A dual-graded Ni-Cr-V-Al medium-entropy alloy and a preparation method thereof

Abstract

The present invention discloses a dual-graded Ni-Cr-V-Al medium entropy alloy and a preparation method thereof. The microstructure of the Ni-Cr-V-Al medium entropy alloy includes a grain heterogeneous structure and a precipitate heterogeneous structure; the elemental percentage composition expression is: M d Al p , where 80% ≤ d < 100%, 0 < p ≤ 20%, and d + p = 100%; M includes the following elemental percentage composition: Ni ≤ 80%, Cr ≤ 60%, V ≤ 60%, and Ni, Cr, and V are all not 0%, and the elemental percentage contents of Cr and V are both less than the elemental percentage content of Ni. The medium entropy alloy obtains a dual heterogeneous structure by micro-alloying with Al element to hinder recrystallization and simultaneously promote hierarchical precipitation, so as to achieve the synergistic strengthening effect of heterogeneous strengthening, precipitation strengthening, and dislocation strengthening, and has high yield strength and high tensile strength while maintaining good plasticity.

Description

Technical Field

[0001] The present invention relates to a dual-graded Ni-Cr-V-Al medium-entropy alloy and a preparation method thereof, and in particular to a method for regulating the strength and plasticity of the Ni-Cr-V-Al medium-entropy alloy by combining alloying treatment, pre-deformation processing and heat treatment process, and belongs to the field of metal material design and preparation. Background Art

[0002] Metal structural parts are an indispensable part of modern engineering and are widely used in large-scale engineering fields such as construction, aerospace and automobile manufacturing. For most structural parts, having excellent strength and plasticity at the same time is the primary requirement. Heterostructured materials are considered to be an effective way to take into account the strength and plasticity of metal materials. Heterostructured materials are a new type of material composed of heterogeneous regions with significantly different mechanical or physical properties. The significant feature of this material is that it can make the performance of the material exceed the expected effect of simple mixing rules, and produce excellent performance between different regions at the same time. For example, Zhao et al. found that by simply increasing the structural gradient in pure Cu, its strength and work hardening can be improved simultaneously. Wu et al. obtained a heterogeneous lamellar structure in Ti alloy to make the alloy have both ultrafine grain strength and coarse grain plasticity. However, how to produce a more effective heterostructure under existing conditions to obtain an excellent combination of strength and plasticity is still a challenging task.

[0003] Among the existing heterostructures, partial recrystallization and multi-level precipitation are two types of heterostructures with significant strengthening effects. The former can significantly improve the yield strength of the material through back stress strengthening and pre-dislocation strengthening; the latter can achieve ultra-high tensile strength through back stress strengthening and precipitation strengthening. However, due to the limited dislocation storage capacity, the work hardening rate of the partially recrystallized heterostructure is often lower than that of the fully recrystallized variant; and multi-level precipitation often leads to a sharp deterioration of its plasticity due to the lack of coordinated deformation capacity and stress concentration problems.

[0004] The above-mentioned methods for regulating the mechanical properties of medium / high entropy alloys are mostly focused on obtaining single-level heterogeneity of grains or precipitate phases through thermal action (heat treatment), force action (deformation processing) and the combination of the two. However, there is a lack of research on the method of combining the content of alloying elements with heat treatment and deformation processing to obtain a double heterogeneous structure of grain heterogeneity and precipitate phase heterogeneity to achieve the regulation of the mechanical properties of medium / high entropy alloys.

[0005] In addition, although the above methods for obtaining the mechanical properties of single-stage heterogeneous alloys with heterogeneous grains or precipitation phases have their own advantages, there are still many problems that need to be solved:

[0006] 1. The work hardening rate of partially recrystallized single-stage heterostructure alloys is often lower than that of fully recrystallized alloys due to the decreased dislocation storage capacity.

[0007] 2. Precipitation phase heterogeneity often leads to a rapid decrease in plasticity due to lack of coordinated deformation and stress concentration.

