Growth of Grain-Boundary Precipitates

Abstract

A model descriptive of the growth of precipitates which nucleate at grain boundaries and grow pre ferentially along them (grain‐boundary allotriomorphs) is studied. The sequential processes during growth are presumed to be volume diffusion of solute to the grain boundary, transport along the grain boundary to the precipitate, and interfacial diffusion and deposition over the surface of the growing allotriomorph. An analysis of this mode of growth is applied to the results of recent observations of the lengthening and the thickening rates of θ‐allotriomorphs in an Al‐4% Cu alloy. On the basis of these measurements the diffusivity within the grain boundary D_b is found to be 0.1 exp (−18480/RT) cm²sec⁻¹, while the diffusivity within the allotriomorph‐matrix interface, D₈, is D₈=1.8 exp (−22020/RT) cm² sec⁻¹ when the mole fraction of solute in this interface is taken to be the average of that in θ and in α at the boundary, or D₈=0.06 exp (−12740/RT) cm² sec⁻¹ when this surface concentration is assumed to be simply that in α at the boundary. The latter value of D₈ is in good agreement with that obtained in a previous analysis where the same value was used for the interface concentration.