A method for improving the strength, plasticity and corrosion resistance of high entropy alloys

Abstract

The present application provides a method for improving the strength, plasticity and corrosion resistance of high entropy alloys, comprising the following steps: performing multiple friction stir processing on a high entropy alloy matrix to obtain an equiaxed structure on the high entropy alloy matrix, wherein the equiaxed structure has 15% to 50% twins; the proportion of grain boundaries with a mismatch angle of not less than 15 degrees in the equiaxed structure is more than 85%. The present application can effectively improve the strength, plasticity and corrosion resistance of high entropy alloys. The method of the present invention has a short process, and a large piece of fine-grained or even ultra-fine-grained structure with a high proportion of twins can be directly obtained from the as-cast structure by friction stir processing. The method of the present invention is simple, efficient, low-cost, and highly automated, and is very suitable for the industrial production and application of high-performance new materials.

Description

Technical Field

[0001] The present application relates to the field of material technology processing, and specifically to a method for improving the strength, plasticity and corrosion resistance of high entropy alloys. Background Art

[0002] With the development and progress of technology, human beings have been applying materials more and more deeply, and the requirements for materials have been continuously improved. Traditional metal materials can no longer meet the application in engineering. Therefore, it is urgent to break through the traditional metal materials and develop new types of alloys to meet engineering applications. As a new type of material, high entropy alloys have a unique solid solution structure that makes them exhibit four effects different from traditional alloys: high entropy effect, lattice distortion effect, slow diffusion effect and "cocktail" effect. As a result, they show a series of excellent properties such as excellent mechanical properties, radiation resistance, corrosion resistance, and oxidation resistance, which has attracted widespread attention from scholars. Therefore, high entropy alloys show potential application prospects in hydrogen storage materials, radiation-resistant materials, and thermoelectric materials.

[0003] However, the application of cast high entropy alloys in the prior art is significantly limited due to their low yield strength and poor corrosion resistance, and the methods of improving the performance of the material in the prior art cannot effectively ensure the strength, plasticity and corrosion resistance of the material.

[0004] Therefore, the present invention provides a method for improving the strength, plasticity and corrosion resistance of high entropy alloys, which can effectively improve the strength, plasticity and corrosion resistance of high entropy alloys. Summary of the invention

[0005] Therefore, the technical problem to be solved by the present application is to provide a method for improving the strength, plasticity and corrosion resistance of high entropy alloys, which can effectively improve the strength, plasticity and corrosion resistance of high entropy alloys.

[0006] In order to solve the above problems, the present application provides a method for improving the strength, plasticity and corrosion resistance of a high entropy alloy, comprising the following steps:

[0007] The high entropy alloy substrate is subjected to multiple friction stir processing to obtain an equiaxed structure on the high entropy alloy substrate, wherein the equiaxed structure has 15% to 50% of twins; the proportion of grain boundaries with a mismatch angle of not less than 15 degrees in the equiaxed structure is more than 85%, and a high proportion of twin boundaries combined with high-angle grain boundaries can simultaneously improve the strength, plasticity and corrosion resistance of the high entropy alloy.

[0008] Furthermore, in the equiaxed structure, the grain size is 0.2μm to 5μm. Such a fine grain size can greatly increase the strength of the high-entropy alloy, and the fine grain size combined with the 15% to 50% high proportion of twins and more than 85% high-angle grain boundaries (mismatch angle is not less than 15 degrees) in the equiaxed structure through multiple stir friction processing can further greatly improve the plasticity and corrosion resistance of the high-entropy alloy, thereby further greatly improving the strength, plasticity and corrosion resistance of the high-entropy alloy.

[0009] Furthermore, the preparation method of the high entropy alloy matrix includes the following steps: welding at least two high entropy alloy substrates to each other to form a high entropy alloy matrix, and the welding method is a stir friction welding method. The welding method has the same process principle and process steps as the stir friction processing, and can be implemented on the same equipment. The welding method can weld small plates into large-size plates, thereby breaking through the size limitation of the high entropy alloy plate, and the microstructure of the stir friction welding weld zone is consistent with the structure of the stir friction processing zone.

[0010] Furthermore, during the multiple-pass friction stir processing of the high-entropy alloy substrate, an inert gas is used to protect and cool the high-entropy alloy substrate. Since the high-entropy alloy and the stirring tool generate heat due to intense friction during the friction stir processing, the high-entropy alloy is prone to oxidation and grain coarsening when deformed at high temperature. Therefore, this process can avoid oxidation of the high-entropy alloy and cool the high-entropy alloy in time, thereby avoiding excessive coarsening of the grains at high temperature.

[0011] Furthermore, the tool used in the multi-pass stir friction processing of the high entropy alloy matrix is ​​a stirring tool; the material of the stirring tool is any one of metal ceramics, tungsten-based alloys, cubic boron nitride, and cemented carbide with high hardness and good wear resistance, so as to ensure that the stirring tool is not severely worn and fails during the intense friction between the stirring tool and the high-entropy alloy with high strength and hardness.

