Rapid-heating, Quenching And Transformation Research Articles

Correlation between J enhancement and phase homogeneity in Sn-Ge co-doped Nb3Al superconducting wires

In response to the growing demand for critical current density (Jc) in superconducting wires, Sn-Ge co-doped Nb3Al wires were fabricated by the rapid-heating, quenching and transformation (RHQT) method. Physical Property Measurement System-Vibrating Sample Magnetometer (PPMS-VSM) was used to perform magnetic measurements at low-temperature and external field to estimate Jc and superconducting transition temperature (Tc). Backscattered electron (BSE) image acquisition and EDS analysis of the polished surface by FESEM were used to determine phase homogeneity and elemental distribution. A significant positive correlation was found between Jc (up to 7 T at 8 K) and ΔTc. Compared with binary sample, doubled Jc was obtained in the low-dose Sn-Ge co-doped Nb3Al wire, reaching 3.01 × 103 A/mm2, 1.46 × 103 A/mm2 and 0.39 × 103 A/mm2 at 5 T and 8 K, 10 K and 12 K, respectively. The increase in Jc can be attributed to the enhancement of the homogeneity of the superconducting phase, especially the distribution of Al element therein.

Microstructure and Composition Comparison of Ti‐ and Ta‐Doped Nb3Al Superconducting Wires Fabricated by RHQT Process

Herein, the microstructure and composition of the Ti‐ and Ta‐doped Nb3Al superconductor fabricated by rapid heating, quenching, and transformation (RHQT) process are compared. After rapid heating and quenching (RHQ) process, body‐centered cubic (bcc) phase is formed in all the wires, showing dark stripe and bright contrast morphology in the backscattered electron (BSE) images. After transformation heat‐treatment, all wires consist of A15 phase and show the same morphology as that of the quenched samples. Near‐stoichiometric composition is shown in 3 at% Ti‐ and Ta‐doped Nb3Al samples, but when doping amount is 10 at%, Ti‐doped sample has constant Nb content of 67 at% and Ta‐doped sample has Al + Ta content of 25 at%. Superconducting properties of both 3 at% Ti‐ and Ta‐doped Nb3Al samples are comparable and much superior than that of 10 at% doped samples, including critical temperature (Tc), critical current density (Jc), and irreversible field (Birr). Though 10 at% Ta‐doped Nb3Al has higher Tc than 10 at% Ti‐doped sample, it presents wider ΔTc value and lower Jc performance. When increasing the measurement temperature from 8 to 10 K, the decay ratio of Birr value for Ti‐doped Nb3Al superconductor follows the scaling law; however, the Ta‐doped samples are not the case.

Study of RHQT-processed Nb3Al multifilamentary rectangular tape strand to be applied to a fusion magnet

Nb3Al superconducting wire shows an excellent strain tolerance compared with Nb3Sn wire and is promising conductor for a future fusion magnet. We investigated about the changes of the superconducting property and the wire flexibility by rectangular tape-shaped deformation on the Rapid Heating/Quenching and Transformation (RHQT) processed Nb3Al multifilamenatry wires. The improvement of the wire flexibility will be enable to the future complicated coil winding through the "React and Winding" process. The Rapid Heating/Quenching (RHQ) treatment was performed on a round-shaped Nb/Al precursor wire, and Nb/Al filaments has converted to the ductile bcc Nb-Al supersaturated solid solution (bcc-SS) phase after the RHQ (Rapid heating and quenching) treatment. The round-shaped wire after the RHQ treatment could be easily cold flat-rolled to the rectangular tape-shape (width: 4.6 mm, thickness: 0.2 mm) at room temperature. The phase transformation from bcc-SS phase to stoichiometric A15-Nb3Al phase by the post-annealing was confirmed. Critical current (Ic) and critical current density (Jc) at 4.2 K and 18 T of the RHQT processed Nb3Al rectangular tape strand was obtained approximately 180 A and 200 A/mm2, respectively. Jc property above 18 T in the rectangular tape strand were clearly improved compared to the round wire strand, and the upper critical magnetic field (Hc2) above 26 T was also obtained. In addition, no Ic deterioration by applying a bending strain up to 0.66% was observed. The wire flexibility of the RHQT processed Nb3Al strand could be improved by the change of wire shape from round wire to rectangular tape. We found that the Nb3Al rectangular tape stand will be desirable and suitable material to apply to a future huge fusion magnet through the React & Winding process.

