Atomistic study of phase transformation and plastic behaviors of gradient nanocrystalline NiTi shape memory alloy

Enhancing the strength and fatigue resistance of NiTi shape memory alloy (SMA) through grain refinement has become a hot research topic. However, the increase in strength of fine grains will significantly inhibit the phase transformation and reduce the recoverable deformation amplitude. The introduction of gradient grain distribution can simultaneously maintain the strength of fine grains and the ductility of coarse grains. In this study, the phase transformation and plastic deformation behavior of gradient nanocrystalline NiTi SMA are simulated using the molecular dynamics method. The simulation results indicate that the stress-induced phase transformation process in gradient nanocrystalline NiTi SMA is spatially homogeneous, while the gradient-distributed stress at grain boundaries promotes synchronous phase transformation in both fine-grained and coarse-grained regions. Conversely, the temperature-induced phase transformation exhibits localized behavior. The superelastic behaviour of gradient nanocrystalline NiTi exhibits a significant tensile-compressive asymmetry, which is dominated by different types of martensitic variants produced by different loading conditions. Whereas, as the gradient rate increases, the plastic deformation at the grain boundaries decreases and the residual strain decreases. Meanwhile, the increase of gradient rate weakens the resistance to dislocation slip, promotes dislocation proliferation and generates plugging phenomenon, and thus enhances the yield strength. In this study, the microscopic mechanism of gradient nanocrystalline NiTi SMA combining excellent superelasticity and high yield strength is revealed based on atomic scale analysis, and the designability of their mechanical properties is verified by regulating the grain gradient distribution, which provides a theoretical basis for the development of high-performance NiTi SMA.