A niobium-titanium superconducting wire having a three-dimensional continuous pinning network and a method of manufacturing the same

Abstract

The application belongs to the technical field of superconducting material processing, and discloses a niobium-titanium superconducting wire with a three-dimensional continuous pinning network and a preparation method thereof. The preparation method of the application first constructs a metastable powder containing metastable alloy elements, and then performs cold deformation processing after densification by pipe hot extrusion to form a fiber-shaped framework. Then, through solid-state decomposition, the pinning phase is precipitated in situ along the fiber framework to form a three-dimensional continuous and through nano pinning network. The metastable alloy element is an M element in a supersaturated solid solution state or an amorphous state, and the M element is selected from at least one of Si, Ge, Sn, Al and Ga. The proportion of M in the metastable powder is 0.5wt%-5wt%. The niobium-titanium superconducting wire prepared by the application has a three-dimensional continuous pinning network. Compared with traditional isolated pinning particles, the critical current density of the wire is increased by about 60%-81% and 76%-96% under 8T and 9T, respectively, and the high-field superconducting performance is significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of superconducting material processing technology, and relates to a niobium-titanium superconducting wire with a three-dimensional continuous pinning network and its preparation method. Background Technology

[0002] High-field magnet technology is a supporting technology for cutting-edge fields such as superconducting power, controlled nuclear fusion, and high-energy physics. Low-temperature superconducting wires, as the core material for high-field magnets, directly determine the working efficiency and operational stability of these magnets due to their superconducting properties. Among various low-temperature superconducting wires, NbTi superconducting wires, with their excellent mechanical properties, good processing characteristics, and mature mass production processes, have been widely used in various medium- and low-field superconducting magnet devices. The critical current density of a superconducting wire is a core indicator for measuring its superconducting performance. For NbTi superconducting wires, its critical current density performance depends on the magnetic flux pinning within the superconducting matrix, and the magnetic flux pinning effect is determined by the distribution density, structural morphology, and pinning strength of the pinning centers.

[0003] Currently, two main technical approaches have emerged to enhance flux pinning effects and increase critical current density in high-field applications: multiple cold working-aging heat treatment and heterogeneous artificial pinning phase introduction. The multiple cold working-aging heat treatment process suffers from drawbacks such as cumbersome processing steps, long production cycles, and high mass production costs. Furthermore, its narrow control window means that even slight deviations in processing parameters can cause significant fluctuations in wire performance between batches, resulting in poor mass production stability. Additionally, the α-Ti precipitate formed by this process has relatively small property differences from the NbTi superconducting matrix, limiting the pinning energy level. Under high magnetic field conditions, the flux binding capacity significantly decreases, leading to a marked drop in pinning efficiency and making it unsuitable for high-field magnet applications.

[0004] The introduction of heterogeneous artificial pinning phases can avoid the drawbacks of the aforementioned processes. The mainstream method is the powder-packing method, which introduces heterogeneous elements such as Fe into the NbTi alloy system. Through high-temperature diffusion reactions, artificial pinning particles are generated inside the matrix, thereby constructing differentiated pinning centers and enhancing high-field pinning performance. However, this technology also has significant shortcomings. The heterogeneous pinning particles generated by the reaction are mostly distributed in an isolated and dispersed form, lacking effective connecting structures between particles, and unable to form an integral pinning system. Magnetic flux lines can slip and creep through the unpinned blank areas between particles, resulting in limited magnetic flux pinning effect under high-field conditions and a limited increase in critical current density. In addition, the hard heterogeneous phase generated by the reaction has high hardness, which can easily cause processing defects such as cracks and wire breaks during the mechanical drawing and forming of long wires, reducing the yield and structural integrity of the wires, and making it difficult to achieve stable industrial-scale production of long wires.

