Carbon microalloyed refractory high-entropy alloy and preparation method thereof

Abstract

The invention provides a carbon microalloyed refractory high-entropy alloy and a preparation method thereof, the chemical formula of the alloy is NbaTabHfcWdCe, the atomic percentage of the elements a: b: c: d: e is (40%-47.5%): (40%-47.5%): (5%-15%): (0-5%): (0.5%-1.5%), a + b + c + d + e is 100, and the C element forms a carbide to play a role in strengthening a second phase. The invention further provides a preparation method of the alloy. The alloy disclosed by the invention obtains excellent room-temperature tensile plasticity and relatively high compressive yield strength at high temperature, has excellent comprehensive mechanical properties, and can meet the service requirements in an ultra-high-temperature environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of materials, in particular, to a carbon micro-alloyed refractory high-entropy alloy and a preparation method thereof. BACKGROUND

[0002] Refractory metals and refractory alloys are indispensable high-temperature structural materials in large national heavy equipment such as space engine combustion chambers, supersonic aircraft nose tips, and extreme environment military equipment, and have a temperature bearing capacity far exceeding that of nickel-based high-temperature alloys. A new branch of the alloy, refractory high-entropy alloy, has more excellent high-temperature mechanical properties. However, the room-temperature brittleness problem of refractory high-entropy alloys is one of the biggest bottlenecks that limits their application, making it difficult to cold and hot deformation processing of as-cast refractory high-entropy alloys, so that they cannot be put into large-scale production and application, and also making the finished product (such as recrystallized state) refractory high-entropy alloy prone to early cracking failure during assembly and service.

[0003] Oxygen or oxide-induced brittleness is one of the most common sources of room-temperature brittleness of refractory high-entropy alloys, because the influence of oxygen exists in raw materials, preparation environment, processing environment, and service environment of any refractory high-entropy alloy system. In existing domestic and foreign researches, NbTaMoW, TiZr 0.5 Hf 0.5 VNbTa 0.5 Mo 0.5 , (VNb) 80 (TaTi) 20 and other refractory high-entropy alloys all show the correlation between grain boundary oxygen element enrichment or grain boundary oxide distribution and intergranular brittle fracture.

[0004] However, the room-temperature brittleness of refractory high-entropy alloys, especially tensile brittleness, is still difficult to break through. Firstly, improper alloy composition design, when Mo / W content is too high, the intrinsic brittleness of the alloy becomes the dominant factor of room-temperature brittleness, which can only be compressed at room temperature but cannot be stretched, and the application prospect is greatly limited. Secondly, oxide-induced brittleness is also a common form of oxygen embrittlement of refractory high-entropy alloys. The current carbon / boron micro-alloying strategy can only deal with atomic-level oxygen embrittlement, and there is a lack of attempts to address whether it is still effective for oxide-induced brittleness. Thirdly, the strategy of active element coated foil can only prevent the infiltration of oxygen during post-processing, but cannot deal with the existing embrittling oxides in bulk alloys. In addition, some means to alleviate room-temperature oxygen embrittlement are not conducive to high-temperature strength, for example, the current carbon / boron micro-alloying exists in the form of atoms, which cannot obtain carbide / boronide strengthening, or the alloy composition is designed as Ti-Zr-Hf-Nb-Ta system, which avoids room-temperature oxygen embrittlement to the greatest extent, but greatly sacrifices high-temperature strength, which is contrary to the application goal of refractory high-entropy alloys.

[0005] Therefore, it is crucial to develop a refractory high-entropy alloy that can effectively overcome the oxygen embrittlement problem and has excellent room temperature tensile plasticity and high temperature strength.

[0006] After searching, it was found that the patent with application publication number CN111334697A discloses a W-Ta-Mo-Nb-C high-temperature high-entropy alloy and a preparation method thereof. The high-temperature high-entropy alloy is composed of W, Ta, Mo, Nb, and C in atomic ratio or non-equivalent atomic ratio. The high-temperature high-entropy alloy has a dual-phase structure of body-centered cubic (BCC) structure and face-centered cubic (FCC) structure, and its room temperature compressive strength and plasticity are significantly improved compared to existing NbMoTaW high-entropy alloys. It has a compressive strength of 585 MPa at 1600°C and a compressive strength of 291 MPa at 1800°C, which is higher than existing high-temperature alloys in terms of high-temperature strength and use temperature range. However, the strategy for improving ultra-high temperature strength in this patent severely damages the room temperature plasticity. SUMMARY

[0007] In view of the defects in the prior art, the purpose of the present application is to provide a carbon micro-alloyed refractory high-entropy alloy and a preparation method thereof.

