Reversible change in transformation temperatures of a Ti-51at%Ni alloy associated with alternating aging

Abstract

Near equiatomic Ti-Ni alloys are the most useful ones in practical applications, because of its interesting properties, such as shape memory effect and superelasticity. Because these properties are closely related to martensitic and R-phase transformations, transformation temperature control is very important for applications of shape memory alloys. Ten years ago Horikawa et al. reported an interesting phenomenon in Ti-Ni alloys, i.e., short-time alternating aging between 713K and 783K in Ti-51at%Ni alloy results in nearly repeatable variation in the transformation temperatures in an up-and-down manner (the transformation temperatures appear to be solely controlled by aging temperatures). In the present study, the authors shall show that the phenomenon still exits even for long-time aging. This suggests that this phenomenon is not due to reversion. The authors shall show that this phenomenon can be simply explained by considering the phase equilibrium between the TiNi matrix and the metastable Ti{sub 3}Ni{sub 4} precipitates at different aging temperatures.