CYCLIC STRESS-STRAIN CHARACTERISTICS OF TI-NI AND TI-NI-CU SHAPE-MEMORY ALLOYS

The authors have examined the responses of three types of Ti-Ni and three types of Ti-Ni-Cu shape memory alloys in a pseudoelastic state to mechanical cycling in hard cycles with a constant e(max) and in soft cycles with a constant sigma(max). It was found that the transformation stress of the B2 parent phase into martensite and the hysteresis (or the amount of energy dissipated during one cycle) diminish while the residual deformation increases as the number of cycles grows. Although the maximum deformation is greater in a soft than in a hard cycle, the critical stress for inducing martensite, at least over the first 10 cycles, declines more slowly in soft than in hard loading cycles. Ternary Ti-Ni-Cu alloys displayed lower transformation deformations and transformation stresses than binary Ti-Ni alloys. In both Ti-Ni and Ti-Ni-Cu alloys, higher nickel contents were found to increase the critical stress for slip, thereby suppressing the residual plastic deformation after the specimens were unloaded and helping to stabilize the cyclic stress-strain curves.