Possible role of the oxygen potential gradient in enhancing diffusion of foreign ions on α-Al₂O₃ grain boundaries

Thermally grown, α-Al₂O₃ external scales formed on alloys after oxidation in pure oxygen at temperatures between 1000° and 1500°C were analyzed using FEG-STEM/XEDS. Alloy dopants such as Y, Zr, La, Hf, and Ti were found to segregate to the α-Al₂O₃ grain boundaries and to the alloy-scale interface. With increasing oxidation time and temperature, the amount of segregant on the oxide grain boundaries near the gas interface increases until a critical level is reached and precipitates begin to nucleate and grow. These observations are a result of the outward transport of dopants from the alloy, through the external alumina scale, to the gas interface. The apparent driving force for the dopant diffusion is the oxygen potential gradient in the growing oxide scale.