Performance of as-reacted and multiple bent (‘pre-bent’) practicalNb3Sn bronze route wires with different architectures

Abstract

Six practical composite wires ofNb3Sn (Furukawa Electric Co, Ltd) with different architectures (designand reinforcement) were compared from the normal state resistanceR, criticaltemperatures Tconset, Tcoffset andΔTc, upper critical fieldBc2 (at 4.2 K) and criticalcurrent density Jc points of view. Wires were as follows: three of near-the-edge reinforcement designwith Nb reinforcement of 0, 21, 50 vol% in the CuNb region, and two of centralreinforcement design with Nb of 21 and 50 vol%. One wire with near the edge50% vol Nb reinforcement had a different reinforcement/superconductor ratio.As-reacted wires show very different patterns ofR,Tconset,Tcoffset,ΔTc,Bc2 and Jc. For the superconducting parameters this is probably due to different 3D thermal residualstrains. Data suggest that the architecture of the as-reacted wire can control residual strainvalues and distribution.During multiple bending of the wires at room temperature (namedpre-bending), introduction of the reinforcement improves relaxation of the 3Dresidual strain and especially of the lateral components. As a consequence,Bc2 andJc versuspre-bending strain, εpb, are enhanced to values closer to those of theNb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforcedwires is higher than for the reinforcement-free wire.Evolution during pre-bending and maximum attained absolute values ofthe superconducting parameters can be grouped roughly as a function ofnear-the-edge or central reinforcement design. Variation of the superconductingparameters suggests that the pre-bent state may depend on the as-reactedone (which is a function of wire architecture and processing history); whenJc waslow as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relativelyhigh Jc enhancement, this parameter attained lower absolute maximum values than for the otherwires.Depending on the wire, when pre-bending strainεpb = 0.8–1%, the enhancementof non-Cu Jc at 4.2 K in the pre-bent wire is by up to 43.5% at 15 T compared to the as-reacted case. The maximum absolutevalue of Jc is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50% vol Nb.Bc2 of these wires was lower than for the other reinforced wires and higher than for thereinforcement-free wire.Pre-bent wires with near-the-edge reinforcement are identified as new and promisingcandidates for fabrication of react-and-wind coils with improved performance.