Effect of seed crystal in directionally solidified high‐entropy (Y _0.2 Gd _0.2 Ho _0.2 Er _0.2 Yb _0.2 )AG/Al ₂ O ₃ eutectic ceramic

Abstract The microstructure refinement and crystallographic orientation control in directionally solidified eutectic ceramics are critical for achieving stable and reliable material performance. This study investigates the evolution of microstructure and crystallographic texture throughout the entire directional solidification process to elucidate the seed‐induced mechanism in high‐entropy (Y 0.2 Gd 0.2 Ho 0.2 Er 0.2 Yb 0.2 ) 3 Al 5 O 12 /Al 2 O 3 ((5RE 0.2 )AG/Al 2 O 3 ) eutectic ceramics fabricated via optical floating zone melting method with four different seed crystals. Within the stable growth zone, (5RE 0.2 )AG/Al 2 O 3 ceramics exhibit an irregular “Chinese Script” morphology composed of (5RE 0.2 )AG and Al 2 O 3 phases. Quantitative evaluation demonstrates that the average eutectic interspacing of four high‐entropy (5RE 0.2 )AG/Al 2 O 3 DSE ceramics in the stable growth zone ranges narrowly from 26.65 to 34.34 µm, indicating that seed crystal selection has a minimal impact on eutectic interspacing. However, the strategic implementation of seed crystal enables selective control over (5RE 0.2 )AG phase orientation (<111> and <110>). The distribution of rare earth elements is critically influenced by seed‐induced solidification dynamics, which are governed by distinct mechanisms dominated by nucleation processes and grain growth competition. The findings demonstrate that seed crystal selection serves as an effective approach for controlling the orientation of Al 2 O 3 ‐based eutectic ceramics, thereby establishing a robust foundation and feasibility for the design of high‐performance eutectic ceramics.