A high-hardness single crystal niobium and its preparation method

Abstract

The present invention relates to the technical field of plastic deformation preparation of metal materials, and provides a high-hardness single-crystal niobium and a preparation method thereof, wherein the martensite content in the metallographic structure thereof is 61-73%. The preparation process includes annealing, pretreatment, and deformation impact steps of the single-crystal niobium. By precisely controlling the annealing parameters and the impact strain rate, the single-crystal niobium undergoes a structural phase transformation to introduce a high-hardness martensite structure, while effectively avoiding impact cracks, and ultimately obtaining crack-free high-hardness single-crystal niobium. The method provided by the present invention has a simple process flow, high efficiency, low cost, and is green, low-carbon, and pollution-free. It can significantly improve the hardness of single-crystal niobium and is applicable to single-crystal raw material niobium with any crystal orientation. The results of the embodiment show that for single-crystal niobium with crystal orientations [100], [110], and [111], the hardness is increased by 24%, 52%, and 42%, respectively, after impact deformation by the method of the present invention.

Description

Technical Field

[0001] The present invention relates to the technical field of plastic deformation preparation of metal materials, and in particular to high-hardness single crystal niobium and a preparation method thereof. Background Art

[0002] Niobium (Nb) metal has excellent properties such as high plasticity, superconductivity and magnetic anisotropy, and is widely used in low-temperature superconductivity, deep-sea polar regions, nuclear magnetic resonance and other fields. In particular, with the rapid development of superconducting high-frequency technology in recent decades, major accelerator devices around the world have generally used superconducting high-frequency cavities made of pure niobium to accelerate various charged particles. Compared with traditional polycrystalline niobium, single-crystalline niobium does not contain grain boundaries (interface defects usually increase the scattering effect on electrons), and its electrical and magnetic functional properties are more excellent. However, as a functional-structural integrated material, the application requirements for single-crystalline niobium are not only limited to functional properties, but also place special emphasis on its mechanical properties such as hardness. Because single-crystalline niobium is relatively soft in texture, it is easy to deform under the action of external loads, which greatly affects its structure and service safety.

[0003] At present, the main strategies for improving the hardness of metals include solid solution / precipitation strengthening, surface strengthening, and phase transformation strengthening. The hardness of the material itself can be improved by adding alloying elements (such as C, Si, Mn, etc.) through solid solution strengthening or precipitation strengthening. However, the improvement range of this method is relatively limited and usually seriously affects the functional properties of the material. Surface strengthening is a gas carburizing / nitriding process under high temperature or vacuum environment. Due to the harsh reaction conditions, it is more suitable for small-scale applications. Phase transformation strengthening usually involves heat treatment of metal materials (such as quenching) to induce a structural phase transformation (such as martensitic transformation) to achieve strengthening. The operation is relatively simple, but it cannot be directly applied to pure metal materials with stable structures.

[0004] In view of this, there is an urgent need to provide a new method that is simple to operate and can effectively improve the hardness of single crystal niobium. Summary of the Invention

[0005] To address the aforementioned issues with single-crystal niobium, the present invention aims to provide a high-hardness single-crystal niobium that is simple to prepare and remains intact and crack-free. This single-crystal niobium exhibits a high martensite content (61-73%), significantly improving its hardness and possessing promising industrial application prospects.

[0006] The specific technical solutions provided by the present invention are as follows:

[0007] A method for preparing high-hardness single-crystal niobium comprises the following steps:

[0008] (1) Annealing single crystal niobium;

[0009] (2) pre-treating the annealed single crystal niobium;

[0010] (3) impact loading is performed on the pretreated single crystal niobium to obtain the high hardness single crystal niobium; the metallographic structure of the high hardness single crystal niobium includes martensite in the shape of a double convex lens, and the martensite is along <110> The crystal orientation groups are uniformly distributed in single crystal niobium; the volume fraction of martensite is 61 to 73%.

[0011] Preferably, in step (1), the annealing conditions are: keeping warm at 600-1200°C for 6-8h; further preferably, the annealing temperature is 800-1000°C to eliminate defect structures such as dislocations inside the material, which is conducive to the subsequent martensitic phase transformation.

