Enhancement of diffusion in deformation-induced non-equilibrium grain boundaries

The nature of the non-equilibrium state of grain boundaries (GBs) is investigated using a Ni-based superalloy with a microduplex γ + γ′ microstructure which is stable against the grain growth at low temperatures by means of a direct diffusion experiment. The diffusion penetration profiles ( 63 Ni) in the as-annealed and pre-cold-deformed (10%) states of the alloy have been measured by the residual activity techniques. The influence of a number of factors (temperatre, time of diffusion anneal and time of preliminary anneal) on the shape of the penetration profiles has been investigated. The profiles for the temperatures above 923 K exhibit a single part associated with GB diffusion with an almost constant slope-∂(Inc)/∂y 6/5 . Below 923 K the GB-related region of the profiles for the pre-cold-deformed samples exhibits two distinct parts with different slopes. The deeply penetrating parts of these profiles are associated with diffusion along non-equilibrium GBs, which disappear after pre-annealing at higher temperatures. On the basis of the diffusion data obtained, the non-equilibrium state of GBs is identified as a metastable state and the process of relaxation of the non-equilibrium GB structure is interpreted as a structural phase transformation.