A CrCoNi-based multi-principal element alloy and its preparation method

Abstract

This application relates to a CrCoNi-based multi-principal element alloy and its preparation method, belonging to the field of metallic materials and surface engineering technology. The composition formula of the CrCoNi-based multi-principal element alloy is (CrCoNi2). 100‑x (AlNb) x x represents the atomic percentage, 1 ≤ x ≤ 10; the CrCoNi-based multi-principal alloy comprises an FCC matrix and coherent L12-type nanoprecipitates distributed within the FCC matrix; the CrCoNi-based multi-principal alloy exhibits a gradient grain structure with gradually increasing grain size from the surface to the core, and residual compressive stress gradually decreasing along the depth direction within the gradient grain structure. This CrCoNi-based multi-principal alloy combines high strength, high plasticity, excellent wear resistance, and performance stability under extreme high and low temperature environments.

Description

Technical Field

[0001] This application relates to the fields of metallic materials and surface engineering technology, and in particular to a CrCoNi-based multi-principal alloy and its preparation method. Background Technology

[0002] Multi-principal alloys, especially face-centered cubic (FCC) CrCoNi alloys, exhibit excellent strength and toughness matching at room temperature and low temperature, making them promising candidate materials for aerospace, polar equipment, nuclear fusion engineering, and other fields. However, existing CrCoNi alloys still have the following drawbacks: (1) insufficient strength and wear resistance; (2) difficulty in simultaneously achieving high strength and high plasticity; and (3) poor stability in extreme environments such as high temperatures.

[0003] Therefore, it is necessary to develop a new type of CrCoNi alloy that combines high strength, excellent plasticity and wear resistance, and can be stably used in extreme environments. Summary of the Invention

[0004] To address the aforementioned problems, the objective of this application is to provide a CrCoNi-based multi-principal alloy and its preparation method, thereby solving the problems of insufficient strength and wear resistance, difficulty in simultaneously achieving high strength and high plasticity, and poor stability in extreme environments such as high temperatures in existing CrCoNi-based multi-principal alloys.

[0005] In one aspect, embodiments of this application provide a CrCoNi-based multi-principal element alloy, the composition expression of which is (CrCoNi2). 100-x (AlNb) x ; Among them, the CrCoNi-based multi-principal-element alloy includes a matrix with a face-centered cubic structure and coherent L12-type nano-precipitates distributed in the matrix; CrCoNi-based multi-principal-element alloys have a gradient grain structure with gradually increasing grain size from the surface to the core, and there is residual compressive stress that gradually decreases along the depth direction within the gradient grain structure.

[0006] In the above technical solution, by selecting specific atoms and designing their proportions in the CrCoNi-based multi-principal alloy, a low stacking fault energy FCC matrix and a high-temperature stable, high-density coherent L12-type nanoprecipitates ((Cr,Co,Ni)3(Al,Nb) type) are formed. Combined with the gradient grain structure from the surface to the core and the residual compressive stress decreasing along the depth direction, the CrCoNi-based multi-principal alloy has excellent comprehensive properties, including high strength, high plasticity, excellent wear resistance, and performance stability under extreme environments.

[0007] The gradient grain structure with gradually increasing grain size and the residual compressive stress that gradually decreases along the depth direction work together to effectively improve the surface hardness of the material and inhibit crack initiation and propagation, giving the alloy high strength, high toughness, and excellent wear resistance. Meanwhile, the low stacking fault energy FCC matrix ensures the alloy's good plastic deformation capability; the high-temperature stable coherent L12-type nanoprecipitates and the surface gradient grain structure work together to ensure that the alloy maintains excellent strength and wear resistance at both room temperature and high temperature.

[0008] The CrCoNi-based multi-principal alloy in this application has a yield strength of over 1450 MPa, a uniform elongation of over 20%, and exhibits extremely low wear rate at both room temperature and 600°C.

[0009] In some embodiments, the gradient grain structure, from the surface to the core of the CrCoNi-based multi-principal alloy, sequentially includes a nanocrystalline or ultrafine grain layer, a submicron-sized deformed grain layer, and an undeformed fine grain structure.

[0010] In the above technical solution, the surface nanocrystalline or ultrafine crystalline layer can effectively improve the surface hardness of the alloy and enhance the wear resistance of the material. The internal undeformed fine crystalline structure can provide sufficient strength, and the intermediate submicron crystalline deformation layer serves as a transition layer, which can further improve the plastic deformation capacity of the alloy.

[0011] In some embodiments, the depth of the gradient grain structure is greater than 200 μm, and more specifically 200 μm to 400 μm.

