Tough niobium-titanium alloy material, preparation method and application of tough niobium-titanium alloy material to preparation of central conductor

Abstract

The invention discloses a ductile niobium-titanium alloy material, a preparation method and application of the ductile niobium-titanium alloy material to preparation of a central conductor. The ductile niobium-titanium alloy material comprises, by weight, 44.5%-47.5% of Nb, 47.0%-50.0% of Ti, 1.0%-2.0% of Zr, 0.8%-2.0% of Mo, 0.2%-0.6% of V, 0.1%-0.3% of Al, 0.05%-0.12% of Si, 0.015%-0.025% of C, smaller than or equal to 0.035% of O, smaller than or equal to 0.01% of N, smaller than or equal to 0.25% of Fe + impurities and the balance Ti. The preparation process comprises the steps of double-vacuum melting, double-stage pre-homogenization heat treatment, gradient temperature control thermal machining and double-stage aging heat treatment, a fine grain beta matrix + dispersed alpha phase + nano MC carbide synergistic strengthening structure is obtained, and 3-10% of metastable beta phase is reserved. The central conductor made of the material has longitudinal and transverse absorption energy of not less than 180 J / cm and not less than 178 J / cm at-273 DEG C, longitudinal and transverse performance difference of not more than 8%, yield strength of not less than 780 MPa, salt spray corrosion resistance of not less than 1000 hours, and no obvious corrosion, and is suitable for manufacturing the central conductor of the low-temperature superconducting wire.

Description

Technical Field

[0001] This invention belongs to the field of cable materials and engineering technology, and relates to an Nb-Ti based composite alloy material. Through multi-element alloying and multi-scale grain / precipitation structure control, it achieves excellent performance at extreme low temperatures (-273℃), and is particularly suitable for the manufacture of seamless tubes for the center conductor of low-temperature superconducting wires. Background Technology

[0002] When using existing Nb-Ti alloys as central conductors, Nb-462%Zr or Nb-Ti-Mo formulations are typically employed, combined with traditional 950–1050℃ solution treatment and single-stage aging. However, practical engineering experience shows that:

[0003] When the service temperature is below -60℃, the brittle phases at grain boundaries (such as Ti-rich α phase, Ti2O, and Nb2C) significantly weaken the toughness.

[0004] Hot-rolled tubes exhibit significant texture along the rolling direction, with transverse energy absorption less than 60% of that in the longitudinal direction, making it difficult to meet the requirements for the center conductor of low-temperature superconducting wires.

[0005] The monotonic solid solution system of Nb and Ti has a narrow β→α phase transformation window, and the stability of the β phase is difficult to control uniformly in the manufacturing of long-distance transport pipes, which easily leads to microstructure delamination.

[0006] Therefore, there is an urgent need for a new Nb-Ti based system that can significantly improve low-temperature toughness through alloy design, thermodynamic phase transformation control, and microstructure gradient regulation during rolling, and can mass-produce highly consistent center conductors. Summary of the Invention

[0007] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0008] In view of the problems existing in the fabrication of Nb-Ti alloys as central conductors, the present invention is proposed.

[0009] Therefore, the technical problem solved by the present invention is to solve the problem of insufficient toughness of existing Nb-Ti alloys when they are made into central conductors.

[0010] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a tough niobium-titanium alloy material, comprising the following components by mass percentage: Nb: 44.1-47.5%; Ti: 47.0-50.0%; Zr: 1.0-2.0%; Mo: 0.8-2.0%; V: 0.2-0.6%; Al: 0.1-0.3%; Si: 0.05-0.12%; C: 0.015-0.025%; O: ≤0.035%; N: ≤0.01%; Fe and unavoidable impurities: ≤0.25%.

[0011] As a preferred embodiment of the tough niobium-titanium alloy material described in this invention, the alloy material in its final heat-treated state simultaneously possesses the following microstructure characteristics: (a) equiaxed grains with a gradient orientation along the tube wall thickness direction, an average grain size of 5–12 μm, and a longitudinal and transverse texture difference index of no more than 0.1; (b) dispersed α-phase precipitates with a grain size of 80–150 nm; (c) dispersed MC-type carbides (V, Mo, NbC) with a grain size of 20–50 nm, mainly located at grain boundaries and dislocation lines; and (d) a metastable β-phase residue of 3–10%.

