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 shows 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), {10.12} 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 relative to 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 ∼2973 K. If the shocked material is fully solid at ∼53 GPa peak pressure, the release isentrope is theoretically predicted to intersect the experimental melt curve of magnesium AZ31B-H24 at a pressure of ∼23.2 GPa and a 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. The present diffraction data, however, suggest the possibility that at least some melting may have occurred at a Hugoniot pressure of ∼53 GPa. In that case, the current, and previous, calculations based on legacy data for Mg AZ31B-H24 and classical equations-of-state would overestimate the Hugoniot melt initiation pressure for this alloy.
2025-04-09 09:00:00
