Tin supply and microstructure development during reaction of an external tin process Nb3sn multifilamentary composite

Abstract

The class of “high tin” multifilamentary Nb3Sn superconducting composites depends on the diffusion of tin from a high tin reservoir to the niobium filaments where the superconducting A15 phase grows by solid state reaction. In particular, external tin composites are fully fabricated as niobium filaments in a copper matrix and the wire is subsequently coated with tin prior to reaction. In the work reported a detailed study is made of tin diffusion and microstructure development during the reaction of a 1369 filament external tin composite wire consisting of 37 × 37 bundled filaments of 3.5 μm diameter. During the initial low-temperature anneal stage the formation and evolution of copper-tin intermetallic phases is followed. High-temperature reaction anneals were then carried out at 755 and 588° C. The rapid conversion of e-phase to α-Cu is accompanied by diffusion of tin towards the centre of the composite and the growth of Nb3Sn A15 phase layers on the niobium filaments. The variation of tin composition and layer thickness is reported for different stages of reaction. In addition, the average composition of the A15 layer is measured as a function of the radial position of the filament, and the tin concentration gradient is measured within the A15 layer for the outermost filaments of the composite. The results show clearly the very strong dependence of the A15 layer growth rate on the tin concentration in the matrix. As a result, for an isothermal anneal at 755° C the layer growth is highy non-uniform across the composite. Furthermore it can be seen that additional inhomogeneity of the layer composition arises as a consequence of the bundled geometry of the composite. An important practical observation is that when the layer thickness approaches the filament radius the A15 compound is still far from stoichiometry particularly for filaments near the centre of the composite. The results overall emphasize the need for a detailed understanding of tin supply and compound growth before optimum heat-treatment procedures can be prescribed for a particular external tin design. A comparison of anneals at 755 and 588° C indicates that the relative rates of tin diffusion and A15 layer growth changes strongly with temperature; this suggests that a combination of anneals at different temperatures might be necessary for the optimization of the superconducting properties of high tin composites.