Inelastic Deformation Mechanisms in Shock Compressed Polycrystalline Pure Magnesium at Temperatures Approaching Melt

Abstract

In this paper, we investigate inelastic deformation mechanisms in polycrystalline commercially-pure (99.9%) magnesium samples shock-compressed using reverse-geometry normal plate-impact experiments at elevated temperatures. The favorability of orientation for the possible deformation modes (i.e. basal slip, prismatic slip, pyramidal I & II slip and extension/contraction twinning) is investigated using Schmid Factor analysis using the EBSD data. The study is an extension of a recent study by the authors [Wang et al. in J Dyn Behav Mater 3:497–509, 2017], and is motivated by the need to better understand the underlying inelastic deformation mechanisms operative in shock-compressed Mg at incipient plasticity at near-melt temperatures. Electron Backscatter Diffraction analysis of the as-received polycrystalline magnesium samples at room temperature reveal that the $$\langle 10\bar{1}0\rangle$$ directions (normal to the prismatic planes) are parallel to the impact direction, while the [0001] poles are aligned with the radial directions (RD). Because of the unfavorable orientation for basal slip and the high Critical Resolved Shear Stresses (CRSS) for the other non-basal slip systems, extension twinning is observed to be the preferred deformation mode in the samples. In addition, results from samples tested at 400 °C, 500 °C, 610 °C and 630 °C, indicate progressive grain coarsening and a notable increase in extension twin activity at the two highest test temperatures used in the study, i.e. 610 °C and 630 °C. In general, for polycrystalline hcp metals, an increase in grain size and test temperatures are understood to promote dislocation-mediated slip and not dislocation twinning; however, in the present experiments, at the two highest test temperatures a notable increase in extension twin area fraction is observed, indicating an increased resistance (suppression) of dislocation slip at incipient inelasticity. These results, along with the experimentally measured particle velocity profiles at the two highest test temperature experiments (610 °C and 630 °C), provide evidence for transition in dislocation-based slip mechanisms from being thermally activated to viscous drag mediated under the normal shock compression and elevated temperature conditions of the present experiments.