In-situ formation of SiC-reinforced Al-Si alloy composites using methane gas mixtures
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INTRODUCTION
METAL-MATRIX composites (MMCs), especially lightweight alloy-matrix composites, have had many applications in recent decades, because of their superior properties to traditional materials. Discontinuously reinforced aluminum alloy composites (DRACs), a group of very important MMCs, are rapidly emerging as new materials for aerospace and automotive industries due to their relatively low cost compared with continuously reinforced composites.[1–6] For example, since late 1980s, DRACs have been used in automotive engines, brake pads, and other parts. Because 12 to 17 pct of the weight of an automobile is made of cast iron, there has been the potential to substitute DRACs for cast iron, in order to reduce automotive weight, fuel consumption, and pollutant emissions. Industrial applications depend on the economic acceptance and technological quality of DRACs. Their applications for wearing parts usually require large, reinforced particles, and those for structural parts require very fine reinforced particles. Table I shows the yield-strength dependence on reinforced particle size, which indicates that a smaller particle size, e.g., less than 10 m, can give a better yield strength. Usually, the aluminum MMC with smaller reinforced particles is much more expensive than that with larger reinforced particles. In order to achieve significant improvement in mechanical properties, reinforced particles should have an average particle size of less than 10 m. A BANQIU WU, formerly Research Associate, Department of Metallurgical and Materials Engineering, The University of Alabama, is Senior Development Engineer, Photronics, Inc., Austin, TX 78728. Contact e-mail: [email protected] RAMANA G. REDDY, ACIPCO Professor, is with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487-0202. Manuscript submitted October 24, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
higher strength generally requires a higher volume fraction of these expensive small particles. Therefore, industrial application of structural DRACs is strongly dependent on significant improvements in particle production technology or in-situ formation technology. The interface between the reinforcement and matrix is one of the most important features in a composite system. It affects the mechanical properties of the composite through mechanisms such as debonding, stress damping, crack deviation, and grain-boundary pinning.[7] During its service, a composite material may experience phase transformation, changes in equilibrium composition, and chemical interaction between the matrix and the reinforcement, which would result in thermodynamic unstability at the interface. For the high-temperature MMCs such as engine materials, interfacial stability plays an important role in the properties of composite materials. It is considered that superior properties can be obtained by an in-situ formed composite with more-thermodynamically stable reinforcements by nucleation and growth from the parent matrix p
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