Accession Number : ADA290298

Title :   Crystallization Behavior and Microstructure Evolution of (Al,Fe)2O3 Synthesized from Liquid Precursors.

Corporate Author : CALIFORNIA UNIV SANTA BARBARA DEPT OF MATERIALS

Personal Author(s) : Polli, Andrew D. ; Lange, Fred F. ; Levi, Carlos G.

PDF Url : ADA290298

Report Date : DEC 1994

Pagination or Media Count : 37

Abstract : Aqueous solutions of Al3+ and Fe3+ nitrates were used to produce (Al,Fe)2O3 amorphous solid solutions with up to 50 mole% Fe2O3. Following subsequent heat treatments, all compositions first crystallized to gamma(Al,Fe)2O3 and those with greater than 20% Fe2O3 ultimately partitioned into a mixture of Al2O3- and Fe2O3-rich alpha phase as predicted by the equilibrium diagram. There were, however, several possible transformation paths between the gamma phase and the final microstructure. Compositions with 10 and 20% Fe2O3 both formed an extended a solid solution which partitioned only sluggishly at 1100 deg C. In contrast, powders with compositions above 30% Fe2O3 formed an intermediate meta table orthorhombic phase (O) based on the equiatomic AlFeO3 compound, but also decomposed eventually into the mixture of two alpha phases. It appears that the alpha 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 approx. 30%Fe2O3. The alpha grain size approx,1 micrometer for compositions less than 5%Fe2O3, and decreases to less than 200 nm for compositions greater than 10%Fe2O3 synthesized at 900 deg C, reflecting the strong effect of Fe2O3 on grain refinement relative to pure Al2O3. The formation of single phase alpha in compositions less than 20% Fe2O3 appears to occur without the prior nucleation of Fe2O3-rich 'seeds', as suggested in the literature, whereas the partitioned alpha mixtures exhibit orientation relationships suggestive of epitaxial nucleation of one phase on the other. An enhanced understanding of the thermodynamics and transformation kinetics of this system emerges from the analysis of the present microstructures. (jg)

Descriptors :   *LIQUIDS, *PRECURSORS, *CRYSTALLIZATION, *IRON, *OXIDES, *ALUMINUM, *EVOLUTION(GENERAL), MICROSTRUCTURE, THERMODYNAMICS, POWDERS, COMPOSITE MATERIALS, PATHS, ORIENTATION(DIRECTION), EPITAXIAL GROWTH, METAL MATRIX COMPOSITES, EQUILIBRIUM(GENERAL), NUCLEATION, MIXTURES, HEAT TREATMENT, CERAMIC MATERIALS, GAMMA RAYS, KINETICS, REFINING, SOLID SOLUTIONS, GRAIN STRUCTURES(METALLURGY), DIAGRAMS, TRANSFORMATIONS.

Subject Categories : Inorganic Chemistry
      Physical Chemistry
      Laminates and Composite Materials
      Crystallography

Distribution Statement : APPROVED FOR PUBLIC RELEASE