Strength of diamond beyond the elastic limit under dynamic compression

Extremely high pressures over a million of atmospheres are required to deform diamonds permanently. Under dynamic high-pressure conditions, even such strong materials lose their strengths so rapidly that the initially pristine lattice transforms into complex dynamics. Here, we report femtosecond x-ray diffraction observations that directly resolve how shock waves deform the crystal lattice in the isotropic nano-polycrystalline form of diamond. The results show that the nano-grain reinforced diamond retains its strength at shock pressures far beyond its elastic limit until it finally approaches zero at 707 GPa, indicating the existence of brittle-ductile transition of nano-polycrystalline diamond under high-strain rate shock compression. The atomic-level deformation process of the diamond observed in this study is a key benchmark for designing high strength materials and simulating their behavior under extreme conditions.