Effect of Copper Content on Superelasticity Characteristics in Ti-Ni and Ti-Ni-Cu Alloy Wires

Abstract

The effect of copper content on superelasticity characteristics in Ti–Ni and Ti–Ni–Cu alloy wires was investigated by performing isothermal cyclic tensile tests at a temperature of Af + 25 K. Specimens were Ti–50Ni, Ti–45Ni–5Cu, Ti–40Ni–10Cu and Ti–37Ni–13Cu(at%), annealed at 673 K for 3.6 ks after cold drawing with 30% reduction. The results show that the changes in residual strain, the critical stress for inducing martensite and the strain energy in all alloys are significant in early cycles, but become insignificant after 10 cycles. The degradation of residual strain and the strain energy during loading increases with decreasing copper content. However, changes in the critical stress for inducing martensite and the dissipated strain energy in Ti–Ni–Cu alloys are insensitive to copper content. Furthermore, in order to clarify the effect of copper content on the degradation of materials functions, the volume fraction of martensite subjected to slip deformation is evaluated by a two-phase model consisting of the parent phase and the martensitic phase connected in series. The volume fraction for a residual strain becomes larger as copper content increases, and it is directly related to the critical stress for inducing martensite and the dissipated strain energ y with number of cycles. Based on these results it can be stated that the volume fraction of martensite subjected to slip deformation is a measure which represents the effects of cyclic deformation and copper content on the degradation of materials functions.