STRUCTURAL CHANGES DURING THE OXIDATION OF CHROMIUM YTTRIUM ALLOYS
FOREIGN TECHNOLOGY DIV WRIGHT-PATTERSON AFB OH
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The oxidation behavior at 1100--1450C of 99.9-pure chromium and chromium-yttrium alloys containing 0.5, 1.0 or 2.0 yttrium was studied. It was found that yttrium improves the oxidation resistance of chromium and the oxidation rate of all the alloys tested, especially that of the alloy containing 0.5 yttrium, was much lower than that of pure chromium. On all the alloys tested, a dense tightly adhering oxide layer was formed, while the oxide layer on pure chromium easily peeled off. In pure chromium, a certain quantity of chromium oxides and nitrides was formed to a depth of 650 microns in a metal oxidized at 1450C for 9 hr. In chromium-yttrium alloys, the amount of chromium oxides was much smaller and the nitrides were absent altogether. The penetration of oxygen and nitrogen into pure chromium proceeds mostly along the grain boundaries. This was not observed in the chromium-yttrium alloys. It is believed that yttrium improves the oxidation resistance of chromium primarily by a refining effect.
- Properties of Metals and Alloys