Effect of particle-size distribution on the properties of high-volume-fraction SiC p -Al-based composites
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NTRODUCTION silicon carbide–reinforced metalmatrix composites (MMCs) have a high potential for advanced engineered materials that have been developed and recently qualified for use in aerospace structures, inertial guidance systems,[1] and lightweight optical assemblies.[2,3,4] Such materials generally exhibit greater a Young’s modulus, higher strengths, and better resistance to creep than the unreinforced alloy; they can also be tailored to match the coefficients of thermal expansion (CTEs) of other materials, including stainless steel, silicon wafers, aluminaum, etc.[5,6,7] Many different technologies for production of SiC/Al composites have emerged over the years. A few of these evolved from methods originally developed for structural applications wherein the amount of SiC particulate was restricted to volume fractions of 0.15 to 0.25, while some are more suited for the higher particulate volume fractions required for optimum CTE matching to optical assembly materials. The powder-metallurgy (PM) process is based on well-established technology[8,9] whereby the SiC particulates are blended with aluminum powder and then consolidated into a fully dense product by vacuum hot pressing. A limitation of this approach is that the maximum volume fraction of SiC particulates that can be incorporated is 0.5 to 0.55, thus placing a limit on the CTE reduction achievable. Some other drawbacks of the process include relatively high costs due to the variety of processing steps involved in the production of a fully dense product, as well as the need for subsequent machining of the final component from the forged or extruded stock. Casting processes, which include pressureless infiltration,[10] gas-pressure infiltration,[11,12,13] and squeeze casting,[14,15] have been utilized to achieve a high volume fraction of SiC reinforcement levels. A primary drawback of pressureless infiltration is believed to be the need to use specific Al-Mg alloy compositions and a nitrogen-protecting furnace during heating, in order to allow the molten alloy to effectively wet and, thus, infiltrate the SiC. Additionally, since
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CHANG-YOU CHEN, Graduate Postdoctoral Candidate Student, and CHUEN-GUANG CHAO, Professor, are with the Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30049, Republic of China. Manuscript submitted July 26, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
the infiltration relies solely on capillary action, it is possible that certain locations of the preform will be incompletely infiltrated. Squeeze casting is a method that applies pressure to push molten alloy into preforms made of ceramic particle. Preforms can be manufactured by a number of known ceramic processing routes, including injection molding, dry pressing, and slip casting. Several studies have presented information on MMCs produced by the squeeze-casting process.[14,15] Most reports investigated the effect of reinforcements that have monosized particles on the properties of MMCs. Lewis and Goldman[16] argue
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