ROLE OF METAL-OXIDE INTERFACIAL REACTIONS ON THE INTERACTIONS BETWEEN OXIDATION AND DEFORMATION E. Andrieu, B. Pieraggi and A.F. Gourgues* ENSCT, UPRES-A CNRS 5071, 31077 Toulouse Cedex, France *Centre des Mate ´riaux, ENSMP, Corbeil, France (Received in final form May 15, 1998) Introduction The influence of chemical environment on mechanical properties and behaviour of materials can be illustrated by many examples. At high temperature, the strong effect of oxygen and oxidation on the creep and fatigue properties of Ni-base superalloys is well documented and has been previously discussed and reviewed. 1–3 The different types of damage induced by oxidation for long-term exposures at high temperatures have been clearly determined and analysed. 4–8 At lower temperatures, i.e. at temperatures which do not exceed 0.6 T m (K) or about 650°C for Ni-base superalloys, such an analysis is more difficult. The diffusion of reacting and/or detrimental species through the oxide scale or within the underlying alloy is not the only operative process. 9 –11 The interfacial reaction steps associated with scale growth are among the processes which can play an important role at intermediate temperatures. As illustrated by the recent analysis by Hirth, Pieraggi and Rapp, 12–14 the scale-alloy interface must be able to annihilate or to provide the flux of defects involved in the diffusion processes acting in both the oxide scale and the underlying alloy. Therefore, the goal of the present article is to present some possible mechanisms based on the role of the scale-alloy interface in scale growth, which permit an analysis and explanation of the observed effect of chemical environment on creep and crack growth rate for some industrial Ni-base alloys. Interfacial Reactions at the Metal-Scale Interface The following description of the interfacial reactions occurring at the metal-scale interface is restricted to the case of cationic growing scales, which typically corresponds to the growth of a NiO or Ni-rich oxide scale on Ni-base alloys. Using the Kro ¨ger-Vink notation, with the underlined species belonging to the metallic lattice, the interfacial reactions involved in the growth of a NiO scale are: O ads 3 O O X + V  Ni = 2h ˙ (1) for the formation of oxide lattice at the gas-oxide interface, and V  Ni + 2h ˙ + Ni alloy 3 Ni Ni X (2) for the consumption of the metal lattice at the metal-oxide interface. 13, 14 Pergamon Scripta Materialia, Vol. 39, Nos. 4/5, pp. 597– 601, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 + .00 PII S1359-6462(98)00202-4 597