Effect of Vacancies on Dynamic Response and Spallation in Single-Crystal Magnesium by Molecular Dynamic Simulation

Abstract

The effect of vacancies on dynamic response and spallation in single-crystal magnesium (Mg) is investigated by nonequilibrium molecular dynamics simulations. The initial vacancy concentration (Cv) ranges from 0% to 2.0%, and the shock loading is applied along [0001] and [10–10] directions. The simulation results show that the effects of vacancy defects are strongly dependent on the shock directions. For shock along the [0001] direction, vacancy defects have a negligible effect on compression-induced plasticity, but play a role in increasing spall damage. In contrast, for shock along the [10–10] orientation, vacancy defects not only provide the nucleation sites for compression-induced plasticity, which mainly involves crystallographic reorientation, phase transition, and stacking faults, but also significantly reduce spall damage. The degree of spall damage is probably determined by a competitive mechanism between energy absorption and stress attenuation induced by plastic deformation. Void evolution during spallation is mainly based on the emission mechanism of dislocations. The {11–22} <11–23> pyramidal dislocation facilitates the nucleation of void in the [0001] shock, as well as the {1–100} <11–20> prismatic dislocation in the [10–10] shock. We also investigated the variation of spall strength between perfect and defective Mg at different shock velocities. The relevant results can provide a reference for future investigations on spall damage.

In situ measurement of damage evolution in shocked magnesium as a function of microstructure

“…As spall strength can vary widely depending on loading conditions, we hypothesize that the variation between our works in sample thicknesses, loading stresses, and peak strain rates during experiments makes a direct comparison of single-crystal trends between the two works invalid. In addition, at the comparatively low impact velocities used in this study (0.4 mm μs −1 to 0.6 mm μs −1 ), there is some agreement with our observed spall strength trends in MD work ( 25 ).…”

“…Both the [112¯0] and [101¯0] orientations show long, thin extensions of void damage away from the spall plane, in line with observations in MD simulations in ( 45 ), which note the nucleation of voids at planar stacking fault interactions and the nonspherical nature of the resulting voids. In addition, MD investigations performed in ( 25 ) identify that void shape and growth are dominated by different mechanisms depending on the crystal orientation, noting the domination of {112¯2}{112¯3} pyramidal dislocations and twinning in the [0001] orientation, the domination of {11¯00}{112¯0} prismatic dislocations, and a lack of twinning, in the [101¯0] case, as well as the presence of nonspherical voids in both single-crystal orientations. While we note that previous MD work cannot be compared quantitatively with the experimental work herein because of inherent differences in model assumptions, nonetheless, it can provide valuable information concerning trends and insight into mechanisms.…”

“…Extensive experiments and simulations have been performed to investigate the effects of relevant parameters on spall strength. It has been found that loading conditions (10)(11)(12), crystal orientation (13)(14)(15)(16), structure (17,18), grain boundaries (19)(20)(21)(22)(23), defects (24,25), and anisotropy (26) have an effect on material behavior and void damage evolution under spall loading. Many analytical void damage models have been developed that attempt to predict spall strength and void evolution over time (5,27).…”