[0008] 3. It is difficult to obtain an ideal bipolar heterostructure only through heat treatment and deformation processing, and the alloy performance enhancement effect is limited. Summary of the invention

[0009] In view of the problems that the partially recrystallized single-stage heterostructure alloys of medium / high entropy alloys in the prior art have low dislocation storage capacity, resulting in low processing strain hardening rate, and the precipitation phase heterogeneity cannot meet the performance requirements of both strong and plasticity due to the lack of coordinated deformation and stress concentration problems, and it is difficult to obtain an ideal bipolar heterostructure only through heat treatment and deformation processing, and the alloy performance enhancement effect is limited. The first object of the present invention is to provide a dual-graded Ni-Cr-V-Al medium-entropy alloy, which has partial recrystallization and multi-stage precipitation heterostructures, can provide plastic deformation and deformation resistance at the same time, and has good comprehensive strength and toughness.

[0010] The second object of the present invention is to provide a method for preparing a dual-graded Ni-Cr-V-Al medium-entropy alloy. The method obtains a dual heterogeneous structure of grain isomerism and precipitation phase isomerism in the Ni-Cr-V-Al medium-entropy alloy by regulating the content of alloying elements and combining heat treatment and deformation processing, thereby achieving its strengthening and toughening. The method has the advantages of convenient preparation, simple process, and safe use.

[0011] In order to achieve the above technical objectives, the present invention provides a dual-graded Ni-Cr-V-Al medium entropy alloy, wherein the microphase of the medium entropy alloy comprises a grain heterogeneous structure and a precipitation phase heterogeneous structure; the atomic percentage composition expression is: M d Al p , wherein 80%≤d<100%, 0<p≤20%, d+p=100%; M comprises the following atomic percentage composition: Ni≤80%, Cr≤60%, V≤60%, and Ni, Cr and V are not 0%, and the atomic percentage contents of Cr and V are both less than the atomic percentage content of Ni.

[0012] The content of Ni in the alloying elements of the present invention is higher than that of Cr and V, which is conducive to the subsequent discharge of Cr from the solid solution to form a Cr-rich BCC precipitation phase when an element with a more negative mixing enthalpy with the Ni element is added. At the same time, sufficient Ni can be used as a forming element of the solute B2-NiAl precipitation phase, and finally meets the material basis required for the dual-grade heterogeneous structure. At the same time, the Al element in the present invention can produce a drag effect on the grain boundary as a solute atom, and the generated precipitation phase produces a pinning effect on the grain boundary to hinder the growth of the grains. In addition, the distribution of the Al element in the solid solution and the precipitation phase forms the B2-NiAl phase, and the characteristics of the adjacent precipitation of the two precipitation phases of the Cr-BCC precipitation phase are finally obtained. Multi-level precipitation, therefore, by controlling the Al content in the alloy within a suitable range, the ratio of the crystallized and uncrystallized regions in the alloy can be controlled, so that the alloy has a bipolar heterogeneous structure, so that the alloy has a good work hardening rate while maintaining excellent plastic deformation. More preferably, M comprises the following atomic percentage composition: the percentage content of Ni element is 40-60%, the percentage content of Cr element is 15-40%, the percentage content of V element is 10-40%, and the percentage content of Al element is 7-10%.

[0013] As a preferred solution, the M further comprises at least one of C, B, Si, P, Ga, In, Sn, Pb, Ge, As, Sb, Te, Fe, Co, Mn, Cu, Zn, Au, Ag, Pt, Pd, Cd, Ru, K, Ta, W, Mo, Nb, Ti, Hf, Mg, Y or Ca in an atomic percentage of not more than 60%. The addition of the preferred M element can further regulate the volume fraction and size of the heterostructure multi-level precipitation phase to enhance its precipitation strengthening, or act as a solid solution element to further promote its solid solution strengthening.

[0014] The present invention also provides a method for preparing a dual-graded Ni-Cr-V-Al medium-entropy alloy, which comprises the steps of: performing surface cleaning pretreatment on raw materials including M and Al, and then sequentially performing vacuum melting, casting, homogenization treatment, pre-deformation processing and annealing treatment to obtain the alloy.