[0012] As a preferred embodiment of the tough niobium-titanium alloy material described in this invention, wherein the molar ratio of Nb / Ti is 0.94 to 1.00.

[0013] As a preferred embodiment of the tough niobium-titanium alloy material described in this invention, wherein: the MC-type carbides precipitate in situ during aging, and the distribution density is 5 × 10⁻⁶. 8 ~2×10 9 pcs / mm³.

[0014] As a preferred embodiment of the tough niobium-titanium alloy material described in this invention, the alloy material has a Charpy absorption work of ≥180 J / cm² in the longitudinal direction at -273℃, and the difference between longitudinal and transverse properties does not exceed 8%.

[0015] To solve the above-mentioned technical problems, the present invention also provides the following technical solution: a method for preparing a tough niobium-titanium based alloy material, using the above-mentioned raw materials, comprising the following steps:

[0016] (a) Double vacuum melting: High-melting-point elements Nb, Ti, and Mo are first melted in a vacuum induction melting furnace, and then Zr, V, Al, and Si are added. The melting temperature is 1820±15℃, the vacuum degree is ≤3 Pa, and the mixture is stirred for 12 to 18 minutes. Then, vacuum consumable arc remelting is used with a current of 4.7 to 5.2 kA and a remelting rate of 3.0 to 3.4 kg / min.

[0017] (b) Pre-homogenization heat treatment: First, hold at 1050℃ for 12 hours, then lower to 850℃ and hold for 8 hours, then furnace cool to 500℃ and air cool.

[0018] (c) Gradient temperature controlled thermomechanical processing: Forging begins in the β+α′ dual-phase region at 930~960℃, and rapid deformation occurs in the pure β region at 980~1000℃ during the intermediate stage. Before unloading, the temperature is reduced to 900℃ for precision forging. The piercing temperature is 940±5℃. After piercing, the material is directly put into hot rolling with a first pass temperature of 930℃ and a last pass temperature of 840℃.

[0019] (d) Heat treatment: The solution treatment temperature is 975±5℃×45 minutes, and the rapid oil cooling is used; the first aging treatment temperature is 470℃×4 hours, and the water cooling is used; the second aging treatment temperature is 550℃×7 hours, and the air cooling is used; the tube wall of the obtained material exhibits an orientation gradient grain structure and retains 3 to 10% metastable β phase.

[0020] In a preferred embodiment of the preparation method of the tough niobium-titanium alloy material described in this invention, the total deformation in the forging stage is 35-50%, and the single-pass deformation in each hot rolling pass is 12-18%.

[0021] As a preferred embodiment of the preparation method of the tough niobium-titanium alloy material described in this invention, a temperature drop gradient method is used during piercing and hot rolling to control the grain orientation gradient difference of the tube wall cross section to be within 10%.

[0022] To solve the above-mentioned technical problems, the present invention also provides the following technical solution: a central conductor, made of the above-mentioned tough niobium-titanium based alloy material, wherein the tube has a Charpy absorption work of ≥180J / cm² in the longitudinal direction and ≥178J / cm² in the transverse direction at -273℃, a yield strength of ≥780MPa, and no obvious corrosion points after 1000 hours of salt spray test.

[0023] As a preferred embodiment of the central conductor described in this invention, the central conductor is used for fabrication of low-temperature superconducting wires.

[0024] This invention provides a tough niobium-titanium alloy material and its application in the preparation of central conductors, which has the following beneficial effects:

[0025] Multi-component microalloying system: Based on Nb-Ti-Zr-Mo, the ternary regulating component V+Al+Si is innovatively introduced to achieve carbide refinement and α phase precipitation rate control, thus achieving both strength and low-temperature toughness.