[0012] In the above technical solutions, a wide depth range is beneficial for achieving a continuous transition of gradient nanostructures, thereby further improving the alloy's strength, plasticity, wear resistance, and high-temperature stability. In some embodiments, the average grain size of the CrCoNi-based multi-principal alloy is less than 50 nm in the range of 0–500 nm from the surface.

[0013] In the above technical solution, the smaller grain size is beneficial to further improve the surface hardness and wear resistance of the alloy, and further suppress crack initiation and propagation. In some embodiments, the maximum residual compressive stress of the CrCoNi-based multi-principal alloy is ≥1.0 GPa in the range of 0 to 50 μm from the surface. In the above technical solution, the higher residual compressive stress can further suppress crack initiation and propagation, and further improve the plastic deformation performance and wear resistance of the alloy. In some embodiments, the average size of the coherent L12-type nanoprecipitates is <30 nm.

[0014] In the above technical solution, the coherent L12 type nano-precipitates are smaller in size, which is beneficial for their compatibility with the FCC matrix and further improves the plasticity and high-temperature thermal stability of the alloy; moreover, the smaller the size, the higher the density, which is beneficial for further improving the strength and toughness of the alloy.

[0015] Secondly, embodiments of this application provide a method for preparing the above-mentioned CrCoNi-based multi-principal-element alloy, comprising the following steps: According to the atomic percentage of the CrCoNi-based multi-principal alloy, the raw materials are mixed and smelted to obtain alloy ingots; wherein, the composition expression of the CrCoNi-based multi-principal alloy is (CrCoNi2). 100-x (AlNb) x x is the atomic percentage, 1≤x≤10; The alloy ingot is subjected to rolling deformation, solution treatment and aging treatment in sequence to obtain an intermediate; The intermediates were subjected to pre-strain treatment in sequence, and then surface mechanical grinding was performed at a temperature of -196℃ to -50℃. In the above technical solution, this application employs a specific atomic composition and ratio. First, an alloy ingot with uniform atomic-scale mixing is obtained through mixed melting. Then, after rolling deformation and solution treatment, a uniform FCC single-phase structure is obtained. Following aging treatment, a high-density, uniformly distributed coherent L12-type nanoprecipitates are obtained. The design of CrCoNi2 facilitates compositional segregation during aging treatment, leading to the formation of a low-stack-energy CoCrNi matrix. The addition of Al and the refractory element Nb regulates the valence electron concentration and atomic size difference of the alloy, forming a low-stack-energy FCC matrix and a high-temperature stable coherent L12-type nanoprecipitates. Next, a pre-strain-low-temperature surface mechanical polishing treatment is employed. The pre-strain treatment introduces high-density dislocations and internal stress fields, which helps reduce the critical stress for subsequent plastic deformation. Surface mechanical polishing at low temperature significantly reduces the stacking fault energy of the alloy, activates the twin-induced grain refinement mechanism, and introduces intense plastic deformation into the alloy surface layer. The synergistic effect of these two processes forms a gradient grain structure and a gradually decreasing residual compressive stress along the depth direction in the alloy.

[0016] Thus, the low stacking fault energy FCC matrix, high temperature stable coherent L12 nanoprecipitates formed by this preparation method, and the synergistic effect of gradient grain structure and residual compressive stress gradually decreasing along the depth direction enable the alloy to have high strength, high plasticity, excellent wear resistance and performance stability under extreme environments.

[0017] In some implementations, the pre-strain treatment is pre-stretching or pre-rolling, with a strain of 2% to 5%. In the above technical solution, the 2%~5% pre-stretching or pre-rolling treatment can introduce uniformly distributed high-density dislocations inside the material, providing a basis for grain refinement in the subsequent surface mechanical grinding process, further increasing the depth of the gradient grain structure, and improving the level and depth of residual compressive stress.

[0018] In some embodiments, the process parameters for surface mechanical polishing include: using bearing steel balls with a diameter of 3mm to 5mm, a vibration frequency of 20kHz to 50kHz, and a processing time of 10min to 60min.

[0019] In the above technical solution, the process parameters of surface mechanical polishing are within a suitable range, which is conducive to further improving the depth of the gradient grain structure and improving the level and depth of residual compressive stress.

[0020] In some embodiments, the solution treatment includes: holding at 1000℃~1200℃ for 5min~20min followed by quenching.

[0021] In the above technical solution, the temperature and time of the solution treatment are within the above range, which is conducive to fully dissolving the nascent L12 phase in the alloy, forming a highly supersaturated FCC solid solution, and is also conducive to controlling the grain size within a suitable range. In conjunction with the subsequent aging treatment, it further improves the strength and plasticity of the alloy.

