IN-SITU SEM STUDY OF CAVITY GROWTH DURING HIGH TEMPERATURE OXIDATION OF -(Ni,Pd)Al D. Oquab and D. Monceau CIRIMAT UMR 5085 CNRS/UPS/INP, Ecole Nationale Supe ´rieure de Chimie de Toulouse, 31077 Toulouse Cedex, France (Received November 15, 2000) (Accepted in revised form January 30, 2001) Keywords: Coatings; Diffusion; ESEM (environmental SEM), kinetics; NiAl alloys; Oxidation; Surfaces and interfaces; SIMS 1. Introduction -NiAl (B2-structure) is used as coating on superalloys for gas turbine applications because it offers a good resistance to oxidation at high temperature by forming a compact alumina scale [1]. Pt is added to NiAl to increase its resistance to hot corrosion and to improve scale adherence. Pd-modified -NiAl coatings were also developed [2] with a cost reduction objective and to reduce the precipitation of brittle intermetallic phases. Voids formation at the oxide/metal interface during nickel aluminide oxidation has been reported and discussed by several authors [3–9]. Similar voids appear also in FeAl [1] and were previously observed in MCrAlY’s [10,11]. The geometry of these voids is related to the crystallographic orientation of the grains. Their formation leads to a loss of adherence and promotes scale spalling upon cooling of the specimens. Indeed, several mechanisms of void formation were or could be proposed: 1/a Kirkendall effect due to the different diffusivities between Ni and Al in -NiAl, as proposed in Ni3Al [12] or NiAl [3,6]; 2/a vacancy condensation during cooling, similarly to the Anthony’s experiment in aluminum alloys [13]; 3/a vacancy condensation due to the large change of the equilibrium vacancy concentration [14] when the aluminum content in the intermetallic NiAl decreases during oxidation. This work presents in-situ observations of voids formation and evolution during high temperature oxidation, and then, contributes to the understanding of the mechanism of this degrading phenomenon. 2. Experimental Procedures The aluminide bond coats investigated in the present study were provided by SNECMA SERVICE and are detailed in a previous paper [8]. The microstructure of the external surface of as-received bond coats is illustrated in Fig. 1-a. The bond coats deposited on disc specimens are characterized by a regular equiaxed microstructure with an average grain size of about 50 m, which is approximately equal to the bond coat thickness. Fig. 1-b reveals that the bond coat grain boundaries are marked by a 5 to 10 m wide bulged zone. High temperature oxidation was conducted in air during 6h. SIMS analysis were obtained with a CAMECA IMS4F/6F apparatus. A SETARAM TAG24S equipment was used for thermogravimetry under air vacuum. Standard scanning electron microscopy was performed on a LEO 435VP system in conventional mode along with a PGT (imix-PC) system for the EDS analysis. The isothermal in-situ Scripta mater. 44 (2001) 2741–2746 www.elsevier.com/locate/scriptamat 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00959-9