Non-Uniform Bronze Formation in Internal Tin ${\rm Nb}_{3}{\rm Sn}$ Wire

The bronze phase transformations during the heat treatment of two multifilamentary internal tin Nb₃Sn wires for high field applications made by Luvata are investigated and compared with some model single core wires. Non-uniform bronze profiles, non-uniform Sn distributions in different bundles, and Sn concentration gradient inversion were observed. Their possible formation mechanisms and consequences are discussed in relation to crystal orientation, Ti addition and microstructure. Wire manufacturing processes such as wire drawing impart preferred orientations on the components. The effect of wire texture on Cu-Sn interdiffusion and non-uniform bronze formation was investigated and believed not to be the cause of the non-uniform bronze profiles. 3-dimensional X-ray microtomography is applied to explain the non-uniformity in different bundles by analyzing the porosity formed during reactive diffusion of the low-melting phases. Ti is often added as a ternary addition, either in the Sn core or as Nb-Ti. In the Luvata wires investigated, Ti is present in Sn as the intermetallic compound Ti₆Sn₅. Its dissolution through a ternary CuSnTi phase, possibly via reaction with CU₃S₁₁, has been observed. EDS mapping of the Ti distribution suggests that it is likely to be responsible for the inversion of the Sn concentration gradient. The potential advantage of adding Ti in Sn over using Nb-Ti alloy is discussed. Critical temperature measurements were performed to relate superconducting properties to the phase transformations, composition changes and non-uniform bronze formation.