Microstructure and Mechanical Properties of Heavily Worked and Recrystallized Copper for Low-Temperature Superconductor Applications

Oxygen-free high-conductivity (OFHC) copper is generally used as a matrix for mechanical and electrical failure support of Nb3Sn superconducting filaments. This matrix provides mechanical stability to the filaments and a path for large currents in the event of an unwanted large temperature rise. Equal channel angular extrusion (ECAE) is an excellent way to increase the strength of bulk copper without an overwhelming degradation of resistivity caused by alloying. A stronger Cu may be beneficial for fabrication of Nb3Sn multifilamentary wires because of its ability to arrest nonuniform deformation of the precursor Nb filaments as the wire is drawn down. For the work reported, 25.4 × 25.4 × 254 mm 3 bars of OFHC Cu were processed by ECAE to plastic strains of 16 following four different ECAE routes, sectioned by wire EDM, and characterized for strength, resistivity, and microstructure. Results indicate routes E and Bc may be the best for strengthening. Postprocessing rolling was found to further increase strength but also result in the increase in resistivity. Additionally, recrystallization was studied for selected routes and it was found to have an inverse relationship with the amount of cold work. The resistivity ratio (R298 K/R77 K) has a visible drop for the most severe deformation cases.