We report on atomic-scale analyses using high-resolution scanning transmission electron microscopy (HR-STEM), atom-probe tomography (APT), and first-principles calculations to study grain-boundary (GB) segregation behavior of Nb 3 Sn coatings on Nb, prepared by a vapor-diffusion process for superconducting radiofrequency (SRF) cavity applications. The results reveal Sn segregation at GBs of some Nb 3 Sn coatings, with a Gibbsian interfacial excess of ~10-20 Sn atoms/nm 2 . The interfacial width of Sn segregation at a GB is ~3 nm, with a maximum concentration of ~35 at.%. HR-STEM imaging of a selected [12 ̅ 0] tilt GB displays a periodic array of the structural unit at the core of the GB, and firstprinciple calculations for the GB implies that excess Sn in bulk Nb 3 Sn may segregate preferentially at GBs to reduce total internal energy. The amount of Sn segregation is correlated with two factors: (i) Sn supply; and (ii) the temperatures of the Nb substrate and Sn source, which may affect the overall kinetics including GB diffusion of Sn and Nb, and the interfacial reaction at Nb 3 Sn/Nb interfaces. An investigation of the correlation between the chemistry of GBs and Nb 3 Sn SRF cavity performance reveals no significant Sn segregation at GBs of high-performance Nb 3 Sn SRF cavities, indicating possible effects of GB segregation on the quality (Q 0 ) factor of Nb 3 Sn SRF cavities. Our results suggest that the chemistry of GBs of Nb 3 Sn coatings for SRF cavities can be controlled by grain-boundary engineering, and can be used to direct fabrication of high-quality Nb 3 Sn coatings for SRF cavities.

Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP® and a lower non-Cu Jc (12 T, 4.2 K) (2500–2700 A mm−2 versus 2900–3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1–2 μm sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.

High critical current density (J c ) Nb 3 Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a J c (12 T, 4.2 K) up to ~2500 A/mm 2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible J c must be addressed by forming the maximum fraction of high J c small-grain (SG) A15 and minimizing low J c large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial J c improvement.

Nb 3 Sn wires are now very close to their final optimization but despite its classical nature, detailed understanding of the role of Ta and Ti doping in the A15 is not fully understood. Long thought to be essentially equivalent in their influence on H c2 , they were interchangeably applied. Here we show that Ti produces significantly more homogeneous chemical and superconducting properties. Despite Ta-doped samples having a slightly higher T c onset in zero-field, they always have a wider T c -distribution. In particular, whereas the Ta-doped A15 has a T c -distribution extending from 18 down to 5-6 K (the lowest expected T c for the binary A15 phase), the Ti-doped samples have no A15 phase with T c below ∼12 K. The much narrower T c distribution in the Ti-doped samples has a positive effect on their in-field T c -distribution too, leading to an extrapolated µ 0 H c2 (0) 2 Tesla larger than the Ta-doped one. Ti-doping also appears to be very homogeneous even when the Sn content is reduced in order to inhibit breakdown of the diffusion barriers in very high J c conductors. The enhanced homogeneity of the Ti-doped samples appears to result from its assistance of rapid diffusion of Sn into the filaments and by its incorporation into the A15 phase interchangeably with Sn on the Sn sites of the A15 phase. a) Electronic mail: tarantini@asc.magnet.fsu.edu 2 Application of large quantities of Nb 3 Sn conductors first for ITER 1 and now for the High Luminosity upgrade of the Large Hadron Collider (LHC) 2 has motivated major recent R&D and raised multiple questions about how best to optimize the superconducting strand, 3 especially with respect to balancing the conflicting requirements of small hysteretic loss, high critical current density (J c ) and a high residual resistance ratio (RRR) in the Cu stabilizer. 4 However, few recent studies have investigated the fundamental properties of the A15 phase in these newer wires, which has a composition range with T c varying from 18 K (stoichiometric) to 6 K on the Sn-poor (18at.%Sn) side of the binary. Controlling this composition range is crucial to optimizing Nb 3 Sn properties. The classical nature of Nb 3 Sn implies broadly well-known properties. 5,6 For example the very similar effect of Ti and Ta in enhancing H c2 was clearly shown many years ago by Suenaga et al., 7 who also noticed a slight increase of T c upon doping that was greater for Ta. Ta, Ti or Ti+Ta doping are now standard for enhancing the in-field performance. 8 However, for a long time Ti and Ta were both thought to substitute on the Nb site, but more recent studies suggested that Ti can substitute on the Sn site. 9,10 A comparison of the in-field physical and compositional properties of Ti and Ta-doped A15 phase in the most modern, highest-J c wires taking into account these factors has not been performed yet. Maintaining a high RRR in the Cu stabilizer is crucial but frequently in conflict with the need to obtain the largest possible quantity of high quality (i.e. homogeneous A15 close to the stoichiometri...