High purity niobium (Nb) is a technologically important material for large-scale accelerator and nano-scale quantum computing applications in microwave frequency range. The high thermal conductivity and low resistivity of Nb are critical to the high performance at temperature range of 0.01–4.0 K. The presence of interstitials such as O, N, H, and C act as scattering centers and alter the mean free path, reducing resistivity and thermal conductivity, and contributing significantly to Nb's thermal performance for temperatures of 2.0 K and above. The residual resistivity ratio (RRR), defined as the ratio of the normal state resistivity at 300 K to that at 4.2 K (Nb, $T_{c}$ = 9.2 K), is an accepted direct estimate of the impurity content of fully recrystallized Nb. Complete re-crystallization of Nb is challenging unless very high temperatures are employed, which is often impractical, hence, in practice, dislocation and dislocation structures impact the thermal performance of Nb due to strong phonon scattering contributions. This paper reports on the degradation of thermal conductivity and RRR of high purity Nb large grain, single crystal with fixed impurity, varying strain, and dislocation content levels. Experimental thermal conductivity data fits the Boltzmann transport equation incorporating dislocation density.

An investigation into the effects of high cycle fatigue on REBCO coils along their axis of rotation was performed to establish design limits. Three coils were tested in liquid nitrogen via a hydraulic materials testing machine. The coils were fabricated with the same type of REBCO tape but had different co-wind materials and dimensions. The results showed that for a coil with REBCO tape wider than its co-wind, where the loads bear directly on the conductor, had 5% permanent critical current degradation at pressures near 100 MPa. A significant improvement was observed in a wax impregnated coil, which aided to distribute loads, with a 1.2% permanent critical current reduction after 50,000 load cycles to 125 MPa. Additionally, a coil with co-wind wider than its REBCO tape was also improved and reached 5% reduction after 50,000 cycles at pressures up to 100 MPa and continued loading to 151 MPa.

During the NHMFL 32 T magnet project, a program to develop a reliable method for making soldered lap joints between REBCO tape conductors was initiated. A standard process was adopted during the program, and then applied during coil fabrication and for quality control (QC) of incoming conductor. For each unique REBCO conductor piece procured for the 32 T project, a lap solder joint was made using the standard process and tested for resistance in liquid nitrogen. A total of 211 lap joints were made and tested. For the NHMFL 40 T magnet project, a similar study was performed using the same standard process on conductors procured for test coils. A total of 38 lap joints were made and tested. Results from both data sets are reported, and compared with findings from the initial study and published findings by others. In the stacked double-pancake construction adopted by the NHMFL for REBCO coils, each double-pancake is connected in series to adjacent modules with a crossover connection, made from an assembly of several REBCO tapes placed in parallel and soldered across the terminal ends of each adjacent module. Resistances of the crossovers in a 32 T test coil and from test articles and test coils made during the 40 T project, are reported here. The resistivity of the crossovers is estimated and compared with the findings from the lap joint measurements.

High field magnets require the development and fabrication of large quantities of conductors with both high strength and high electrical conductivity. This combination of properties can be obtained from copper matrix composites either in macroscopic or microscopic form. Deformation can strengthen these composites further by either inducing dislocations or refining microstructure. During deformation, the strengthening component either retains its original shape or flows with the matrix, depending on its original hardness. In general, a non-deformable component is initially harder than one that deforms with the matrix. Co-deformation in both component and matrix leads to very high strength levels that are significantly greater than those that can be achieved in composites strengthened by non-deformable components. Thus, to properly choose a system for application in high field magnets, we must consider the detailed mechanisms of strengthening that are operative in materials with ultra-fine scale microstructure. In this paper, we compare composites strengthened by either deformable or non-deformable components and describe parameters for the design and fabrication of materials selected for high field magnets.

