Functional fatigue and restoration in superelastic NiTi shape-memory alloys

Functional fatigue in NiTi-based shape-memory alloys (SMAs), a critical barrier to their widespread adoption for a variety of technologies, remains a key challenge with incomplete mechanistic understanding. Here we investigate functional fatigue and its restoration in superelastic NiTi SMAs with wide-ranging grain sizes and subjected to elastocaloric cycling under varying maximum applied stresses (σ_max). Results show that larger grain sizes and/or higher σ_max significantly exacerbate the kinematic irreversibility caused by the fatigue-induced increased dislocation density and martensite retention. It is demonstrated that functional restoration can be achieved through a simple overheating treatment (‘healing’) after cycling, which reverts the retained martensite into austenite for subsequent transformation while preserving dislocations. Retained martensite alone lowers the critical forward transformation stress during cycling, but its effect is fully reversible by healing, irrespective of grain size and σ_max. Both dislocations and retained martensite impair the cyclic transformation capacity of the material, leading to elastocaloric degradation. The contribution of retained martensite, which can be revoked by healing for elastocaloric restoration, increases with σ_max and eventually outweighs the influence of dislocations; refinement in the grain size accelerates this transition. The work provides quantitative insights into the micro-mechanisms underlying functional fatigue and restoration in NiTi SMAs, advancing the development of sustainable elastocaloric technologies.