[0022] In some implementations, the aging process includes: quenching after treatment at 600℃~800℃ for 8h~24h.

[0023] In the above technical solution, the temperature and time of the aging treatment are within the above range, which is conducive to the precipitation of fine, high-density, and diffusely distributed coherent L12-type nanoprecipitates, and also conducive to maintaining the stability of the matrix grains.

[0024] In some implementations, the rolling deformation is cold rolling or hot rolling, with a total deformation of 70% to 90%.

[0025] In the above technical solution, the matrix grains can be effectively refined through large plastic deformation, and a certain number of dislocations can be introduced into the material. This is beneficial for the formation of a uniform FCC matrix and high-density, uniformly distributed coherent L12 nano-precipitates during subsequent solution treatment and aging treatment. This is beneficial for further improving the strength, plasticity, wear resistance and performance stability of the alloy under extreme conditions.

[0026] In some embodiments, the alloy ingot is further subjected to a homogenization treatment before rolling deformation, which includes holding at 1100℃~1250℃ for 5h~24h.

[0027] In the above technical solution, homogenization treatment is beneficial to forming a uniform, fine-grained primary structure and promotes the full diffusion of Al and Nb atoms. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a process flow diagram of a method for preparing a CrCoNi-based multi-principal-element alloy provided in an embodiment of this application.

[0030] Figure 2 Figure (a) is an electron backscatter diffraction (EBSD) pattern of the gradient grain structure of the CrCoNi-based multi-principal alloy in Example 1 of this application; Figure (b) is a transmission electron microscope (TEM) image of the coherent L12-type nanoprecipitates of the CrCoNi-based multi-principal alloy in Example 1 of this application.

[0031] Figure 3 Figure (a) is a transmission electron microscope (TEM) image of the grain structure on the surface of the CrCoNi-based multi-principal alloy in Comparative Example 2 of this application; Figure (b) is an electron backscatter diffraction (EBSD) image of the grain structure on the surface of the CrCoNi-based multi-principal alloy in Comparative Example 2 of this application.

[0032] Figure 4 The images show the room temperature tensile curves of the CrCoNi-based multi-principal-element alloys and single-phase CrCoNi-based alloys in Examples 1, 1, and 2 of this application.

[0033] Figure 5 Figure (a) shows the friction and wear curve of the CrCoNi-based multi-principal alloy in Example 1 of this application at room temperature; Figure (b) shows the friction and wear curve of the CrCoNi-based multi-principal alloy in Example 1 of this application at 600°C. Detailed Implementation

[0034] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the CrCoNi-based multi-principal-element alloy and its preparation method thereof, but some unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0035] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0037] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0038] Multi-principal alloys, especially face-centered cubic (FCC) CrCoNi alloys, exhibit excellent strength and toughness matching at room temperature and low temperature, making them promising candidate materials for aerospace, polar equipment, nuclear fusion engineering, and other fields. However, existing CrCoNi alloys still have the following defects: (1) Insufficient strength and wear resistance: Single-phase CrCoNi-based alloys with equiatomic ratios have low yield strength (usually between 300MPa and 500MPa) and poor wear resistance, making it difficult to meet the requirements of high load-bearing capacity and long service life for key moving parts of high-end equipment (such as bearings, gears, and ship propellers). (2) Difficulty in simultaneously achieving high strength and high plasticity: In order to improve strength, existing research often introduces non-coherent hard second phases (such as oxides and carbides) or performs severe plastic deformation, but the non-coherent phase interface is prone to become a crack initiation point, which seriously damages the plasticity of the material; while simple fine-grain strengthening or gradient structure design can improve strength, but the improvement on wear resistance is limited. (3) Poor stability in extreme environments, such as high temperatures: Some reinforcing phases introduced into the existing CrCoNi alloy system, such as the Ni3(Al, Ti) type L12 phase, are prone to coarsening or dissolution at temperatures above 650°C, resulting in a sharp decline in the high-temperature performance of the alloy.

[0039] Based on this, the first aspect of this application provides a CrCoNi-based multi-principal element alloy, the composition formula of which is (CrCoNi2). 100-x (AlNb) x x is the atomic percentage, 1≤x≤10; wherein, the CrCoNi-based multi-principal alloy includes a matrix with a face-centered cubic structure and coherent L12-type nanoprecipitates distributed in the matrix; the CrCoNi-based multi-principal alloy has a gradient grain structure with the grain size gradually increasing from the surface to the core, and there is a residual compressive stress that gradually decreases along the depth direction within the gradient grain structure.

