A method for refining the size of potassium bubbles in a W-K alloy rod by electron beam heat loading

Abstract

The present application relates to a method for refining the size of potassium bubbles in W-K alloy rods by electron beam heat loading, comprising the following steps: (1) cutting the W-K alloy rods into samples with a thickness of not more than 1 mm, and polishing the samples to make the sample surface smooth and bright; (2) placing the samples obtained by step (1) in an electron beam device for cyclic electron beam heat loading treatment, wherein the absorption power density of the electron beam heat loading is 10-20 MW / m 2 and the scanning frequencies of the electron beam in the x direction and the y direction are 35 kHz and 26 kHz respectively. The present application refines the tubular potassium bubbles in the W-K alloy to a nanoscale row distribution by applying an electron beam heat load with a suitable power density. The purpose of refining the potassium bubbles is achieved without changing the microstructure of the tungsten matrix, and the overall mechanical properties of the material are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material microstructure optimization, and particularly relates to a method for refining the size of potassium bubbles in W-K alloy rods by using electron beam heat loading. BACKGROUND

[0002] Metal tungsten (W) is widely concerned due to its high melting point, good thermal conductivity, low tritium retention rate, high temperature strength and other advantages, and is considered as one of the most promising high-temperature structural materials, especially as the best candidate material for the plasma-facing components of the nuclear fusion reactor. However, due to the inherent brittleness of W, such as low-temperature brittleness (high ductile-brittle transition temperature (DBTT) ~ 400 o C) and irradiation brittleness, the application of pure W in the fusion reactor is severely limited. The service environment of the plasma-facing wall material (PFM) in the nuclear fusion reactor is extremely harsh, especially under high thermal load. The thermal load damage resistance behavior of tungsten material is closely related to its mechanical properties, including strength, DBTT and thermal conductivity. The thermal stress generated by the temperature gradient in the heating and cooling process is the main reason for the formation of surface cracks of the material under thermal loading. When the temperature is lower than the DBTT, the thermal stress is greater than the yield strength of the material, or when the temperature is higher than the DBTT, the thermal stress is greater than the ultimate strength of the material, which will cause the material to crack. High thermal conductivity can reduce the extreme temperature gradient caused by thermal load, thereby reducing the thermal stress inside the material. Therefore, to solve the problem of brittleness of W material, it is necessary to fundamentally improve the strength and ductility of W material, which is of great significance to the engineering application of W material.

[0003] Potassium (K) doping as a method of dispersion strengthening of tungsten material has been successfully applied in tungsten lamp tubes for many years. In potassium-doped tungsten wire, during drawing, spherical sintering bubbles are elongated into ellipsoids or long tubes, and during high-temperature annealing, long-tube potassium bubbles with a large aspect ratio break to form rows of potassium bubbles. The row-shaped distribution of fine potassium bubbles is beneficial to improving the performance of the material. Because W-K alloy has excellent performance, such as thermal conductivity comparable to pure tungsten, excellent thermal shock resistance, high recrystallization temperature, low DBTT, etc., it has great application prospects as a PFM in fusion reactors. Industrial production of W-K alloy first sintered into blanks at a high temperature of 2800°C in a hydrogen atmosphere, and then subjected to hot rolling or forging at 1700°C to improve its density. During sintering, potassium bubbles migrate to the grain boundaries due to their instability, and high sintering temperature and long sintering time can lead to the agglomeration and growth of potassium bubbles at the grain boundaries. During subsequent hot plastic deformation, due to the deformability of potassium bubbles, the agglomerated and grown potassium bubbles deform with the deformation of the tungsten matrix, forming ellipsoids or long tubes. Part of the long-tube potassium bubbles with a large aspect ratio break due to Rayleigh instability during intermediate annealing, forming rows of fine potassium bubbles. Large-size long-tube potassium bubbles weaken the dispersion strengthening effect of potassium bubbles and are not conducive to improving the performance of the material.

[0004] The prior art discloses some methods for refining potassium bubbles in W-K alloy, for example, patent number CN106676358A discloses a W-K / W-K-Y alloy with nano-potassium bubble distribution and a preparation method thereof. By means of powder metallurgy, the size and distribution of potassium bubbles in tungsten are controlled, the size of potassium bubbles is controlled to be 20-130 nm and distributed in the grain boundaries and the grains, and the strength and thermal shock resistance of the material are improved by using nano-potassium bubbles to pin dislocations. Although the above technical solution obtains nano-potassium bubbles in the tungsten matrix, the distribution of potassium bubbles at the grain boundaries and in the grains is random, and row-shaped potassium bubble strings cannot be formed, which limits the pinning of potassium bubbles to grain boundaries and dislocations and the improvement of material performance. Moreover, the sample size obtained by SPS sintering is small, engineering application cannot be realized, and the size of potassium bubbles and the density of the sample are difficult to be optimized simultaneously.