[0012] Furthermore, the stirring tool includes a shoulder and a stirring needle; the diameter of the shoulder is 10 to 20 mm. Since an excessively large tool shoulder size can easily lead to excessive heat input during the stir friction processing of the stirring tool, the high-temperature formability of the high-entropy alloy is poor and the grains are excessively coarsened; while an excessively small tool shoulder size leads to too low heat generation during the stir friction processing, and the material to be processed cannot be fully softened, and thus processing cannot be carried out. Only a suitable shoulder size can ensure the effective forming of the high-entropy alloy and a smaller grain size.

[0013] Further, the high entropy alloy substrate is subjected to a multi-pass friction stir processing, including the following steps: the rotation speed of the stirring tool is 100 to 800 rpm; and / or the travel speed of the friction stir processing is 25 to 300 mm / min. Since the high entropy alloy has a high hardness and a large friction coefficient, it is very easy to overheat at a high speed during the friction stir processing, resulting in failure to form. Therefore, compared with the friction stir processing range of traditional aluminum alloys and magnesium alloys, the rotation speed and travel speed parameter range is smaller. Only within a smaller suitable rotation speed and travel speed parameter range can the high entropy alloy be effectively formed.

[0014] Furthermore, the width of the overlap zone between two adjacent friction stir processing passes is 1 / 3 to 2 / 3 of the width of each friction stir processing pass. If the overlap zone width is too small, there will be an unprocessed area between the two passes, and the performance of the local unprocessed area is too low to be used. If the overlap zone width is too large, the structure of a certain processing area will be affected by the heat of subsequent multiple processing passes, that is, if the overlap zone width is too small, it is easy to cause the entire processing area to be affected by multiple heat accumulations, thereby causing a significant increase in grain size, a significant impact on the ratio of twin boundaries and high-angle grain boundaries (mismatch angle is not less than 15 degrees), and resulting in too low production efficiency. Therefore, the width of the overlap zone must be within an appropriate range to ensure the structure required in this aspect, thereby improving the performance of the high entropy alloy.

[0015] Furthermore, after multiple friction stir processing, the yield strength of the high entropy alloy obtained is 500-1000 MPa, the elongation is 20-55%, and the corrosion current density is less than 0.18 μA / cm 2 , the corrosion potential is greater than -0.25V.

[0016] Furthermore, the high entropy alloy matrix is ​​a CoCrFeNi-based high entropy alloy. Due to its lower stacking fault energy, the CoCrFeNi-based high entropy alloy is more likely to form a high proportion of twin boundaries during multi-pass friction stir processing, which is more conducive to improving the strength, plasticity and corrosion resistance of the high entropy alloy.

[0017] The present application provides a method for improving the strength, plasticity and corrosion resistance of a high entropy alloy. The alloy obtained in the present application is a large-sized block fine grain with both high strength, plasticity and corrosion resistance.

[0018] The advantages of the present invention are:

[0019] The present application is a method for improving the strength, plasticity and corrosion resistance of a large-sized bulk fine-grained high-entropy alloy with both high strength, plasticity and corrosion resistance. The preparation process is as follows: the present application adopts a method combining stir friction welding and stir friction processing to stir friction weld the cast high-entropy alloy, and then stir friction processes the welded alloy for multiple passes, thereby obtaining a large-sized bulk high-entropy alloy with a fine-grained structure with a high proportion of twin boundaries. Grain refinement and a high proportion of twin boundaries improve the strength and plasticity matching of high-entropy alloys, while grain refinement improves the density and stability of the passivation film. The high proportion of twin boundaries, as low-coincidence position lattice grain boundaries, have low interfacial energy, which can reduce the sensitivity to corrosion, thereby improving the corrosion resistance of high-entropy alloys. Therefore, the present invention combines the advantages of welding and large plastic deformation, can break through the size limitation of high entropy alloy plates, and obtain large-size block fine-grained high entropy alloys that simultaneously improve the mechanical properties and corrosion resistance of cast high entropy alloys. The preparation method has an extremely short process, and large-block fine-grained or even ultra-fine-grained structures can be directly obtained from the cast structure by one-time welding + processing. It is simple and efficient, low cost, and highly automated, and is very suitable for the industrial production and application of high-performance new materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a preparation flow chart for improving the strength, plasticity and corrosion resistance of high entropy alloys.

[0021] Figure 2 It is an actual macroscopic morphology diagram of friction stir welding and multi-pass friction stir processing in Example 1;

[0022] Figure 3 This is the microstructure diagram of the high entropy alloy processing area obtained by using the method of the present application to improve the strength, plasticity and corrosion resistance of high entropy alloys in Example 1.

[0023] Figure 4 It is the tensile curve of the high entropy alloy processing area and the cast high entropy alloy prepared by the method of the present application for improving the strength, plasticity and corrosion resistance of high entropy alloy in Example 1.

