The impact of the composition and solidification rate on the microstructure and the crystallographic orientations of Al₂O₃-YAG-ZrO₂ eutectic solidified by the micro-pulling down technique

Directional solidification experiments have been carried out in eutectic and off eutectic composition in Al₂O₃-YAG-ZrO₂ system using the micro-pulling down (μ-PD) technique. The formation of different microstructures of stable phases, over a range of velocities studied, is discussed. During the solidification process, coupled growth microsctrucure can occur at off eutectic composition. Colonies and dendrites were analysed as a function of the composition and velocity. Increasing the solidification rate resulted in a smaller grain size and instability of the crystallization interface with respect to ZrO₂ segregation at the solidification front. Increasing ZrO₂ in the eutectic composition showed enhanced tendency for colony and dendrite formation. When the velocity was extended to the rapid solidification regime (>1.75 mm min^-1), the geometrical pattern microstructure was found to grow and a change in the growth orientation of Al₂O₃ corresponding to c-axis (0 0 01) was observed. Whatever the pulling rate, during the eutectic solidification the Al₂O₃ and ZrO₂:Y phases' growth competition was observed, and 〈100〉 YAG orientation was grown along the pulling direction.