[0005] Therefore, it is of great significance to develop a method for preparing niobium-titanium superconducting wires that can construct a continuous flux pinning network and possess both excellent high-field superconductivity and mass production capability. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a niobium-titanium superconducting wire with a three-dimensional continuous pinning network and its preparation method. The preparation method first constructs a metastable powder containing metastable alloying elements, which is then densified by hot extrusion in tubes followed by cold deformation to form a fibrous framework. Solid-state decomposition causes the pinning phase to precipitate in situ along this fibrous framework. Due to the geometric constraints of the precursor fibers, the precipitated pinning phase is continuously distributed in three dimensions along the original fiber path, thus forming a nanoscale pinning network. The metastable alloying element is M, which is in a supersaturated solid solution state or an amorphous state. M is selected from at least one of Si, Ge, Sn, Al, and Ga, and its proportion in the metastable powder is 0.5wt%~5wt%. The niobium-titanium superconducting wire prepared by this invention possesses a three-dimensional continuous pinning network. Compared to traditional isolated pinned particles, the high-field critical current density is significantly improved, with increases of approximately 60%~81% and 76%~96% at 8T and 9T, respectively, demonstrating significantly superior high-field superconducting performance.

[0007] On one hand, the present invention provides a method for preparing a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, specifically including the following steps: S1: Preparation of metastable powder: The Ti content in the Nb-Ti-M alloy powder or strip is controlled at 40%~55%, the M content is controlled at 0.5%~5%, and the balance is Nb and unavoidable impurities. M is selected from at least one of Si, Ge, Sn, Al, and Ga (each raw material particle size ≤100μm). The alloy powder is prepared by gas atomization or by single-roller spin quenching. The alloy powder is sieved, and the undersize powder with a particle size of less than 75μm is taken as metastable powder. The strip is mechanically crushed or low-energy ball milled to obtain powder with a particle size of less than 50μm, thus obtaining metastable powder.

[0008] Furthermore, the Ti content in the Nb-Ti-M alloy powder or strip is 46%~48%, and the M content is 1.0%~3.0%. After sieving, the undersize powder with a particle size of less than 45μm is collected.

[0009] Furthermore, regarding impurity content, the O content is controlled below 800 ppm, the C content is controlled below 300 ppm, and the N content is controlled below 200 ppm.

[0010] Furthermore, the cooling rate of the atomization is not less than 10. 3 K / s, preferred cooling rate 10 4 ~10 6K / s. The atomizing medium is high-purity Ar gas or Ar-He mixed gas, the atomizing pressure is 3MPa~8MPa, the diameter of the guide tube is 2mm~6mm, and the superheat of the alloy melt is 100℃~250℃.

[0011] Furthermore, the linear velocity of the roller surface during single-roll quenching is 20 m / s to 50 m / s, and the resulting strip thickness is 15 μm to 60 μm. The melt spraying temperature is 1200℃ to 1500℃, and the spraying pressure is 0.1 to 0.5 MPa.

[0012] Furthermore, the alloy powder or strip is required to have no M element forming an equilibrium compound phase, no second phase precipitation, and to form a metastable single-phase structure.

[0013] S2: Tubing and Hot Extrusion: The metastable powder is loaded into a metal casing. The loading process is carried out with the assistance of a vibrating table or mechanical tapping. After loading, the casing is evacuated to 10°C. -2 The material is heated to 450℃~520℃ and held for 1min~3min per millimeter of wall thickness. The composite rod is obtained by hot extrusion densification. The material is then heated to 450℃~520℃ and held for 1min~3min per millimeter of wall thickness.

[0014] Furthermore, the metal sheath is an oxygen-free copper tube, and the inner wall of the metal sheath is lined with a barrier layer. The barrier layer material is selected from pure Nb, Nb alloy, or Ta, and the thickness of the barrier layer accounts for 5% to 20% of the sheath wall thickness. The barrier layer prevents Cu and NbTi from undergoing interfacial reaction during subsequent heat treatment.

[0015] Furthermore, the compacted density of the powder after loading reaches 60% to 75% of the theoretical density.

[0016] Furthermore, the hot extrusion is carried out at 300℃~550℃, the extrusion ratio is controlled within the range of 8:1~25:1, and the extrusion speed is 5mm / s~30mm / s. The relative density of the bar billet after hot extrusion reaches over 98%.

[0017] Furthermore, the hot extrusion is carried out at 450℃~520℃, and the extrusion ratio is controlled in the range of 12:1~18:1.

[0018] Furthermore, the composite rod material maintained metastable characteristics after extrusion, without significant decomposition or crystallization.

[0019] S3: Cold deformation processing, which involves cold drawing or cold rolling of composite rods to elongate powder or thin strip particles into fiber form along the deformation direction, forming a spatially entangled and closely contacted precursor fiber framework.