[0040] As an example, x is any single point value or any value between two points in the range of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

[0041] "CrCoNi-based multi-principal element alloys" refer to a type of multi-component solid solution alloy formed by adding elements such as Al and Nb, with chromium (Cr), cobalt (Co), and nickel (Ni) as the core principal elements. These are also known as medium / high entropy alloys. In this application, the CrCoNi-based multi-principal element alloy has a disordered face-centered cubic solid solution composed of Cr, Co, and Ni, i.e., an FCC matrix with a face-centered cubic structure, which is the continuous phase and supporting matrix of the alloy. The coherent L12-type nanoprecipitates, which are (Cr,Co,Ni)3(Al,Nb) type (mainly Ni3(Al, Nb) type), are ordered face-centered cubic superlattice nano-ions that precipitate from the supersaturated FCC matrix over time, maintaining a completely coherent interface with the matrix, and are the reinforcing phase of the alloy.

[0042] "Gradient grain structure" refers to a three-dimensional continuous structure formed on the surface of an alloy, where the grain size gradually increases from the nanometer scale at the surface to the micrometer scale at the core. It belongs to the gradient structure of the matrix material and has no obvious interface. Residual compressive stress refers to the internal stress that remains inside the material and puts the material surface in a state of compression when no external force is applied. The gradient grain structure is the carrier of residual compressive stress, and the two work together.

[0043] Understandably, conventional methods and equipment can be used to test the coherent L12-type nanoprecipitates, gradient grain structure, and residual compressive stress in CrCoNi-based multi-principal element alloys. For example, transmission electron microscopy (TEM) can be used to observe the size, morphology, and uniformity of the coherent L12-type nanoprecipitates; electron backscatter diffraction (EBSD) can be used to observe the depth and grain size distribution of the gradient grain structure; and X-ray diffraction (XRD) and focused ion beam digital image correlation (FIB-DIC) can be used to test the residual compressive stress on the alloy surface.

[0044] In this application, through specific atomic selection and sizing design, a low stacking fault energy FCC matrix and a high-temperature stable, high-density coherent L12-type nanoprecipitates ((Cr,Co,Ni)3(Al,Nb) type) are formed. Combined with the gradient grain structure from the surface to the core and the residual compressive stress decreasing along the depth direction, the CrCoNi-based multi-principal element alloy has excellent comprehensive properties, including high strength, high plasticity, excellent wear resistance and performance stability under extreme environments.

[0045] Among them, with (CrCoNi) x (AlTiNb) 100-x Compared to the previous system, this application's design (CrCoNi2) 100-x (AlNb) x Furthermore, the atomic ratio is controlled to avoid introducing Ti atoms. This is because while the introduction of Ti helps to form a strengthening phase, it also significantly increases the stacking fault energy of the alloy and promotes the formation of incoherent or semi-coherent phases (such as the Laves phase). This will directly lead to the following irreversible adverse effects: (1) suppressing the activation of deformation twins, making it difficult for the material to undergo twin-induced grain refinement during strong plastic deformation; (2) reducing the localization ability of plastic deformation during surface mechanical treatment, making it difficult for the gradient structure to extend to deeper layers; (3) forming incoherent interfaces, increasing the tendency for crack initiation, and weakening the synergy between plasticity and wear resistance. In contrast, this application only introduces Al and Nb atoms to maintain a low stacking fault energy in the matrix, while forming a high-density coherent L12 nano-precipitate phase through Al and Nb, thereby achieving a synergistic structure of "low stacking fault energy matrix + highly stable coherent precipitate phase". This structure not only ensures the precipitation strengthening effect, but also provides the necessary conditions for twin deformation and gradient structure construction.

[0046] Furthermore, the gradient grain structure with gradually increasing grain size from the surface to the core, combined with the residual compressive stress that gradually decreases along the depth direction, effectively improves the surface hardness of the material and inhibits crack initiation and propagation, giving the alloy high strength, high toughness, and excellent wear resistance. Simultaneously, the low stacking fault energy FCC matrix ensures the alloy's good plastic deformation capability; the high-temperature stable coherent L12-type nanoprecipitates and the surface gradient grain structure work together to maintain excellent strength and wear resistance at both room temperature and high temperature.

[0047] The CrCoNi-based multi-principal alloy provided in this application has a yield strength of over 1450 MPa, a uniform elongation of over 20%, and exhibits extremely low wear rate at both room temperature and 600°C.

[0048] In some embodiments, the gradient grain structure, from the surface to the core of the CrCoNi-based multi-principal-element alloy, sequentially includes a nanocrystalline or ultrafine-grained layer, a submicron-sized deformed layer, and an undeformed fine-grained structure.

