Stress-induced martensite formation under high strain rate loading in TiNi shape memory alloy

The functional-mechanical behavior of shape memory alloys is affected by both temperature and strain rate. Depending on the strain rate, strain can occur through different channels: irreversible dislocation-based plastic strain or reversible phase strain due to martensitic phase transformation and / or martensite reorientation. The formation of stress-induced martensite, in particular, depends on the strain rate; the higher the strain rate, the lower the proportion of stress-induced martensite. This means that the phase transformation caused by an external stress is sensitive to the loading rate. The study investigates the effect of the strain variation on heating and cooling of TiNi specimens after high strain rate deformation in austenitic, pre-martensitic, and mixed-phase states. The high strain rate deformation was performed in tensile mode at different rates using the modified Kolsky method. Quasistatic deformation tests were conducted on a universal testing machine at identical temperatures and up to matching residual strains. The study shows how the strain rate dependence of stress-induced martensite formation affects the strain variation during the realization of one-way and two-way shape memory effects. The material behavior can be explained by the hypothesis that the volume fraction of stress-induced martensite decreases with increasing strain rate.