The Nb3Sn layers formation at diffusion annealing of Ti-doped multifilamentary Nb/Cu–Sn composites

• In bronze-processed superconductors Nb3Sn phase nucleates in bulk of Nb filaments.
• Less columnar Nb3Sn grains are formed in the wires with Ti-doped Nb filaments.
• The Nb3Sn grain sizes increase with increasing Ti concentration in Nb filaments.
• Ti doping of bronze matrixes results in the best structure of Nb3Sn layers.

The structure and morphology of superconducting Nb3Sn layers in multifilamentary Nb–Ti/Cu–Sn and Nb/Cu–Sn–Ti composite wires after various regimes of the diffusion annealing have been studied by transmission and scanning electron microscopy. The effect of the doping mode and Ti concentration on the thickness, structure and morphology of Nb3Sn layers is pronounced even after the first (low-temperature) stage of the diffusion annealing. After various regimes of the annealing the wires with doped bronze matrix (Nb/Cu–Sn–0.24Ti) demonstrate wider zones of fine equiaxed Nb3Sn grains with narrow grain size distributions compared to those with Ti-doped Nb filaments, which is of great importance for an attainment of the highest critical current densities Jc.