Improved critical current density of Cu-doped Nb3Al wire through optimized Rapid-Heating Quenching and Transformation method

We have prepared 2.5% Cu-doped Nb3Al wire by the Rapid-Heating Quenching and Transformation (RHQT) method with optimized annealing. XRD results show that A15 and supersaturated bcc phase are formed after high current RHQ process, and the supersaturated bcc phase is transformed into A15 phase after conventional annealing (800 °C\\10 h). We observed the microstructures at different annealing time by field-emission scanning electronic microscope (FESEM) and found that σ phase is generated from the decomposition of the metastable A15 phase after prolonged annealing due to its lower free energy at 800 °C, and gradually accumulates Cu to form the Cu-rich σ phase after 20 h. The low-field superconducting properties were also given through magnetic measurement. Both Tc and Bc2 decreased slightly along with the increase of σ phase volume when the annealing is prolonged, while Jc reached its peak after 20 h\\800 °C annealing, which were 1836 A/mm2 at 4.2 K and 5 T and 822 A/mm2 at 8 K and 5 T for 800 °C\\20 h annealing, about 34% and 36% higher than that of the conventional transformed sample, respectively. The calculated Fpmax of the samples with optimized annealing condition also increased accordingly. The fitting analysis of fp using the Dew-Hughes model showed that the pinning mechanism was surface pinning. The additional diffusion channels for Cu provided by newly generated interfaces of σ and Nb(Al) particles enhance the Cu segregation on the A15 boundaries and may be the source of the pinning force improvement.

Microstructure and Superconducting Properties of Rapid Heating, Quenching, and Transformation (RHQT) Powder‐in‐Tube Nb3Al Wires Doped with Sn Element

Properties of rapid heating, quenching, and transformation (RHQT)‐treated powder‐in‐tube (PIT) Nb3Al1−xSnx wires are studied herein. After the RHQ process, the body‐centered cubic (bcc) phase is formed in the Sn‐doped Nb3Al wires. The transformed superconducting A15 phase shows a uniform microstructure and homogenous component distribution feature. With the increase of the Sn doping level, the critical transition temperature (Tc), critical current density (Jc), and irreversibility field (Birr) of the superconducting wires increase first and then reduce, achieving the highest Tc value of 17.6 K and the smallest ΔTc of 0.6 K in the 4% Sn doped Nb3Al wire. The 4% Sn addition helps to improve the layer Jc of the sample to 2.1 × 105 A cm−2 at 8 K and 5 T, which is about 2.5 times that of the pure Nb3Al sample. Furthermore, the Nb3Al1−xSnx (x = 0.04) wire gives the highest Birr value of 17.22, 13.23, and 9.11 T at 8, 10, and 12 K, respectively. The addition of Sn helps to achieve stoichiometric Nb3Al A15 phase and formation of precipitations with the size of 10 nm, which might act as the flux‐pinning center.

