A niobium tri-tin conductor and a thermal strain relief preparation method and application thereof

Abstract

This invention relates to a niobium-3Sn conductor, its method for heat-strain-relieving preparation, and its application. It belongs to the field of niobium alloy superconducting conductor preparation technology, and solves the problems of existing Nb3Sn conductor heat treatment processes failing to effectively alleviate residual thermal shrinkage stress after winding and easily leading to excessive coarsening of Nb3Sn grains. The method for heat-strain-relieving preparation of the niobium-3Sn conductor includes the following steps: S1, winding an Nb3Sn conductor blank into a magnet coil shape; S2, performing intermediate annealing on the Nb3Sn conductor blank after S1, the annealing temperature of which is lower than the Nb3Sn phase-forming temperature; S3, performing phase-forming heat treatment on the Nb3Sn conductor blank after intermediate annealing in S2, causing Nb and Sn to diffuse and react to form the Nb3Sn superconducting phase; cooling after the phase-forming heat treatment to obtain the Nb3Sn conductor. By adding an intermediate annealing process, the release of winding mechanical stress and the alleviation of thermal stress are achieved.

Description

Technical Field

[0001] This invention relates to the field of niobium alloy superconducting conductor preparation technology, and in particular to a niobium tritin conductor and its thermal strain-controlled release preparation method and application. Background Technology

[0002] The fabrication of Nb3Sn conductors for high-field magnets often employs a "winding followed by heat treatment" process. This involves first winding an Nb-Sn composite wire with a certain degree of mechanical ductility into a magnet coil shape, and then subjecting it to high-temperature heat treatment to induce a diffusion reaction between Nb and Sn, generating the Nb3Sn superconducting phase. However, during heat treatment, differences in the thermal expansion coefficients of the conductor components and the superposition of winding mechanical stresses can lead to residual thermal shrinkage stress, which in turn can cause defects such as conductor microcracks and a decrease in superconducting performance and magnet reliability.

[0003] Currently, heat treatment of Nb3Sn conductors mostly employs a single-stage high-temperature holding process, focusing only on the full formation of the Nb3Sn superconducting phase without addressing the residual thermal shrinkage stress of the conductor after winding. This results in excessive residual thermal shrinkage stress in the conductor. Although some improved processes attempt to reduce stress by adjusting the final heat treatment temperature or cooling rate, the effects are limited and they can easily lead to excessive coarsening of Nb3Sn grains or insufficient formation of the superconducting phase, which in turn damages the superconducting properties of the conductor. Summary of the Invention

[0004] Based on the above analysis, the embodiments of the present invention aim to provide a niobium-tin conductor and its thermal strain-relieving preparation method and application, in order to solve at least one of the problems of existing Nb3Sn conductor heat treatment processes failing to effectively alleviate the residual thermal shrinkage stress of the wound conductor, easily leading to excessive coarsening of Nb3Sn grains or insufficient formation of superconducting phase.

[0005] On one hand, the present invention provides a method for the slow-release preparation of niobium-tin conductors under thermal strain, comprising the following steps: S1, winding the Nb3Sn conductor blank into a magnet coil shape; S2, intermediate annealing is performed on the Nb3Sn conductor blank wound in S1. The annealing temperature of the intermediate annealing is lower than the Nb3Sn phase formation initiation temperature. S3 involves performing a phase-forming heat treatment on the Nb3Sn conductor blank after intermediate annealing in S2, causing Nb and Sn to diffuse and react to form the Nb3Sn superconducting phase; cooling is performed after the phase-forming heat treatment to obtain the Nb3Sn conductor.

[0006] Furthermore, the Nb3Sn conductor preform includes: a central reinforcement and a superconducting core wire precursor; in S2, an intermediate annealing parameter coupling relationship is established, and the intermediate annealing process parameters are set according to the intermediate annealing parameter coupling relationship; the intermediate annealing parameter coupling relationship includes: T = 420 + ΔT1 + ΔT2 - ΔT3; Where: T is the intermediate annealing temperature in °C; ΔT1 is the intermediate reinforcement material correction coefficient; ΔT2 is the superconducting core wire nano-doping correction coefficient; and ΔT3 is the superconducting core wire precursor density correction coefficient.

[0007] Furthermore, the coupling relationship of intermediate annealing parameters also includes: t = 20 + (T - 420) / 2 + 0.3 × N / 10; Where: t is the intermediate annealing holding time, in hours; T is the intermediate annealing temperature, in degrees Celsius; N is the number of superconducting core wire precursors, ranging from 49 to 144.

[0008] Furthermore, in the coupling relationship of intermediate annealing parameters, when the intermediate reinforcement is an Nb-Ti alloy or pure Nb, ΔT1 is 0℃; when the intermediate reinforcement is a Cu-Ni alloy, ΔT1 is +15℃; when the intermediate reinforcement is stainless steel, ΔT1 is +25℃; and when the intermediate reinforcement is a W-Re alloy, ΔT1 is +30℃.

[0009] Furthermore, when the superconducting core wire precursor has no Zr and Hf doping, ΔT2 is 0℃; when the superconducting core wire precursor is doped with Zr alone, ΔT2 is +10℃; when the superconducting core wire precursor is doped with Hf alone, ΔT2 is +15℃; and when the superconducting core wire precursor is doped with both Zr and Hf, ΔT2 is +20℃.

[0010] Furthermore, the number of superconducting core wire precursors N ranges from 49 to 144. When the number of superconducting core wire precursors N ≤ 70, ΔT3 is 0℃; when 70 < N ≤ 120, ΔT3 is 10℃; and when N > 120, ΔT3 is 20℃.

[0011] Furthermore, the intermediate annealing process in S2 includes a heating stage, a holding stage, and a cooling stage; the intermediate annealing process is carried out under an inert atmosphere.