[0020] Furthermore, the cold drawing employs cemented carbide drawing dies or diamond drawing dies. During the drawing process, the wire temperature should not exceed 200°C, and the processing rate per pass is controlled at 10%~20%. The cumulative number of passes is determined based on the required cumulative true strain. The wire temperature is controlled to not exceed 200°C by configuring an intermediate water-cooling or forced air-cooling device.

[0021] Furthermore, the cumulative true strain ε is defined as ε = ln(A0 / A), where A0 is the cross-sectional area before deformation and A is the cross-sectional area after deformation. The value of ε is not less than 4, preferably in the range of 5 to 7, and can reach 8 to 9 when a finer fiber diameter is desired.

[0022] Furthermore, the reduction rate per pass during cold rolling is controlled at 5% to 15%, and the total reduction rate is not less than 90%.

[0023] Optionally, when the degree of work hardening affects subsequent deformation, 1-2 intermediate annealings can be interspersed during cold deformation to restore plasticity. The intermediate annealing temperature is set at 300℃~450℃, and the holding time is 0.5h~2h. After annealing, rapid cooling is required to maintain the metastable state of the material as much as possible.

[0024] Optionally, after the cold deformation process is completed, the material has been fully elongated into a fiber shape with a fiber diameter in the range of 50nm to 500nm, an aspect ratio greater than 50, and a fiber volume fraction accounting for 70% to 90% of the superconducting region area.

[0025] S4: In-situ phase change heat treatment generates a pinning network under a protective atmosphere (high-purity Ar gas, purity ≥ 99.999%) or vacuum (vacuum degree below 10). -3 In-situ phase-change heat treatment under (Pa) conditions causes solid-state decomposition of metastable precursor fibers. The general reaction formula is: metastable Nb-Ti-M → equilibrium NbTi superconducting phase + Ti-M intermetallic compound nanophase. The precipitated Ti-M intermetallic compounds nucleate, grow, and bridge each other in situ along the original fiber framework, forming a three-dimensional continuous pinned network.

[0026] Furthermore, the in-situ phase change heat treatment is a single-temperature treatment or a two-stage temperature treatment, followed by furnace cooling or controlled cooling at a set rate. When the M content is 1.0%~2.5%, a single temperature treatment is used, with a temperature of 550℃~700℃, a holding time of 20h~80h, and a heating rate of 1℃ / min~10℃ / min.

[0027] When the M element content is 2.5%~3.0% or a finer pinned network is desired, a two-stage temperature treatment is employed. The first stage involves holding at 400℃~500℃ for 10h~40h with a heating rate of 1℃ / min~10℃ / min. This temperature range promotes component amplitude modulation decomposition or the formation of diffuse pre-nucleation particles within the fiber. The second stage involves holding at 550℃~700℃ for 10h~50h with a heating rate of 1℃ / min~10℃ / min, allowing the pinned phase to fully nucleate, grow, and stabilize. Direct heating between the two stages is possible without intermediate cooling.

[0028] Furthermore, after the in-situ phase transformation heat treatment, the equivalent size of the Ti-M intermetallic compound nanophase is 5nm~50nm, the average spacing between adjacent nanophases is 10nm~100nm, and the interface between the pinned phase and the NbTi matrix is ​​semi-coherent or coherent.

[0029] S5: Finished product processing, the wire is subjected to low-temperature stress-relief annealing at a temperature of 250℃~400℃ and a holding time of 10h~40h to obtain the finished wire.

[0030] Furthermore, if it is necessary to introduce machining dislocations to enhance pinning, a final pull-out with a small deformation can be applied, with the machining rate controlled at 5% to 15%.

[0031] On the other hand, the present invention claims protection for a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, which is prepared by the above-described method.

[0032] Furthermore, this invention seeks protection for the application of the above-described preparation method in improving the superconducting properties and / or high-field superconducting properties of niobium-titanium superconducting wires, wherein the magnetic induction intensity of the high field is not less than 8T.

[0033] Furthermore, the performance testing of the niobium-titanium superconducting wire revealed that the critical current density of the wire under a 5T conventional magnetic field is not less than 3700 A / mm². 2 Compared to niobium-titanium superconducting wires without added M element, the Jc is increased by approximately 22%. The niobium-titanium superconducting wires prepared in this invention exhibit even more significant advantages under high-field conditions, with critical current densities of no less than 1250 A / mm² at 8T and 9T, respectively. 2 and 900A / mm 2 The high-field Jc increases by approximately 64%~81% and 78%~92%, respectively.

