A method for preparing a centimeter-sized single crystal niobium

Abstract

The present application relates to the technical field of preparation of large-size bulk single-crystal metal materials, and provides a preparation method of single-crystal niobium with a centimeter level. The present application proposes a method for preparing single-crystal niobium with a centimeter level based on low strain + two-stage annealing. The method drives abnormal growth of dominant grains (the size can reach a centimeter level) by cold rolling deformation of niobium ingot and two-stage annealing, and finally obtains single-crystal niobium with a centimeter level through processing and cutting. The method provided by the present application breaks through the bottleneck that it is difficult to obtain centimeter-level grains by conventional recrystallization annealing process, and realizes stable preparation of single-crystal niobium with a centimeter level. In addition, compared with the electron beam floating zone melting method commonly used in the industry for preparing single-crystal niobium, the present application significantly reduces the equipment investment and production cost, has higher process flexibility and preparation efficiency, and has a wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of large-size bulk single-crystal metal material preparation technology, and in particular to a method for preparing centimeter-scale single-crystal niobium. Background Technology

[0002] Niobium (Nb) possesses a high superconducting critical temperature (9.2 K) and excellent radio frequency (RF) properties, making it a core material for fabricating superconducting RF accelerator cavities. It is widely used in cutting-edge technologies such as low-temperature superconductivity, particle accelerators, free-electron laser devices, and quantum computing. In these applications, grain boundaries significantly reduce the operational stability and superconducting performance of niobium. Therefore, practical applications typically require niobium materials to have the largest possible grain size, with single-crystal niobium being the optimal morphology. Thus, the fabrication of large-size, highly crystal-intact single-crystal niobium is crucial for supporting the research and development and performance improvement of related high-end equipment.

[0003] Recrystallization annealing is a common method for adjusting the grain morphology and size of metallic materials. It has advantages such as conventional equipment, simple process, and scalability, and is widely used in the microstructure control and performance optimization of metallic materials. However, when traditional recrystallization annealing is used for grain control of niobium, it is difficult to break through the centimeter-level grain size limitation, and it is impossible to directly produce single-crystal niobium that meets the requirements of high-end applications through subsequent processing.

[0004] Currently, the mainstream technology for preparing high-purity bulk single-crystal niobium is electron beam levitation zone melting, which involves locally melting polycrystalline niobium rods with an electron beam and using the surface tension of the melt to support and move the molten zone to achieve single-crystal growth. However, electron beam melting equipment is expensive, the process is complex, and the production efficiency is low, resulting in high prices for single-crystal niobium products, making it difficult to meet the needs of large-scale applications. Summary of the Invention

[0005] In view of this, the present invention provides a method for preparing centimeter-scale single-crystal niobium. The preparation method provided by the present invention is simple to operate, low in cost, and high in production efficiency, achieving stable preparation of centimeter-scale single-crystal niobium.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A method for preparing centimeter-scale single-crystal niobium includes the following steps: The cast niobium ingot is subjected to cold rolling and pretreatment in sequence to obtain niobium billet; the cumulative deformation of the cold rolling is 5~10%; The niobium billet is subjected to a two-stage annealing process to obtain centimeter-sized single-crystal niobium, or to obtain polycrystalline niobium with a grain size of centimeters. When obtaining polycrystalline niobium with a grain size of centimeters, the process further includes cutting individual grains in the polycrystalline niobium to obtain the centimeter-sized single-crystal niobium. The two-stage annealing process includes a first-stage annealing and a second-stage annealing, wherein the temperature of the first-stage annealing is 750~900℃ and the temperature of the second-stage annealing is 1200~1500℃.

[0007] Preferably, the reduction per cold rolling pass is ≤0.2mm.

[0008] Preferably, the pretreatment includes: cutting the rolled niobium obtained after cold rolling to obtain niobium blocks; and grinding the niobium blocks to obtain the niobium billet.

[0009] Preferably, the niobium block has a width of 10-30 mm and a length of 10-30 mm.