[0012] Furthermore, during the heating stage in S2, the heating rate is 40~60℃ / h, and the temperature is uniformly increased to the intermediate annealing temperature.

[0013] Furthermore, the heat preservation stage in S2 is as follows: heat preservation at the intermediate annealing temperature, the heat preservation time is calculated based on the coupling relationship of the intermediate annealing parameters, and the heat preservation time range is 20~65h.

[0014] Furthermore, the cooling stage in S2 is as follows: cooling begins after the heat preservation stage ends, with the cooling rate controlled at 10~20℃ / h, and the final temperature drops to 300~350℃.

[0015] On the other hand, the present invention provides a niobium-tin conductor prepared by the above-mentioned thermal strain slow-release preparation method.

[0016] On the other hand, the present invention also provides a niobium tritin conductor prepared by the above-mentioned thermal strain slow release preparation method or the application of the above-mentioned niobium tritin conductor in a high-field magnet.

[0017] This invention can achieve at least one of the following beneficial effects: 1. This invention addresses the common industry problem of excessive residual thermal shrinkage stress in Nb3Sn conductors, leading to decreased superconductivity and structural instability, in existing "winding followed by heat treatment" processes. It abandons the traditional "single high-temperature phase formation" approach and breaks the industry's inherent perception that "residual thermal shrinkage stress is an inevitable byproduct of Nb3Sn conductor heat treatment and cannot be effectively alleviated." Instead, it proposes a phased heat treatment method of "intermediate annealing stress relief + final high-temperature phase formation," achieving layered control of "stress relief - phase preparation - full phase formation." By adding an intermediate annealing treatment and controlling the annealing temperature strictly below the Nb3Sn phase formation initiation temperature, it ensures that the superconducting phase does not precipitate prematurely. This releases the mechanical stress of winding, alleviates thermal stress, and homogenizes the conductor's microstructure before the large-scale formation of the Nb3Sn superconducting phase. By advancing the stress relief step, residual stress is reduced from the source, while simultaneously laying the foundation for subsequent phase formation reactions. Ultimately, it achieves synergistic optimization of stress relief, full superconducting phase formation, and grain refinement, improving the overall performance and reliability of the Nb3Sn conductor.

[0018] 2. This invention abandons the traditional extensive process of uniform annealing parameters. Instead, it establishes a coupling relationship between the core parameters of intermediate annealing for Nb3Sn conductors with different structures and doping systems. This enables multi-parameter coordinated control, avoids the limitations of single parameter adjustment, and performs differentiated stress-relieving intermediate annealing. This allows Nb3Sn conductors with different structures and doping systems to effectively release winding mechanical stress, alleviate thermal stress, and achieve uniformity of conductor microstructure.

[0019] 3. By setting a cooling stage after intermediate annealing and heat preservation, the present invention can release internal thermal stress in stages, stabilize and uniform microstructure, and maintain stress relief effect; smooth the transition temperature range, reduce the impact of temperature changes in subsequent processes, protect the integrity of interlayer interfaces; constrain element diffusion behavior, avoid component segregation, and ensure the quality of subsequent phase formation.

[0020] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objectives and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer, exemplary embodiments of the present invention will be described below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. For clarity and brevity, not all features of actual implementations are described in the specification.

[0022] An embodiment of the present invention provides a method for the slow-release preparation of a niobium-tin conductor under thermal strain, comprising the following steps: S1, the Nb3Sn conductor blank that has not undergone phase-forming reaction is wound into the shape of a magnet coil to obtain a shaped coil blank; S2, the formed coil blank of S1 is subjected to intermediate annealing treatment, and the annealing temperature of the intermediate annealing treatment is lower than the Nb3Sn phase formation initiation temperature; S3 involves performing a phase-forming heat treatment on the formed coil blank after the intermediate annealing treatment in S2, causing Nb and Sn in the formed coil blank to diffuse and react to form the Nb3Sn superconducting phase; after the phase-forming heat treatment is completed, the coil blank is cooled to obtain an Nb3Sn conductor with low residual stress and high superconductivity.

[0023] This invention addresses the common industry problem of excessive residual thermal shrinkage stress in Nb3Sn conductors, leading to decreased superconductivity and structural instability, in existing "winding followed by heat treatment" processes. It abandons the traditional "single high-temperature phase formation" approach and breaks the industry's inherent perception that "residual thermal shrinkage stress is an inevitable byproduct of Nb3Sn conductor heat treatment and cannot be effectively alleviated." Instead, it proposes a phased heat treatment method of "intermediate annealing stress relief + final high-temperature phase formation," achieving layered control of "stress relief - phase preparation - full phase formation." By adding an intermediate annealing process and controlling the annealing temperature strictly below the Nb3Sn phase formation initiation temperature, it ensures that the superconducting phase does not precipitate prematurely. This releases the mechanical stress of winding, alleviates thermal stress, and homogenizes the conductor's microstructure before the large-scale formation of the Nb3Sn superconducting phase. By advancing the stress relief step, residual stress is reduced from the source, while simultaneously laying the foundation for subsequent phase formation reactions. Ultimately, it achieves synergistic optimization of stress relief, full superconducting phase formation, and grain refinement, improving the overall performance and reliability of Nb3Sn conductors.

[0024] According to some embodiments of the present invention, in S1, the Nb3Sn conductor blank that has not undergone phase-forming reaction is precisely wound into a magnet coil shape of a preset specification according to the target working condition requirements. The winding process is controlled to ensure uniform winding tension and avoid local stress concentration.

[0025] According to some embodiments of the present invention, the Nb3Sn conductor blank used in S1 includes: a central reinforcement, a superconducting core wire precursor, a Sn-Ti reactive alloy layer, a barrier layer, a copper stabilizing matrix, and an outer sheath. The central reinforcement is located at the axis of the Nb3Sn conductor blank. The Sn-Ti reactive alloy layer is wrapped around each superconducting core wire precursor, wherein the Ti content is 1wt%~2wt%. Multiple superconducting core wire precursors are arranged in a ring around the central reinforcement. The barrier layer is wrapped around the periphery of each superconducting core wire precursor. The copper stabilizing matrix is ​​wrapped around the sub-component composed of the central reinforcement and multiple superconducting core wire precursors. The outer sheath is wrapped around the copper stabilizing matrix.