[0015] As a preferred solution, the surface cleaning pretreatment adopts ethanol ultrasonic washing. Ethanol ultrasonic washing can remove organic pollutants on the surface of raw materials Ni, Cr, V, M, Al, and reduce the introduction of impurity elements into the alloy material.

[0016] As a preferred solution, during the vacuum smelting process, the metal raw materials are stacked into the smelting furnace from top to bottom in the order of melting points. This operation can ensure that the high melting point metal raw materials are fully melted.

[0017] As a preferred solution, the vacuum melting is achieved by an arc melting furnace, and the conditions for the arc melting are: the arc current is 370-400A, the melting is repeated 5-8 times, and the melting time each time is 60-120s.

[0018] As a preferred solution, the homogenization treatment conditions are: in an inert atmosphere of 0.4 to 0.6 atmospheres, at a temperature of 1423 to 1523 K, for 1.5 to 2 hours. Before entering the inert atmosphere, it is necessary to evacuate to 2×10 - 1 Pa or less, the inert atmosphere used in the present invention is argon.

[0019] As a preferred solution, the conditions of the pre-deformation processing are: the total rolling deformation is 60-85%, the pass deformation is 5-10%, and the rolling temperature is room temperature. Under the selected deformation processing conditions, the outer dimensions of the sample can be processed with a large change range, and finally a plate rolling sample with a large rolling amount is obtained, which can store more energy for the sample. In addition, due to the different deformation amounts of grains between different orientations, the structural and energy conditions required for the original heterogeneous structure for subsequent recrystallization and multi-level precipitation. Too low rolling deformation may limit the formation of bipolar heterogeneous structures. Excessive rolling amount will cause cracks in the rolled plate in the later stage of rolling, seriously deteriorating the plasticity of the material.

[0020] As a preferred scheme, the conditions of the annealing treatment are: atmospheric atmosphere, temperature of 1223-1323K, and time of 3-20min. The present invention can control the influence of the Al content in the Ni-Cr-V-Al medium-entropy alloy on the formation of the bipolar heterostructure by strictly controlling the annealing temperature and time, and finally obtain the optimal Al content required for the ideal bipolar heterostructure. Too low recrystallization temperature or shorter recrystallization time will lead to too many unrecrystallized areas, seriously deteriorating plasticity. Too high recrystallization temperature or longer recrystallization time will lead to too few unrecrystallized areas, and the strengthening effect is not good. Further preferably, the temperature is 1250-1273K and the time is 5-10min.

[0021] As a preferred solution, cooling during the homogenization and annealing processes is performed by water quenching.

[0022] The present invention provides a method for preparing a dual-grade Ni-Cr-V-Al medium-entropy alloy, comprising the following steps:

[0023] 1) High-purity metal raw materials Ni, Cr, V, and Al are mechanically removed from the surface oxide layer and then accurately weighed, and the weighed raw materials are cleaned with alcohol ultrasonic cleaning;

[0024] 2) placing the metal raw materials into a vacuum arc melting furnace for melting, and using vacuum suction casting and copper mold cooling technology to obtain a columnar sample alloy;

[0025] 3) homogenizing the columnar sample after vacuum sealing in a muffle furnace;

[0026] 4) Performing pre-deformation processing on the homogenized sample;

[0027] 5) The pre-deformed sample is subjected to a short-time annealing treatment to obtain the product.

[0028] Compared with similar processes in the prior art, it has the following obvious advantages:

[0029] (1) The preparation method of the dual-graded Ni-Cr-V-Al medium-entropy alloy provided by the present invention has the advantages of simple process and safe use.

[0030] (2) The main elements of the dual-graded Ni-Cr-V-Al medium-entropy alloy provided by the present invention are ordinary pure metal raw materials, and Al is a common light alloying element, and the production cost is low.

[0031] (3) The bipolar Ni-Cr-V-Al medium entropy alloy provided by the present invention can effectively hinder recrystallization and promote multi-stage precipitation by increasing the Al content to obtain a partially recrystallized and multi-stage precipitation heterogeneous structure. This bipolar heterogeneous structure can provide plastic deformation through the recrystallization area and deformation resistance through non-recrystallization and multi-stage precipitation, so as to obtain good comprehensive strength and toughness.

