Spalling modes and mechanisms of shocked nanocrystalline NiTi at different loadings and temperatures

Spalling damage is one of the typical forms of material dynamic failure, of which the damage modes and physical mechanisms are directly related to the design of material impact protection. NiTi alloys have already been used in some dynamic extreme environments. However, its dynamic failure behavior and mechanism under high pressure and high strain rate are still unclear. In this study, nonequilibrium molecular dynamics simulations (NEMD) were used to explore the spalling modes and physical mechanisms of shocked nanocrystalline NiTi (nc-NiTi) at different loadings and temperatures. The results show that its spalling modes include classical-spall without melting state, multi-spall with partial-melting or partial-disorder states and micro-spall with complete melting or disorder states according to the thermodynamic states and melting characteristics of the spall region. It was found that the multi-spall or micro-spall depends on the homogeneity of the main Voronoi polyhedrons (VPs) of the short-range order (SRO) under different shock stresses, and that the spall strength at T0=1000 K decreases more regularly than that of T0=300 K with the increase of shock loading velocity Up. Moreover, increasing the initial ambient temperature T0 from 300 K to 1000 K not only reduced the spall strength of nc-NiTi, but also reduced the critical stress required for different spalling modes. This is attributed to the temperature softening effect occupying a dominant position during the spalling, including the grain boundary diffusion and the change of the thermodynamic paths due to the increase of initial ambient temperature T0.