[0026] Gradient orientation grain design: The thermomechanical processing temperature drop curve design can create an orientation gradient in the tube wall thickness direction, breaking the traditional transverse shortcomings of rolled tubes;

[0027] Dynamic phase transformation path: Two-stage heat treatment makes the precipitate particle size distribution more dispersed and retains the metastable β phase, which significantly increases the crack tip passivation ability;

[0028] Longitudinal and transverse performance consistency: Through the synergistic effect of gradient organization and residual metastable phase, the longitudinal and transverse performance difference is ≤8%, while the existing technology is generally 15~25%. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments 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. Wherein:

[0030] Figure 1 The flowchart illustrates the method for preparing the tough niobium-titanium based alloy material provided by this invention. Detailed Implementation

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0033] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0034] Existing Nb-Ti alloys have the following problems when used as central conductors:

[0035] When the service temperature is below -60℃, the brittle phases at grain boundaries (such as Ti-rich α phase, Ti2O, and Nb2C) significantly weaken the toughness.

[0036] Hot-rolled tubes exhibit significant texture along the rolling direction, with transverse energy absorption less than 60% of that in the longitudinal direction, making it difficult to meet the requirements for the center conductor of low-temperature superconducting wires.

[0037] The monotonic solid solution system of Nb and Ti has a narrow β→α phase transformation window, and the stability of the β phase is difficult to control uniformly in the manufacturing of long-distance transport pipes, which easily leads to microstructure delamination.

[0038] To address the aforementioned issues, this invention provides a tough Nb-Ti based alloy with a composite multi-scale structure. Through dual-interval microalloying, multi-step thermomechanical treatment, and dynamic adjustment of the phase transformation path, a structure is obtained that is synergistically strengthened by gradient-oriented grains, nano-dispersed precipitation, and metastable residual phase. This results in a central conductor with an absorption energy ≥180 J / cm² at -273℃, a yield strength ≥780 MPa, and a longitudinal and transverse performance difference ≤8%.

[0039] Specifically, the present invention provides a tough niobium-titanium alloy material, comprising the following components by mass percentage:

[0040] Nb: 44.1–47.5%;

[0041] Ti: 47.0–50.0%;

[0042] Zr: 1.0~2.0%;

[0043] Mo: 0.8–2.0%;

[0044] V: 0.2~0.6%;

[0045] Al: 0.1–0.3%;

[0046] Si: 0.05~0.12%;

[0047] C: 0.015~0.025%;

[0048] O: ≤0.035%;

[0049] N: ≤0.01%;

[0050] Total Fe and unavoidable impurities: ≤0.25%.

[0051] It should be noted that the alloy material in the final heat-treated state simultaneously exhibits the following microstructure characteristics: (a) equiaxed grains with a gradient orientation along the tube wall thickness direction, with an average grain size of 5–12 μm and a longitudinal and transverse texture difference index of no more than 0.1; (b) dispersed α-phase precipitates with a grain size of 80–150 nm; (c) dispersed MC-type carbides (V, Mo, NbC) with a grain size of 20–50 nm, mainly located at grain boundaries and dislocation lines; and (d) a metastable β-phase residue of 3–10%.

[0052] Furthermore, the molar ratio of Nb / Ti is 0.94 to 1.00.

[0053] Furthermore, MC-type carbides precipitate in situ during the aging process, with a distribution density of 5 × 10⁻⁶. 8 ~2×10 9 pcs / mm³.

[0054] Furthermore, the Charpy absorption work of this alloy material at -273℃ is ≥180 J / cm² in the longitudinal direction, and the difference in longitudinal and transverse properties does not exceed 8%.

[0055] It should be noted that the analysis from the perspective of the general mechanism of action is as follows: element Mechanism of action Nb Forming a β matrix provides high-temperature strength and corrosion resistance. Ti Forming a continuous solid solution with Nb improves toughness. Zr Grain boundary stabilizers inhibit grain boundary embrittlement phases. Mo β-phase stabilizing elements broaden the processing window and enhance solid solution strengthening. V During aging, stable MC-type carbides are formed, grains are refined, and high-temperature creep resistance is improved. Al Adjusting the β→α phase transition kinetics to accelerate recrystallization Si Improve the density of oxide film C Formation of nano-carbide precipitates with V and Mo O Control oxide nucleation to prevent coarse oxides from becoming embrittled. N Limiting nitriding embrittlement Fe+ impurities Inevitable elements

[0056] Additionally, the present invention also provides a method for preparing a tough niobium-titanium based alloy material, using the above-mentioned raw materials, comprising the following steps:

[0057] (a) Double vacuum melting: High-melting-point elements Nb, Ti, and Mo are first melted in a vacuum induction melting furnace, and then Zr, V, Al, and Si are added. The melting temperature is 1820±15℃, the vacuum degree is ≤3 Pa, and the mixture is stirred for 12 to 18 minutes. Then, vacuum consumable arc remelting is used with a current of 4.7 to 5.2 kA and a remelting rate of 3.0 to 3.4 kg / min.

