The significance of diffusional flow in ultrafine-grained materials

Plastic deformation of crystalline materials at elevated temperatures can occur by intragranular dislocation motion or grain boundary related processes of diffusion creep. At relatively low stresses, where intragranular dislocation activity is limited, diffusional creep is likely to be a dominant mode of deformation. Diffusional creep involves vacancy flow either through the lattice (Nabarro-Herring creep) or along grain boundaries (Coble creep), and the process leads to Newtonian flow. Models for such processes have generally assumed simple grain shapes-spheres, cubes or cylinders-to obtain the appropriate equations for steady-state creep rate. In the present investigation, diffusion creep is modeled for Kelvin's tetrakaidecahedron-which represents a realistic grain shape in polycrystalline materials-and compared to the predictions of the earlier models. In ultrafine-grained materials in the nanocrystalline range, triple grain junctions offer an alternate path for vacancy diffusion. The existing models for Nabarro-Herring and Coble diffusion creep are modified to account for matter transport by triple line diffusion, and the possible implications are discussed for ultrafine-grained materials