RESEARCH ON THE STRAIN RATE DEPENDENCE OF PHASE TRANSFORMATION PATTERN EVOLUTION IN NITI ALLOYS

NiTi alloys exhibit a pronounced nonlinear thermomechanical coupling during phase transition under non-isothermal loading conditions, accompanied by a variety of interfacial pattern changes. This unique phenomenon is related to the interface thickness and strain rate. When the applied strain rate increases to a certain level, it may disrupt the traditional isothermal nucleation and growth, which has not been previously studied. In this work, we observed the evolution of transformation patterns in nanocrystalline NiTi polycrystalline strips with different thicknesses under various strain rates, using high-speed camera technology and digital image correlation. Based on the Ginzburg-Landau theory, we introduced the interface scale and the time scale of latent heat release and conduction, established the momentum balance equation and heat equation, and used perturbation analysis to study the formation and evolution of patterns in the early stage of phase transition. Unlike traditional systems with constant external fields, the perturbation analysis of our driving system is more challenging, with a continuously changing uniform substrate and consideration of the time scale of the driving rate. The results show that the critical strain rate is determined by the interface thickness. In the early stage of phase transition, the pattern wavelength follows a -1/6 power-law relationship with the strain rate. When the strain rate exceeds the critical value, pattern formation is suppressed, and the phase transition grows uniformly. In the later stage of phase transition, the patterns evolve into ordered nucleation and domain growth, with the characteristic domain spacing following a -1/2 power-law relationship with the strain rate. The phase transition mode shifts from unstable nucleation and growth at low strain rates to spontaneous growth at high strain rates, reflecting the breakdown of the nucleation and growth paradigm under thermomechanical coupling.