Microstructure Evolution and R-Phase Transformation in NiTi Shape Memory Alloy Processed by Constrained Groove Pressing and Ageing Treatment

Abstract

R-phase transformation has been known to originate from thermomechanical processing, ageing and microalloying various elements in NiTi. R-phase transformation has been known for high cyclic fatigue resistance, higher stability but in short range of 1%–2% strain thus, possess high potential for sensors, actuators, dampers and elastocaloric cooling applications. Constrained groove pressing (CGP) is well known sheet metal severe plastic deformation technique for grain refinement. In this study, the microstructure evolution and R-phase transformation has been analyzed first time in NiTi sheet processed by CGP and ageing treatment. The CGP leads to severe plastic deformation and grain refinement while post ageing resulted into Ni4Ti3 precipitation. This gives rise to improvement and stabilization of R-phase transformation, larger R-phase transformation and higher martensite stress during cyclic loading as compared to the water quenched NiTi alloy. These improvements will help to extend nitinol sheet-based applications domain which utilize R-phase transformation.