[0026] The central reinforcement can be any one of Nb-Ti alloy, pure Nb, stainless steel, Cu-Ni alloy, and W-Re alloy.

[0027] According to some embodiments of the present invention, in S2, differentiated intermediate annealing treatment is performed on Nb3Sn conductor blanks with different structures and different doping systems. Specifically, an intermediate annealing parameter coupling formula can be established, and the intermediate annealing process parameters can be set differently according to the intermediate annealing parameter coupling formula for Nb3Sn conductor blanks with different structures and different doping systems to achieve multiple effects such as mechanical stress release, microstructure homogenization, and pre-diffusion activation.

[0028] According to some embodiments of the present invention, the coupling relationship of intermediate annealing parameters includes: T=420+ΔT1+ΔT2-ΔT3(1); Where: T is the intermediate annealing temperature in °C; ΔT1 is the intermediate reinforcement material correction coefficient; ΔT2 is the nano-doping correction coefficient; and ΔT3 is the superconducting core wire precursor density correction coefficient.

[0029] Specifically, for ΔT1, when the intermediate reinforcement is an Nb-Ti alloy or pure Nb, ΔT1 is 0℃; when the intermediate reinforcement is a Cu-Ni alloy, ΔT1 is +15℃; when the intermediate reinforcement is stainless steel, ΔT1 is +25℃; and when the intermediate reinforcement is a W-Re alloy, ΔT1 is +30℃.

[0030] For ΔT2, when there is no Zr and Hf doping in the superconducting filament precursor, ΔT2 is 0℃; when there is only Zr doping in the superconducting filament precursor, ΔT2 is +10℃; when there is only Hf doping in the superconducting filament precursor, ΔT2 is +15℃; when there is a combination of Zr and Hf doping in the superconducting filament precursor, ΔT2 is +20℃.

[0031] For ΔT3, the number of superconducting core wire precursors N ranges from 49 to 144. When the number of superconducting core wire precursors N ≤ 70, ΔT3 is 0℃; when 70 < N ≤ 120, ΔT3 is 10℃; and when N > 120, ΔT3 is 20℃.

[0032] The intermediate annealing temperature is determined using the intermediate annealing parameter coupling formula of this invention, and the intermediate annealing temperature range is between 400 and 470°C. This method allows for precise control of the intermediate annealing temperature, ensuring it remains below the Nb3Sn phase formation initiation temperature of 580°C. This avoids the premature formation of a hard and brittle Nb3Sn phase that hinders stress relief, while simultaneously promoting the initial diffusion of Sn atoms, thus ensuring a balance between stress relief and phase preparation. This ensures that no Nb3Sn superconducting phase is formed during this stage, achieving only stress relief and element pre-diffusion activation.

[0033] According to some embodiments of the present invention, the coupling relationship of intermediate annealing parameters further includes: t=20+(T-420) / 2+0.3×N / 10(2); Where t is the intermediate annealing holding time in hours; T is the intermediate annealing temperature in degrees Celsius; and N is the number of superconducting core wire precursors, ranging from 49 to 144.

[0034] The embodiments of the present invention abandon the traditional extensive process of uniform annealing parameters. For Nb3Sn conductors with different structures and different doping systems, a coupling relationship of intermediate annealing core parameters is established to achieve multi-parameter synergistic control, avoid the limitations of single parameter adjustment, and perform differentiated stress-relieving intermediate annealing. This allows Nb3Sn conductors with different structures and different doping systems to achieve effective release of winding mechanical stress, relief of thermal stress, and homogenization of conductor microstructure.

[0035] According to some embodiments of the present invention, the intermediate annealing process includes a heating stage, a holding stage, and a cooling stage. That is, the Nb3Sn conductor blank is heated to the intermediate annealing temperature, held at the intermediate annealing temperature for a certain period of time, and then cooled after the holding period.

[0036] The intermediate annealing process is carried out entirely in an inert atmosphere, for example, using argon as a protective atmosphere, and controlling the oxygen content in the furnace to ≤10ppm to prevent conductor oxidation.

[0037] According to some embodiments of the present invention, in S2, during the heating stage of the intermediate annealing treatment, the heating rate is 40~60℃ / h, and the temperature is uniformly increased to the calculated intermediate annealing temperature. Exemplary heating rates are 40℃ / h, 45℃ / h, 50℃ / h, 55℃ / h, and 60℃ / h.

[0038] According to some embodiments of the present invention, in S2, the heat preservation stage of the intermediate annealing treatment is: heat preservation at the intermediate annealing temperature, the heat preservation time is calculated according to the coupling relationship of intermediate annealing parameters, and the heat preservation time range is 20~65h.

[0039] In embodiments of the present invention, the holding time is calculated using the aforementioned intermediate annealing parameter coupling formula, and the holding time is controlled between 20 and 65 hours. If the holding time is too short (less than 20 hours), the dislocations and distortion defects generated during winding inside the conductor cannot fully slide and recover, the residual mechanical stress is not completely released, and there are still local stress concentration areas. At the same time, the pre-diffusion degree of Sn and Ti elements is extremely low, the activation effect of the reaction interface is poor, and problems such as uneven phase formation, local phase loss, and large differences in grain growth are likely to occur during the subsequent high-temperature phase formation process. Ultimately, this leads to high residual stress in the conductor, local distortion of the core wire, attenuation of high-field critical current density, and increased hysteresis loss, making it impossible to achieve the performance optimization objective. If the heat preservation time is too long (greater than 65 hours), the stress relief will reach saturation. Further extending the heat preservation time will not have any beneficial effect. On the contrary, it will cause excessive diffusion of Sn elements and excessive growth of matrix grains, resulting in severe softening of the oxygen-free copper matrix, a decrease in the mechanical load-bearing capacity of the conductor, and a deterioration of the residual resistivity ratio (RRR). At the same time, excessively long low-temperature heat preservation will induce the initiation and growth of micropores and micro-defects at the interface, destroying the interfacial bonding between the core wire and the matrix. During subsequent high-temperature phase formation, core wire deformation and minor fractures are likely to occur, reducing the structural integrity and service stability of the conductor. Moreover, it will significantly increase production energy consumption and preparation cycle, which is not conducive to industrial mass production.

