The interactive role of inclusions and SiC reinforcement on the high-cycle fatigue resistance of particle reinforced met
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I. INTRODUCTION
FOR years, one of the primary means of improving the fatigue resistance of aluminum alloys and other commercial metals has been the elimination or minimization of hard second-phase inclusions.[1–4] In an apparent contradiction, the advent of metal matrix composites (MMCs), specifically, particle reinforced aluminum, has been affected, in part, by the improvements in fatigue strength brought about by the incorporation of hard second-phase reinforcement particles (e.g., SiC). The incorporation of a significant volume fraction of ceramic particles in aluminum alloys has been shown to improve high-cycle fatigue resistance.[5–9] This has been demonstrated for a wide variety of reinforcement types and aluminum alloy matrices. An example of this is shown in Figure 1, a plot of high-cycle fatigue resistance vs reinforcement volume fraction in a 2080/SiC/30p composite.[5] The improvement in fatigue resistance can be explained in terms of load transfer to the stiffer ceramic reinforcement that reduces the stresses in the matrix. Researchers have found that fatigue crack initiation sites in these materials are generally at the surface of the specimen.[5,6,9] Several studies in particle-reinforced aluminum alloys have also shown that fatigue crack initiation may also occur at particle clusters or abnormally large particles.[10,11,12] Li and Ellyin[11] found that in an Al2O3 particle-reinforced Al 6061 alloy, damage in the form of matrix cracking was localized and triggered at large particles, particle clusters, N. CHAWLA, formerly Research Fellow, Department of Materials Science and Engineering, University of Michigan, is Assistant Professor, Department of Chemical, Bio, and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006. C. ANDES is former Research Fellow, Department of Materials Science and Engineering, University of Michigan. L.C. DAVIS, Manager, Physics Department, Ford Research Laboratory, and J.E. ALLISON, Senior Technical Staff Specialist, Scientific Research Laboratory, are with Ford Motor Co., Dearborn, MI 48121-2053. J.W. JONES, Associate Dean for Undergraduate Education, is with the College of Engineering, University of Michigan, Ann Arbor, MI 48109. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
or large particles with sharp edges surrounded by large reinforcement-free areas. This localized damage leads to initiation and extension of short fatigue cracks. This implies that the fatigue strength of the matrix may not be the only factor controlling fatigue resistance of the composite. In general, fatigue initiation at defects is often associated with premature fatigue crack initiation, which would seem to indicate that the increase in high-cycle fatigue resistance in MMCs may be related to improvement in resistance to fatigue
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