Amorphous structure in single-crystal magnesium under compression along the c axis with ultrahigh strain rate

Abstract

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.