[0040] According to some embodiments of the present invention, in S2, the cooling stage of the intermediate annealing process is as follows: cooling begins after the heat preservation stage ends, the cooling rate is controlled at 10~20℃ / h, and the temperature is finally reduced to 300~350℃.

[0041] The embodiments of this invention, by incorporating a cooling stage, can release internal thermal stress in stages, stabilize and homogenize the microstructure, and maintain the stress-relieving effect; a smooth transition temperature range reduces the impact of temperature changes in subsequent processes, protecting the integrity of interlayer interfaces; and constrain element diffusion behavior, preventing component segregation and ensuring the quality of subsequent phase formation. If cooling is not performed and subsequent phase-forming heat treatment is carried out directly after the holding stage, severe thermal stress will be generated, easily causing core wire deformation and interface cracking; the released stress will rebound and recover, rendering the annealing effect ineffective; simultaneously, disordered element diffusion will lead to component inhomogeneity, ultimately resulting in a significant decrease in superconducting performance and structural stability.

[0042] For example, the final cooling endpoint temperatures are 300℃, 310℃, 320℃, 330℃, 340℃, and 350℃. Embodiments of the present invention gradually reduce the temperature from the intermediate high-temperature range of the intermediate annealing to the medium-temperature range of 300~350℃, enabling the conductor components to gradually shrink according to their differences in thermal expansion coefficients. This releases the thermal stress accumulated during the annealing process in stages, preventing a sudden accumulation of internal stress and maintaining the integrity of the interface between the core wire, barrier layer, and copper substrate.

[0043] For example, the cooling rate during the cooling stage is 10℃ / h, 12℃ / h, 15℃ / h, 18℃ / h, and 20℃ / h. By precisely controlling the heating and cooling rates, this invention can avoid excessive grain coarsening, ensure the full formation of the Nb3Sn superconducting phase and grain refinement, thereby increasing the critical current density (Jc) of the conductor and balancing stress relief with optimization of superconducting performance.

[0044] The embodiments of the present invention limit the heating rate, holding time, cooling rate and cooling endpoint temperature, so that the parameters work together to avoid insufficient stress relief, grain coarsening and secondary stress caused by thermal shock, effectively release the mechanical stress accumulated during the winding process, and alleviate the residual thermal shrinkage stress caused by thermal shrinkage mismatch and composition inhomogeneity during the formation of Nb3Sn.

[0045] According to some embodiments of the present invention, the Nb3Sn conductor blank in step S1 can be an Nb3Sn conductor blank prepared by the bronze method, the internal tin method, or the powder-in-tube method. The method of the present invention is adaptable to Nb3Sn conductors prepared by different methods, ensuring that stress relief and superconducting performance optimization can be achieved for different types of conductors, and has process versatility.

[0046] According to some embodiments of the present invention, in step S1, the Nb3Sn conductor blank undergoes surface cleaning treatment after winding to remove surface impurities and oxide layers. After surface cleaning treatment, the Nb3Sn conductor blank is placed in a vacuum heat treatment furnace and evacuated to a specified vacuum level to avoid conductor oxidation during heat treatment, thus laying the foundation for subsequent heat treatment.

[0047] After the Nb3Sn conductor blank is cooled to its final cooling temperature, it is heated to the temperature for phase-forming heat treatment. This invention seamlessly integrates the intermediate annealing process with the existing final phase-forming heat treatment, requiring no new specialized equipment or significant equipment modifications. It can be achieved simply by adjusting the parameters of the vacuum heat treatment furnace. The process is simple, highly controllable, and balances process compatibility with industrial feasibility.

[0048] According to some embodiments of the present invention, in step S3, the phase-forming temperature of the phase-forming heat treatment is 640~700℃, the holding time is 180~220h, and the heating rate is 60~90℃ / h. The phase-forming treatment promotes the full diffusion reaction of Nb and Sn to generate a dense and uniform Nb3Sn superconducting phase. After the reaction is completed, the temperature is uniformly cooled to room temperature at a cooling rate of 70~120℃ / h, ultimately obtaining an Nb3Sn conductor with low residual stress and high superconductivity. Furthermore, argon gas is introduced throughout the phase-forming heat treatment to prevent oxidation.

[0049] For example, the phase formation temperature is 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, and 700℃; the holding time is 180h, 190h, 200h, 210h, and 220h; the heating rate is 60℃ / h, 70℃ / h, 80℃ / h, and 90℃ / h; and the cooling rate is 70℃ / h, 80℃ / h, 90℃ / h, 100℃ / h, 110℃ / h, and 120℃ / h.

[0050] The embodiments of this invention employ gradient-controlled temperature phase-forming heat treatment. The coil blank after S2 intermediate annealing undergoes segmented, precisely temperature-controlled phase-forming heat treatment. Through precise three-stage control of heating, isothermal treatment, and cooling, Nb and Sn elements are fully diffused to generate a uniform and dense nanoscale Nb3Sn superconducting phase. After the phase-forming heat treatment, gradient-controlled rate furnace cooling is used to avoid secondary thermal stress caused by rapid temperature changes, ultimately obtaining a high-performance Nb3Sn conductor with low residual stress, fine grains, and high flux pinning ability. The phase-forming heat treatment process and the intermediate annealing process in this embodiment of the invention form a synergistic matching relationship. The longer the intermediate annealing holding time and the higher the temperature adaptability, the lower the activation energy of the subsequent phase-forming reaction, the better the phase uniformity, and the more significant the grain refinement effect.

