Postscript

The case study, as developed here, is an oversimplification. Magnet design is, today, very sophisticated, involving nested sets of coils (up to 9 deep), with geometry the most important variable. But the selection scheme for winding materials, has validity: when pulses are long, electrical resistivity is the primary consideration; when they are short, it is strength. Similar considerations enter the selection of materials for very high-speed motors, for bus-bars and for relays. For further information, see (Herlach, 1988).

There are four ways of giving strength to highly-conducting metals: alloying, cabling, macrocomposites and microcomposites. Alloying generally lowers the conductivity too much. Copper cabled with stainless steel wire or aluminum with high-strength steel, gives the strength of one material with the conductivity of the other. Macrocomposites of copper clad in stainless steel, copper-beryllium or titanium have promise. Microcomposites are made by co-drawing two-phase castings or wire bundles of Cu-Nb, Cu-Ta, Cu-V or Cu-Ag, giving sub-micron filaments of one phase in the other, with strength greater than that predicted by the rule of mixtures. All are used to produce high-performance materials for magnet and motor windings.