Abstract
A two-wave shock structure---elastic precursor followed by an inelastic compression wave---is observed in single crystal and polycrystalline diamond laser shock compressed to peak stresses as high as 800 GPa. The Hugoniot elastic limits are measured to be 80 $(\ifmmode\pm\else\textpm\fi{}12)$, 81 $(\ifmmode\pm\else\textpm\fi{}6)$, and 60 $(\ifmmode\pm\else\textpm\fi{}3)\text{ }\text{GPa}$ for the $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$ orientations of single crystals with the directional dependence attributable to the relative increase in strength under confining stress. These values imply a single crystal yield strength approximately 1/3 of theoretical predictions. The measurements reveal clear deviations from an elastic-plastic response upon dynamic yielding with significant relaxation toward an isotropic stress state for shock stresses of at least 160 GPa. Previously reported signatures of melting at 700--800 GPa along the diamond Hugoniot may be related to the transition from a two-wave to a single-wave structure, supporting the interpretation that melting begins at lower stresses $(\ensuremath{\sim}600\text{ }\text{GPa})$ with the appearance of an optically reflecting phase of carbon.
Published Version
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