[0008] In the first aspect of the present application, a carbon micro-alloyed refractory high-entropy alloy is provided, and the chemical formula of the alloy is Nb a Ta b Hf c W d C e , the atomic percentage of each element a : b : c : d : e = (40% ~ 47.5%) : (40% ~ 47.5%) : (5 ~ 15%) : (0 ~ 5%) : (0.5% ~ 1.5%), and a+b+c+d+e=100, and the C element forms carbide for second phase strengthening.

[0009] Optionally, the alloy takes Nb and Ta as matrix elements, Hf and W as solid solution strengthening elements, and Hf and C as carbide forming elements.

[0010] Optionally, the alloy has a body-centered cubic matrix phase and a face-centered cubic structure carbide dispersed precipitate phase.

[0011] Optionally, the alloy exhibits a cast microstructure morphology of equiaxed crystals and subgrains, and the carbide is discontinuously and dispersedly distributed in the grain boundaries, subgrain boundaries, and intracrystalline.

[0012] In the second aspect of the present application, a preparation method of the carbon micro-alloyed refractory high-entropy alloy is provided, and the method comprises: According to the designed atomic percentage, the required metal raw material blocks and TaC raw material blocks are weighed respectively; The titanium ingot is first placed in a crucible, and then the metal raw material block and the TaC raw material block are sequentially placed in the crucible according to the order of the melting point from low to high, and vacuum arc melting is performed to obtain a master alloy ingot, that is, the carbon micro-alloyed refractory high-entropy alloy.

[0013] Optionally, the required metal raw material block and the TaC raw material block are weighed according to the designed atomic percentage, and the purity of the required metal raw material block and the TaC raw material block is not less than 99.9 wt.%.

[0014] Optionally, before the vacuum arc melting, the method further comprises: extracting vacuum and introducing protective gas, the vacuum degree is 5.0*10 -3 Pa, and the gas pressure of the protective gas is 0.2-0.5 kPa.

[0015] Optionally, the vacuum arc melting comprises: accelerating the element homogenization of the alloy melt by external electromagnetic stirring during the vacuum arc melting.

[0016] Optionally, the vacuum arc melting comprises: after each alloy button ingot is completely cooled after being melted, the alloy button ingot is turned over, and the alloy button ingot is repeatedly melted for 8-12 times until no visible unmelted particles are present in the high-temperature melt, and the melting current is slowly reduced until the melting is stopped, to obtain the master alloy ingot.

[0017] Optionally, the melting time is 2-3 min, and the melting current is 380-420 A.

[0018] The carbon micro-alloyed refractory high-entropy alloy provided in the application comprehensively considers the interaction of various metal elements and the non-metal element C, has the balance of the main elements Nb and Ta to ensure the room temperature plasticity and the high-temperature strength to a certain extent, has the solid solution strengthening of the elements Hf and W and the second phase strengthening of the carbide to the high-temperature strength, and utilizes the C element itself and the carbonization to eliminate the oxygen and the oxide, effectively overcomes the room temperature brittleness, obtains excellent room temperature tensile plasticity, significantly improves the compression yield strength of the alloy at high temperature, has excellent comprehensive mechanical properties, can meet the service requirements in the super-high-temperature environment, and is expected to promote the industrial application of the carbide strengthened refractory high-entropy alloy in the fields of aerospace, military industry and the like.