Rare earth Barium Copper Oxide (REBCO) coated conductors are promising candidates for high field (>25 T) user magnets. However, as the demand for higher fields increase, so does the potential to overstrain the conductors being used. Coated conductor substrates, such as 310s stainless steel and the super-alloy Hastelloy C276, serve as the backbone for mechanical strength in these conductors. Both substrate alloys share similar properties when optimally processed into strips prior to manufacturing of the REBCO coated conductor. We find that with subsequent REBCO manufacturing processes the strength of the substrate changes, the magnitude of which depends on whether Hastelloy C276 or 310s stainless steel is used. In this study, we investigate the stress-strain variability found in coated conductors and how the manufacturing process affects the mechanical properties. The manufacturing step of concern is the short time that the substrate is exposed to high temperature (700 to 800 C) during the REBCO deposition process. To better relate manufacturing processes and mechanical properties, we subjected bare substrates to different heat treatments at 700, 750, and 800 C for 15 minutes each. With post heat-treatment room-temperature tensile tests, we found that the 310s stainless steel substrate was sensitive to the variations of time and temperature, exhibiting yield strength reductions of 20 to 50 % depending on the heat treatment. By contrast, Hastelloy C276 did not weaken and initially showed strengthening effects with exposure to the lower temperature heat treatments. Coated conductor manufactures may prefer 310s stainless steel as their substrate due to cost and availability, however, moving to Hastelloy C276 will offer better mechanical robustness and reproducibility of mechanical properties within their coated conductor.

High-temperature superconducting magnet coils made with Sumitomo Type HT-NX are complex composite structures composed of Bi-2223 conductor filaments, silver matrix, solder, nickel-alloy laminations, polymer insulation, and epoxy or wax. The mechanical properties of these composites are required inputs to a correct stress analysis. Measurements of the desired properties are performed on representative model test specimens. Mechanical test specimens composed of several layers of insulated conductor are prepared by cutting to length, stacking and epoxy impregnation. Mechanical tests are performed in liquid nitrogen and liquid helium. Elastic constants are found from tensile strain measurements in the conductor longitudinal, or coil hoop, direction and from compressive strain measurements in the conductor transverse, or coil radial and axial directions.

Nitronic 40 forged shells are typically used for structural reinforcement in high field pulse magnet design and applications. To better understand the mechanical performance of this versatile high strength austenitic steel a series of mechanical tests were conducted. Tensile were performed at 295 K, 77 K and 4 K, and cryogenic fracture mechanics tests were performed at 77 K and 4 K. The effect of temperature on strength, ductility, toughness and fatigue crack growth rate are evaluated. Microstructure and composition effects are also presented and discussed.

High-strength copper conductor composites have been used in pulse magnet applications and as internal reinforcement in superconductor wires. The required property is high strength while maintaining high conductivity. Heavily worked pure copper has a strength of 450-500 MPa. Increasing the strength of copper by alloying additions without affecting the conductivity drastically is a significant challenge. Mixing pure copper with a higher modulus, higher strength tantalum alloys is an option to meet the challenge. This work reports on the use of a tantalum-tungsten alloy to strengthen high-purity copper through a multi-filament approach. The tantalum tungsten has a high modulus and very low solubility in copper. A composite wire containing 133 filaments of Ta-W (25% by volume) has a conductivity of 80% International Annealed Copper Standard and ultimate tensile strength of over 650 MPa with a modulus over 140 GPa. The wire has good ductility and toughness. These results are comparable or better than current options, and the potential to improve this product is explored.

Within the framework for establishing standards of test methods for superconducting technical wires, various standards have been issued by the International Electrotechnical Commission (IEC) (standard documents IEC 61788-1 to -20). Following the successful round robin test (RRT) for tensile testing REBCO wires at room temperature (Osamura K et al 2014 Supercond. Sci. Technol. 27 085009), this effort is extended to tensile test HTS wires at cryogenic temperatures and is coordinated by the CryoMaK lab at Karlsruhe Institute of Technology. Five different commercially available REBCO wires from five different manufacturers and one BiSCCO wire from another supplier were provided by the Versailles Project on Advanced Materials and Standards (http://vamas.org) for testing. Samples were distributed between eight participating labs from five different countries for testing according to the specified guidelines. After the test results were delivered by all participants, the data were evaluated with statistical tools to investigate the main source of scatter and its magnitude in the test results. The final goal of the RRT is issuing an ISO/IEC standard for a cryogenic temperature tensile test for REBCO wires. In this report the results of the RRT for tensile testing REBCO wires at cryogenic temperatures are presented and discussed.

