Effect of doping mode and composite geometry on the structure of nanocrystalline $\mathrm{Nb_3Sn}$ layers in superconducting $\mathrm{Nb/Cu\text{-}Sn}$ composites

Multifilamentary superconducting bronze-processed $\mathrm{Nb/Cu\text{-}Sn}$ composites doped with $\mathrm{Ti}$ have been studied by transmission and scanning electron microscopy. The effect of doping mode (artificial doping of Nb filaments by inserts of $\mathrm{NbTi}$ alloy or $\mathrm{Ti}$ doping of the bronze matrix) and external wire diameter on the morphology of $\mathrm{Nb}$ filaments and $\mathrm{Nb_3Sn}$ layers and on grain sizes of the superconducting phase has been revealed. Correlation between the average grain size of $\mathrm{Nb_3Sn}$ phase and critical current density of a composite is demonstrated.