Microstructure and mechanical properties of directionally solidified Al2O3/GdAlO3 eutectic ceramic prepared with horizontal high-frequency zone melting

Directionally solidified eutectic oxide ceramics are very promising as a next-generation structural material for ultrahigh-temperature applications, above 1600?°C, owing to their outstanding properties of high corrosion resistance, oxidation resistance, high fracture strength and toughness, and high hardness. Herein, Al₂O₃/GdAlO₃ eutectic ceramic was prepared with horizontal high-frequency induction zone melting (HIZM), and the effects of the processing parameters on the eutectic microstructure and mechanical properties were investigated. The results indicated that the directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic was composed only of the Al₂O₃ phase and GdAlO₃ phase penetrating mutually, and the Al₂O₃ phase was the substrate in which the GdAlO₃ phase was embedded. As the solidification rate increased from 1 to 5?mm/h, the eutectic microstructure underwent a transformation from an irregular pattern to a relatively regular “rod” or “lamellar” pattern, and the eutectic spacing constantly decreased, reaching a minimum value of 0.5?μm. The eutectic ceramic hardness and fracture toughness at room temperature increased continuously, reaching 23.36?GPa and 3.12?MPa?m^1/2, which were 2.3 times and 2.5 times those of the sintered ceramic with the same composition, respectively. Compared with the samples obtained from vertical high-frequency induction zone melting, the orientation of eutectic phases along the growth direction decreased significantly, and the size uniformity of the GdAlO₃ phase became poorer in the samples prepared with HIZM at the same solidification rate; nevertheless, the hardness and fracture toughness of the samples increased by 11% and 63%, respectively.