Thermally grown α-phase external scales which were formed on alloys by oxidation in pure O, at temperatures of between 1000 and 1500C, were analyzed. Alloy dopants, such as Y, Zr, La, Hf and Ti, were found to segregate to the α-phase grain boundaries and to the alloy/scale interface. With increasing oxidation time and temperature, the amount of segregant at oxide grain boundaries, near to the gas interface, increased until a critical level was reached and precipitates began to grow. This was a result of the outward transport of dopants from the alloy, and through the external alumina scale, to the gas interface. The apparent driving force for dopant diffusion was the O potential gradient in the growing oxide scale.

B.A.Pint, A.J.Garratt-Reed, L.W.Hobbs: Journal of the American Ceramic Society, 1998, 81[2], 305-14