[0024] Figure 5 The electrochemical curves of the high entropy alloy processing area and the cast entropy alloy prepared by the method of the present application for improving the strength, plasticity and corrosion resistance of the high entropy alloy. DETAILED DESCRIPTION

[0025] According to the embodiment of the present application, Figure 1-5As shown, a method for improving the strength, plasticity and corrosion resistance of a high entropy alloy comprises the following steps: performing multiple friction stir processing on a high entropy alloy substrate to obtain an equiaxed structure on the high entropy alloy substrate, wherein the equiaxed structure has 15% to 50% twins; the proportion of grain boundaries with a mismatch angle of not less than 15 degrees in the equiaxed structure is more than 85%. A high proportion of twin boundaries combined with high-angle grain boundaries can simultaneously improve the strength, plasticity and corrosion resistance of the high entropy alloy.

[0026] The present application is a method for improving the strength, plasticity and corrosion resistance of a large-sized block fine-grained high-entropy alloy with both high strength, plasticity and corrosion resistance. The preparation process is as follows: The present application adopts a method combining stir friction welding and stir friction processing to stir friction weld the cast high-entropy alloy, and then stir friction processes the welded alloy for multiple passes, thereby obtaining a large-sized block high-entropy alloy with a fine-grained structure with a high proportion of twin boundaries. The manufacturing process is as follows: Figure 1 As shown. Grain refinement and a high proportion of twin boundaries improve the strength-plasticity matching of high-entropy alloys, while grain refinement improves the density and stability of the passivation film. A high proportion of twin boundaries, as lattice grain boundaries with low overlap positions, have low interfacial energy, which can reduce the sensitivity to corrosion, thereby improving the corrosion resistance of high-entropy alloys. Therefore, the present application has the advantages of both welding and large plastic deformation, can break through the size limitations of high-entropy alloy plates, and obtain large-size bulk fine-grained high-entropy alloys that simultaneously improve the mechanical properties and corrosion resistance of cast high-entropy alloys. The preparation method has an extremely short process, and large-scale fine-grained or even ultra-fine-grained structures can be obtained directly from the cast structure by welding + processing once. It is simple and efficient, low cost, and highly automated, and is very suitable for the industrial production and application of high-performance new materials.

[0027] This application uses a traditional stirring tool with a stirring needle, and uses stir friction welding to first connect small plates into large plates, and then perform multiple stir friction processing on the large plates. Since it is a multiple-pass stir friction processing, each additional pass is equivalent to an annealing heat input, so this application can obtain fine grains, a high proportion of high-angle grain boundaries and a high proportion of twins after stir friction welding + multiple-pass stir friction processing, thereby changing the strong plasticity and corrosion resistance. The stir friction welding and stir friction processing used in this application use the same tools and the same processing process. In this way, the stir friction tool in this application can not only connect small-sized plates into large-sized plates, but also improve the strong plasticity and corrosion resistance of the noble alloy.

[0028] The present application also discloses some embodiments, in which the grain size is 0.2 μm to 5 μm in the equiaxed structure. At this grain size, such a fine grain size can greatly increase the strength of the high entropy alloy, and the fine grain size combined with the 15% to 50% high proportion of twins and more than 85% of high-angle grain boundaries (mismatch angle is not less than 15 degrees) in the multi-pass stir friction processing equiaxed structure can further greatly improve the plasticity and corrosion resistance of the high entropy alloy, thereby further greatly improving the strength, plasticity and corrosion resistance of the high entropy alloy.

[0029] The present application also discloses some embodiments, and the preparation method of the high entropy alloy matrix comprises the following steps: at least two high entropy alloy substrates are welded to each other to form a high entropy alloy matrix, and the welding method is a stir friction welding method. That is, the present application adopts a method combining stir friction welding and stir friction processing, and the welding method has the same process principle and process steps as the stir friction processing, and can be implemented on the same equipment, and the welding method can weld small plates into large-sized plates, thereby breaking through the size limit of the high entropy alloy plate, and the microstructure of the stir friction welding weld zone is consistent with the structure of the stir friction processing zone. The cast high entropy alloy is subjected to stir friction welding, and then the welded alloy is subjected to multiple stir friction processing, and the coarse cast structure is transformed into a fine equiaxed fine-grained structure with a high proportion of twin boundaries, and a large-sized block fine-grained high entropy alloy is prepared. The processing area is subjected to room temperature tensile test and electrochemical corrosion test to obtain excellent strength-plasticity matching and corrosion resistance.

[0030] The present application also discloses some embodiments, in which inert gas protection is used and the high entropy alloy substrate is cooled during the multi-pass friction stir processing of the high entropy alloy substrate. Argon gas protection is performed during the friction stir processing process to prevent surface oxidation on the one hand and reduce the temperature of the processing area on the other hand. Since the high entropy alloy and the stirring tool generate heat due to intense friction during the friction stir processing, the high entropy alloy is prone to oxidation and grain coarsening when deformed at high temperature, so this process can avoid oxidation of the high entropy alloy and timely cool the high entropy alloy, which can avoid excessive coarsening of the grains at high temperature.

