Nanometer TiB2 reinforced CoCrNi medium-entropy alloy and preparation method thereof

Abstract

The invention discloses a nano TiB2 particle reinforced CoCrNi medium-entropy alloy and a preparation method of the nano TiB2 particle reinforced CoCrNi medium-entropy alloy. The preparation method comprises the following steps: firstly, respectively drying CoCrNi medium entropy alloy powder and nano TiB2 particles, then mechanically mixing, and finally carrying out ball milling treatment to obtain composite material powder. And under the condition of no substrate preheating, the composite material powder is subjected to additive manufacturing forming according to key parameters optimized in a specific range. The preparation method optimizes a pretreatment process and improves the quality of raw materials; and laser additive manufacturing process parameters are optimized, the strengthening effect of the nano TiB2 particles is fully exerted, and a high-performance material can be prepared without preheating the substrate. The average grain size of the obtained alloy is smaller than 3 microns, the density reaches 99.6%, the room-temperature ultimate tensile strength is larger than 1200 MPa, the yield strength is larger than 1000 MPa, the mechanical property is excellent, material guarantee is provided for key equipment application, and application of the CoCrNi medium-entropy alloy in the field of high-strength requirements is expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of new material processing, and particularly relates to a nano-TiB2 reinforced CoCrNi medium entropy alloy and a preparation method thereof. Background Art

[0002] Medium entropy alloys are considered potential candidates to replace traditional metallic materials due to their excellent mechanical, physical, and chemical properties. Among numerous medium entropy alloys, CrCoNi medium entropy alloy has attracted increasing attention due to its unprecedented properties. Especially at room temperature and low temperature, due to the plastic deformation mode, namely the {111}1 / 6<112> partial dislocations that dissociate early under tension and the subsequent activated deformation twins and transformation-induced plasticity effect, the CrCoNi medium entropy alloy has excellent ductility and fracture toughness. In particular, the formation of deformation twins caused by the low stacking fault energy of the (111) crystal plane is one of the main deformation mechanisms of the CrCoNi medium entropy alloy at low temperature. At the same time, the CrCoNi medium entropy alloy can form a stable and thick passivation film on the surface, having excellent corrosion resistance. Although the CrCoNi medium entropy alloy with a single face-centered cubic crystal structure has good ductility and fracture toughness, its yield strength is usually lower than 400 MPa, which is not sufficient to meet many engineering applications. How to improve the yield strength of the CrCoNi medium entropy alloy has become a key problem restricting its development.

[0003] In order to further improve the yield strength of the CrCoNi medium entropy alloy, adding ceramic particles to the matrix to construct a metal matrix composite material is an effective method. Ceramic particles are considered excellent candidates for strengthening pure metallic materials due to their low density, high hardness, high elastic modulus, and excellent wear resistance, and have been successfully used in additive manufacturing of iron-based composite materials, nickel-based superalloy matrix composite materials, and aluminum-based composite materials. Among numerous ceramic particles, nano-TiB2 particles are considered potential reinforcing phases due to their excellent mechanical properties (hardness ~30 GPa, elastic modulus ~550 GPa, flexural strength above 500 MPa), high-temperature stability (melting point 3225 °C), high oxidation resistance and corrosion resistance, as well as good thermal conductivity and electrical conductivity (60 - 120 W·m-1·K-1 and 106 S·cm-1 respectively). However, so far, the prior art has not reported how to obtain a CrCoNi medium entropy alloy with fine grains, high density, and ultra-high strength by reinforcing the CrCoNi medium entropy alloy with nano-TiB2 particles based on the laser additive manufacturing technology (i.e., Laser Power Bed Fusion, abbreviated as LPBF). Summary of the Invention

[0004] Based on this, it is necessary to provide a nano-TiB2 reinforced CoCrNi medium entropy alloy with fine grains, high density and ultra-high strength and its preparation method for the above technical problems. The proposal of this invention fills the gap of nano-TiB2 particle reinforced CrCoNi medium entropy alloy and further expands the application of CrCoNi medium entropy alloy in key engineering fields with high strength requirements such as aerospace.

[0005] In the first aspect, the present invention provides a preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy with fine grains, high density and ultra-high strength, which includes: Mix the CrCoNi medium entropy alloy powder with nano-TiB2 particles to obtain a mixed powder; Perform ball milling on the mixed powder; Perform additive manufacturing on the ball-milled composite powder; Among them, in step (1), before mixing, both the CrCoNi medium entropy alloy powder and the nano-TiB2 particles are subjected to drying treatment.

[0006] Furthermore, the drying temperature of the drying treatment is 75-85 °C and the drying time is 0.8-1.2 h.

[0007] Furthermore, the ball-milled composite powder is not dried before additive manufacturing.

[0008] Furthermore, the average particle size of the nano-TiB2 is 20-40 nm.

[0009] Furthermore, the mass fraction of the added nano-TiB2 is 4 wt%. Selecting an addition amount of 4 wt% of TiB2 can obtain excellent comprehensive mechanical properties. When the addition amount is less than 4 wt%, the TiB2 particles may not be sufficient to provide sufficient strengthening effect. When it exceeds 4 wt%, the excessive TiB2 may cause aggregation between particles, resulting in agglomeration, which may cause local stress concentration and an increase in matrix brittleness, and even affect the subsequent product performance. It should be noted that the above mass fraction is based on the total mass of the alloy powder + nano-particles.

[0010] Furthermore, the specific optimization range of the key process parameters of additive manufacturing: laser power 180-250 W, scanning speed 600-1000 mm / s.