[0019] Other technical effects brought by the additional features will be further described in the corresponding embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0020] Other features, objects and advantages of the application will become more apparent from the following detailed description of non-limiting embodiments with reference to the attached drawings: Figure 1 Flow chart of the process of designing, preparing and testing of carbon micro-alloyed refractory high-entropy alloys according to an exemplary embodiment; Figure 2 Room temperature tensile engineering stress-strain curve of high-entropy alloys according to an exemplary embodiment without carbon addition; Figure 3 Fracture SEM morphology and grain boundary TEM characterization results of room temperature tensile brittle fracture of high-entropy alloys according to an exemplary embodiment without carbon addition; Figure 4 X-ray diffraction patterns of high-entropy alloys prepared in Examples 1, 2, 3; Figure 5 Optical microscope characterization results of high-entropy alloys prepared in Example 1; Figure 6 Optical microscope characterization results of high-entropy alloys prepared in Example 2; Figure 7 Optical microscope characterization results of high-entropy alloys prepared in Example 3; Figure 8 Room temperature tensile engineering stress-strain curves of high-entropy alloys prepared in Examples 1, 2, 3; Figure 9 Grain boundary TEM characterization results of high-entropy alloys prepared in Example 2; Figure 10 Compressive engineering stress-strain curves of high-entropy alloys prepared in Examples 1, 2, 3 and without carbon addition at 1200℃; Figure 11 Comparison of high temperature yield strength of high-entropy alloys prepared in Examples 1, 2, 3 and without carbon addition with that of several commercial refractory alloys. DETAILED DESCRIPTION

[0021] The present application will be described in detail below with specific embodiments. The following examples will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. The parts not described in detail in the following examples can be realized by using existing technology.

[0022] The existing solutions to the problem of room temperature brittleness, especially tensile brittleness, of refractory high-entropy alloys are difficult to balance high-temperature strength, and cannot obtain good comprehensive mechanical properties, thereby limiting the industrial application of refractory high-entropy alloys. For example, CN111334697A, due to the formation of a too high volume fraction of carbide ceramic phase in the eutectic reaction with the BCC matrix, the high-entropy alloy can only obtain plasticity in compression test at room temperature, and cannot obtain plasticity in tensile test. Again, due to the cracks that have been generated in the compression process being pressed together by the compression stress and thus being difficult to expand, compression is not a sufficient criterion for whether the alloy truly has plasticity, so only when a refractory high-entropy alloy has good high-temperature strength and considerable room temperature tensile plasticity, it has a better practical application prospect. Based on the above problems, the embodiments of the present application provide a carbon micro-alloyed refractory high-entropy alloy to solve the above problems.

[0023] In an embodiment of the present application, a carbon micro-alloyed refractory high-entropy alloy is provided, and the chemical formula of the alloy is Nb a Ta b Hf c W d C e , the atomic percentage of each element a : b : c : d : e = (40% ~ 47.5%) : (40% ~ 47.5%) : (5 ~ 15%) : (0 ~ 5%) : (0.5% ~ 1.5%), and a+b+c+d+e=100, and the C element forms carbide to play a second phase strengthening role.

[0024] Specifically, in the carbon micro-alloyed refractory high-entropy alloy in the embodiments of the present application, the composition range is obtained by Calculation of Phase Diagrams (CALPHAD) assisted alloy composition design. The alloy selects four high-melting-point transition metal elements Nb, Ta, Hf, W and a small amount of non-metallic additive element C. Starting from the excellent plasticity and BCC phase stability of Nb and Ta matrix elements, it is determined that the atomic ratio of Nb and Ta is 1:1 or close to 1:1 in the extended composition interval, combining the qualitative prediction of solid solution strengthening, the high-temperature and room-temperature solid solution strengthening effect of Hf and W elements, and the carbide forming ability of Hf and C, and finally combining the upper limit of Hf, W and C addition (about 15 at.%, 5 at.% and 1.5 at.% respectively) required by phase stability and plasticity, the composition range of the refractory high-entropy alloy in the embodiments of the present application is determined.

[0025] It should be noted that when the atomic percentage of Hf is 0, carbides can also be formed, but the formed is no longer HfC (hafnium carbide), but other carbides such as Nb2C (diniobium carbide), Ta2C (ditantalum carbide), etc. This is because: first, the tendency of each metal element in the alloy of these systems to combine with C to form carbides is ranked from strong to weak as the fourth group (Hf) > the fifth group (Nb, Ta) > the sixth group (W), so in the case of Hf content being 0, only compounds between C and Nb and Ta will be formed. In the Nb-C phase diagram and the Ta-C phase diagram, the high-temperature region is the bimetallic carbide compound, and only in the low-temperature region is the monometallic carbide. Since the electric arc melting technology adopted is a continuous cooling preparation process based on high-temperature solidification and subsequent cooling, it is not a thermodynamic equilibrium, so it tends to form Nb2C and Ta2C. The key is that these two carbides are always easy to grow thick and long needle-shaped in the as-cast refractory alloy, and their crystal structure is a poor hexagonal symmetry, which significantly damages the mechanical properties, especially the plasticity. Therefore, the atomic percentage of Hf is set to 5-15% to form HfC. This carbide is not only easy to grow small, but also has a good cubic structure, so it is more beneficial to performance improvement.