[0005] In addition, it is reported in the literature that high-performance W-K alloy rods with high density are prepared by rotary forging, and exhibit excellent performance. However, there are still a certain amount of long-diameter ratio tubular potassium bubbles in the prepared W-K alloy rods, although it is proved that these potassium tubes are refined after subsequent high-temperature isothermal annealing, but the required annealing temperature is high, for example, after annealing at 1350℃ for 1 hour, although part of the tubular potassium bubbles are refined, but long potassium tubes can still be observed, and high-temperature annealing also affects the distribution of dislocations in the matrix, promotes the rearrangement and annihilation of dislocations, and greatly reduces the strength of the material. Therefore, it is necessary to explore a more efficient and suitable method for refining potassium bubbles and obtaining W-K alloy with dispersed nanometer second phase by exploring the process, so as to provide an excellent technical scheme for preparing high-performance W-K alloy without reducing the high strength of W-K alloy. SUMMARY

[0006] The purpose of the present application is to solve the problem of the weakening of the potassium bubble dispersion strengthening effect caused by the existence of large-sized tubular potassium bubbles in W-K alloy, and to provide a method for refining potassium bubbles in W-K alloy while maintaining the basic microstructure of the tungsten matrix, wherein the refined potassium bubbles are nanoscale and arranged in rows. The method is simple, convenient and fast, and the obtained W-K alloy has excellent mechanical properties.

[0007] The present application realizes the above-mentioned purpose by the following technical scheme:

[0008] The present application provides a method for refining the size of potassium bubbles in W-K alloy rods by electron beam heat loading, comprising the following steps:

[0009] (1) cutting the W-K alloy rod into a sample with a thickness not greater than 1mm, and polishing the sample to make the sample surface smooth and flat;

[0010] (2) placing the original sample obtained by step (1) in an electron beam device for cyclic electron beam heat loading treatment, wherein the absorption power density of the electron beam heat loading is 10-20 MW / m2, and the scanning frequency of the electron beam in the x direction and the y direction is 35 kHz and 26 kHz, respectively.

[0011] As a further optimization scheme of the present application, the K content in the W-K alloy rod is 68-70 ppm.

[0012] As a further optimization scheme of the present application, the length of the sample is 30mm, and the width is 25mm.

[0013] As a further optimization scheme of the present application, in step (1), the sample is polished with 240-3000 mesh SiC sandpaper.

[0014] As a further optimization scheme of the present application, in step (2), the cyclic electron beam heat loading refers to single electron beam loading duration of 1 second, cooling time of 10 seconds, and cyclic loading of 50 times.

[0015] As a further optimization scheme of the present application, the step (2) is performed in a vacuum environment.

[0016] As a further optimization scheme of the present application, in step (2), the sample is subjected to electron beam heat loading at an absorption power density of 10-20 MW / m 2 , and the surface temperature ranges from 1000-1499 K.

[0017] In the existing W-K rod or W-K plate prepared by plastic deformation, due to the deformability of potassium bubbles, the large-size potassium bubbles formed in the sintering process, especially the potassium bubbles at the grain boundaries, form long tubular potassium bubbles during the plastic deformation process. Although the tubular potassium bubbles with a large aspect ratio will be broken due to Rayleigh instability during subsequent high-temperature annealing process, forming a row of potassium bubbles, but there are still some tubular potassium bubbles with a relatively small aspect ratio in the W-K alloy. The present application utilizes the method of electron beam heat loading to apply cyclic and non-steady-state heat load on the W-K alloy, so that the tubular potassium bubbles are broken due to Rayleigh instability, and form a nanoscale row of potassium bubble strings.

[0018] In addition, the literature reports that the ~ 95 nm potassium bubbles in the W-K alloy can be refined to 89 nm by isothermal annealing at 1350 DEG C for 1 h. However, after annealing at a slightly higher temperature (1450 o C for 1 h, the potassium bubbles grow significantly due to the long annealing time, and the potassium bubble size is ~ 137 nm. However, the literature does not report the change of the mechanical properties of the material after annealing. Unlike the high-temperature isothermal annealing for refining potassium bubbles in the literature, isothermal annealing is a thermal equilibrium state. Long-time high-temperature annealing will promote the movement of dislocations, and the dislocations around the potassium bubbles broken due to Rayleigh instability will rearrange and annihilate. In addition, long-time high-temperature annealing will also promote the rearrangement and annihilation of the original dislocations in the tungsten matrix, greatly reducing the strength of the material. The electron beam heat loading of the present application can realize the rapid heating and cooling of the sample, and the tubular potassium in the W-K alloy is broken under the drive of heat, and the process of refining the nanoscale potassium bubbles is a non-thermal equilibrium state. At the same time, due to this non-thermal equilibrium state, there are dislocations at the interface between the refined potassium bubbles and the tungsten matrix that have not been annihilated in time, and these dislocations play an important role in improving the mechanical properties of the material. In addition, the temperature rise caused by the electron beam loading is not high (~ 1165 K), which will not significantly affect the arrangement of the original dislocations in the tungsten matrix.

