Grain-Boundary Diffusion of 57 Co in Ultrafine-Grained Niobium after Severe Plastic Deformation

The grain-boundary diffusion of Co in ultrafine-grained Nb subjected to severe plastic deformation by high pressure torsion was studied by layer-by-layer radiometric analysis. The coefficient of grain-boundary diffusion has been determined for several temperatures. It has been shown that diffusion along grain boundaries proceeds much faster than that in coarse-grained niobium with relaxed boundaries, which is explained by the formation of "nonequilibrium" grain boundaries upon severe plastic deformation, which act as paths for ultrafast diffusion. Upon heating, recovery occurs in nonequilibrium grain boundaries, which makes their properties approach those of the general high-angle boundaries of a coarse-grained material.