Room-temperature tensile behaviour of bronze-processed multi-filamentary Nb3Sn superconducting materials

Structure and its relation to fracture behaviour of multi-filamentary Nb₃Sn superconducting composite materials prepared by the bronze method were studied by tensile testing at room temperature. There were two types of fracture mode. Type I showed high elongation, accompanied by apparent plastic deformation of composites as a whole and the Nb₃Sn layer exhibited multiple fracture. Type II showed no apparent plastic deformation and the composites fractured in a brittle manner. Type I occurred when the fraction of the Nb₃Sn layer was small and the drop of load-bearing capacity due to fracture of Nb₃Sn layer could be compensated mainly by strain hardening of ductile constituents of Nb, Cu-Sn and Cu. On the other hand, Type II occurred when the fraction of Nb₃Sn layer was large and the fracture of the Nb₃Sn layer caused fracture of composites as a whole. To describe the tensile strength of composites for both types, a model was proposed, which explained well the experimental results. It was found that the strength of the Nb₃Sn layer decreases with increasing diameter of composites and with increasing annealing temperature and time.