Influence of metallurgical factors on superconducting current densities in ‘‘bronze-processed’’ Nb3Sn multifilamentary wires

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Abstract
A number of metallurgical factors influencing superconducting properties, primarily critical current densities, of bronze‐processed Nb3Sn multifilamentary wires were examined for a set of wires in which the ratio of bronze to Nb, R, varied from 2.0 to 3.7. The morphology and the size of the Nb3Sn grains were found to be strongly influenced by R and the temperature at which the compound was formed. A qualitative model for the growth of the compound layer is proposed accounting for the microstructural variations in the wires. The critical‐current density Jc of the wires was the highest for those heat treated at 725 °C for 192 h, and was essentially independent of R. The highest Jc was also found in the wires with ∼7‐μm filaments, compared with those with ∼3‐ and ∼15‐μm filaments. The decreases in Jc with the reduction of the filament size to 3 μm was correlated with the spread in the distribution of the filament sizes. By examination of the present and the previous data on Jc of multifilamentary Nb3Sn wires, it is concluded that values of Jc of ∼800 A/mm2 at 10 T and ∼500 A/mm2 at 12 T can be easily achieved in bronze‐processed wires without an undue amount of effort in fabrication, but to achieve significantly higher values (∼1200 A/mm2 or higher at 10 T) may require extensive quality control in the manufacturing of these composite wires.