[0058] It should be noted that:

[0059] I. Vacuum Induction Melting:

[0060] 1. Process conditions:

[0061] Vacuum degree ≤3 Pa. Before melting, the furnace cavity is evacuated and filled with high-purity argon (99.999%) for 10 min to remove residual moisture.

[0062] The order of adding materials is as follows: First, add high-melting-point components such as Nb (purity ≥99.9%) and Mo (purity ≥99.8%), and heat to 1720℃ until they are completely melted. Then add Ti (purity ≥99.85%), Zr (purity ≥99.9%), V, Al, Si, etc., and then raise the temperature to 1820±15℃.

[0063] The stirring method was electromagnetic stirring at a frequency of 20 Hz for 18 minutes to ensure uniform element distribution.

[0064] Online detection (spectral method) of oxygen and nitrogen content confirmed that O ≤ 0.034% and N ≤ 0.009% at the end of smelting.

[0065] 2. Principle Explanation: VIM can avoid air pollution, inhibit the absorption of oxygen, nitrogen, and hydrogen, and allow the components to diffuse fully in the liquid state. Pre-melting of high-melting-point elements can prevent Ti and Zr from being burned off by localized overheating.

[0066] 3. When compiling statistics for experimental batches:

[0067] Single smelting weight: 30 kg;

[0068] Sample chemical composition homogeneity (6-point sampling, ICP analysis): fluctuation range of each element ≤ ±0.03 wt.%;

[0069] Total impurities (C, O, N, Fe): ≤0.28 wt.%;

[0070] II. Vacuum self-consuming arc remelting:

[0071] It should be noted that:

[0072] 1. Process conditions:

[0073] Current 4.7–5.2 kA, voltage 25–28 V;

[0074] Remelting rate: 3.0–3.4 kg / min;

[0075] The distance between the electrode and the molten pool is controlled at 20~25mm and is adjusted in real time by an automatic control system.

[0076] After remelting, the billet diameter is 180 mm, and 25 mm is trimmed off from the head and tail.

[0077] 2. Principle explanation: VAR can remelt under a protective atmosphere, transforming the columnar crystals of the solidified structure into equiaxed crystals, while allowing residual inclusions to float to the surface, thereby reducing the inclusion density (to ≤0.02 inclusions / mm²).

[0078] 3. When statistically analyzing experimental batches (microstructure):

[0079] Cooling rate range: 1.5~2.0℃ / s;

[0080] No shrinkage cavity defects were detected in the ingot (ultrasonic testing UT100% qualified).

[0081] Check grain size: average approximately 220 μm (as-cast state);

[0082] (b) Pre-homogenization heat treatment: First, hold at 1050℃ for 12 hours, then lower to 850℃ and hold for 8 hours, then furnace cool to 500℃ and air cool.

[0083] It should be noted that:

[0084] 1. Process conditions:

[0085] The furnace was held at 1050℃ for 12 hours (heating rate approximately 10℃ / min), and the furnace gas was high-purity argon.

[0086] Cool to 850℃ and keep warm for 8 hours

[0087] Cool the furnace to 500°C, then air cool.

[0088] 2. Principle Explanation:

[0089] The high-temperature stage of 1050℃ promotes the diffusion of Zr and Mo from the enrichment region to the matrix;

[0090] The subsequent 850℃ holding temperature was controlled within the metastable range of the β→α′ transformation, which allowed some α′ phase to precipitate uniformly, breaking the continuity of columnar crystals and providing heterogeneous nucleation sites for subsequent forging nucleation.

[0091] Cooling the product to below 500°C before removing it from the furnace can reduce the risk of thermal stress cracking.

