Structures and residual stresses of Cr fibers in Cu-15Cr in-situ composites
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INTRODUCTION
THE Cu-based in-situ composites, which consist of Cu (fcc) matrices reinforced with hard bcc or fcc metals, such as Ta,[1] Nb,[1,2–4] Cr,[1,5–7] Fe,[8,9] and Ag,[10] have been developed to meet the increasing industrial requirements for materials with higher strength and higher electrical conductivity. These so-called in-situ composites are generally manufactured by vacuum casting or powder metallurgy followed by heavy cold drawing or rolling and are superior to the artificial composites such as carbon fiber reinforced copper composites due to their economic competitiveness and cold deformability. Among these Cu-based in-situ composites, the Cu15Cr composites (15 mass pct Cr, balance Cu) have drawn a lot of attention recently.[5–7,11,12] The hard, refractory Cr phase, which is dispersed within the Cu phase after casting and forging, can be elongated by cold drawing. The good deformability of these composites is attributed to the dynamic deformation-recovery-recrystallization behavior of the Cu matrix. The heavy cold working could produce dislocations in both the Cu and Cr phases. To obtain higher electrical conductivity, the as-drawn Cu15Cr in-situ composites are required to be aged at intermediate temperatures in order to precipitate the Cr dissolved in the Cu matrix. The effects of aging temperature and holding time on the electrical and mechanical properties of Cu-15Cr in-situ composites have been reported.[12] The tensile strength of the composites decreases with increasing aging temperature, and the conductivity reaches a peak value after aging at an optimum temperature. The residual stress in the reinforcement is a very important factor as it affects the strength and work hardening of the metal matrix composites. Although many efforts have been made to explore the properties of Cu-15Cr in-situ composites, the residual stresses in the Cr fibers that result from either cold working after drawing or thermal expansion mismatch during aging have been largely ignored to date. Residual internal stress is a common phenomenon in SHOUJIN SUN, Research Fellow, is with the Materials Processing Division, National Research Institute for Metals, Tsukuba 305-0047, Japan. Manuscript submitted May 22, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanically and thermally inhomogeneous materials, especially composites. The residual stresses in composites are generated from the differential thermal expansion coefficients after the composites are heat treated or fabricated at an elevated temperature (thermal residual stresses) or from cold deformation (mechanical residual stresses). Both the mechanical and thermal residual stresses can be produced in Cu-15Cr in-situ composites due to the heavy cold drawing and the following aging treatment. Ramamurty et al.[13] developed a new method to directly measure the thermal misfit strain in continuous SiC fiberreinforced Ti matrix composites. Specifically, the matrix is selectively etched and the exposed SiC fibers extend because the residual stresses are released. The
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