Reducing functional fatigue, transition stress and hysteresis of NiTi micropillars by one-step overstressed plastic deformation

The effect of one-step overstressed compressive plastic deformation on functional fatigue of polycrystalline NiTi is investigated. Cuboidal micropillars are subjected to one-step plastic deformation at 1.8 GPa with residual strain of 3.5% and then cyclically compressed at 1 GPa for 10⁶ cycles. Compared with as-received micropillars, the total residual strain, transition stress and hysteresis loop area of the plastically-deformed micropillar are reduced by 74%, 52% and 67%. Microstructure analysis reveals that the observed changes originate from saturated dislocation structures and residual nanosized martensite in the one-step plastic deformation. The former suppresses further dislocation formation in the subsequent cyclic phase transition and thus enhances the cyclic stability, while the latter reduces the overall transition stress and the hysteresis dissipation through the created residual stress and the direct martensite growth. The proposed method is simple and effective in reducing the functional fatigue and enhancing the cooling performance of NiTi for elastocaloric refrigeration.