[0026] As shown in Figure 1 After arc melting preparation, sample cutting, mechanical testing and micro characterization, it is verified that the designed refractory high-entropy alloy indeed overcomes the room temperature brittleness based on the self-elimination of C elements and the oxygen and oxide of carbides, has excellent tensile plastic deformation ability at room temperature, and has outstanding high-temperature strength. The total elongation rate reaches 11.4-13.31% in room temperature tensile, the tensile strength reaches 718.9-807.0 MPa, the yield strength reaches 381-432 MPa in 1200℃ compression, and the compression elongation rate can exceed 60%, indicating that carbon micro-alloying successfully suppresses the room temperature brittleness of the alloy without carbon addition. The alloy has excellent room temperature tensile plasticity and high temperature strength, can meet the requirements of aerospace and other ultra-high temperature extreme service, and can be applied to engine combustion chambers, supersonic aircraft nose tips, military weapon parts, etc.

[0027] The above embodiments of the present application comprehensively consider the interaction of various metal elements and non-metal element C, have the most important elements Nb and Ta to balance the room temperature plasticity and the high temperature strength to a certain extent, have the solid solution strengthening of Hf and W elements and the second phase strengthening of carbide to the high temperature strength, and utilize the C element itself and the carbonization to oxygen and oxide, effectively overcome the room temperature brittleness, obtain excellent room temperature tensile plasticity, significantly improve the compression yield strength of the alloy at high temperature, have excellent comprehensive mechanical properties, and meet the service requirements in the super-high temperature environment, and are expected to promote the industrialized application of the carbide strengthened refractory high-entropy alloy in the fields of aerospace, military industry and the like.

[0028] In some specific embodiments of the present application, the alloy takes Nb and Ta as the main matrix elements, takes Hf and W as the solid solution strengthening elements, and takes Hf and C as the carbide forming elements.

[0029] In some specific embodiments of the present application, the alloy has a body-centered cubic (BCC) matrix phase and a carbide dispersed precipitate phase with a face-centered cubic (FCC) structure.

[0030] In some specific embodiments of the present application, the alloy has a cast microstructure morphology of equiaxed crystals and subcrystals, the carbide is relatively fine, and the carbide is discontinuously and dispersedly distributed at the grain boundaries, subgrain boundaries and inside the grains.

[0031] Specifically, the grain boundaries refer to equiaxed grain boundaries, and the inside the grains refers to the inside of the equiaxed crystals. According to the transmission characterization results, the size (width direction) of the carbide is 10-500 nm, and most of them are concentrated in 40-100 nm; the length direction size is about several hundred nanometers.

[0032] In the above embodiments of the present application, the matrix of the alloy is a body-centered cubic (BCC) structure phase, and the MC carbide phase with a face-centered cubic (FCC) structure is dispersedly distributed at the grain boundaries, subgrain boundaries and inside the grains.

[0033] Specifically, MC refers to the type of carbide, that is, one C and one M, and M represents a metal atom.

[0034] It should be noted that in other embodiments, the distribution and morphology of the matrix and the carbide may be different from the above, but the effect of eliminating room temperature oxygen embrittlement and high temperature strengthening can still be achieved.

[0035] Based on the same technical concept, another embodiment of the present application provides a preparation method of the carbon micro-alloyed refractory high-entropy alloy in any of the above embodiments, and continues to refer to Figure 1 As shown in the figure, the method adopts a vacuum arc melting technology, and includes the following steps: S1, weighing the required metal raw material blocks and TaC raw material blocks according to the designed atomic percentage; S2, first placing a titanium ingot in a crucible, then placing the metal raw material blocks and the TaC raw material blocks in the crucible in order of increasing melting point, and performing vacuum arc melting to obtain a master alloy ingot, which is a carbon micro-alloyed refractory high-entropy alloy.

