Microstructural stability and orientation relationships of directionally solidified Al2O3-Er3Al5O12-ZrO2 eutectic ceramics up to 1600 °C

Al2O3-Er3Al5O12-ZrO2 eutectic ceramic rods were directionally solidified using the laser floating zone method at different growth rates, 25, 350 and 1200 mm/h. The microstructure obtained, in terms of both morphology and phase size, was strongly dependent on the growth rate. However, electron backscatter diffraction experiments showed that the growth directions were the same for all the processing rates, [0 0 0 1]Al2O3//[1 0 0]EAG//[1 0 0]ZrO2[0 0 0 1]Al2O3//[1 0 0]EAG//[1 0 0]ZrO2. The microstructural stability was investigated up to 1600 °C as a function of the growth rate. Ceramics with the largest phase size presented high stability, their microstructure remaining substantially unchanged at the highest annealing temperature for 100 h. Eutectics processed at higher growth rates and with a finer microstructure showed coarsening after heat treatments. The sample grown at 350 mm/h coarsened at 1450 °C whereas the eutectic solidified at 1200 mm/h thickened at 1400 °C. The growth directions remain unaffected for all growth rates. The mechanisms of microstructural coarsening were investigated.