The influence of matrix microstructure
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I.
INTRODUCTION
T H E search for low-density materials that possess attractive properties has led automotive and aerospace engineers to consider particle-reinforced aluminum composites. Successful implementation of these materials will require an extensive knowledge of microstructure/property relationships, especially in the area of fatigue. Numerous studies have been performed on the smooth-bar fatigue behavior of composite materials tested under stress-controlled conditions with various stress ratios, a variety of matrix materials, and various volume fractions of SiC and A1203 particle reinforcements.lJ-~ q The majority of these studies concluded that the incorporation of reinforcement improved the overall fatigue strength. However, several of these investigations have shown that this improvement depends on stress level and test method~ and in fact, under certain conditions, reinforcement can lead to a reduction in resistance to cyclic loading (e.g., Reference 1). Material quality has also been a concern since crack initiation has often been associated with processing-related intermetallic inclusions, SiC clusters, large SiC particles, shrinkage holes, and/or exogenous defects. It has recently been reported that improved fatigue lives can be attained by a reduction in these processing defects, m Since aluminum composites are often based on conventional aluminum alloys, a brief review of the present G.M. VYLETEL, formerly Graduate Student, Department of Materials Science and Engineering, The University of Michigan, is Materials Engineer, Johnson Controls, Inc., Plymouth, MI 48170. J.E. ALLISON, Staff Scientist, is with the Ford Research Laboratory, Ford Motor Company, Dearborn, MI 4812L D.C. VAN AKEN, formerly Assistant Professor, Department of Materials Science and Engineering, The University of Michigan, is Associate Professor, University of Missouri-Rolla, Rolla, MO 65401, Manuscript submitted March 12, 1993. METALLURGICAL TRANSACTIONS A
knowledge of the microstructure/property relationships in conventional precipitation-hardened aluminum alloys is warranted. The cyclic response of aluminum alloys is very dependent on microstructure. Alloys strengthened by shearable precipitates are often cyclically unstable, and the typical cyclic response shows a region of hardening to a peak stress followed by cyclic softening) ~2'~3] In contrast to this behavior, a stable cyclic response is observed in aluminum alloys strengthened by nonshearable precipitates. In general, alloys strengthened by these strong dislocation barriers do not cyclically harden or soften unless the plastic strain amplitudes are very large. 1~4] Despite the significant difference in cyclic response between naturally aged and artificially aged aluminum alloys, the fatigue life of these materials has been found to be virtually independent of the aging condition. tlSA6'171 This behavior has been attributed to a competition between two strengthening mechanisms; the strengthening effects of the precipitates are balanced by the loss of the solute streng
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