[0036] Specifically, in S1, Nb, Ta, Hf, and W metal raw material blocks and TaC carbide raw material blocks are weighed according to the designed atomic percentage. It should be noted that when the atomic percentage of Hf is 0, the Hf metal raw material block does not need to be weighed. In S2, a titanium ingot is first placed in a crucible, and then each metal block and carbide block is placed in order of increasing melting point. The carbide block is placed in the middle of the raw materials to ensure that the raw material block with the highest melting point is closest to the arc during melting, so that it completely melts at the highest temperature. The specific positions are from bottom to top: Hf→Nb→TaC→Ta→W. The master alloy ingot (small size button ingot) obtained by the preparation method is a carbon micro-alloyed refractory high-entropy alloy that is stretchable at room temperature and has high temperature and high strength.

[0037] It should be noted that since the amount of C added is very small, theoretically, any carbide between carbon and niobium, tantalum, hafnium, and tungsten can be used as raw material, and none of them will cause the addition of excessive amounts of certain metal elements. In the specific experimental process, on the one hand, due to the high purity of TaC, there are few harmful impurities such as oxygen, which is beneficial to the performance of the alloy. On the other hand, the actual chemical formula of all the above carbides generally deviates from the theoretical chemical formula, that is, a large number of metal atoms will replace the C atoms on the sublattice of C, and the ingredients will be difficult to calculate and determine the specific amount of raw materials. However, TaC can achieve an actual Ta:C ratio of about 1:1, making it easier to take materials. In other embodiments, other types of carbides can also be selected for use according to experimental needs.

[0038] In some specific embodiments of the present application, the required metal raw material blocks and TaC raw material blocks are weighed according to the designed atomic percentage, wherein: the carbon element in the alloy is added in the form of a blocky carbide TaC, and the purity of the required metal raw material blocks and TaC raw material blocks is not less than (greater than or equal to) 99.9 wt.%.

[0039] Specifically, to remove impurities on the surface of the metal blocks, surface treatment such as grinding and pickling is performed before weighing, so that the Nb, Ta, Hf, and W metal blocks can minimize impurities under the current original purity. After cleaning with an alcohol solution and drying, a precision electronic balance with a precision of 0.001 g is used to weigh the raw materials to ensure that the ratio of each raw material block is as close to the nominal ratio as possible.

[0040] In order to avoid oxidation of the refractory high-entropy alloy, in some embodiments of the present application, before vacuum arc melting, the following steps are included: vacuum extraction and introduction of protective gas, the vacuum degree is 5.0*10 -3 Pa, and the gas pressure of the protective gas is 0.2-0.5 kPa (such as 0.4 kPa).

[0041] Specifically, because the refractory high-entropy alloy is more easily oxidized at high temperature, before vacuum arc melting, vacuum extraction and introduction of protective argon gas are performed, wherein the vacuum degree is 5.0*10 -3 Pa, the gas pressure of the argon gas is 0.4 kPa, and the purity of the argon gas is 99.999%, thereby effectively preventing oxygen in the environment from entering the alloy during the melting process and avoiding the impact on the performance of the alloy.

[0042] It should be noted that the vacuum degree and the gas pressure of the protective gas are related to the vacuum arc melting furnace used, and in other embodiments, according to the performance of the equipment and considering the oxygen content in the melting environment, the vacuum degree and the gas pressure of the protective gas can also be appropriately adjusted.

[0043] In order to improve the melting quality, in some embodiments of the present application, vacuum arc melting is performed, including: accelerating the element homogenization of the alloy melt by external electromagnetic stirring during the vacuum arc melting process.

[0044] Specifically, in the process of multi-pass melting, external electromagnetic stirring can accelerate the element homogenization of the melt, and ensure that each refractory raw material block is completely melted and uniformly melted.

[0045] In some embodiments of the present application, vacuum arc melting is performed, including: after each alloy button ingot (master alloy ingot) is completely cooled after melting, the alloy button ingot is flipped and repeatedly melted for 8-12 times until there is no visible unmelted particle in the high-temperature melt, and the melting current is slowly reduced until the power is turned off at the end of the last melting, thereby obtaining a master alloy ingot.

[0046] Specifically, in this step, if the current is reduced too fast, too many solidification defects may be caused, or the typical solidification dendritic segregation structure state is maintained. At the end of the last melting, the melting current is slowly reduced until the power is turned off, for example, the current is uniformly reduced from the melting current of 380-420 A, and it takes about 40-60 seconds to reduce to about 100 A or below, and then the power is turned off, which not only can reduce the generation of casting defects, but also can change the as-cast structure of the prepared refractory high-entropy alloy from the typical solidification structure (dendrite) to equiaxed crystal and subcrystal, which is more conducive to the comprehensive mechanical properties.

