Microstructure and orientation of quasi-unidirectionally solidified A1_{2O_{3/YAG eutectic composites

Unidirectionally solidified A12O3/YAG (Y3A15O12) eutectic composites prepared by the Bridgman method have a preferable orientation relationship with equilibrium boundaries between A12O3 and YAG phases. The microstructure of a quasi-unidirectionally solidified A12O3/YAG eutectic composite was investigated to reveal the formation of the equilibrium boundary at early stages of the solidification. At the first stages of solidification, no specific orientation of A12O3 and YAG phases along the direction of the thermal gradient existed. During progress of the solidification, A12O3 phase gradually orientated to a certain crystallographical direction along the thermal gradient, and then YAG phase orientated to a certain crystallographical direction with respect to the A12O3 phase. In the next stage of the solidification, the eutectic lamellar structure is arranged along the thermal gradient, and the characteristic crystallographic orientation relationships between A12O3 and YAG along the thermal gradient are identified.