A method for preparing a nanocrystalline layer on the surface of refractory metal tungsten

Abstract

The present invention belongs to the technical field of material surface treatment, and specifically relates to a method for preparing a nanocrystalline layer on the surface of refractory metal tungsten. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten provided by the present invention comprises the following steps: applying pressure to the surface of refractory metal tungsten in a vacuum environment or a protective atmosphere, and heat-treating the surface of refractory metal tungsten during the pressure application process, wherein the pressure is 7.5 to 50 MPa. Under the combined action of surface pressure and temperature, the original coarse-grained structure of the surface layer of refractory metal tungsten spontaneously re-nucleates and crystallizes to form a surface nanocrystalline layer that is firmly bonded to the matrix. The surface nanocrystalline layer preparation method provided by the present invention does not require the surface material of refractory metal tungsten to undergo strong plastic deformation or rapid heating and cooling treatment, and does not have high requirements on the surface state of the metal. The preparation method is simple and efficient, highly operable, and convenient for mass production.

Description

Technical Field

[0001] The invention belongs to the technical field of material surface treatment, and in particular relates to a method for preparing a nanocrystalline layer on the surface of refractory metal tungsten. Background Art

[0002] Tungsten metal has outstanding performance advantages such as high melting point, high hardness, high elastic modulus, high thermal conductivity, low thermal expansion coefficient, low sputtering rate, low vapor pressure and low tritium retention rate. It is widely used in the fields of high temperature resistance, friction resistance, erosion resistance, and ablation resistance. However, tungsten metal is brittle and has a high ductile-brittle transition temperature. When irradiated by high-energy rays, it will produce bubbles, nanowires and other phenomena. Under the action of high-voltage arcs, large-sized ablation pits may also be generated locally, causing premature failure of the workpiece. Therefore, in order to give full play to the performance advantages of tungsten metal and increase the service life of tungsten metal workpieces under actual working conditions, it is necessary to perform surface nano-processing on tungsten metal to form a nanocrystalline layer of a certain thickness on the surface of the workpiece, so as to further improve the strength, wear resistance, fatigue resistance, radiation resistance, ablation resistance and other properties of the surface layer material.

[0003] Due to its high melting point, high hardness and difficulty in plastic deformation, metallic tungsten is generally prepared by powder metallurgy and mechanically processed by grinding. It is difficult to prepare nanocrystalline layers on the surface of refractory metals such as tungsten by using metal surface nanomaterial technology based on strong plastic deformation of the surface layer material (ultrasonic shot peening, ultrasonic impact, supersonic particle bombardment, mechanical grinding, mechanical rolling) or non-equilibrium rapid heating and cooling (laser heating, ion irradiation). Therefore, it is still of great theoretical significance and practical application value to continue to explore and develop methods for preparing nanocrystalline layers on the surface of refractory metals such as tungsten that are firmly bonded to the substrate, simple and efficient, highly operational, and easy to mass-produce.

[0004] The Chinese patent application with publication number CN115716130A filed on February 28, 2023, discloses a method for surface nano-sizing of refractory metal tungsten, which uses a process of pressure-assisted mechanical polishing combined with vacuum heat treatment to achieve surface nano-sizing of coarse-grained tungsten. The pressure-assisted metallographic polishing process will introduce high-density dislocations in the surface structure of coarse-grained tungsten, and the surface structure containing high-density dislocations will undergo recovery and recrystallization during the subsequent vacuum heat treatment process, thereby forming stable nanocrystals. The thickness of the surface nanocrystalline layer formed is 200 to 400 nm, and the nanocrystalline grain size is about 200 to 400 nm. This method for surface nano-sizing of metal tungsten requires first performing pressure-assisted mechanical polishing on the surface of coarse-grained metal tungsten, and has high requirements for the surface state of refractory metal tungsten. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a nanocrystalline layer on the surface of refractory metal tungsten, so as to solve the problem that the original surface state of refractory metal tungsten has high requirements during the nano-crystalization process of the surface of refractory metal tungsten.

