Methodology to design the interfaces in SiC/Al composites
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I. INTRODUCTION
CARBIDES, due to their high strength, modulus, and low thermal expansion coefficient, have been used as reinforcements for various metal matrix composites (MMCs) including cermets. However, a frequent problem encountered in fabricating Al alloy-based composites reinforced with carbides, such as SiC,[1–6] B4C,[7,8,9] as well as graphite,[10] can be the formation of Al4C3 at the carbide/Al matrix interface as a result of the interfacial reaction between Al matrices and carbides. Al4C3 is known to be very brittle and unstable, resulting in the degradation of mechanical properties of composites.[11] In addition, due to the hydrophilic nature of Al4C3,[12,13] composites containing Al4C3 could be very sensitive to some corrosive environments.[14,15] Therefore, formation of Al4C3 during composite fabrication has to be avoided. Extensive studies to develop methods for suppressing the formation of Al4C3 have been carried out in recent years. Examples of these methods are the Si addition into the matrix,[1–3,16] coating of SiC,[15,17,18] and passive oxidation of SiC.[14,16,19] To date, Si addition has been the most successful method for suppressing the formation of Al4C3 during fabricating SiC/Al composites and is currently employed to produce commercial SiC/Al composites and cermets via stir casting and pressureless infiltration. One drawback of this method is that the matrix has to contain a large amount of Si. The normal Si contents within SiC/Al composites produced by this method range from 10 to 18 pct depending on processes.[6,20,21] Considering that both the strength and ductility of aluminum alloys usually decrease with increasing Si contents, SiC/Al composites containing high Si may not be suitable for some structural applications where high toughness, as well as strength, is required. Coating of SiC with metallic materials[15,17,18] has been satisfactory to some extent, both for preventing the detrimental interfacial reaction and enhancing the wetting characteristics. Although interfacial characteristics can be tailored JAE-CHUL LEE and HO-IN LEE, Principal Research Scientists, Division of Metals, JAE-PYOUNG AHN, Research Scientist, Division of Ceramics, and JAE-HYEOK SHIM, Research Scientist, Division of Metals, are with the Korea Institute of Science and Technology (KIST), Seoul, 136-791 Korea. ZHONGLIANG SHI, Associate Professor, is with the State Key Lab for Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai, 200030 China. Manuscript submitted March 7, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
easily using this method, the high cost involved in the process acts as a major obstacle for economic production of MMCs. In addition, this process cannot guarantee the structural stability of the interfaces at elevated temperature for a prolonged time since the coating materials usually react with the matrix alloys, causing unintended matrix composition. Another alternative to avoid the formation of Al4C3 is the passive oxidation of SiC combined with matrix modification. When SiC is
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