[0049] Among them, the "nanocrystalline or ultrafine-grained layer" is the surface region in the gradient grain structure where plastic deformation is most intense and the grain refinement is the highest. The grain size is usually less than 100nm, providing the highest hardness and achieving the wear resistance of the alloy. The "submicron-grained deformation layer" is a transition layer structure located between the nanocrystalline or ultrafine-grained layer and the undeformed fine-grained structure in the core. The grain size is usually 100nm~1μm, which is conducive to achieving a smooth transition of deformation. The "undeformed fine-grained structure" refers to the original matrix structure that is less affected by plastic deformation. The grain size is usually greater than 1μm, providing the alloy with basic plasticity, toughness and load-bearing capacity.

[0050] In some embodiments, the depth of the gradient grain structure is greater than 200 μm. Further, the depth of the gradient grain structure is 200 μm to 400 μm. As an example, the depth of the gradient grain structure is any single value or any value between two points selected from 200 μm, 250 μm, 300 μm, 350 μm, and 400 μm.

[0051] In some embodiments, the average grain size of the CrCoNi-based multi-principal-element alloy in the range of 0–500 nm from the surface is less than 50 nm, for example, 50 nm, 30 nm, etc. It can be understood that an average grain size of less than 50 nm corresponds to a nanocrystalline layer in a gradient grain structure.

[0052] In some embodiments, the maximum residual compressive stress of the CrCoNi-based multi-principal-element alloy within a range of 0 to 50 μm from the surface is ≥1.0 GPa, such as 1.0 GPa, 1.1 GPa, 1.2 GPa, 1.3 GPa, 1.4 GPa, etc.

[0053] In some embodiments, the average size of the coherent L12 nanoprecipitates is <30 nm. As an example, the average size of the coherent L12 nanoprecipitates is any one value or any value between two values ​​from 5 nm, 10 nm, 20 nm, and 30 nm.

[0054] The second aspect of this application also provides a method for preparing the above-mentioned CrCoNi-based multi-principal element alloy. Figure 1 This is a process flow diagram illustrating a method for preparing a CrCoNi-based multi-principal element alloy, as provided in this application embodiment. Please refer to [link / reference]. Figure 1 The preparation method includes the following steps: S10: The raw materials are mixed and smelted according to the atomic percentage of the CrCoNi-based multi-principal alloy to obtain an alloy ingot; wherein, the composition expression of the CrCoNi-based multi-principal alloy is (CrCoNi2). 100-x (AlNb) x x is the atomic percentage, 1≤x≤10.

[0055] As an example, x is any single point value or any value between two points in the range of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

[0056] In some embodiments, the mixed melting step includes: using metallic Cr, Co, Ni, Al, and Nb as raw materials, placing them in a vacuum suspension melting furnace according to the proportions, and repeatedly melting them 3-4 times under argon protection to obtain an alloy ingot with uniform composition.

[0057] Compared to stepwise melting processes, such as melting the CrCoNi matrix first and then introducing the precipitated metal, this application employs a one-time, all-component melting process. This allows for uniform mixing of elements at the atomic scale during the liquid stage, preventing local segregation of elements such as Nb and Al. This provides a uniform compositional basis for the formation of high-density, diffusely distributed, coherent L12 nano-precipitates during subsequent aging. It also helps maintain the precipitate size at the nanoscale (<30 nm), rather than producing coarse precipitates. Stepwise melting methods tend to lead to uneven element distribution, affecting the nucleation kinetics of the precipitate and making it difficult to obtain a nano-precipitate structure with "high density + high uniformity + strong coherence."

[0058] S20: The alloy ingot is subjected to rolling deformation, solution treatment and aging treatment in sequence to obtain an intermediate.

[0059] In some embodiments, the rolling deformation is cold rolling or hot rolling, with a total deformation of 70% to 90%. Preferably, cold rolling is performed at room temperature.

[0060] In some embodiments, the solution treatment includes: holding at 1000℃~1200℃ for 5min~20min followed by quenching.

[0061] Solution treatment can be carried out in a tube furnace; water quenching is the preferred method for quenching.

[0062] Understandably, the solution treatment can be performed once or multiple times.

[0063] As an example, the solution treatment temperature is any one value or any value between two values ​​from 1000℃, 1050℃, 1110℃, 1150℃, and 1200℃; the treatment time is any one value or any value between two values ​​from 5min, 10min, 15min, and 20min.

[0064] In some embodiments, the aging treatment includes: quenching after treatment at 600℃~800℃ for 8h~24h.

[0065] As an example, the aging treatment temperature can be any one value or any value between two points from 600℃, 650℃, 700℃, 750℃, and 800℃; the treatment time can be any one value or any value between two points from 8h, 10h, 15h, 20h, and 24h.