[0051] The Nb3Sn conductor prepared by the method of this invention exhibits an axial residual stress below 81.4 MPa, which is 40% to 60% lower than that of traditional phase formation processes. The critical current density at 4.2 K and 20 T is 806 A / mm². 2 Compared to traditional processes, the overall performance is improved by 9% to 20%, significantly reducing defects such as conductor microcracks and superconducting filament breakage. It also solves the problem of reduced superconducting performance caused by excessive stress in existing processes, and significantly improves the structural stability of the conductor and the service life of the magnet.

[0052] An embodiment of the present invention also provides a niobium-tin conductor, which is prepared by the above-described thermal strain sustained-release preparation method.

[0053] Embodiments of the present invention also provide an application of niobium-tin conductor in high-field magnets, for example, in high-field magnets with a magnetic field strength ≥12T, and in high-field magnets with a magnetic field strength ≥20T.

[0054] The niobium-tin conductor prepared by the above method can effectively avoid damage caused by stress failure in high-field magnets during low-temperature operation and magnetic field loading by alleviating residual thermal shrinkage stress, reducing internal defects in the conductor, and improving the structural stability of the conductor. This significantly extends the service life of high-field magnets and reduces the cost of magnet preparation and maintenance.

[0055] The technical solution of the present invention will be further illustrated below with specific embodiments.

[0056] Example 1 The specific steps of the niobium-tin conductor thermal strain sustained-release preparation method in this embodiment are as follows: S1. Winding and Surface Cleaning: A Nb3Sn conductor blank (0.8 mm in diameter) prepared by the bronze method is selected and wound into a magnet coil shape to obtain a shaped coil blank. The blank is then ultrasonically cleaned with 100W anhydrous ethanol for 15 minutes to remove surface oil and oxide layers. After drying, it is placed in a vacuum heat treatment furnace and evacuated to a vacuum level of 1×10⁻⁶. -3 Pa. The Nb3Sn conductor preform comprises: a central reinforcement, a superconducting core wire precursor, a Sn-Ti reactive alloy layer (Ti content 1.5wt%), a Ta barrier layer, an oxygen-free copper stabilizing matrix, and a CuNi alloy outer sheath; the central reinforcement is located at the axis of the Nb3Sn conductor preform, the Sn-Ti-based reactive alloy layer is wrapped around each superconducting core wire precursor, multiple superconducting core wire precursors are arranged in a ring around the central reinforcement, the Ta barrier layer is wrapped around the periphery of each superconducting core wire precursor, the oxygen-free copper stabilizing matrix is ​​wrapped around the sub-component composed of the central reinforcement and multiple superconducting core wire precursors, and the CuNi alloy outer sheath is wrapped around the oxygen-free copper stabilizing matrix.

[0057] The intermediate reinforcement is an Nb-Ti alloy, the number of superconducting core wire precursors is N=60, and the superconducting core wire precursors are free of Zr and Hf doping.

[0058] S2, Intermediate annealing treatment: The intermediate annealing temperature and holding time are calculated based on the intermediate annealing parameter relationship, where ΔT1=0℃, ΔT2=0℃, and ΔT3=0℃. The calculation results are as follows: T = 420 + 0 + 0 - 0 = 420℃; t=20+(420-420) / 2+0.3×60 / 10=21.8h.

[0059] Based on the above calculation results, intermediate annealing is performed, including: (1) Heating stage: The formed coil blank is heated and the heating rate is controlled at 50℃ / h to raise the furnace temperature to the intermediate annealing temperature of 420℃. (2) Heat preservation stage: heat preservation at 420℃ for 21.8h to carry out stress relief and microstructure homogenization treatment; (3) Cooling stage: Control the cooling rate to 15℃ / h and reduce the furnace temperature to 300℃.

[0060] S3. Final phase formation heat treatment: Control the heating rate to 60℃ / h, heat to the phase formation heat treatment temperature of 640℃, and hold for 180h; then cool down to room temperature at a rate of 70℃ / h to complete the heat treatment.

[0061] Testing revealed that the Nb3Sn conductor treated in this embodiment exhibited an axial residual stress of -81.4 MPa (compared to -135.7 MPa for a conventionally treated conductor without this process), representing a 40% reduction in residual stress. The critical current density at 4.2 K and 20 T was 810 A / mm². 2 This process improves upon conventional methods by 10%, with no obvious microcracks on the conductor surface and good structural stability.

[0062] Example 2 The specific steps of the niobium-tin conductor thermal strain sustained-release preparation method in this embodiment are as follows: S1. Winding and Surface Cleaning: A Nb3Sn conductor blank (0.8 mm in diameter) prepared by the bronze method is selected and wound into a magnet coil shape to obtain a shaped coil blank. The blank is then ultrasonically cleaned with 100W anhydrous ethanol for 15 minutes to remove surface oil and oxide layers. After drying, it is placed in a vacuum heat treatment furnace and evacuated to a vacuum level of 1×10⁻⁶. -3 Pa. Among them, the intermediate reinforcement of the Nb3Sn conductor blank is Cu-Ni alloy, the number of superconducting core wire precursors is N=100, the superconducting core wire precursors are single doped with Zr, and other structures are the same as in Example 1.

[0063] S2, Intermediate annealing treatment: The intermediate annealing temperature and holding time are calculated based on the intermediate annealing parameter relationship, where ΔT1=+15℃, ΔT2=+10℃, and ΔT3=10℃. The calculation results are as follows: T = 420 + 15 + 10 - 10 = 435℃; t=20+(435-420) / 2+0.3×100 / 10=30.5h.

