Effect of yttria doping on the microstructure and mechanical properties of Al2O3–FeAl2O4 nanocomposites developed via solid state precipitation

We have recently reported the production of Al2O3-matrix nanocomposites via solid state precipitation of nanosized FeAl2O4 particles within the matrix grains during aging of Al2O3–10 wt.% Fe2O3 solid solutions in a reducing atmosphere (N2 + 4% H2). In addition to these nanoparticles, however, coarse micron-sized FeAl2O4 particles were present along the matrix grain boundaries. In the present work, we show that the addition of ∼250 ppm yttria to the solid solutions suppressed the development of these intergranular particles, reducing their size by a factor of ∼2 with optimum aging. A fracture toughness improvement by 45% and flexural strength improvement by 50% with respect to monolithic Al2O3 were recorded with the yttria-containing nanocomposite developed by aging for 20 h at 1450 °C. Aging also improved the hardness with respect to the solid solution. The change in fracture mode in the presence of the nanosized intragranular particles was believed to be the major contributing factor towards the improvement in toughness and therefore the strength. The higher strengths obtainable in the presence of yttria were attributed to the reduction of intergranular precipitate size relative to yttria-free nanocomposites.