Insight into faceted-nonfaceted transition of directionally solidified eutectic ceramic composites by laser floating zone melting and infrared imaging

Faceted-nonfaceted transition was found during directional solidification of an Al₂O₃/Y₃Al₅O₁₂(YAG)/ZrO₂ eutectic ceramic composite at the pulling rate over 260 μm/s by laser floating zone melting. This transition, for the first time, optimized the colony structure of faceted eutectic composite materials, resulting in a refined and homogeneous microstructure consisting of both rod and lamella. Infrared imaging was employed to uncover temperature field evolution of the melt during the transition relying on an interesting measurement calibration strategy. In addition to an in-depth discussion about this evolution, we revealed that the transition was triggered by a critical undercooling at the solid-liquid interface. A 3D model of this novel eutectic structure was established, by which the mechanism of structural optimization achieved by the transition was discussed. We supposed that the growth orientation with the smallest index could firstly occur faceted-nonfaceted transition under a certain undercooling degree, and subsequently predominated the growth of faceted eutectic composite, which contributed to the optimization of faceted eutectic colony structure.