[0064] Based on the above calculation results, intermediate annealing is performed, including: (1) Heating stage: The Nb3Sn conductor billet is heated and the heating rate is controlled at 50℃ / h to raise the furnace temperature to the intermediate annealing temperature of 435℃. (2) Heat preservation stage: heat preservation at 435℃ for 30.5h to carry out stress relief and microstructure homogenization treatment; (3) Cooling stage: Control the cooling rate to 15℃ / h and reduce the furnace temperature to 320℃.

[0065] S3. Final phase formation heat treatment: Control the heating rate to 75℃ / h, raise the temperature to 660℃ for phase formation heat treatment, and hold for 200h; then cool down to room temperature at a rate of 90℃ / h to complete the heat treatment.

[0066] Testing revealed that the Nb3Sn conductor treated in this embodiment exhibited an axial residual stress of -72.3 MPa (compared to -135.7 MPa for a conventionally treated conductor without this process), representing a 46.8% reduction in residual stress. The critical current density at 4.2 K and 20 T was 855 A / mm². 2 It improves upon conventional processes by 16.2%; the conductor core wire has a uniform structure, free from microcracks and distortion, and exhibits superior mechanical stability and superconducting performance.

[0067] Example 3 The specific steps of the niobium-tin conductor thermal strain sustained-release preparation method in this embodiment are as follows: S1. Winding and Surface Cleaning: A Nb3Sn conductor blank (0.8 mm in diameter) prepared by the bronze method is selected and wound into a magnet coil shape to obtain a shaped coil blank. The blank is then ultrasonically cleaned with 100W anhydrous ethanol for 15 minutes to remove surface oil and oxide layers. After drying, it is placed in a vacuum heat treatment furnace and evacuated to a vacuum level of 1×10⁻⁶. -3 Pa. Among them, the intermediate reinforcement of the Nb3Sn conductor blank is W-Re alloy, the number of superconducting core wire precursors is N=140, the superconducting core wire precursors are composite doped with Zr and Hf, and other structures are the same as in Example 1.

[0068] S2, Intermediate annealing treatment: The intermediate annealing temperature and holding time are calculated based on the coupling relationship of the intermediate annealing parameters, where ΔT1 = +30℃, ΔT2 = +20℃, and ΔT3 = 20℃. T = 420 + 30 + 20 - 20 = 450℃; t=20+(450-420) / 2+0.3×140 / 10=39.2h.

[0069] Based on the above calculation results, intermediate annealing is performed, including: (1) Heating stage: The Nb3Sn conductor blank is heated and the heating rate is controlled at 50℃ / h to raise the furnace temperature to the annealing temperature of 450℃; (2) Heat preservation stage: Heat preservation at 450℃ for 39.2h to carry out stress relief and microstructure homogenization treatment; (3) Cooling stage: Control the cooling rate to 15℃ / h and reduce the furnace temperature to 350℃.

[0070] S3. Final phase formation heat treatment: Control the heating rate to 90℃ / h, raise the temperature to 690℃ for phase formation heat treatment, and hold for 220h; then cool down to room temperature at a rate of 120℃ / h to complete the heat treatment.

[0071] Testing revealed that the Nb3Sn conductor treated in this embodiment exhibited an axial residual stress of -65.8 MPa (compared to -135.7 MPa for a conventionally treated conductor without this process), representing a 51.5% reduction in residual stress. The critical current density at 4.2 K and 20 T was 880 A / mm². 2 It improves upon conventional processes by 19.7%; the conductor grains are refined and uniform, the interface bonding is tight, there are no stress concentration defects, and the overall performance is optimal.

[0072] Example 4 The only difference between this embodiment and Embodiment 1 is the process parameters for the final phase-forming heat treatment in S3; all other steps and the conductor blank are the same as in Embodiment 1. Specifically: The heating rate was controlled at 70℃ / h, and the temperature was raised to 640℃ and held for 190h. Then, the temperature was lowered to room temperature at a rate of 80℃ / h to complete the heat treatment.

[0073] The Nb3Sn conductor prepared in this embodiment has an axial residual stress of -80.2 MPa, which is 40.9% lower than that prepared by conventional processes; the critical current density is 806 A / mm². 2 It improves upon conventional processes by 9.5%; the conductor structure is uniform, free of microcracks and interface defects, and its overall performance is stable.

[0074] Example 5 The only difference between this embodiment and Embodiment 2 is the process parameters for the final phase-forming heat treatment in S3; all other steps and the conductor blank are the same as in Embodiment 2. Specifically: The heating rate was controlled at 85℃ / h, and the temperature was raised to 660℃ and held for 210h. Then, the temperature was lowered to room temperature at a rate of 110℃ / h to complete the heat treatment.

[0075] The Nb3Sn conductor prepared in this embodiment has an axial residual stress of -71.5 MPa, which is 47.3% lower than that prepared by conventional processes; the critical current density is 862 A / mm². 2 This process improves upon conventional methods by 17.0%; Nb and Sn diffusion is sufficient, the superconducting phase is dense and uniform, and the grain size consistency is good.

[0076] Example 6 The only difference between this embodiment and Embodiment 3 is the process parameters for the final phase-forming heat treatment in S3; all other steps and the conductor blank are the same as in Embodiment 3. Specifically: The heating rate was controlled at 62℃ / h, and the temperature was raised to 690℃ and held for 220h. Then, the temperature was lowered to room temperature at a rate of 125℃ / h to complete the heat treatment.

[0077] The Nb3Sn conductor prepared in this embodiment has an axial residual stress of -64.9 MPa, which is 52.2% lower than that of conventional processes; the critical current density is 886 A / mm². 2 It improves upon conventional processes by 20.3%; it has a high proportion of superconducting phase, excellent flux pinning ability, and tight, defect-free interface bonding.

