Effect of the fabrication route on the phase and volume changes during the reaction heat treatment of Nb3Sn superconducting wires

Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently built and developed using Nb3Sn superconductors. This technology requires a reaction heat treatment of the magnet coils, during which Nb3Sn is formed from its ductile precursor materials (wind and react approach). The Nb3Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the phase changes during the reaction heat treatment. 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 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 evolution in the three wire types is similar, with respectively solid state interdiffusion of Cu and Sn, Cu6Sn5 formation, and Cu6Sn5 peritectic transformation. Strong differences in phase evolutions in the different wires are found when temperatures exceed 450 °C. The volume changes of the conductor during Reaction Heat Treatment (RHT) are a main challenge 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 different Nb3Sn wire types. 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.