应力作用下 NiTi 形状记忆合金微结构演化的相场模拟及其本征应变率敏感性

NiTi shape memory alloy, a typical smart and functional material, has been widely applied in various engineering fields due to its excellent superelasticity and shape memory effect originated from reversible thermo-elastic martensite transformation. The phase-field method is a powerful computational approach for modeling and predicting the mesoscale morphology and microstructural evolution of materials. It is employed to describe the microstructural evolution via a set of order parameters that are continuous in both time and space. In this study, a new non-isothermal phase-field model was established based on the time-dependent Ginzburg-Landau kinetic equation. In particular, an additional grain boundary energy term was introduced into the local free energy density to consider the contribution from the grain boundary of a polycrystalline NiTi shape memory alloy system. In order to understand the underlying microscopic mechanisms for the superelastic deformation, the microstructural evolution and the overall mechanical behavior of both single-crystalline and polycrystalline NiTi shape memory alloys were numerically investigated under tensile loading and unloading at 290 K. After that, the intrinsic strain-rate sensitivity of nanocrystalline NiTi shape memory alloy was studied with the grain size of 60 nm at low strain rates (0.0005−15 s⁻¹). The results show that the martensitic transformation in the single crystalline NiTi shape memory alloy is uniform. No austenite-martensite interface was formed during the computation. Superelastic deformation was simulated by a nanocrystalline NiTi phase-field model. Such behavior is achieved through the nucleation and expansion of martensite bands during uniaxial tensile loading as well as the disappearance of martensite bands during unloading. In comparison, the single-crystalline NiTi shape memory alloy processes larger hysteresis area and better superelastic deformation ability than the polycrystalline NiTi shape memory alloy under the same external loading condition. Noticeable strain-rate sensitivity was exhibited in stress-strain relation of the nanocrystalline NiTi shape memory alloys under low-to-medium strain-rate loadings. The phase-transformation stress increases with the rise of implemented strain rate. Such strain-rate dependence is a result of the competition in the phase-field model between the speed of martensitic domain evolution and the speed of external loading.