Influence of Ge addition on Nb_3Sn layer growth and electromagnetic properties in internal tin processed wires

摘要

In order to investigate the effect of Ge addition to the Cu matrix on the microstructure and the critical current density, four kinds of internal tin processed Nb_3Sn wires with pure Cu and Cu ― 0.2, 0.4 and 0.6 wt.% Ge alloys were drawn to 0.8 mm diameter. The microstructure and critical current of internal tin processed Nb3Sn wires that were heat treated at temperatures ranging from 680 to 740 °C for 240 h were investigated. The Ge addition to the matrix did not make workability worse. A Ge rich layer in the Cu-Ge matrix suppressed the growth of the Nb_3Sn layer and promoted grain coarsening. The high AC loss which is disadvantage of internal tin processed Nb_3Sn compared with the bronze method wire decreased significantly by adding Ge, which resulted from increased resistance between Nb_3Sn filaments caused by segregation of Ge-rich layer. The longer heat treatment, the lower the net-J_c result in the Ge added wires because of large grains due to abnormal grain growth.