Phase growth patterns for Al₂O₃/GdAlO₃ eutectics over wide ranges of compositions and solidification rates

Phase selection and growth characteristics of directionally solidified Al₂O₃/GdAlO₃ (GAP) faceted eutectic ceramics are investigated over wide ranges of compositions and solidification rates to explore the eutectic coupled zone. Through the observation of the quenched solid-liquid interface, the competitive growth of primary faceted Al₂O₃ phase, primary non-faceted GAP phase and Al₂O₃/GAP eutectic with different morphologies is detected. Microstructure transitions from wholly eutectic to primary Al₂O₃ (GAP) dendrite plus eutectic and then to wholly eutectic are found in Al₂O₃-20 mol% Gd₂O₃ hypoeutectic (Al₂O₃-26 mol% Gd₂O₃ hypereutectic) ceramics with the increase of solidification rate. The dendrite growth of faceted Al₂O₃ and non-faceted GAP phases are well predicted by KGT model, which have introduced appropriate dimensionless supersaturation Ω to characterize the anisotropic growth of dendrites. Based on the maximum interface temperature criterion, the competitive growth of primary phase and eutectic is analyzed theoretically and the predicted coupled zone of Al₂O₃/GAP eutectic ceramics is in good agreement with the experimental results. Besides, the influence of microstructure with these different morphologies on the flexural strength of Al₂O₃/GAP eutectic ceramics is studied.