[0032] (4) The dual-graded Ni-Cr-V-Al medium entropy alloy provided by the present invention can achieve a yield strength of 1243 MPa and a tensile strength of 1521 MPa while maintaining a certain elongation. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the synchrotron radiation diagram of the alloys of Example 1 of the present invention and Comparative Examples 1 and 2.

[0034] Figure 2 The alloy matrix Ni used in Comparative Example 1 of the present invention 60 Cr 20 V 20 organizational morphology.

[0035] Figure 3 The alloy matrix Ni used in Comparative Example 1 of the present invention 60 Cr 20 V 20 Mechanical properties.

[0036] Figure 4When the alloying element Al (9 at.%) is added in Example 1 of the present invention, that is, (Ni 60 Cr 20 V 20 ) 91 Al9 alloy microstructure morphology.

[0037] Figure 5 When the alloying element Al (9 at.%) is added in Example 1 of the present invention, that is, (Ni 60 Cr 20 V 20 ) 91 Mechanical properties of Al9 alloy.

[0038] Figure 6 is the present invention embodiment 2 (Ni 58.14 Cr 19.38 V 19.38 Ti 3.10 ) 97 Al3 alloy microstructure morphology.

[0039] Figure 7 is the present invention embodiment 2 (Ni 58.14 Cr 19.38 V 19.38 Ti 3.10 ) 97 Mechanical properties of Al3 alloy.

[0040] Figure 8 When the alloying element Al (9 at.%) is added in Comparative Example 2 of the present invention, that is, (Ni 60 Cr 20 V 20 ) 91 Microstructure of Al9 alloy at annealing temperature of 1373K.

[0041] Fig. 9 When the alloying element Al (9 at.%) is added in Comparative Example 2 of the present invention, that is, (Ni 60 Cr 20 V 20 ) 91 Mechanical properties of Al9 alloy at annealing temperature of 1373K.

[0042] Fig.10 When the alloying element Al (9 at.%) is added in Example 3 of the present invention, that is, (Ni 60 Cr 20 V 20 ) 91 Mechanical properties of Al9 alloy at annealing temperature of 1223K. DETAILED DESCRIPTION

[0043] In order to facilitate the understanding of the present invention, the content of the present invention will be described more comprehensively and carefully below in combination with the accompanying drawings and preferred embodiments of the specification, but the protection scope of the present invention is not limited to the following specific embodiments.

[0044] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0045] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0046] Example 1

[0047] The present invention (Ni 60 Cr 20 V 20 ) 91 Preparation and properties of Al9 (9Al alloy) medium entropy alloy, including the following steps:

[0048] (1) High-purity (≥99.9%) Ni, Cr, V, and Al are used as alloy smelting raw materials. Each element is accurately weighed and proportioned, with Ni being 36.136 g, Cr being 10.671 g, V being 10.455 g, and Al being 2.738 g, with a total weight of 60.000 g. Each weighed metal raw material is then cleaned by ultrasonic vibration in alcohol, and then placed on filter paper and dried by a heating lamp for later use.

[0049] (2) The dried raw materials are stacked in the water-cooled copper crucible of the vacuum arc melting furnace in the order of melting point from top to bottom, with Al at the bottom, V and Ni in the middle, and Cr with the highest melting point at the top. Then close the furnace door and evacuate the furnace chamber to 1×10 -3 Pa, and then fill the furnace with half atmosphere pressure. Then evacuate the furnace again to 1×10 -3 Pa, then fill the furnace with half atmosphere pressure, ignite the tungsten electrode head, and start smelting. Before smelting the target alloy, move the ignited tungsten electrode head to the titanium ingot and stay there for 30 seconds. At this time, the arc current is 120A to absorb free oxygen, nitrogen and other gases in the furnace. Finally, slowly increase the current density to 385A, and after the target alloy is melted, the arc is maintained for 120 seconds. After the alloy cools, turn it over, repeat the smelting 8 times, and after smelting 4 times, vacuum it again and fill it with half atmosphere pressure argon. After the master alloy is fully uniform, take out the alloy.

