Improvement of high field performances of bronze-processed Nb3Sn superconductors by titanium addition to the matrix

Addition of a small amount of Ti to the Cu-Sn alloy matrix of bronze-processed Nb3Sn superconductors has been found to remarkably enhance both the formation rate of the Nb3Sn layer and Hsub(e2). Fundamental studies on the mechanism of increase in Hsub(e2) and that in the Nb3Sn layer formation rate as well as metallurgical studies have been made. Composites consisting of Nb core and Cu-7 at % Sn-1 at % M (M = Ti, Cr and Fe), Cu-7 at % Sn-(0.2, 0.35, 0.5 and 1.5) at % Ti matrix alloys were worked into single-core wires 0.5 mm in diameter. Multifilamentary wires with different amounts of Ti addition to the matrix were also produced. Among additives of transition metals, Ti, Cr and Fe, only Ti showed large positive effects on the Nb3Sn layer formation rate and Hsub(e2), while Cr and Fe showed negative effects on them. A small amount of Ti addition makes crystal grains of the Cu-Sn alloys very fine. The Ti addition also makes Nb3Sn crystal grains fine for the wires reacting at 1073 K, while it slightly coarsens Nb3Sn grains for the wires reacting at 1023 K. Hsub(e2) (4.2 K) shows the maximum when the Ti concentration in the Nb3Sn layer is between 1.0 and 2.0 at %. Measurements of the normal state resitivity, rhosub(n), and (dHsub(e2)/dT) sub(T = Tsub(e)) together with calculations proved that the enhancement in Hsub(e2) due to the Ti addition was caused mainly by the increase in rhosub(n). (author)