Nucleation and growth of Al 2 O 3 /metal composites by oxidation of aluminum alloys
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V. Jayaram Assistant Professor, Indian Institute of Science, Bangalore, India
K. C. Vlach Assistant Research Engineer, University of California, Santa Barbara, California 93106
C. G. Levi Associate Professor of Materials and Mechanical Engineering, University of California, Santa Barbara, California 93106
R. Mehrabian President, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 (Received 25 May 1989; accepted 1 May 1991)
The nucleation and growth mechanisms during high temperature oxidation of liquid A l - 3 % Mg and A l - 3 % M g - 3 % Si alloys were studied with the aim of enhancing our understanding of a new composite fabrication process. The typical oxidation sequence consists of an initial event of rapid but brief oxidation, followed by an incubation period of limited oxide growth after which bulk A12O3/A1 composite forms. A duplex oxide layer, MgO (upper) and MgAl 2 O 4 (lower), forms on the alloy surface during initial oxidation and incubation. The spinel layer remains next to the liquid alloy during bulk oxide growth and is the eventual repository for most of the magnesium in the original alloy. Metal microchannels developed during incubation continuously supply alloy through the composite to the reaction interface. During the growth process, a layered structure exists at the upper extremity of the composite, consisting of MgO at the top surface, MgAl 2 O 4 (probably discontinuous), Al alloy, and finally the bulk A12O3 composite containing microchannels of the alloy. The bulk oxide growth mechanism appears to involve continuous formation and dissolution of the Mg-rich oxides at the surface, diffusion of oxygen through the underlying liquid metal, and epitaxial growth of A12O3 on the existing composite body. The roles of Mg and Si in the composite growth process are discussed. I. INTRODUCTION The interaction of gases with liquid metals has long been a subject of interest in the understanding and control of casting defects. In general, these interactions are regarded as undesirable except in metal refining processes where impurities are eliminated by selective oxidation in the molten state. Typical examples of the latter include the oxidation of carbon during steelmaking and the purification of lead by removing impurities, e.g., Sn, Bi, and Sb, as drosses. However, one is usually concerned in these cases with minimizing the loss of the base metal by reaction with the oxidizer, and the temperature is consequently limited to modest superheating above the melting point. In contrast, a new ceramic/metal composite formation process (known commercially as Directed Melt Oxidation or DIMOX and developed by Lanxide Corporation of Newark, DE) is specifically 1964 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 9, Sep 1991 Downloaded: 03 Apr 2015
designed to enhance and exploit the oxidation of the solvent at temperatures significantly above the liquidus of the alloy.1'2 In the new compositing process, a ceramic preform (particulate or fiber) is continuously infiltrated by a molten alloy that un
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