The deformation mechanism of nanocrystalline Ni (with grain sizes in the range of 30-100 nm) at ultrahigh strain rates ( > 10~7 s~(-1)) was investigated. A laser-driven compression process was applied to achieve high pressures (20-70 GPa) on nanosecond timescales and thus induce high-strain-rate deformation in the nanocrystalline Ni. Postmortem transmission electron microscopy examinations revealed that the nanocrystalline structures survive the shock deformation, and that dislocation activity is a prevalent deformation mechanism for the grain sizes studied. No deformation twinning was observed even at stresses more than twice the threshold for twin formation in micron-sized polycrystals. These results agree qualitatively with molecular dynamics simulations and suggest that twinning is a difficult event in nanocrystalline Ni under shock-loading conditions.
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