers f defects which are favurable fr y» transfrmatin will be reduced, and the strength f the parent phase will decrease (see Chapter 5). Thus, the tendency fr slip defrmatin will be enhanced, leading t the degradatin f the shape memry effect. (2) the effect f annealing temperature n yy reverse transfrmatin The shape recvery prcess is assciated with >y reverse transfrmatin. Frm bth the ptical and electrn micrscpic results, it is evident that the size f the martensite plates is large when the annealing temperature is high. The size f the plate will determine the magnitude f the plastic accmmdatin strain surrunding the plate and therefre resistance t reverse transfrmatin shuld increase with increasing size f the martensite plate. Based n the experimental data, it can be seen that the ptimal annealing temperature i abut 873K.

109 ********************************************************************** Chapter 4 The effect f precipitatin n shape memry effect ********************************************************************** 4.1 Results Intrductin It has been reprted[l-5, 7-13, 36, 40, 59-61] that strengthening f the austenitic ma is beneficial fr shape memry effect in ferrus shape memry allys. The strengthenin methds include ausfrming, cld wrking and annealing, rdering and ausaging. Fr example, the rdering f the austenite in Fe-Pt allys exerts a prfund effect n the martensitic transfrmatin, with a transitin frm thick plate t thin plate martensi significant decrease in transfrmatin temperature[60]. The thin plate martensite beha thermelastically and shws a small transfrmatin hysteresis. One f the several fact which is likely t cntribute t the enhanced transfrmatin reversibility and the assciated shape memry behavir is a significant increase in the flw strength f the austenite as a result f rdering. The stiffened parent phase is mre able t accmm transfrmatin strains elastically, leading t the maintenance f a cherent r

110 49 semicherent martensite/austenite interface which can mve freely in either the frward r reverse directins with small stress r temperature changes. The effectiveness f austenite strengthening in imprving transfrmatin reversibilit als been demnstrated fr ther ferrus allys, ntably Fe-C-Ni-Ti allys, which can be "ausaged" t frm cherent Ni 3 Ti precipitates and thus prduce a stiffened austenitic phase in which martensitic transfrmatin ccurs with reduced hysteresis. Thermalmechanical training schedules which cnditin the austenite defect structure have als prved useful in prmting shape recvery in Fe-Mn-Si allys. The additin f substantial amunts f interstitial carbn t these allys has als prvided a ptent means fr strengthening the austenite and appears, surprisingly, t be effective under certain cnditins in prmting shape memry behavir. Furthermre, Kajiwara[l] has reprted nearly cmplete shape memry assciated with thin plate martensite in an Fe-31%Ni- 0.4%C ally, fllwing austenite strengthening by ausfrming. This chapter cnsiders the effect f precipitatin strengthening f austenite n the memry effect f Fe-Mn-Si based shape memry allys Shape memry effect A bending test was used t characterise the shape memry effect in the allys studied. The samples were slutin treated at 1473K fr 1 hur, and quenched int ht water (abut 353K) t avid the frmatin f thermal martensite. Then the samples were aged at 773 K fllwed by quenching int ht water. Figure 4.1 shws the schematic diagram fr the ageing prcedure. Transmissin electrn micrscpy bservatin revealed the present f Ni3Ti precipitatin in the relevant allys, Figure 4.2 shws the mrphlgies f the precipitates. Figure 4.3 and Figure 4.4 shw results fr the reference allys (2# and 6#) and allys (34#, 35#, 36# and 37#) in which precipitatin f y'-ni 3 Ti was btained by ausaging. T determine the effect f precipitatin, the same heat treatment was used fr bth sets f allys (Fe-Mn-Si and Fe-Mn-Si-Cr-Ni based allys). Figure 4.3 and Figure 4.4 indicate that the recvery strains were lwer in the presence f

111 1473K 1 t H 353K, ht water Time Figure 4.1 Schematic diagram shwing the ageing prcedure

112 loonm Figure 4.2 TEM micrgraph shwing Ni3Ti precipitates in Ally 37# (specimen aged 773K fr 0.5 hur).

113 CO > u " Bending strain (%) IUU g ; Bending strain (%) 6# V 36# (b) 37# Figure 4.3 The effect f precipitatin n the shape memry effect f (a) Fe-Mn-Si based and (b) Fe-Mn-Si-Cr-Ni based shape memry allys. Aging cnditin: 773K, 30 minutes.

114 100 T" 1 2 Bending strain (%) $ v ' a ed 04 ft «> O 04 u bu Bending strain (%) Figure 4.4 The effect f precipitatin n the shape memry effect f (a) Fe-Mn-Si based and (b) Fe-Mn-Si-Cr-Ni based shape memry allys. Aging cnditin: 773K, 60 minutes.

115 precipitates, i.e. that precipitatin degrades the shape memry effect. It shuld be mentined that the shape memry capacity f slutin treated samples (allys 34#, 35#, 36#, 37#) is very clse t their reference allys (allys 1# and 6#) Transfrmatin behaviur The effect f precipitatin n the shape memry effect may be explained in terms f the fllwing three factrs: (1) y» transfrmatin temperature, (2) the amunts f stress induced martensite and residual austenite after defrmatin, and (3) the interactin f precipitates and Shckley partial dislcatins The effect f precipitatin n phase transfrmatin temperature The ht rlled allys were slutin treated at 1472K fr 1 hur, and the Ms temperature are listed in Table 4.1. The Ms temperatures f the slutin treated allys were measured by DSC, but in rder t reverse any existing martensite whilst preventing ageing, the samples were rapidly heated up t 573K and then cled t 373K at 50K/min. The Ms temperatures were btained by scanning the heat flw frm 373K t 223K at lok/min. It is evident that the additin f Ni and Ti results in a slight decrease in Ms temperature. Table 4.1 Ms temperatures f Ni3Ti precipitatin strengthened Fe-Mn-Si based allys 2# 34# 35# 6# 36# 37# Fe-31Mn- Fe-31Mn- Fe-31Mn- Fe-13Mn- Fe-13Mn- Fe-13Mn- Ally 6Si 6Si-2.6Ni- 6Si-2Ni- 5Si-10Cr- 5Si-10Cr- 5Si-10Cr- 0.7Ti 0.55Ti 6Ni 8.6Ni- 9.5Ni-lTi 0.7Ti Ms(K)

116 51 The changes in the Ms temperatures f allys 34# and 36# as a functin f ageing time are shwn in Figure 4.5. The Ms temperature increases slightly with increasing ageing time. This effect is prbably related t the precipitatin f Ni3Ti resulting in the lss f slute Ni and Ti frm the austenitic matrix The measurements f stress induced martensite by X-ray diffractin The effect f precipitatin n stress induced y > transfrmatin can be reflected by amunt f stress induced martensite after defrmatin. In the present study, X-ray diffractin and ptical micrscpy were used t measure the amunt f martensite. Rm temperature X-ray measurements were made n samples subjected t ageing at 773K fr 1 hur. Figure 4.6 and Figure 4.7 shw the changes in integrated intensities f the (1011 )e and (200)y reflectins with bending angle, which can be interpreted as strain. It is clear that the results fr slutin treated samples f ally 34# (befre ageing) are clse t thse f the reference allys (2#).The higher intensities f the martensite reflectin and the lwer intensities f the austenite reflectin fr allys 2# and 34# cmpared with allys 6#, 36# and 37#, are cnsistent with the Ms temperatures recrded in Figure 4.5 and Table 4.1. Bth the abslute value and the rate f increase f the intensity f the (10ll) reflectin markedly decreased after precipitatin had ccurred. Furthermre, the rate f decrease in the intensity f the (200)y reflectin is als lwer when precipitatin ccurs. These results indicate that martensite is induced by bending defrmatin at rm temperature, and that the vlume fractin f martensite is greatly reduced when precipitates f y-ni3ti are present. These data are cnsistent with Figures 4.3 and 4.4 which shw that precipitatin suppresses stress induced y -> transfrmatin and causes significant degradatin f the shape memry effect The measurement f stress induced martensite by ptical micrscpy Figure 4.8 gives phtmicrgraphs f allys 6#, 36# and 37# after 2% defrmatin at rm temperature. The allys were aged at 773K fr 1 hur befre being defrmed. It is clear that the amunt f stress induced martensite is less in the presence f the

117 330 & P cd u 34# 36# s Ageing time (hurs) Figure 4.5 Effect f aging n the Ms temperature ( aging temperature: 773K)

118 2200 2# 34# 34# unaged Bending Angle U VI a # 36# 37# 40 Bending Angle r Figure 4.6 Effect f precipitatin n integrated intensities f (101 l)g after aging 1 hur at 773K

119 VI c < 2# 34# 34# unaged Bending Angle 80 tuuu - 1 (200) y c V} a t ^ ^ ~ - X^^T 0 6# 36# 37# \ I... j Bending Angle Figure 4.7 Effect f precipitatin n integrated intensities f (200)y after aging 1 hur at 773K.