[0031] The present application also discloses some embodiments, wherein the tool used in the multi-pass friction stir processing of the high entropy alloy substrate is a stirring tool; the material of the stirring tool is any one of metal ceramics, tungsten-based alloys, cubic boron nitride, and cemented carbide. The tungsten-based alloy is any one of W-Re or W-La2O3. The material of the stirring tool is any one of metal ceramics, tungsten-based alloys, cubic boron nitride, and cemented carbide with high hardness and good wear resistance, so as to ensure that the stirring tool is not severely worn and ineffective during the intense friction between the stirring tool and the high-entropy alloy with high strength and hardness.

[0032] The present application also discloses some embodiments, wherein the stirring tool comprises a shoulder and a stirring needle; the diameter of the shoulder is 10 to 20 mm. Since an excessively large tool shoulder size may easily lead to excessive heat input during the friction stir processing of the stirring tool, the high-temperature formability of the high-entropy alloy is poor and the grains are excessively coarsened; while an excessively small tool shoulder size may lead to too low heat generation during the friction stir processing, and the material to be processed cannot be fully softened, so that the processing cannot be performed. Only by using the shoulder size of the above size can the high-entropy alloy be effectively formed and have a smaller grain size.

[0033] The present application also discloses some embodiments, wherein a multi-pass friction stir processing of a high entropy alloy substrate comprises the following steps: the rotation speed of the stirring tool is 100 to 800 rpm; and / or the travel speed of the friction stir processing is 25 to 300 mm / min. Due to the high hardness and friction coefficient of the high entropy alloy, it is very easy to overheat at a high speed during the friction stir processing, resulting in failure to form, and a higher requirement for the rotation speed and travel speed parameter range is required. Only within the above rotation speed and travel speed parameter range can the high entropy alloy be effectively formed and the beneficial effects of the present application be achieved.

[0034] The present application also discloses some embodiments, in which the width of the overlap zone between two adjacent friction stir processing processes is 1 / 3 to 2 / 3 of the width of each friction stir processing process. If the overlap zone width is too small, there will be an unprocessed area between the two passes, and the performance of the local unprocessed area is too low to be applied. If the overlap zone width is too large, the organization of a certain processing area will be affected by the heat of subsequent multiple processing passes, that is, if the overlap zone width is too small, it is easy to cause the entire processing area to be affected by multiple heat accumulations, thereby causing a significant increase in grain size, a significant impact on the ratio of twin boundaries and high-angle grain boundaries (mismatch angle is not less than 15 degrees), and causing too low production efficiency. Therefore, the width of the overlap zone must be within the above range to ensure the organization required by the present application, thereby improving the performance of the high entropy alloy.

[0035] Furthermore, the yield strength of the cast high entropy alloy is between 100 and 300 MPa, the elongation is between 5-45%, the corrosion current density is greater than 0.3 μA / cm2, and the corrosion potential is less than -0.3 V. The yield strength of the multi-pass processing area is between 500 and 1000 MPa, the elongation is between 20-55%, the corrosion current density is less than 0.18 μA / cm2, and the corrosion potential is greater than -0.25 V. The smaller the corrosion current density and the greater the corrosion potential of the high entropy alloy, the higher the corrosion resistance of the high entropy alloy. Therefore, after multi-pass friction stir processing, the strong plasticity and corrosion resistance are greatly improved.

[0036] Furthermore, the high entropy alloy matrix is ​​a CoCrFeNi-based high entropy alloy. Due to its lower stacking fault energy, the CoCrFeNi-based high entropy alloy is more likely to form a high proportion of twin boundaries during multi-pass friction stir processing, which is more conducive to improving the strength, plasticity and corrosion resistance of the high entropy alloy.

[0037] The present application also discloses some embodiments, wherein the yield strength of the high entropy alloy obtained after multiple friction stir processing is 500-1000 MPa, the elongation is 20-55%, and the corrosion current density is less than 0.18 μA / cm 2 , the corrosion potential is greater than -0.25V. The high entropy alloy obtained by this application can greatly improve the strength, plasticity and corrosion resistance.

[0038] The fine-grained or ultrafine-grained high-entropy alloy obtained in the present application has a twin boundary ratio between 15% and 50%, a high-angle grain boundary ratio above 85%, and a higher twin boundary ratio and a high-angle grain boundary ratio than high-entropy alloys prepared by other processing methods.

[0039] The high entropy alloy of the present application has excellent strength-plasticity matching and corrosion resistance. The yield strength of the cast alloy is between 100 and 300 MPa, the elongation is between 5 and 45%, and the corrosion current density is greater than 0.3 μA / cm 2 , the corrosion potential is less than -0.3V. The yield strength of the multi-pass processing area is between 500 and 1000MPa, the elongation is between 20-55%, and the corrosion current density is less than 0.18μA / cm 2 , the corrosion potential is greater than -0.25 V. Compared with the cast high entropy alloy, the yield strength of the multi-pass processing area is increased by at least one time, and the excellent elongation is maintained. The multi-pass processing area has a higher corrosion potential and a lower corrosion current density, which solves the problem of low strength and poor corrosion performance of the cast high entropy alloy.

