The evolution of Nb recrystallization in a Cu matrix: morphology of Nb and modulation of its critical fields in a wire configuration

We report on the microstructural evolution in Cu15%Nb multifilamentary wires upon annealing and the corresponding effects on their magnetic properties. During annealing at temperatures higher than 800 deg. C thermal instability mechanisms take place in the microstructure of the Cu15%Nb composite, leading to the spheroidization of niobium filaments. Another important consequence of annealing is the recrystallization of Nb, that has a straightforward influence on the magnetization data. When compared to the as-drawn composite, in the annealed Cu15%Nb samples the bulk upper critical field Hc2(T) is reduced significantly, while the lower critical field Hc1(T) is increased as a function of the annealing temperature and time. This indicates that we may employ this process as an efficient method to modify the basic superconducting parameters of Nb, namely the coherence length ξ(T) and the penetration depth λ(T)