The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part
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I.
INTRODUCTION
PARTICLE-REINFORCED aluminum alloys are being viewed with great interest by the automotive industry. Their attractive specific properties and the ability to tailor such properties as elastic modulus and thermal expansion coefficient have led engineers to consider this class of materials for numerous automotive applications.m Successful use of these metal matrix composites will require an extensive knowledge of processing-microstructure-property relationships. In order to optimize the properties for a desired application, knowledge is needed on how variables such as grain size, matrix chemistry, precipitate structure, and reinforcement particle characteristics (i.e., size, shape, and volume fraction) affect the mechanical properties of particle-reinforced aluminum alloys. The current state of knowledge on factors affecting the fatigue behavior of particle-reinforced MMCs has been recently reviewed.t2] In general, particle reinforcement increases the stress-controlled fatigue resistance; t3 87however, these improvements are dependent on stress level and testing method. Under total strain and plastic strain-controlled conditions, it has been found that reinforcement leads to a decrease in fatigue life.iT.8,91
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. J.E. ALLISON is Principal Research Scientist with the Ford Research Laboratories, Ford Motor Company, Dearborn, MI 48121. D.C. VAN AKEN is Associate Professor with the Dept. of Metallurgical Engineering, University of Missouri-Rolla, Rolla, MO 65401. 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 MATERIALS TRANSACTIONS A
Matrix precipitate structure (naturally aged vs peak-aged) has been shown to have little influence on the fatigue life behavior of particle-reinforced aluminum alloys.tS,m 1~Similar results have also been observed in peak-aged and naturally aged unreinforced aluminum alloys.t8,12161 The similarity in smooth bar fatigue life of these two microstructural conditions has been suggested to result from a competition between two strengthening mechanisms, that is, the strengthening effects of the precipitates v s the loss of solute strengtheningY 21 Another proposed rationale is that the cyclic work hardening in the naturally aged condition creates a material that has a similar fatigue resistance to the peak-aged material,rsJ Recent investigations indicate that the similarity may result from equivalent fatigue-crack propagation behaviors that appear to dominate total fatigue life in smooth bar specimens, r161 Grain size effects on the fatigue behavior of composites have not been explored explicitly; however, it
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