On the diffusional growth of compounds with narrow homogeneity range in multiphase binary systems

A general analysis of the problem of diffusional growth of n compounds with narrow homogeneity range in a binary system is presented. Both constituents are assumed to be mobile. The end members of the diffusion couple can be any combination of pure elements (when terminal solubility is negligible), saturated terminal solid solutions and binary compounds. The kinetic equations follow from the coupling between chemical reactions at phase boundaries and partitioning of the diffusion flux between two adjacent layers. Different kinds of parabolic rate constant are used to describe the growth of compound layers in various conditions and the relationships between these quantities are established. In particular, the rate constant of the second kind for the exclusive growth of a given compound is a function of the n rate constants of the first kind measured on the complete couple where all the intermediate phases grow simultaneously. The rate constant of the second kind is related to the diffusion properties and the thermodynamic stability of the phase. The equivalence between the present approach and the purely diffusional model of Wagner is shown. Most of the models proposed in the literature can be obtained as special cases of the present analysis. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.