Crystallographic-orientation-dependence plasticity of niobium under shock compressions

Plastic deformation mechanism of metals under shock compressions is one of longstanding interests in compression science and materials related field. In this work, shock responses of Nb under the shock compressions along [001], [110] and [111] directions are investigated using large-scale nonequilibrium molecular dynamics (NEMD) simulations. A new reliable embedded atom method (EAM) potential is specially developed for purposes of studying the deformation twinning under high pressures. Our results indicate that the shock wave front exhibits split two wave structures in all three shock direction. In contrast to traditional understandings, the overdriven pressure of [110] is smaller than that of [001], which is attributed to their different twinning threshold and growth speed. The deformation twins are considerably more pronounced for the shock along [001] and [110] directions than that along [111] direction, which take place along {112}111 systems. Twins nucleate at the shock front and rapidly grow accompanied by dislocation nucleation and multiplication between the twins. Different twinning mechanisms for the shock along [001] and [110] directions are identified. For the shock along [001], the deformation twin is formed through successive movements of atoms on the alternative (112) plane along the [110] and [113] direction. For the shock along [110], the atoms on each (112) plane directly move along the [111] direction layer by layer. Using a lattice model combined with the transition state theory, we find that uniaxial compressions along specific directions play the key role for the twinning mechanism.