The Relationship Between Oxide Grain Morphology and Growth Mechanisms for Fe‐Cr‐Al and Fe‐Cr‐Al‐Y Alloys

The progressive development of the scale morphology on an Fe‐27% Cr‐4%Al alloy, and similar alloys containing 0.008%, 0.023%, and 0.82%Y, after oxidation in 1 atm oxygen at 1200°C 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 scales.