High‐Temperature Environmental Stability of the Compounds in the Al2O3–Y2O3 System

Heat treatments in several environments were performed on a series of compounds in the Al2O3 and Y2O3 system: Al2O3Y3Al5O12 eutectic, Y3Al5O12, YAlO3, Y4Al2O9, and Y2O3. The yttrium aluminates were found to be stable at high temperatures under vacuum and in air. However, when they were heat-treated under vacuum in proximity to SiC, degradation was observed. This was found to be primarily a result of carbothermal reduction. In a similarly reducing environment without Si, the yttrium aluminates, and Al2O3 and Y2O3, all exhibited degradation by carbothermal reduction. Based upon the experimental results, a degradation mechanism for yttrium aluminates was proposed.

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