ENHANCEMENT OF DIFFUSION IN DEFORMATION-INDUCED NONEQUILIBRIUM GRAIN-BOUNDARIES

The nature of the non-equilibrium state of grain boundaries (GBs) is i nvestigated using a Ni-based superalloy with a microduplex gamma + gam ma' microstructure which is stable against the grain growth at low tem peratures by of a direct diffusion experiment. The diffusion penetrati on profiles (Ni-63) in the as-annealed and pre-cold-deformed (10%) sta tes of the alloy have been measured by the residual activity technique s. The influence of a number of factors (temperature, time of diffusio n anneal and time of preliminary anneal) on the shape of the penetrati on profiles has been investigated. The profiles for the temperatures a bove 923 K exhibit a single part associated with GB diffusion with an almost constant slope -partial derivative(In c)/partial derivative(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. Th e deeply penetrating parts of these profiles are associated with diffu sion 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 inter preted as a structural phase transformation.