A Three-Dimensional Fractal-Based Study of the Effects of the Complex Interface between Bi ₂Sr ₂CaCu ₂O _x Filaments and the Ag Matrix on the Mechanical Behavior of Composite Round Wires

Bi ₂Sr ₂CaCu ₂O _x/Ag/Ag-alloy (Bi2212) multifilamentary composite round wires (RWs) are the only RW candidate for fabricating superconducting magnets generating magnetic fields greater than 25 T. The complicated microstructure of the RWs, however, affects not only the electrical transport but also the mechanical behavior. To establish the correlation between the macroscopic electromechanical properties and the microstructure, an accurate characterization of the Bi2212 composite RW microstructure is important. Here, we develop a 3-D fractal-based model of the complex interface between Bi2212 filaments and the Ag matrix and determine its impact on RW mechanical behavior. The results indicate that such a complex interface strongly affects the local mechanical behavior rather than macroscopic behavior. Specifically, it causes the stress to be highly nonuniform in localized regions. At and/or around thin 'Plate-like Extensions' (PEs) and 'Bridge Connections' (BCs), the stress peaks under a simple uniaxial load. The results conclude that mechanical failure likely initiates at PEs, BCs, and the interface between them and the Ag matrix.