[0010] Preferably, the holding time for the first stage of annealing is 1 to 2 hours.

[0011] Preferably, the heating rate from room temperature to the temperature of the first stage annealing is 5~10℃ / min.

[0012] Preferably, the holding time for the second stage of annealing is 8 to 12 hours.

[0013] Preferably, the heating rate from the temperature of the first stage annealing to the temperature of the second stage annealing is 3~5℃ / min.

[0014] Preferably, the second stage of annealing further includes a cooling stage, which is: first cooling to 200-300°C at a rate of 3-5°C / min, and then taking out the sample for air cooling.

[0015] Preferably, the number of grains in the polycrystalline niobium is 3 to 5.

[0016] This invention provides a method for preparing centimeter-scale single-crystal niobium, comprising the following steps: sequentially cold-rolling and pre-treating a cast niobium ingot to obtain a niobium billet; the cumulative deformation of the cold rolling is 5-10%; subjecting the niobium billet to a two-stage annealing process to obtain centimeter-scale single-crystal niobium, or to obtain polycrystalline niobium with a grain size of centimeters; when obtaining polycrystalline niobium with a grain size of centimeters, the method further includes cutting individual grains in the polycrystalline niobium to obtain the centimeter-scale single-crystal niobium; the two-stage annealing process includes a first-stage annealing and a second-stage annealing, wherein the temperature of the first-stage annealing is 750-900℃, and the temperature of the second-stage annealing is 1200-1500℃. This invention proposes a method for preparing centimeter-scale single-crystal niobium based on low strain + two-stage annealing, by controlling the deformation of the cast niobium ingot and combining it with a two-stage annealing process, driving abnormal growth of individual grains. The first-stage annealing is carried out under relatively low temperature conditions, causing primary recrystallization of the material, transforming the deformed grains back into uniform and fine equiaxed grains.

[0017] The second-stage annealing is carried out at significantly higher temperatures, triggering abnormal grain growth driven by excess surface energy at the grain boundaries. During this process, a few grains with growth advantage continuously engulf surrounding smaller grains, achieving abnormal grain growth and ultimately resulting in grain sizes on the centimeter scale.

[0018] In summary, this invention drives abnormal grain growth within the material by controlling strain and annealing methods, achieving stable preparation of centimeter-scale single-crystal niobium and overcoming the technical problem of limited grain size obtained by traditional recrystallization annealing. Furthermore, compared with the electron beam levitation zone melting method commonly used in the industry for preparing single-crystal niobium, this invention significantly reduces equipment investment and production costs, possesses higher process flexibility and preparation efficiency, and has broad application prospects. Attached Figure Description

[0019] Figure 1 Metallographic photographs of the samples obtained in Examples 1-3 and Comparative Example 1, wherein (a) Example 1, (b) Example 2, (c) Example 3, and (d) Comparative Example 1; Figure 2 The inverse pole figure results of backscattered electron diffraction (EBSD) of the samples obtained in Examples 1-3 and Comparative Example 1 are shown, where (a) Example 1, (b) Example 2, (c) Example 3, and (d) Comparative Example 1. Detailed Implementation

[0020] This invention provides a method for preparing centimeter-scale single-crystal niobium, comprising the following steps: The cast niobium ingot is subjected to cold rolling and pretreatment in sequence to obtain niobium billet; the cumulative deformation of the cold rolling is 5~10%; The niobium billet is subjected to a two-stage annealing process to obtain centimeter-sized single-crystal niobium, or to obtain polycrystalline niobium with a grain size of centimeters. When obtaining polycrystalline niobium with a grain size of centimeters, the process further includes cutting individual grains in the polycrystalline niobium to obtain the centimeter-sized single-crystal niobium. The two-stage annealing process includes a first-stage annealing and a second-stage annealing, wherein the temperature of the first-stage annealing is 750~900℃ and the temperature of the second-stage annealing is 1200~1500℃.