120 20 urn Figure 4.8 The effect f precipitatin n the amunt f stress induced martensite (bending strain = 2%). (a) 6# ally, (b) 36# ally, (c) 37# ally

121 52 precipitates. The thickness f the martensite plates als decreased when precipitatin ccurs, suggesting that Ni 3 Ti precipitates retard the grwth f stress induced martensite. Etch pits aligned alng {111} planes are very evident in allys 36# and 37# which cntained precipitates f Ni 3 Ti. Hwever, similar pits, but with a much lwer density, are als present in ally 6# which des nt cntain precipitates, s the rigin f the etch pits remains uncertain. 4.2 Discussin The effect f precipitatin n the amunt f stress induced martensite Because f the lw stacking fault energy f Fe-Mn-Si based shape memry allys, the perfect dislcatin 1/2<110>fcc is easily split int tw 1/6<112>f cc type Shckley partial dislcatins n the {lll}fcc plane. A stacking fault frms between tw Shckley partials. The mtin f Shckley partials and the verlapping f stacking faults frm martensite. Therefre, the mbility f Shckley partials is cnsidered t be the key factr in martensite frmatin. The results f the experiments described in Sectin 4.1 shw that precipitatin is nt favurable t the mvement f Shckley partials. Figure 4.9 shws a TEM micrgraph which indicates that the cherent precipitates can act as barriers t the mvement f Shckley partials. Pinning f these dislcatins is likely t retard stress induced y» transfrmatin in respect t bth nucleatin and grwth. The precipitates inhibit dislcatin mtin which is necessary fr nucleatin f martensite, and as the thickness f stress induced martensite plates in the allys with precipitates is reduced, it appears that precipitatin als suppresses the grwth f martensite The effect f precipitatin n the shape memry effect The precipitatin makes stress induced y» transfrmatin mre difficult, leading t a decrease in the amunt f martensite at a cnstant bending strain. The precipitates als retard the reverse mvement f Shckley partials. Fr these reasns the shape memry capacity was degraded in ausaged Fe-Mn-Si based shape memry allys.

122 loonm Figure 4.9 TEM micrgraph shwing precipitate pinning f Shckley partials.

123 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * # ; ).. ( ; Chapter 5 Mechanical behaviur ********************************************************************* 5.1 Results The strength f allys The tensile strengths f Fe-Mn-Si based shape memry allys are quite high cmpared with mild steels, and can vary with annealing cnditins and defrmatin temperature[8]. In the present study, the strengths f the allys were studied by ten testing at rm temperature, except fr the investigatin f the effect f defrmati temperature n the strength f allys. The yield strengths in the current research we defined by the 0.2% prf stress. Three allys were selected fr detailed study: the reference allys Fe-Mn-Si (1#) and Fe-Mn-Si-Cr-Ni (6#), and Fe-20Mn-6Si-7Cr-lCu (28#) The effect f annealing n ally strength Figure 5.1 shws the changes in strength f allys 1#, 6# and 28# with increasing annealing temperature (annealing time f 30 minutes). It is clearly seen that the tens

124 m i# # cd fe g a «900 H ' a u -C i r i T T en en en en en en r- r- c- r~ t r- O P- 00 a\ Annealing temperature en r- 700' a I ««r~~r ^ ^ ^ ^ ^ en en en en en en en r- r- r- r- t t- r- r-- \ CN Annealing temperature E3 28# n g H ' g «a ^ «75 r- en f- en r~ en C ^0 r-~ j= 1 1 en en r> r- \ «en r-^-< Annealing temperature 1 en r- CN Figure 5.1 The effect f annealing temperature n the tensile strengths f allys 1#, 6# and 28# at rm temperature.

125 strength decreases with increasing annealing temperature. By raising the annealing temperature t 1273K, the strength drps mre than loompa. The yield strength belw the Md temperature f Fe-Mn-Si based shape memry allys can be cnsidered as the critical stress t induce martensite, which is als largely influenced by annealing cnditin. Figure 5.2 shws the effect f annealing temperature n the yield strength f allys 1#, 6# and 28#. The results fr the three allys are very similar. The yield strength decreases with increasing annealing temperature when the annealing temperature is lwer than 1073K, but it increases sharply when annealing temperature increases frm 1073K t 1173K, and then it decreases again. Based n these results, it may be cncluded that annealing in the range 773 K t 1073K prvides a suitable cnditin fr the frmatin f stress induced martensite, leading t a gd shape memry effect. Figures 5.3 and 5.4 shw the changes f elngatin and reductin in area f tensile samples with increasing annealing temperature. There are n bvius trends in these tw prperties with annealing temperature, but ally 6# is clearly the mst ductile f the three allys The effect f defrmatin temperature n the strength f allys The strength f allys usually decreases with increasing defrmatin temperature, and the same trend is fund Fe-Mn-Si based shape memry allys. The results are summarised in Figure 5.5. By raising the defrmatin temperature frm rm temperature t 473K, the strengths f the three allys shwn in Figure 5.5 decrease by mre than 200MPa. Cntrary t the usual trend f decreasing strength with increasing temperature, the yi stress f Fe-Mn-Si based shape memry allys des nt decrease mntnically with increasing temperature. Figure 5.6 shws the temperature dependence f the yield stress fr allys 1#, 6# and 28#. It is clearly seen that a psitive temperature dependence f the

126 ' H 6# cd OH 400- t a VI 2 "5 J3 en en en en en en en r-~ t- r- r~ r- r~ r- VO t 00 ON. O > ' CN Annealing temperature a T it it en en en en en en en r- r- 00 ON x it i CN Annealing temperature # cd 0H 6 C c T3 U en en en en t- VO t- r- 00 r- ON en O ut en it en r- r- i Annealing temperature Figure 5.2 The effect f annealing temperature n the yield strengths f allys 1#, 6# and 28# at rm temperature.

127 40 1# 60 6# c Id c W 6 e td C O s ' a *> T"T j ' ^ ^J W ^ it en en en en en en en t t f- t & r- r- CN 00 ON Annealing temperature T3 1 it m e~ vo * en l- r- * en r~ 1 1 it it en en r- r- ON 1 1 it it en en r- r- CN Annealing temperature 40' 28# 30- * ' ' ' * '! i 1. 1.'!! : ;! : es i 20- :: cd ba G O 10- a ^ ^ ^ Ui * ^ * en en en en en en en t t. t c t c t VO t"- 00 ON O CN Annealing temperature Figure 5.3 The effect f annealing temperature n the elngatin f allys 1#, 6# and 28#.

128 E ] 1 # 80-6# 30- rr! TT: 20- :'y. : : : : : c n - I a <u a *> i 1 'f i 1 i 1 * * r"r en en en en en en r^ r- r- 00 ON t-» O t- r- en CN a u r v", r' 'i en ^ en ^ en ^ en it, en en en r- r- t-~ r- r-- r-~ r- r-- ON CN Annealing temperature Annealing temperature 1 28* r Tvl a i-h a u J3 1 1 x it, 1 it I I I l it iti it a en en en en en en en t~- r- r- t~- r-» r- r- v r- 00 ON O CN Annealing temperature Figure 5.4 The effect f annealing temperature n the area f reductin f ally 1#, 6# and 28#.

