Intermittent healing for alleviating the functional fatigue and restoration of the elastocaloric effect in superelastic NiTi shape memory alloy

Functional fatigue in the superelastic NiTi shape memory alloys occurs due to the accumulation of dislocations and retention of martensite with the cyclic loading. These mechanisms reduce the amount of the material available for the stress-induced transformation and, thus, lower the elastocaloric effect that originates from the stress-induced latent heat variations. In this study, the individual contributions of the micromechanisms responsible for the functional fatigue in superelastic NiTi at different maximum tensile stress (σmax) are critically examined. Results show that the elastocaloric effect degrades significantly with cycling, and the saturated degraded value increases with σmax; the steady-state adiabatic temperature change is unexpectedly non-proportional to σmax. An overheating treatment (‘healing’) after mechanical fatigue reverts the retained martensite into austenite, making it available for subsequent transformation and restoring the elastocaloric effect significantly. Such a restoration increases exponentially with σmax. Consequently, the steady-state elastocaloric effect of the healed NiTi is proportional to σmax and can reach more than twice that of NiTi without healing. The work sheds light on the physical origins of elastocaloric degradation of superelastic NiTi and also provides a feasible method for ameliorating functional fatigue.