A novel mechanical criterion and interpretation for dual stress plateau phenomenon in NiTi alloy under tension

Numerous studies have investigated Lüders band and transformation stress plateau in NiTi. However, localized plastic deformation (LPD) of martensite and the stress drop before the second stress plateau remain poorly understood. In this study, drawing an analogy to Lüders band propagation, we observed LPD band nucleation propagation in a microstructure with 253 nm grain size by in-situ Digital Image Correlation (DIC). Based on displacement conservation, we propose a mechanical criterion for inelastic loading in NiTi. This criterion accurately predicts the stress drop associated with LPD band nucleation and movement, providing a theoretical foundation. Furthermore, we systematically explain, for the first time, the abnormal strain softening effect responsible for the second stress plateau during martensitic plastic deformation. By offering new insights into martensitic transformation and LPD mechanisms, this research advances the understanding of dual stress plateaus and LPD in NiTi.