129 Cd I bo G OO cd 60 a i 1 1 r i 1 r Temperature (K) Temperature (K) cd CU C i 1 r r Temperature (K) Fgiure 5.5 The effect f defrmatin temperature n the tensile strengths f allys 1#, 6# and 28#.

130 cd bo G s ^ a a e a a D a a i# 6# 28# i i i i Temperature (K) Fgiure 5.6 The effect f defrmatin temperature n the yield strengths f allys 1#, 6# and 28#.

131 yield stress is bserved belw abut 380K. This phenmenn is assciated with stress induced y >e transfrmatin. 5.2 Discussin Annealing temperature effect The tensile strengths f the allys investigated decreased with increasing annealing temperature, pssibly in part because f the eliminatin f strain hardening assciated with prir ht rlling. Transmissin electrn micrscpy (Figures 3.16 and 3.17) als shwed a decreasing defect density with increasing annealing temperature. The change in yield stress with increasing annealing temperature is different frm that strength, as shwn in Figure 5.2. The yield stress at rm temperature (belw Md) in Fe-Mn-Si based shape memry allys reflects the critical stress t induce martensite. Therefre, the annealing prcess must affect stress induced y > transfrmatin. Based n the results btained frm studying the mechanism f martensitic transfrmatin, it is knwn that the transfrmatin is clsely related t the mvement f Shckley partials and the verlapping f stacking faults. Accrding t the thery f transfrmatin thermdynamics and kinetics, internal stresses can enhance the driving frce fr transfrmatin and defects can act as nucleatin sites. When the annealing temperature is lw, a relatively high density f defects (particularly dislcatins) culd prvide barriers fr transfrmatin by retarding the mvement f Shckley partials. When the annealing temperature is t high, internal stress relief decreases the driving frce fr transfrmatin, and the lw density f defects results in a decrease in the nucleatin rate. Frm an verall cnsideratin, annealing at temperature frm 773K t 1073K prvides a suitable cnditin fr stress induced y-»e transfrmatin Defrmatin temperature effect The yield stress f the allys examined varies with defrmatin temperature. A peak ccurs arund 380K which can be clarified as the critical temperature T'. Belw T', the

132 yield stress reflects the critical stress fr martensite frmatin. The driving frce (applied stress) needed fr martensite frmatin increases with increasing defrmatin temperature, due t the parent phase (austenite) becming mre stable and the stacking fault energy increasing with increasing temperature[53]. Therefre, as the yield stress increases, abve T\ slip defrmatin ccurs rather than martensite frmatin. The yield stress decreases with increasing defrmatin temperature, which is cnsistent with the general trend fr steels. The tensile strength f the allys gradually decrease with increasing defrmatin temperature frm rm temperature t 473 K, which can be explained by the fllwing tw reasns: (1) the amunt f stress induced martensite decreases with increasing defrmatin temperature, leading t a decrease in the cntributin f phase transfrmatin strengthening. (2) the mbility f dislcatins is enhanced by raising the defrmatin temperature, which als causes a strength decrease.

133 ********************************************************# ;j.# ; l s;h #.^* ;l.^;t:. 1. Chapter 6 Crrsin testing *******************************************^^^^^^^^^^^^^^.^;1. ;i c^;(. ;js;). ;. Sj. :(.^;(. >i. 6.1 Results The crrsin resistance f traditinal Fe-Mn-Si ternary allys is very pr, and is barrier t industrial applicatin. Fr this reasn, Fe-Mn-Si-Cr-Ni stainless steel ba shape memry allys have been develped. In the current study, Fe-Mn-Si-Cr-Cu, Fe- Mn-Si-Al-Cu and Fe-Mn-Si-Cr-Ni-Cu crrsin resistant allys have been develped, and crrsin tests were carried n these allys Immersin crrsin testing The cnventinal immersin test was used in which the crrsin rate is described by weight reductin rate (g/m2hr) f the test sample. This is cnsidered t be a simple efficient methd. The immersin time in the present research was 4 hurs, and the tes temperature was 25 C.

134 Crrsin resistance in hydrchlric acid Samples were immersed in hydrchlric acid at rm temperature fr 4 hurs. Figure 6.1 shws the results fr an Fe-Mn-Si ally (1#) and an Fe-Mn-Si-Al-Cu ally (10#). It is clearly seen that the crrsin rate f ally 10# is much lwer than that f ally 1 The crrsin rate f ally 10# remained nearly cnstant with increasing cncentratin f hydrchlric acid. This means that allying a Fe-Mn-Si ternary ally with abut l%cu and 1%A1 can markedly imprve the crrsin resistance t hydrchlric acid. An Fe-Mn-Si-Cr-Ni ally (6#) and tw Fe-Mn-Si-Cr-Ni-Cu allys (7# and 8#) were selected fr investigating the effect f cpper n crrsin resistance f Fe-Mn-Si-Crallys. It has been reprted that the additin f cpper can imprve the crrsin resistance f stainless steel, particularly t sulphuric acid [53,54,62-64]. The crrs test results fr allys 6#, 7# and 8# in hydrchlric acid at rm temperature are illustrated in Figure 6.2, which indicates that the crrsin resistance f allys 7# a is much higher than that f 6# ally. Hwever, there is little difference between ally (l%cu) and ally 8# (2%Cu). This suggests that the additin f l%cu is adequate. The Fe-Mn-Si-Cr-Cu allys develped in this wrk were als tested fr crrsin resistance. The crrsin resistances in hydrchlric acid f allys 25#, 28# and 29# selected frm the Fe-Mn-Si-Cr-Cu system are shwn in Figure 6.3. The results shw that these allys have gd crrsin resistance t hydrchlric acid. The crrsin rates f these ally are lwer that f ally 6# (Fe-Mn-Si-Cr-Ni system). It is als can be seen that the crrsin resistance f Fe-Mn-Si-Cr-Cu allys increases with increasing Cr cntent Crrsin resistance in sulphuric acid Sulphuric acid is a strng crrsin medium. The immersin test results fr ally 1# (F Mn-Si system) and ally 10# (Fe-Mn-Si-Al-Cu system) are shwn in Figure 6.4. The crrsin rate increases with increasing cncentratin f sulphuric acid frm 5% t 20%. The results shw that allying with Al and Cu can significantly imprve crrsin resistance f Fe-Mn-Si ternary allys. Figure 6.5 shws the crrsin resistance f

135 HC1 (%) Figure 6.1 Crrsin resistance in hydrchlric acid (allys 1# and 10#)

136 CN,6 I 04 G O c O fc O U HC1 (%) Figure 6.2 Crrsin resistance in hydrchlric acid (allys 6#, 7# and 8#) HC1 (%) Figure 6.3 Crrsin resistance in hydrchlric acid (allys 25#, 28# and 29#)

137 CN s 39 < u G O I i < U HSO (%) 2 4 V ' Figure 6.4 Crrsin resistance in sulphuric acid (allys 1# and 10#). CN E 39 I u G O c O U H 2 S0 4 (%) Figure 6.5 Crrsin resistance in sulphuric acid (allys 6#, 7# and 8#).

