Pergamon PII S 1359-6454(96)00054-7 Acta mater. Vol. 44, No. 10, pp. 4033-4038, 1996 Copyright ~ 1996 Acta Metallurgica Inc. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 1359-6454/96 $15.00 + 0.00 CALCULATION OF PRECIPITATE DISSOLUTION ZONE KINETICS IN OXIDISING BINARY TWO-PHASE ALLOYS P. CARTER, B. GLEESON and D. J. YOUNG School of Materials Science and Engineering, The University of New South Wales, Sydney. NSW 2052, Australia (Received 25 September 1995; in revised form 17January 1996) Abstract~xidation of a binary alloy at appropriate oxygen potentials leads to the selective oxidation of one component. When that component is concentrated in a dispersed precipitate within a two-phase alloy, dissolution of the precipitate phase will occur when external oxidation takes place. In this paper a diffusional analysis is used to predict the width of the precipitate-dissolutionzone (Xd) formed during such an oxidation process. Solutions to the case of both a stationary and a mobile alloy/scale interface are obtained. The predicted values of X~ derived from both the present and previous theoretical treatments are then compared with recently obtained experimental results on the oxidation behaviour of a Ni-Ni~Si alloy. Predicted values of Xd calculated using the present model were in very close agreement with those obtained experimentally. This is in contrast to the values predicted from previously derived models, which significantly underestimated Xd. Copyright © 1996 Acta Metallurgica Inc. I. INTRODUCTION Most of the existing theories on alloy oxidation are specific to single-phase alloys. However, many of the commercial alloys used for high-temperature appli- cations are multiphase [1-16]. For example, nickel- base superalloys used for gas turbine components are typically based on a two-phase microstructure of ~,'-Ni3(AI, Ti) precipitates in a y-Ni matrix. In addition to multiphase alloys, many multiphase coatings are used for high temperature applications [14]. The most common of these are the plasma sprayed, Ni-Co-Cr-A1-Y overlay coatings, which are usually two-phase mixtures of the fl-NiAl and 7-Ni structures. Despite the considerable practical importance of multiphase alloys and coatings, there have been relatively few theoretical treatments of their oxidation behaviour [17-23]. Moreover, the treatments which do exist have not yet been compared to experimentally obtained results. Previous studies of multiphase alloy oxidation have been concerned primarily with determining the alloy scaling kinetics and characterising the resulting scale morphologies [1-12]. The results have shown that the oxidation of multiphase alloys is complex and highly variable. For instance, Gesmundo and Gieeson [17] have recently identified the following three broad categories of two-phase, binary-alloy oxidation behaviour: (1) alloys in which each phase oxidises independently forming a two-phase scale; (2) alloys in which the two phases oxidise cooperatively to form a homogeneous, single-phase scale; and (3) alloys in which the precipitate phase, rich in the more reactive solute element, acts as a reservoir for the continued, exclusive growth of the solute-metal oxide scale. This last form of oxidation behaviour has been observed in a number of systems such as chromia-forming Co-Cr:3C6 alloys [16], and is characterised by the formation of an alloy subsurface zone in which the carbide precipitates are dissolved. The situation is shown schematically in Fig. 1. The aim of the present study is to provide a diffusional analysis for the kinetics of precipitate-dissolution zone formation. Previous theoretical treatments have been pre- sented by Wahl [18], Wang et al. [19], Wang [20] and Gesmundo et al. [21-23] for the oxidation of a two-phase alloy in which the second phase acts as a solute-metal reservoir. All these treatments are semiquantitative, and are aimed at developing an expression for the critical solute-metal content, N~', necessary for the exclusive formation of its oxide scale. The treatments are analogous to but more complicated than Wagner's [24] treatment for predicting N* for a single-phase alloy. The treatment by Wahl [18] is the simplest of the multiphase treatments, as it assumes a linear concentration gradient of the reactive solute component in the precipitate-dissolution zone. As part of Wahl's treatment, the following expression for the depletion- zone thickness, Xd, was obtained x~ = ~ - - ° o (1) ~/ Ng ' where DE is the interdiffusion coefficient in the dissolution zone, t is time, and N~B and Ng are the atom fractions of B in the matrix and bulk alloy, respectively. 4033