Effect of Cu doping on properties of RHQT processed powder in tube Nb3Al wires

Powder-in-tube (PIT) Nb3(Al1-xCux)(x = 0–0.04) superconducting wires were fabricated by rapid heating, quenching and transformation (RHQT) process. With the addition of Cu, disordered Nb3Al A15 phase with stoichiometric composition was formed in the quenched wires and atomic long range ordering occurred during the low temperature annealing process, resulting in improvement of Tc, Jc and Birr performance compared to the pure Nb3Al sample. The highest Tc of 18.0 K, narrowest ΔTc of 0.6 K and best layer Jc of about 30481 A/cm2 @12 K, 4T was found in the Nb3(Al0·97Cu0.03) wire, which is nearly four times of pure Nb3Al sample. The Nb3(Al0·97Cu0.03) wire also shows the highest Birr value of 25.7T at 8 K and 12.9T at 12 K, respectively. The formed A15 phase exhibit uniform microstructure and composition. Flux pinning of the Nb3(Al1-xCux) wires follows the surface pinning mechanism, where grain boundary and stacking faults were considered as pinning centers.

Performance of Nb3Al superconducting wires with kilometer-grade length after treatment via reel-to-reel joule heating

In view of practical manufacturing of Nb3Al materials, it is a triumph that we have fabricated kilometer-grade single-filament jelly-roll Nb3Al precursor wires on this occasion and then investigated the effects of large heating current on superconducting properties of the sample wires treated by rapid heating, quenching and transformation (RHQT). The optimized RHQ current explored in this experiment is 270 A, in which samples show some distinct properties, an abrupt superconducting transition, an excellent onset superconducting transition temperature (Tc) of 18.2 K and a relatively high critical current density (Jc) of 6.68 × 104 A/cm2 at 10 K and 7 T. Since the current variation keeping wires ductile after quenching comes clear, continuous kilometer-class wiring under large current heating will be real as long as the proportion of Nb and Al in precursor is improved or by other means Nb/Al composite in RHQT Nb3Al is enhanced. This work has really pushed the commercial application of Nb3Al materials.

Effect of Joule heating current on phase formation and superconducting properties based on Nb3Al for applications in nuclear fusion magnet energy

With extremely high critical current density (Jc) and excellent strain tolerance, Nb3Al superconductor is considered as an alternative to Nb3Sn for application of high-field magnets. However, owing to their complex structure, Nb3Al superconducting wires can hardly meet the requirement of engineering application at present. In this work, we have reported the manufacture of a single core-structured jelly-roll Nb3Al precursor long wire. While using homemade rapid heating and quenching (RHQ) equipment, we have successfully carried out RHQ heat-treatment of Nb3Al wire samples with various heating conditions, especially the effect of the Joule heating currents by rapid heating, quenching and transformation (RHQT) method on the phase formation and the superconducting properties of the wires were investigated.

Fabrication of Nb3Al superconducting wires by utilizing the mechanically alloyed Nb(Al)ss supersaturated solid-solution with low-temperature annealing

High-performance Nb3Al superconducting wire is a promising candidate to the application of high-field magnets. However, due to the production problem of km-grade wires that are free from low magnetic field instability, the Nb3Al wires made by rapid heating, quenching and transformation (RHQT) are still not available to the large-scale engineering application. In this paper, we reported the properties of the in situ powder-in-tube (PIT) Nb3Al superconducting wires, which were made by using the mechanically alloyed Nb(Al)ss supersaturated solid solution, as well as the low temperature heat-treatment at 800°C for 10h. The results show that Nb3Al superconductors in this method possess very fine grains and well superconducting properties, though a little of Nb2Al and Nb impurities still keep being existence at present work. At the Nb3Al with a nominal 26at.% Al content, the onset Tc reaches 15.8K. Furthermore, a series of Nb3Al wires and tapes with various sizes have been fabricated; for the 1.0mm-diameter wire, the Jc at 4.2K, 10T and 14T have achieved 12,700 and 6900A/cm2, respectively. This work suggests it is possible to develop high-performance Cu-matrix Nb3Al superconducting wires by directly using the Nb(Al)ss supersaturated solid-solution without the complex RHQT heat-treatment process.

