Effect of an abrupt change in pulling rate on microstructures of directionally solidified Al ₂ O ₃ -Er ₃ Al ₅ O ₁₂ eutectic and off-eutectic composite ceramics

Directionally solidified microstructures of Al ₂ O ₃ -Er ₃ Al ₅ O ₁₂ eutectic and off-eutectic in situ composite ceramics were explored under abrupt-change pulling rate conditions. Corresponding temperature distributions and interface locations were studied. In eutectic composition, fluctuation of eutectic spacing occurred when the pulling rate increased abruptly. A gradually increase or abrupt increase in eutectic spacing was observed when the pulling rate decreased abruptly. In hypoeutectic and hypereutectic compositions, formation of the primary phases were suppressed when the pulling rate increased abruptly from 10 µm/s to 100 µm/s, while primary phases precipitated when the pulling rate decreased abruptly from 100 µm/s to 10 µm/s. The interface altitude decreased after the pulling rate increased abruptly, but increased after the pulling rate decreased abruptly. The liquid composition restriction (around the eutectic composition) at the eutectic interface plays an important role in the suppression of the primary dendrite and coupled eutectic oxides can be obtained in off-eutectic compositions even under higher solidification rate conditions.