Damping behavior of discontinuously reinforced ai alloy metal-matrix composites
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I.
INTRODUCTION
STRUCTURAL materials that exhibit high damping characteristics are useful in the passive attenuation of noise and vibration in structures. This has driven investigators toward the development of materials possessing high damping capacity, in combination with high stiffness and low density. Due to the demonstrated ability of A1 alloys to satisfy the latter two requirements, they have emerged as prime candidates to serve as the basis of such a material.t~] Aluminum alloys, however, are essentially low damping materials and, hence, are limited in their application and performance in a dynamic environment. Accordingly, investigators have attempted to improve the damping capacity of A1 alloys not only through the addition of alloying elements[21 and heat treatment [3,4] but also through the use of metal-matrix composite (MMC) technology. This class of materials, which incorporates a nonequilibrium reinforcing phase into the matrix alloy, may exhibit high damping through the addition of reinforcing phases that possess high intrinsic damping or that dramatically modify the matrix microstructure in such a way as to increase damping. Some of the work on MMCs has proceeded in the direction of continuous fiber-reinforced A1 alloys. Fiber-reinforced MMCs have been extensively investigated for decades as a result of the attractive physical and mechanical properties, such as high specific modulus, strength, fracture toughness, and thermal stability, that can be obtained
E.J. LAVERNIA, Associate Professor, R.J. PEREZ, Graduate Student, and J. ZHANG, Assistant Specialist, are with the Department of Chemical Engineering and Materials Science, University of California-Irvine, Irvine, CA 92717. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee. METALLURGICALAND MATERIALSTRANSACTIONS A
through the use of continuous reinforcements.[ s,61 Accordingly, the damping behavior and mechanisms of continuous graphite fiber-reinforced MMCs have also received much attention.t7-n] Such reinforcements, however, have been shown to provide limited or slight improvements in the resultant damping capacity of MMCs. t12,131Moreover, extensive industrial applications of these MMCs have been hindered by the high manufacturing costs that are associated with available reinforcement fibers. More recently, discontinuously reinforced MMCs have been investigated by a number of researchers as a means of efficiently increasing damping capacityY2-151 In applications not requiring extreme loading or thermal conditions, such as in automotive components, discontinuously reinforced MMCs have been shown to offer substantial improvements in mechanical properties, v,s,6,~6,171The family of discontinuous reinforcements used in MMCs includes short fibers, whiskers, and particulates. SiC whisker (SiCk) reinforced MMCs have shown attractive combinations of strength, fracture toughness, and thermal
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