Ultra high strength nanocomposite conductors for pulsed magnet windings

A process based on cold drawing was developed to elaborate high strength nanocomposite wires for pulsed magnet windings. The best results have been obtained for Cu/Nb nanocomposites composed of a copper matrix embedding 9.10/sup 6/ continuous parallel niobium fibers, with a diameter of 40 nm: their ultimate tensile stress is 1950 MPa and /spl rho/=0.6 /spl mu//spl Omega/.cm at 77 K. TEM and HREM studies characterized the nanocomposite structure: the Nb fibers are nanowhiskers embedded in a copper matrix with semi-coherent interfaces. 3D tomographic analysis and in-situ deformations showed that the Orowan mechanism is controlling the dislocations behaviour in this structure. Optimized conductors are developed: the new Cu/Nb nanocomposites will contain 4.10/sup 9/ Nb fibers with smaller diameter d/sub Nb/ (down to 10 nm) to increase the whisker effect, proportional to 1/d/sub Nb/. Concurrently, Cu/Ta conductors are fabricated. The theoretical strength of a whisker is /spl mu//2/spl pi/ (/spl mu/ is the shear modulus). Since /spl mu//sub Ta//spl ap/2/spl mu//sub Nb/, the Cu/Ta UTS should be enhanced. However, the drawing of Cu/Ta billets lead to the formation of a macroscopic roughness at the Cu/Ta interface and the fracture of Ta. This phenomenon is interpreted in terms of stress driven rearrangement (Grinfeld instabilities) and solutions are given to prevent its formation.