[0047] In some embodiments of the present application, the melting time is 2-3 min, and the melting current is 380-420 A.

[0048] It should be noted that if the melting time is too short, it is not conducive to melting through and melting evenly, and if the melting time is longer, it may damage the power supply box (i.e. the welding machine) of the electric arc melting, so the melting time is set to 2-3 min. If the melting current is lower than 380A, the entire (or most of) alloy ingot cannot be melted through, only the upper half is in the molten pool state, and the lower half is still solid, which is very unfavorable for the chemical uniformity and organizational uniformity of the entire alloy ingot, and if the melting current exceeds 420A, the excessive agitation of the molten pool will cause too much argon gas to be involved in the Marangoni convection into the molten pool, resulting in subsequent formation of porosity defects, and the excessive current will also damage the power supply box.

[0049] For example, vacuum arc melting is performed, and before each time the raw material block is melted, the titanium ingot is melted 2-3 times to absorb the oxygen that may be left in the melting furnace, so as to avoid oxidation of the alloy during subsequent alloy melting; after the titanium ingot is melted, the raw material block is slowly preheated and then heated and melted to prevent the raw material block from splashing; during the melting process, external electromagnetic stirring is used to accelerate the element homogenization of the alloy melt; after each alloy button ingot is completely cooled after being melted, it is turned over and repeatedly melted 8-12 times, each time for 2-3 min, and the melting current is 380-420 A. Repeated melting can reduce or even avoid the influence of the difference in melting time and melting current on the distribution of alloy elements, the organizational structure and the performance. The melting points of the raw materials used in the embodiments of the present application span a large range (2200-3880 ℃), and the preheating time, melting time, melting current, and electromagnetic stirring intensity need to be comprehensively considered in combination with the performance of the vacuum arc melting furnace and the size of the raw material block, so that each raw material block is fully melted and evenly melted, thereby realizing the composition and organizational uniformity of the final alloy product.

[0050] It should be noted that the melting method can also use water-cooled copper crucible suspension melting, etc. in addition to the above-mentioned vacuum arc melting. The water-cooled copper crucible suspension melting is very similar to the laboratory-level vacuum arc melting in the above-mentioned embodiments of the present application in terms of technology and process, and can be implemented by referring to CN201410482071.X.

[0051] In the above-mentioned embodiments of the present application, the carbon element micro-alloying method is low in cost and simple in process, effectively solves the common room temperature brittleness problem of refractory high-entropy alloys, is conducive to large-scale industrial promotion, and is expected to be applied to service environments in extreme high-temperature environments such as aerospace.

[0052] The preferred features in the above embodiments can be used alone in any embodiment, and can also be used in combination on the premise of not conflicting. In addition, the parts not described in detail in the embodiments can be implemented by using the prior art.

[0053] The present application will be further described in conjunction with specific application examples / comparative examples, so as to better understand the above technical solutions of the present application. It should be understood that the following are only some examples and do not limit the present application.

[0054] To illustrate the overcoming of room temperature oxygen embrittlement of the carbon micro-alloyed room temperature stretchable and high temperature high strength refractory high-entropy alloy, a comparative example is provided.

[0055] Comparative example First, high-purity metal raw material blocks are used for vacuum arc melting to prepare carbon-free micro-alloyed Nb (40%~47.5%) Ta (40%~47.5%) Hf (5%~15%) W (0%~5%) alloy, specifically Nb 44 Ta 44 Hf 10 W2. After the tensile specimen of the alloy is obtained by wire cutting, the alloy exhibits brittleness in room temperature tensile test, without macroscopic yield phenomenon, and premature failure occurs in the macroscopic elastic deformation stage, as shown in Figure 2 . The fracture morphology of the carbon-free micro-alloyed alloy is observed by scanning electron microscopy (SEM), and the grain boundary of the carbon-free micro-alloyed alloy is observed by transmission electron microscopy (TEM), and it can be seen that the room temperature tensile fracture mode of the carbon-free micro-alloyed alloy is intergranular brittle fracture, and a large number of quasi-continuous, coarse, monoclinic hafnium dioxide (m-HfO2) brittle oxides distributed in the grain boundary are the intergranular crack origins, as shown in Figure 3 . The above is the specific form of room temperature oxygen embrittlement of refractory high-entropy alloy when carbon-free micro-alloying occurs.