[0050] (3) Grinding the alloy melted in step (2) to remove the surface oxide layer with a grinder, then cleaning it twice with ultrasonic vibration in alcohol, drying it with a hair dryer, placing it in a wiped vacuum melting furnace, and screwing it into a suction casting copper mold to perform suction casting on the alloy to obtain a 10×10×80 mm medium entropy alloy rectangular sample;

[0051] (4) Place the sample obtained by suction casting in step (3) into a glass tube with a groove at the waist and insert a glass stopper. Place the glass tube on a tube sealing machine and evacuate the vacuum to 2×10 -3 Pa, then fill with half an atmospheric pressure of argon, repeat the above operation at least 5 times. Then open the hydrogen and oxygen machine to strictly weld the glass tube and the glass plug. After the glass tube is cooled, put it into the muffle furnace for homogenization treatment at a temperature of 1473K for 2 hours. Or get a uniform alloy. After the homogenization treatment, throw the glass tube directly into a bucket for water cooling, and ensure that the glass tube is cracked in the water during this process, so that the sample can be water-cooled.

[0052] (5) After the homogenization treatment in step (4), the surface oxide layer of the sample is removed, and then the sample is subjected to room temperature rolling treatment. The total rolling deformation is 85%, and the rolling deformation of each pass is 5-10%.

[0053] (6) The sample after rolling in step (5) is directly placed in a muffle furnace for annealing at a temperature of 1273 K for 10 minutes under the cooling condition of water cooling. Finally, the target sample is obtained.

[0054] The product of this embodiment was tested by tissue observation method. 60 Cr 20 V 20 ) 91 The phase composition of Al9 (9Al alloy) medium entropy alloy after annealing is as follows Figure 1 As shown in Figure 1, it is an ordered BCC-B2+BCC+FCC three-phase structure. Figure 4 As shown, the grains are divided into recrystallized areas and non-recrystallized areas. This embodiment is in an incompletely recrystallized state at this temperature. The shapes of the precipitated phases in the non-recrystallized area include spherical and rod-shaped shapes, and a partial recrystallization and multi-level precipitation double-graded heterogeneous structure is obtained. Figure 5 ), the yield strength of this material is 1151MPa, the tensile strength is 1448MPa, the uniform elongation is 17%, and the tensile elongation is 18%.

[0055] Example 2

[0056] The present invention (Ni 58.14 Cr 19.38 V 19.38 Ti 3.10 ) 97Preparation and properties of Al3 (6TiAl alloy) medium entropy alloy, including the following steps:

[0057] (1) High-purity (≥99.9%) Ni, Cr, V, Ti, and Al are used as alloy smelting raw materials. Each element is accurately weighed and proportioned, with Ni being 36.310 g, Cr being 10.722 g, V being 10.505 g, Ti being 1.575 g, and Al being 0.888 g, with a total weight of 60.000 g. Each weighed metal raw material is then cleaned by ultrasonic vibration in alcohol, and then placed on filter paper and dried by a heating lamp for later use.

[0058] (2) The dried raw materials are stacked in the water-cooled copper crucible of the vacuum arc melting furnace in the order of melting point from top to bottom, with Al at the bottom, Ti and Ni in the middle, and V and Cr with the highest melting points at the top. Then close the furnace door and evacuate the furnace chamber to 1×10 -3 Pa, and then fill the furnace with half atmosphere pressure. Then evacuate the furnace again to 1×10 -3 Pa, then fill the furnace with half atmosphere pressure, ignite the tungsten electrode head, and start smelting. Before smelting the target alloy, move the ignited tungsten electrode head to the titanium ingot and stay there for 30 seconds. At this time, the arc current is 120A to absorb free oxygen, nitrogen and other gases in the furnace. Finally, slowly increase the current density to 385A, and after the target alloy is melted, the arc is maintained for 120 seconds. After the alloy cools, turn it over, and repeat the smelting 8 times. After smelting 4 times, vacuumize again and fill with half atmosphere argon. After the master alloy is fully uniform, take out the alloy. Steps (3) to (5) are consistent with Example 1.

