Acta Materialia 50 (2002) 525–535 www.elsevier.com/locate/actamat On the diffusional growth of compounds with narrow homogeneity range in multiphase binary systems V. Buscaglia a,* , U. Anselmi-Tamburini b a Institute of Physical Chemistry of Materials, National Research Council, via De Marini 6, I-16149 Genoa, Italy b Dept. of Physical Chemistry and C.S.T.E./CNR, University of Pavia, v.le Taramelli 16, I-27100 Pavia, Italy Received 2 August 2001; received in revised form 28 September 2001; accepted 30 September 2001 Abstract 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 par- titioning 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.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Theory & modeling (kinetics, transport, diffusion); Diffusion (bulk); Solid-state reactions; Growth kinetics 1. Introduction When two different materials are put in intimate contact at high temperature, interdiffusion occurs under the effect of the gradient of the chemical potential. In many systems, interdiffusion is accompanied by the formation of intermediate phases and the overall process is described as * Corresponding author. Tel. +39-010-6475708; fax: +39- 010-6475700. E-mail address: buscaglia@icfam.ge.cnr.it (V. Buscaglia). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00367-6 reactive diffusion [1–4]. There are many techno- logically important processes where multiphase diffusional growth is involved. Well known examples include high-temperature corrosion of metals and alloys [5], realization of intermetallic protective coatings by diffusional annealing [5], modification of the surface properties of titanium alloys by nitriding or carbonitriding [6], welding and joining of dissimilar materials [7]. In general, reactive phase formation has a strong influence on fabrication, properties and eventually in the failure of materials and devices. Diffusional growth of a single binary phase from