The kinetics and mechanism of the thickening of κ plates in a Cu-Si alloy

Abstract

Hot stage, polarized light microscopy studies have shown that hcp κ plates, formed in a fcc α Cu-Si matrix, thicken discontinuously. Abrupt, micron-sized increases in thickness take place during the initial minutes of transformation and occasionally thereafter. At all other times, the rate of thickening is, at most, exceedingly small. There are very few misfit dislocations at the broad face of κ plates. The combination of these experimental results and the very small free energy change attending κ precipitation rule out both conventional heterogeneous nucleation and most diffusional growth processes as significant contributors to the development of κ plates. The principal mechanism of this transformation appears to be the formation of bundles of stacking faults, evidently in response to cooling stresses. The fault bundles are then enriched to the composition of the κ phase by the inward diffusion of silicon.