Atomistic investigation of grain refinement induced by shear localization in nanocrystalline NiTi alloy during dynamic failure

This study is dedicated to conducting non-equilibrium molecular dynamics simulations to explore the grain-refined phenomenon involved in a nanocrystalline nickel-titanium (NiTi) alloy subjected to high-speed impact. The results demonstrate that there is no shear localization produced in the sample at an impact velocity of 0.5 km/s, whereas severe shear deformation concentrations are located in individual grains when the velocity increases to 1.0 km/s, indicative of the presence of shear deformation band. It was discovered that the shear deformation bands are stemmed from the interior of individual grains, but grain boundaries hinder their development. With continuous loading, dislocations pass through the initial grain boundaries by the transmission mode and gather around the bands, promoting a transformation of dislocation type from Other to 1/2 <111>. After entering the grain interior, dislocations move along the shear banding direction until traversing the whole grain. As a result, new grain boundaries are formed at some specific locations belonging for shear deformation bands. Dislocations exhibit a glide motion on the new grain boundaries, instead of crossing them. A combined action of these two modes contributes to grain refinement, which is closely associated with shear localization. In addition, voids were found to nucleate on both initial and new grain boundaries within the regions close to the site at the highest tensile stress. On the contrary, the void growth leads to the grain mergence adjacent to the voids, suggesting that there is an inhibitory effect of void growth on grain refinement.