[0040] The preparation process of the method of the present application is simple. The ingot can be prepared into a large-sized fine-grained or ultrafine-grained high-entropy alloy component by combining simple stir friction welding with multi-pass stir friction processing. Compared with other fine-grained preparation methods, it has the advantages of high efficiency, short process, low cost and the ability to produce large-sized plates, and is suitable for industrial production.

[0041] Example 1

[0042] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiMn high entropy alloy;

[0043] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.7 mm and a shoulder diameter of 12 mm.

[0044] Processing process: The above-mentioned stirring needle is used to stir friction weld two pieces of cast alloy at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two pieces of cast alloy into a high entropy alloy matrix, i.e., a large-sized plate. The large-sized plate after welding is subjected to multiple stir friction processing using the same processing parameters, and the overlap zone width is 1 / 2; a large-sized block high entropy alloy with a grain size of 1.9 microns, a high-angle grain boundary ratio of 91% and a twin boundary ratio of 19% is obtained. The specific macroscopic morphology and processing zone organization diagram are shown as follows: Figure 2 and Figure 3 shown.

[0045] Technical effect: Figure 4 As shown, the large-size block high entropy alloy obtained by the method of combining stir friction welding and stir friction processing in this embodiment has a yield strength of 539MPa, a tensile strength of 722MPa, and an elongation of 41%. In addition, in the electrochemical experiment carried out in 3.5% NaCl solution at room temperature, its corrosion potential is -0.25V, and the self-corrosion current density is 0.1μA / cm 2 .

[0046] Example 2

[0047] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiMn high entropy alloy;

[0048] Processing process: Two pieces of cast alloys are welded into a high entropy alloy matrix, i.e., a large-sized plate, by using a vacuum electron beam. The parameters of the vacuum electron beam are: voltage 125KV, current 6mA, and welding speed 9.5mm / s. The large plate after electron beam welding is subjected to multiple friction stir processing. The friction stir processing tool is made of metal ceramic material. The stirring needle of the stirring tool has a conical shape, a needle length of 1.7mm, and a shoulder diameter of 12mm. The friction stir processing parameters are: the rotation speed is 400rpm, the welding speed is 75mm / min, and the overlap zone width is 2 / 3; a large-sized block high entropy alloy with a grain size of 2.5 microns, a high-angle grain boundary ratio of 91% and a twin boundary ratio of 28% is obtained.

[0049] Technical effect: The large-size block high entropy alloy obtained by the method of combining vacuum electron beam welding with multi-pass friction stir processing in this embodiment has a yield strength of 520MPa, a tensile strength of 720MPa, and an elongation of 39%. In addition, in the electrochemical experiment carried out in 3.5% NaCl solution at room temperature, the corrosion current density is 0.07μA / cm2 and the corrosion potential is -0.21V.

[0050] Comparative Example 1

[0051] The matrix material of this comparative example is a 2 mm thick original cast CoCrFeNiMn high entropy alloy plate, and the cast high entropy alloy plate is not subjected to any other processing.

[0052] Test results: The mechanical properties and corrosion resistance of the 2mm thick cast CoCrFeNiMn high entropy alloy were tested. The yield strength of the cast CoCrFeNiMn high entropy alloy is 194MPa, the tensile strength is 323MPa, and the elongation is 14%. Its corrosion potential is -0.31V, and the self-corrosion current density is 0.37μA / cm2.

[0053] Technical effects and analysis of Example 1, Example 2 and Comparative Example 1: The coarse structure and casting defects of the cast alloy lead to its lower strength and poor corrosion resistance. In contrast, the large-sized block high-entropy alloy obtained by the method of combining stir friction welding and stir friction processing in Example 1, and the large-sized block high-entropy alloy obtained by combining vacuum electron beam welding and multi-pass stir friction processing in Example 2 have an equiaxed fine-grained structure with a high proportion of high-angle grain boundaries and twin boundaries, thereby significantly improving the strength and plasticity of the alloy at the same time. In addition, grain refinement improves the density and stability of the passivation film, and a high proportion of twin boundaries, as lattice grain boundaries with low overlap positions, have low interfacial energy, which can reduce the sensitivity to corrosion, thereby improving the corrosion resistance of the high-entropy alloy. It shows that after splicing small plates into large-sized plates by different welding methods, and then using multiple stir friction processing, the effect of the present application can be achieved.

[0054] Example 3

[0055] The matrix material of this embodiment is two 1.2 mm thick cast CoCrFeNiAl 0.3 High entropy alloys;

[0056] Processing tool: stirring needle, the shape of the stirring needle is conical, the stirring needle is made of W-Re alloy, the needle length of the stirring needle is 1.0 mm, and the shoulder diameter is 20 mm.

