Effect of hot rolling on microstructure and transformation cycling behaviour of equiatomic NiTi shape memory alloy

In this study, a near equiatomic NiTi shape memory alloy was hot rolled at 800 degrees C using the thickness reductions of 30 and 50%. Optical and transmission electron microscopy, together with X-ray diffraction were used to demonstrate the microstructural changes associated with the hot rolling at different thickness reductions. Repeated transformation cycling was employed to investigate the evolution of R phase during cycling. Microstructural observations revealed the presence of deformation twins embedded in an elongated grain matrix in the hot rolled material. Moreover, it was found that with increasing degree of thickness reduction, the size and number of deformation twins increased throughout the grains. After transformation cycling, higher values of Delta H-A -> R were noticed in the 50% hot rolled material in comparison to the annealed condition. Transmission electron microscopy microstructure of the 50% hot rolled material transformation cycled up to 50 cycles showed that R phase packets are frequently observed near grain boundaries. The observation of R phase packets near the grain boundaries in the hot rolled material was attributed to the high stress concentrations at these regions.