A Study on the Superelastic Recovery Mechanism of Nanocrystalline NiTi Alloy After Plastic and Cyclic Deformation

Large deformation and cyclic loading-unloading can result in significant degradation of superelasticity in nanocrystalline NiTi alloys, primarily due to dislocation accumulation and retain martensite. In this study, the influence of various plastic deformation on the recovery and evolution of superelasticity following direct current heat treatment (DCHT) was systematically investigated using in-situ digital image correlation (DIC) techniques. The effect of varying degrees of plastic deformation on the cyclic stability of superelastic behavior was evaluated after applying optimized DCHT process. Nanocrystalline NiTi alloy wires were categorized into four groups based on imposed residual strain levels: (i) 7.5%, (ii) 21%, (iii) 33%, and (iv) 44%. It is demonstrated that DCHT is capable of effectively restoring superelasticity in NiTi alloys subjected to severe plastic deformation and 50 cycle loading-unloading, with observed stress hysteresis recovery ranging from 63 to 115%. These findings provide novel insights into the multi-stage recovery mechanisms of functional properties in nanocrystalline NiTi shape memory alloys and highlight the potential of DCHT for enhancing long-term performance in practical applications.