Processing, microstructure and mechanical properties of directionally-solidified Al2O3–Y3Al5O12–ZrO2 ternary eutectics

Rods of 1–2 mm diameter of Al2O3/Y3Al5O12/ZrO2 in the ternary eutectic composition (65 mol% Al2O3, 19 mol% ZrO2, 16 mol% Y2O3) were grown by the laser-heated floating-zone method at growth rates between 10 and 1000 mm/h. At low growth rates the rods presented a Chinese script microstructure formed by an interpenetrating network of Al2O3 (40%) and Y3Al5O12 (42%) domains of similar size, with smaller cubic Y2O3-stabilized ZrO2 (18%) domains at the Al2O3/Y3Al5O12 interfaces. Compressive residual stresses of 200 MPa were measured in the alumina phase by piezo-spectroscopy and their thermo-elastic origin was validated by self-consistent simulations. The phase spacing and the residual stresses decreased with increasing growth rate and this influenced the hardness and the fracture toughness. The fine and homogeneous microstructure of the ternary eutectic rods led to an outstanding flexure strength of 2.3 GPa at ambient temperature, which was retained up to 1473 K, and then decreased rapidly to 1.2 GPa at 1700 K.