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 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.

Constitutive Equation with Consideration of Slip-Deformed Martensite in the Deforming of Ti–Ni Shape Memory Alloy

“…8,9) It has also been reported that start and finish temperatures of reverse transformation increase with increasing prestrain. [10][11][12][13] In this work, plastic deformation after pre-deforming is considered using the volume fraction of slip-deformed martensite.…”

Effect of Pre-Strain on Transformation and Deformation Behavior in Ti–50 at%Ni Shape Memory Alloy

“…Then, the volume fraction of slip-deformed martensite with various pre-strains was defined as the eq. (1), 12,13) where, E A and E M are elastic moduli of parent phase and martensitic phase, respectively. Figure 2(a) shows a schematic diagram of stressstrain curve in the heating test under the constrained strain condition.…”

“…Authors have defined the remained fraction of martensitic phase in the parent phase as the volume fraction of slip-deformed martensite , and have clarified that the value of can be used as a criterion of pre-strain induced dislocation. 12,13) And also, elastic strain energy is stored in the materials by the martensitic deformation, and the energy promotes the reverse transformation, then it acts to restrain the martensite from transforming. [7][8][9][10] Therefore, if the elastic strain energy is released by the deformation of the martensitic phase, the transformation temperature increases.…”

Effect of Cold Working on Transformation and Deformation Behavior after Pre-Deforming in Ti–50 at%Ni Shape Memory Alloy

“…The present authors have reported that the residual martensite occurred by the slip deformation represents the index of dislocation. 8,9) However, the systematic study about these influences is not yet done enough.…”