Strength and ductility of heavily drawn bundled Cu-Nb filamentary microcomposite wires with various Nb contents
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I. INTRODUCTION
THE microstructure and strengthening mechanisms of Cu-based microcomposites incorporating a phase have been studied extensively.[1–13] For such materials, the bcc phase such as Nb, Ta, Fe, and Cr is initially present as primary dendrites in the copper matrix. Following extensive mechanical deformation of Cu-Nb, for example, niobium dendrites transform into fine niobium ribbons as a result of the ^110& niobium texture upon drawing.[1,2,3] This nanostructure contributes to the ultrahigh strength of Cu-Nb microcomposites. The strength of heavily deformed Cu-Nb exceeds that predicted by the rule-of-mixtures, and a fundamental understanding of the strengthening mechanisms involved has been the subject of much discussion. [1–12] Spitzig and his coworkers[1,2,3] suggest a barrier strengthening model, while Funkenbusch and Courtney[4] and Courtney[5] believe that stored dislocations have a role in substructure hardening. Hangen and Raabe[14] recently proposed an analytical model for the calculation of the yield strength (YS) of Cu-Nb microcomposites. Hong and Hill[15,16] found that the ratio of yield stresses at 295 and 77 K is close to the ratio of Young’s moduli in Cu base microcomposites, suggesting that athermal obstacles significantly affect the YS. Hong[17] also suggested that the contribution of the substructure strengthening associated with dislocations, subgrains, and/or grains to the strength of Cu-Nb filamentary microcomposite decreases rapidly with increasing drawing strain, and the microstructural scale of heavily drawn Cu-Nb filamentary microcomposites is limited by Nb filaments. These observations favor the barrier strengthening model over the dislocation accumulation model to predict the YS.[15,16,17] S.I. HONG, Associate Professor, and H.S. KIM, Assistant Professor, are with the Department of Metallurgical Engineering, Chungnam National University, Taejon 305-764, Korea. M.A. HILL, Staff Member, is with the Materials Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted January 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Spitzig et al.[18] noted that a 44 pct increase in strength of Cu-Nb could be obtained using a bundling and drawing process. This involves filling a container with small diameter wires and subsequently redrawing. Using this technique, it is possible to obtain higher strengths in larger diameter wires. The large diameter wire is desirable for magnet applications to avoid electrical breakdown and disintegration of the wire. Spitzig et al., however, observed that on subsequent cold drawing of the bundled wires, the strength increased at a slower rate than that obtained on continuous cold drawing of the subelemental wire, and the strength differential decreased. They reported that upon subsequent cold drawing of the bundled wire to a diameter of 0.15 mm, the ultimate tensile strength (UTS) is only 10 pct greater than that of the subelemental wire continuously drawn to the same diameter.[18] They suggested that a slow strengthening
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