Shock-loading response of 6061- T6 aluminum metal-matrix composites
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INTRODUCTION
IN the last decade, a substantial amount of research has been conducted on Al-alloy-matrix composites in order to develop not only better composites with improved strengths and ductility but also more reliable models that can predict the stress-strain responses of those compositesY -4j It has been well documented that when a large difference in the coefficients of thermal expansion (CTE) exists between the matrix and reinforcement in metal-matrix composites (MMCs), internal stresses can develop, which are sufficiently high to generate dislocations at the reinforcement/matrix interface55~ Numerous observations have been made of the dislocation-punching phenomenon via transmission electron microscopy (TEM), which have shown a variety of different dislocation substructures and dislocation-punching mechanisms in composites/6-9] An important consequence of this phenomenon is that the metal matrix adjacent to the reinforcement becomes strain hardened as the dislocation density increases, thereby increasing the
KENNETH S. VECCHIO, Assistant Professor, is with the Materials Science Group, Department of Applied Mechanics and Engineering Sciences, University of California-San Diego, La Jolla, CA 92093-0411. GEORGE T. GRAY III, Team Leader and Staff Member, is with the Materials Research and Processing Science Department, Los Alamos National Laboratory, Los Alamos, NM 87545. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
strength and reducing the subsequent plastic flow of the matrix. How the reinforcing phase strengthens the ductile metal matrix and what, if any, effect the reinforcement has on the work-hardening behavior of the matrix remain tinclear. The difficulty in assessing the role of the reinforcement on both the strength and work-hardening behavior is in separating the modulus and strength effects of the reinforcement from the substructure changes (i.e., strain hardening) introduced into the matrix resulting from the thermal expansion mismatch between the matrix and the reinforcement. These two effects cannot easily be separated, since the introduction of the reinforcing phase during processing generally necessitates the use of elevated temperatures and, therefore, the development of internal stresses and their subsequent relaxation via dislocation generation at the reinforcement interfaces. The focus of this study is on aluminum-alloy composites reinforced with ceramic particulates of either alumina or mullite. These MMCs represent a class of materials that (1) contains two distinctly different constituents in terms of structural, physical, and mechanical properties, morphology, volume fractions, etc.; (2) exhibits elastic and plastic anisotropy; and (3) achieves some of its properties as a result of interracia
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