Large-scale molecular-dynamics simulations of nanocrystalline model microstructures were performed to elucidate the dislocation and grain-boundary mechanisms of plastic deformation in nanocrystalline materials. These simulations reveal how and why, at some critical grain size of the order of about 20 nm, the conventional dislocation-slip mechanism in fcc metals becomes ineffective in favor of a grain-boundary based deformation mechanism. In agreement with recent experimental observations, the simulations show that for the larger grain sizes, complete dislocations nucleate from the grain boundaries and grain junctions. Following their nucleation, these extended dislocations travel across the grains until they annihilate in some other grain boundary. A variety of well-known dislocation-dislocation interaction processes are identified and, for larger plastic strains (of typically > 5%), extensive deformation twinning is observed. For the smallest grain sizes and in the absence of grain growth, these materials are found to deform via a mechanism involving an intricate interplay between grain-boundary sliding and grain-boundary diffusion.
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Work supported by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under Contract W-3l-l09-Eng-38.

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