Formation of Nb3Sn Nanocrystalline Layers in Cu-Sn/Nb Superconducting Composites Under Different Diffusion Annealing Modes

Nb3Sn-based superconductors are used in high-field magnets of various research projects, including magnetic systems of Tokamak thermonuclear fusion and accelerators for high-energy physics. An important task aimed at improving the current-carrying characteristics of the wire is to study the effect of design features and alloying of multifilamentary strands and the heat treatment modes on the formation of superconducting layers with an optimal structure, namely, with fine equiaxed grains of Nb3Sn phase with close to stoichiometry composition. This study presents the results of scanning and transmission electron microscopy examination of the structure and morphology of Nb3Sn layers in multifilamentary superconducting bronze-processed strands, with coupled Nb filaments doped with Ti, after various heat treatment (diffusion annealing) modes. The study used single-stage and two-stage annealing with different temperatures and durations.