138 allys 6#, 7# and 8# t sulphuric acid. The effect f cpper n crrsin is very bvius fr stainless steel based shape memry allys. The results indicate that by adding 1-2% cpper t an Fe-Mn-Si-Cr-Ni ally, the crrsin resistance t sulphuric acid is increased t the extent that the ally can successfully resist up t 20% sulphu acid; whereas, in the absence f cpper, the crrsin rate is quite high even if the cncentratin f acid is 5%. The crrsin resistance t sulphuric acid f Fe-Mn-Si-Cr- Cu allys (25#, 28# and 29#) is illustrated in Figure 6.6. The crrsin rates increase with increasing cncentratin f sulphuric acid and the results suggest that these all nly have gd crrsin resistance t dilute sulphuric acid. The crrsin resistance enhanced by raising the Cr cntent, with ally 29# shwing the best crrsin resistance. Mrever, sme Fe-Mn-Si-Cr-Cu allys, such as allys 28# and 29#, have similar crrsin resistance t stainless steel based allys (ally 6#, cmpare Figures and 6.6) Crrsin resistance in 3.5% NaCl slutin Salt slutin als can strngly attack steels. In the current study, the samples were immersed in 3.5% NaCl slutin fr 100 hurs at rm temperature. The allys selected were 1#, 6#, 7#, 10#, 25#, 28# and 29#. The test results are shwn in Figure 6.7. Allys 6# and 7# shwed the best crrsin resistance. The effect f cpper additin n crrsin resistance t 3.5% NaCl slutin is nt marked. Fr the Fe-Mn-Si-Cr-Cu series (25#, 28# and 29#), the crrsin resistance increased with increasing Cr cnten The results suggest that allying with Cu ffers n advantages fr marine applicatins, whereas Cr is effective in increasing crrsin resistance in this case Ptentistatic tests In rder t study the effect f cpper n the crrsin resistance f Fe-Mn-Si-Cr-Ni allys in detail, ande plarizatin tests were cnducted n allys 6#, 7# and 8#. The gd resistance f stainless steels t crrsin is a cnsequence f their capacity t becme passivated, which can be reflected in the andic plarizatin curve. Figure 6.8

139 CN s t I G O c O t U Figure 6.6 Crrsin resistance in sulphuric acid (allys 25#, 28# and 29#). 3.5%NaCl slutin, 100 hurs is CN to I S-H G O *w* g U i * ^.. 1# 10# 6# 7# 25# 28# 29# Figure 6.7 Results f crrsin tests in 3.5% NaCl slutin.

140 IT\ 1 I I 11II11 1 I I III If 1 I I lllll Current Density (ua/cm ) Figure 6.8 Andic plarizatin curve f Fe-Mn-Si-Cr-Ni based shape memry allys

141 shws the andic plarizatin curves fr the allys investigated. The andic current density is prprtinal t the crrsin rate f the ally. The passivatin f these al ccurs ver a wide ptential range. It is clearly seen that increasing the cpper cnte facilitates passivatin, i.e. shifts the andic plarizatin curve in a favurable dire Figure 6.9 shws the measured results f the critical current density ( an index f gen crrsin) and passive current density ( an index f passive film frmatin). These results shw that the additin f cpper imprves the general crrsin resistance f F Mn-Si-Cr-Ni allys, and makes the passive film mre stable Scanning electrn micrscpy The crrsin surface f allys was studied by using the scanning electrn micrscpe. The type and degree f crrsin can be reflected frm the crrsin surface. All the t pieces were grund n 1200# silicn carbide paper befre testing. The crrsin prducts were cleaned ff using a sft tthbrush. Figure 6.10 shws the crrsin surfaces f allys 1# and 10# after immersin in hydrchlric acid fr 4 hurs at rm temperature. It is evident that the surface f 1# deeply etched, whereas the crrsin f 10# ally was very limited. The scratches frm grinding n silicn carbide paper can still be seen. There was sme pitting crrsin revealed n the surface f 10# ally. This suggests that allying Fe-Mn-Si ternary all with Cu and Al can significantly enhance their crrsin resistance t hydrchlric aci The crrsin surfaces f allys 6#, 7# and 8# after testing in hydrchlric acid are shwn in Figure 6.11, which indicates that pitting crrsin is the main crrsin damage fr these allys. Allys 7# and 8# shw better resistance t hydrchlric acid attack than 6# ally. Similar results were btained fr the Fe-Mn-Si-Cr-Cu system, which is shwn in Figure Pitting crrsin can be clearly seen n the test surface but there was nt much difference between allys 25# and 28#, and ally 29# shwed the best crrsin resistance.

142 CN I c C 8 4» G B u (a) Ally 6# Ally 7# AUy 8# 5000 I I I 1 I I 1 1 I P'F'fM rm (b) ^ g 2000 G t JzZ Ally 6# Ally 7# Ally 8# Figure 6.9 The measured results fr (a) the passive current density (b) and the critical current density

143 l L (a) ill >M#ltfi (b) 10 um Figure 6.10 The crrsin surface f 1# and 10# allys after immersin test in 10% hydrchlric acid fr 4 hurs at rm temperature.

144 (a) 10 urn Figure 6.11 The crrsin surface f 6#, 7# and 8# allys after immersin test in M)% hydrchlric acid fr 4 hurs at rm temperature.

145 loiim Figure 6.12 The errsin surface f 25#, 28# and 29# allys after immersin test in 10% hydrchlric acid fr 4 hurs at rm temperature.

146 The surfaces f samples after the immersin test in sulphuric acid were als bserved by scanning electrn micrscpy. Figure 6.13 shws the results fr allys 1# and 10# afte crrsin testing in sulphuric acid fr 4 hurs at rm temperature. The highly crr surface f 1# ally reveals that Fe-Mn-Si ternary allys have very pr resistance t sulphuric acid crrsin. The surface f ally 10# shws a damaged xidised layer, but this layer culd prtect the steel frm crrsin t sme extent. The crrsin prduc mrphlgies f allys 6#, 7# and 8# are shwn in Figure 6.14; ally 8# shwed the best crrsin resistance t sulphuric acid. It is clearly seen that the grain bunda 6# ally were attacked, but thse f allys 7# and 8# were nt attacked. Because ally 7# and 8# were develped frm ally 6# by allying with cpper, the results may indicate that cpper can prevent intergranular penetratin f sulphuric acid. The cr surfaces f allys 25#, 28# and 29# after immersin in sulphuric acid are illustrated Figure The results shw that the crrsin resistance f Fe-Mn-Si-Cr-Cu allys clse t that f Fe-Mn-Si-Cr-Ni stainless steel based shape memry allys. Figure 6.16 shws the crrsin surfaces f selected allys (1#, 10#, 6#, 25#, 28# and 29#) after immersin in 3.5% NaCl slutin at rm temperature fr 100 hurs. It is clear that th crrsin resistance f Fe-Mn-Si-Cr-Cu allys t NaCl slutin is very gd, and the crrsin resistance is enhanced with increasing the Cr cntent, allys 6#, 28# and 29 nly shw minr attack n the surface. It is als can be seen that there is a layer n surface f ally 10#, and this culd be the reasn that it has better crrsin resis NaCl slutin than ally 1#. Hwever, this layer is nt impervius, and the imprvement f resistance t the attack in NaCl slutin is nt significant. 6.2 Discussin The effect f allying elements n crrsin resistance The new ptential allys, such as 10#, 25# and 28# are develped based n Fe-Mn-Si ternary allys and cmbinatin f the allying elements Cr, Cu and Al. The Fe-Mn-Si- Cr-Ni-Cu allys are based n Fe-Mn-Si-Cr-Ni stainless steel allys. The allying elements play an imprtant rle in imprving the crrsin resistance.

147 (a) V> (b) 10 im Figure 6.13 The crrsin surface f 1# and 10# allys after immersin test in 10% sulphuric acid fr 4 hurs at rm temperature.

148 v»v.* ' 10 urn Figure 6.14 The crrsin surface f 6#, 7# and 8# allys after immersin test in 10% sulphuric acid fr 4 hurs at rm temperature.

149 (b) loixm Figure 6.15 The crrsin surface f 25#, 28# and 29# allys after immersin test in 10% sulphuric acid fr 4 hurs at rm temperature.

