Growth of grain boundary precipitates in Al-4% Cu by interfacial diffusion

Abstract

The lengthening rates and the thickening rates of θ precipitates which nucleate at grain boundaries and grow preferentially along them, known as grain boundary allotriomorphs, were measured in the temperature range 200°–325°C in an Al-4 % Cu alloy. Both the lengthening rates and thickening rates were found to be orders of magnitude too rapid to be controlled by the volume diffusion of Cu toward the growing allotrimorphs. Analyses of these kinetic data indicate that the thickening of θ allotriomorphs is controlled by the rate of diffusion of Cu along the α : θ boundary comprising the broad faces of the allotriomorphs. The lengthening of these crystals proceeds by volume diffusion of Cu to the α:α grain boundaries, followed by grain boundary diffusion to the advancing edges of the allotriomorphs. From the thickening kinetics, the interphase boundary diffusion coefficient was found to be D αθ = 0.23 exp (−12,800/ RT), in order of magnitude agreement with diffusivities along disordered grain boundaries in other metals.