Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior. Herein, the microstructure transformation of faceted Al₂O₃/Er₃Al₅O₁₂ thermal emission eutectic composite ceramics is explored over wide ranges of compositions (13.5 mol%–22.5 mol% Er₂O₃) and solidification rates (2–200 µm/s). Entirely coupled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%–22.5 mol% Er₂O₃ at low solidification rates. The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion. In addition, the mechanisms of irregular eutectic spacing selection and adjustment under different solidification rates are revealed based on Magnin–Kurz model. A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling. According to the well microstructure tailoring, the flexural strength of Al₂O₃/Er₃Al₅O₁₂ eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation. The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refining microstructure. The as-designed Al₂O₃/Er₃Al₅O₁₂ composites with microstructural tailoring have great potential as integrations of structural and functional materials.