Evaluation of Superelastic Behavior by Residual Martensite Volume Fraction in Ti-50.6at%Ni Shape Memory Alloy

Abstract

The effects of maximum strain, testing temperature and shape memory treatment temperature on superelasticity characteristics in Ti-Ni alloy wires were investigated. The isothermal cyclic tensile tests were carried out at temperatures of 343, 353 and 363K. Specimens were Ti-50.6at%Ni, annealed at 623, 673, 723 and 773K for 3.6ks after cold drawing with 34% reduction. The results show that the changes in the critical stress for inducing martensite, the dissipated strain energy and residual strain are significant in early cycles, but become insignificant after 20 cycles. The degradation of the critical stress for inducing martensite and the dissipated strain energy increase with increasing testing temperature and shape memory treatment temperature. However, these changes are insensitive to maximum strain. Furthermore, in order to clarify the effects of maximum strain, testing temperature and shape memory treatment temperature on the degradation of materials functions, the residual martensite volume fraction subjected to slip deformation was evaluated by a two-phase model consisting of the parent phase and the martensitic phase connected in series. Increase in residual strain against the residual martensite volume fraction becomes larger as shape memory treatment temperature increases. The residual martensite volume fraction represents the changes in the critical stress for inducing martensite and the dissipated strain energy with number of cycles, maximum strain, testing temperature and shape memory treatment temperature. Based on these results it can be stated that the residual martensite volume fraction subjected to slip deformation is capable of representing the effects of cyclic deformation, maximum strain, testing temperature and shape memory treatment temperature on the degradation of materials functions.