The age-hardening characteristics of an AI-6061/AI 2 O 3 metal matrix composite
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INTRODUCTION
THE development of metal matrix composites, whereby either continuous or discontinuous reinforcements are introduced into a metal matrix, represents a well-established method for improving the strength and stiffness of a material. However, a continuous reinforcement is expensive to fabricate and generally leads to markedly anisotropic properties in service. As a result of economic constraints together with the need for more isotropic materials, the high-volume applications for metal matrix composites are usually restricted to the use of discontinuous particulate-reinforced composites fabricated by powder metallurgy or liquid metallurgy procedures. ~] When a particulate reinforcement is introduced into an aluminum alloy matrix, the composite can be age hardened and there may be a significant acceleration in the kinetics of precipitation in comparison with the unreinforced matrix alloy. 12-~3] This acceleration is attributed to a decrease in the incubation time required for the nucleation event and a concomitant increase in the solute diffusivity and, therefore, in the rate of precipitate growth. 17,~~ However, there are also experimental data on discontinuous 6-alumina fiber-reinforced A1-6061 matrix composites showing that the total level of hardening attained via aging is very much reduced in the composites in comparison with the unreinforced A1-6061 matrix due to, it was suggested, a decrease in the JINGTAO WANG, formerly Visiting Scholar, Kyushu University, is Associate Professor, Xian Institute of Metallurgy and Construction Engineering, Xian 710055, People's Republic of China. MINORU FURUKAWA, Associate Professor, is with the Department of Technology, Fukuoka University of Education, Munakata, Fukuoka 811-41, Japan. ZENJI HORITA and MINORU NEMOTO, Associate Professor and Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan. YAN MA and TERENCE G. LANGDON, Research Assistant Professor and Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted September 29, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
quenched-in vacancy concentration in the composites because of the large number of vacancy sinks at the fiber-matrix interfaces. 1141Also, experiments on AI-Cu, A1-Cu-Mg, and A1-Mg-Si composites have shown that the age hardening may be substantially decreased by using a reinforcement of 20 vol pct of A1203 particles with an average size of - 1 /zm, and, in some composites, there is no age hardening but rather there may be age softening. 1.5,161 These various trends are not consistent but, nevertheless, the general observation in most particulatereinforced metal matrix composites that the aging kinetics are enhanced because of particulate reinforcement is usually attributed to an increase in the matrix dislocation density arising from the mismatch in the values of the coefficient of thermal expansion (
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