[0019] In summary, the present application has the following advantages:

[0020] (1) The present application refines the tubular potassium bubbles in W-K alloy to nanoscale row distribution by applying electron beam heat load with suitable power density. The purpose of refining potassium bubbles is achieved without changing the microstructure of tungsten matrix (including grain morphology, size, dislocation arrangement), and the comprehensive mechanical properties of the material are improved.

[0021] (2) The present application refines the size of potassium bubbles in W-K alloy by adopting the method of cyclic electron beam heat loading, which is simple in process, low in time and energy consumption, reliable in repeatability, and can realize batch processing. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is the cross-sectional scanning electron microscope image of the original sample and the sample after electron beam heat loading of examples 1-3 provided by the present application.

[0023] Figure 2 is the potassium bubble size distribution graph along the radial and axial directions of the original sample and the sample after electron beam heat loading of examples 1-3 provided by the present application (in the figure, (a) corresponds to the statistical results of the size of potassium bubbles in the original sample; (b)-(d) correspond to the statistical results of the size of potassium bubbles in the samples after electron beam heat loading of examples 1-3).

[0024] Figure 3 is the tensile stress-strain curve graph of the sample after electron beam heat loading of examples 1-3 provided by the present application. DETAILED DESCRIPTION

[0025] It is necessary to point out here that the following detailed description is only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application according to the above application content.

[0026] Example 1

[0027] The present embodiment provides a method for refining the size of potassium bubbles in W-K alloy rod by electron beam heat loading, which specifically comprises the following steps:

[0028] (1) The W-K alloy rod is cut into a sample with a size of 30 mm (length) x 25 mm (width) x 0.75 mm (thickness) by electric spark, the sample surface is polished on 240-3000 mesh SiC sandpaper to obtain a bright and smooth surface, which is recorded as an original sample, and the potassium content in the W-K alloy rod is 68-72 ppm;

[0029] (2) The original sample obtained in step (1) is placed in an electron beam device, and the electron beam heat loading conditions are set, the absorption power density of the electron beam heat loading is set to 10 MW / m2, the scanning frequencies of the electron beam in the x direction and the y direction are 35 kHz and 26 kHz respectively, the single electron beam loading duration is 1 second, the cooling time is 10 seconds, the heat loading mode of 50 cycles is adopted, and an infrared camera is used to monitor the surface temperature of the W-K sample during the heat loading process;

[0030] (3) When the vacuum degree of the electron beam sample cavity is better than 5x10-4 mbar, the electron beam heat loading test is started, and the sample after electron beam heat loading is obtained, and the infrared camera measures that the surface temperature of the sample is about 1000 K.

[0031] Example 2

[0032] The embodiment provides a method for refining the size of potassium bubbles in a W-K alloy rod by using electron beam heat loading, and specifically comprises the following steps:

[0033] (1) The W-K alloy rod is cut into a sample with a size of 30 mm (length) x 25 mm (width) x 0.75 mm (thickness) by electric spark cutting, the surface of the sample is polished on 240-3000 mesh SiC sandpaper to obtain a bright and smooth surface, and the sample is recorded as an original sample, and the potassium content in the W-K alloy rod is 68-72 ppm;

[0034] (2) The original sample obtained in step (1) is placed in an electron beam device, and the electron beam heat loading conditions are set, the absorption power density of the electron beam heat loading is set to 13 MW / m2, the scanning frequencies of the electron beam in the x direction and the y direction are 35 kHz and 26 kHz respectively, the single electron beam loading duration is 1 second, the cooling time is 10 seconds, the heat loading mode of 50 cycles is adopted, and an infrared camera is used to monitor the surface temperature of the W-K sample during the heat loading process;

[0035] (3) When the vacuum degree of the electron beam sample cavity is better than 5x10-4 mbar, the electron beam heat loading test is started, and the sample after electron beam heat loading is obtained, and the infrared camera measures that the surface temperature of the sample is about 1165 K.

[0036] Example 3

[0037] The embodiment provides a method for refining the size of potassium bubbles in a W-K alloy rod by using electron beam heat loading, and specifically comprises the following steps:

[0038] (1) The W-K alloy rod was cut into a sample with a size of 30 mm (length) x 25 mm (width) x 0.75 mm (thickness) by electric spark cutting, the surface of the sample was polished on a 240-3000 mesh SiC sandpaper to obtain a bright and smooth surface, which was recorded as the original sample, and the potassium content in the W-K alloy rod was 68-72 ppm;

[0039] (2) The original sample obtained in step (1) was placed in an electron beam device, and the electron beam heat loading conditions were set. The absorption power density of the electron beam heat loading was set to 20 MW / m2, the scanning frequencies of the electron beam in the x direction and the y direction were 35 kHz and 26 kHz, respectively, the single electron beam loading duration was 1 second, the cooling time was 10 seconds, and the heat loading was cycled 50 times. At the same time, an infrared camera was used to monitor the surface temperature of the W-K sample during the heat loading process.