[0021] This invention involves sequentially cold rolling and pretreatment of a cast niobium ingot to obtain a niobium billet. In this invention, the purity of the cast niobium ingot is preferably above 99.99%; the dimensions of the cast niobium ingot are preferably 20 mm × 40 mm × 7.5 mm; the cumulative deformation of the cold rolling is preferably 5-10%, specifically 5%, 6%, 8%, or 10%; the reduction per pass in the cold rolling is preferably ≤0.2 mm; the cold rolling is preferably performed using a twin-roll mill. This invention introduces appropriate strain energy through multi-pass, small-deformation rolling, avoiding excessive deformation that could lead to excessively high strain energy, thereby affecting subsequent recrystallization and abnormal grain growth.

[0022] In this invention, the pretreatment preferably includes: cutting the rolled niobium obtained after cold rolling to obtain niobium blocks; grinding the niobium blocks to obtain the niobium billet; the niobium blocks are rectangular niobium blocks; the width of the niobium blocks is preferably 10~30mm, and the length is preferably 10~30mm; the grinding is preferably done with sandpaper, specifically #180, #500, and #800 grit sandpaper can be used in sequence; this invention removes the oxide scale and defect layer on the surface of the niobium blocks by grinding; in a specific embodiment of this invention, the grinding process preferably further includes cleaning the niobium blocks; the cleaning includes sequentially performing deionized water cleaning and ethanol ultrasonic cleaning; the ethanol ultrasonic cleaning time is preferably 3~5 minutes; this invention ensures the cleanliness of the specimen surface through cleaning.

[0023] After obtaining the niobium billet, the present invention performs a two-stage annealing process on the niobium billet to obtain centimeter-sized single-crystal niobium, or to obtain polycrystalline niobium with a grain size of centimeters. When obtaining polycrystalline niobium with a grain size of centimeters, the process further includes cutting individual grains in the polycrystalline niobium to obtain the centimeter-sized single-crystal niobium. In the present invention, the two-stage annealing process includes a first-stage annealing and a second-stage annealing. The temperature of the first-stage annealing is 750~900℃, specifically 800 or 850℃, and the holding time is preferably 1~2h. The heating rate from room temperature to the temperature of the first-stage annealing is preferably 5~10℃ / min. The present invention enables the material to undergo sufficient primary recrystallization through the first-stage annealing, thereby transforming the non-uniform structure formed after deformation into a recrystallized matrix with fine and uniform grains.

[0024] In this invention, the temperature of the second-stage annealing is 1200~1500℃, specifically 1250, 1300, or 1400℃, and the holding time is 8~12h, specifically 9, 10, or 11h. The heating rate from the first-stage annealing temperature to the second-stage annealing temperature is preferably 3~5℃ / min, specifically 4℃ / min. During the second-stage annealing process, grains with growth advantages migrate rapidly through grain boundaries, engulfing surrounding fine grains to achieve abnormal growth, thereby enabling grain sizes to reach the centimeter level. Traditional recrystallization annealing methods typically place the deformed niobium material directly at high temperatures for a single long-term holding, resulting in low grain growth efficiency, limited size, and poor process reliability. This invention improves grain growth efficiency and process stability through a staged annealing method, effectively shortens the annealing time, and achieves stable preparation of centimeter-sized single-crystal niobium.

[0025] In this invention, the second stage of annealing preferably includes a cooling stage, which is preferably: first cooling to 200-300°C at a rate of 3-5°C / min, and then taking out the sample for air cooling.

[0026] In this invention, the two-stage annealing process is preferably carried out in a tube furnace; the two-stage annealing process is preferably carried out under a protective atmosphere, which is preferably argon; the argon gas inlet rate is preferably 100 sccm, and the gas pressure in the tube furnace is preferably <0.02 MPa.

