  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.