[0066] Understandably, the aging treatment can be performed once or multiple times. Preferably, the alloy after solution treatment is held at 720°C and 620°C for 8 hours respectively, followed by water quenching.

[0067] In some embodiments, before rolling deformation, the alloy ingot is further subjected to a homogenization treatment, which includes holding at 1100℃~1250℃ for 5h~24h.

[0068] As an example, the homogenization temperature is any one value or any value between two points from 1100℃, 1150℃, 1200℃, and 1250℃; the processing time is any one value or any value between two points from 10h, 15h, 20h, and 24h. S30: The intermediate is subjected to pre-strain treatment in sequence, and then surface mechanical grinding is performed at a temperature of -196℃ to -50℃.

[0069] In some embodiments, the pre-straining treatment is pre-stretching or pre-rolling, with a strain of 2% to 5%.

[0070] Preferably, the pre-strain treatment is pre-stretching along the rolling direction, that is, applying plastic tensile deformation along the forward direction of the sheet during rolling, which is beneficial for introducing a uniformly distributed high-density dislocation within the material. For example, the strain is 2%, 3%, 4%, 5%, etc.

[0071] In some embodiments, the process parameters for surface mechanical polishing (SMAT) include: using bearing steel balls with a diameter of 3 mm to 5 mm, a vibration frequency of 20 kHz to 50 kHz, and a processing time of 10 min to 60 min. For example, the diameter of the bearing steel balls may be 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, etc.; the vibration frequency may be 20 kHz, 30 kHz, 40 kHz, 50 kHz, etc.; and the processing time may be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, etc.

[0072] Preferably, the pre-strained plate is immersed in liquid nitrogen (-196°C) for sufficient cooling before surface mechanical grinding.

[0073] Understandably, surface mechanical polishing (SMAT) is an effective surface deformation strengthening technique, but it is mainly applicable to metal material systems with single phase or low strengthening phase content. For alloys containing high-density nano-precipitates, the critical stress for twin nucleation during deformation is very high because the precipitates significantly increase the stacking fault energy. Traditional room temperature SMAT technology is difficult to refine the surface grains and usually only forms a limited deformation layer on the surface, making it difficult to form a sufficiently deep gradient grain structure.

[0074] In this application, a "pre-strain-low temperature surface mechanical polishing (PC-SMAT)" method is used to synergistically regulate the stacking fault energy and difficult twinning deformation in precipitation-strengthened CrCoNi-based alloys. By introducing high-density dislocations and internal stress fields through pre-strain, the critical stress for subsequent plastic deformation is reduced. The low-temperature environment significantly reduces the stacking fault energy of the alloy and promotes the activation of deformation twins. The synergistic effect of the two methods activates the "twin-induced grain refinement mechanism" that is difficult to occur in precipitation-strengthened alloys, enabling the achievement of deep gradient nanostructures exceeding 200 μm and residual compressive stress as high as 1.5 GPa.

[0075] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0076] Example 1 This embodiment provides a CrCoNi-based multi-principal-element alloy, the preparation method of which includes the following steps: (1) Smelting: Using high-purity (>99.9%) metals Cr, Co, Ni, Al, and Nb as raw materials, according to (CrCoNi2) 91 The atomic percentages of (AlNb)9 were determined, with the atomic ratio of Cr:Co:Ni:Al:Nb being 23:23:45:4.5:4.5. All the prepared raw materials were placed in a vacuum suspension melting furnace and repeatedly melted four times under argon protection to obtain a homogeneous alloy ingot.

[0077] (2) Homogenization treatment: The alloy ingot is solution treated at 1200℃ for 6 hours to obtain a homogenized alloy.

[0078] (3) Rolling deformation: The homogenized alloy is cold rolled at room temperature, with a total deformation of 80%.

[0079] (4) Solution treatment: The cold-rolled sheet is placed in a tube furnace and solution treated at 1100℃ for 10 min, followed by water quenching to obtain a uniform FCC single-phase structure.

[0080] (5) Aging treatment: Then, the phases were aged at 720°C and 620°C for 8 hours respectively, followed by water quenching to obtain high-density, diffusely distributed L12-Ni3(Al, Nb) coherent nano-precipitates.

[0081] (6) Pre-strain: The aged alloy sheet is pre-stretched along the rolling direction with a strain of 3%.

[0082] (7) Low-temperature SMAT: After the pre-strained plate is immersed in liquid nitrogen (-196℃) and cooled completely, it is subjected to SMAT treatment. The SMAT process parameters are: using bearing steel balls with a diameter of 3mm, vibration frequency of 20kHz, and treatment time of 10min. After treatment, the sample is taken out and allowed to return to room temperature.

