Measurement of the strains induced upon thermal oxidation of an alumina-forming alloy

The mechanisms that govern the formation of a thermally grown alumina on Al-containing super-alloys has been the subject of much recent speculation, because of the key influence of this oxide on the durability of thermal barrier systems for gas turbines. Among the important issues to be resolved are the location of the new oxide growth (interface, surface or internal grain boundary), the strains induced by the growth on non-planar surfaces and the associated stresses. To address some of the uncertainties, an experimental probe is designed. The experiments are based on the curvature changes, Δκ, that occur on curved thin foils, caused by differences in strain accommodation on convex and concave surfaces. Experiments performed on thin FeCrAlY foils provide a vivid illustration of the strains induced upon growth, demonstrating a large increase in the radius of curvature upon oxidation. The sign and magnitude of the changes affirm that most of the new oxide forms at the interface.