Phase-field simulation on grain-size dependent fracture of cyclically loaded NiTi-SMA

Based on crystal plasticity theory, a new non-isothermal fracture phase field model was proposed, incorporating various inelastic deformation mechanisms in NiTi shape memory alloy (SMA). The crack propagation of NiTi-SMA under cyclic loading was simulated by addressing its one-way shape memory effect (OWSME) and super-elasticity (SE). The effects of stress-induced martensite transformation (MT), temperature-induced MT, martensite reorientation (MR), and plastic deformation on the crack propagation of NiTi-SMA were examined. The simulated results indicate that dissipation caused by MT, MR, and plastic deformation effectively reduces the crack propagation rate. The fracture mode (crack propagation path) of NiTi-SMA is strongly correlated with the distribution of grain boundaries. As the grain size increases, the crack propagation rate in the super-elastic NiTi systems increases, and the fracture mode gradually transitions from the transgranular fracture to the intergranular one. However, the crack propagation path in the OWSME NiTi system exhibits independence on grain size, and the crack propagation rate within the OWSME system is slightly lower than that in the SE system. The difference of fracture behavior between the super-elastic NiTi system and shape memory NiTi system can be explained from the perspective of microstructure evolution.