[0012] Preferably, in step (2), the pretreatment method is to grind, polish and ultrasonically clean the annealed single crystal niobium in sequence; the grinding is preferably: coarse grinding and fine grinding are performed successively using metallographic sandpaper with grit sizes of #180, #500 and #800; the polishing is preferably: polishing the sample to a mirror surface level using a diamond polishing spray with a grit size of W2; the cleaning agent used in the ultrasonic cleaning is preferably alcohol, the ultrasonic power is preferably 200 W, and the ultrasonic time is preferably 5 minutes; the above parameters are to ensure that oil stains on the surface of the single crystal niobium are removed and uneven deformation of the sample is avoided.

[0013] Preferably, in step (3), the strain rate of the impact is 1×10 3 ~6×10 3 s -1 , the temperature is room temperature.

[0014] Preferably, in step (3), the device used for the impact is a Hopkinson pressure bar device, which has good impact stress uniformity and a wide range of measurable strain rates, so as to effectively impact deform single crystal niobium at the required impact rate.

[0015] Further preferably, the end face size of the single crystal niobium is smaller than the end face sizes of the incident rod and the transmission rod in the Hopkinson pressure bar device.

[0016] Preferably, the crystal orientation of the single crystal niobium is

[100] ,

[110] or

[111] ; and the purity of the single crystal niobium is not less than 99.9%.

[0017] Preferably, the surface roughness of the single crystal niobium is less than 0.03 μm; and the roughness of both end surfaces of the single crystal niobium is less than 0.02.

[0018] Preferably, the C content in the single crystal niobium is 25-88 ppm, the H content is less than 1 ppm, the O content is 15-230 ppm, the N content is less than 5 ppm, and the Ag content is less than 0.9 ppm.

[0019] More preferably, the single crystal niobium before impact has a body-centered cubic (BCC) structure. The present invention has no special requirements on the source of the single crystal niobium, and commercially available single crystal niobium can be used.

[0020] The beneficial effects of the present invention are:

[0021] (1) The high-hardness single-crystal niobium involved in the present invention is obtained by subjecting commercially available single-crystal niobium to an impact deformation process to induce a martensitic phase transformation. By precisely controlling the annealing parameters and the impact strain rate, a significant increase in the material hardness is achieved, ultimately resulting in a complete, crack-free, high-hardness single-crystal niobium.

[0022] (2) The impact deformation process is used to induce martensitic phase transformation. This process is simple, efficient, low-cost, green, low-carbon and pollution-free.

[0023] (3) This method is applicable to single crystal raw materials of niobium with any crystal orientation, and is expected to be extended to niobium-based single crystals and polycrystalline alloys, such as niobium-titanium alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The hardness indentation profiles of single crystal niobium with

[100] crystal orientation at five different positions (corresponding to different indentation depths) before and after impact in Example 1 are shown, with the top one being before impact and the bottom one being after impact.

[0025] Figure 2 The hardness indentation diagrams of single crystal niobium with

[110] crystal orientation at five different positions (corresponding to different indentation depths) before and after impact in Example 2, where the top one is before impact and the bottom one is after impact;

[0026] Figure 3 The hardness indentation diagrams of single crystal niobium with

[111] crystal orientation at five different positions (corresponding to different indentation depths) before and after impact in Example 3, where the top one is before impact and the bottom one is after impact;

[0027] Figure 4 These are SEM morphologies of martensite revealed by surface corrosion after impact deformation of single crystal niobium in three crystal orientations in Examples 1 to 3;

[0028] Figure 5 This is the SEM morphology of the single crystal niobium after impact deformation in Comparative Example 4;

[0029] Figure 6 These are the XRD characterization results of single crystal niobium in three crystal orientations before and after impact in Examples 1 to 3. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to specific examples and accompanying drawings. It should be understood that these examples are intended to illustrate the present invention only and are not intended to limit the scope of the present invention. The experimental methods in the following examples, where specific conditions are not specified, are generally carried out under conventional conditions or under conditions recommended by the manufacturer.

[0031] Example 1

[0032] The single crystal niobium used in this embodiment has a crystal orientation of

[100] and a crystal orientation deviation of <1°. It is disc-shaped with a surface roughness of <0.03 μm, a diameter of 10 mm, and a thickness of 4 mm. The roughness of the two end surfaces of the sample is less than 0.02, and the purity is greater than 99%. The impurity element contents are as follows: C 25 ppm, H <1 ppm, O 15 ppm, N <5 ppm, and Ag <0.9 ppm.

