Effect of Annealing Temperature and Time on Martensitic Transformation Temperatures and Mechanical Properties of the Ti-50.7 at % Ni Shape Memory Alloy

The influence of temperature and time of recrystallization annealing on the characteristic temperatures of martensitic transformation and mechanical properties has been studied using Ti-50.7 at % Ni shape memory alloy in the form of wire after cold deformation by drawing at room temperature. Six regimes of post-deformation annealing have been considered for the alloy at various temperatures and holding times; as a consequence, structures with various sizes of recrystallization grain have been obtained. Its size is determined by electron backscatter diffraction (EBSD); it has been revealed that it increases from 2.5 mu m to 9 mu m upon an increase in both the annealing temperature (600-700 degrees C) and the holding time (0.5-5.0 h). The characteristic temperatures of forward and reverse martensitic transformations have been determined by differential scanning calorimetry. It has been demonstrated that, as a consequence of a threefold increase in the size of recrystallized grain, the martensite start temperature decreases and the temperature range of reverse martensitic transformation is broadened. The results of mechanical tension tests at ambient temperature evidence that the increase in grain size results in a decrease in the dislocation yield strength and an increase in the yield strength. It has been determined that the dislocation yield strength is defined by the Hall-Petch law and the phase yield strength by the position of testing temperature with regard to the temperature of the start (or peak) of forward martensitic transformation. While recommending the heat treatment regime for certain products, these competing factors should be taken into account along with the temperatures of reverse martensitic transformation responsible for the temperatures of shape recovery.