The microstructure and critical current characteristic of a bronze-processed multifilamentary Nb3Sn superconducting wire

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Abstract
The superconducting phase within a bronze‐process, multifilamentary Nb3Sn superconducting wire is formed by reaction at the interface between the Nb filaments and the bronze matrix. The maximum current that can be carried by the wire is known to depend on the time and temperature of the heat treatment as well as on the transverse magnetic field. In the work reported here a commercial Airco wire containing 2869 Nb filaments of 3–5 μm diameter in a matrix with a bronze/Nb ratio of three was given a variety of reaction heat treatments. The microstructure of the reacted layer was analyzed as a function of heat treatment, and found to be divisible into three concentric shells that are morphologically distinct. The central shell consists of fine equiaxed grains. Its areal fraction, grain size, and composition depend on the heat treatment, and appear to determine the critical current. The best combination of grain size and composition, and the highest critical current, is obtained with an intermediate reaction temperature (700–730 °C). A further improvement in both microstructure and critical current is achieved by double‐aging the wire, starting the reaction at 700 °C and finishing it at 730 °C. The relation between microstructure and heat treatment is interpreted in light of the apparent mechanism of the reaction, which is revealed by high resolution analyses of the reacted layer. The relation between microstructure and properties is consistent with current understanding of the influence of grain size and stoichiometry on the behavior of type II superconductors.