Measurement of the critical temperature transition and composition gradient in powder-in-tube Nb3Sn composite wire

The T-c transitions in a Nb3Sn PIT composite wire made by ShapeMetal Innovations, Holland, have been extensively studied for multiple heat treatments. The composite contains 504 very uniform filaments, each one being a Nb tube that contains the Sn-rich phases and Cu needed to form Nb3Sn. Parallel measurements of the T-c transition and chemical composition are very interesting in such a composite because the filament is magnetically transparent, the lowest T-c phase lying on the outside of each filament, permitting a volume-dependent T-c to be extracted, which can be compared to the measured composition profile. Our results show that the Nb3Sn composition and T-c gradients established over most of the layer are smaller than would be expected. The T-c profile becomes rather flat after 8 hours, but acquires a steep tail after the Nb3Sn consumes the Nb sheath and comes in contact with the pure Cu matrix. This design of composite appears to have an inbuilt A15 phase composition gradient, no matter what the reaction. However, excellent properties are obtained even in the presence of such gradients.