Dynamic Failure Behavior of Nanocrystalline Cu at Atomic Scales

Large-scale molecular dynamics (MD) simulations are used to investigate the effects of microstructure and loading conditions on the dynamic failure behavior of nanocrystalline Cu. The nucleation, growth, and coalescence of voids is investigated for the nanocrystalline metal with average grain sizes ranging from 6 nm to 12 nm (inverse Hall-Petch regime) for conditions of uniaxial expansion at constant strain rates ranging from 4x107 s - 1 to 1010 s - 1. MD simulations suggest that the evolution of voids can be described in two stages: The first stage corresponds to the nucleation of voids and the fast linear initial growth of all the individual voids. The second stage of void growth corresponds to the steady (slower) growth and coalescence of the void aggregates/clusters. The evolution of void fraction is found to be strongly dependent on the loading strain rates, but is less dependent on the grain size of the nanocrystalline metal. Higher strain rates require larger plastic strains to nucleate voids, whereas the larger grain sizes require lower plastic strains to nucleate voids in the inverse Hall-Petch regime. The spall strength of the nanocrystalline metal is less affected by the grain size, but is strongly affected by the loading strain rates.