Fabrication of Nb3Al superconducting bulks by mechanical alloying method

Since it can directly prepare the Nb–Al supersaturated solid solution, mechanical alloying is thought as a promising method to make high-performance Nb3Al superconductors at a low temperature annealing condition, without the complicated rapid heating, quenching and transformation (RHQT) process. In this paper, we investigate the effects of milling time, the content of Al and annealing temperature on phase formation and superconducting properties of mechanical alloying Nb3Al bulks in detail. The study results show that Nb–Al supersaturated solid solution could be obtained by high energy ball milling, as long as the Nb–Al blended powder is ball milled for 1h, even the amorphous phase appears with the ball milling time prolonging to 10h, the Nb–Al intermetalics do not come out either. Amorphous phase is hardly beneficial to synthesizing the Nb3Al phase, instead, it will make the products impurity. By optimizing the milling time, elements composition and annealing temperature , pure Nb3Al phase is obtained and the highest onset superconducting transition temperature (Tc-onset) reaches 15.8K and the critical current density (Jc) 106A/cm2 at 8K without outer field. This paper also discusses the main reasons that affect the superconducting properties of mechanical alloying Nb3Al superconductor.

A new RHQT Nb3Al superconducting wire with a Ta/Cu/Ta three-layer filament-barrier structure

To suppress the low-magnetic-field instability (flux jumps in low magnetic fields) of a rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductor, we had previously modified the cross-sectional design of an RHQT Nb3Al by adopting a Ta filament-barrier structure. Unlike Nb barriers, Ta barriers are not superconducting in magnetic fields at 4.2 K so that they electromagnetically decouple filaments. However, small flux jumps still occurred at 1.8 K, which is a typical operating temperature for the magnets used in high-energy particle accelerators. Furthermore, poor bonding at the Ta/Ta interface between neighboring Ta-coated jelly-roll (JR) filaments frequently caused precursor wires to break during drawing. To overcome these problems, we fabricated a new RHQT Nb3Al wire with a Ta/Cu/Ta three-layer filament-barrier structure for which an internal stabilization technique (Cu rods encased in Ta are dispersed in the wire cross section) was extended. Removing the Ta/Ta interface in the interfilamentary barrier (JR filament/Ta/Cu/Ta/JR filament) allowed precursor wires to be drawn without breaking. Furthermore, the Cu filament barrier electromagnetically decoupled filaments to suppress flux jumps at 1.8 K. The ductile Cu layer also improved the bending strain tolerance of RHQT Nb3Al.

Stability Test of RHQT Nb3Al Cable-in-conduit Conductor

The Japan Atomic Energy Agency (JAEA) and National Institute for Material Science (NIMS) have been collaborating in the development of high-performance (i.e., more than 16 T and 80 kA) Nb3Al cable-in-conduit (CIC) conductor prepared using the rapid-heating, quenching and transformation (RHQT) process, and aiming for the application of this conductor to a demonstration plant. The technical issue relating to application of the RHQT Nb3Al strands to a fusion magnet is stabilization against perturbation. NIMS has developed a technique to attach a copper stabilizer using electroplating, and a sub-scale CIC conductor is developed using this conductor. JAEA performed a stability test of the developed CIC conductor to demonstrate the efficiency of this copper stabilization technique. The initial perturbation was applied via inductive heating, whose energy was calibrated using a calorimetric method. The measured stability margin is sufficiently high compared to that of a similar NbTi CIC conductor previously tested by the authors. In addition, a heat transfer coefficient to define the so-called limiting current is estimated to be about 1 kW/m2K, a sufficiently high value that is almost the same as that of a CIC conductor consisting of normal Cu stabilized strands. From these experimental results, it can be concluded that the copper stabilizer works efficiently from the viewpoint of stability, thus offering a solution to the remaining technical issues relating to the RHQT CIC conductor. On this basis, we can say that the RHQT Nb3Al CIC conductor is the most promising candidate for application to a magnet in the demonstration plant.

