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Degradation of an Ni-Ti alloy during cyclic loading

It has long been recognized that Ni-Ti shape memory alloys (SMAs) behave pseudoelastically above the austenite finish temperature (Af) with an initially very weak strain-hardening character during the transformation. Under conditions of cyclic loading with a maximum strain, (epsilon) max, the critical stress to initiate stress-induced martensitic (SIM) transformation decreases, the strain-hardening rate increases, residual strain accumulates and the hysteresis energy progressively decreases over many cycles of loading. Hence the hysteresis energy available for dissipation gradually decreases during cycling. In this paper, some modeling features are discussed to address the effects of dislocation arrays generated in the parent phase as well as the distribution of transformation product/interfaces. Several uniaxial experiments are reported on Ni-Ti to highlight the path dependence of the cyclic deformation behavior and progressive decrease of dissipated energy. Implications for multiaxial loading behavior also are discussed.