150 20 urn Figure 6.16 The crrsin surfaces f selected allys after immersin in 3.5% NaCl slutin fr 100 hurs at rm temperature. (a) 1#, (b) 10#, (c) 6#, (d) 25#, (e) 28#, (f) 29#

151 The effect f Cu cntent Cu is sluble in austenitic steels t at least 4% by weight. In the present study, the Cu cntent were varied frm 1% t 3%. The results f immersin tests in hydrchlric ac and sulphuric acid shw that Cu prmtes crrsin resistance in these tw acids. Th additin f Cu, hwever, did nt appreciably imprve the crrsin resistance f al t 3.5% NaCl slutin, as shwn in Figure 6.7. This result suggests that Cu bearing allys ffers n imprvements fr marine applicatins. Electrchemical measurements (Figure 6.8) shwed that Cu shifts the andic plariza curve f Fe-Mn-Si-Ni-Cr allys in a favurable directin. The critical current dens drps markedly with increasing Cu cntent, suggesting that general crrsin resist f the allys is imprved, since the andic current density is prprtinal t the rate f the ally. The passive current density als decreased with increasing Cu c suggesting that the passive films frmed n Cu bearing allys are mre stable. Frm verall view pint f the effect f Cu n prperties, such as shape memry effect, h wrkability and crrsin resistance, the additin f 1% Cu is favurable fr Fe-Mnbased shape memry allys The effect f Cr The results f immersin tests n Fe-Mn-Si-Cr-Cu allys shw that the crrsin resistance f allys t hydrchlric acid, sulphuric acid and 3.5% NaCl slutin are enhanced by raising the Cr cntent. Allying irn based allys with Cr cnsiderably facilitates passivatin. The stability f the passive film increases with increasin cntent. Hwever, if Cr cntent exceeds 10% in Fe-Mn-Si-Cr-Cu allys, ferrite can frm in sme allys, which can degrade the shape memry effect The effect f Ni

152 By cmparing the crrsin resistance f allys 7# and 29# (Ni bearing and nn-ni bearing), it is clearly seen that 7# ally shws better crrsin resistance t ac 3.5% NaCl slutin. This phenmenn can be explained by the cmbinatin f Cr and Ni markedly facilitating passivatin (Cr-Ni steels are easier t passivate than pla chrmium steels) and lwering the crrsin rate[53].

153 64 ******************************************************************* Chapter 7 Thermmechanical training Sic Sic Sic SlC SlC Sic sk Sic S'k SlC Sic SlC S^C SlC Sic SlC StC S4C SlC SlC SlC SlC SlC SlC SlC sfc SlC SlC SlC Sic Sic SlC SlC Sic Sic sic sic Sic sic sic 2JC SlC sic sic sic Sic? sic? sic sic sic! sic? sic? sic SlC sfc? sic? sic? sic sic? sfc sic? slf ^Jf sic??& "JrZ ^ It has been reprted[29,39,41,49,51] that thermmechanical training can significan imprve the shape memry effect f Fe-Mn-Si based allys The prper training prcess can suppress slip defrmatin thrugh strengthening f the austenitic matri and lwer the stress fr inducing martensite by creating dislcatin structures are favurable t martensitic transfrmatin. Hwever, the mechanism f thermmechanical training is still nt clear. This chapter reprts the experiments undertaken, presents discussin n the factrs influencing thermmechanical trainin and explains the mechanism f training thrugh the analysis f mechanical characteristics and micrstructure.

154 Results The effect f thermmechanical training n shape memry effect In the current study, thermmechanical training was carried ut by tensile tests, the pre-annealing cnditin fr all samples being 873K (600 C) fr 10 mins. The variables selected fr investigating the effect f thermmechanical training wer fllws: (1) Recvery annealing temperature: the temperature range was set frm 673K t 1073K, (2) Amunt f strain: the strain during training was set frm 2% t 6.5%, (3) Strain rate: the crss-head speed is used t study the effect f strain rate training, ranged frm 0.2rnm/min t 8mm/min. Thermmechanical training was carried n the allys indicated belw Fe-Mn-Si ternary shape memry ally Figure 7.1 shws the effect f thermmechanical training n the recvery rati f ally 1# (Fe-Mn-Si), fr a training strain f 2%. It is clear that the magnitude recvery rati is strngly dependent n the recvery annealing temperature. Nearl cmplete shape recvery was btained after 4 training cycles when the recvery annealing temperature ranged frm 773K t 873K. After 4 cycles, the recvery rati remained cnstant with increasing number f training cycles. When the recvery annealing temperature is lwer than 773K (673K and 723K), the imprvement in shape memry effect decreased, becming lwer with a lwer recvery annealing temperature. The recvery rati reached a peak value after 2 r 3 training cycles then decreased with increasing training cycles. The recvery rati was als lwer

155 100 t$ cd S-i < > cj e 2% strain 10 minutes recvery annealing 673K 723K 773K 823K 873K 973K training cycle (N) 8 10 Figure 7.1 The effect f thermmechanical training with 2% strain n the shape memry effect f an Fe-Mn-Si ally (ally 1#)

156 when the recvery annealing temperature was increased t 973K, shwing a gradual decrease after 4 training cycles. 66 The results fr thermmechanical training with 4% pre-strain are shwn in Figure 7.2. The best results were btained when the recvery annealing temperature was again between 773K and 873K, but an increased number f training cycles was needed t achieve nearly cmplete shape recvery. The recvery rati apprached a maximum value after 5 training cycles and then remained cnstant with increasing training cycles. When the recvery annealing temperature was 673K, the recvery rati decreased dramatically with increasing training cycles after 2 cycles, falling even lwer than the riginal value. This phenmenn suggests that unrecvered structure induced by training accumulates with increasing training cycles, and affects y<-> phase transfrmatin. When the recvery annealing temperature was 973K, the shape memry effect was als significantly imprved by thermmechanical training. Hwever, the imprvement was still nt as marked as that when the recvery annealing temperature was in the range 773K t 873K. Frm the abve results, it is seen that the ptimum recvery annealing temperature lies in the range f 773K t 873K. Figures 7.3 and 7.4 shw the changes f recvery rati with increasing training cycles when the training strain is 5.5% and 6.5%, respectively. The figures shw that the recvery rati increases markedly with increasing number f training cycles, but less than 80% recvery was achieved. When the strain is ver 4%, slip defrmatin and intersectin f martensite plates increases with increasing strain, leading t greater difficulty in the reverse mtin f Shckley partial dislcatins. It can als be seen that the imprvements in the recvery rati are similar when the recvery annealing temperatures are 773K, 873K and 973K, but the results fr thermmechanical training at 873K are slightly better.

157 I > 4% strain 10 minutes recvery annealing 673K 723K 773K 823K 873K 973K training cycle (N) -r Figure 7.2 The effect f thermmechanical training with 4% strain n the sha recvery rati f an Fe-Mn-Si ally (ally 1#)

158 ^ 2 a " 5.5% strain 10 minutes recvery annealing Training cycles (N) -r 6 Figure 7.3 The effect f thermmechanical training with 5.5% strain n the shape recvery rati f an Fe-Mn-Si ally (ally 1#) 100 ^ cci IH t > % strain 10 minutes recvery annealing 2 4 Training cycles (N) Figure 7.4 The effect f thermmechanical training with 6.5% strain n the shape recvery rati f an Fe-Mn-Si ally (ally 1#)

159 67 Figure 7.5 shws the changes f recvered strain and residual strain (unrecvered strain) with increasing number f training cycles fr training strains f 2%, 4%, 5.5% and 6.5%. The recvery annealing cnditin was 873K fr lornin. Figure 7.5 shws that the recvery strain fr 2% and 4% training strain initially increases rapidly after 3 r 4 training cycles, and increases mre slwly afterwards. Fr the larger training strain f 5.5% and 6.5% the recvery strain peaks after abut 3 cycles then decreases slightly. Fr the higher training strains, slip defrmatin and the intersectin f martensite plates increase, causing difficulty fr the reverse mtin f Shckley partial dislcatins. It als can be seen that the largest recvery strain (abve 4%) is btained fr a training strain f 5.5%. Based n the results btained, the shape memry effect f a prperly trained sample is significantly imprved. In Figure 7.6 the measurements f the shape recvery rati f trained samples (873K, lornin, 4 times) and untrained samples are pltted as a functin f pre-strain. The recvery rati f trained samples is much higher than that f untrained samples Fe-Mn-Si-Cr-Ni shape memry ally Ally 6# (Fe-Mn-Si-Cr-Ni system) was chsen fr thermmechanical training, because it has a very gd shape memry effect in the initial (untrained) state as discussed in Chapter 3. Figure 7.7 shws the changes f recvery rati with increasing training cycles fr a training strain f 2%. It is clearly seen that the recvery rati f untrained samples is very high, arund 90%. In this case, thermmechanical training is almst independent f recvery annealing temperature. After a few training cycles, 100% shape recvery can be btained fr all recvery annealing cnditins. When the training strain increases t 4%, the results f thermmechanical training becmes ttally different, as shwn in Figure 7.8. Althugh the recvery rati f

160 a S-l training cycle (N) (a) training cycle (N) (b) a a C/3 ~ Q recvery strain - residual strain training cycle (N) (c) I ' training cycle (N) (d) Figure 7.5 The influence f thermmechanical training n the recvery strain and residual strain fr ally 1#.

