Superconducting properties of cold-worked core-wires on the basis of A15-phases

The preparation and the superconducting properties of core-wires on the basis of the A15-phases Nb3Sn, Nb3Al, and Nb3Al0.76Ge0.24 are reported. Using the powdered, already prepared phase as starting material for filling of the tube, a strong decrease of the transition temperature occurs owing to the technological drawing process, and the transition curve shows two distinct transitions. The low-temperature transition is ascribed to a phase which is formed by a residual stress-induced, partial transformation of the A15-phase on cooling to helium temperatures. The temperature behaviour of the resistance shows a clear anomaly near 80 K. By heat treatments the second transition in the transition curve may be influenced only insignificantly as to Nb3Al or Nb3Al0.76Ge0.24, respectively, while it may be nearly completely eliminated as to Nb3Sn.