Ultrahigh flow strength in shocked nanopolycrystalline diamond.

Extreme pressures and temperatures create conditions that allow even hard and brittle materials to flow plastically. Despite extensive research, the limits of flow strength under such conditions remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ x-ray diffraction experiments and large-scale molecular dynamics simulations, we demonstrate that stacking fault-mediated strengthening enables shock-compressed nanopolycrystalline diamond to achieve a peak flow strength of 92 ± 3 GPa at a stress of 212 ± 6 GPa. Our findings show that extreme conditions can unlock ultrahigh strength via a complex array of competing deformation mechanisms and thermodynamic effects.