Phase selection of the Al2O3-Y2O3 system controlled by nucleation

There are two eutectic reactions in the Al2O3-rich portion of the Al2O3-Y2O 3 pseudo-binary system: one is the Al2O3-YAG (yttrium-aluminum-garnet) equi librium eutectic reaction, the other is the Al2O3-YAP (yttrium-aluminum-per ovskite) metastable reaction. The selection of eutectic systems was examine d, in terms of maximum melt temperatures before solidification and of cooli ng rates, for specimens in a composition range of 13.5 to 28.5 mol%Y2O3. YA G nucleation did not occur st a cooling rate of more than 5 K/s when eutect ic composition specimens were cooled from 2173 K. Heating of the melt to te mperatures above 2273 K also inhibited YAG nucleation. The inhibition of YA G nucleation led to the production of undercooling melt below the metastabl e eutectic temperature, and consequently resulted in a metastable solidific ation path. Selection of the eutectic system was controlled by YAG nucleati on. At around 2273 K no heat release was detected by optical DTA measuremen t, although the effect of the maximum melt temperature on YAG nucleation su ggested some change in the melt at about 2273 K.