Stress relaxation during superelastic behavior of TiNi shape memory alloy

Martensitic and reverse transformations in TiNi shape memory alloy subjected to tension test at room temperature are investigated using a thermovision camera. Stress relaxation stages during the loading and unloading branches are induced, respectively. A very large stress drop, caused by the phase transformation, is observed during the stress relaxation in both the course of loading and unloading, while, for the temperature-controlled test, in the unloading branch the stress increases under constant strain; due to the recovery. The non-uniform temperature distribution reflecting the inhomogeneous phase transformation at its initial stage is observed to start again at the end of the relaxation break and develop during the reloading process.