Oxidation of Ti[sub n+1]AlX[sub n] (n=1-3 and X=C, N)

Abstract

In this, Part I of a two-part study, a generalized model for the oxidation of the ternary compounds, where and X is carbon and/or nitrogen, is proposed. In all cases, the oxidation products in the 800-1100°C temperature range are rutile in which some Al is dissolved, i.e., where and The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of and ions through the layer. The C and N atoms are presumed to diffuse through the reaction layers and oxidize. The basic premises of the model are that the subjection of the layer to an oxygen chemical potential gradient results in its demixing, with the dissolving into the rutile at the low oxygen partial pressure and its precipitation as at the high partial pressure side. If extensive, the demixing results in the formation of layers of porosity, through which the ions cannot diffuse but the ions can. The resulting microstructures can be highly striated where three layers; an -rich layer, an -rich layer, and a porous layer repeat numerous times. Comparison with previously published results on the oxidation of leaves little doubt that dissolution of the Al in the reaction layer enhances the oxidation kinetics. This is most probably accomplished by an increase in the oxygen vacancy concentration. The fact that the oxide scales are not fully dense is also believed to play an important role in enhancing the oxidation kinetics. © 2001 The Electrochemical Society. All rights reserved.