Effect of flat-roll forming on critical current density characteristics and microstructure of Nb/sub 3/Al multifilamentary conductors
We have developed a nearly-stoichiometric Nb/sub 3/Al multifilamentary superconductor by exploiting the transformation from bcc supersaturated-solid-solution Nb(Al)/sub ss/. This is a candidate conductor for 1 GHz NMR magnets. Flat-roll forming, an effective way to increase the packing factor of the coil and thereby the coil-current-density, has been carried out on the multifilamentary Nb(Al)/sub ss//Nb wire, and its effects are compared with that of the conventional Nb tube processed conductor. A Nb(Al)/sub ss//Nb wire, 0.74 mm in diameter, was successfully deformed into a tape conductor (the thinnest case: 0.15/sup t//spl times/2.2/sup w/ mm) without mechanical fracture. The flat-roll forming creates very little anisotropy in critical parameters and scarcely degrades the J/sub c/ properties. Excess flat-rolling causes a wide distribution of Nb(Al)/sub ss/ filament areas and thus reduces the n-index in the voltage-current characteristic. However, it is probably possible to retain the high value of the n-index of a wire, by appropriately designing the overall-aspect-ratio of a flattened conductor.