In-situ synthesis of acicular Al₂O₃ toughened (Y_0.2Yb_0.2Gd_0.2Eu_0.2Er_0.2)₃Al₅O₁₂ high-entropy ceramics by amorphous crystallization

In this work, we present a novel strategy for in-situ synthesis of acicular Al₂O₃ toughened (Y_0.2Yb_0.2Gd_0.2Eu_0.2Er_0.2)₃Al₅O₁₂ high-entropy ceramics (AATY) via an amorphous crystallization route. The amorphous powders (APs) composed of Al₂O₃ and multiple rare-earth oxides were rapidly synthesized using an Al-Al(NO₃)₃ combustion synthesis-assisted water atomization method, yielding micro-spherical particles characterized by uniform elemental distribution. Upon heat treatment, the APs crystallized into (Y_0.2Yb_0.2Gd_0.2Eu_0.2Er_0.2)₃Al₅O₁₂ high-entropy phase and α-Al₂O₃ phase. The crystallization temperature of high-entropy phase was significantly reduced to 891 °C. Subsequent spark plasma sintering (SPS) consolidated APs into a dense AATY with dual-phase structure, wherein the self-generated Al₂O₃ exhibited a fine-grained, acicular morphology with random orientations dispersed throughout the high-entropy matrix. This unique microstructure imparted excellent mechanical properties, achieving a fracture toughness of 7.12 ± 0.42 MPa m^1/2 and a Vickers hardness of 17.17 ± 0.62 GPa. This study provides valuable insights into the phase evolution of quenched oxides and the microstructural design of rare-earth aluminate high-entropy ceramics.