Ductility enhancement in unreacted internal-Sn Nb3Sn through low-temperature anneals

It is shown that a lack of tensile ductility in as-drawn superconducting wires at room temperature can lead to damage during magnet fabrication steps such as cabling and coil winding. The tensile elongation of 0.42 mm internal-Sn Nb₃Sn conductor in the as-drawn unreacted condition is limited to less than 3% at room temperature. The tensile elongation is substantially enhanced through low-temperature annealing treatments at 200°C, increasing to as high as 10% after 24 h at 200°C. This effect is attributed to increased work hardening capacity from softening, which reduces yield strengths up to 50% while maintaining 90% of the tensile strength. Fracture surfaces of annealed wires reveal increased adhesion between the Sn cores and the surrounding Cu as well as an increase in ductile void coalescence as the failure mechanism within the Cu. Preliminary results on the prereaction strain sensitivity of reacted wire critical currents confirm the increased strain tolerance in annealed wires