[0057] Processing process: stir friction welding is performed on two pieces of cast alloy at a rotation speed of 100rpm and a welding speed of 25mm / min to weld the two pieces of cast alloy into a high entropy alloy matrix, i.e., a large-sized plate. The large-sized plate after welding is processed multiple times using the same processing parameters, and the overlap width is 1 / 2. A large-sized block high entropy alloy with a grain size of 0.3 microns, a high-angle grain boundary ratio of 89% and a twin boundary ratio of 15% is obtained.

[0058] Technical effect: This embodiment obtains a yield strength of 1000 MPa and maintains an elongation of 20%. In addition, in an electrochemical experiment conducted in a 3.5% NaCl solution at room temperature, the corrosion current density is 0.15 μA / cm2 and the corrosion potential is -0.24V.

[0059] Comparative Example 2

[0060] The matrix material of this comparative example is two 1.2 mm thick cast CoCrFeNiAl 0.3 High entropy alloys;

[0061] Processing tool: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.0 mm and a shoulder diameter of 20 mm.

[0062] Processing process: Two pieces of cast alloy were friction stir welded at a rotation speed of 50 rpm and a welding speed of 10 mm / min.

[0063] Technical effect and analysis: It is impossible to obtain a defect-free weld with the above-mentioned lower rotation speed. This shows that although lowering the heat input can obtain better performance, too low heat input will result in the failure of the weld to form. Therefore, maintaining a reasonable matching of rotation speed and welding speed is an important factor in obtaining excellent performance of large-sized bulk high-entropy alloys.

[0064] Comparative Example 3

[0065] The matrix material of this comparative example is two 1.2 mm thick cast CoCrFeNiAl 0.3 High entropy alloys;

[0066] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.0 mm and a shoulder diameter of 25 mm.

[0067] Processing process: Two pieces of cast alloy were friction stir welded at a rotation speed of 100 rpm and a welding speed of 25 mm / min.

[0068] Technical effect and analysis: Too large a shoulder size leads to excessive heat generation at the shoulder, resulting in obvious tunnel defects in the processing area. Therefore, maintaining a reasonable shoulder size is an important factor in obtaining excellent performance of large-size bulk high-entropy alloys.

[0069] Example 4

[0070] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiMn high entropy alloy;

[0071] Processing tools: The stirring tool is made of W-Re alloy, the stirring needle of the stirring tool is 1.7mm long and the shoulder diameter is 10mm.

[0072] Processing process: The two cast alloys were subjected to friction stir welding at a rotation speed of 800 rpm and a welding speed of 300 mm / min to weld the two cast alloys into a high entropy alloy matrix, i.e., a large-sized plate. The large-sized plate after welding was subjected to multiple processing with the same processing parameters, and the overlap width was 1 / 2. A large-sized block high entropy alloy with a grain size of 4 microns, a high-angle grain boundary ratio of 93% and a twin boundary ratio of 50% was obtained.

[0073] Technical effect: This embodiment obtains a yield strength of 500 MPa and maintains an elongation of 55%. In addition, in an electrochemical experiment conducted in a 3.5% NaCl solution at room temperature, the corrosion current density is 0.06 μA / cm2 and the corrosion potential is -0.13V.

[0074] Comparative Example 4

[0075] The matrix material of this comparative example: two 2 mm thick cast CoCrFeNiMn high entropy alloys;

[0076] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.7 mm and a shoulder diameter of 10 mm.

[0077] Processing process: Two pieces of cast alloy were friction stir welded at a rotation speed of 1000 rpm and a welding speed of 400 mm / min to obtain a large-sized plate. The large-sized plate after welding was processed multiple times using the same processing parameters, and the overlap width was 1 / 2;

[0078] Technical effect: The yield strength of the processed area is only 300MPa, and the tensile strength is 490MPa, which is not significantly improved compared with the parent material strength.

[0079] Technical effect and analysis: Excessive heat input leads to coarse grain size and does not significantly improve the mechanical properties of the material. Therefore, maintaining a reasonable rotation speed and welding speed match is an important factor in obtaining excellent performance of large-size bulk high-entropy alloys.

[0080] Comparative Example 5

[0081] The matrix material of this comparative example is: two 2 mm thick cast CoCrFeNiMn high entropy alloys;

[0082] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.7 mm and a shoulder diameter of 8 mm.

[0083] Processing process: Two pieces of cast alloy were friction stir welded at a rotation speed of 800 rpm and a welding speed of 300 mm / min to obtain large-sized plates. The large-sized plates after welding were processed in multiple passes using the same processing parameters, and the width of the overlap area was 1 / 2;

[0084] Technical effect and analysis: Too small shoulder size leads to insufficient heat generation in the shoulder, and low heat input cannot obtain a defect-free processing area. Therefore, maintaining a reasonable shoulder size is an important factor in obtaining excellent performance of large-size bulk high-entropy alloys.

[0085] Example 5

[0086] The matrix material of this embodiment is: two pieces of 1.5 mm thick cast CoCrFeNiMn high entropy alloy;

[0087] Processing tools: The stirring tool is made of metal ceramics. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.2 mm and a shoulder diameter of 12 mm.

