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 being built and developed, using Nb$_3$Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb$_3$Sn is formed from its ductile precursor materials (a “wind and react” approach). The Nb$_3$Sn 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) Nb$_3$Sn 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, Cu$_6$Sn$_5$ formation, and Cu$_6$Sn$_5$ 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 Nb$_3$Sn 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 Nb$_3$Sn wire. Unlike the Nb$_3$Sn wire length changes, the wire diameter evolution is characteristic for each Nb$_3$Sn 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 Nb$_3$Sn formation.