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

基于Ginzburg-Landau动力学控制方程建立了NiTi形状记忆合金非等温相场模型，实现了对NiTi合金内应力诱导马氏体相变的数值模拟。同时将晶界能密度引入系统局部自由能密度，从而考虑多晶系统中晶界的重要作用。数值计算了单晶和多晶NiTi形状记忆合金在单轴机械载荷作用下微结构的动态演化过程和宏观力学行为，并重点研究了晶粒尺寸为60 nm的NiTi纳米多晶在低应变率下（0.0005～15 s?1）力学行为的本征应变率敏感性。研究结果表明，单晶NiTi合金系统高温拉伸-卸载过程中马氏体相变均匀发生，未形成奥氏体-马氏体界面。而纳米多晶系统在加载阶段出现了马氏体带的形成-扩展现象，在卸载阶段出现了马氏体带的收缩-消失现象。相同外载作用过程中，NiTi单晶系统的宏观应力-应变曲线具有更大的滞回环面积，拥有更优的超弹性变形能力。计算结果显示，在中低应变率下纳米晶NiTi形状记忆合金应力-应变关系表现出较明显的应变率相关性，应变率升高导致材料相变应力提升。这一应变率相关性主要源于相场模型中外加载荷速率与马氏体空间演化速度的相互竞争关系。

Phase-field simulation of microstructural dynamics in NiTi shape memory alloys and their intrinsic strain rate sensitivities

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.