[0056] High-purity metal raw material blocks and carbide raw material blocks are used for vacuum arc melting to prepare Nb (40%~47.5%) Ta (40%~47.5%) Hf (5%~15%) W (0%~5%) C (0.5%~1.5%) alloy, specifically as in Examples 1-3.

[0057] Example 1 In this example, Nb 43.7 Ta 43.7 Hf 10 W2C 0.6 is prepared.

[0058] Example 2 In this example, Nb 43.6 Ta 43.6 Hf 10 W2C 0.8 is prepared.

[0059] Example 3 In this example, Nb 43.5 Ta 43.5Hf 10 W2C1.

[0060] The phase composition and microstructure of the alloy samples prepared in the above embodiments were analyzed, and the room temperature tensile properties were tested.

[0061] As Figure 4 shown, the X-ray diffraction analysis results show that the refractory high-entropy alloys in the above embodiments of the present application are all based on BCC phase; as Figure 5 , Figure 6 , Figure 7 shown, the optical microscope images after chemical corrosion show that the refractory high-entropy alloys in the above embodiments of the present application all present equiaxed grain and subgrain morphology, and the grain boundary, subgrain boundary and intragranular discontinuous distribution of the BCC matrix phase have carbide phase, and the carbide size is relatively small.

[0062] As Figure 8 shown, the refractory high-entropy alloys after carbon micro-alloying in the above embodiments of the present application all obtain good room temperature tensile plasticity, and eliminate room temperature brittleness, wherein the yield strength of the alloy in Example 1 (Nb 43.7 Ta 43.7 Hf 10 W2C 0.6 ) is 653.3 MPa, the tensile strength is 743.1 MPa, and the total elongation is 12.7%; the comprehensive performance of the alloy in Example 2 (Nb 43.6 Ta 43.6 Hf 10 W2C 0.8 ) is the most excellent, the yield strength is 682.2 MPa, the tensile strength is 807.0 MPa, and the total elongation is 13.31%; the yield strength of the alloy in Example 3 (Nb 43.5 Ta 43.5 Hf 10 W2C1) is 602.7 MPa, the tensile strength is 718.9 MPa, and the total elongation is 11.4%.

[0063] As Figure 9 (a) shown, taking Example 2 as an example, the grain boundary of the carbon micro-alloyed refractory high-entropy alloy was characterized by TEM, and Figure 9 (d-e) selected area electron diffraction (SAED) under two different crystal band axes, combined with Figure 9 (b-c) energy spectrum (EDS) analysis, confirmed that the grain boundary discontinuous phase of the alloy is FCC-MC type carbide, and there is no oxide at the grain boundary, indicating that the phase origin leading to the room temperature oxygen embrittlement of the alloy has been eliminated.

[0064] The sample of the refractory high-entropy alloy in the non-carbon addition state (Nb 44 Ta 44 Hf10 W2), Example 1 (Nb 43.7 Ta 43.7 Hf 10 W2C 0.6 ), Example 2 (Nb 43.6 Ta 43.6 Hf 10 W2C 0.8 ), Example 3 (Nb 43.5 Ta 43.5 Hf 10 W2C1) were tested at 1200℃, as shown in Table 1, the yield strength of the sample without carbon addition was 287 MPa, the yield strength of Example 1 sample was 381 MPa, the yield strength of Example 2 sample was 432 MPa, and the yield strength of Example 3 sample was 419 MPa. It can be seen that the carbides formed by carbon micro-alloying significantly improve the high-temperature compression yield strength of refractory high-entropy alloys, and the highest increase can reach 50.5%. Moreover, the high-temperature deformation softening resistance (i.e. the descending rate of the compression engineering stress-strain curve in the plastic deformation section) of Example 1 and Example 2 is significantly better than that of the sample without carbon addition. Figure 10 The 1200℃ compression yield strength data of the refractory high-entropy alloy sample without carbon addition, Example 1, Example 2 and Example 3 were compared with the yield strength data of many traditional refractory alloys at high temperature, as shown in Table 2, although the 1200℃ yield strength of a small part of traditional refractory alloys such as WC-3009 is better than that of the sample without carbon addition, but after carbon micro-alloying, the 1200℃ yield strength of the refractory high-entropy alloy in the embodiments of the present application is better than that of all traditional refractory alloys.