[0083] Example 2 This embodiment provides a CrCoNi-based multi-principal element alloy, the preparation method of which is largely the same as that in Example 1, the difference being: In step (7) low-temperature SMAT, the SMAT process parameters are: using bearing steel balls with a diameter of 3mm, vibration frequency of 20kHz, and processing time of 30min.

[0084] Comparative Example 1 This comparative example provides a CrCoNi-based multi-principal alloy, the preparation method of which differs from that of Example 1 in that: pre-straining and low-temperature SMAT treatment are not performed, i.e., steps (6) and (7) are not included.

[0085] Comparative Example 2 This comparative example provides a CrCoNi-based multi-principal element alloy, the preparation method of which differs from that of Example 1 in that: No pre-strain treatment is performed, and SMAT is performed at room temperature, i.e., step (6) is not included. In step (7), the aged board is directly subjected to SMAT treatment.

[0086] Performance testing and results analysis The performance of the CrCoNi-based multi-principal element alloys prepared in the above embodiments and comparative examples was tested. The test results are shown in Table 1. The specific test methods are as follows: (1) Microstructure The size, morphology, and uniformity of the coherent L12 nanoprecipitates were observed using transmission electron microscopy (TEM); the depth and grain size distribution of the gradient grain structure were observed using electron backscatter diffraction (EBSD).

[0087] (2) Residual compressive stress The residual compressive stress on the alloy surface was tested using focused ion beam-digital image correlation (FIB-DIC).

[0088] (3) Mechanical properties The alloy samples were machined into dog-bone shaped tensile specimens using a CNC wire EDM machine. The gauge length was 12.5 mm, the width was 3 mm, and the thickness was approximately 1 mm. The sample surfaces were smoothed with SiC sandpaper to remove wire cutting marks and surface oxide layers. Uniaxial tensile tests were conducted using a CMT5105 microcomputer-controlled electronic universal testing machine. All tests used a constant strain rate of 5 × 10⁻⁶. -4 s -1 We obtained a room temperature tensile curve of strain-stress, and obtained the yield strength, tensile strength and uniform elongation from the curve.

[0089] (4) Friction and wear performance Alloy samples were subjected to circumferential (3 mm radius) sliding friction and wear tests on an AntonPaar THT-800 high-temperature friction and wear testing machine. A ball-disc dry sliding wear test was used, with Φ6 mm silicon nitride ceramic balls as the grinding material. Wear tests were conducted in atmospheric environments (approximately 25 °C, RT) at room temperature and 600 °C (approximately 70% relative humidity), with a friction sliding speed of 0.1 m / s, a load of 5 N, and a sliding distance of 1000 m (approximately 53,000 revolutions). After the experiments, the friction coefficient curves were recorded; and the wear trajectory morphology was measured using a 3D profilometer. The wear rate was calculated based on the wear volume.

[0090] Table 1 Performance test results of CrCoNi-based multi-principal element alloys

[0091] Figure 2 Figure (a) is the electron backscattering diffraction (EBSD) pattern of the gradient grain structure of the CrCoNi-based multi-principal element alloy in Example 1 of this application. Figure 2 As can be seen from the above, the CrCoNi-based multi-principal alloy in Example 1 of this application has a gradient grain structure from the surface to the core, and the depth of the gradient grain structure is greater than 200 μm; the average grain size in the range of 0~500 nm from the surface is less than 50 nm.

[0092] Figure 2 Figure (b) is a transmission electron microscope (TEM) image of the coherent L12-type nanoprecipitates in the CrCoNi-based multi-principal-element alloy of Example 1 of this application. Figure 2As can be seen, there are high-density, uniformly distributed coherent L12-type nanoprecipitates inside the FCC matrix, with an average size of <20nm.

[0093] Figure 3 Figure (a) is a transmission electron microscope (TEM) image of the grain structure on the surface of the CrCoNi-based multi-principal-element alloy in Comparative Example 2 of this application; Figure (b) is an electron backscattering diffraction (EBSD) pattern of the grain structure on the surface of the CrCoNi-based multi-principal-element alloy in Comparative Example 2 of this application. Figure 3 As can be seen from the data, in Comparative Example 1, no pre-straining and low-temperature treatment were performed. Instead, only conventional SMAT treatment was used. The grain refinement of the alloy surface was insufficient, with almost no grain refinement and no obvious gradient grain structure.

