Ultra high strength nanocomposite conductors for pulsed magnet windings

A process based on cold drawing was developed to elaborate high strength na nocomposite wires for pulsed magnet windings. The best results have been ob tained for Cu/Nb nanocomposites composed of a copper matrix embedding 9.10( 6) continuous parallel niobium fibers, with a diameter of 40 nm: their Ulti mate Tensile Stress is 1950 MPa and rho = 0.6 mu Omega.cm at 77K. TEM and HREM studies characterized the nanocomposite structure: the Nb fibe rs are nanowhiskers embedded in a copper matrix with semi-coherent interfac es. 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 conta in 4,10(9) M, fibers with smaller diameter d(Nb) (down to 10 nm) to increas e the whisker effect, proportional to 1/d(Nb). Concurrently, Cu/Ta conductors are fabricated. The theoretical strength of a whisker is mu/2 pi (mu is the shear modulus). Since mu(Ta) approximate to 2 mu(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 inter face and the fracture of Ta. This phenomenon is interpreted in terms of str ess driven rearrangement (Grinfeld instabilities) and solutions are given t o prevent its formation.