[0092] 3. Statistical analysis of experimental batches (EDS area scan analysis):

[0093] The macroscopic segregation coefficients of Zr and Mo decreased from 1.08 in the smelting state to 1.01;

[0094] Metallographic observation: The α′ phase is diffusely distributed with a density of approximately 0.6 × 10⁻⁶. 8 Pieces / mm²;

[0095] Ingot hardness change: HV (0.5 kg) increased from 262 to 275 (metastable region hardening);

[0096] (c) Gradient temperature controlled thermomechanical processing: Forging begins in the β+α′ dual-phase region at 930~960℃, and rapid deformation occurs in the pure β region at 980~1000℃ during the intermediate stage. Before unloading, the temperature is reduced to 900℃ for precision forging. The piercing temperature is 940±5℃. After piercing, the material is directly put into hot rolling with a first pass temperature of 930℃ and a last pass temperature of 840℃.

[0097] It should be noted that:

[0098] I. Forging Stage

[0099] 1. Process conditions:

[0100] The first large deformation (10-15% reduction / pass) is applied at an initial temperature of 930-960℃ (β+α′ two-phase region).

[0101] The intermediate stage involves rapid deformation at 980–1000℃ (fully in the β-zone) (15% reduction per pass, forming speed 0.2 s). -1 )

[0102] The final stage involves precision forging at 900℃ (with a reduction of 8-10% per pass).

[0103] 2. Principle Explanation:

[0104] Deformation in the initial two-phase region can shear the original α′ phase and promote grain refinement;

[0105] Rapid deformation in the pure β region reduces the texture's single orientation along <110>.

[0106] Low-temperature precision forging refines and eliminates stress concentration in the deformation zone.

[0107] 3. When compiling statistics for experimental batches:

[0108] Deformation energy consumption decreased by 7% (better plasticity in the β region);

[0109] Average grain size after forging: 8~10μm (significantly refined);

[0110] II. Perforation + Hot Rolling

[0111] 1. Process conditions:

[0112] Perforation temperature 940±5℃;

[0113] The first hot rolling pass is 930℃, and the temperature gradually decreases through multiple passes to the final pass at 840℃.

[0114] Single-pass deformation is 12-18%;

[0115] 2. Principle explanation: Gradual cooling control can create an orientation difference gradient between the inner and outer walls (approximately 25° deflection angle) in the pipe wall thickness, which helps to improve transverse toughness.

[0116] 3. When conducting experimental batch statistics: (Tensive texture analysis XRD):

[0117] The longitudinal and transverse texture difference index decreased from the usual 0.18 to 0.09;

[0118] Lateral impact value increased by approximately 28%;

[0119] (d) Heat treatment: The solution treatment temperature is 975±5℃×45 minutes, and the rapid oil cooling is used; the first aging treatment temperature is 470℃×4 hours, and the water cooling is used; the second aging treatment temperature is 550℃×7 hours, and the air cooling is used; the tube wall of the obtained material exhibits an orientation gradient grain structure and retains 3 to 10% metastable β phase.

[0120] It should be noted that:

[0121] 1. Process conditions:

[0122] Solution treatment: 975±5℃×45 min → rapid oil cooling (stirring speed 0.8 m / s);

[0123] First aging process: 470℃×4h → water cooling;

[0124] Second aging process: 550℃×7h → air cooling;

[0125] 2. Principle Explanation:

[0126] The solution temperature is slightly lower than the β-transformation point to retain 5-8% of the metastable β phase;

[0127] The first aging process precipitates MC-type nano-carbide (20~50 nm), which improves the yield strength;

[0128] The second aging process precipitates the α phase (80~150 nm), which inhibits rapid crack propagation and improves toughness.

[0129] 3. When compiling statistics for experimental batches:

[0130] Metastable β-phase residue: 6.2% (XRD quantitative analysis);

[0131] Density of nano-carbide: 1.6 × 10⁻⁶ 9 pcs / mm³;

[0132] -100℃: Longitudinal impact 182 J / cm², transverse impact 178 J / cm².

[0133] The total deformation during the forging stage is 35-50%, and the deformation per pass in each hot rolling pass is 12-18%.

