Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic trans
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INTRODUCTION
THE shape memory effect (SME) is now a well-known phenomenon associated with reversible martensitic transformations. The fact that the SME has been discovered in many materials with typical martensitic transformations implies that the martensite itself, thermoelastic or nonthermoelastic, is inherently crystallographically reversible and capable of exhibiting the SME under certain conditions. The question is now becoming how to make full use of crystallographic reversibility for a large and stable SME. ~J Compared with the well-understood typical shape memory alloys, such as NiTi and Cu-based alloys, the Fe-based alloys with "nonthermoelastic" martensitic transformations have recently become attractive both practically and theoretically, t1-~~ Among them are the Fe-based alloys showing the face-centered cubichexagonal close-packed (fcc-hcp) martensitic transformation, u-Tl Unfortunately, however, one of the critical problems with these alloys is the rather low fully reversible prestrain (usually less than 2 pct). Nevertheless, according to the shear model for the fcc-hcp martensitic transformation whereby hcp e-martensite can be formed b y shearing along (112}y on every other {111}y plane of the fcc 7 austenite, at least 10 pct reversible prestrain is potentially available. Another problem which would possibly make the practical application of such shape memory alloys "questionable" is the thermomechanical stability of the SME, since it is intimately related with the motion of Shockley partial dislocations. Further, there is controversy surrounding the Fe-based shape memory alloys in terms of whether superelasticity and the two-way SME exist in such systems as with typical shape memory alloys. Although most investigators agree that these phenomena are unlikely because of the distinctive nature of the y --> e transformation in contrast to other thermoelastic a l l o y s , 11'2'3] this conclusion may be premature due J.H. YANG, Research Associate, H. CHEN, and C.M. W A Y M A N , P r o f e s s o r s , are with the D e p a r t m e n t o f Materials Science and Engineering, University of Illinois, Urbana, IL 61801. Manuscript submitted March 18, 1991. METALLURGICAL TRANSACTIONS A
to the lack of adequate experimental and theoretical evidence. Actually, our knowledge on nonthermoelastic martensitic transformations of Fe-based alloys showing the SME is far from the point reached with the thermoelastic martensitic transformation and its SME in typical shape memory alloys, such as NiTi and Cu-based alloys. A large potential may be expected in Fe-based systems and may be developed by a better understanding of the deformation and transformation mechanism. Based on newly developed Fe-based alloys with f c c hcp martensitic transformations, the present work concerns three aspects: (1) SME and associated behavior, (2) martensitic transformation behavior under the influence of a magnetic transition, and (3) the microstructural features of the SME. In light of experimental results, fundamental factors on the impro
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