Creep strengthening in a discontinuous SiC-Al composite
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I.
INTRODUCTION
METALmatrix composites (MMCs), in which a rigid ceramic reinforcement is embedded in a ductile metal or alloy matrix, offer many advantages in applications where high strength, high modulus, and good conductivity are desirable. Of several available MMC systems, particulate- or whisker-reinforced aluminum alloys are of interest partly because they can be subjected to conventional metal working processes and partly because they, unlike continuously reinforced MMCs, exhibit near-isotropic properties. In recent years, the creep characteristics of discontinuous silicon carbide- (SIC-) reinforced aluminum alloys have been the subject of several investigations t~-81 that have aimed at (a) assessing the potential of these composites for use as high-temperature materials and (b) identifying the nature and origin of deformation mechanisms that may control creep in such materials. As a result of these investigations, t~-8j it has been shown that the creep behavior of SiC-reinforced A1 alloys is unusual in two ways. First, the stress dependence of the KYUNG-TAE PARK, formerly Research Associate, Materials Section, Department of Mechanical and Aerospace Engineering, University of California, is with the Research Institute of Industrial Science and Technology (RIST), Pohang, 796-600, Korea. FARGHALLI A. MOHAMED, Professor, is with Materials Science & Engineering, the Department of Chemical and Biochemical Engineering, University of California, Irvine, CA 92717. This article is based on a presentation made in the symposium entitled "Creep and Fatigue in Metal Matrix Composites" at the 1994 TMS/ASM Spring meeting, held February 28-March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASMMSD Composite Materials Committee.
METALLURGICAL AND MATERIALS TRANSACTIONS A
creep rate, as expressed by the value of the stress exponent n, is high and, in some cases, t/'5] variable; n varies between 7 to 25. Second, the temperature dependence of the steady-state creep rate, which is measured by the creep activation energy, Q, is much larger than that for self-diffusion in aluminum; Q varies from 200 kJ/mol to 500 kJ/mol, t2,5~In addition, considerable research efforts t~-~~ have been made to develop semiempirical approaches and analytical treatments that attempt to provide a description of creep strengthening in SiC-A1 composites. Table I summarizes these approaches and treatments. Despite the significance of the preceding results, approaches, and treatments (Table I), the role played by the SiC particulate- or whisker-reinforcement phase during the creep of discontinuous SiC-A1 composites is not well-understood. First, the creep behavior of these composites resembles that of dispersion-strengthened (DS) alloys t~1'~21 with respect to the high-stress exponent, n, and the high apparent activation energy, Qa- This resemblance suggests that the creep behavior of these composites, like that of DS alloys, may be explained in terms of the existence of a threshold stress for creep, r0. In this case, t
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