Recent developments in the research of shape memory alloys

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518 K. Otsuka, X. Ren/Intermetallics 7 (1999) 511±528Fig. 12. Strain-temperature curves of Ti-±48.2Ni thin ®lm under variousconstant stresses containing GP zone precipitates. The specimenwas heat-treated at 745 K for 1 h, which is slightly above the crystallizationtemperature 737 K [61].this precipitation on Ti-rich side, refer to the originalpapers for those.Thus, it has become possible to make Ti±Ni thin ®lmswith good mechanical properties, but one problem forpractical use is the control of the composition of thin®lms [65]. Since it seems that the composition is di€erenteven on the same substrate and the transformationtemperature is very sensitive to composition, compositioncontrol will be an important problem in the future.Furthermore, for the application to micromachines, thesubstrate must be Si chips. Thus, how to avoid the constraintdue to the large di€erence in the thermal expansioncoecient of the thin ®lm and substrate will be thesecond problem [65]. As the third, we expect furtherdevelopment of applications of thin ®lms, since manyapplications are not disclosed as yet [68].3. Developments in shape memory alloys in general3.1. Defect densities in intermetallicsÐare there anyconstitutional vacancies in intermetallics?As mentioned earlier, most of SMAs are in factintermetallic compounds (shape memory intermetallics)because they are ordered alloys and many of themremain ordered near to the melting temperature (e.g.Ti±Ni, Au±Cd, etc.). Therefore, point defects we discusshere include both vacancy and anti-site defect (ASD).In comparison to the relatively recent knowledgeabout the role of point defects on mechanical propertiesof structural intermetallics for high temperature applications[69], the important role of point defects in shapememory intermetallics has been widely known for manydecades, because they strongly a€ect nearly all aspectsof martensitic transformation. This is evidenced by thestrong dependence of M s temperature, transformationhysteresis, and even the structure of martensite, on theconcentration of point defects, which can be variedeither by composition change or by heat-treatment (e.g.quenching). For example, in many alloys (e.g. Cu±Zn, Ti±Ni, Cu±Al, Fe±Pd,...) only 1 at% change in compositionvaries the M s temperature by as much as tens of or evenover a hundred degrees. Sometimes a very small changein composition can even change the resultant martensitestructure (e.g., Au±47.5Cd and Au±49.5Cd have di€erentmartensite structure). Quenching also gives rise tostrong change in transformation temperature and evenchange in martensite structure, as well as an increase intransformation hysteresis. Point defects also play a centralrole in determining the puzzling martensite aginge€ect, which will be discussed in the next section.Lattice dynamics plays a key role in martensitictransformations, as manifested by frequently observedlattice softening prior to martensitic transformation[70,71] Therefore, we expect that the e€ect of pointdefects on MT must stem from their in¯uence on latticedynamics through changing elastic constants and phononenergies, as well as anharmonic energies. A typical exampleof this has been given in Section 2.3, which showedhow an addition of Cu to Ti±Ni changes the elastic constantsand ultimately the structure of martensite.Despite the importance of point defects, quantitativeinformation about their concentrations and compositiondependence in SMAs (intermetallics) is rare. Thismay be due to the experimental diculty in measuringdefect concentrations. Only Au±Cd furnace-cooled b 2alloy has been measured systematically in its wholehomogeneity range (Cd=4652 at%) [72], as shown inFig. 13. The vacancy concentration is determined by acombination of density measurement and x-ray latticeparameter determination, and the concentrations ofother defects (Au±ASD and Cd±ASD) are determinedby best-®tting the experimental vacancy-compositionrelation with a theoretical model. A notable fact fromthis result is that vacancy concentration in slow-cooledAu-Cd is rather high (>10 3 ), and is extremely high onCd-rich side (10 2 ), compared with that in pure metalsor disordered alloys even in their quenched state. Thisfeature is common to many intermetallics with the openB2 structure.
K. Otsuka, X. Ren/Intermetallics 7 (1999) 511±528 519Fig. 13. Composition dependence of point defect (vacancy/ASD) concentrations for Au±Cd alloys [77]. Experimentally measured vacancy concentrationsare shown with square symbol [72].Intermetallics are usually divided into three distincttypes according to their defect types [73]: triple defect(TRD) type, ASD type and hybrid type. For TRD typeintermetallics (e.g. Ni±Al, Co±Al, Fe±Al) vacancy isthought to be a necessary constituent to stabilize the B2structure, thus a very high concentration of vacancy isconsidered to be stable down to absolute zero. Theminimum vacancy concentration z for TRD intermetallicsis z ˆ 2x on one side from stoichiometry(where x is the composition deviation from stoichiometry)as shown by the Bradley±Taylor (BT) [87] line inFig. 1. Such stable vacancy is usually called `constitutionalvacancy'. On the other hand, in ASD type andhybrid intermetallics vacancy is unstable at low temperatures,thus its concentration is below BT line. Au±Cdapparently belongs to the hybrid type and it contains alarge amount of frozen-in vacancy.For over 60 years it has been widely believed thatTRD intermetallics such as Ni±Al, Co±Al and Fe±Alcontain constitutional vacancies. However, recent carefulmeasurements on these intermetallics found thateven these `typical' TRD intermetallics have vacancyconcentration appreciably below the BT-line [74±76],which is the lower limit for constitutional vacancy toexist. This behavior is qualitatively similar to the case ofAu±Cd. This clearly suggests that vacancy is unstable atlow temperature, thus gives a strong challenge to thenotion of constitutional vacancy. Based on theseexperimental ®ndings, we showed that no constitutionalvacancy exists in any intermetallics, and the di€erence invacancy-frozen temperature (which depends on orderingenergy) gives rise to three apparently di€erent types ofintermetallics [77].3.2. Martensite aging e€ectÐstabilization and rubber-likebehaviorMartensite aging e€ect is a puzzling phenomenonunclear for over 60 years, and has been found in Au±Cd,Au±Cu±Zn, Cu±Zn±Al, Cu±Al±Ni, and In±Tl. It involvestwo time-dependent phenomena during martensiteaging. One is called martensite stabilization, the other iscalled rubber-like behavior (RLB). Martensite stabilizationrefers to the phenomenon that martensiteappears to be more stable with respect to the parentphase during aging so that the reverse transformationtemperature is increased. The rubber-like behaviorrefers to the phenomenon that martensite exhibits arecoverable or pseudo-elastic deformation behaviorafter aging together with an increase in critical stress, asexempli®ed in Fig. 14(A). The most puzzling problemwith the RLB is why there should exist a restoring force,because martensite deformation involves only twinningand there is no phase transformation involved, unlikethat of superelasticity of the parent phase due to stressinducedmartensitic transformation. The microstructurechange underlying the RLB is a reversible one, that is,the original martensite domain pattern is recovered after
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Recent developments in the research of shape memory alloys

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