Effects of strain rate on the superelasticity of polycrystalline NiTi shape memory alloy with microvoids: constitutive modeling and molecular dynamics

Effects of strain rate and grain size on the superelastic behaviors of polycrystalline NiTi shape memory alloy with microvoids are investigated based on theoretical analysis and molecular dynamics simulation. Firstly, a new constitutive model which is able to reproduce the strain rate and grain size dependence of stress-strain responses is proposed. The proposed model incorporates a transformation function similar to the Gurson-Tvergaard-Needleman potential and takes the presence of microvoids and void growth into account. Secondly, the mechanisms of martensitic transformation, the microstructure evolution during deformation and the superelastic responses at different strain rates and porosity levels are revealed at the atomic level. The simulated results by molecular dynamics demonstrate that the superelasticity of polycrystalline NiTi exhibits a strong dependence on the grain size, the volume fraction of microvoids and the strain rate. The transformation flow stress and dissipation energy density are found to be sensitive to the strain rate and the porosity level; the gradually decreasing grain size exerts an inhibitory influence on the stress-induced martensitic forward and reverse transformation. Higher strain rate and lower porosity have the ability to increase the critical transformation stress and the overall stress level. At last, adopting the parameters obtained from atomic simulation, the proposed model's capability in capturing the strain rate and grain size-dependent superelastic properties of polycrystalline NiTi-containing microvoids is validated.