Study of microstructural and resistivity changes during thermal processing of tin bronze for superconducting applications

Electrical resistivity measurements have been performed for annealed Cu–Sn wires with compositions of 2, 5, 10 and 13.5 wt% Sn from room temperature to 700 °C. The annealing behaviour of cold-worked samples of these compositions has been investigated using resistometry, microhardness and optical microscopy. All samples undergo a hardness reduction of around 100 HV (300 g) on annealing. However, the room temperature resistivities differ greatly between the cold-worked and annealed condition only for the 10 and 13.5 wt% Sn samples, for which large resistance changes occur in the 300–500 °C range. Recrystallisation is delayed to around 400 °C in the high-tin samples; interaction between annealing and Cu3Sn (epsilon phase) precipitation is proposed as an explanation for the resistivity behaviour of these samples. The implications of these resistivity results for resistometric monitoring and optimisation of bronze route Nb3Sn superconducting wires are summarised.