Composite core Nb3Sn wires: preparation and characterization

Powder metallurgically processed composite-core Nb₃Sn wires, a novel type with filaments containing controlled amounts of normal conducting inclusions elongated in the wire axis, e.g., Ta or NbTi, have recently been developed. Composite cores replace the Nb cores, which are inserted either in a CuSn or in a Cu matrix for producing wires by the bronze route or by the internal Sn diffusion process, respectively. The ductile inclusions are deformed during the wire drawing process to long ribbons being <100 nm apart. If the reaction temperature is sufficiently low, the Ta or NbTi shows little reaction with the Nb₃Sn phase but reduces the A15 grain growth in the radial direction. The resulting enhanced pinning at the grain boundaries (artificial pinning) leads to higher critical-current densities, particularly in the field range B<14 T. A difference between the observed field dependence for Ta and NbTi inclusions in the present configuration is explained by different degrees of prestress. The achievement of higher J_c values at 10-12 T may be advantageous for their application in fusion magnets