Synchrotron X-ray study of bulk lattice strains in externally loaded Cu-Mo composites
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I. INTRODUCTION
THE load-bearing capacity of a metal matrix composite (MMC) is dictated by the load transfer occurring from the compliant, soft matrix to the stiff, hard reinforcements.[1] The load partitioning ratio between matrix and reinforcement remains constant as long as both components behave in a linear elastic manner. Upon plastic deformation of the matrix, the partitioning ratio changes and the reinforcing phases carry a greater portion of the load. Thus, the composite becomes mechanically more efficient as a higher portion of the applied load is carried by the reinforcing phase. However, with increasing stress in the reinforcement, that phase may fracture or debond from the matrix, thus reducing the load born by the reinforcement and the overall load-bearing capacity of the composite, and also often leading to macroscopic failure of the composite. Experimental measurement of load partitioning between the individual phases of MMCs can thus give a wealth of information on the micromechanical evolution of composites during deformation. Such measurements have been performed by neutron diffraction in aluminum reinforced with various ceramic particles,[2,3,4] in NiTi reinforced with TiC particles,[5,6] and in a model composite consisting of copper reinforced with molybdenum particulates.[7] Neutron diffraction techniques, however, require long measurement times and large diffraction volumes because of the small neutron flux available at existing thermal neutron sources and the inherently weak interaction of neutrons with crystalline solids.[8] The availability of high-energy, high-intensity X-rays from third-generation synchrotron research facilities has allowed the use of X-rays for such bulk strain measurements. Recently, Daymond and Withers,[9] and Korsunsky et al.[10] have described a novel synchrotron X-ray transmission technique by which, as they demonstrated in their studies on ALEXANDER WANNER, Akademischer Rat, is with the Institute fu¨r Metallkunde, Universita¨t Stuttgart, D-70174 Stuttgart, Germany. DAVID C. DUNAND, Associate Professor, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Manuscript submitted February 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
fine-grained aluminum matrix composites, bulk strain measurements can be performed at much reduced exposure times and in small diffracting volumes, thus allowing for investigating the time dependence of lattice strains and for mapping such strains with a lateral resolution of a fraction of a millimeter. We have further developed this technique and applied it to study lattice strains in copper-molybdenum composites.[11] The first goal of the present work is to establish a detailed methodology for lattice strain measurements in mechanically loaded samples. As there is only limited experience with this technique and as the methods to evaluate the raw data are not standardized, the present article examines in detail the experimental procedures and data evaluation techniques. The major
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