Abstract: We herein reported the effects of growth rate on the microstructure, crystallographic texture and mechanical properties of a novel directionally solidified high-entropy (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₃Al₅O₁₂/Al₂O₃ ((5RE_0.2)AG/Al₂O₃) eutectic ceramics. All (5RE_0.2)AG/Al₂O₃ ceramics demonstrate outstanding microstructural stability across the growth rates from 10 mm/h to 180 mm/h and maintain the fine irregular morphology resembling “Chinese script” even at the highest growth rate of 180 mm/h. The average interspacing (λ) decreases with increased growth rate (v), following λv^0.5=68.8 µm^1.5 s^-0.5. Crystallographic orientation relationships remain stable across the wide range of growth rates, ranging from 10 mm/h to 180 mm/h, with crystallographic texture relationships: ($\overline{2} 11$) or ($2 \overline{1} 1$) (5RE_0.2)AG // (0001) Al₂O₃ and <111> or <011> (5RE_0.2)AG // <$ 10 \overline{1} 0$> Al₂O₃. We found that high-entropy modification enhanced the distortion tolerance at the phase interface of (5RE_0.2)AG/Al₂O₃ ceramics, ranging from -4.70% to 11.54%, and it facilitates the rapid approach to a stable relationship between growth orientation and the phase interface. Hardness and fracture toughness increase with enhancing the growth rate, rising from 13.6±0.1 GPa to 16.5±0.1 GPa and from 2.8±0.5 MPa m^1/2 to 3.4±0.3 MPa m^1/2, respectively. The findings highlight the exceptional microstructure stability and crystallographic relationship formation capability of high-entropy Al₂O₃-based eutectic ceramic.

Key words: High entropy, Eutectic oxides, Growth rate, Microstructure evolution, Mechanical properties