Micro-texture and residual stress evolution in shot peened superelastic Ni-Ti/Ni-Ti-Co shape memory alloys

Ni-Ti based shape memory alloys (SMAs) are popularly known for its biomedical applications due its excellent superelastic property. In order to modify the mechanical properties of the base Ni-Ti alloy, ternary alloying elements (e.g. Co) are generally added. The Co addition to Ni-Ti base alloy has helped in increasing the plateau stress of the hysteresis curve. Shot peening is one of the most widely used techniques to improve the fatigue life of SMAs by inducing compressive residual stress. It was believed that the shot peening has both equally affected the residual stress and texture of the alloy. And there are instances where certain texture orientation has played key role in improving the fatigue life of shape memory alloys. The main objective of this study is to understand the micro-texture evolution and residual stress generation supported by stress-induced phase transformation during shot peening process. The alteration of the shot-peened surface is assessed using X-Ray diffraction (XRD) and Electron Back Scatter Diffraction (EBSD) techniques. More compressive residual stress was induced in shot peened Ni-Ti (NT) alloys compared to Ni-Ti-Co (NTC) alloy due to more stress-induced phase transformation. The band like features generated during shot peening of Ni-Ti as was found to be missing in case of shot peened Ni-Ti-Co alloy. The incomplete phase transformation in shot peened Ni-Ti-Co alloys gave rise to coarser grain as compared to peened Ni-Ti alloy. The shot peening has resulted in preferred orientation of grains along the 111 direction that has helped in inducing maximum compressive residual stress.