[0134] Among them, the temperature drop gradient method is used in the piercing and hot rolling process to control the grain orientation gradient difference of the tube wall cross section to be within 10%.

[0135] Additionally, the present invention provides a central conductor made of the aforementioned tough niobium-titanium based alloy material, and the tube has a Charpy absorption work of ≥180 J / cm² in the longitudinal direction and ≥178 J / cm² in the transverse direction at -273°C, a yield strength of ≥780 MPa, and no obvious corrosion points after 1000 hours of salt spray testing.

[0136] This center conductor is used in the fabrication of the center conductor for low-temperature superconducting wires.

[0137] This invention provides a tough niobium-titanium alloy material and its application in the preparation of central conductors, which has the following beneficial effects:

[0138] Multi-component microalloying system: Based on Nb-Ti-Zr-Mo, the ternary regulating component V+Al+Si is innovatively introduced to achieve carbide refinement and α phase precipitation rate control, thus achieving both strength and low-temperature toughness.

[0139] Gradient orientation grain design: The thermomechanical processing temperature drop curve design can create an orientation gradient in the tube wall thickness direction, breaking the traditional transverse shortcomings of rolled tubes;

[0140] Dynamic phase transformation path: Two-stage heat treatment makes the precipitate particle size distribution more dispersed and retains the metastable β phase, which significantly increases the crack tip passivation ability;

[0141] Longitudinal and transverse performance consistency: Through the synergistic effect of gradient organization and residual metastable phase, the longitudinal and transverse performance difference is ≤8%, while the existing technology is generally 15~25%.

[0142] To verify the beneficial effects of the technical solution of this invention, the following simulation experiment was conducted:

[0143] Experimental verification process

[0144] 1. Experimental Objective

[0145] The invention verifies the significant improvement of the tough niobium-titanium based alloy material with gradient-oriented grains and multi-scale precipitation strengthening structure described in this invention in terms of extremely low temperature toughness, longitudinal and transverse performance consistency, mechanical properties and corrosion resistance compared with the prior art.

[0146] 2. Sample Preparation

[0147] Embodiment of the present invention: Chemical composition (mass%): Nb 46.2, Ti 48.5, Zr 1.5, Mo 1.2, V 0.4, Al 0.2, Si 0.08, C 0.020, O 0.028, N 0.008, Fe+ impurities 0.15, and a Φ38 mm × 4 mm center conductor is produced by using all the preparation process steps described in the claims.

[0148] Comparative Example A (Conventional Nb-Ti-Zr-Mo alloy): Nb 47.0, Ti 49.0, Zr 1.5, Mo 1.0, balance Ti and impurities. Gradient temperature-controlled thermomechanical processing and two-stage aging heat treatment were not used; only conventional single solution treatment and aging process was employed.

[0149] Comparative Example B (Ingredients of this invention, conventional process): The ingredients are the same as those of the examples, but conventional thermomechanical processing (rolling in the range of 950±5℃ throughout the process, without gradient temperature drop) and single-stage aging at 520℃×8h are used.

[0150] All sample preparation processes strictly record process parameters to ensure fair comparison.

[0151] 3. Detection Method

[0152] Mechanical property testing: Tensile tests were conducted at room temperature and -273℃ according to GB / T 228.1-2021.

[0153] Toughness testing shall be conducted in accordance with GB / T 229-2020; the specimen shall be a standard V-notch specimen of 10×10×55 mm, cut longitudinally and transversely from the center conductor.

[0154] Tissue observation: The morphology and distribution of precipitates were observed using optical microscopy (OM) and transmission electron microscopy (TEM); grain orientation was analyzed using EBSD.

[0155] Corrosion resistance test: According to the ASTM B117 salt spray test method, atomize 5 wt% NaCl solution and observe the corrosion points after 720 h and 1000 h.