[0006] To achieve the above objectives, the technical solution of the method for preparing a nanocrystalline layer on the surface of refractory metal tungsten of the present invention is as follows:

[0007] A method for preparing a nanocrystalline layer on the surface of refractory metal tungsten includes the following steps: applying pressure to the surface of the refractory metal tungsten in a vacuum environment or a protective atmosphere, and heat treating the surface of the refractory metal tungsten during the pressure application process, wherein the pressure is 7.5 to 50 MPa.

[0008] The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten provided by the present invention improves the existing technology and causes the coarse-grained structure of the surface of refractory metal tungsten to spontaneously re-nucleate and crystallize through the coupling effect of surface pressure and temperature. Under the combined action of pressure and temperature, the lattice of the surface structure of refractory metal tungsten gradually expands and deforms, lattice defects and grain boundaries gradually increase, the coarse-grained structure spontaneously re-nucleates and crystallizes, and the grain size gradually decreases, thereby generating a surface nanocrystalline layer with a certain thickness and firmly bonded to the matrix. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten provided by the present invention does not require the metal surface material to undergo strong plastic deformation, does not require the metal surface to undergo non-equilibrium rapid heating and cooling treatment, and does not require the metal surface to undergo polishing treatment, thereby reducing the requirements for the metal surface state. In addition, the preparation method is simple and efficient, highly operable, low-cost, and convenient for mass production.

[0009] In order to further improve the grain refinement rate, preferably, the heat treatment includes a heating process, a holding process and a cooling process; the holding temperature of the holding process is 900-1100° C., and the holding time of the holding process is 1-3 hours.

[0010] In order to further increase the speed of grain refinement, preferably, the heating rate of the heating process is 5 to 15° C. / min.

[0011] In order to effectively prevent the grain growth of the surface nanocrystalline layer, preferably, the cooling process adopts natural cooling along with the furnace.

[0012] In order to further improve the uniformity of the nanocrystalline layer on the surface of the refractory metal tungsten and reduce surface defects, preferably, the surface of the refractory metal tungsten is subjected to surface grinding before the pressure is applied, and the surface roughness R of the refractory metal tungsten after grinding is a Less than 1.0μm.

[0013] In order to further remove pollutants such as oil, impurities, oxygen elements, etc. on the metal surface, preferably, the surface is cleaned after grinding, and the surface cleaning includes first soaking and cleaning in dilute hydrochloric acid and / or hot alkaline solution, and then performing ultrasonic vibration cleaning in deionized water and / or anhydrous ethanol.

[0014] In order to clean the metal surface pollutants more thoroughly, preferably, the mass fraction of the dilute hydrochloric acid is 5-10 wt.%, and the hot alkaline solution is a sodium hydroxide or potassium hydroxide aqueous solution at a temperature of 50-90°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a surface scanning electron microscope image of the metal tungsten with a surface nanocrystalline layer prepared in Example 1 of the present invention;

[0016] Figure 2 This is a surface scanning electron microscope image of the metal tungsten with a surface nanocrystalline layer prepared in Example 2 of the present invention;

[0017] Figure 3 This is a scanning electron microscope image of the fracture surface of the metal tungsten with a surface nanocrystalline layer prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0018] The technical concept of the method for preparing a nanocrystalline layer on the surface of refractory metal tungsten provided by the present invention is as follows:

[0019] A method for preparing a nanocrystalline layer on the surface of refractory tungsten metal comprises the following steps: applying pressure to the surface of the refractory tungsten metal in a vacuum environment or a protective atmosphere, and heat treating the surface of the refractory tungsten metal during the pressure application process, wherein the pressure is 7.5 to 50 MPa. Under the combined effects of the surface pressure and temperature, the lattice of the surface structure of the refractory tungsten metal gradually expands and deforms, lattice defects and grain boundaries gradually increase, and the original coarse-grained structure spontaneously renucleates and crystallizes, thereby forming a surface nanocrystalline layer that is firmly bonded to the substrate. As the surface pressure increases and the heat treatment temperature rises, the degree of grain refinement and the speed of refinement of the refractory tungsten metal surface layer also accelerate due to the increased driving force. The method for preparing the nanocrystalline layer on the surface of the refractory tungsten metal of the present invention does not require drastic plastic deformation of the metal surface, non-equilibrium rapid heating and cooling of the metal surface, or polishing of the metal surface, thereby reducing the requirements for the metal surface condition. The preparation method is simple, efficient, highly operable, and low-cost, making it easy to mass produce.

