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The yield strength of Cu–Nb filamentary microcomposites was predicted as a function of Nb content by modifying the barrier strengthening model. To predict the variation of the yield strength with Nb content, the interfilamentary spacing was calculated as a function of Nb content on the basis of the assumption that Nb filaments are distributed regularly along the sides of triangular unit cells. The yield stress can be described as the sum of the substructural strengthening component and the filament boundary strengthening term. The good agreement between the prediction and the experimental data suggests that the strength increase in Cu–Nb filamentary microcomposites with increasing Nb content results mostly from increasing the volume fraction of Nb filaments, which act as barriers to plastic flow.
I. INTRODUCTION
Recently there has been a renewed interest in high magnetic field research, and an international effort exists to develop long-pulse high-field magnets.1–3 The magnitude of the fields produced by these magnets is limited by the performance of conductor materials. The conductor must possess high conductivity to reduce Joule heating and high strength to resist Lorentz forces from the magnetic field. The conductivity of these materials should be greater than 60% International Annealed Copper Standard (IACS), while the tensile strength should approach 1 GPa.1–5 The most promising conductor materials for use in magnet designs are microcomposites, which include copper–niobium and copper–silver. These materials possess three to four times the strength of hardened copper while remaining highly conductive. Both the mechanical and electrical properties of these microcomposites depend upon the composition, degree of cold work, and intermediate annealing treatments. 4,5 Roomtemperature tensile strengths in excess of 1 GPa with a conductivity of 60–70% IACS have been achieved in these microcomposites.4–8 The microstructure and strengthening mechanisms of Cu-based microcomposites incorporating a bodycentered-cubic (bcc) phase have been studied extensively.6–16 For such materials, the bcc phase is initially present as primary dendrites in the copper matrix. Following extensive mechanical deformation of Cu–Nb, niobium dendrites transform into fine niobium ribbons as a result of the 〈110〉 niobium texture upon drawing.6–8 This nanostructure contributes to the ultrahigh strength of Cu–Nb microcomposites. The strength of heavily deJ. Mater. Res., Vol. 15, No. 9, Sept 2000
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formed Cu–Nb exceeds that predicted by the rule-ofmixtures (ROM), and a fundamental understanding of the strengthening mechanisms involved has been the subject of much discussion.6–14 Spitzig and his co-workers6–8 suggest a barrier strengthening model, while Funkenbusch and Courtney9,10 believe that stored dislocations have a role in substructural hardening. Hangen and Raabe17 recently proposed an analytical model for the calculation of the yield strength of Cu–Nb microcomposites. Recently, Hong and Hil
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