Real-time observation of twinning, detwinning, and melting in shock-loaded AZ31B-H24 magnesium alloy

Time-resolved, synchrotron x-ray diffraction methods coupled to gas and powder gun shock experiments are used to study twinning, detwinning, and melting in magnesium alloy AZ31B-H24. The x-ray diffraction data show that when this cold-rolled polycrystalline metal is shock-compressed to 9 GPa in the rolling direction (corresponding to a pressure-temperature state well within the hcp-phase region), ${10\overline{1}2}$ extension twinning is triggered. Upon release from this shocked state, detwinning is observed. For a shock pressure of 53 GPa, diffraction data show a fully amorphous phase at late times after impact, suggesting complete melting upon release has occurred. Results from theoretical-numerical modeling augment the x-ray diffraction data when the metal is shock-compressed to 53 GPa, predicting a corresponding peak Hugoniot temperature of 2990 K. If the shocked material is fully solid at 53 GPa peak stress, the release isentrope is theoretically predicted to intersect the experimental melt curve of magnesium AZ31B-H24 at a pressure of 23.2 GPa and temperature of 2380 K. The solid-liquid phase transition initiates at this intersection point. Calculated results show that when the release pressure and temperature subsequently drop to 4.8 GPa and 1360 K along a fit to legacy experimental melt data (i.e., melt curve) for magnesium AZ31B-H24, the solid-liquid transition concludes; afterwards the fully liquid-phase metal releases isentropically to zero stress.