[0027] In this invention, centimeter-sized single-crystal niobium or centimeter-sized polycrystalline niobium can be directly obtained after two-stage annealing. When centimeter-sized single-crystal niobium is obtained, it can be directly subjected to crystallographic orientation and wire cutting to obtain single-crystal niobium with the required orientation and specific shape to meet the usage requirements. When centimeter-sized polycrystalline niobium is obtained, the process also includes cutting inside a single grain of the polycrystalline niobium to obtain the centimeter-sized single-crystal niobium.

[0028] In this invention, the number of grains in the polycrystalline niobium is 3 to 5. In a specific embodiment of this invention, abnormally grown grains can be identified by metallographic observation, and then cut within them; the metallographic observation can specifically be optical microscopy or scanning electron microscopy based on backscattered electron diffraction (EBSD); in a specific embodiment of this invention, if the diameter of at least one grain in the polycrystalline niobium reaches 1 cm or more, it is considered to meet the process requirements.

[0029] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0030] Example 1 (1) Select commercially available cast niobium ingots (purity 99.99%) with dimensions of 20 mm × 40 mm × 7.5 mm. Use a twin-roll mill for room temperature cold rolling, strictly control the reduction of each pass to ≤0.2 mm, and roll the thickness from 7.5 mm to 7.125 mm, with a corresponding cumulative deformation of 5%. Cut the resulting rolled polycrystalline niobium into rectangular blocks of 10 mm × 10 mm, and polish them sequentially with #180, #500, and #800 grit sandpaper, and finally clean them with alcohol using ultrasonic cleaning.

[0031] (2) The niobium billet was annealed using a tube furnace. The cleaned sample was placed in the tube furnace and argon gas was introduced. The gas inlet rate was controlled at 100 sccm and the furnace pressure was <0.02 MPa. First, the temperature was raised to 850℃ at a rate of 10℃ / min and held for 1 h. Then, the temperature was raised to 1250℃ and held for 9 h. After that, the temperature was lowered to 300℃ at a rate of 4℃ / min. The sample was then taken out and air-cooled.

[0032] The obtained samples were subjected to metallographic etching. Figure 1 In Figure (a), the metallographic structure of the sample obtained in Example 1 is shown. The results show that the sample surface is smooth and there are no visible grain boundaries. It exhibits a highly consistent metallic luster and is preliminarily determined to be a single grain. Figure 2 In Figure (a), the inverse pole figure of the sample obtained in Example 1 was obtained by electron backscatter diffraction (EBSD) characterization. Different colors represent grains with different crystal orientations, and the interfaces between different colors represent grain boundaries between grains. Figure 2 The characterization results in (a) show that the entire sample is the same color, which indicates that the original polycrystalline grain boundaries of the sample have completely disappeared and the entire sample has been transformed into a single complete grain with high crystal integrity.

[0033] Example 2 The other conditions are the same as in Example 1, except that the cumulative deformation of cold rolling in step (1) is 10%, and the material cutting shape size is 15 mm × 10 mm.

[0034] Figure 1 (b) is a metallographic photograph of the sample obtained in Example 2. Figure 2In the middle (b), the inverse pole figure of the sample obtained in Example 2 is characterized by EBSD. It can be seen that the sample contains 3 grains, of which the red area constitutes a large grain with a significantly dominant size, covering most of the sample area; the blue area and the orange area in the upper left corner are two relatively smaller independent grains. Figure 1 and Figure 2 The results in (b) show that the sample obtained in this embodiment has formed large grains at the centimeter scale.

[0035] Example 3 The other conditions are the same as in Example 1, except that the material is cut into a shape of 20 mm × 20 mm.

[0036] Figure 1 (c) is a metallographic photograph of the sample obtained in Example 3. Obvious grain boundaries can be observed on the sample surface. At this time, there is one very large grain and four smaller grains. Figure 2 (c) is the inverse pole figure of the sample obtained in Example 3 characterized by EBSD. Different colors in the figure represent different crystal orientations. The green area corresponds to a very large grain with a size of centimeters, while the red, blue, pink and orange areas correspond to the other four smaller grains. Figure 1 and Figure 2 The results in (c) show that the sample obtained in this embodiment has formed a centimeter-scale large grain structure consisting of 5 grains, among which the green extra-large grains can be directly used for subsequent single crystal cutting and extraction.

