Selection of eutectic systems in Al2O3 –Y2O3 ceramics
There are two eutectic reactions in the Al2O3-rich portion of the Al2O3–Y2O3 pseudo-binary system; one is the equilibrium Al2O3–YAG eutectic reaction at 1826°C, and the other is the metastable Al2O3–YAP eutectic reaction at 1702°C. Selection of the Al2O3–YAG and the Al2O3–YAP eutectics was examined in terms of cooling rate, nucleation temperature and maximum melt temperature. When the melt was cooled from 2100°C at any cooling rate, it always nucleated below the Al2O3–YAP eutectic temperature, therefore the Al2O3–YAP eutectic was selected. The Al2O3–YAG eutectic was selected when the melt was cooled from 1900°C at a cooling rate of less than 1 K s−1. The selection of the two eutectic systems was determined by the nucleation temperature, although the maximum holding temperature of the melt and the cooling rate significantly affected the nucleation temperature. The structure of the melt, such as coordination of oxygen and chemical order when being heated to 2100°C may affect the nucleation behavior.