The research and development of ultrahigh-temperature structural materials and advanced preparation technologies that can work stably in extreme environments at >1600 ℃ for a long time is a fundamental guarantee for improving the thrust weight ratio, prolonging the service life and enhancing the reliability of aircraft engines, and become a strategic focus of international competition. At high melting points, high strength, and excellent oxidation and corrosion resistance, ultra-high temperature oxide ceramics are regarded as one of the most promising high-temperature structural materials in high-temperature oxidation environments. However, the microstructure of oxide ceramics prepared by conventional powder sintering technology shows typical polycrystalline structure characteristics. Structural defects (i.e., grain boundary glass phases and weak bonding interfaces) seriously weaken the high-temperature strength, structural stability, and creep resistance of oxide ceramics, making it difficult to fully demonstrate the superior oxidation resistance and corrosion resistance of oxide ceramics in high-temperature and oxygen-enriched environments. In recent years, alumina-based eutectic ceramics prepared by directional solidification technology, based on the principle of liquid-solid transformation, have shown a breakthrough in the field of advanced structural materials. This technology diverges from the conventional ceramic sintering concept and achieves a creative combination of melt growth technology and the exceptional properties of oxide ceramics. However, the solidification characteristics of oxide eutectic ceramics under high-temperature gradients are not well understood, and the forming technology for large-size components is not yet mature. The poor thermal conductivity and high melt viscosity of oxides make it easy to produce solidification defects, such as cracks, pores, and segregation, during the solidification process. The eutectic phases of oxide grown in small planes show an intense anisotropy in microstructure and properties, but they cannot achieve precise control of crystal orientation. The poor toughness of oxide eutectic ceramics also limits their application process.

In this review, the principle and development status of four solidification forming technologies, including the Czochralski method, laser suspension zone melting, laser directional energy deposition, and laser powder bed melting, are summarized, and the formation mechanism and control strategy of defects in the solidification process are also represented. This review focuses on the solidification structure evolution and homogenization method of oxide eutectic ceramics, as well as the influencing factors and control mechanism of crystal orientation selection, as well as the research progress of room-temperature/high-temperature mechanical properties and failure mechanism, high-temperature microstructure stability and high-temperature CMAS corrosion properties of oxide ceramics. Finally, the development trend of high-temperature oxide eutectic ceramics in three aspects of high gradient solidification forming, non-equilibrium solidification defect suppression and multi-level synergistic strengthening and toughening is prospected.

A high-gradient directional solidification and complex component laser additive manufacturing technology for ultra-high temperature alumina-based eutectic ceramics is established. The mechanisms of defect formation during the solidification process is revealed. This research enables the efficient fabrication of high-density, large-sized, and ultra-fine eutectic ceramic samples, as well as complex components. The non-equilibrium rapid solidification microstructural characteristics and evolution mechanisms of multi-phase oxide eutectic ceramics are revealed. The eutectic-dendrite transition rate and the range of the eutectic coexistence zone are obtained. The crystal orientation selection and competitive growth patterns induced by seed crystals in eutectic ceramics are elucidated. A stable coupled growth model for the faceted eutectic ceramics is established. The evolution mechanisms of high-temperature performance of eutectic ceramics under extreme oxidative corrosion conditions are elucidated. The fabricated porous eutectic ceramics (porosity of 34%) achieve a high bending strength that is an international record (i.e., 497 MPa at room temperature; 324 MPa at 1500 ℃). The high-density eutectic ceramics exhibit a bending strength of up to 2500 MPa at room temperature. After 500-h thermal exposure at 1500 ℃, the microstructural coarsening rate is ≤0.002 μm/h, and the corrosion depth from molten oxide exposure at 1500 ℃ for 100 h is ≤130 μm, demonstrating a superior high-temperature mechanical and environmental performance.

Ultra-high temperature oxide eutectic ceramics have superior oxidation resistance, corrosion resistance, microstructure stability, and high-temperature mechanical properties. They are expected to be one of the new high-temperature structural candidate materials, serving for an extended period in high-temperature oxidation-corrosion environments. However, the engineering application of oxide eutectic ceramics faces key bottleneck problems, including the immature forming technology for large-size, complex eutectic ceramic components, the difficulty in inhibiting solidification defects, and low toughness. It is thus necessary to continue to promote the research process of the integration of shape control and property control of large-scale complex components of oxide eutectic ceramics, mainly in the following three aspects, i.e., 1) It is urgent to develop efficient solidification forming technology for large-scale complex ceramic components, and explore the solidification characteristics of high melting point multi-phase oxide eutectic ceramics under high temperature gradient; 2) The effects of process parameters and component size on eutectic structure and solidification defects under non-equilibrium solidification conditions should be explored, and the methods of microstructure refinement and solidification structure inhibition should be established; and 3) It is necessary to clarify the failure mechanism of oxide eutectic ceramics and develop a multi-scale strengthening and toughening technology suitable for high melting point ceramic materials to achieve a synergistic improvement of the strength and toughness of eutectic ceramics.