[0094] Figure 4 These are room temperature tensile curves of CrCoNi-based multi-principal-element alloys and single-phase CrCoNi-based alloys from Examples 1, 1, and 2 of this application. Figure 4 As can be seen from Table 1, the yield strength of the alloy in Example 1 is 1495 MPa, the tensile strength is 1733 MPa, and the uniform elongation is 23%; while the yield strength of the alloy in Comparative Example 1 is only 1150 MPa and the tensile strength is only 1600 MPa; although the yield strength of the alloy in Comparative Example 2 is as high as 1623 MPa, the tensile strength is only 1683 MPa, and the uniform elongation is only 5%.

[0095] Figure 5 Figure (a) shows the friction and wear curve of the CrCoNi-based multi-principal alloy in Example 1 of this application at room temperature; Figure (b) shows the friction and wear curve of the CrCoNi-based multi-principal alloy in Example 1 of this application at 600°C. Figure 5 As can be seen from Table 1, the wear rate of this alloy at room temperature is as low as 1.05 × 10⁻⁶. -5 mm 3 The wear rate is reduced by 2-3 orders of magnitude compared to the untreated single-phase CrCoNi alloy, and by 42% compared to Comparative Example 1 (without pre-straining and low-temperature SMAT treatment). Even at 600℃, it still maintains a wear rate of 1.75 × 10⁻⁶ N·m. -6 mm 3 / (N•m), which means it exhibits excellent self-lubricating and anti-wear properties at both room temperature and high temperature, and the wear rate is reduced by 62% compared with Comparative Example 1 without pre-strain and low temperature SMAT treatment.

[0096] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A CrCoNi-based multi-principal-element alloy, characterized in that, The composition formula of the CrCoNi-based multi-principal element alloy is (CrCoNi2). 100-x (AlNb) x x is the atomic percentage, 1≤x≤10; The CrCoNi-based multi-principal alloy includes a matrix with a face-centered cubic structure and coherent L12-type nanoprecipitates distributed within the matrix. The CrCoNi-based multi-principal alloy has a gradient grain structure with gradually increasing grain size from the surface to the core, and there is a residual compressive stress that gradually decreases along the depth direction within the gradient grain structure.

2. The CrCoNi-based multi-principal-element alloy according to claim 1, characterized in that, From the surface to the core of the CrCoNi-based multi-principal-element alloy, the gradient grain structure sequentially includes a nanocrystalline or ultrafine grain layer, a submicron-sized deformed grain layer, and an undeformed fine grain structure.

3. The CrCoNi-based multi-principal-element alloy according to claim 1 or 2, characterized in that, The depth of the gradient grain structure is greater than 200 μm, and further 200 μm to 400 μm. And / or, the average grain size of the CrCoNi-based multi-principal alloy is less than 50 nm in the range of 0 to 500 nm from the surface.

4. The CrCoNi-based multi-principal element alloy according to claim 1, characterized in that, The maximum residual compressive stress of the CrCoNi-based multi-principal alloy within a range of 0~50μm from the surface is ≥1.0GPa.

5. The CrCoNi-based multi-principal-element alloy according to claim 1, characterized in that, The average size of the coherent L12 type nanoprecipitates is <30 nm.

6. A method for preparing a CrCoNi-based multi-principal-element alloy, characterized in that, Includes the following steps: According to the atomic percentage of the CrCoNi-based multi-principal alloy, the raw materials are mixed and smelted to obtain an alloy ingot; wherein, the composition expression of the CrCoNi-based multi-principal alloy is (CrCoNi2). 100-x (AlNb) x x is the atomic percentage, 1≤x≤10; The alloy ingot is subjected to rolling deformation, solution treatment and aging treatment in sequence to obtain an intermediate; The intermediate was subjected to pre-strain treatment in sequence, and then surface mechanical grinding was performed at a temperature of -196℃ to -50℃.

7. The preparation method according to claim 6, characterized in that, The pre-strain treatment is pre-stretching or pre-rolling, with a strain of 2% to 5%.

8. The preparation method according to claim 7, characterized in that, The process parameters for the surface mechanical grinding treatment include: using bearing steel balls with a diameter of 3mm to 5mm, a vibration frequency of 20kHz to 50kHz, and a processing time of 10min to 60min.

9. The preparation method according to claim 7, characterized in that, The solution treatment includes: holding at 1000℃~1200℃ for 5min~20min followed by quenching; And / or, the aging treatment includes: quenching after treatment at 600℃~800℃ for 8h~24h.

10. The preparation method according to claim 7, characterized in that, The rolling deformation is either cold rolling or hot rolling, and the total deformation is 70%~90%; And / or, prior to the rolling deformation, the alloy ingot is further subjected to a homogenization treatment, the homogenization treatment comprising: holding at 1100℃~1250℃ for 5h~24h.

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