Synergistic effects of strain rate and temperature on the superelastic–plastic behaviors of nanocrystalline NiTi shape memory alloy: A molecular dynamic study

Molecular dynamics simulations are employed to investigate the superelastic–plastic behaviors of nanocrystalline NiTi shape memory alloy subjected to tensile strains of up to 12%. The effects of strain rate (6 × 108/s–4.8 × 109/s) and temperature (400–600 K) on the recoverable phase transformation and irrecoverable plastic deformation during loading are examined. The findings indicate that reducing grain size, increasing temperature, and strain rate all serve to suppress martensitic transformation. The plastic deformation observed in nanocrystalline NiTi exhibits a close relationship with grain-boundary sliding, the generation of disordered structures, and the proliferation of dislocations. The stress–strain behaviors display an obvious strain-rate hardening effect in terms of both the critical transformation stress and the yield stress of the martensite phase. Additionally, it is found that the strain-rate sensitivity is weakened by decreasing the grain size. At higher temperatures, the temperature-softening during plastic deformation is closely related to the sliding of grain-boundary atoms. More pronounced grain-boundary sliding leads to an increase in disordered structures and the destruction of the crystal structure. The evolution of the microstructure manifests that the strain-rate hardening effect competes significantly with the temperature-softening effect, particularly at high temperatures and high strain rates. Higher temperatures are capable of weakening the strain-rate hardening effect, while higher strain rates tend to alleviate the temperature-softening phenomenon. Meanwhile, strain rate and temperature exert coupling effects on the residual strain in nanocrystalline NiTi after unloading, by influencing both the residual martensite and unrecoverable disordered structures.