Phase Separation in Alumina-Chromia
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Phase Separation in Alumina-Chromia M. S. McIntosh, T. H. Sanders, Jr., and J. M. Hampikian School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0245 Abstract The alumina-chromia system shows complete mutual solubility and is represented by an isomorphous phase diagram. However, the alumina-chromia system exhibits an asymmetric miscibility gap under 1300oC. Using existing data from the literature, the alumina-chromia system was assessed using thermodynamic modeling by Kim and Sanders [1]. Regular and subregular solution models for the liquid and solid phases were used to define the phase boundaries for the miscibility gap in this system. Using this thermodynamic representation of the miscibility gap to select temperatures of interest, 75 mole percent Al2O3 samples were synthesized via combustion of powders, followed by pressing into pellets and heat-treated for various times and temperatures. Both X-ray and TEM analysis showed evidence of spinodal decomposition after heat-treatment. X-ray analysis showed that decreasing the heat-treatment temperature increases the compositional difference between the phases present. The experimentally observed microstructures exhibit lamella-like structures that vary in spacing from 8nm to 3nm as the heat-treatment temperature varies from 400oC to 800oC. Introduction Since many high temperature alloys contain significant amounts of aluminum and chromium, it is important to fully understand the alumina-chromia phase diagram. Alumina and chromia are both protective oxides, in that diffusion through their oxide scales is slow and they generally have excellent adherence to their respective metals. Both alumina and chromia have the corundum crystal structure and share a common oxygen ion. The Al3+ and Cr3+ ions are also chemically similar with a size difference less that 15% according to the Shannon Prewitt scale for six-fold coordination [2]. The isomorphous nature of the Al2O3-Cr2O3 phase diagram was first put forth by Bunting [3]. Later experiments done at different pressures on this system showed varying boundaries of the miscibility gap [4-8]. Kaufman and Nesor [9] calculated the liquidus and solidus curves for this system but their interaction parameter was negative at low temperatures so no miscibility gap was generated. Degterov and Pelton [10] also calculated the phase diagram for this system, using regular and subregular solution models for the liquid and solid phases respectively. Their miscibility gap was wide in comparison to the data generated by Roy and Barks [7] though their chemical spinodal was more in line with that data. Kim and Sanders [2] also did their calculation using the regular and subregular solution models for the liquid and solid phases respectively, but used data from the more recent experiments done by Sitte [8] and from solidus and liquidus data [3, 11]. They
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