The Influence of Implanted Yttrium on the Microstructures of Chromia Scales Formed on a Co‐45 Weight Percent Cr Alloy

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Abstract
During the early stages of oxidation of a Co‐45 weight percent Cr alloy that has been implanted with a high dose of Y, some of the Y is incorporated into the film that forms on the alloy surface, and some diffuses back into the alloy. The yttrium in the alloy is incorporated into the oxide film during subsequent oxidation. The latter process occurs slowly, and accounts for the present of Y in the oxide near the alloy/oxide interface after 25h of oxidation at 1000°C. The Y in the oxide is present both as Y cations, which segregate to oxide grain boundaries and which account for most of the Y in the oxide, and as particles of . The particles dissociate during growth of the oxide film to yield and additional Y cations, which also segregate to oxide grain boundaries. Segregation of Y cations to oxide grain boundaries causes a change in the growth mechanism of the oxide, which causes a change in the microstructure of the oxide. It also inhibits grain growth by a solute‐drag effect, thereby yielding an oxide with a much finer grain size.

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