Phase Selection of the Al2O3-Y2O3 System Controlled by Nucleation

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.