Recent experiments have demonstrated that plastic strains in nanocrystalline aluminum and gold films with grain sizes on the order of 50 nm are partially recoverable. To reveal the mechanisms behind such strain recovery, we perform large scale molecular dynamics simulations of plastic deformation in nanocrystalline aluminum with mean grain sizes of 10, 20, and 30 nm. Our results indicate that the inhomogeneous deformation in a polycrystalline environment results in significant residual stresses in the nanocrystals. Upon unloading, these internal residual stresses cause strain recovery via competitive deformation mechanisms including dislocation reverse motion/annihilation and grain-boundary sliding/diffusion. By tracking the evolution of each individual deformation mechanism during strain recovery, we quantify the fractional contributions by grainboundary and dislocation deformation mechanisms to the overall recovered strain. Our analysis shows that, even under strain rates as high as those in molecular dynamics simulations, grainboundary-mediated processes play important roles in the deformation of nanocrystalline aluminum.grain-boundary diffusion ͉ grain-boundary sliding ͉ molecular dynamics simulation
Nanoindentation induces versatile deformation mechanisms. The situation is examined here for the case of nanocrystalline Cu by means of parallel molecular dynamics simulations. The indenter is applied to the grain boundary (GB) or to the grain interior. The inter-atomic interaction of Cu is modelled by both the Lennard-Jones potential and the embedded atom method potential. The burst and arrest of stacking faults hold the key for the plastic deformation of nanocrystalline Cu under nanoindentation, in clear contrast to the case of nanoindentation on single-crystal Cu. The following descending order of indentation resistance is found: single crystal (or polycrystal of large grain size), nanocrystal when indented on the grain interior, and nanocrystal indented on the GB.
Through massively parallel molecular dynamics simulations for the evolution of U-shaped dislocation in face-centered cubic aluminum, conventional and noncoplanar ͑in a helical form͒ evolutions of dislocation segments are revealed at the atomistic scale. The two different evolutions are closely related to the Frank-Read multiplication mechanism. The possibility of noncoplanar process is quantitatively analyzed using a combination of continuum dislocation dynamics theory and atomistic simulations. The cross-slip mechanism involving in the noncoplanar evolution is supported by examining its energy barrier and critical stress. It is suggested that the operations of two different evolutions are dictated by the strain rate and the crystal size.
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