We synthesized four Fe(67_X)Cr31CoXSi2 (X = 9, 14, 19, and 24 at.%) medium-entropy alloys and investigated the effect of Co on the connection between magnetic properties and spinodal structure. When the value of X was increased from 9 to 14 at.%, both the volume fraction of alpha 1 phase and the composition difference between alpha 1 and alpha 2 phases increased, leading to an increase in coercivity. This increase also resulted in an increase in the size of alpha 1 phase, leading to a decrease in remanence. When the value of X was increased beyond 14 at.%, both coercivity and remanence were significantly enhanced, even though the volume fraction of the alpha 1 phase remained unchanged. We attribute this enhancement to (1) intensification of the composition fluctuation in the spinodal structure, (2) increase in lattice misfits, and (3) refinement of the alpha 1 phase. Our results will be valuable in future magnet design.

In low-Ag Cu matrix alloys, the presence of coarse discontinuous precipitates may limit strength. We demonstrated that discontinuous precipitation was suppressed, and continuous precipitation was en-hanced by the doping of Cu-6 wt%Ag with Sc. A high-volume fraction of continuous precipitates, which nucleated on {111} planes, led to a 55 MPa increase in strength, with only a slight decrease in electri-cal conductivity. The addition of Sc inhibited the nucleation of discontinuous precipitates by causing the Sc and the Ag to co-segregate onto grain boundaries, thus forming a thin intermetallic compound layer between grains. After deformation, both discontinuous and continuous precipitates were drawn into Ag fibers. The combination of deformation strain and doping caused an increase in density and a decrease in the diameter of Ag fibers, resulting in about 205 MPa increase in doped samples when the deformation strain reached 4.9. The thinner, denser Ag fibers in the doped samples also caused higher electron scatter-ing at interfaces, leading to electrical conductivity that was 11% IACS lower than in non-doped samples. For reference, 100% IACS (International Annealed Copper Standard) is equivalent to 1.7241 mu ⠂ cm.(c) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Although much effort has been devoted to the study of the relationship between properties and modulated structure in FeCrCo alloys, the connection between their structure and composition at atomic scale and their magnetic and mechanical properties at macroscale remains largely unclear. We explored this connection by tracing the evolution of microstructure and composition in FeCrCo (using data from STEM-HAADF/EDS) and correlating this evolution to the development of magnetic and mechanical properties during the step aging process. The results indicated that, during the decomposition of alpha into alpha 1 and alpha 2, the size of the alpha 1 phase increased (from 10 to 36 nm) as step aging progressed. The volume fraction of the alpha 1 phase increased in early stages of spinodal decomposition, then remained unchanged in later stages. The alpha 1 phase was rich in Fe and Co. By contrast, the alpha 2 phase was rich in Cr. The composition difference between alpha 1 and alpha 2 increased during the first six stages but decreased in the last stage. A close examination of the atomic structure showed that the lattice misfit increased as step aging progressed. Both magnetic and hardness properties increased as step aging pro-gressed, but remanence and hardness decreased slightly in the final stage.

The construction of magnets with ever higher magnetic fields requires ever stronger and tougher reinforcement materials. Nitronic stainless steels are sometimes used for this purpose. Avoiding the sensitization temperature of these steels during fabrication is of critical importance because excessive exposure to this temperature may make them sensitive to corrosion. Even though these steels have exceptionally high mechanical strength and satisfactory toughness for use at room temperatures, low cryogenic temperatures may reduce their toughness to the degree that they are rendered unsatisfactory as reinforcement materials. This low toughness at cryogenic temperature in Nitronic steels often stems from the microstructure established during fabrication, even when the heat treatment profile follows the required protocol. Even materials that have not previously been exposed to sensitization temperatures during fabrication may later be exposed during the construction of magnets. Consequently, quality control of reinforcement materials must include both toughness tests and microstructure analyses. This paper reports recent studies of the effect of component volume and microstructure on cryogenic temperature toughness in Nitronic-type stainless steels. By comparing four different shells, we established that the presence of grain boundary particles caused lower impact fracture energy but had no significant impact on mechanical strength.

