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 A f + 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 energy 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.
The effects of cyclic deformation and copper content on the thermo-mechanical characteristics in Ti-Ni-Cu alloys were investigated. Thermo-mechanical cyclic tests were conducted for various strains at a fixed heating temperature. Specimens were 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 change of functions such as residual strain and the strain energy is significant in early cycles, but it becomes insignificant after 100 cycles. Also, the change of functions with number of cycles shows the dependence on copper content. In order to clarify the effects of cyclic deformation and copper content on the degradation of functions, the volume fraction of slip-deformed martensite is evaluated by a two-phase model consisting of the parent phase and the martensitic phase connected in series. The volume fraction of slip-deformed martensite represents the variation of the residual strain and the degraded recovery strain energy with number of cycles and copper content. Based on these results, it is concluded that the volume fraction of slip-deformed martensite is a measure which represents the effects of cyclic deformation and copper content on the degradation of functions.
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