Crystallization behavior and microstructure evolution of (Al,Fe)(2)O-3 synthesized from liquid precursors

Aqueous solutions of Al3+ Fe3+ nitrates were pyrolyzed to produce (Al,Fe)(2)O-3 amorphous solid solutions with up to 50 mol% Fe2O3. Following subsequent heat treatments, all compositions first crystallized to gamma-(Al,Fe)(2)O-3 and those with greater than or equal to 20% Fe2O3 ultimately partitioned into a mixture of Al2O3 and Fe2O3-rich alpha phases as predicted by the equilibrium diagram. The transformation path between the gamma phase and the final microstructure was dependent on composition, Materials with 10%-20% Fe2O3 formed extended alpha solid solutions which partitioned only sluggishly at 1100 degrees C. In contrast, materials with compositions above approximate to 30% Fe2O3 formed an intermediate metastable orthorhombic phase (O) based on the equiatomic AlFeO3 compound. The O-phase also decomposed into a mixture of two alpha phases upon further heating. It appears that the alpha(1) + alpha(2) mixture may evolve directly from gamma for compositions near 30% Fe2O3, but clearly forms from O as the Fe content increases. Resistance to partitioning also reaches a minimum at approximate to 30% Fe2O3. At 900 degrees C, the size of the alpha grains was greater than or similar to 1 mu m for compositions less than or similar to 5% Fe2O3, and decreased to less than or similar to 200 nm for compositions greater than or similar to 10% Fe2O3, reflecting the strong effect of Fe2O3 on grain refinement relative to pure Al2O3. Contrary to literature, the formation of single-phase alpha in compositions less than or similar to 20% Fe2O3 appears to occur without the prior nucleation of Fe2O3-rich ''seeds.'' Partitioned alpha mixtures exhibit orientation relationships suggestive of epitaxial nucleation of one phase on the other. The present results bear on important thermodynamic and kinetic issues related to the synthesis of fine-grained polycrystalline alumina fibers.