High‐Temperature Environmental Stability of the Compounds in the Al ₂ O ₃ –Y ₂ O ₃ System

Abstract

Heat treatments in several environments were performed on a series of compounds in the Al 2 O 3 and Y 2 O 3 system: Al 2 O 3 Y 3 Al 5 O 12 eutectic, Y 3 Al 5 O 12 , YAlO 3 , Y 4 Al 2 O 9 , and Y 2 O 3 . 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 Al 2 O 3 and Y 2 O 3 , all exhibited degradation by carbothermal reduction. Based upon the experimental results, a degradation mechanism for yttrium aluminates was proposed.