Structure and room-temperature deformation of alumina fiber-reinforced aluminum
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INTRODUCTION
C O M P A R E D to elastic deformation, which is well predicted, or at least closely bounded by mechanical analysis, tl-71 the mechanical behavior of metal matrix composites is more difficult to predict when the matrix deforms plastically. This is due, in large part, to a more complex mechanical analysis, which is accomplished either by modifying models for fully elastic composites, t2'8-14J or by using numerical methods assuming macroscopic laws for plastic f l o w . t4'15-32] In addition to the increased complexity of analysis, a second complication arises from the fact that the matrix intrinsic postyield mechanical behavior is often affected by the reinforcement, in contrast with elastic constants, which vary little with the microstructure and local stress in each phase of the composite. This additional complicating factor is at the root of several discrepancies that exist between mechanical analysis and experimental data. These include numerous cases where, after identical processing, the composite mechanical behavior varies with the size of the reinforcement. I33-471 No such scale dependence can be predicted by continuum mechanics if intrinsic mechanical properties of matrix and reinforcement remain the same. Therefore, unless the matrix chemistry changes with reinforcement size, the cause for these variations in composite properties must be metallurgical. The reinforcement diameter is, in other words, on the scale of substructural or microstructural features of the matrix and interferes with these in a manner where its comparative size matters.
J.A. ISAACS, formerly with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Research Engineer, Aluminium Ranshofen, Gembh, Ranshofen, Austria. A. MORTENSEN, Associate Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted May 14, 1991. METALLURGICAL TRANSACTIONS A
With continuous fiber-reinforced composites stressed uniaxially parallel to their fibers, mechanical analysis of composite behavior is simple enough to allow measurement of matrix in situ properties (although significant experimental scatter will result from the dominant loadbearing role of the fibers). Hill tgJ gives two relatively close bounds for the instantaneous composite moduli in this configuration, which are independent of fiber packing or geometry and are valid even for high fiber volume fractions. Pedersen has also shown consistency of meanfield models with these bounds, t2,11,12~ In the present investigation, we take advantage of this fact to study metallurgical aspects of the plastic deformation of aluminum in a composite, with the aim to understand its in situ mechanical behavior. Because aluminum is an attractive matrix for composites from an engineering standpoint, there have been several previous investigations of the axial behavior of aluminum and its alloys reinforced with continuous fibers. Studies of continuous alumina fiber-reinfor
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