Effects of thermomechanical treatments on the martensitic transformation and critical stress of Ti-50.2Â at.% Ni alloy

In this research, the effects of cold work and annealing temperature on the martensitic transformation, mechanical properties, and the critical stress of a near equiatomic TiNi alloy were investigated. For this purpose, differential scanning calorimetry and tensile tests were used. A solution annealed strip with the chemical composition of Ti-50.2 at.% Ni was cold rolled to the reductions of 20% and 40% separately. They were subsequently annealed at a temperature range between 300 and 700 °C for 1 h. For the 40% cold rolled sample, the occurrence of A ↔ M transformation was inhibited due to the severe plastic deformation; therefore, differential scanning calorimetry of the sample was completely flat and no transformation peak was detected. Annealing at 400 °C made the temperature difference between the peaks of A → R and R → M transformations reach the maximum value, in comparison to other annealing temperatures. The influence of the annealing temperature on the mechanical properties of the alloy was also investigated. Features of the plateau region in the stress-strain curves of the specimens showed that the critical stress and the slope of plateau region decreased by increasing the annealing temperature. Then, they reached the constant values by annealing at above 500 °C. It was proposed that the mechanism of deformation in the plateau region of the annealed samples at temperatures lower than the recrystallization temperature is the reorientation of martensite and the elastic deformation of the reoriented martensite. Above the recrystallization temperature annealing, just a reorientation of martensite takes place.