Study of ${\rm Nb}_{3}{\rm Al}$ Wires for High-Field Accelerator Magnet Applications

Rapid heating, quenching and transformation (RHQT)-processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires possess better strain tolerance than Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires. However, RHQT-processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires so far developed show lower non-copper critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) than recently developed high-J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires in the field range of 12-15 T. Therefore, the development of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires having enhancing J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> in this field range is a major concern for accelerator magnet designers. Among a number of fabrication parameters that might affect J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> properties, we analysed the effect of thickness of Al and Nb foils in the jelly-roll-shaped filaments embedded in the precursor wires. Mechanical and superconducting properties of these wires with various Al-foil thicknesses in the filaments were investigated.

Magnetization studies of RHQT-processed Nb3Al wires for high-field accelerator magnet applications

Rapid heating/quenching and transformation (RHQT)-processed Nb3Al wires possess better strain tolerance than Nb3Sn wires and exhibit similar high-field properties. Therefore, Nb3Al wires might be promising candidates for use in future high-field accelerator magnets. For this reason, we have been developing RHQT-processed Nb3Al wires for a number of years. During this development, magnetization measurements on several samples have been carried out as a function of either temperature or the magnetic field. This paper presents some of the results of the magnetization measurements performed.

Quench and Normal Zone Propagation Characteristics of RHQT-Processed ${\rm Nb}_{3}{\rm Al}$ Wires Under Cryocooler-Cooling Conditions

The minimum quench energy (MQE) and normal zone propagation velocity (NZPV) of three kinds of Nb3Al superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density (J c); high MQE was obtained for low J c. It is assumed that J c has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration.

Development of Ta-Matrix ${\rm Nb}_{3}{\rm Al}$ Wire for Next-Generation Accelerator Magnet

We have been developing rapid heating, quenching and transformation (RHQT) processed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires for next-generation accelerator magnets that require magnetic fields between 12 and 17 T. However, it has recently been found that the conventional Nb-matrix wire shows a rather strong magnetic instability at low fields. Taking this into consideration, we have begun developing Ta-matrix Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires. We have fabricated a precursor wire with a Ta-matrix volume fraction of 0.8, and the mechanical properties of the RHQ-processed wire were examined. The effects of the RHQ condition and the area reduction (AR) after RHQ on the critical current density ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) , and magnetization of the wire have also been examined. This paper describes the properties for the Ta-matrix wire in comparison with those of the Nb-matrix wire.

Reduction of the Matrix Ratio for Improvement of the Non-Cu $J_{\rm c}$ of ${\rm Nb}_{3}{\rm Al}$ Wires

We attempted to reduce the matrix ratio down to 0.5 and 0.25 for RHQT (rapid-heating, quenching and transformation) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires, which is directly related to improvement of the non-Cu J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . The Ta matrix was chosen to improve the wire strength. It was found that the matrix ratio of 0.25 is too small to carry out RHQ treatment although drawing was possible by using cassette-roller dies (CRDs) as well as conical dies. The reduction of the matrix ratio down to 0.5 can also be achieved. However, the scale-up would make the drawing very difficult. It would be necessary for the scale-up to improve adhesion among the jelly-roll (JR) cores and also the outer Ta sheath in the multi-billet. The RHQ treatment was possible for the wire with a matrix ratio of 0.5. However, it was successful only when the wire diameter was less than 1.13 mm. It is necessary to suppress the experienced bend strain in the operation. The non-Cu J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> was found to be improved by about 20% by the reduction of the matrix ratio, compared to conventional Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al wires.

