Nanocrystalline metals are polycrystalline metals with grain sizes in the nanometer range. They have attracted significant interest in recent years due to their unique mechanical and electrical properties. The main objective of this thesis is to develop continuum-scale descriptions of nanoscale deformation and failure mechanisms in nanocrystalline metals. The research has focused on three specific aspects: the influence of grain boundary mechanisms on the grain-size dependence of the yield stress, the influence of grain boundary friction on the response to shock loading and the increased ductility accompanied by increased strength observed in ultrafine crystals with embedded growth nanotwins. A phenomenological model considering grain boundary sliding and accommodation as uncoupled dissipative deformation mechanisms is proposed to describe the constitutive behavior of grain boundaries. In agreement with atomistic models and experiments, tensile test simulations using the numerical model predict the inverse Hall-Petch effect, i.e. a dependence of the yield stress on the inverse square root of the grain size with a negative slope. In addition, the model suggests that the observed discrepancy between atomistic and experimental results may be partially related to rate dependence effects.
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