Comprehensive microstructure regularization mechanism and microstructure–property stability at 1773 K of directionally solidified Al₂O₃/GdAlO₃ eutectic ceramic composite

Melt grown Al₂O₃/GdAlO₃(GAP) eutectic ceramic composite is considered as a promising ultra–high temperature structural material in fields as aerospace, power generation and so on. In order to tune the properties of directionally solidified Al₂O₃/GAP eutectic ceramic composite, its microstructure evolution is investigated over a wide range of compositions and solidification rates by laser floating zone melting method (LFZM). With the increase of solidification rate and the enrichment of Al₂O₃ phase, the eutectic microstructure undergoes a transformation from “Chinese script” irregular morphology to rod–like regular and complex regular morphology. The synergistic effects of composition and solidification rate on the microstructure regularization have been quantitatively characterized by the proportion of regular eutectic. High solidification rate (>200 μm/s) is beneficial to the formation of isothermal interface, partly inhibiting the anisotropic growth of Al₂O₃ faceted phase, which further promotes the coupling growth of the eutectic phases at the front of the interface and facilitates the microstructure regularization. The stability of eutectic spacing is verified to coincide with the Ostwald ripening relationship when the eutectic ceramic composite is thermally exposed at 1773 K for long–term (250 h). The microstructure coarsening rate is less than 0.003 μm/h, and the hardness slightly decreases about 4%, and the fracture toughness remains almost unchanged, which indicates excellent thermal stability of microstructure and property for the Al₂O₃/GAP eutectic ceramic composite at high temperature.