THE INFLUENCE OF Y ON THE STRUCTURE AND GROWTH-MECHANISM OF ALUMINA SCALES

Small additions of yttrium to NiAl result in a decreasing oxygen transport through the Al2O3 scale on NiAl at 1270 K, as has been shown by nuclear reaction analysis (18O(p, α)15N). A higher concentration, e.g. 0.5 wt%, has the opposite effect. Yttrium segregates to the grain boundaries in the alloy, and an enhanced Y concentration has been observed by means of EPMA in the oxide scale above the grain boundaries of the alloy. Transition aluminium oxides are shown to be formed in the initial stage at 1170 K with (glancing angle) x-ray diffraction. Only α-Al2O3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al2O3 to the stable α-Al2O3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al2O3 scales are short-circuit diffusion of oxygen and/ or aluminium. At 1170 K, while transition oxides are still present, Al transport predominates. At higher temperatures, when the oxide has the α-Al2O3 structure, oxygen transport is enhanced. This would also explain the retardation of oxygen transport by the addition of Y, since Y was found to prevent the formation of the most stable form of Al2O3.