Plasticity of continuous fiber-reinforced metals
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I. INTRODUCTION
REINFORCEMENTS used to produce metal-matrix composites are generally on the order of several micrometers in size, a dimensional scale commensurate with that of microstructural features that intervene in the plastic flow of pure or alloyed metals, such as cells, twins, subgrains, precipitates, or dislocation tangles. Since matrix regions delineated by such reinforcements are of a similar size scale for usual reinforcement volume fractions, it is plausible that the plastic flow mechanisms and the in situ flow stress of the matrix of a composite would differ from those of the same unreinforced metal or alloy, processed identically and having experienced the same prior strain history. One of the best known examples of such an influence of a reinforcement on matrix plasticity is found in the copper– tungsten system. Tungsten fibers 20 mm or less in diameter have been documented to raise the apparent work-hardening rate of pure copper to spectacularly high values, which approach its elastic modulus and depend both on fiber diameter and fiber volume fraction.[1,2] These data have formed the focus of several publications and theoretical analyses; these are reviewed in detail by Pedersen,[3,4] and still remain somewhat controversial. More recent work dealing with particle-reinforced metals has also shown an influence of reinforcement size on composite flow stress.[5–15] Since no such size effect arises from PAVEL BYSTRICKY, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Senior Engineer with CeraNova Corporation, Franklin, MA 02038. HENRIK ˚ BJERREGARD, formerly on leave from the Materials Department, Risø National Laboratory, Roskilde, Denmark DK-4000, is Project Manager with Danisco Sugar, DK-1001 Copenhagen K, Denmark. ANDREAS MORTENSEN, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, is with the Department of Materials, Swiss Federal Institute of Technology in Lausanne, CH-1015 Lausanne, Switzerland. Manuscript submitted May 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
continuum mechanical analysis, this indicates that reinforcements cause alterations in dislocation creation and flow within these reinforced metals compared with their unreinforced counterparts. Various dislocation mechanisms, which are either inspired from prior work on dispersion-hardened metals or are specific to particle-reinforced or short fiberreinforced metals, have thus been proposed to interpret these data. In particular, a commonly invoked influence of the reinforcement on the matrix substructure and flow stress is dislocation punching, caused by differential thermal contraction between matrix and reinforcement (e.g., References 8 through 10 and 15 through 17) Thus, regardless of reinforcement geometry, studies of the plastic deformation of metal-matrix composites containing reinforcements several micrometers in diameter show that matrix plasticity is indeed strongly altered by the r
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