[0034] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages: (1) This invention provides a method for preparing niobium-titanium superconducting wires with a three-dimensional continuous pinning network. The method first constructs a metastable powder containing metastable alloying elements, then densifies it through hot extrusion in tubes followed by cold deformation processing to form a fibrous framework. Then, through solid-state decomposition, the pinning phase precipitates in situ along the fibrous framework, forming a three-dimensional continuous nano-pinning network. The metastable alloying element is element M in a supersaturated solid solution state or an amorphous state. Element M is selected from at least one of Si, Ge, Sn, Al, and Ga, and the proportion of M in the metastable powder is 0.5wt%~5wt%. This invention, through extensive experimental screening, determined that element M is Si, Ge, Sn, Al, or Ga. This element can form high-melting-point, high-hardness intermetallic compounds with Ti, and its equilibrium solid solubility in NbTi is extremely low, resulting in a large driving force for solid-state decomposition, which is beneficial for forming a continuous network. While other metallic elements (such as Fe, Zr, and Hf) can form a second phase, the interfacial relationship and distribution morphology between the precipitated phase and the matrix are insufficient to meet the requirements of a three-dimensional continuous network, resulting in a pinning improvement effect far inferior to that of this invention. Furthermore, experiments revealed that when the amount of metal added is below 0.5%, the volume fraction of the pinned phase is insufficient; when the amount of metal added is above 5%, work hardening is severe and coarse equilibrium phases are easily formed, both of which degrade performance. The niobium-titanium superconducting wire prepared by this invention possesses a three-dimensional continuous pinning network, and the critical current density of this wire under a 5T conventional magnetic field is not less than 3700 A / mm². 2 Compared to traditional isolated pinned particles, niobium-titanium superconducting wires show a Jc improvement of no less than 22%.

[0035] (2) The pinning network of the niobium-titanium superconducting wire of the present invention is generated through in-situ decomposition of precursor fibers. The decomposition reaction is carried out under the constraint of the fiber skeleton, resulting in small and controllable pinning phase size, which improves the pinning efficiency. This gives the niobium-titanium superconducting wire of the present invention excellent high-field critical current density. Its critical current density at 8T and 9T is not less than 1250A / mm². 2 and 900A / mm 2 The high-field Jc increases by approximately 60%~81% and 76%~96%, respectively, resulting in significantly superior high-field superconducting performance. The increased high-field critical current density of the wire allows for magnet designs with higher winding current densities or more compact winding structures, thereby reducing the amount of superconducting wire used and the thermal load on the cryogenic system, and lowering the overall cost of the device.

[0036] (3) The preparation method of this invention reduces the traditional preparation process of repeated cold working-heat treatment cycle to "one large deformation + one in-situ phase transformation heat treatment", which simplifies the process, enhances the process stability, and is suitable for industrial mass production. In addition, the obtained pinned phase is generated in-situ during the heat treatment process. During the processing, the wire is basically free of a large number of hard particles, with low deformation resistance and good plasticity, which helps to reduce wire breakage and realize the preparation of long wires. The simplification of the process and the shortening of the preparation cycle are conducive to breaking through the bottleneck of long-cycle supply of superconducting wires for large scientific devices. The key parameters of the short process are well controllable, which facilitates the replication and large-scale scale-up of production lines and can realize stable batch preparation. It can provide engineeringable and scalable superconducting wire solutions for fields such as high-field accelerators, fusion engineering test reactors, and next-generation high-field magnetic resonance imaging systems. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the structure of an Nb-Ti-M superconducting wire with a three-dimensional continuous pinning network. The labels in the figure are as follows: 1 represents metastable powder, 2 represents the barrier layer, and 3 represents the oxygen-free copper tube. Detailed Implementation

[0039] The technical solution of the present invention will be described below with reference to embodiments. However, the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods and detection methods described in each embodiment are conventional methods; unless otherwise specified, the reagents and materials can be purchased commercially. Unless otherwise specified, the percentages in the following embodiments refer to mass percentages.

[0040] Example 1 This embodiment provides a method for preparing a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, specifically including the following steps: S1: Preparation of metastable powder. The alloy ratio is Nb-46.5wt%Ti-1.8wt%Si. The raw materials are prepared into alloy powder by vacuum induction melting gas atomization. High-purity Ar gas is used as the atomization medium. The diameter of the guide tube is 4mm. The superheat of the alloy melt is 150℃. The atomization pressure is 5MPa and the cooling rate is 10. 4 ~10 5 The obtained alloy powder is sieved at K / s, and the powder with a particle size of less than 45μm is taken to obtain supersaturated metastable powder.

