Internal Stress in High-Strength CuAg Conductor

Resistive magnets with ultrahigh magnetic fields require composite conductors (almost all based on Cu) with optimized combinations of mechanical strength and electrical conductivity. In the fabrication of these conductors, the lower the melting point of the alloys, the easier they are to cast. Among conductors with melting points below the melting point of Cu, those of Cu-Ag achieve the highest mechanical strength. During cold-rolling, which is the final step for making these Cu-Ag conductors, small Ag precipitates elongate into a high density of fine Ag fibers, thus producing the high strength of the material. In this study, ultimate tensile strength values reached >850 MPa when composites were rolled to a reduction-in-thickness of >97% and spacing between fibers was reduced to less than 50 nm, generating high internal stresses. In these composites, the ratio of ultimate strength in the transverse direction to that in the longitudinal direction was about 1.13, indicating anisotropy. We speculate that such anisotropy in mechanical strength may lead to an internal-stress anisotropy at macroscale that could later complicate the manufacture of Bitter plates. In order to optimize the manufacturing process, we quantified the relationship between internal stress and strength anisotropy in Cu-24wt% Ag.