161 < > <0 20- trained samples untrained samples 2 4 Pre-strain(%) Figure 7.6 Effect f pre-strain n the recvery rati f trained samples and untrained samples fr ally 1#. t I- t > 04 Training cycle (N) Figure 7.7 The effect f thermmechanical training n shape memry effect f ally 6# (2% training strain)-

162 100 < 90-2 > <u 04 Training cycle (N) Figure 7.8 The effect f thermmechanical training n shape memry effect f ally 6# (4% training strain)

163 68 untrained samples is mre than 70%, which is cnsiderably higher than that f Fe- Mn-Si ternary allys, the imprvement f the shape memry effect by thermmechanical training is nt as bvius as Fe-Mn-Si ternary allys. When the recvery annealing temperature is 673K, the recvery rati f ally 6# drps rapidly with increasing training cycles after 3 cycles, which is similar t the results f Fe-Mn- Si ternary allys. Increasing the recvery annealing temperature t 1073K reduces the recvery rati cmpared t recvery annealing in the range K. Figure 7.9 shws the changes f recvery strain and residual strain with increasing training cycles when the training strain is 4%, and the recvery annealing cnditin is 873K fr lomin. These results clearly shw that the recvery strain after training is lwer than that f ally 1# (Fe-Mn-Si system) Fe-Mn-Si-Cr-Ni-Cu allys The Fe-Mn-Si-Cr-Ni-Cu allys were develped frm Fe-Mn-Si-Cr-Ni allys by adding Cu in rder t imprve the crrsin resistance. Figures 7.10 and 7.11 shw the changes f recvery rati f the selected Fe-Mn-Si-Cr-Ni-Cu ally (7#) with increasing training cycles fr training strains f 2% and 4%, respectively. It is clearly seen that shape memry effect f ally 7# is slightly enhanced by thermmechanical training except fr recvery annealing at 673K fr 10 min. The shape memry effect f untrained samples was very similar t that f ally 6# (Fe-Mn-Si-Cr-Ni system) Fe-Mn-Si-Al-Cu allys The Fe-Mn-Si-Al-Cu allys were develped frm Fe-Mn-Si ternary allys by adding Cu and Al. In the current study, ally 10# (a Fe-Mn-Si-Al-Cu ally) was selected fr investigating the effect f thermmechanical training n shape memry effect, as illustrated in Figures 7.12 and 7.13 fr training strains f 2% and 4%, respectively. The results shw that ally 10# has a very gd shape memry effect in the initial (untrained) state. The untrained recvery ratis fr 2% and 4% pre-strain are abut

164 6 CI -, 2 a a VI 1-2 residual strain 4% strain recvery strain T 4 training cycle (N) 10 Figure 7.9 The influence f thermmechanical training n recvery strain and residual strain (ally 6#, 4% training strain)

165 100' 2% strain 60' Training cycle (N) Figure 7.10 The effect f thermmechanical training n shape memry effect f ally 7 (2% training strain) 100 i t > O training cycle (N) Figure 7.11 The effect f thermmechanical training n shape memry effect f ally 7# (4% training strain)

166 100' 100> ed > u at 10# 2% strain 673K 10 min 4) > u OS # 2% strain 773K 10 min T r 4 Training cycle (N) T "i 1 r Training cycle (N) 100 p^ 100 ~ ^ =ffi= -ffl > u A 10# 2% strain 873K 10 min T T > u u ct 25- -ffl- 10# 2% strain 973K 10 min T Training cycle (N) Training cycle (N) Figure 7.12 The effect f thermmechanical training n shape memry effect f ally 10# (2% training strain)

167 100 4% strain 60 r 4 Training cycle (N) Figure 7.13 The effect f thermmechanical training n shape memry effect f ally 10# (4% training strain).

168 69 90% and 80%, respectively, which are much better than the recvery ratis fr Fe- Mn-Si ternary allys. This suggests that the additin f Al and Cu can imprve that shape memry effect f Fe-Mn-Si allys. Fr 2% training strain, the recvery rati increased with increasing training cycles, and apprached 100% (cmplete recvery) fr all recvery annealing cnditin after a few training cycles. Hwever, in the case f 4% training strain, the recvery rati increased slightly with increasing number f training cycles, except fr recvery annealing at 673K fr lornin, which resulted in a steep fall in recvery rati Fe-Mn-Si-Cr-Cu allys Tw allys (25# and 28#) were selected frm the Fe-Mn-Si-Cr-Cu system fr studying the influence f thermmechanical training n the shape memry effect. Fe- Mn-Si-Cr-Cu allys shwed gd shape memry effect and crrsin resistance. The cst is als lwer than that f stainless steel based shape memry allys. The influence f thermmechanical training n the shape memry effect f ally 25# are illustrated in Figures 7.14 and 7.15 fr training strains f 2% and 4%, respectively. As fr the previus allys, thermmechanical training using the 673K fr lomins recvery annealing cnditin did nt imprve the shape memry effect. Fr 2% training strain, the shape memry effect f ally 25# was markedly imprved by thermmechanical training when the recvery annealing temperature was between 773K and 973K, with the shape recvery rati being abve 95% after a few cycles f training. In the case f 4% training strain, the shape memry effect was significantly enhanced by thermmechanical training when the recvery annealing temperature was in the range 773K t 873K. Annealing at 873K was mst effective fr imprvement f the shape memry effect, with the shape recvery rati being mre than 95% after a few training cycles. When the recvery annealing temperature was higher than 873K, the training was less effective.

169 100 $ < > <u 04 Training cycle (N) Figure 7.14 The effect f thermmechanical training n the shape memry effect f ally 25# (2% training strain) Training cycle (N) Figure 7.15The effect f thermmechanical training n the shape memry effect f ally 25# (4% training strain)

170 70 In rder t study the effect f strain rate n thermmechanical training, different crss head speeds during tensile testing were emplyed in the current study, the recvery annealing cnditin was 873K fr lomin. The results are shwn in Figure 7.16, which clearly shws that the training results btained frm different crss head speeds (0.02mm/min t 8mm/min) are similar, with the lw speeds (0.02mm/min r 0.2mm/min), appearing t be mre favurable fr the imprvement f shape memry effect. This result indicate that strain rate des extent an effect n stress induced y-> transfrmatin, prducing up t 10% increase in recvery rati fr 1-2 rders f magnitude decrease in crss-head speed. Figures 7.17 and 7.18 shw the changes f recvery rati f ally 28# with increasing thermmechanical training cycles fr training strains f 2% and 4%, respectively. The results indicate that ally 28# has a very gd shape memry effect in the initial state, and the 100% shape recvery can be easily btained with 2% training strain. In the case f 4% training strain, recvery annealing at medium temperatures (frm 773K t 873K) was mst effective fr the imprvement f shape memry effect. The effect f training strain n thermmechanical training f ally 28# is shwn in Figure The recvery rati gradually decreased with increasing training strain. Hwever, the recvery strain is the mst imprtant factr in cmmercial applicatins, and Figure 7.20 shws the changes f recvery strain with increasing training cycles fr different training strains. This figure clearly shws that recvery strain increases with increasing training strain, and saturates at abut 6% training strain, with a maximum recvery strain f abut 5.4%. Hwever, fr untrained samples, the recvery strain appears t reach the saturatin value at abut 5% strain, and less than 3.5% strain is recvered. These data indicate that it is mst effective t carry ut thermmechanical training using abut 6% training strain.

