Solidification and shape casting of Al₂O₃–YAG eutectic ceramics from the undercooled melt produced by melting Al₂O₃–YAP eutectics

There are two eutectic systems in the Al2O3-rich portion of the Al2O3–Y2O3 system: one is the Al2O3–YAG equilibrium eutectic system and the other is the Al2O3–YAP metastable eutectic system. Heating the Al2O3–YAP metastable eutectic structure up to temperatures above the metastable eutectic temperature but below the equilibrium eutectic temperature produced the undercooled melt. Solidification in the equilibrium path immediately followed the undercooled melt formation. The solidification in the equilibrium path along with the melting of the metastable eutectic structure resulted in a fine and uniform equilibrium eutectic structure with lamellar spacing of less than 1 μm. The equilibrium eutectic structure was not affected by the metastable eutectic structure used for the undercooled melt formation. Shape casting and joining of alumina rods were demonstrated using the undercooled melt produced by the melting of the Al2O3–YAP metastable eutectic structure. The fine eutectic structure was obtained throughout the castings. Coupling of the solidification and the melting achieved a higher growth rate than conventional solidification from the undercooled melt. The skewed coupled growth zone due to the kinetic effect contributed to formation of the fine eutectic structure at the metastable eutectic composition.