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The progressive development of the a-A1203 scale morphology on an Fe-27%Cr-4%A1 alloy, and similar alloys containing 0.008%, 0.023%, and 0.82%Y, after oxidation in I atm oxygen at 1200~ has been investigated. After oxidation for several hours, the oxide present on the yttrium-free alloy is convoluted and consists of largely equiaxed grains which can be classified according to a number of grain types. However, with additions of yttrium to the alloy, distinct changes occur in the oxide morphology, with a progressively more columnar morphology developing as the yttrium level is increased. The implications of these grain morphologies are considered in light of various hypotheses about bulk transport and short-circuit diffusion of aluminum and oxygen ions in the scale. It is emphasized that such consideration is important in planning detailed transport measurements in a-Al~O3 scales.

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