[0041] S2: Tubing and Hot Extrusion: The metastable powder is loaded into an oxygen-free copper sheath (lined with a 0.5mm thick pure Nb barrier layer) with an outer diameter of 60mm and an inner diameter of 50mm. The loading process is carried out with the assistance of a vibrating table. After loading, the compacted density of the powder reaches 68% of the theoretical density. After loading, the sheath is evacuated to 10°C. -2 Below Pa, the material is sealed using vacuum electron beam welding (welding current 10mA, accelerating voltage 60kV) to obtain a package. The package is then heated to 480℃, with a holding time calculated as 2 minutes per millimeter of the package wall thickness (5mm). At the same temperature, it is hot-extruded and densified at an extrusion ratio of 16:1 and an extrusion speed of 10mm / s. The exit diameter of the hot extrusion is 15mm. Through the above process control, the precipitation of solute elements is suppressed throughout the process, and the initial supersaturated solid solution metastable structure of the composite rod is still retained.

[0042] S3: Cold deformation processing involves multiple cold drawing passes of the composite rod. During drawing, the wire temperature does not exceed 200℃, and the processing rate per pass is controlled at 15%, with a total drawing diameter of 2.0 mm and a cumulative true strain of approximately 4.03. Through this cold deformation process, the original Nb-Ti-M metastable powder particles within the composite rod are elongated along the deformation direction, forming a fiber morphology. Based on the cross-sectional shrinkage ratio before and after deformation, the fiber diameter is calculated to be in the range of 20–300 nm, with an aspect ratio greater than 50. Simultaneously, based on the powder tap density and extrusion ratio, the fiber volume fraction accounts for approximately 85% of the superconducting region area.

[0043] S4: In-situ phase change heat treatment generates a pinning network. The temperature is increased to 620℃ at a rate of 5℃ / min, held for 60h, and then cooled in the furnace. After the above in-situ phase change heat treatment, the metastable precursor fibers undergo solid-state decomposition. The general reaction formula is: metastable Nb-Ti-M → equilibrium NbTi superconducting phase + Ti-M intermetallic compound nanophase. Since the decomposition reaction is confined within the original fiber framework, the precipitated Ti-M intermetallic compounds nucleate, grow, and bridge each other in situ along the fiber axis, naturally forming a three-dimensional continuous pinning network. Based on the nucleation-growth kinetics calculation of solid-state decomposition under this process condition, the equivalent size of this Ti5Si3 nanophase is approximately 15~35nm, and the interface between it and the equilibrium NbTi superconducting matrix exhibits a semi-coherent relationship.

[0044] S5: Finished product processing, the wire is drawn to a diameter of 1.8mm in a single pass with a pass rate of 19%, followed by low-temperature stress-relief annealing at 350℃ for 20 hours to obtain the finished wire.

[0045] Performance testing: Jc (based on non-copper area) was tested at liquid helium temperature of 4.2K under different back fields and compared with commercially available Nb-46.5wt%Ti wire (diameter 1.8mm) that has undergone multiple heat treatments. The results are shown in Table 1.

[0046] Table 1. Performance comparison results of niobium-titanium superconducting wire with three-dimensional continuous pinning network and existing wires.

[0047] As shown in Table 1, the niobium-titanium superconducting wire prepared by the method of the present invention has a superior critical current density, with Jc increasing by about 22% at 5T, and by about 64% and 78% at 8T and 9T, respectively. This indicates that the niobium-titanium superconducting wire of the present invention has a more significant advantage in critical current density under high field conditions.

[0048] Example 2 This embodiment provides a method for preparing a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, specifically including the following steps: S1: Preparation of metastable powder. The alloy ratio is Nb-47wt%Ti-2.5wt%Ge. The raw material is quenched into a thin strip with a thickness of about 30μm using a single roller. The roller surface linear speed is 30m / s, the spraying temperature is 1300℃, and the spraying pressure is 0.2MPa. The obtained thin strip is crushed into powder with a particle size of less than 50μm, which is a supersaturated metastable powder.