[0078] Example 7 The specific steps of the niobium-tin conductor thermal strain sustained-release preparation method in this embodiment are as follows: S1. Winding and Surface Cleaning: A Nb3Sn conductor blank (0.8 mm in diameter) prepared by the bronze method is selected and wound into a magnet coil shape to obtain a shaped coil blank. The blank is then ultrasonically cleaned with 100W anhydrous ethanol for 15 minutes to remove surface oil and oxide layers. After drying, it is placed in a vacuum heat treatment furnace and evacuated to a vacuum level of 1×10⁻⁶. -3 Pa. Among them, the intermediate reinforcement of the Nb3Sn conductor blank is pure Nb, the number of superconducting core wire precursors is N=70, and the superconducting core wire precursors are single-doped with Hf.

[0079] S2, Intermediate annealing treatment: The intermediate annealing temperature and holding time were calculated based on the intermediate annealing parameter relationship, where ΔT1=0℃, ΔT2=+15℃, and ΔT3=0℃; the calculation results are as follows: T = 420 + 0 + 15 - 0 = 435℃; t=20+(435-420) / 2+0.3×70 / 10=29.6h; Based on the above calculation results, intermediate annealing is performed, including: (1) Heating stage: control the heating rate at 50℃ / h, and heat to 435℃; (2) Insulation stage: Insulate at 435℃ for 29.6 hours; (3) Cooling stage: control the cooling rate to 15℃ / h, and cool down to 330℃.

[0080] S3. Final phase formation heat treatment: Control the heating rate to 78℃ / h, heat to 670℃, and hold for 195h; then cool down to room temperature at a rate of 95℃ / h to complete the heat treatment.

[0081] Testing revealed that the axial residual stress of the Nb3Sn conductor prepared in this embodiment was -76.1 MPa, a reduction of 43.9% compared to conventional processes; the critical current density was 834 A / mm². 2 It improves upon conventional processes by 13.0%; the conductor structure is uniform and free of microcracks, the overall process is highly adaptable, and all performance characteristics are at an excellent level.

[0082] Comparative Example 1 Nb3Sn conductor blanks with the same specifications and preparation method as in Example 2 were selected and subjected to conventional heat treatment processes, i.e., no intermediate annealing was performed, and phase formation heat treatment was carried out directly. The heating rate of the phase formation heat treatment was 10℃ / min, the holding time was 100h at 650℃, and the cooling rate was 4.5℃ / min.

[0083] Testing revealed that the Nb3Sn conductor treated in this comparative example exhibited an axial residual stress of -135.7 MPa and a critical current density of 736 A / mm² under conditions of 4.2 K and 20 T. 2 Furthermore, there are a few microcracks on the conductor surface, with a defect rate greater than 5%.

[0084] Comparative Example 2 This comparative example uses bronze Nb3Sn conductor blanks (0.8 mm in diameter) of the same specifications and batch as in Example 2. The winding and surface cleaning processes, the heating and holding stages in the intermediate annealing process, and the final phase formation heat treatment process are all consistent with those in Example 2. The difference is that the cooling stage in the intermediate annealing process is omitted, and the temperature is directly increased after the intermediate annealing and holding process to enter the final phase formation heat treatment stage.

[0085] Testing revealed that the axial residual stress of the Nb3Sn conductor prepared in this comparative example was -98.6 MPa, representing a reduction of only 27.3%; the critical current density under 4.2 K / 20 T conditions was 785 A / mm². 2 The performance improvement is significantly limited.

[0086] Comparative Example 3 This comparative example uses bronze Nb3Sn conductor blanks of the same specifications and batches as in Example 2. The winding and surface cleaning processes, intermediate annealing temperatures, heating and cooling rates, cooling endpoints, and phase formation heat treatment processes are all consistent with those in Example 2. The only difference is that the intermediate annealing holding time is adjusted to 15 hours.

[0087] Testing revealed that the axial residual stress of the Nb3Sn conductor prepared in this comparative example was -112.6 MPa, which was only 17.0% lower than that prepared by conventional processes; the critical current density was 742 A / mm², which was 2.1% lower than that prepared by conventional processes.

[0088] Comparative Example 4 This comparative example uses bronze Nb3Sn conductor blanks of the same specifications and batches as in Example 2. The winding and surface cleaning processes, intermediate annealing temperatures, heating and cooling rates, cooling endpoints, and phase formation heat treatment processes are all consistent with those in Example 2. The only difference is that the intermediate annealing holding time is adjusted to 70 hours.

[0089] The Nb3Sn conductor prepared in this comparative example has an axial residual stress of -89.3 MPa, which is 34.2% lower than that prepared by conventional processes; and a critical current density of 776 A / mm², which is 4.2% higher than that prepared by conventional processes.

[0090] Comparative Example 5 This comparative example uses bronze Nb3Sn conductor blanks of the same specifications and batches as in Example 2. The winding and surface cleaning processes, intermediate annealing temperatures, holding times, heating and cooling rates, and phase formation heat treatment processes are all consistent with those in Example 2. The only difference is that the cooling endpoint in the intermediate annealing cooling stage is adjusted to 280°C.

[0091] The Nb3Sn conductor prepared in this comparative example has an axial residual stress of -95.8 MPa, which is 29.4% lower than that prepared by conventional processes; and a critical current density of 791 A / mm², which is 4.9% higher than that prepared by conventional processes.

[0092] The axial stress and critical current density test results for each embodiment and comparative example are shown in Table 1.

[0093] Table 1 Performance test results of Nb3Sn conductors in each embodiment and comparative example

[0094] As shown in Table 1, Comparative Example 1, due to the lack of intermediate annealing, has a large axial residual stress in the conductor and a low critical current density, resulting in a significant reduction in conductor structural stability and superconducting performance.

