An investigation of strain hardening and creep in an AI-6061/AI 2 O 3 metal matrix composite
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INTRODUCTION
W H E N a particulate reinforcement is used in the fabrication of a metal matrix composite, there is usually a significant increase in the dislocation density within the matrix of the composite due to differences in the values of the coefficients of thermal expansion between the matrix and the reinforcement. [~-5] In practice, there is experimental evidence that the geometry of the reinforcement may play an important role in generating dislocations due to the thermal expansion mismatch. 16] There have been numerous reports on the role of particulate reinforcements in various aluminum alloy matrices. To date, much of the experimental evidence suggests that there may be an acceleration in age hardening in the composites by comparison with the unreinforced aluminum matrix alloys t2,3,7 141 due, it has been suggested, 13'~21to a decrease in the incubation time for the nucleation event and a concomitant increase in the solute diffusivity and, thus, in the rate of growth of the precipitates. Although the reports of an acceleration in age hardening appear to be well documented, there are also reports of conflicting trends. For example, Friend and Luxton I]5) observed a reduction in the level of hardening achieved via aging in an A1-6061 matrix composite reinforced with discontinuous 8-alumina fibers, and Ikeno et al.IJ6,171 reported an absence of age hardening and even some age softening in a series of A1-Cu, AI-Cu-Mg, and MINORU FURUKAWA, Associate Professor, is with the Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-41, Japan. JINGTAO WANG, formerly Visiting Scholar, Kyushu University, is Associate Professor, Department of Metallurgy, Xian Institute of Metallurgy and Construction Engineering, Xian 710055, People's Republic of China. ZENJI HORITA, Associate Professor, and MINORU NEMOTO, Professor. are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan. YAN MA, Research Assistant Professor, and TERENCE G. LANGDON, Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted February 21, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
A1-Mg-Si alloys reinforced with 20 vol pct of A1203 particles, having an average size of - 1 /_tm. An earlier report described a series of experiments conducted on an A1-6061 metal matrix composite, reinforced with - 2 0 vol pct of alumina-based microspheres, to investigate the characteristics of the age-hardening behavior, t18} In these experiments, it was confirmed that the presence of the microspheres substantially increases the dislocation density within the matrix of the composite in the as-quenched state by comparison with the unreinforced A1-6061 matrix alloy. This increase in dislocation density is of the order of two orders of magnitude, from --2.4 x 101] m -2 in the as-quenched unreinforced matrix alloy to - 1 . 5 x 1013 m 2 in the as-quenched composite. N
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