Nb₃Sn 線材のための実用ブロンズ合金

―組織と冷間加工性―

抄録

Copper-Tin (Cu-Sn) bronze alloy is the key material for bronze-processed Nb ₃ Sn superconducting wires. The Osaka Alloying Works in Japan has established a unique melting process for large-scale bronze alloys called the “Mizuta method". In this process, the graphite crucible containing hot molten metal is cooled by passing it directly through a water shower. A general mold casting is not needed, so only slight oxidation of the hot molten metal occurs. Since a uni-directional solidification process is used, a very homogeneous tin concentration without severe inverse segregation in the longitudinal direction of the ingots is obtained. In this paper, we investigate the microstructure and mechanical performance details of the practical bronze having Sn concentrations of 14, 15 and 16 mass%. Elongation, 0.2% toughness, Vickers hardness at room temperature, and the limit of continuous cold-drawability as a function of intermediate annealing were studied. Degradation of cold-drawability was excessive, with a rather low annealing temperature of 400^oC. We found that numerous fine precipitates appeared at the intra- and inter-alpha grains after low-temperature annealing. The transmission electron microstructure analysis revealed that those precipitates were the delta phase (Cu₄₁Sn₁₁). The delta precipitates may behave as a pinning site of the slip motion for the plastic deformation of bronze alloys. We have to re-recognize that the intermediate annealing temperature is a very important parameter for avoiding wire breakage during the industrial production of bronze-processed Nb ₃ Sn superconductors.