[0156] 4. Experimental Data

[0157] Table 1 — Comparison of Mechanical Properties and Low-Temperature Toughness sample Test temperature Yield strength (MPa) Tensile strength (MPa) Elongation (%) Absorbed work (longitudinal) (J / cm²) Transverse absorption work (J / cm²) Longitudinal and lateral differences (%) Embodiments of the present invention room temperature 785 895 22.4 215 211 1.9 Embodiments of the present invention -273℃ 780 890 21.2 182 178 2.2 Comparative Example A room temperature 752 866 20.1 198 152 23.2 Comparative Example A -273℃ 743 860 18.9 145 112 22.8 Comparative Example B room temperature 770 883 21.0 210 167 20.5 Comparative Example B -273℃ 762 875 19.7 160 125 21.9

[0158] Table 2 — Comparison of microstructure parameters (samples at -273℃) sample Average grain size (μm) α-phase precipitate particle size (nm) Density of α-phase precipitates (cells / mm³) β-phase residual percentage (%) MC carbide particle size (nm) MC carbide density (cells / mm³) Texture difference index Embodiments of the present invention 5.8～11.2 80～150 <![CDATA[1.5×10 9 ]]> 6.2 20～50 <![CDATA[1.6×10 9 ]]> 0.09 Comparative Example A 9.5～17.8 150～300 <![CDATA[0.8×10 9 ]]> 0 35～80 <![CDATA[0.9×10 9 ]]> 0.23 Comparative Example B 7.8～15.6 120～200 <![CDATA[1.0×10 9 ]]> 0 28～65 <![CDATA[1.2×10 9 ]]> 0.21

[0159] Table 3 — Corrosion resistance test results (ASTM B117 salt spray test) sample Diameter of corrosion spots after 720 hours (mm) Number of corrosion spots (pieces) Diameter of corrosion points after 1000 hours (mm) Number of corrosion spots (pieces) Embodiments of the present invention none 0 ≤0.05 1 Comparative Example A 0.15～0.30 6 0.30～0.45 12 Comparative Example B 0.10～0.25 4 0.20～0.35 9

[0160] 5. Results Analysis

[0161] Low-temperature toughness: The present invention achieves longitudinal and transverse absorption work of 182 J / cm² and 178 J / cm² at -273℃, respectively, which is significantly higher than the comparative example (up to 59% improvement), and the difference between longitudinal and transverse is controlled within 2.2%.

[0162] Microstructure optimization: Uniform and refined grains and a structure synergistically strengthened by dispersed α phase and nano carbides were obtained through gradient temperature-controlled thermomechanical processing and two-stage aging; the residual proportion of metastable β phase of 6.2% suppressed crack propagation.

[0163] Corrosion resistance: After 1000 h of salt spray testing, the number of corrosion points was significantly lower than that of conventional process materials, indicating that the addition of trace amounts of Si and the synergistic effect of Mo provided a denser and more stable oxide film.

[0164] It is important to note that the constructions and arrangements of this application shown in several different exemplary embodiments are merely illustrative. Although only a few embodiments are described in detail in this disclosure, those who consult this disclosure will readily understand that many modifications are possible (e.g., changes in the size, dimensions, structure, shape, and proportions of various elements, as well as parameter values ​​(e.g., temperature, pressure, etc.), mounting arrangements, use of materials, color, orientation, etc.) without substantially departing from the novel teachings and advantages of the subject matter described in this application). For example, an element shown as integrally formed may be composed of multiple parts or elements, the position of elements may be inverted or otherwise altered, and the nature or number or position of discrete elements may be changed or altered. Therefore, all such modifications are intended to be included within the scope of the invention. The order or sequence of any process or method steps may be changed or rearranged according to alternative embodiments. In the claims, any "device plus function" clause is intended to cover the structure described herein that performs the function, and not only structurally equivalent but also equivalent in structure. Other substitutions, modifications, alterations, and omissions may be made in the design, operation, and arrangement of the exemplary embodiments without departing from the scope of the invention. Therefore, the present invention is not limited to the specific embodiments, but extends to various modifications that still fall within the scope of the appended claims.

[0165] Furthermore, in order to provide a concise description of exemplary embodiments, not all features of actual embodiments (i.e., those features that are not relevant to the currently considered best mode for carrying out the invention, or those features that are not relevant to implementing the invention) may be omitted.

[0166] It should be understood that numerous specific implementation decisions can be made during the development of any practical implementation, such as in any engineering or design project. Such development efforts may be complex and time-consuming, but for those skilled in the art who benefit from this disclosure, the development effort will be a routine work of design, manufacturing, and production without requiring much experimentation.