[0065] Figure 11

[0066] In summary, based on the application requirements and technical bottlenecks of refractory high-entropy alloys, the above-mentioned embodiments of the present application use phase diagram calculation to assist alloy composition design, consider the properties of each element itself and the interaction between them, and provide a carbon micro-alloyed refractory high-entropy alloy with excellent room temperature tensile plasticity and high temperature strength and a preparation method thereof. The alloy eliminates the room temperature brittleness in the state without carbon addition, obtains excellent room temperature tensile plasticity, and the total elongation is more than 10%. At the same time, the alloy has more excellent high temperature strength than the alloy without carbon addition, the compression yield strength at 1200℃ is 381~432 MPa, and is better than most traditional refractory alloys. The above-mentioned embodiments of the present application solve the problem of room temperature brittleness of refractory high-entropy alloys, obtain good room temperature tensile plasticity, and at the same time improve the compression yield strength at high temperature. The alloy is prepared by vacuum arc melting, and the cost of carbon micro-alloying is low and the operation is simple, which is beneficial to industrial large-scale promotion, and is expected to be applied to aerospace and other extreme service environments. ​​

[0067] The above describes some specific embodiments of the present application. It needs to be understood that the present application is not limited to the above specific embodiments, and various modifications or changes can be made by those skilled in the art within the scope of the claims, which does not affect the essence of the present application. The above preferred features can be used in any combination as long as they are not in conflict with each other.

Claims

1. A carbon microalloyed refractory high-entropy alloy, characterized in that, The chemical formula of the alloy is Nb a Ta b Hf c W d C e , the atomic percentage of each element a:b:c:d:e=(40%-47.5%):(40%-47.5%):(5-15%):(0-5%):(0.5%-1.5%), and a+b+c+d+e=100, the C element forms carbide to play a second phase strengthening effect.

2. The carbon microalloyed refractory high-entropy alloy of claim 1, wherein, The alloy takes Nb and Ta as matrix elements, Hf and W as solid solution strengthening elements, and Hf and C as carbide forming elements.

3. The carbon microalloyed refractory high-entropy alloy of claim 1, wherein, The alloy has a body-centered cubic matrix phase and a carbide dispersed precipitated phase with a face-centered cubic structure.

4. The carbon microalloyed refractory high-entropy alloy of claim 1, wherein, The alloy presents a cast microstructure morphology of equiaxed crystals and subcrystals, and the carbide is discontinuously and dispersedly distributed at grain boundaries, subgrain boundaries and within grains.

5. A method of producing the carbon microalloyed refractory high-entropy alloy according to any one of claims 1 to 4, characterized in that, The method comprises the following steps: The required metal raw material blocks and TaC raw material blocks are weighed according to the designed atomic percentage; The titanium ingot is first placed in a crucible, and then the metal raw material blocks and the TaC raw material blocks are sequentially placed in the crucible according to the order of the melting points from low to high, and vacuum arc melting is performed to obtain a master alloy ingot, which is a carbon micro-alloyed refractory high-entropy alloy.

6. The method of claim 5, wherein the carbon microalloyed refractory high-entropy alloy is characterized by, The required metal raw material blocks and TaC raw material blocks are weighed according to the designed atomic percentage, and the purity of the metal raw material blocks and the TaC raw material blocks is not less than 99.9 wt.%.

7. The method of claim 5, wherein the carbon microalloyed refractory high-entropy alloy is characterized by, Before the vacuum arc melting, including: vacuum extraction and into the protective gas, the vacuum degree is 5.0 x 10 -3 Pa, the gas pressure of the protective gas is 0.2-0.5 kPa.

8. The method of claim 5, wherein the carbon microalloyed refractory high-entropy alloy is characterized by, The vacuum arc melting comprises the following steps: accelerating the element homogenization of the alloy melt by external electromagnetic stirring during the vacuum arc melting.

9. The method of claim 5, wherein the carbon microalloyed refractory high-entropy alloy is characterized by, The vacuum arc melting comprises the following steps: after each alloy button ingot is completely cooled after being melted, the alloy button ingot is turned over, and the alloy button ingot is repeatedly melted for 8-12 times until no visible unmelted particles are present in the high-temperature melt, and the melting current is slowly reduced until the melting is stopped to obtain the master alloy ingot.

10. The method of claim 9, wherein the carbon microalloyed refractory high-entropy alloy is characterized by, The melting time is 2-3 min, and the melting current is 380-420 A.

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