[0020] In a specific embodiment, the surface grinding is to grind the surface of the metal workpiece using a grinding tool such as a grinding machine equipped with diamond grinding wheels of different mesh sizes or different specifications to obtain a refractory metal tungsten surface with different roughness. The grinding machine can be a surface grinder, a cylindrical grinder, an internal hole grinder, a centerless grinder, etc. The surface of the refractory metal tungsten after grinding can have different surface roughness according to specific usage requirements. When the metal surface is required to have a high degree of nano-crystallization, uniform structure, and fewer defects, the metal surface roughness is as small as possible, generally R a Less than 0.4μm; when the requirements for the uniformity of the metal surface structure and the number of defects are not high, the roughness of the metal surface can be appropriately relaxed. Generally, R a 0.4-1.0μm.

[0021] In a specific embodiment, the pressure value of the vacuum environment is less than 1×10 -1 Pa, the pressure of the protective atmosphere gas is 1×10 5 Pa, the protective atmosphere gas is argon or helium with a purity greater than 99.99%, and the protective atmosphere gas can be filled in after vacuuming. The vacuum environment and protective atmosphere environment can further reduce the impact of impurity elements in the environment on the microstructure and uniformity of the nanocrystalline layer on the surface of the refractory metal tungsten.

[0022] In a specific embodiment, the pressure is applied before the refractory metal tungsten is heated to an elevated temperature.

[0023] In a specific embodiment, the pressure can be applied by a hydraulic system or by a dedicated fixture. The pressure is released by removing the pressure applied by the hydraulic system or removing the dedicated fixture that fixes the refractory metal workpiece such as tungsten.

[0024] In a specific embodiment, the pressure relief operation is performed after the refractory metal tungsten is cooled to room temperature.

[0025] In a specific embodiment, after the heat treatment of the refractory metal tungsten is completed, its surface is cleaned, and the cleaning is performed by soaking the refractory metal tungsten in deionized water and / or anhydrous ethanol.

[0026] In a specific embodiment, the preparation of the nanocrystalline layer on the surface of the refractory metal tungsten can be carried out in a vacuum hot pressing furnace, or in a vacuum furnace or an atmosphere furnace, and the cooling is carried out by natural cooling along with the furnace.

[0027] In a specific embodiment, the prepared refractory metal tungsten with a surface nanocrystalline layer can be further processed into required parts, shapes and sizes by using processes such as wire electric discharge cutting, welding, and grinding.

[0028] In a specific embodiment, the metal tungsten is a metal tungsten plate in a hot-rolled annealed state conventionally shipped from the manufacturer.

[0029] The embodiments of the present invention are further described below with reference to specific examples. Unless otherwise specified, the chemical reagents involved in the following examples are all commercially available conventional products.

[0030] 1. The specific embodiment of the method for preparing the nanocrystalline layer on the surface of refractory metal tungsten of the present invention is as follows:

[0031] Example 1

[0032] The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten in this embodiment includes the following steps:

[0033] (1) Surface grinding and surface cleaning: A tungsten metal plate with a size of 40 mm × 40 mm × 2 mm was ground using a surface grinder equipped with a diamond grinding wheel. The surface roughness of the tungsten plate after grinding was R a ≈0.9μm, soak the surface-ground tungsten plate in dilute hydrochloric acid (10wt.%) and / or hot alkaline solution (10wt.% NaOH aqueous solution at 50°C) for 8 hours, then soak it in deionized water and / or anhydrous ethanol for 5 minutes and perform ultrasonic cleaning at the same time to remove oil, impurities, oxygen and other contaminants on the surface of the tungsten plate;

