Deformation and diffusion modes in nanocrystalline materials

An overview of theoretical models of plastic deformation and diffusion processes in nanocrystalline (nano-grained) materials is presented. The key experimentally detected facts - abnormal Hall-Petch relationship, anomalously fast diffusion, grain rotations, plastic flow localisation, high-strain-rate superplasticity, etc. - in nanocrystalline materials are discussed. Special attention is paid to theoretical models which describe such plastic deformation mechanisms (modes) as lattice dislocation slip, grain boundary (GB) sliding, GB diffusion creep, triple junction diffusion creep and rotational deformation in nanocrystalline materials with emphasis on competition and interaction between these deformation mechanisms and explanation of the key experimentally detected facts. Theoretical models of diffusion enhancement associated with transformations of GB defects in nanocrystalline materials are also considered.