Distribution of Alumina in Aluminum Prediction Based on Thermodynamic and Diffusion Analysis
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recently, the solubility of oxygen in aluminum was thought to be less than 1 atom in 1035 to 1040 atoms. This corresponded to less than 1 atom of oxygen in the entire world supply of aluminum since the metal was first extracted.[1] In contrast, it was known that aluminum was full of aluminum oxide, alumina. This apparent paradox was taken to indicate that the oxide in the metal could not have occurred as a result of a chemical reaction between oxygen in solution and the metal but must have been incorporated mechanically. Broadly, this seems to be true; the surface film of oxide on the liquid metal, resulting from contact with air, is entrained into the bulk metal during casting and is in the form of films (actually, double films called bifilms), which are extremely detrimental to properties.[1] Similar incorporated oxide films are to be expected in other consolidation processes such as power metallurgy and spray forming.[2] As will be shown in the course of this article, the solubility of O in liquid Al is low in the region of 107 but is vastly greater than the values of 1035 to 1040 mentioned earlier. This massive revision in solubility prompts a re-examination of existing assumptions. It seems reasonable to speculate what form this newly
YOUN-BAE KANG is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 37673, Korea. Contact e-mail: [email protected] JOHN CAMPBELL is with the Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK. Manuscript submitted November 10, 2016. Article published online March 28, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
discovered source of alumina might take and whether it might cause current alumina bifilm theory to be revised. This communication provides a brief examination of this question. There have been few experimental attempts to determine O content in liquid Al saturated by alumina,[3] but the results reported so far have not been reliable because of unclear experimental conditions.[4] This is partly due to very low O content in liquid Al, which often lies below the limits of analysis, or it may be attributed to the fact that suspended alumina particles cause inaccuracy in the analysis. Recently, Paek et al.[5] attempted to determine the solubility of O in liquid Al (in equilibrium with alumina) in the temperature range 1673 K to 1873 K (1400 C to 1600 C). Due to the inherent difficulty in accurate analysis of soluble O (free from suspended alumina particles), Paek et al. employed a CaO-Al2O3 flux on top of the liquid Al in an effort to remove the alumina particles floating up to the surface. Uncertainty of the analysis using the inert gas fusion–infrared absorptiometry technique was found to be significantly improved when using the flux. The experimentally determined O solubility in the temperature range of 1673 K to 1873 K (1400 C to 1600 C) was approximately 60 to 130 mass ppm. As will be evident later in this account, this level of oxygen is two orders of magnitude h
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