Strengthening by grain boundaries is commonly attributed to the dislocation pile-up at the boundary describing the effect of grain size, i.e. the Hall-Petch relation. In this work, the importance of the boundary structure is emphasized, along with grain size, in predicting the flow stress using alternative deformation mechanisms. The effect of boundary structure on the plastic deformation of metals is modeled by computing the aggregate response of composite grains in the visco-plastic self-consistent scheme. Assuming a planar boundary, the composite grain model accounts for the interaction between neighboring grains by satisfying the compatibility and equilibrium constraints across the boundary. For silver with 2 mum grains, in-situ Transmission Electron Microscopy studies suggest that annealing twin boundaries are sources for lattice dislocations. These sources contribute to an extended yield point, modeled using a formulation for slip system hardening that accounts for the evolution of mobile and forest dislocation densities---depicting boundary-dislocation and dislocation-dislocation interactions, respectively. In addition, the composite grain model is applied to predict the unique rolling texture of Cu/Nb nanostructured multilayers occurs due to the confined layer slip of single Orowan loops. The model regards each grain as a composite composed of Cu and Nb crystals. A hardening effect is introduced to account for the interaction between glide and interface dislocations. This unconventional hardening promotes symmetry of slip activity and consequently slows evolution of the rolling texture for Cu/Nb nanolayers.
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