Mechanical behavior of cast particulate SiC/AI (A356) metal matrix composites
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INTRODUCTION
D I S C O N T I N U O U S fiber (whisker or particulate)reinforced metal matrix composites (MMCs) have attracted more and more attention from both academic institutions and industrial sectors in recent years. In addition to their superior mechanical properties to conventional metals and alloys, these types of MMC materials can now be produced economically by conventional metallurgical techniques, such as direct-casting, [11 and they are usually machinable after fabrication. Furthermore, they can be isotropic in mechanical properties, whereas continuous fiber-reinforced MMCs are usually unisotropic. As for continuous fiber-reinforced composites, the rule of mixture is usually employed as the simplest, yet reasonably accurate, method to estimate the stiffness of the composites.t2] However, in the case of discontinuous fiber-reinforced MMCs, the estimation of the composite stiffness is more complicated and the rule of mixture is usually not applicable. Unlike in the continuous fiber MMCs, the metal matrix in discontinuous reinforcement MMCs is not only the load transferer, but also the load bearer. In addition to the high stiffness and the volume fraction, the distribution of the short fibers or particulates, their orientations, and their aspect ratios are all extremely important factors in determining the properties of the composites. In short, the strengthening mechanisms should no longer be the same in the discontinuous reinforcement MMCs. Hence, there have been numerous articles discussing the strengthening mechanisms in this type of material, and most of them are concerned with studying the increase of the elastic constant (stiffness)
ZHIRUI WANG, Assistant Professor, and RUBY J. ZHANG, Research Assistant, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 1A4. Manuscript submitted September 10, 1990. METALLURGICAL TRANSACTIONS A
and yield strength. A typical example is Arsenault and co-workers work in which the increased strength observed in SiC (short fiber)/A1 composites was accounted for by a high dislocation density in the aluminum matrix. This high dislocation density was produced by the substantial difference in the coefficient of thermal expansion (CTE) between SiC and and the A1 matrix.J3.4] Christman and Suresh tS] also observed the increase in dislocation density due to the addition of SiC reinforcement in the aluminum matrix. However, they pointed out that whether the anomalously high strength of the composite was due to the increased dislocation density was not yet clear. Using a different approach, Takao and Taya [6] have recently studied the effect of the nonuniformity of the fiber aspect ratio and found that the use of the mean value of the aspect ratio (l/d) for prediction of the stiffness can be justified provided that the distribution of the fiber orientation angle is not widely scattered. Apparently, more work is needed to understand the strengthening mechanisms in this type of material. Furthermore, it should be poin
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