Part of the process for fabricating high field magnets using exceptionally strong reinforcement materials involves subjecting these materials to deformation at room temperature beyond the yield point. During the operation of the magnets, further plastic deformation may also occur at cryogenic temperatures. In most face-centered-cubic (fcc) materials, strain-hardening occurs after plastic deformation, further increasing strength. This paper reviews some of our recent work concerning the effect of microstructure on both yielding and strain-hardening in 316LN, a modified stainless steel, and in Haynes 242, a Ni-Mo-Cr alloy. Although both materials have fcc matrix, Haynes 242 yielded at higher stress with a lower strain hardening rate than 316LN at a low strain range. Because of high nitrogen content, 316LN showed pronounced yield point elongation. At cryogenic temperatures, both materials showed higher strain-hardening rates.

The high strength conductors used in pulsed magnets in the US National High Magnetic Field Laboratory (MagLab) are manufactured from Cu matrix composites. One of the composites is made from particle-reinforced Cu. The fabrication of these composite conductors requires high deformation strain, which creates high densities of dislocations and reduced particle spacing. Both mechanical strength and electrical conductivity can be predicted from particle spacing and dislocation density. When dislocation density reaches a certain value, the particle size, distribution, and shape become important to mechanical properties. We studied the particle size, distribution, and shape in high-strength conductors with respect to the properties of the conductors. The two most important factors related to the above parameters were dislocations near the interface between particle and matrix and stress concentration near the particles. By engineering these variations, the properties of the conductors can be optimized. This paper reports our understanding of the relationship between critical properties and particle distribution in composite conductors for high field pulsed magnets.

Zylon fibers, which are made of high-strength polymer, are used at the National High Magnetic Field Laboratory for structural reinforcement of high field pulsed magnet coils. Most polymers are subject to chain scission, which is the result of aging degradation of the fibers in the absence of chemical agent. We studied chain scission of Zylon fibers stored with and without visible light. No mechanical strength degradation was found in Zylon stored without exposure to visible light for 8 years. Prolonged exposure to lab-lighting rendered formation of surface defects, resulting in reduction in mechanical strength.

No-insulation (NI) superconducting REBCO magnets have advantages of self-quenchprotection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (R-c). In an effort to control R-c we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films We present experimental results of R-c of an as-received sample under cyclic contact pressure of 2.5-25 MPa up to 30 000 cycles. After an initial increase in R-c . for the first 10-20 cycles, R-c . decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low R-c . resulting from a previously high number of load cycles. We also studied R-c . of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase R-c, we experimented with electro- and electroless plating of Ni, Cr, and Ni-P. We also measured R-c . with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases R-c by about three orders of magnitude; while the Cu plated SS interlayer only increases R-c by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol (R) C solution. This controlled oxidation allowed the R-c . to be controlled over a wide range.

The ITER central solenoid (CS) components are currently being manufactured. This Nb3Sn superconducting magnet will provide the magnetic flux swing required to induce up to 15 MA as plasma current. It includes six identical coils, called modules, stacked on top of each other to form a solenoid, enclosed inside a structure split into nine subsets, to provide vertical precompression and mechanical support. High mechanical stresses in materials and high voltages call for the use of structural materials with high strength and toughness and high dielectric strength insulating materials, respectively. The pulsed operation imposes materials with high fatigue strength at cryogenic temperatures. Unlike for the structure, where large existing manufacturing tools were usable, the modules required the construction of a dedicated manufacturing line. A comprehensive qualification programme is performed at the manufacturers before applying procedures for the production of the CS components. The main characteristics of the CS components, their manufacturing routes and the different elements of the qualification programme are described. The overall plan for the manufacture is reported. The status of the first series production components manufactured is presented as well as the planned delivery schedule to the ITER site.

