Ultra strong conductors for pulsed high field magnets
The main requirement for conductor wires in pulsed non-destructive high-field magnets is the optimum superposition of high mechanical strength, high electrical conductivity and sufficient ductility. While high ultimate tensile strength is required to resist the Lorentz force, the high electrical conductivity limits the Joule heating and, hence, determines the attainable pulse time. Adequate ductility is necessary for a crack-free deformation of the wire to rectangular cross-section by drawing or profile rolling and for crack-free bending in winding the coil.
The ultimate tensile strength of the most advanced electrical conductors presently available for pulsed magnets is of the order of 0.6-1.2 GPa for wires with an electrical conductivity of 50-80% IACS (International Annealed Copper Standard). The conductor materials are dispersion-strengthened copper, Cu/stainless steel macrocomposites (Cu core with steel jacket) as well as microcomposites based on alloys of the binary Cu-Nb and Cu-Ag systems. In order to obtain the required strength level, the manufacturing method generally involves heavy cold deformation for microstructural refinement. The characteristic feature of the Cu/stainless steel macrocomposites is the distinct separation of the tasks between the components. Therefore, the resulting strength and conductivity are roughly proportional to the volume fractions of the constituent materials. The outstanding feature of the cold deformed microcomposites is their very high tensile strength, which is much higher than expected from a simple rule of mixtures.
Currently, there is a large discrepancy between the tensile strength achieved in laboratories for round wires with small diameters and the tensile strength of conductors realized with appropriate rectangular cross-sections. These discrepancies are due to the insufficient knowledge of the microstructural conditions for obtaining the high strength values and of the formation mechanisms of material defects during the deformation process and during the transition from round to rectangular cross-section.
Our research activities focus on the development of micro- and macrocomposites with
- microcomposite Cu-Ag alloys
- macrocomposites with high-nitrogen stainless steel and pearlitic steel jackets, respectively
In addition, the potential of
produced by mechanical alloying and hot compaction is investigated
Ultimate tensile strength vs. conductivity for various Cu based conductor materials - including ours (red) - the encirceled region represents the target region which is aimed at for conductor materials in pulsed high field magnets.