Application of an external magnetic field during heat treatment affects the hardness of magnetic Fe-Cr-Co alloys. The microstructure and composition at atomic scale, as well as the hardening mechanisms need extensive studies. Using atomic resolution STEM, we investigated the effect of a 3 T magnetic field on the spinodal decomposition, as well as microstructure and hardness in step-aged Fe27wt. %Cr15wt. %Co samples. Spinodal decomposition resulted in a homogeneous α phase transforming into an Fe-Co rich α1 phase and a Cr rich α2 phase. Although α1 and α2 showed distinct contrast at low magnification Z-contrast images, close examination at atomic scale of the samples showed no sharp α1/α2 interfaces. Inside each phase, composition fluctuations occurred. A 3 T external magnetic field during step aging increased the size of the α1 phase and introduced microstructural anisotropy, which is desired for permanent magnet applications, In addition, the spinodal decomposition increased the hardness. Annealing in a 3 T magnetic field decreased the hardness but increased the ductility, which is desirable for manufacturing permanent magnets. The change in hardness values is attributed to the composition fluctuations of Fe, Cr and Co, and we further discuss the mechanisms for composition fluctuations and hardness.

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.

Fe-50 vol%Pt multilayers that were prepared by accumulative roll-bonding were annealed in vacuum for 2 h at various temperatures between 723 K and 823 K. Some samples were annealed without any high magnetic field (HMF). Others were subjected HMF up to 31.2 T, applied in either in-plane or out-of-plane directions. The annealing partially transformed the original multilayered structure into a polycrystalline hard-magnetic FePt magnetic phase with ordered L1(0) structure. Textured FePt/Fe nanocomposite foils were formed under HMF. Magnetic anisotropic behavior was observed in the annealed FePt/Fe composite because of the textured nanostructure. HMF annealing decreased grain size and increased the (0 0 1) texture along the HMF direction. In the sample annealed with out-of-plane HMF, both coercivity and maximum energy product were enhanced because of higher (0 0 1) out-of-plane texture and finer nanoscaled grain sizes compared to other samples. An inplane HMF appeared to strongly inhibit phase transformation. This inhibition likely resulted from a change in Gibbs free energy and the diffusion inhibition caused by HMF.

This paper presents simulation results for double nanohole and inverted bowtie nanoapertures optimized to resonate in the short-wave infrared regime (1050 nm and 1550 nm). These geometries have shown great promise for trapping nanoparticles with applications in optical engineering, physics, and biology. Using a finite element analysis tool, we found that the outline length for inverted bowtie nanoapertures in a 100 nm thick gold film with a 20 nm gap dimension having an optimized transmission resonance for 1050 nm and 1550 nm optical wavelengths is 106.5 nm and 188.5 nm, respectively. With the same gap size, the radii of the circles for the double nanohole nanoapertures are 72 nm and 128 nm. The near-field enhancements of the two structures are almost the same, while the double nanohole geometries have a 20% larger full width at half-maximum than the inverted bowtie. Next, by studying the effect of changing the inner radii of the inverted bowtie corners, we found that the difference between 2 nm and 6 nm corner radii can blue-shift the optical resonance by up to 45 nm. As a result of not having any inner corners, the double nanohole structure requires less precise fabrication and therefore could potentially have a higher successful yield of nanoapertures during the manufacturing process. Lastly, we will show experimental results that confirm the optical resonance of the nanoapertures at 1550 nm. These results will enable better performance and signal-to-noise ratio in nanoaperture trapping for the short-wave infrared wavelength regime.

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) REBCO magnets have many advantages. They are self-protecting, therefore do not need quench detection and protection which can be very challenging in a high Tc superconducting magnet. Moreover, by removing insulation and allowing thinner copper stabilizer, NI REBCO magnets have significantly higher engineering current density and higher mechanical strength. On the other hand, NI REBCO magnets have drawbacks of long magnet charging time and high field-ramp-loss. In principle, these drawbacks can be mitigated by managing the turn-to-turn contact resistivity (Rc). Evidently the first step toward managing Rc is to establish a reliable method of accurate Rc measurement. In this paper, we present experimental Rc measurements of REBCO tapes as a function of mechanical load up to 144 MPa and load cycles up to 14 times. We found that Rc is in the range of 26-100 uOhm-cm2; it decreases with increasing pressure, and gradually increases with number of load cycles. The results are discussed in the framework of Holm's electric contact theory.