[0059] (6) The sample after rolling in step (5) is directly placed in a muffle furnace for annealing at a temperature of 1273 K for 10 minutes under the cooling condition of water cooling. Finally, the target sample is obtained.

[0060] The product of this embodiment was tested by tissue observation method. 58.14 Cr 19.38 V 19.38 Ti 3.10 ) 97 Al3 microstructure Figure 6 As shown, the grains are divided into recrystallized areas and non-recrystallized areas. This embodiment is in an incompletely recrystallized state at this temperature. The shapes of the precipitated phases in the non-recrystallized area include spherical and rod-shaped shapes, and a partial recrystallization and multi-level precipitation double-graded heterogeneous structure is obtained. Figure 7 ), the yield strength of this material is 1243MPa, the tensile strength is 1521MPa, the uniform elongation is 13%, and the tensile elongation is 14%.

[0061] Example 3

[0062] The present invention (Ni 60 Cr 20 V 20 ) 91 The preparation and properties of Al9 (9Al alloy) medium entropy alloys include the following steps:

[0063] Steps (1) to (5) are consistent with those in Example 1.

[0064] (6) The sample after rolling in step (5) is directly placed in a muffle furnace for annealing at a temperature of 1223 K for 10 minutes under the cooling condition of water cooling. Finally, the target sample is obtained.

[0065] The product of this embodiment was tested by tissue observation method. 60 Cr 20 V 20 ) 91 Al9 (9Al alloy) has been tested (such as Fig.10 ), its yield strength: 1452MPa, tensile strength: 1655MPa, uniform elongation: 7%, tensile elongation: 7%. Comparative Example 3, when the annealing temperature is reduced, the yield strength and tensile strength of the alloy material increase, but the plasticity decreases significantly. This is because the temperature is too low, the area of ​​the non-recrystallized region in the alloy is too large, resulting in too many misalignments reserved for alloy deformation, and excessive stress concentration at the interface between the precipitated phase and the matrix during the later deformation process, which ultimately leads to premature fracture of the sample and poor plasticity.

[0066] Comparative Example 1

[0067] The Ni of the present invention 60 Cr 20 V 20 The preparation and properties of the (0Al alloy) medium entropy alloy include the following steps:

[0068] (1) High-purity (≥99.9%) Ni, Cr, and V are used as alloy smelting raw materials. Each element is accurately weighed and proportioned, with Ni being 37.864g, Cr being 11.181g, and V being 10.955g, with a total weight of 60.000g. Each weighed metal raw material is then cleaned by ultrasonic vibration in alcohol, and then placed on filter paper and dried with a heating lamp for later use.

[0069] (2) The dried raw materials are stacked in a water-cooled copper crucible of a vacuum arc melting furnace in descending order of melting point, with Ni at the bottom, V in the middle, and Cr having the highest melting point at the top; the remaining conditions and steps are the same as those in Example 1.

[0070] The product of this embodiment was tested by tissue observation method.60 Cr 20 V 20 (0Al alloy) The phase composition of the medium entropy alloy after annealing is face-centered cubic (FCC) single phase, such as synchrotron radiation Figure 1 Its organization is as shown. Figure 2 As shown, the grains are coarse equiaxed grains (average grain size is 14.7 μm). This example is a matrix control sample, and the matrix is ​​in a completely recrystallized state at this temperature. Figure 3 ), the yield strength of the material is 487MPa, the tensile strength is 975MPa, the uniform elongation is 43%, and the tensile elongation is 50%. It can be seen that when the alloy does not contain Al element, at the same annealing temperature, the Ni prepared in Example 1 is 60 Cr 20 V 20 The yield strength and tensile strength of (0Al alloy) are significantly lower than those of the 9Al alloy in Example 1. This is mainly because at this temperature, the solute atoms in the alloy have a smaller drag effect on the grain boundaries and cannot form a bipolar heterogeneous structure.

[0071] Comparative Example 2

[0072] The present invention (Ni 60 Cr 20 V 20 ) 91 The preparation and properties of Al9 (9Al alloy) medium entropy alloys include the following steps:

[0073] Steps (1) to (5) are consistent with those in Example 1.

