Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^®Nb₃Sn wires

Abstract

Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb<sub>3</sub>Sn wires, in particular a critical current density (J<sub>c</sub>) of more than 1500 A mm<sup>-2</sup> at 16 T and 4.2 K with an effective filament diameter (D<sub>eff</sub>) of less than 20 μm. The restacked-rod-process (RRP<sup>®</sup>) is the technology closest to meeting these demands, with a J<sub>c</sub>(16 T) of up to 1400 A mm<sup>-2</sup>, residual resistivity ratio > 100, for a sub-element size D<sub>s</sub> of 58 μm (which in RRP<sup>®</sup> wires is essentially the same as D<sub>eff</sub>). An important present limitation of RRP<sup>®</sup> is that reducing the sub-element size degrades J<sub>c</sub> to as low as 900 A mm<sup>-2</sup> at 16 T for D<sub>s</sub>= 35 μm. To gain an understanding of the sources of this J<sub>c</sub> degradation, we have made a detailed study of the phase evolution during the Cu-Sn 'mixing' stages of the wire heat treatment that occur prior to Nb<sub>3</sub>Sn formation. Using extensive microstructural quantification, we have identified the critical role that the Sn-Nb-Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C-400 °C range—greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu<sub>6</sub>Sn<sub>5</sub>) and the more desirable ϵ phase (Cu<sub>3</sub>Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb<sub>3</sub>Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn<sub>2</sub> on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb<sub>3</sub>Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ϵ phase through better Cu-Sn mixing. At a D<sub>s</sub> of 41 μm, the Nausite control heat treatment increases the J<sub>c</sub> at 16 T by 36%, reaching 1300 A mm<sup>-2</sup> (i.e. 2980 A mm<sup>-2</sup> at 12 T), and moving RRP<sup>®</sup> closer to the FCC targets.