Effects of composition and solidification rate on growth striations in laser floating zone melted Al₂O₃/GdAlO₃ eutectic ceramics

Growth striations with a period of 283 seconds are observed in Al₂O₃/GdAlO₃ (GAP) directionally solidified eutectic ceramics prepared by laser floating zone melting (LFZM) process. The effects of composition and solidification rate on the microstructure of growth striations are investigated, aiming to reveal the formation mechanism of growth striations. The results show that a number of coarse primary Al₂O₃ phases are produced preferentially at growth striations in hypoeutectic ceramics and primary GAP phases segregate in hypereutectic ceramics. Simultaneously, homogeneous “Chinese script” eutectic microstructures are formed between two adjacent growth striations in Al₂O₃/GAP eutectic ceramics, affected very few by the composition segregations at growth striations. Moreover, it is also found that the interval of periodic growth striations has a liner relationship with solidification rate, but has no relevance with composition. The width of growth striations, however, mainly depends on composition that the larger the composition deviates from eutectic composition, the coarser the phases precipitate at growth striations. Based on the detailed examinations of solid-liquid interface right at growth striations, the growth striations induced by Marangoni convection are strongly affected by constitutional undercooling in Al₂O₃/GAP off-eutectic ceramics.