[0074] (6) The sample after rolling in step (5) is directly placed in a muffle furnace for annealing at a temperature of 1373 K for 10 minutes under the cooling condition of water cooling. Finally, the target sample is obtained.

[0075] The product of this embodiment was tested by tissue observation method. 60 Cr 20 V 20 ) 91 The phase composition of Al9 (9Al alloy) medium entropy alloy after annealing is as follows Figure 1 As shown in Figure 1, it is an ordered BCC-B2+BCC+FCC three-phase structure. Figure 8 As shown in the figure, it is a completely recrystallized state, and the precipitated phase is irregular polygonal, mainly distributed at the trigeminal grain boundary. Figure 5), the yield strength of the material is 669MPa, the tensile strength is 1162MPa, the uniform elongation is 30%, and the tensile elongation is 34%. Compared with Example 2, when the annealing temperature is increased, the yield strength and tensile strength of the alloy material are significantly reduced. This is because too high a recrystallization temperature will lead to too few unrecrystallized areas in the alloy, resulting in poor strengthening effect.

[0076] Comparative Example 3

[0077] The only difference between Comparative Example 3 and Example 1 is that the percentage ratio of Ni, Cr and V in the M element is 1:1:1, and the other steps and conditions are the same. When it comes to step (5), the alloy material cannot be rolled due to its excessive brittleness.

Claims

1. A dual-graded Ni-Cr-V-Al medium entropy alloy, characterized in that: The microphases include grain heterostructure and precipitation phase heterostructure; The atomic percentage composition expression is: M d Al p , where 90%≤d≤93%, 7≤p≤10%, d+p=100%; M comprises the following atomic percentage composition: the percentage content of Ni element is 40-60%, the percentage content of Cr element is 15-40%, and the percentage content of V element is 10-40%, and the atomic percentage content of Cr and V are both less than the atomic percentage content of Ni; The precipitate phase heterogeneous structure is a multi-level precipitate heterogeneous structure finally obtained by combining the characteristics of adjacent precipitation of B2-NiAl precipitate phase and Cr-BCC precipitate phase; The grain heterogeneous structure is a partially recrystallized structure.

2. A dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 1, characterized in that: The M also contains at least one of C, B, Si, P, Ga, In, Sn, Pb, Ge, As, Sb, Te, Fe, Co, Mn, Cu, Zn, Au, Ag, Pt, Pd, Cd, Ru, K, Ta, W, Mo, Nb, Ti, Hf, Mg, Y or Ca with an atomic percentage of not more than 35%.

3. A method for preparing a dual-graded Ni-Cr-V-Al medium-entropy alloy according to claim 1 or 2, characterized in that: The raw materials including M and Al are subjected to surface cleaning pretreatment, and then vacuum melting, casting, homogenization treatment, pre-deformation processing and annealing treatment are carried out in sequence to obtain the product.

4. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 3, characterized in that: The surface cleaning pretreatment adopts ethanol ultrasonic washing.

5. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 3, characterized in that: During the vacuum smelting process, the metal raw materials are piled into the smelting furnace from top to bottom in the order of melting points.

6. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 5, characterized in that: The vacuum melting is achieved by an arc melting furnace, and the conditions of the arc melting are: the arc current is 370-400 A, the melting is repeated 5-8 times, and the melting time each time is 60-120 seconds.

7. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 3, characterized in that: The homogenization treatment conditions are: in an inert atmosphere of 0.4-0.6 atmospheres, at a temperature of 1423-1523K, for 1.5-2h.

8. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 3, characterized in that: The conditions of the pre-deformation process are as follows: the total rolling deformation is 60-85%, the pass deformation is 5-10%, and the rolling temperature is room temperature.

9. The method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 3, characterized in that: The annealing treatment conditions are: air atmosphere, temperature of 1223-1323 K, and time of 3-20 min.

10. A method for preparing a dual-graded Ni-Cr-V-Al medium entropy alloy according to claim 7 or 9, characterized in that: During the homogenization and annealing processes, cooling is performed by water quenching.

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