[0049] S2: Tubing and Hot Extrusion: The metastable powder is loaded into an oxygen-free copper tube with an outer diameter of 60 mm and an inner diameter of 50 mm. The oxygen-free copper tube has a wall thickness of 5 mm and is lined with a 1 mm thick Ta-45wt%Nb alloy tube barrier layer. The loading process is carried out with the assistance of mechanical tapping. After loading, the powder compaction density reaches 65% of the theoretical density. After loading, the encapsulation is evacuated to 10... -2 The pressure is below Pa, and then welded (welding current 15mA, accelerating voltage 80kV) to obtain a package. The package is heated to 550℃, and the holding time is calculated as 3min per millimeter of the package wall thickness, for a total of 15min. At the same temperature, it is hot-extruded and densified at an extrusion ratio of 12:1 and an extrusion speed of 8mm / s. The exit diameter of the hot extrusion is 17.3mm, resulting in a composite rod.

[0050] S3: Cold deformation processing involves multiple cold drawing passes of the composite rod. During drawing, the wire temperature does not exceed 200℃, and the processing rate per pass is controlled at 12%, with a total diameter of 1.5mm and a cumulative true strain of approximately 5.18. Through cold deformation processing, the Ge metastable fibers are elongated along the deformation direction, forming a fiber morphology. Based on the section shrinkage ratio before and after deformation, the aspect ratio is 80, and the fiber volume fraction accounts for approximately 82% of the superconducting region.

[0051] S4: In-situ phase change heat treatment generates a pinned network. Two-stage temperature treatment is used. In the first stage, the temperature is increased to 480℃ at 1℃ / min and held for 30h to form Ge-rich and Ge-poor regions. In the second stage, the temperature is increased to 650℃ at 1℃ / min and held for 30h. Since the decomposition reaction is confined to the inside of the protofibrillary framework, Ti5Ge3 nanophase is precipitated in-situ in the Ge-rich region, naturally forming a three-dimensional continuous network. Based on the nucleation-growth kinetics, the pinned phase size is estimated to be 8~25nm, forming a double continuous network.

[0052] S5: Finished product processing, the wire is subjected to low-temperature stress-relief annealing at 400℃ for 10 hours to obtain the finished wire.

[0053] Performance testing: At 4.2K, Jc is 3850 A / mm at 5T. 2 At 8T, Jc is 1350 A / mm 2 980A / mm at 9T 2 .

[0054] Example 3 This embodiment provides a method for preparing a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, specifically including the following steps: S1: Preparation of metastable powder. The alloy ratio is Nb-46wt%Ti-1.0wt%Si-1.2wt%Sn. The raw materials are prepared into alloy powder by vacuum induction melting gas atomization. The atomization medium is an Ar-He mixed gas (Ar:He=7:3). The diameter of the guide tube is 3mm. The superheat of the alloy melt is 200℃. The atomization pressure is 8MPa and the cooling rate is 10. 4 ~10 5 The obtained alloy powder is sieved at K / s, and the powder with a particle size of less than 45μm is taken to obtain supersaturated metastable powder.

[0055] S2: Tubing and Hot Extrusion: The metastable powder is loaded into an oxygen-free copper sheath (lined with a 0.25mm thick pure Ta barrier layer) with an outer diameter of 60mm and an inner diameter of 50mm. The loading process is carried out with the assistance of a vibrating table. After loading, the compacted density of the powder reaches 70% of the theoretical density. After loading, the sheath is evacuated to 10°C. -2 Below Pa, electron beam welding (5mA current, 50kV voltage) is performed to obtain the package. The package is then heated to 450℃ and held for 5 minutes at a temperature calculated as 1 minute per millimeter of the package wall thickness. At the same temperature, it is then hot-extruded and densified at an extrusion ratio of 25:1 and an extrusion speed of 20mm / s. The exit diameter of the hot extrusion is 12mm. Through the above process control, the precipitation of solute elements is suppressed throughout the process, and the initial supersaturated solid solution metastable structure of the composite rod is still retained.

[0056] S3: Cold deformation processing, the composite rod is cold drawn in multiple passes, the wire temperature does not exceed 200℃ during the drawing process, the processing rate of each pass is controlled at 15%, the total drawing is to a diameter of 1.9mm, and the cumulative true strain is about 4.13. The fiber diameter is about 50~400nm, the aspect ratio is greater than 60, and the fiber volume fraction is about 80%.