[0037] Comparative Example 1 The other conditions are the same as in Example 1, except that the cumulative deformation of cold rolling in step (1) is 25%.

[0038] Figure 1 The middle (d) is a metallographic photograph of the sample obtained in Comparative Example 1. It can be observed that after etching, grains with different brightness and darkness can be observed on the sample surface, showing obvious polycrystalline characteristics. Figure 2 In the middle (d), the inverse pole figure of the sample obtained in Comparative Example 1 is characterized by EBSD. Different colors in the figure represent different crystal orientations, and each continuous area of ​​the same color corresponds to an independent grain. It can be observed that there are some slightly larger grains in the figure, but a large number of small grains of different colors are still distributed around them. Figure 1 and Figure 2 The results in (d) show that although the sample obtained in Comparative Example 1 also showed abnormally large grains, a large number of small grains that were not absorbed remained, exhibiting a mixed crystal structure of "large grains + small grains", and failing to achieve the goal of preparing centimeter-sized grains.

[0039] In summary, this invention proposes a method for preparing centimeter-scale single-crystal niobium based on recrystallization annealing. Commercially available niobium ingots are subjected to low-strain, two-stage annealing to drive anomalous grain growth, resulting in individual grains reaching centimeter-scale sizes. Subsequent crystallographic orientation and wire cutting of these anomalous grains yield single-crystal niobium with the desired orientation and specific shape. This invention utilizes only conventional rolling and heat treatment equipment, eliminating the need for complex and expensive single-crystal growth devices such as electron beam levitation melting, significantly reducing preparation costs and process barriers. The entire process is simple and highly reproducible, providing a reliable and economical technical path for the large-scale preparation and engineering application of single-crystal niobium in cutting-edge fields such as superconducting radio frequency cavities and quantum computing devices.

[0040] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing centimeter-scale single-crystal niobium, characterized in that, Includes the following steps: The cast niobium ingot is subjected to cold rolling and pretreatment in sequence to obtain niobium billet; the cumulative deformation of the cold rolling is 5~10%; The niobium billet is subjected to a two-stage annealing process to obtain centimeter-sized single-crystal niobium, or to obtain polycrystalline niobium with a grain size of centimeters. When obtaining polycrystalline niobium with a grain size of centimeters, the process further includes cutting individual grains in the polycrystalline niobium to obtain the centimeter-sized single-crystal niobium. The two-stage annealing process includes a first-stage annealing and a second-stage annealing, wherein the temperature of the first-stage annealing is 750~900℃ and the temperature of the second-stage annealing is 1200~1500℃.

2. The preparation method according to claim 1, characterized in that, The reduction per cold rolling pass is ≤0.2mm.

3. The preparation method according to claim 1, characterized in that, The pretreatment includes: cutting the rolled niobium obtained after cold rolling to obtain niobium blocks; and grinding the niobium blocks to obtain the niobium billet.

4. The preparation method according to claim 3, characterized in that, The niobium block has a width of 10-30 mm and a length of 10-30 mm.

5. The preparation method according to claim 1, characterized in that, The holding time for the first stage of annealing is 1~2 hours.

6. The preparation method according to claim 1, characterized in that, The heating rate from room temperature to the temperature of the first stage annealing is 5~10℃ / min.

7. The preparation method according to claim 1, characterized in that, The holding time for the second stage of annealing is 8~12 hours.

8. The preparation method according to claim 1, characterized in that, The heating rate from the temperature of the first stage annealing to the temperature of the second stage annealing is 3~5℃ / min.

9. The preparation method according to claim 1, characterized in that, The second stage of annealing also includes a cooling stage, which is: first, cooling to 200-300°C at a rate of 3-5°C / min, and then taking out the sample for air cooling.

10. The preparation method according to claim 1, characterized in that, The polycrystalline niobium contains 3 to 5 grains.

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