SELF-DIFFUSION IN ALPHA-AL2O3 AND GROWTH-RATE OF ALUMINA SCALES FORMED BY OXIDATION - EFFECT OF Y2O3 DOPING

In many cases, alumina scales are assumed to grow predominantly by oxy gen diffusion, but some authors have found that the growth can be cont rolled by aluminium diffusion. These mechanisms can be modified by act ive elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of a lumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion c oefficients. In order to obtain information about the alumina scale gr owth mechanism, oxygen (O-18) and aluminium (Al-26) self-diffusion coe fficients in Al2O3 were determined in the same materials and in the sa me experimental conditions, thus allowing a direct comparison. For bot h isotopes, bulk and sub-boundary diffusion coefficients were determin ed in single crystals of undoped alumina. Grain-boundary diffusion coe fficients have been computed only for oxygen diffusion in polycrystals . Oxygen diffusion has been also studied for yttria-doped alpha-alumin a in the lattice, sub-boundaries and grain boundaries. Oxygen and alum inium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxy gen diffusion. Yttria doping induces a slight increase of the oxygen b ulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self- diffusion nor grain boundary self-diffusion can explain the growth rat e of alumina scales. Such a situation is compared to the case of Cr2O3 .