Direct diffusion experiments were performed on a Ni-based super-alloy which had a micro-duplex microstructure that was stable against grain growth at low temperatures. The diffusion penetration profiles of 63Ni, in as-annealed and pre cold-deformed (10%) states of the alloy, were measured by using the residual activity technique. It was found that the profiles for temperatures above 923K exhibited a single portion which was associated with grain boundary diffusion. The almost constant slope was equal to -∂(ln[c])/∂y6/5. At temperatures below 923K, the grain boundary-related region of the profile for pre cold-deformed samples exhibited 2 distinct parts with differing slopes. The deeply penetrating parts of these profiles were associated with diffusion along non-equilibrium grain boundaries. These disappeared after pre-annealing at higher temperatures. On the basis of these data, the non-equilibrium state of grain boundaries was identified as being a metastable state, and the process of relaxation of the non-equilibrium grain boundary structure was explained in terms of a structural phase transformation.

Enhancement of Diffusion in Deformation-Induced Non-Equilibrium Grain Boundaries L.G.Kornelyuk, A.J.Lozovoi, I.M.Razumovskii: Philosophical Magazine A, 1998, 77[2], 465-74