The fabrication, properties and microstructure of Cu–Ag and Cu–Nb composite conductors

The properties and the microstructure of cold-drawn Cu–Ag and Cu–Nb composites have been investigated for their potential use as conductors in pulsed high field magnets. Cu–Nb and Cu–Ag are among the leading candidates for the 100T magnet to be built in cooperation between NHMFL and LANL. These conductors show a combination of high conductivity, high strength, adequate workability, and the final section size obtainable for the wire. The conductors are operated in the temperature range from liquid nitrogen to room temperature, and therefore the mechanical and electrical properties were measured at both −196 and +25°C. The 100T magnet requires conductors with various cross-sections. The conductors with small cross-sectional areas (6–12 mm2) require higher strength. Thus, Cu–Nb composites with cross-sectional areas between 6 and 12 mm2 were fabricated by bundling-and-drawing and characterized. The Cu–Ag composite was developed as one of the candidates for large cross-section applications. It was produced by casting, forging, rolling and cold drawing to a final cross-section of up to 45 mm2. In addition to the ability to fabricate coils for the magnet, we have considered the service life of the materials in regard to both cyclic loading and thermal fatigue. Examination of the initial portion of the stress–strain curve of cold-worked materials reveals that the internal stresses developed during the fabrication influence the mechanical response of the materials. Thus the properties of the drawn materials have been monitored as a function of cyclic loading and thermal annealing cycles. An important aspect of the selection of materials for high field magnets is the ability to obtain the material in large cross-sections and thus the work in this paper is concerned with the development of suitable production routes as well as the relationship between the structure and the properties of processed composites.