[0057] S4: In-situ phase change heat treatment generates a pinned network. The temperature is increased to 580℃ at 10℃ / min, held for 80h, and then cooled in the furnace. After the above process, a large number of Ti5Si3 and Ti3Sn mixed nanophases with equivalent sizes of 10~30nm are generated in-situ within the fiber framework of the material. The two coexist to form a three-dimensional continuous network, which is semi-coherent with the matrix interface.

[0058] S5: Finished product processing, the wire is drawn to a diameter of 1.8mm in a single pass with a pass rate of 10.3%, followed by low-temperature stress-relief annealing at 350℃ for 20 hours to obtain the finished wire.

[0059] Performance testing: Jc is 3920 A / mm at 4.2K and 5T. 2 At 8T, Jc is 1410 A / mm 2 At 9T, Jc is 1000 A / mm 2 Under ±1T, 50Hz alternating conditions, the AC loss is reduced by about 25% compared to conventional wires.

[0060] Example 4 To verify the validity of the parameter range, this embodiment uses the same steps as Example 1, but the M content is set to the upper limit of 4.8 wt% (close to 5%), with a specific ratio of Nb-46 wt% Ti-4.8 wt% Si. Due to the high M content, a two-stage temperature treatment is used (first stage 450℃ (5℃ / min) / 20h, second stage 600℃ (5℃ / min) / 30h). The final wire Jc at 8T is 1250 A / mm. 2 900 A / mm at 9T 2 This indicates that good results can still be achieved within the upper limit range.

[0061] Comparative Example 1 The preparation method for this comparative example is the same as that in Example 1, except that it does not contain Si, and the alloy ratio is Nb-46.5wt%Ti. Because it does not contain M, a metastable precursor cannot be formed, and the final microstructure contains only a small amount of dispersed α-Ti precipitate.

[0062] Performance testing: Jc is only 610 A / mm at 4.2K and 8T. 2 This value is lower than that of the commercially available comparison cable in Table 1 (780 A / mm). 2The reason is that the commercial comparison wires have undergone multiple optimized aging heat treatments, resulting in finer and more uniform α-Ti precipitation, while this comparative example only underwent the same single annealing as Example 1, which failed to fully precipitate and strengthen the wires, thus resulting in a poor pinning effect.

[0063] Comparative Example 2 The preparation method for this comparative example is the same as in Example 1, except that element M is replaced with Fe at a dosage of 1.8 wt%, while the rest remains the same. Due to the high solid solubility of Fe in NbTi and the poor coherence between the precipitated phase (such as FeTi) and the matrix interface, a continuous network cannot be formed, resulting in a final Jc of 780 A / mm at 8T. 2 This is far lower than the 1280 A / mm in Example 1. 2 This indicates that the M element selected in this invention can significantly improve wire performance.

[0064] Comparative Example 3 The preparation method for this comparative example is the same as in Example 1, but the Si addition is increased to 6.0 wt%, exceeding the upper limit of this invention. Severe work hardening occurs during cold drawing, resulting in frequent wire breakage and making mass production impossible; even if samples are barely prepared, the Jc at 8T is only about 850 A / mm. 2 Furthermore, the pinned phases are coarse (size > 100 nm) and the continuous network is incomplete. This indicates that excessive addition of M element will deteriorate processability and pinning performance.

[0065] As described above, the basic principles, main features, and advantages of the present invention have been well described. The above embodiments and specifications are merely descriptions of preferred embodiments of the present invention, and the present invention is not limited to the above embodiments. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit and scope of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A method for preparing a niobium-titanium superconducting wire with a three-dimensional continuous pinning network, characterized in that, include: Prepare Nb-Ti-M metastable powder, where M is selected from at least one of Si, Ge, Sn, Al, and Ga; The Nb-Ti-M metastable powder was packed into a metal casing, heated, and then hot-extruded to obtain a composite rod. The composite rod is cold-deformed to elongate the internal Nb-Ti-M metastable powder particles into a fiber shape along the deformation direction, thus obtaining a composite line with a precursor fiber framework. The composite wire is subjected to in-situ phase transformation heat treatment to cause the metastable precursor fiber to undergo solid-state decomposition to form a pinning network. After annealing, the niobium-titanium superconducting wire with a three-dimensional continuous pinning network is obtained.

