An elastic–viscoplastic model of deformation in nanocrystalline metals based on coupled mechanisms in grain boundaries and grain interiors

An elastic–viscoplastic model based on coupled deformation mechanisms of grain-boundary sliding, grain-boundary diffusion, grain-interior diffusion and grain-interior plasticity is developed to investigate the mechanical behavior of nanocrystalline metals. Application of the model to nanocrystalline copper shows that overall plastic deformation from the coupled deformation mechanisms depends on both grain size and strain rate. A sharp strain-rate sensitivity transition occurs from about 0.01 to 1, as strain rates change to below 10−6s−1 in nanocrystalline copper. Grain-boundary sliding and diffusion dominate inelastic deformation when the strain-rate sensitivity approaches 1. For grain sizes larger than a critical value about 15 nm, increasing slip resistance in grain interiors causes the strength to increase as the grain size is reduced. However, further reduction in grain sizes to below 15 nm results in softening in strength due to enhanced grain-boundary sliding and diffusion.