EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Commercial superelastic polycrystalline NiTi has big potential in compression-based elastocaloric cooling technology but suffers high transition stress, large hysteresis and functional degradation in cyclic phase transition. In this study, we enhance the cyclic stability together with the reduction of transition stress and hysteresis by one-step overstressed compressive plastic deformation. Cuboidal NiTi micropillars were one-step plastically deformed by flat-end tip nanoindentation to residual strain of 3.5% at the stress of 1.8 GPa and then subjected to cyclic phase transition under 1 GPa compressive stress. Compared with the as-received micropillars, the transition stress and hysteresis loop area of the overstressed micropillar are reduced by 52% and 67%, and the total residual strain in the 1 million phase transformation cycles is only 0.8%, 4 times smaller than that of the as-received micropillar (3.1%). Microstructure analysis reveals that the saturated dislocation wall structure and locked residual nanosized martensite in the plastic deformation are responsible for the achieved property. The former partitions the B2 grain into many small dislocation free pockets and suppresses the further formation of dislocation thus enhances the cyclic stability, while the latter reduces the transformation stress and hysteresis through direct growth of the martensite without nucleation and residual stress. The presented one-step overstressed plastic deformation is simple and effective in significantly reducing the functional fatigue, transition stress and hysteresis dissipation of NiTi.