[0034] (2) Vacuum pressurization: The metal tungsten plate obtained after surface grinding and surface cleaning in step (1) is placed between the upper and lower pressure heads of a vacuum hot pressing furnace. A pressure of 12.5 MPa is applied to the tungsten plate through the hydraulic system of the vacuum hot pressing furnace itself, and then the furnace is evacuated to a pressure of 1×10 -1 Pa;

[0035] (3) Heat treatment: Heat the tungsten plate to 900°C at a heating rate of 15°C / min and keep it at 900°C for 3 hours. After the heat preservation is completed, turn off the heating power supply. After the tungsten plate cools to room temperature with the furnace, unload the pressure.

[0036] (4) Cleaning: Open the furnace door, take out the cooled tungsten plate, and clean it in deionized water to obtain a refractory metal tungsten plate with a nanocrystalline layer on the surface;

[0037] (5) Processing: The tungsten plate with the surface nanocrystalline layer was processed into a sample of a specified shape using the wire-cut electric discharge process.

[0038] Example 2

[0039] The method for preparing the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in Example 1, except that the pressure applied to the tungsten plate in step (2) is 10 MPa, and the holding time in step (3) is 1 hour.

[0040] Example 3

[0041] The method for preparing the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in Example 1, with the only difference being that the pressure applied to the tungsten plate in step (2) is 7.5 MPa.

[0042] Example 4

[0043] The method for preparing the nanocrystalline layer on the surface of refractory metal tungsten in this embodiment is basically the same as that in Example 2, with the only difference being that the holding time in step (3) is 3 hours.

[0044] Example 5

[0045] The method for preparing the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in Example 2, with the only difference being that the holding time in step (3) is 2 hours.

[0046] Example 6

[0047] The preparation method of the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in embodiment 1, except that the surface roughness R of the tungsten plate after grinding in step (1) is a ≈0.7μm.

[0048] Example 7

[0049] The preparation method of the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in Example 1, except that: in step (2), the furnace is evacuated to a pressure of 1×10 -1 Pa, then fill the furnace with protective gas until the pressure reaches 1×10 5 Pa, the protective gas is argon with a purity of 99.999%.

[0050] Example 8

[0051] The method for preparing the nanocrystalline layer on the surface of refractory metal tungsten in this embodiment is basically the same as that in Example 1, except that the heating rate in step (3) is 5°C / min and the holding temperature is 1100°C.

[0052] Example 9

[0053] The method for preparing the nanocrystalline layer on the surface of refractory metal tungsten in this embodiment is basically the same as that in Example 1, except that the concentration of the dilute hydrochloric acid in step (1) is 5 wt.%, and the temperature of the hot alkaline solution is 90°C.

[0054] Example 10

[0055] The method for preparing the nanocrystalline layer on the surface of the refractory metal tungsten in this embodiment is basically the same as that in Example 1, with the only difference being that the pressure applied to the tungsten plate in step (2) is 50 MPa.

[0056] 2. The experimental example of the method for preparing the nanocrystalline layer on the surface of refractory metal tungsten of the present invention is as follows:

[0057] Experimental Example 1

[0058] In this experimental example, the SEM morphology of the nanocrystalline layer on the surface of the tungsten plate after pressure heat treatment in Examples 1 to 7 was obtained by scanning electron microscopy (SEM), and the average size of the grains in the surface nanocrystalline layer was calculated based on the SEM morphology and scale. The specific calculation method is as follows: the obtained refractory metal tungsten plate with a surface nanocrystalline layer is cut into 4 to 6 test samples using an electric spark wire cutting process. The test samples are 5 mm in length and 14 mm in width. After the test samples are processed, they are placed in a scanning electron microscope and imaged by secondary electrons to obtain the organizational morphology of the nanocrystalline layer on the surface of the tungsten plate, as shown in FIG. Figure 1 and Figure 2 According to the scale bar length, the sizes of 300 to 500 nano-grains were measured and counted, and the average and standard deviation of the grain sizes were calculated. The results are shown in Table 1.

