Dans la continuité du séminaire proposé le 27 Novembre 2019, les résultats de l’étude de l’influence de la structuration de fils de Cu/Sn/Nb sur le chemin de réaction vers l’obtention du composé Nb₃Sn supraconducteur sont publiés ce mois-ci dans la revue “Superconducting Science and Technology”.

Abstract

Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently being built and developed, using Nb₃Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb₃Sn is formed from its ductile precursor materials (a “wind and react” approach). The Nb₃Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the phase changes during RHT. By combining in situ differential scanning calorimetry, high energy synchrotron x-ray diffraction, and micro-tomography experiments, we have acquired a unique data set that describes in great detail the phase and microstructure changes that take place during the processing of restacked rod process (RRP), powder-in-tube (PIT), and internal tin (IT) Nb₃Sn wires. At temperatures below 450 °C the phase evolutions in the three wire types are similar, with respectively solid state interdiffusion of Cu and Sn, Cu₆Sn₅ formation, and Cu₆Sn₅ peritectic transformation. Distinct differences in phase evolutions in the wires are found when temperatures exceed 450 °C. The volume changes of the conductor during RHT are a difficulty in the production of Nb₃Sn accelerator magnets. We compare the wire diameter changes measured in situ by dilatometry with the phase and void volume evolution of the three types of Nb₃Sn wire. Unlike the Nb₃Sn wire length changes, the wire diameter evolution is characteristic for each Nb₃Sn wire type. The strongest volume increase, of about 5%, is observed in the RRP wire, where the main diameter increase occurs above 600 °C upon Nb₃Sn formation.