Materials used as reinforcement for conductors in high-field magnets require both a high capacity for load bearing and a high resistance to deformation under stress; that is, a high value for tensile strength and a high modulus of elasticity. In addition, compatibility between the magnet conductor and any proposed reinforcement materials has to be carefully evaluated in terms of their capability for thermal expansion, stability at high temperature, resistance to oxidation, and crack propagation. We investigated a number of (nickel based and nickel-cobalt based) superalloys designed for high-temperature applications. These superalloys have higher Young's modulus than the stainless steels that are currently used as reinforcement materials for high-field magnets. Our test materials were subjected to thermo-mechanical processing that strengthens the alloys by forming very fine particles within them. Our initial work focused on changes that occured in the alloy during deformation at either cryogenic or room temperature. Because we observed distinct interfaces between the particles and the matrix, we decided that these materials could be described as precipitate-strengthened alloys. Both the strengthening component area and the matrix had more resistance to plastic deformation at cryogenic temperatures, than at room temperatures. In some cases, we further enhanced the strength of the alloy by doping them with other elements. In all the cases, these alloys permitted more efficient performance of conductors by shareing more of the load than would be possible with stainless steel reinforcement materials. This paper outlines the properties of these new alloys and establishes their compatibility with certain conductors commonly used for high-field magnets.

In 2005, the Committee on Opportunities in High Magnetic Fields issued a challenge to develop a 30 T high-resolution NMR magnet. In response, the National High Magnetic Field Laboratory (NHMFL) is investigating all three commercially available high-temperature superconductors including REBCO, Bi-2212 and most recently, a reinforced Bi-2223 conductor supplied by Sumitomo Electric, designated Type HT-NX. Recent investigations of Type HT-NX conductor at the NHMFL and by others suggest that operation at hoop stress above 400 MPa, and total strain above 0.7% may be feasible. We have fabricated a test coil from a single 240 m length of HT-NX. The coil was successfully operated to 19.5 T in a 14 T background field, with a total applied strain of 0.8% and coil current density of 243 A/mm(2). The coil was cycled 20 times from half the design current to full current without observed degradation.

Plastic deformation under constant load (creep) in austenitic stainless steels has been measured at temperatures ranging from 4 K to room temperature. Low-temperature creep data taken from past and unreported austenitic stainless steel studies are analyzed and reviewed. Creep at cryogenic temperatures of common austenitic steels, such as AISI 304, 310 316, and nitrogen-strengthened steels, such as 304HN and 3116LN, are included. Analyses suggests that logarithmic creep (creep strain dependent on the log of test time) best describe austenitic stainless steel behavior in the secondary creep stage and that the slope of creep strain versus log time is dependent on the applied stress/yield strength ratio. The role of cold work, strain-induced martensitic transformations, and stacking fault energy on low-temperature creep behavior is discussed. The engineering significance of creep on cryogenic structures is discussed in terms of the total creep strain under constant load over their operational lifetime at allowable stress levels.

For the past couple decades, 316LN stainless steel has remained the “go-to” alloy for structural components intended for cryogenic temperature service, partially because of its favorable mechanical properties, but also because of the data available in the literature for T = 4 K. In recent years, some interest has arisen to investigate and develop stronger and tougher alloys for cryogenic structural components, particularly for magnet systems like ITER. This study presents new 4 K fatigue crack growth rate (FCGR) and fracture toughness data for Nitronic® 50 and JK2LB stainless steels, compiles existing data for these alloys, and compares them with 316LN data found in literature. This study intends to further expand the existing cryogenic data set for these alloys, clarify key differences between them to better facilitate mechanical design, and potentially bolster further alloy development.

We study here the effect of axial strain on the degradation of the critical current Ic for bare and reinforced, overpressure processed Bi-2212 conductors. We show that reinforcement markedly improves the conductor’s stress limit, doubling it from ~150 MPa in the bare conductor to ~300 MPa when reinforced. We find also that certain processes used to reinforce the conductor slightly reduce the Ic degradation strain limit from ~0.6% to ~0.4%. Stress vs strain data taken from the samples studied here has been used to create a finite element model to explore the feasibility of using a reinforced Bi-2212 strand (produced by Solid Material Solutions) in a small test coil. The model predicts an IC limited coil with a maximum hoop strain of 0.31%, well below the experimentally verified strain limit, and is designed to lead to Bi-2212 coils that are not strain limited, but Ic limited.