Conductors in pulsed magnets in the U.S. National High Magnetic Field Laboratory (MagLab) are rectangular cross section wires of relatively large thickness. During the manufacture of the magnets, some of these conductors are wound to small-diameter coils (less than 15 mm). Because of the large thickness and the small bending radii for winding, the wires undergo large bending strain, sometimes causing breakage. We studied the bending behavior of high-strength conductor wires. In most materials, maximum bending strain can usually be calculated from elongation values obtained in tensile tests. In this paper, however, the maximum bending strain exceeded the elongation of most of our high-strength wires; therefore, we could not estimate bending strain from elongation. The large bending strain that occurs during the manufacture of coils in pulsed magnets causes an increase in residual strain and a decrease in packing factor. Due to the anticlastic effect, the cross section of the wire changes from rectangular to keystone-shaped, with the keystone angle up to 10 $^{\circ}$ . This introduces gaps into the coils, thus reducing the magnetic field by an amount that must be taken into an account. In both tensile- and compressive-strained regions, we observed significant microstructure changes. Certain properties, such as tensile mechanical strength and electrical conductivity, depend directly on microstructure. This paper summarizes our work on both geometry and microstructure evolution in conductors exposed to different bending strain values.

CuAg in situ composite conductors are used as conductors for Florida Bitter magnets and potential candidates for pulsed magnets in the National High Magnetic Field Laboratory, and are likely exposed to temperatures higher than ambient during operations in magnets. The conductors are fabricated by cold rolling that introduces lattice distortions and high densities of interfaces in a unit volume. High temperature exposure by the conductors may affect the characteristics of the lattice distortions and the interfaces. The lattice distortion and density of the interface affects the mechanical properties of the conductors, such as the tensile and yield strength, as well as the electrical conductivity of the composites. Understanding the performance of the conductors after they are exposed to high temperature heat treatments helps one to make good use of them for magnets and to manufacture conductors to meet the requirements of the magnets, particularly when the magnetic stress reaches the limit of the mechanical strength of the conductors. The goal of this research is to understand the microstructure evolution of the Cu16at%Ag after the high temperature heat treatment and to relate such microstructural features to mechanical tensile strength and electrical conductivities.

ITER high J C Nb3Sn wires are designed to compensate for the performance degradation due to transverse electromagnetic force in cable-in-conduit-conductor. J C characterization of these wires, especially as a function of axial strain, is very important. We performed I C irreversibility strain epsivirr-epsivmax measurements and geometry examinations on two types of ITER high J C Nb3Sn wires. epsivirr-epsivmax is measured in a 18 T magnetic field by a straight pull device developed at the NHMFL. Consistent difference in epsivirr between the two types of strands is observed. We found that the epsivirr-epsivmax is ~ 0.3% for the type of strands with higher J C and ~ 0.4% for that with lower J C . This can be correlated to the difference in their geometry.

Nb3Sn superconductors are examined to select proper conductors for hybrid magnets outserts (that is a superconductor solenoid wound on the outside of a Bitter magnet) at the National High Magnetic Field Laboratory. The materials are subject to both thermal and mechanical stresses when in service. The stresses are complex due to the thermal contraction of the conduit applying extra stresses to the conductors. The final thermal stress levels can be well beyond the yield stress of some components in the superconductor composite and therefore plastic flow occurs in these components, such as Cu stabilizer and Cu-Sn matrix. If the plastic deformation strain is large, deformation strain in Nb3Sn superconductor filaments can be difficult to estimate. The differences in conductor design also render the estimation of the strain status difficult. It is well documented that the critical current is strongly affected by strain in the Nb3Sn superconductors. Therefore, we have characterized the microstructure and critical current of the superconductors with respect to the strain of various Nb3Sn superconductors. Other relevant properties are also measured and related to the fabrication, design and microstructure of the conductors. This paper reports our efforts in characterization of the Nb3Sn superconductors for hybrid magnets.