Niti and NiTi-TiC composites: Part III. shape-memory recovery
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I.
INTRODUCTION
A s reviewed recently by, e.g., References 1 through 11, the shape-memory behavior exhibited by many allotropic, ordered alloys is the result of the thermoelastic nature of their phase transformation. In near-equiatomic NiTi alloys, the low-temperature B 19' phase (martensite M) consists of equal fractions of 24 variants with different crystallographic orientations. The B19' structure deforms by twinning up to tensile strains of about 8 pct, resulting in a strong texture, as the optimally oriented variants grow at the expense of the less-favorably oriented variants. Upon heating above the Aj temperature and complete allotropic transformation of the twinned B19' phase (martensite M') to the untwinned high-temperature B2 phase (austenite /3), the strain accumulated by twinning of the martensite is recovered. If, upon subsequent cooling, the original martensite M with equal volume fractions of the 24 possible variants is formed, the recovered strain is retained. This shape-memory effect (SME) can be repeated if the martensitic sample is again mechanically deformed. However, if, upon cooling, the martensite M' with oriented variants is formed so as to relax internal elastic stresses, the transformation is biased, some of the strain recovered upon heating is lost, and the sample adopts a shape intermediate between those of the deformed and undeformed states. This so-called two-way shape-memory effect (TWSME) can be repeated upon further thermal cycling between the austenite and martensite temperature ranges of the matrix. Both memory effects, SME and
K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Development Engineer with Raychem Corp., Menlo Park, CA 94025. D.C. DUNAND, AMAX Assistant Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 14, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
TWSME, have been exploited in applications such as actuators, connectors, heat engines, control systems, active dampers, self-erectable structures, and medical devices, t4,u-ls] The TWSME relies on biasing the formation of martensite by elastic stresses, which can originate from external or internal sources. For example, an elastic spring or a weight attached to a NiTi element can provide an external biasing force, resulting in a device with TWSME functionality. Cl6'lT'xs] The extemal bias can also originate from an elastic layer applied as a coating on the surface of the NiTi element. Hombogen and co-workers[~9,2~reported enhanced SME and TWSME for silicone-coated NiTi strips and springs. Finally, the biasing stresses can be intemal, for instance, produced by stiff particlesE2n or dislocations.t4] In the latter case, mechanical training for the TWSME involves slip deformation of the martensite and arrangement of the dislocations such that they are in equilibrium with the deformed martensitic structure. The stress field of
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