Microstructural morphology and irregular eutectic growth of directionally solidified Al₂O₃/YAG eutectic in situ composite

Directionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) eutectic in situ composite was fabricated by the laser zone remelting technique. The eutectic morphology, phase composition, interface structure, microstructure evolution and phase precipitation rule were analyzed by SEM, XRD, EDS and TEM. The irregular microstructure morphology was quantitatively characterized by fractal dimension, and the irregular growth mechanism of the oxide eutectic was discussed. The results show that the composite only consists of homogeneously distributed Al₂O₃ and YAG phases with well matched interfaces, which interweave each other and grow coupledly, showing a 'Chinese script' structure. The YAG phase is the primary phase during solidification. The eutectic spacing is highly refined with increasing the laser scanning rate and the minimal spacing is down to 0.2 μm. At low rate, the eutectic tends to show typical lamellar irregular eutectic structure and has evident fractal characteristic, whereas, when the rate reaches a high value of 2000 μm/s, the cellular and dendritic structures appear and the fractal characteristic is weaken. The faceted/faceted eutectic growth derived from the large kinetic undercooling of laser rapid solidification and high entropies of fusion of eutectic phases is the most factor to form the complexly irregular eutectic morphology.