[0059] Table 1 Statistical results of the grain size of the nanocrystalline layer on the surface of the tungsten plate after pressure heat treatment in Examples 1 to 7

[0060] Average grain size and standard deviation, nm Example 1 247±56 Example 2 300±57 Example 3 266±74 Example 4 270±63 Example 5 256±63 Example 6 253±60 Example 7 258±64

[0061] The measurement statistics in Table 1 show that the average grain size of the nanocrystalline layer on the surface of the tungsten plate obtained in Examples 1 to 7 is approximately 247 to 300 nm. The grain size of the nanocrystalline layer on the surface of the tungsten plate obtained in Example 1 is significantly smaller than that of Examples 3 and 4, indicating that the greater the pressure, the finer the grain size of the nanocrystalline layer. The grain size of the nanocrystalline layer on the surface of the tungsten plate obtained in Example 5 is significantly smaller than that of Example 2, indicating that appropriately extending the holding time is conducive to the refinement of the grain size of the nanocrystalline layer. The grain size of Examples 5 and 4 is not much different, indicating that when the holding time is too long, the effect on grain refinement gradually decreases.

[0062] Experimental Example 2

[0063] In this experimental example, the fracture morphology of the tungsten plate with a surface nanocrystalline layer in Example 1 was obtained by scanning electron microscopy (SEM). Figure 3As shown. It can be seen from the figure that the coarse grain size of the matrix structure inside the tungsten plate is about 2 to 6 μm, while the average grain size of the surface nanocrystalline layer is about 247 nm. That is, after pressure heat treatment, the grains of the surface structure of the tungsten plate are refined to 1 / 10 to 1 / 20 of the grains of the internal matrix structure. It was found through the fracture structure morphology that the thickness of the transition area between the surface nanocrystals and the coarse crystal structure inside the tungsten plate is about 2 to 3 μm, and the structure in the transition area is uniform, the transition is continuous, and it is firmly bonded to the internal coarse crystal structure. This shows that the pressure heat treatment process described in the present invention can prepare a nanocrystalline layer with a certain thickness on the surface of refractory metals such as tungsten, and the nanocrystalline layer has a continuous transition with the internal matrix structure, is firmly bonded, and is not easy to fall off.

[0064] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to replace some of the technical features therein with equivalents. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method for preparing a nanocrystalline layer on the surface of refractory metal tungsten, characterized in that: The following steps are involved: The surface of the refractory metal tungsten is subjected to surface grinding treatment, and then pressure is applied to the surface of the refractory metal tungsten in a vacuum environment or a protective atmosphere. During the pressure application process, the surface of the refractory metal tungsten is subjected to heat treatment, wherein the pressure is 7.5 to 50 MPa; the heat treatment includes a heating process, a heat preservation process, and a cooling process; The insulation temperature of the insulation process is 900-1100° C., and the insulation time of the insulation process is 1-3 hours.

2. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten according to claim 1, characterized in that: The heating rate of the heating process is 5-15°C / min.

3. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten according to claim 1, wherein: The cooling process adopts natural cooling along with the furnace.

4. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten according to any one of claims 1 to 3, characterized in that: Surface roughness R of refractory metal tungsten after grinding a Less than 1.0μm.

5. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten according to claim 4, characterized in that: After the surface is ground, the surface is cleaned. The surface cleaning includes first immersing and cleaning in dilute hydrochloric acid and / or hot alkaline solution, and then performing ultrasonic vibration cleaning in deionized water and / or anhydrous ethanol.

6. The method for preparing a nanocrystalline layer on the surface of refractory metal tungsten according to claim 5, characterized in that: The mass fraction of the dilute hydrochloric acid is 5-10 wt.%, and the hot alkaline solution is a sodium hydroxide or potassium hydroxide aqueous solution at a temperature of 50-90°C.

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