Recent progress in the development of ultra high strength "continuous" Cu/Nb and Cu/Ta conductors for non-destructive pulsed fields higher than 80 T

High strength and high conductivity Cu/Nb nanofilamentary wires are elaborated for the winding of resistive coils that produce pulsed high magnetic fields. The structure of the "continuous" Cu/Nb nanocomposites is a Cu matrix containing continuous parallel Nb filaments with a diameter in the nanometer range. The best first generation conductors possessed at 77 K an ultimate tensile strength of 2 GPa and a resistivity of 0.6 μΩ.cm for a 0.04 mm² section. The aim of this work was to link the macroscopic properties to the structure at the nanometer scale in order to define an optimized geometry. Structural studies and in-situ tensile tests in a transmission electron microscope revealed the effect of confinement on the plasticity of the finest Cu channels and of the Nb nanowires. The effect of the nanometer size was modeled to predict the mechanical and the electrical properties. The good agreement between the simulations and the experimental data allowed the definition of the optimization parameters that were used to elaborate the new generation continuous Cu/Nb nanocomposites: the best result was achieved for a Cu/28% vol. Nb conductor with a 5 mm² section, containing 52.2×10⁶ Nb nanofilaments with a diameter of 140 nm: at 77 K, the UTS is 1.9 GPa and the resistivity is 0.58 μΩ.cm. These optimized reinforced conductors can be used to build a pulsed coil generating fields higher than 80 T in a coilin-coilex system. The continuous nanocomposites are further improved by the following two ways: (i) increasing the number of Nb nanowhiskers to 85⁵=4.4×10⁹; (ii) using other b.c.c. reinforcing metals with higher shear modulus (tantalum). An innovating and promising structure is also under development: the "co-cylindrical" Cu/Nb/Cu nanocomposites, where the Cu matrix contains 85ⁿ Nb nanotubes filled with Cu nanowhiskers.