Ultra-fast visualisation of plasticity in polycrystalline MgO under shock compression

MgO is an important planetary material and computational studies has shown some fascinating mechanical behaviour of this material. At very low strain rates (10^-16 /s) and high pressures, the slow diffusion of oxygen impedes dislocation recovery and strengthens MgO dramatically.  At high pressures above 50 GPa, a change in deformation mechanism is predicted where the slip system changes from [110](110) to [110](100). Here, we demonstrate the mechanical behaviour of MgO at conditions up to 160 (20) GPa and 2000 (300)K. We use laser shock compression along with ultra-fast diagnostics at the European XFEL to probe how the rapidly changing pressure-temperature conditions affect the dominant deformation mechanisms in polycrystalline MgO. These observations, coupled with elasto-viscoplastic self-consistent (EVPSC) simulations, unequivocally prove that MgO attains plastic regime in the nano-seconds scale which we can then study and model in terms of strength and deformation mechanisms. The experiments point to a rich intricate mechanical behaviour in shocked polycrystalline ceramics for the first time, which may have profound impact on the viscosity and rheological behaviour of Earth and Earth-like exo-planets.

This work is the result of experiments performed under the EuXFEL 6659 community proposals led by J. Eggert and G. Morard.