Microstructural evolution of a nanotwinned steel under extremely high-strain-rate deformation

Nanotwinned metals are promising structural materials for resisting impact due to their excellent combination of strength and ductility. In this study, the microstructural evolution of a nanotwinned steel under extremely high-strain-rate ballistic impact was systematically investigated by nanoindentation as well as detailed electron microscopy characterization. It is found that the nanotwin structure remains similar after ballistic impact, while secondary twinning activates in a limited portion of grains. In contrast, dislocation gliding is the main plasticity mechanism in the nanotwinned steel during ballistic impact, which leads to substantial increase of hardness in the severely-deformed region close to the fracture surface. Dislocation multiplication is promoted during ballistic impact due to the phonon drag effect, resulting in a hardness increment that exceeds the maximum value achieved in quasi-static tension. In addition, recrystallization occurs in the nanotwinned steel during ballistic impact due to the significant temperature increase when the hot bullet contacted and transferred sufficient heat to the nanotwinned steel.