Shock Response in Metals: Collective Nature of Plasticity in Mediating Phase Transformation

An open question in the behavior of metals subjected to shock is the nature of the deformation that couples to the phase transformation process. Experiments to date, which are largely ex situ, cannot discriminate between the role of known deformation processes such as twinning or dislocations accompanying a phase change, and modes that can become active only in extreme environments. I will discuss insights from atomistic simulations which show that a deformation mode not present in static conditions plays a dominant role in mediating plastic behavior in hcp metals and determines the course of the transformation. Simulations for Titanium demonstrate that a 90o lattice reorientation results in the collective action of dislocations and deformation twins preceding the transformation. The resultant domain boundary motion associated with this mode is the cause of the faster plastic response under shock compared to deformation twinning or dislocation line based slip. I will suggest how in situ coherent diffraction measurements at facilities such as LCLS may validate these findings. I will close by discussing plans for next-decade XFEL facilities, such as MaRIE (Matter Radiation in Extremes), that will generate much harder x-rays to probe the temporal and spatial evolution of polycrystalline microstructure.