The effect of matrix microstructure on cyclic response and fatigue behavior of particle-reinforced 2219 aluminum: Part I
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I.
INTRODUCTION
PARTICLE-REINFORCED aluminum alloys are being considered for applications in the automotive industryY] These materials are candidates for components such as brake rotors, cylinder liners, connecting rods, and pistons. To assure the structural integrity of these components, an understanding of the mechanical behavior, in particular the fatigue behavior, of these materials at the typical service temperatures is required. Research on the isothermal elevated temperature fatigue behavior of particle-reinforced aluminum alloys has been rather limited. At room temperattire, the addition of reinforcing particles generally improves the stress-controlled fatigue life of aluminum alloys,t2] At elevated temperatures (200 ~ however, the fatigue life of 2080/SiC/20p-T4 has been reported to be inferior to that of unreinforced 2080-T4.I31 In addition, cyclic softening has been observed in both reinforced and unreinforced 2080 isothermally fatigued at 200 ~ and 300 oC.[3]
Since matrix effects are important in particle-reinforced alloys, a knowledge of the elevated temperature fatigue behavior of the unreinforced material is beneficial. In this case, a few studies have been reported on the elevated temperature fatigue behavior of unreinforced precipitationhardened aluminum alloys.W8~ The elevated temperature cyclic response of A1-4Cu strengthened with semicoherent 0' precipitates exhibits cyclic softening at both 200 ~ and 250 ~ This cyclic softening has been attributed to the
G.M. VYLETEL, formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, is Design Engineer with Advanced Vehicle Technologies, Ford Motor Company. D.C. VAN AKEN is Associate Professor with the Dept. of Metallurgical Engineering, University of Missouri Rolla, Rolla, MO 65401. J.E. ALLISON is Principal Research Scientist with the Ford Research Laboratories, Ford Motor Company, Dearborn, MI 48121. 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/ASM-MSD Composite Materials Committee. METALLURGICALAND MATERIALSTRANSACTIONSA
decomposition of the 0' strengthening precipitates to form the equilibrium constituent phase, 0-A12Cu. Cyclic deformation appeared to markedly increase the rate of 0' decomposition.t4] Plastic deformation, in general, was found to increase the rate of 0' to 0 decomposition and alter the morphology of the precipitates, tg,l~ Normally, in undeformed samples, the 0 crystals form with angular shapes (i.e., rods and plates); however, in the presence of cyclic straining, the 0 precipitates grow in an isotropic manner (i.e., spheroids), t41. Despite these changes, the basic mechanism of the 0' to 0 transformation remained unchanged from those reported for undeformed specimens.tX2.X31In an A1-CuLi alloy, cyclic softening was observed from the commencement of c
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