171 100 Training cycle (N) Figure 7.16 The effect f strain rate n thermmechanical training fr ally 25# (recvery annealing cnditin: 873K fr lornin.).

172 100 t ccj t > <u % strain r 4 Training cycle (N) Figure 7.17 The effect f thermmechanical training n the shape memry effect f al 28# (2% training strain) 100 $ c3 < > u 04 Training cycle (N) Figure 7.18 The effect f thermmechanical training n the shape memry effect f ally 28# (4% training strain)

173 100 2% strain 6.5% strain Training cycle (N) Figure 7.19 The effect f training strain n the shape recvery rati f ally 28# (recvery annealing cnditin: 873K fr lornin.) 2% strain a '53 a VI t > t 04-4% strain O 5% strain A 5.5% strain ffl 6% strain 6.5% strain Training cycle (N) Figure 7.20 The effect f training strain n recvery strain during thermmechanical training f ally 28# (recvery annealing cnditin: 873K fr lornin.).

174 71 The results demnstrate that a prper training prcess can significantly enhance the shape memry effect f Fe-Mn-Si based shape memry allys. As the shape memry effect is gverned by y<-> transfrmatin, the effect f thermmechanical training n y<r+e transfrmatin is critical, as the fllwing discussin indicates The effect f training n phase transfrmatin temperatures Phase transfrmatin temperatures (Ms, As and Af) directly reflect the characteristi f y<r*e transfrmatin, and the effect f thermmechanical training was studied measuring the transfrmatin temperatures by DSC and TMA. Figure 7.21 illustrates the changes f Ms, As and Af f ally 1# (Fe-Mn-Si system) with increasing training cycles when the training strain was 4%. The figure clearly shws that the Ms and As temperatures remain nearly cnstant with increasing number f training cycles. Hwever the Af temperature was influenced by training. Fr a recvery annealing temperature f 673K, the Af temperature increased with increasing number f training cycles, and rse by nearly 50K (frm 643K t 692K) after five training cycles. The Af temperature was higher than the recvery annealing temperature after 3 training cycles. This bservatin supprts the prpsal that the accumulatin f unrecvered structure causes bstacles t the reverse mtin f Shckley partial dislcatins, which makes the >y transfrmatin mre difficult, leading t the degradatin f shape memry effect. Fr recvery annealing temperatures higher than 773K (773K t 973K), the Af temperature decreased with increasing number f training cycles, and appeared t reach a saturatin value after 5 r 6 training cycles. The Af decreased by nearly 40K, and it is inferred that the structure (especially the dislcatin structure) created by training is favurable fr yy reverse transfrmatin, leading t imprvement in the shape memry effect. Similar results were als btained fr ally 6# (Fe-Mn-Si-Cr-Ni system) and ally 28# (Fe-Mn-Si-Cr-Cu system), as shwn in Figures 7.22 and 7.23, respectively.

175 Af u. O c E As Ms b Training cycle (N) Training cycle (N) Training cycle (N) Training cycle (N) Figure 7.21 The effect f thermmechanical training n the phase transfrmatin temperatures f ally 1# fr 4% training strain determined by DSC and TMA.

176 % u c E Training cycle (N) Training cycle (N) Training cycle (N) Training cycle (N) Figure 7.22 The effect f thermmechanical training n the phase transfrmatin temperatures f ally 6# fr 4% training strain

177 u E 400 H Q s ca 400 H Training cycle (N) Training cycle (N) Training cycle (N) r Training cycle (N) Figure 7.23 The effect f thermmechanical training n the phase traansfrmatin temperatures f ally 28# fr 4% training strain.

178 72 It shuld be nted that the DSC was carried ut by cycling a sample after prestraining. Heating at lok/min causes reversin f stress-induced martensite ver the range As-Af, and subsequent cling at lok/min establishes the Ms f thermally induced martensite in the trained sample. If the sample were cled under stress, the Ms temperature will be different (higher), because f the cmbined effect f the stress and the dislcatin structure generated by training. Stress acting n riented martensite nuclei created by the same pattern f stress used in training, catalyses stress induced martensite frmatin The effect f training n mechanical behaviur Because stress induced y > transfrmatin is critical fr shape memry effect f Fe- Mn-Si based shape memry allys, the mechanical behaviur f allys can reflect the transfrmatin behaviur. In the current study, the mechanical behaviur f Fe-Mn-Si based allys was examined by tensile testing. Stress induced y-» transfrmatin is strngly dependent n the defrmatin temperature. If the defrmatin temperature is higher than the Md temperature, y-> transfrmatin will be ttally replaced by slip defrmatin. T btain a gd shape memry effect, it is imprtant t suppress slip defrmatin. The critical stress fr martensite frmatin and that fr slip defrmatin are functins f temperature. The frmer shws a psitive temperature dependence and the latter has a negative dependence with the magnitude f these stresses becming equal at a certain temperature (T). These stresses can be apprximated by yield stresses. The temperature dependence f yield stresses fr allys 1# and 28# are shwn in Figure The critical stress fr martensite frmatin and that fr slip defrmatin crrespnd t the yield stresses at T<T' and T>T', respectively. It is clearly seen that the results fr the tw allys are similar. In rder t prevent slip defrmatin, the defrmatin temperature must be lwer than T.

179 cd VI VI s Critical stress fr martensite frmatin 1# untrained samples i i Critical stress fr slip defrmatin T' i i Temperature (K) 500' cd CM VI VJ c/3 400' Critical stress fr martensite frmatin Critical stress fr slip defrmatin # untrained samples Temperature (K) Figure 7.24 Temperature dependence f yield stress f allys 1# and 28#.

180 73 The influence f thermmechanical training n yield stresses f allys has been investigated in the current study. The critical stresses fr martensite frmatin and slip defrmatin were measured as yield stresses at rm temperature and 473K, respectively. Figure 7.25 shws the changes f yield stresses f ally 1# with increasing number f training cycles at rm temperature when the training strains ar 2% and 4%. The changes f yield stress are largely dependent n the recvery annealing temperature. When the recvery annealing temperature is 673K, the yield stress increases with increasing number f training cycles, which means the critical stress fr martensite frmatin increases, leading t the degradatin f shape memry effect. When the recvery annealing temperature is in the range f 773K t 1073K, the critical stress fr martensite frmatin decreases with increasing number f training cycles, and appraches a saturated value after several training cycles, leadi t imprvement f the shape memry effect. On the basis f these results, annealing at 873K is the mst effective heat treatment. The effectiveness f training diminishes when the recvery annealing temperature increases t 1073K, suggesting that the dislcatin structure created by training, which is favurable fr the frmatin f stress induced martensite, recvered during annealing. These results are cnsistent with thse f the training effect n shape memry effect which were discussed previusly. Similar results were btained fr the ther allys, as shwn in Figure 7.26 t Figure Based n these results, it can be cncluded that the training ef n allys 1#, 25# and 28# is greater than that n allys 6# and 10#, which is cnsistent with the results f the effect f training n shape memry effect. Figure 7.30 illustrates the changes f yield stresses f allys 1# and 28# at 473K wi increasing number f training cycles fr the training strain f 4%. The yield stress 473K (which is higher than the Md temperature) represents the critical stress fr sli defrmatin (strength f matrix). It was fund that the yield stress remained almst cnstant with increasing number f training cycles when the recvery annealing temperature was abut 823K. The austenitic matrix was strengthened when the

181 100-1# 2% training strain T 4 Training cycle (N) 10 1#,4% training strain 10 Training cycle (N) Figure 7.25 The effect f thermmechanical training n the yield stress at rm temperature fr ally 1#.