[0095] In Comparative Example 2, due to the elimination of the intermediate cooling buffer, high-temperature phase formation was directly performed after high-temperature annealing, resulting in severe secondary thermal stress inside the conductor. This caused some micro-dislocations to spring back, reduced structural uniformity, and the presence of micropores and stress concentration defects at a small number of core wire interfaces. The hysteresis loss was higher than that in Example 2, and the structural stability and superconducting performance were significantly deteriorated. This fully demonstrates the key role of the intermediate gradient cooling process in stress relief and performance improvement in this invention.

[0096] In Comparative Example 3, the intermediate annealing holding time was insufficient, resulting in incomplete recovery of dislocations and lattice distortions generated during winding, and incomplete release of mechanical stress. The pre-diffusion activation effect of Sn and Ti elements was poor, leading to uneven subsequent phase formation reactions, localized phase loss, large differences in grain size, and a significant deterioration in superconducting performance. In Comparative Example 4, the intermediate annealing holding time was too long, stress relief had reached saturation, and additional holding had no positive effect. Excessive diffusion of Sn elements caused coarse and softened copper matrix grains, reducing the mechanical properties of the conductor. Simultaneously, micropores formed at the interface, disrupting interface integrity and ultimately leading to a significant drop in superconducting performance.

[0097] In Comparative Example 5, the intermediate annealing cooling endpoint was too low, amplifying the differences in thermal shrinkage among the conductor components and causing additional thermal stress to accumulate again in the low-temperature range. The temperature gradient was too large, causing the stress released in the early stage to rebound. At the same time, the element diffusion behavior was abnormal at low temperatures, resulting in local component segregation, poor phase uniformity, and a significant decrease in both stress relief and superconducting performance.

[0098] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a niobium-tin conductor with sustained thermal strain, characterized in that, Includes the following steps: S1, winding the Nb3Sn conductor blank into a magnet coil shape; S2, the Nb3Sn conductor blank wound in S1 is subjected to intermediate annealing treatment, and the annealing temperature of the intermediate annealing treatment is lower than the Nb3Sn phase formation initiation temperature. S3, the Nb3Sn conductor blank after intermediate annealing in S2 is subjected to phase-forming heat treatment to induce Nb and Sn diffusion reaction to generate Nb3Sn superconducting phase; after the phase-forming heat treatment is completed, it is cooled to obtain Nb3Sn conductor; The Nb3Sn conductor preform includes: a central reinforcement and a superconducting core wire precursor; In S2, an intermediate annealing parameter coupling relationship is established, and the intermediate annealing process parameters are set according to the intermediate annealing parameter coupling relationship; the intermediate annealing parameter coupling relationship includes: T = 420 + ΔT1 + ΔT2 - ΔT3; Where: T is the intermediate annealing temperature in °C; ΔT1 is the intermediate reinforcement material correction coefficient; ΔT2 is the superconducting core wire nano-doping correction coefficient; and ΔT3 is the superconducting core wire precursor density correction coefficient.

2. The thermal strain sustained-release preparation method according to claim 1, characterized in that, The intermediate annealing parameter coupling formula also includes: t = 20 + (T - 420) / 2 + 0.3 × N / 10; Where: t is the intermediate annealing holding time, in hours; T is the intermediate annealing temperature, in degrees Celsius; N is the number of superconducting core wire precursors, ranging from 49 to 144.

3. The thermal strain sustained-release preparation method according to claim 1, characterized in that, In the coupling relationship of the intermediate annealing parameters When the intermediate reinforcement is an Nb-Ti alloy or pure Nb, ΔT1 is 0℃; when the intermediate reinforcement is a Cu-Ni alloy, ΔT1 is +15℃; when the intermediate reinforcement is stainless steel, ΔT1 is +25℃; when the intermediate reinforcement is a W-Re alloy, ΔT1 is +30℃. And / or, when the superconducting filament precursor has no Zr and Hf doping, ΔT2 is 0℃; when the superconducting filament precursor is doped with Zr alone, ΔT2 is +10℃; when the superconducting filament precursor is doped with Hf alone, ΔT2 is +15℃; when the superconducting filament precursor is doped with both Zr and Hf, ΔT2 is +20℃. And / or, the number N of the superconducting core wire precursors ranges from 49 to 144. When the number N of the superconducting core wire precursors is ≤70, ΔT3 is 0°C; when 70 < N ≤ 120, ΔT3 is 10°C; when N > 120, ΔT3 is 20°C.

4. The thermal strain sustained-release preparation method according to claim 1, characterized in that, The intermediate annealing process described in S2 includes a heating stage, a holding stage, and a cooling stage; the intermediate annealing process is carried out under an inert atmosphere.

5. The thermal strain sustained-release preparation method according to claim 4, characterized in that, In the heating stage described in S2, the heating rate is 40~60℃ / h, and the temperature is uniformly increased to the intermediate annealing temperature.

6. The thermal strain sustained-release preparation method according to claim 4, characterized in that, The heat preservation stage described in S2 is: heat preservation at the intermediate annealing temperature, the heat preservation time is calculated according to the coupling relationship of the intermediate annealing parameters, and the heat preservation time range is 20~65h.

7. The thermal strain sustained-release preparation method according to claim 4, characterized in that, The cooling stage described in S2 is as follows: after the heat preservation stage ends, the cooling begins, and the cooling rate is controlled at 10~20℃ / h, eventually cooling down to 300~350℃.

8. The method for preparing thermally strained sustained-release material according to claim 1, characterized in that, The phase formation temperature of the phase formation heat treatment in S3 is 640~700℃, the holding time is 180~220h, and the heating rate is 60~90℃ / h.

9. A niobium-tin conductor, prepared by the thermal strain-controlled release preparation method according to any one of claims 1-8.

10. The application of a niobium tritin conductor prepared by the thermal strain controlled release preparation method according to any one of claims 1-8, or the niobium tritin conductor according to claim 9, in a high-field magnet.

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