Beneficial influence of Hf and Zr additions to Nb4at%Ta on the vortex pinning of Nb ₃ Sn with and without an O source

Here we show that addition of Hf to Nb4Ta can significantly improve the high field performance of Nb ₃ Sn, making it suitable for dipole magnets for a machine like the 100 TeV Future Circular Collider (FCC). A big challenge of the FCC is that the desired non-Cu critical current density (J _c ) target of 1500 A mm ^-2 (16 T, 4.2 K) is substantially above the best present Nb ₃ Sn conductors doped with Ti or Ta (∼1300 A mm ^-2 in the very best sample of the very best commercial wire). Recent success with internal oxidation of Nb-Zr precursor has shown significant improvement in the layer J _c of Nb ₃ Sn wires, albeit with the complication of providing an internal oxygen diffusion pathway and avoiding degradation of the irreversibility field H _Irr . We here extend the Nb1Zr oxidation approach by comparing Zr and Hf additions to the standard Nb4Ta alloy of maximum H _c2 and H _Irr . Nb4Ta rods with 1Zr or 1Hf were made into monofilament wires with and without SnO ₂ and their properties measured over the entire superconducting range at fields up to 31 T. We found that Group IV alloying of Nb4Ta does raise H _Irr , though O ₂ addition still slightly degrades it. As noted in earlier Nb1Zr work with an O source, the pinning force density F _p is strongly enhanced and its peak value shifted to higher field by internal oxidation. A surprising result of this work is that we found better properties in Nb4Ta1Hf without SnO ₂ , F _pMax achieving 2.35 times that of the standard Nb4Ta alloy, while the oxidized Nb4Ta1Zr alloy achieved 1.54 times that of the Nb4Ta alloy. The highest layer J _c (16 T, 4.2 K) of 3700 A mm ^-2 was found in the SnO ₂ -free wire made with Nb4Ta1Hf alloy. Using a standard A15 cross-section fraction of 60% for modern powder-in-tube and rod restack process wires, we estimated that a non-Cu J _c of 2200 A mm ^-2 is obtainable in modern conductors, well above the 1500 A mm ^-2 FCC specification. Moreover, since the best properties were obtained without SnO ₂ , the Nb4Ta1Hf alloy appears to open a straightforward route to enhanced properties in Nb ₃ Sn wires manufactured by virtually all the presently used commercial routes employed today.