Performance of as-reacted and multiple bent ('pre-bent') practical Nb ₃Sn bronze route wires with different architectures

Six practical composite wires of Nb₃Sn (Furukawa Electric Co, Ltd) with different architectures (design and reinforcement) were compared from the normal state resistance R, critical temperatures T_c ^onset, T_c ^offset and ΔT_c, upper critical field B_c2 (at 4.2K) and critical current density J _c points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/ superconductor ratio. As-reacted wires show very different patterns of R, T _c ^onset, T_c ^offset, ΔT _c, B_c2 and J_c. For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature (named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, B_c2 and J_c versus pre-bending strain, ε_pb, are enhanced to values closer to those of the Nb ₃Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one (which is a function of wire architecture and processing history); when J_c was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high J_cenhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain ε_pb ≤ 0.8-1%, the enhancement of non-Cu J_c at 4.2K in the pre-bent wire is by up to 43.5% at 15T compared to the as-reacted case. The maximum absolute value of J_c is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50%vol Nb. B_c2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.