Atomistic mechanisms of shock-induced spallation in perfect single-crystal NiTi alloy under various stress states

This work focuses on exploring the mechanisms of spallation fracture in a nickel–titanium alloy cylinder, taking into account the evolution of the stress state. A converging shock wave is achieved through the rapid contraction of a potential wall, thereby enabling implosion loading. Along with a stress wave propagating within the material, we observe several spallation planes occurring in sequence, indicative of the emergence of multiple spallation. Interestingly, the sample at nucleation sites is found to remain in a biaxial tensile state in radial and azimuthal stresses for the first spallation, whereas it experiences distinctive behavior in the stress state, characterized by radial compressive stress and azimuthal tensile stress, preparing for the generation of secondary spallation. Moreover, the stress-induced phase transformation during the first spallation drives a heterogeneous distribution of atomic potential energy, beneficial to void nucleation. For the secondary spallation, there is another mechanism of phase transition, which is closely associated with shear deformation. Owing to severe lattice distortion, the resulting atomic misalignment provides nucleation sites for dislocations. The plastic flow induced by dislocation activity is responsible for triggering the development of shear localization. Accordingly, the generation of a deformation band covered with high shear strain leads to an increase in local temperature, and the softening effect contributes to a lower strength of the secondary spallation.