[0033] The single crystal niobium sample was annealed at 1000℃ for 6 hours, and then ground, polished and ultrasonically cleaned for use. At room temperature, the single crystal niobium was subjected to an impact test using a Hopkinson pressure bar device, as follows: the rod system was adjusted with the axis of the launch tube as the reference, and the front and rear center brackets were adjusted using a height gauge, a level, a micrometer, etc. to make the rod end face fit tightly with the bullet end face. Stainless steel gaskets were added between the sample and the incident rod end face, as well as between the sample and the transmission rod end face, and a small amount of vaseline was added between the gasket and the rod. Using bottled high-pressure nitrogen and high-pressure air, the launch tube vent valve was unscrewed and the bullet was pushed into the appropriate position with a soft rod. The impact strain rate was controlled at 1×10 3 s -1 The impacted sample was separated from the stainless steel gasket, and the sample was removed, rinsed, and machined to obtain high-hardness single crystal niobium.

[0034] Example 2

[0035] The single crystal niobium used in this embodiment has a crystal orientation of

[110] and a crystal orientation deviation of <1°. It is disc-shaped with a surface roughness of <0.03 μm, a diameter of 10 mm, and a thickness of 4 mm. The roughness of the two end surfaces of the sample is less than 0.02, and the purity is greater than 99%. The impurity element contents are as follows: C 25 ppm, H <1 ppm, O 15 ppm, N <5 ppm, and Ag <0.9 ppm.

[0036] The operation method of the impact test is the same as that of Example 1.

[0037] Example 3

[0038] The single crystal niobium used in this embodiment has a crystal orientation of

[111] and a crystal orientation deviation of <1°; it is disc-shaped with a surface roughness of <0.03 μm, a diameter of 10 mm, a thickness of 4 mm, an unevenness of less than 0.02 at both end faces of the sample, a purity of more than 99%, and the following impurity element contents: C 25 ppm, H <1 ppm, O 15 ppm, N <5 ppm, and Ag <0.9 ppm.

[0039] The operation method of the impact test is the same as that of Example 1.

[0040] Comparative Example 1

[0041] The single crystal niobium used in this embodiment has a crystal orientation of

[110] and is prepared according to the steps in Example 2, except that it is not annealed.

[0042] Comparative Example 2

[0043] The single crystal niobium used in this embodiment has a crystal orientation of

[110] and is prepared according to the steps in Example 2, with the only difference being that the annealing temperature is 400°C.

[0044] Comparative Example 3

[0045] The single crystal niobium used in this embodiment has a crystal orientation of

[110] and is prepared according to the steps in Example 2, except that the strain rate of the impact is 1×10 2 s -1 .

[0046] Comparative Example 4

[0047] The single crystal niobium used in this embodiment has a crystal orientation of

[110] and is prepared according to the steps in Example 2, except that the strain rate of the impact is 1×10 4 s -1 .

[0048] Test Case

[0049] 1. Hardness test

[0050] The hardness of single crystal niobium in Examples 1-3 and Comparative Examples 1-4 was tested before and after impact. Five different locations on each sample were tested, with penetration depths of 1.0 mm, 1.2 mm, 1.4 mm, 1.6 mm, and 1.8 mm, respectively. The results are shown in Tables 1-5.

[0051] Table 1 Hardness of

[100] single crystal niobium before and after impact in Example 1

[0052]

[0053]

[0054] Table 2 Hardness of

[110] single crystal niobium before and after impact in Example 2

[0055] Location Depth (mm) Hardness before impact (HV) Hardness after impact (HV) 1 1.00 89.4 132.4 2 1.20 88.1 135.5 3 1.40 86.1 146.3 4 1.60 87.1 126.4 5 1.80 88.6 126.7

[0056] Table 3 Hardness of

[111] single crystal niobium before and after impact in Example 3

[0057] Location Depth (mm) Hardness before impact (HV) Hardness after impact (HV) 1 1.00 77.2 108.2 2 1.20 76.5 107.5 3 1.40 78.6 112.1 4 1.60 74.6 103.8 5 1.80 76.2 111.1

[0058] According to Tables 1 to 3, it can be seen that the hardness of single crystal niobium with different orientations at different positions in Examples 1 to 3 is significantly improved after impact. Figures 1 to 3 It can be seen that the hardness indentation profile of single-crystal niobium that undergoes a martensitic transformation after impact is clear, and the profile morphology is essentially consistent at different depths, indicating that the martensite is uniformly distributed within the sample. Based on Examples 1-3, this method is universally applicable to single-crystal niobium raw materials of any crystal orientation.

