Microstructure transformation from irregular eutectic to complex regular eutectic in directionally solidified Al₂O₃/GdAlO₃/ZrO₂ ceramics by laser floating zone melting

Directionally solidified Al₂O₃/GdAlO₃(GAP)/ZrO₂(GdSZ) ternary eutectic ceramics have been grown by laser floating zone melting. The microstructure evolution rule is investigated as a function of growth rate. The as-solidified samples present high density (>99%), smooth surface and high diameter stability free of pores and cracks. Increasing the growth rate from 4 to 100 μm/s, the microstructures undergo a transformation from "Chinese script" irregular eutectic pattern to eutectic colony and to complex regular eutectic patterns (lamellae or rods). The average interphase spacing of the "Chinese script" microstructure presents an inverse square relationship according to λ = 10.46 × ν^-1/2, where the average interphase spacing λ has the dimension of μm and growth rate ν is in μm/s. While the interphase spacing of the complex regular microstructure is finer. The mechanisms of the microstructure evolution and the effects of ZrO₂ phase on the morphology and domain size of microstructure are discussed in detail.