[0088] Processing process: The above-mentioned stirring tool is used to stir friction weld two cast alloys at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two cast alloys into a high-entropy alloy matrix, i.e., a large-size plate. The large-size plate after welding is subjected to multiple stir friction processing using the same processing parameters, and the width of the overlap zone is 1 / 3; a large-size block high-entropy alloy with a grain size of 1.3 microns, a high-angle grain boundary ratio and a twin boundary ratio of 92% and 20% respectively is obtained.

[0089] Technical effect: The large-sized block high entropy alloy obtained by the method of combining stir friction welding and stir friction processing in this embodiment can still obtain a uniform processing area when the overlap area width is 1 / 3. Its yield strength is 560MPa, tensile strength is 750MPa, and the elongation is maintained at 36%. In addition, in the electrochemical experiment carried out in 3.5% NaCl solution at room temperature, the corrosion current density is 0.09μA / cm2 and the corrosion potential is -0.23V.

[0090] Comparative Example 6

[0091] The matrix material of this embodiment is: two pieces of 1.5 mm thick cast CoCrFeNiMn high entropy alloy;

[0092] Processing tool: The stirring tool is made of metal ceramic alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.2 mm and a shoulder diameter of 12 mm.

[0093] Processing process: The above stirring tool is used to stir friction weld two cast alloys at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two cast alloys into a high entropy alloy matrix, i.e., a large-sized plate. The large-sized plate after welding is subjected to multiple friction stir processing using the same processing parameters. The overlap zone width is 1 / 4; the overlap zone grain size is 1.3 microns, and the high-angle grain boundary ratio and twin boundary ratio are 91% and 19% respectively. However, the non-overlap zone structure does not change and is still a cast structure.

[0094] Technical effect: The width of the overlap area is too small, resulting in a shallow processing depth between the two passes, and the effective processing area depth cannot be obtained, resulting in an unprocessed area between the two passes, and the local performance of the unprocessed area is too low.

[0095] Technical effect and analysis: Too small an overlap area leads to insufficient depth of the processing area, and there are local unprocessed areas, which cannot be applied. Therefore, a reasonable overlap area size is an important factor in obtaining the depth of the processing area.

[0096] Example 6

[0097] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiMn high entropy alloy;

[0098] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.7 mm and a shoulder diameter of 12 mm.

[0099] Processing process: The above-mentioned stirring tool is used to stir friction weld two cast alloys at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two cast alloys into a high-entropy alloy matrix, i.e., a large-size plate. The large-size plate after welding is subjected to multiple stir friction processing using the same processing parameters, and the overlap zone width is 2 / 3; a large-size block high-entropy alloy with a grain size of 2 microns, a high-angle grain boundary ratio and a twin boundary ratio of 93% and 25% respectively is obtained.

[0100] Technical effect: The large-sized block high entropy alloy obtained by the method of combining stir friction welding and stir friction processing in this embodiment can still obtain a uniform processing area when the overlap area width is 2 / 3. Its yield strength is 530MPa, tensile strength is 730MPa, and the elongation is maintained at 38%. In addition, in the electrochemical experiment carried out in 3.5% NaCl solution at room temperature, the corrosion current density is 0.08μA / cm2 and the corrosion potential is -0.22V.

[0101] Comparative Example 7

[0102] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiMn high entropy alloy;

[0103] Processing tools: The stirring tool is made of W-Re alloy. The stirring needle of the stirring tool is conical in shape, with a needle length of 1.7 mm and a shoulder diameter of 12 mm.

[0104] Processing process: The above-mentioned stirring needle is used to stir friction weld two cast alloys at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two cast alloys into a high-entropy alloy matrix, i.e., a large-size plate. The large-size plate after welding is subjected to multiple stir friction processing using the same processing parameters, and the overlap zone width is 5 / 6; a large-size block high-entropy alloy with a grain size of 10 microns, a high-angle grain boundary ratio of 84% and a twin boundary ratio of 10% is obtained.

[0105] Technical effect: The width of the overlap area is too large, resulting in the previous processing area being greatly affected by the heat of the next processing area, and its structure is coarse, and excellent performance cannot be obtained.

[0106] Technical effect and analysis: Too large an overlap area causes the processing area to be greatly affected by heat, and its coarse structure cannot achieve excellent performance. Therefore, a reasonable overlap area size is an important factor in achieving excellent performance.

[0107] Example 7

[0108] The matrix material of this embodiment is: two pieces of 1.2 mm thick cast CoCrFeNiAl high entropy alloy;

[0109] Processing tools: The stirring tool is made of metal ceramics, the stirring needle is 1.0mm long and the shoulder diameter is 11mm.

[0110] Processing process: The above-mentioned stirring needle is used to stir friction weld two cast alloys at a rotation speed of 400rpm and a welding speed of 50mm / min to weld the two cast alloys into a high-entropy alloy matrix, i.e., a large-size plate. The large-size plate after welding is subjected to multiple stir friction processing using the same processing parameters, and the overlap zone width is 1 / 2; a large-size block high-entropy alloy with a grain size of 0.5 microns, a high-angle grain boundary ratio and a twin boundary ratio of 90% and 20% respectively is obtained.

