Phase field simulations on the rate- and grain-size-dependent crack propagation of polycrystalline NiTi shape memory alloy

A three-dimensional non-isothermal phase field model of polycrystalline NiTi shape memory alloy (SMA) was established based on the Ginzburg-Landau's theory, which considered the interaction between crack growth and martensite transformation (MT). Then, the crack propagation of polycrystalline NiTi SMA and its dependence on the system's grain size and loading rate were studied by the phase field simulations. The results show that the proposed phase field model can characterize the MT behavior, temperature field changes, and stress-displacement response of NiTi SMA during the crack growth with different grain sizes and loading rates. That is the higher the loading rate, the higher the fracture toughness is; the larger the grain size, the higher the fracture toughness is and the more significant the influence of grain boundary on the crack growth path becomes. Meanwhile, as the grain size increases, the fracture mode gradually changes from the transgranular fracture (for the fine-grained alloy with a grain size approximate to 15 nm) to the intergranular one (for the coarse-grained alloy with a grain size approximate to 100 nm). The mechanisms for the rate dependence of crack propagation are clarified. The influence of the latent heat effect on crack propagation are clarified. The mechanisms for the toughening effects under different grain sizes are clarified. The mechanisms for the change in fracture mode are clarified.