A phase mixture model in which a polycrystallinematerial is regarded as a mixture of a crystalline phase and a grain-boundary phase is presented. The model aims to describe theplastic deformation behaviour of fine-grained materials. Themechanical properties of the crystalline phase are modelled usingunified viscoplastic constitutive relations, which take dislocationdensity evolution and diffusion creep into account. The total strainrate of a crystallite is calculated by summation of the contributionsof dislocation, boundary diffusion and lattice diffusionmechanisms. The deformation mechanism for the grain-boundaryphase is modelled as a diffusional flow of matter through the grainboundary. Using a simple rule of mixtures, the grain sizedependence of the overall plastic deformation behaviour of thematerial is analysed. Rate effects are also investigated. The resultsof the calculations are compared with previously published experimental data.
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