Phase Selection of the Al₂O₃-Y₂O₃ System Controlled by Nucleation

Abstract

There are two eutectic reactions in the Al₂O₃-rich portion of the Al₂O₃–Y₂O₃ pseudo-binary system; one is the Al₂O₃-YAG (yttrium-aluminum-garnet) equilibrium eutectic reaction, the other is the Al₂O₃-YAP (yttrium-aluminum-perovskite) metastable reaction. The selection of eutectic systems was examined, in terms of maximum melt temperatures before solidification and of cooling rates, for specimens in a composition range of 13.5 to 28.5 mol%Y₂O₃. YAG nucleation did not occur at a cooling rate of more than 5 K/s when eutectic composition specimens were cooled from 2173 K . Heating of the melt to temperatures above 2273 K also inhibited YAG nucleation. The inhibition of YAG nucleation led to the production of undercooling melt below the metastable eutectic temperature, and consequently resulted in a metastable solidification path. Selection of the eutectic system was controlled by YAG nucleation. At around 2273 K no heat release was detected by optical DTA measurement, although the effect of the maximum melt temperature on YAG nucleation suggested some change in the melt at about 2273 K.