Shape-Memory Resistivity Behavior of Transformation-Fatigue-Cycled Nickel-Titanium Wires Near the Equiatomic Composition

摘要

The resistivity transformation behavior was investigated as a function of applied tensile stress for equiatomic Ni-Ti wires in the uncycled condition and after millions of stress-temperature cycles (transformation fatigue cycling) in several shape memory heat engine devices. The shape of the resistivity anomaly peak on cooling changes as the stress is increased due to progressive increases in the martensite start temperature and resistivity. As a result of prior transformation fatigue cycling, the stress dependence of the martensite resistivity is reduced, the anomaly peak is broadened, and transformation-induced changes in resistivity and length on cooling and heating take place in two stages instead of one. One hour anneals at 400 exp 0 C and 600 exp 0 C induce different characteristic shapes to the anomaly peak and cause the two-stage behavior for the cycled wire to be eliminated. The results are interpreted in terms of the formation of a rhombohedral phase (R phase) with attendant increases in resistivity. (ERA citation 07:054387)