2. The preparation method according to claim 1, characterized in that, The Nb-Ti-M metastable powder contains 40wt%~55wt% Ti and 0.5wt%~5wt% M. The preparation method of the Nb-Ti-M metastable powder is as follows: Nb, Ti, and M powder raw materials are atomized into alloy powder, sieved, and the powder with a particle size of less than 75 μm is taken as the Nb-Ti-M metastable powder. Alternatively, Nb, Ti, and M powder raw materials can be prepared into thin strips by single-roller spin quenching, and the thin strips can be mechanically crushed or low-energy ball milled into powder with a particle size of less than 50 μm to obtain the Nb-Ti-M metastable powder; The Nb-Ti-M metastable powder has no second phase precipitation inside, forming a metastable single-phase structure.

3. The preparation method according to claim 2, characterized in that, The cooling rate of the atomization is not less than 10. 3 K / s; The atomizing medium is selected as high-purity Ar gas or Ar-He mixed gas. The pressure of the gas atomization is 3MPa~8MPa, the diameter of the guide tube is 2mm~6mm, and the superheat of the alloy melt is 100℃~250℃; The linear velocity of the roller surface during single-roll quenching is 20~50m / s, and the thickness of the thin strip is 15μm~60μm.

4. The preparation method according to claim 1, characterized in that, The metal sheath is an oxygen-free copper tube, and the inner wall of the metal sheath is lined with a barrier layer. The barrier layer material is selected from pure Nb, Nb alloy or Ta, and the thickness of the barrier layer accounts for 5% to 20% of the wall thickness of the sheath. The loading is carried out with the assistance of a vibration table or mechanical tapping, and the compacted density of the powder is 60% to 75% of the theoretical density.

5. The preparation method according to claim 1, characterized in that, The target temperature for heating is 450℃~520℃, and the holding time after heating to the target temperature is calculated as 1min~3min per millimeter of the sheath wall thickness. The hot extrusion is carried out at 300℃~550℃, with an extrusion ratio of 8:1~25:1 and an extrusion speed of 5mm / s~30mm / s. The Nb-Ti-M metastable powder in the hot-extruded composite rod remains metastable.

6. The preparation method according to claim 1, characterized in that, The cold deformation process is cold drawing or cold rolling; During the cold drawing process, the wire temperature shall not exceed 200℃, the processing rate per pass shall be controlled at 10%~20%, and the cumulative number of passes shall be determined according to the required cumulative true strain, wherein the cumulative true strain value shall not be less than 4. The fiber morphology has a fiber diameter of 50nm~500nm, an aspect ratio greater than 50, and a fiber volume fraction accounting for 70%~90% of the superconducting region area.

7. The preparation method according to claim 1, characterized in that, The general reaction formula for the solid-state decomposition is: metastable Nb-Ti-M → equilibrium NbTi superconducting phase + Ti-M intermetallic compound nanophase; The equivalent size of the Ti-M intermetallic compound nanophase is 5nm~50nm, the average spacing between adjacent nanophases is 10nm~100nm, and the interface between the Ti-M intermetallic compound nanophase and the equilibrium NbTi superconducting phase is semi-coherent or coherent. The annealing temperature is 250℃~400℃, and the annealing holding time is 10h~40h.

8. The preparation method according to claim 1, characterized in that, The in-situ phase change heat treatment is either a single-temperature treatment or a two-stage temperature treatment. When the content of M in the Nb-Ti-M metastable powder is 1.0%~2.5%, a single temperature treatment is used; The single-temperature treatment temperature is 550℃~700℃, the holding time is 20h~80h, and the heating rate is 1℃ / min~10℃ / min; When the content of M in the Nb-Ti-M metastable powder is 2.5%~3.0%, a two-stage temperature treatment is adopted; The temperature of the first stage of the two-stage temperature treatment is 400℃~500℃, the holding time is 10h~40h, and the heating rate is 1℃ / min~10℃ / min. The second stage of the two-stage temperature treatment has a temperature of 550℃~700℃, a holding time of 10h~50h, and a heating rate of 1℃ / min~10℃ / min.

9. A niobium-titanium superconducting wire with a three-dimensional continuous pinning network, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the preparation method according to any one of claims 1 to 8 in improving the superconducting properties and / or high-field superconducting properties of niobium-titanium superconducting wires, characterized in that, The magnetic induction intensity of the high field is not less than 8T.

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