[0059] Table 4 Comparison of the effect of different annealing conditions on the hardness improvement of

[110] single crystal niobium in Example 2 and Comparative Examples 1-2

[0060]

[0061] As shown in Table 4, controlling the annealing conditions within the scope of the present invention can effectively increase the hardness of single crystal niobium. In Comparative Examples 1 and 2, when no annealing is performed or a lower annealing temperature (400°C) is used, defects such as dislocations within the material are not fully eliminated. This results in competition with the martensitic transformation during the subsequent impact process, resulting in insufficient martensitic transformation and limited hardness improvement.

[0062] Table 5 Comparison of the effect of different impact rates on the hardness improvement of

[110] single crystal niobium in Example 2 and Comparative Examples 3-4

[0063]

[0064] According to Table 5, it can be seen that controlling the impact strain rate within the range of the present invention can effectively increase the hardness of single crystal niobium. In Comparative Example 3, a lower impact strain rate (1×10 2 s -1 ), which is insufficient to drive the martensitic transformation, resulting in a significant decrease in the hardness improvement rate compared to Example 2. In Comparative Example 4, at a higher impact strain rate (1×10 4 s -1 ), resulting in numerous microcracks within the sample, rendering it unusable. Comparative Examples 3 and 4 demonstrate that martensitic transformation is sensitive to impact strain rate. By controlling the strain rate within the range of the present invention, intact, crack-free, high-hardness single crystal niobium can be obtained.

[0065] 2. Characterization of martensite morphology

[0066] The SEM morphology of the martensite on the surface of the single crystal niobium with three crystal orientations after impact deformation in Examples 1 to 3 is as follows: Figure 4 As shown. Figure 4 It can be seen that a large amount of martensite appears in single crystal niobium with

[100] ,

[110] and

[111] directions after impact.

[0067] The SEM image of the single crystal niobium after impact deformation in Comparative Example 4 is as follows: Figure 5 As shown. Figure 5 It can be seen that cracks appear on the surface of single crystal niobium after impact deformation. This is because a higher strain rate (1×10 4 s -1 ) impact, inducing a large amount of martensite to form rapidly and collide with each other, resulting in a large number of impact cracks in the sample, making it unusable for further use.

[0068] 3. XRD characterization

[0069] The XRD test results of the three single crystal niobium with different crystal orientations before and after impact in Examples 1 to 3 are as follows: Figure 6 As shown. Figure 6 It can be seen that strong martensitic diffraction peaks appear in single crystal niobium with

[100] ,

[110] and

[111] directions after impact, indicating that a large amount of martensitic structure is produced during the impact process.

[0070] The above description is only a preferred embodiment of the present invention. It should be pointed out that several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing high-hardness single-crystal niobium, characterized in that: The following steps are involved: (1) annealing the single crystal niobium; the annealing conditions are: 600-1200° C., and a holding time of 6-8 hours; (2) pre-treating the annealed single crystal niobium; (3) impact loading is performed on the pretreated single crystal niobium to obtain the high hardness single crystal niobium; the metallographic structure of the high hardness single crystal niobium includes martensite in the shape of a double convex lens, and the martensite is along <110> The crystal orientation groups are uniformly distributed in the single crystal niobium; the volume fraction of martensite is 61 to 73%; the strain rate of the impact is 1×10 3 ~6×10 3 s -1 .

2. The preparation method according to claim 1, characterized in that In step (2), the pretreatment steps are: grinding, ultrasonic cleaning and polishing.

3. The preparation method according to claim 1, characterized in that In step (3), the impact temperature is room temperature.

4. The preparation method according to claim 1, characterized in that In step (3), the device used for the impact is a Hopkinson pressure bar device.

5. The preparation method according to claim 4, characterized in that The end face size of the single crystal niobium is smaller than the end face sizes of the incident rod and the transmission rod in the Hopkinson pressure rod device.

6. The preparation method according to claim 1, characterized in that The C content of the single crystal niobium is 25-88 ppm, the H content is less than 1 ppm, the O content is 15-230 ppm, the N content is less than 5 ppm, and the Ag content is less than 0.9 ppm.

7. The preparation method according to claim 1, characterized in that The crystal orientation of the single crystal niobium is [100], [110] or [111]; and the purity of the single crystal niobium is not less than 99.9%.

8. The preparation method according to claim 1, characterized in that The surface roughness of the single crystal niobium is less than 0.03 μm; the unevenness of both end surfaces of the single crystal niobium is less than 0.02.

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