[0111] Technical effect: Friction stir welding combined with multi-pass friction stir processing obtains a processing area with a yield strength of 1000MPa and maintains an elongation of 22%. In addition, in the electrochemical experiment conducted in 3.5% NaCl solution at room temperature, the corrosion current density was 0.14μA / cm2 and the corrosion potential was -0.18V.

[0112] Comparative Example 8

[0113] The matrix material of this embodiment is: two pieces of 2 mm thick cast CoCrFeNiAl high entropy alloy;

[0114] Processing tools: The stirring tool is made of tool steel, the stirring needle is 1.0mm long and the shoulder diameter is 11mm.

[0115] Processing process: The above-mentioned stirring tool was used to perform friction stir welding and multi-pass friction stir processing on two pieces of cast alloys at a rotation speed of 400 rpm and a welding speed of 50 mm / min.

[0116] Technical effect: Due to the high hardness of CoCrFeNiAl high entropy alloy, tool wear is very severe during the stir friction welding process, and tool wear leads to poor welding formability.

[0117] Technical effect and analysis: Using metal ceramic stirring tools with good wear resistance can obtain welding joints and processing areas with good formability, but stirring tools made of tool steel alloys with poor wear resistance will wear severely during the stir friction processing, making the processing area unable to form. Therefore, stirring tools of suitable materials are an important condition for high entropy alloy stir friction processing and stir friction welding.

[0118] Through the above examples 1-7 and comparative examples 1-8, it can be seen that the high entropy alloy with low stacking fault energy is subjected to stir friction welding and multi-pass stir friction processing under the matching of reasonable rotation speed and welding speed, and a large-sized block fine-grained high-entropy alloy is obtained, whose twin boundary ratio is between 15% and 50%, the high-angle grain boundary ratio is above 85%, and the grain size is between 0.2μm and 5μm. While refining the grains, the defects of the cast structure and the density of the structure are significantly improved. The high proportion of twin boundaries improves the strength and plasticity matching of the material while reducing the corrosion sensitivity of the material. Therefore, a large-sized fine-grained high-entropy alloy with high strength, plasticity and corrosion resistance is obtained.

[0119] It is easy for those skilled in the art to understand that, under the premise of no conflict, the above-mentioned advantageous methods can be freely combined and superimposed.

[0120] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application. The above are only preferred implementations of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present application, and these improvements and variations should also be regarded as the protection scope of the present application.

Claims

1. A method for improving the strength, plasticity and corrosion resistance of a high entropy alloy, characterized in that: The steps include: The high entropy alloy substrate is subjected to multiple friction stir processing, so that an equiaxed structure is obtained on the high entropy alloy substrate, wherein the equiaxed structure has 15% to 50% of twins; and the proportion of grain boundaries with a mismatch angle of not less than 15 degrees in the equiaxed structure is more than 85%; Wherein, the high entropy alloy matrix is ​​CoCrFeNiMn high entropy alloy, CoCrFeNiAl 0.3 High entropy alloy, one of CoCrFeNiAl high entropy alloy; The tool used in the multi-pass friction stir processing of the high entropy alloy substrate is a stirring tool; the stirring tool comprises a shoulder and a stirring needle; The width of the overlapped area between two adjacent friction stir processing passes is 1 / 3 to 2 / 3 of the width of each friction stir processing pass.

2. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 1, characterized in that: In the equiaxed structure, the grain size is 0.2 μm to 5 μm.

3. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 1, characterized in that: The method for preparing the high entropy alloy matrix comprises the following steps: welding at least two high entropy alloy substrates to each other to form the high entropy alloy matrix, and the welding method is a stir friction welding method.

4. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 1, characterized in that: During the multi-pass friction stir processing of the high entropy alloy substrate, an inert gas is used to protect and cool the high entropy alloy substrate.

5. The method for improving the strength, plasticity and corrosion resistance of high entropy alloy according to claim 1, characterized in that: The stirring tool is made of any one of metal ceramics, tungsten-based alloys, cubic boron nitride, and cemented carbide.

6. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 5, characterized in that: The diameter of the shaft shoulder is 10 to 20 mm.

7. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 5, characterized in that: The multi-pass friction stir processing of the high entropy alloy substrate comprises the following steps: the rotation speed of the stirring tool is 100 to 800 revolutions per minute; and / or the advancing speed of the friction stir processing is 25 to 300 millimeters per minute.

8. The method for improving the strength, plasticity and corrosion resistance of a high entropy alloy according to claim 1, characterized in that: After the multi-pass friction stir processing, the high entropy alloy has a yield strength of 500-1000 MPa, an elongation of 20-55%, and a corrosion current density of less than 0.18 μA / cm 2 , the corrosion potential is greater than -0.25V.

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