Coupled growth of unidirectionally solidified Al2O3-YAG eutectic ceramics

Unidirectional solidification of the Al2O3-YAG eutectic system in a composition range of 13.5-28.5 mol% Y2O3 was investigated. Growth rates ranged from 1.4 x 10(-7) to 2.8 x 10(-5) m/s with a temperature gradient of roughly 1 x 10(4) K/m. Primary Al2O3 crystals with dendritic shapes were observed at compositions of less than 18.5 mol% Y2O3, while primary YAG crystals were observed at compositions of more than 20.5 mol% Y2O3. Coupled growth occurred only in the specimens of 18.5 mol % Y2O3 (eutectic composition) and 20.5 mol % Y2O3. The decrease of the growth rate down to 1.4 x 10-7 m/s did not extend to the coupled growth zone. The composition range for coupled growth was about 2 mol% Y2O3 around the eutectic composition, and eutectic spacing tended to deviate from the relationship lambda(2)V = constant at a low growth rate. The non-planar interface due to the faceted growth did not cause any temperature decrease of the solidifying front at low growth rates, and consequently coupled growth occurred in the restricted composition range. (C) 2002 Elsevier Science B.V. All rights reserved.