Cu Stabilized $\hbox{Nb}_{3}\hbox{Al}$ Strands for the High Field Accelerator Magnet

Characteristics of developed Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand though the RHQT (rapid heating/quenching and transformation) process are described. One kilometer-long copper stabilized Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al round strands were economically fabricated with ion-plating and electroplating. The copper electroplating was successfully done with the high velocity of 5 m/h. A strong bonding between the copper and the precursor was achieved, and the mechanical rolling test did not show the separation of the copper stabilizer from the precursor. The rolled Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strands showed no degradation both in critical current density and RRR value. Magnetic instability at 4.2 K at low fields was apparently improved on the F3 strand, relative to the previously F1 stand because a tantalum was used for the interfilament matrix of precursor. The large magnetic flux jumps, which were observed with the Fl strand, were suppressed at 4.2 K, although appeared again below 3.0 K. The twisted strands are shown to be effective to reduce the time-dependent eddy current coupling. It was obtained 48 mum of D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> on F3 strand with twist pitch of 45 mm. In addition, the transverse pressure test for the Fl Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al strand was performed at 4.2 K and 12 T. The critical current did not degrade with the transverse pressure up to 210 MPa.

急熱急冷変態法 Nb3Al 超伝導体の急熱急冷条件の最適化

The RHQT (rapid-heating, quenching and transformation) processed Nb3Al superconducting wire has a larger critical current density Jc in high magnetic fields and better strain tolerance than Nb3Sn does. Thus, the Nb3Al wire is one of the most promising candidate superconductors for the large-scale and high-field applications. Although the RHQ optimization is a key to a high Jc through controlling the microchemstry and grain structure of bcc and hence A15 phases, investigated have been only the RHQ parameters of heating current IRHQ and wire speed vwire for a given electrode spacing (100 mm). From a view point of apparatus design a smaller IRHQ is favorable, but a heating time which it takes for a wire to move between electrodes is instead needed to become longer for heating the wire up to 2273 K. Thus vwire must become slower. However, such a slow vwire also causes a slower cooling rate of wire that would partly form undesirable A15 phase at the RHQ treatment. In the present study, an attempt has been made to extend the electrode spacing from 100 mm to 300 mm. This enables a relatively large vwire (high cooling rate) but a small IRHQ, which ensures a heating time enough to react the precursor. The Nb/Al precursor used has been prepared by the double-stacked rod-in-tube process, where the starting material is a 7-core Nb/Al composite of which Al was alloyed with 5 at% Mg. The final size of Al alloy core calculated is 0.56 μm, several times larger than the thickness of Al layer of jelly-roll Nb/Al precursor. The extension of electrode spacing from 100 mm to 300 mm did not degrade Tc of the Nb3Al transformed from bcc supersaturated-solid solution, which was 17.6 K, and allowed the IRHQ to be reduced to ∼1/√3 in comparison to the conventional IRHQ condition with the same vwire. The newly optimized RHQ condition of 0.33 m/s, 48 A for the electrode spacing of 300 mm eventually doubled the Jc (15 T, 4.2 K), at least, for the rod-in-tube processed Nb3Al that has not been mechanically deformed between RHQ and transformation annealing.

Recent topics in high-field A15 superconductors in Japan

A different type of V 3Ga conductors was fabricated in the early stage of high-field A15 conductor development in Japan. Then, Ti-doped Nb 3Sn conductors have been developed, and widely used for high-field generation. An increase of Sn concentration in the bronze produces an appreciable progress in the performance of bronze processed (Nb,Ti) 3Sn conductors. Nb 3Sn wires for an ac application have been also developed through the bronze process. New internal tin(IT) processed (Nb,Ti) 3Sn conductors with modified cross-sectional configurations have been produced, which exhibit large J c in high fields as well as reduced ac loss. Both bronze processed and IT processed (Nb,Ti) 3Sn conductors satisfy the ITER magnet specifications. As for new type Nb 3Sn conductors, powder core and Jelly Roll processed (Nb,Ta) 3Sn wires have been fabricated. Nb 3Al wires with improved performance are prepared by rapid heating, quenching and transformation (RHQT) process. The quenched wire with bcc crystal structure can be deformed after the Cu cladding, and finally reacted at 800 °C for the transformation. Recently, stable reel-to-reel RHQT operation has been performed on the wire over a long length. Meanwhile, V-based high-field A15 compounds such as V 3Ga may need to be reconsidered for a fusion application, since they have much better radioactive characteristics than those of Nb-based A15 compounds.