182 K 773K 823K -B *- 873K 973K Training cycle (N) 500 Training cycle (N) Figure 7.26 The effect f thermmechanical training n the yield stress at rm temperature fr ally 6#.

183 400 VI # 2% training strain 200-f 0 T T Training cycle (N) 450 Training cycle (N) Figure 7.27 The effect f thermmechanical training n the yield stress at rm temperature fr ally 10#.

184 cd C/3 VI 8 VI K 773K 823K 873K 973K 25#, 2% training strain n Training cycle (N) K 773K 823K 13 X ffl *- -ffl *- 873K 973K Training cycle (N) Figure The effect f thermmechanical training n the yield stress at rm temperature fr ally 25#.

185 400 Training cycle (N) 500' 400 cd OH Cfl « #,4% training strain T 4 Training cycle (N) Figure The effect f thermmechanical training n the yield stress at rm temperature fr ally 28#.

186 1000 1#,4% training strain Yield stress at 473K T 4 Training cycle (N) 600' cd OH S Vj VI S C/ a #,4% training strain Yield stress at 473K Training cycle (N) Figure 7.30 The effect f thermmechanical training n the yield stress f allys 1# and 28#at473K.

187 74 recvery annealing temperature was lwer than 773K, and sftened when the recvery annealing temperature was higher than 873K. Therefre, the effect f thermmechanical training n the parent phase (austenite) strength depends n the recvery annealing temperature. Figure 7.31 shws the effect f training n the temperature dependence f yield stres f allys 1# and 28# (4% training strain, repeated 4 times). It is clearly seen that strengthening f the parent phase was negligible when the recvery annealing temperature was arund 873K, the recvery annealing temperature at which the best shape memry effect can usually be btained. This suggests that the benefit f training derives frm lwering the critical stress fr martensite frmatin rather than strengthening f the austenite matrix Metallgraphic features As the shape memry effect f Fe-Mn-Si based allys can be markedly imprved by thermmechanical training, it is apparent that the training prcess must influence stress induced y-» transfrmatin and its reverse transfrmatin. The effect f thermmechanical training n y<-> transfrmatin was investigated by bserving the ptical and electrn micrstructures f the allys Optical micrscpy Figures 7.32 and 7.33 shw the micrstructures f samples f allys 6# and 28# befre training and after training (4 training cycles, recvery annealing temperature 873K). The samples were defrmed 4% in tensin. It was fund that the thickness f the martensite plates decreased after thermmechanical training, whereas the amunt f martensite did nt shw any bvius change. Based n this result, tw pints may be cncluded as fllws: firstly, y-> transfrmatin kinetics are influenced by thermmechanical training, since the number f nucleatin sites fr martensite in trained samples is bviusly greater than in untrained samples, ie., the nucleatin rate

188 1000 cd Vl VI % VI D O I D e 1 D 8 0 a D 0 ffl a 1#,4% tra lining strain D O e a B 0 e m a 673K 773K untrained 873K 973K 0-1 l i I Defrmatin temperature (K) 600' 500- cd e 0 D i a O 0 673K 773K g 3 H is T3 [g ffl a untrained 873K 973K # 4% training strain 1 1 \ Defrmatin temperature (K) Figure 7.31 The effect f training n the temperature dependence f yield stress f allys 1# and 28#. (4% training strain)

189 20 urn Figure 7.32 The micrstructure f specimens f ally 6# after 4% defrmatin. (a) untrained specimen, (b) trained specimen (4% training strain, 4 cycles, recvery annealing temperature 873K)

190 (a) 20 um Figure 7.33 The micrstructure f specimens f ally 28# after 4% defrmatin. (a) untrained specimen, (b) trained specimen (4% training stram, 4 cycles, rec annealing temperature 873K)

191 75 f stress induced martensite in trained sample is higher, and the energy barrier t nucleatin is lwer. Secndly, -ry reverse transfrmatin becmes easier after training because thinner martensite plates cause smaller shape strains, reducing the need fr plastic accmmdatin f the transfrmatin. These tw effects are likely t be respnsible fr the imprvement in shape memry effect Electrn micrscpy Since stress induced y-> martensitic transfrmatin is highly crrelated with stackin faults in the austenitic matrix, the effect f thermmechanical training n the dislcatin structure in the parent phase was studied by transmissin electrn micrscpy. The micrstructures f samples befre training and after 4 training cycles fr allys 6# and 28# are shwn in Figures 7.34 and 7.35, respectively. The training schedule cnsisted f 4% strain, fllwed by annealing at 873K fr 10 minutes. It is clear that stacking faults are highly riented after training, and mre stacking faults are induced by training. These stacking faults can act as "embrys" fr martensite nucleatin accrding t the discussin in Chapter 3. Therefre, the density f nucleatin sites fr martensite in trained samples is higher than that in untrained samples, making stress induced y-» transfrmatin easier, leading t an imprvement f shape memry effect. 7.2 Discussin Factrs influencing training Frm the results btained, it has been cnfirmed that the effectiveness f thermmechanical training is dependent n the recvery annealing temperature, training cycles, the training strain and the strain rate The effect f recvery annealing temperature

192 (a) i> (b) loonm Figure 7.34 The stacking faults structure f specimens f ally 6#. (a) untrained specimen (annealing temperature 873K), (b) trained specimen (4% training strain, 4 cycles, recvery annealing temperature 873K)

193 (a) ^ '!>WHIdHi^ wi>^ fzl^fl^l^. ^v^e^ib^^^ Y^ Dfl Biw^ mmw ^BmmWmmWmmW^mm* V ^J^b^ ^^^. ' ^mmmmmmwmmwnk W^ (b) Lllft HH * t loonm Figure 7.35 The stacking faults structure f specimens f ally 28#. (a) untrained specimen (annealing temperature 873K), (b) trained specimen (4% training strain, 4 cycles, recvery annealing temperature 873K)

194 76 The shape memry effect f Fe-Mn-Si based shape memry allys can be imprved by thermmechanical training, and the recvery annealing temperature plays an imprtant rle in this training. Based n the training results fr selected allys, it was fund that thermmechanical training can significantly imprve the shape memry f allys, except fr recvery annealing treatment at 673K r belw. When the recvery annealing temperature is higher than 773K, the Af temperature and the critical stress fr martensite frmatin decrease with increasing training cycle number, suggesting that energy barriers fr stress induced y-> transfrmatin and its reverse transfrmatin decrease, leading t an imprvement f shape memry effect. When samples were recvery annealed at 673K, the Af temperature and the critical stress fr martensite frmatin increased with increasing training cycle number, suggesting that training under these cnditins increases the frictin assciated with the y<-> transfrmatin, leading t decreased reversibility and a degradatin f shape memry effect. Figure 7.36 shws the micrstructures f parent phases f allys 6# and 28# after 4 training cycles with 4% strain fllwed by annealing at 673K fr 10 minutes. It is clear that the dislcatin density is high, since dislcatin tangles and cell structure can be seen in the austenite. These structures are caused by accumulatin f dislcatins n cycling, making stress induced y > transfrmatin mre difficult. The training results shw that a recvery annealing treatment abut 873K is the mst effective, and that increasing the recvery annealing temperature abve 873K will cause the "training efficiency" t decrease. The likely explanatin is that the higher temperatures result in a decrease in stacking fault density, i.e., nucleatin site density: leading t an adverse influence n stress induced y» transfrmatin. In additin, the strength f the austenite decreases, facilitating irreversible slip defrmatin The effect f training strain The purpse f thermmechanical training is t imprve the shape memry effect f allys, ie., t btain the maximum recvery strain. Frm the training results f allys

195 loonm Figure 7.36 The dislcatin structure f specimens f allys 6# and 28# after 4 cycl training (4% training strain, recvery annealing temperature 673K). (a) ally 6#, (b) ally 28#