Behavior of Ti-based Ternary Compounds in High Tin Bronze Used for Nb₃Sn Superconducting Wires

Abstract

High tin bronze is used as a component of practical Nb₃Sn superconducting wires. In particular, the addition of titanium to bronze is well known to improve superconducting properties as well as the workability of the bronze alloy. In the present study, change in the microstructure of bronze by adding titanium was investigated. The important results are summarized as follows. First, when the Cu-10.5 at%Sn-0.72 at%Ti alloy was held at 550^oC for 1000 min, three phases of primary α, γ and CuSnTi (Ti111) were equilibrated. Second, after holding at 750^oC for 1000 min, three phases of primary α, β and CuSn₃Ti₅ (Ti135) coexisted. Third, during heating from 560^oC to 750^oC, a remarkable phase change was observed near 675^oC, below which the Ti111 phase was dominant, but above which the principal Ti-based compound phase was Ti135. Fourth, in the present region of alloy composition and heat treatment temperature, two invariant reactions of Ti135 + β = α + Ti111 and α + β = γ + Ti111 were recognized to take place in the Cu-Sn-Ti ternary system. Finally, characteristic features of workability at room temperature as well as at elevated temperatures could be well explained in conjunction with the behavior of titanium ternary compounds.