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 3 Sn wires, in particular a critical current density (J c ) of more than 1500 A mm −2 at 16 T and 4.2 K with an effective filament diameter (D eff ) of less than 20 μm. The restacked-rod-process (RRP ® ) is the technology closest to meeting these demands, with a J c (16 T) of up to 1400 A mm −2 , residual resistivity ratio>100, for a subelement size D s of 58 μm (which in RRP ® wires is essentially the same as D eff ). An important present limitation of RRP ® is that reducing the sub-element size degrades J c to as low as 900 A mm −2 at 16 T for D s =35 μm. To gain an understanding of the sources of this J c 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 3 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 rangegreatly 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 6 Sn 5 ) and the more desirable ε phase (Cu 3 Sn), which decomposes at 676°C. After the mixing stages, when heated above 408°C towards the Nb 3 Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn 2 on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb 3 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 s of 41 μm, the Nausite control heat treatment increases the J c at 16 T by 36%, reaching 1300 A mm −2 (i.e. 2980 A mm −2 at 12 T), and moving RRP ® closer to the FCC targets.

Beneficial influence of Hf and Zr additions to Nb4at%Ta on the vortex pinning of Nb3Sn with and without an O source

“…31, while the orange dashed lines are for the best Ti-doped RRP® after the optimized Cu-Sn mixing heat treatment of ref. 8. Our Nb4Ta wire has a layer Jc of 3210 ± 920 A/mm 2 at 12 T and 1250 ± 360 A/mm 2 at 16 T, values typical of those found for Ta-doped RRP® conductors, which range between 3190-5250 A/mm 2 at 12 T and 1400-1880 A/mm 2 at 16 T. The Tidoped RRP® conductor after optimized heat treatment has markedly better 16 T properties, with a layer Jc > 2000 A/mm 2 .…”

“…To achieve such not yet achieved dipole fields requires magnet conductors with the not yet achieved high current density (Jc) of 1500 A/mm 2 at 16 T (4.2 K), while simultaneously maintaining high Cu stabilizer residual resistivity ratio (RRR) ≥ 100 and a large strand diameter of order 1 mm [7]. Recent rethinking of the heat treatment of commercial rod restack process (RRP) conductors has shown that a better optimized Sn mixing heat treatment can allow Jc (16 T, 4.2 K) as high as 1300 A/mm 2 in specially selected 0.8 mm diameter wires, a significant 20% increase over the best prior standard heat treatment [8]. However, real conductor production has significant spread so there is little hope to achieve FCC specification by this route alone.…”

Implications of the strain irreversibility cliff on the fabrication of particle-accelerator magnets made of restacked-rod-process Nb3Sn wires

“…This deserves a study of its own to supplement the results presented here. Effects on the allowable HT window of a simplified HT scheme that has two stages (instead of three) to control the Sn-Nb-Cu Nausite phase are also worthy of a systematic study 19 .…”

“…Reducing the size of Nb 3 Sn sub-elements can mitigate these instabilities significantly, and a sub-element size around 20 μm or less is desirable 1 . However, size reductions below 50 μm in RRP wires have resulted in a dramatic drop of J c 8,19 . In addition, such wires still need further development to become technological conductors that can be readily fabricated in piece-lengths sufficient for applications.…”

Phase Evolution During Heat Treatment of Nb3Sn Wires Under Development for the FCC Study

“…An important recent contribution was the reassessment of heat treatment design for Restacked Rod Process (RRP ® ) wires (Bruker OST) undertaken by Sanabria et al [2], [3]. Conventional heat treatments include steps at around 210 °C and 400 °C, followed by the reaction step that forms Nb3Sn at 650-665 °C (Fig.…”