Dual-phase eutectic ceramics with improved hardness and toughness via nano-coherent high-entropy oxides

Alumina-based eutectic ceramics exhibited high high-temperature strength but their intrinsic brittleness constrained broad structural applications. Here, we broke this limitation by introducing high-entropy rare-earth aluminate (REAlO₃, RE = Gd_0.25Eu_0.25Nd_0.25Sm_0.25) into the Al₂O₃ matrix via directional solidification. The resulting dual-phase eutectics exhibited a unique architecture where single-crystalline Al₂O₃ was interlocked with bicrystalline high-entropy REAlO₃, forming nanoscale coherent grain boundaries (~57.5 nm) and semi-coherent phase boundaries (lattice misfit <5.1%). The tailored microstructure and orientation relationship enabled a synergistic enhancement of Vickers hardness (19.4 GPa) and fracture toughness (5.5 MPa·m^1/2), outperforming all reported binary alumina-based counterparts. The property synergy originated from a cascaded strengthening mechanism that spanned atomic-scale lattice distortion to nanoscale coherent interfaces, coupled with multi-mode toughening via crack deflection, bifurcation, and bridging. Our work establishes a high-entropy eutectic design strategy for engineering ceramics with exceptional mechanical performance under extreme conditions.