[0040] (3) When the vacuum degree of the electron beam sample chamber was better than 5 x 10 -4 mbar, the electron beam heat loading test was started, and the sample after electron beam heat loading was obtained. At the same time, the infrared camera measured the surface temperature of the sample to be ~ 1499 K.

[0041] In order to further verify the effect of electron beam heat loading on refining the potassium bubbles in the W-K alloy, the samples after electron beam heat loading obtained in Examples 1-3 were placed under a scanning electron microscope, and the morphology and size of the potassium bubbles in the W-K alloy after electron beam heat loading were observed and compared with the original sample.

[0042] Figure 1 are cross-sectional scanning electron microscope images of the original sample and the samples obtained in Examples 1-3, Figure 2 are the size distribution diagrams of the potassium bubbles along the radial and axial directions of the original sample and the samples obtained in Examples 1-3, wherein, Figure 2 (a) in (a) is the statistical result of the size of the potassium bubbles in the original sample, Figure 2 (b)-(d) in (b)-(d) are the statistical results of the size of the potassium bubbles in the products of Examples 1-3.

[0043] The results show that a large number of long tubular potassium bubbles exist in the original sample, and the proportion of potassium tubes with a size greater than 200 nm is about 37.7%. In Example 2, after electron beam heat loading at 13 MW / m2, the size of the potassium bubbles in the W-K alloy is refined, the average radial size is reduced from ~ 46 nm to 39 nm, and the proportion of tubular potassium bubbles with an axial size greater than 200 nm is reduced from ~ 37.7% to ~ 17.1%. In Example 3, after electron beam heat loading at 20 MW / m2, the potassium bubbles in the W-K alloy significantly grow, and the average radial size increases to 89 nm.

[0044] In addition, in order to study the effect of the potassium bubble refinement on the mechanical properties of the W-K alloy after the electron beam heat loading, the samples obtained in Examples 1-3 are cut into dumbbell-shaped tensile test samples, the length of the tensile test sample is 16 mm, the width is 4 mm, and the wire cutting marks are removed to the surface smoothness by using 240-3000 mesh SiC sandpaper, in addition, the original sample is also cut into tensile test samples of the same size and shape and is polished in the same way. The four kinds of tensile test samples obtained above are respectively subjected to tensile test, and the stress-strain curves of the W-K alloy after the electron beam heat loading are obtained, as shown in Figure 3 The strain rate is 0.06 mm / min during the tensile test, and the test temperature is 200°C.

[0045] It can be seen from Figure 3 that the elongation of the W-K alloy at 200°C is greatly improved due to the potassium bubble refinement in the W-K alloy after the electron beam heat loading of 10 and 13 MW / m2, and the elongation of the material is improved from ~24.5% to ~31.3% after the electron beam heat loading of 13 MW / m2, the elongation is improved by 26.9%, and the ultimate tensile strength of ~1040 MPa is still maintained.

[0046] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as the limitation of the scope of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application.

Claims

1. A method for refining the size of potassium blisters in a W-K alloy rod by electron beam heat loading, characterized by: The method comprises the following steps: (1) cutting a W-K alloy rod into a sample with a thickness of not more than 1 mm and polishing the sample to a bright and smooth surface; the K content in the W-K alloy rod is 68-70 ppm; (2) The sample obtained in step (1) is placed in an electron beam device and subjected to cyclic electron beam heat loading treatment in a vacuum environment, wherein the absorption power density of the electron beam heat loading is 10-20 MW / m 2 and the scanning frequencies of the electron beam in the x direction and the y direction are 35 kHz and 26 kHz, respectively; the cyclic electron beam heat loading refers to 50 cycles of loading, with a single electron beam loading duration of 1 second and a cooling time of 10 seconds.

2. The method for refining the size of potassium bubbles in a W-K alloy rod by electron beam heating according to claim 1, characterized in that: The length of the sample is 30 mm, and the width is 25 mm.

3. The method for refining the size of potassium bubbles in a W-K alloy rod by using an electron beam to heat load, according to claim 1, wherein in step (1), the sample is polished by using 240-3000 mesh SiC sandpaper.

4. The method for refining the size of potassium bubbles in a W-K alloy rod by electron beam heating according to claim 1, characterized in that: In step (2), the sample is heated by an electron beam with an absorbed power density of 10-20 MW / m 2 and a surface temperature of 1000-1499 K.

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