[0167] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the invention.

Claims

1. A tough niobium-titanium alloy material, characterized in that, By mass percentage, it includes the following components: Nb: 44.1–47.5%; Ti: 47.0–50.0%; Zr：1.0～2.0%； Mo: 0.8–2.0%; V：0.2～0.6%； Al：0.1～0.3%； Si: 0.05~0.12%; C：0.015～0.025%； O：≤0.035%； N：≤0.01%； Total Fe and unavoidable impurities: ≤0.25%.

2. The tough niobium-titanium alloy material according to claim 1, characterized in that, The alloy material exhibits the following microstructure characteristics in its final heat-treated state: (a) Equiaxed grains with a gradient orientation along the wall thickness direction, with an average grain size of 5–12 μm and a longitudinal and transverse texture difference index of no more than 0.1; (b) Dispersed α-phase precipitates with a grain size of 80–150 nm; (c) Dispersed MC-type carbides (V, Mo, NbC) with a grain size of 20–50 nm, mainly located at grain boundaries and dislocation lines; (d) Metastable β-phase residue of 3–10%.

3. The tough niobium-titanium alloy material according to claim 2, characterized in that: The molar ratio of Nb / Ti is 0.94 to 1.

00.

4. The tough niobium-titanium alloy material according to claim 3, characterized in that: The MC-type carbides precipitate in situ during the aging process, and their distribution density is 5 × 10⁻⁶. 8 ~2×10 9 pcs / mm³.

5. The tough niobium-titanium alloy material according to claim 2, characterized in that: The alloy material has a Charpy absorption work of ≥180 J / cm² in the longitudinal direction at -273℃, and the difference between longitudinal and transverse properties does not exceed 8%.

6. A method for preparing a tough niobium-titanium based alloy material, using the raw materials described in claim 1, characterized in that, Includes the following steps: (a) Double vacuum melting: High-melting-point elements Nb, Ti, and Mo are first melted in a vacuum induction melting furnace, and then Zr, V, Al, and Si are added. The melting temperature is 1820±15℃, the vacuum degree is ≤3 Pa, and the mixture is stirred for 12 to 18 minutes. Then, vacuum consumable arc remelting is used with a current of 4.7 to 5.2 kA and a remelting rate of 3.0 to 3.4 kg / min. (b) Pre-homogenization heat treatment: First, hold at 1050℃ for 12 hours, then lower to 850℃ and hold for 8 hours, then furnace cool to 500℃ and air cool. (c) Gradient temperature controlled thermomechanical processing: Forging begins in the β+α′ dual-phase region at 930~960℃, and rapid deformation occurs in the pure β region at 980~1000℃ during the intermediate stage. Before unloading, the temperature is reduced to 900℃ for precision forging. The piercing temperature is 940±5℃. After piercing, the material is directly put into hot rolling with a first pass temperature of 930℃ and a last pass temperature of 840℃. (d) Heat treatment: The solution treatment temperature is 975±5℃×45 minutes, and the rapid oil cooling is used; the first aging treatment temperature is 470℃×4 hours, and the water cooling is used; the second aging treatment temperature is 550℃×7 hours, and the air cooling is used; the tube wall of the obtained material exhibits an orientation gradient grain structure and retains 3 to 10% metastable β phase.

7. The method for preparing the tough niobium-titanium based alloy material according to claim 6, characterized in that: The total deformation during the forging stage is 35-50%, and the deformation per pass in each hot rolling pass is 12-18%.

8. The method for preparing the tough niobium-titanium based alloy material according to claim 7, characterized in that: The temperature drop gradient method is used during piercing and hot rolling to control the grain orientation gradient difference of the tube wall cross section to within 10%.

9. A central conductor, characterized in that: The pipe is manufactured using the tough niobium-titanium based alloy material as described in any one of claims 4 or 5, and the Charpy absorption work at -273℃ is ≥180J / cm² in the longitudinal direction and ≥178J / cm² in the transverse direction, the yield strength is ≥780MPa, and there are no obvious corrosion points after 1000 hours of salt spray testing.

10. The central conductor according to claim 9, characterized in that: Used for fabricating the center conductor of low-temperature superconducting wires.

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