Real-time observation of twinning-detwinning in shock-compressed magnesium via time-resolved in situ synchrotron XRD experiments

Both engineered and natural materials may be subjected to extremes of elevated pressure, temperature, and strain rate due to shock loading; examples include ballistic impact of projectiles on armor, planetary impacts, and high-speed machining operations. Experimental techniques for ascertaining the macroscopic response of materials to shock loading are well established, but insight into fundamental mechanisms of deformation requires the ability to characterize the evolution of microstructure in real time. Experiments in which specimens are recovered and characterized after shock loading have been widely used to understand structure-property relationships. But these shock recovery experiments only reveal information at the end state of the material, from which the evolution of the structure during unloading must be inferred. There is always the possibility that the structure of the material continues to evolve during unloading, leading to a potential misunderstanding of the structural evolution during shock compression and release. In this paper we describe the results of shock recovery and time-resolved in situ x-ray diffraction studies of deformation twinning in an extruded fine-grained AMX602 magnesium alloy. The samples were shock compressed along the plate normal and extrusion directions, then released back to ambient conditions. Analysis of the microstructure before and after shock loading indicates a substantial change in crystallographic texture reflecting substantial deformation twinning. Texture evolution from in situ synchrotron x-ray diffraction measurements show significant twinning during shock compression followed by detwinning during stress release. These results not only provide insight into the complex twinning-detwinning behavior of this particular alloy, but also illustrate the utility of in situ characterization for bridging the knowledge gap between shock recovery experiments and the transient behavior of materials during shock loading more generally.