Effect of mechanical cycling on the pseudoelasticity characteristics of Ti-Ni and Ti-Ni-Cu alloys

This paper presents the findings of an experimental study of how mechanical cycling of Ti-Ni and Ti-Ni-Cu shape memory alloys in the pseudoelastic (PE) state affects their residual elongation after unloading, their critical stress for martensite formation and their hysteresis or amount of energy dissipated during one cycle. Specimens were cycled in two basic modes: hard loading cycles at a constant e(max) and soft ones at a constant sigma(ms). In the hard cycling the authors further investigated how the PE characteristics respond to various strain rates and how the strain rate changes. Each of the examined alloys was cycled in the PE deformation mode at a temperature where each specimen can be deformed at the same constant critical stress for martensite formation in the first cycle of the test. As the number of cycles increases, the residual strain e(o) grows, while both the stress sigma(ms) for martensite transformation and the hysteresis W decrease. The rate at which e(o) grows depends on sigma s, sigma(ms) during cycling and the type of cycling mode. By considering the two factors sigma(s) and sigma(ms), the rather complicated effect of cyclic deformation on the PE characteristics was explained. Cycling at higher strain rates has been found to increase the residual elongation left after the specimen is unloaded and to cause a more raped decline of the critical stress for martensite formation as cycling continues. After changes in the elongation rate the stability of the cyclic stress-elongation diagram depends on the amount of residual elongation present and on the stability of that diagram during the first cycling at the original elongation rate.