Dislocation evolution and peak spall strengths in single crystal and nanocrystalline Cu

The dynamic evolution and interaction of defects under the conditions of shock loading in single crystal and nanocrystalline Cu are investigated using a series of large-scale molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock simulations that result in deformation and failure. These stages correspond to: the initial shock compression (I); the propagation of the compression wave (II); the propagation and interaction of the reflected tensile wave (III); and the nucleation, growth, and coalescence of voids (IV). The effect of the microstructure on the evolution of defect densities during these four stages is characterized and quantified for single crystal Cu as well as nanocrystalline Cu with an average grain size of 6 nm, 10 nm, 13 nm, 16 nm, 20 nm, and 30 nm. The evolution of twin densities during the shock propagation is observed to vary with the grain size of the system and affects the spall strength of the metal. The grain sizes of 6 nm and 16 nm are observed to have peak values for the twin densities and a spall strength that is comparable with the single crystal Cu.