[0011] Furthermore, the best key process parameters of additive manufacturing: laser power 180 W, scanning speed 600 mm / s.

[0012] Furthermore, other process parameters of additive manufacturing also include: filling line spacing 0.07 mm, layer thickness 0.03 mm.

[0013] The calculation formula for volume energy density is VED = P / vht, where P represents the laser power, v represents the laser scanning speed, h is the filling line spacing, and t is the layer thickness. In the present invention, the laser power is 180 - 250 W, the scanning speed is 600 - 1000 mm / s, the filling line spacing is 0.07 mm, and the layer thickness is 0.03 mm. The corresponding range of volume energy density is 85.71 - 174.60 J / mm 3 。

[0014] Furthermore, the ball milling treatment adopts a planetary ball milling method, the ball milling time is 110 - 130 min, and the rotational speed of the ball mill is 250 - 350 rpm. Even further, before the additively manufactured forming of the ball-milled composite powder, no drying is carried out because re-drying may have unpredictable effects on the powder morphology, grain boundary state, and even phase structure, such as causing surface oxidation, lattice stress release, etc., which may damage the original intention of the material design and thus affect the performance and finished product quality during additively manufactured forming.

[0015] In the second aspect, a nano-TiB2 reinforced CoCrNi medium entropy alloy with fine grains, high density, and ultra-high strength is obtained by the preparation method of the nano-TiB2 reinforced CoCrNi medium entropy alloy as described in the first aspect. Its average grain size < 3 μm, the density is as high as 99.6%, the ultimate tensile strength at room temperature > 1200 MPa, and the yield strength > 1000 MPa.

[0016] Furthermore, in step (3), there is no substrate preheating step before forming.

[0017] Compared with the prior art, the present invention has the following beneficial effects: In the preparation method of the present invention, before mixing the powders, the CrCoNi medium entropy alloy powder and the nano-TiB2 particles are first dried (the drying temperature is 75 - 85 °C and the drying time is 0.8 - 1.2 h), then placed in a powder mixer for sufficient mixing, and then ball milling treatment is carried out. This improved additive manufacturing pretreatment process not only reduces the drying temperature and time, but also ensures the uniform distribution of TiB2 particles in the powder, effectively avoiding the agglomeration of nano-particles, improving the density and performance consistency, and providing a high-quality raw material basis for subsequent processing. The TiB2 particles are more uniformly distributed in the CrCoNi medium entropy alloy, effectively activating the Orowan strengthening, dislocation pinning, and grain refinement mechanisms. The interfacial bonding with the matrix alloy is stronger, significantly inhibiting particle agglomeration and pore formation. In addition, the high melting point and thermal stability of TiB2 make it more structurally stable during the preparation process of high-energy beam flows such as LPBF, reducing the generation of reaction phases and brittle products, and further improving the finished product quality and consistency.

[0018] The preparation method of the present invention optimizes the key forming process parameters (laser power and scanning speed) within a specific range during the laser additive manufacturing process, effectively determines the optimal preparation parameters, and thus fully exerts the strengthening effect of TiB2 nanoparticles as the reinforcing phase. In addition, the preparation method of the present invention can prepare a crack-free composite material with a relative density as high as 99.6% without using a substrate preheating process before forming.

[0019] In the nanometer TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by the present invention, during the deformation process, since the nanometer TiB2 particles effectively refine the grain structure as heterogeneous nucleation sites, the average grain size is only 2.71 μm, thus significantly triggering the fine-grain strengthening effect and improving the overall strength of the material.

[0020] In the nanometer TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by the present invention, during the deformation process, due to the entanglement of high-density dislocations with each other and the interaction between dislocations and uniformly distributed nanometer TiB2 particles, the dislocation strengthening and Orowan strengthening effects are significantly triggered, thereby further improving the overall strength of the material. The ultimate tensile strength at room temperature can reach 1262 MPa, and the yield strength can reach 1020 MPa.

[0021] The CrCoNi medium-entropy alloy-based composite material prepared by the present invention not only exhibits excellent mechanical properties, and its strength at room temperature is significantly better than that of existing high-entropy alloy-based composite materials and other ceramic particle-reinforced CrCoNi medium-entropy alloy-based composite materials, thus providing a solid material guarantee for the application of key equipment under high-load working conditions. Description of the Drawings

[0022] In order to more clearly illustrate the solutions in the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings.

[0023] Figure 1 It shows the comparison of the mechanical properties of the nanometer TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared in the embodiments of the present invention under different laser additive manufacturing volume energy density conditions, as well as the engineering stress-strain curve of the material obtained under optimized laser additive manufacturing process parameters; Figure 2 It shows the change trend of the relative density of the nanometer TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared in the embodiments of the present invention under different laser additive manufacturing volume energy density conditions; Figure 3Shows the grain size distribution, kernel average orientation difference distribution and geometrically necessary dislocation density of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by optimizing the laser additive manufacturing process parameters in the embodiments of the present invention; Figure 4 Is the bright-field transmission electron microscopy image of the as-cast microstructure of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by optimizing the laser additive manufacturing process parameters in the embodiments of the present invention; Figure 5 Is the bright-field transmission electron microscopy image of the deformed microstructure of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by optimizing the laser additive manufacturing process parameters in the embodiments of the present invention; Figure 6 Is the strength comparison chart of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared in the embodiments of the present invention, the existing high-entropy alloy-based composites and other ceramic particle-reinforced CrCoNi medium-entropy alloy-based composites. Detailed implementation manners

[0024] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0025] In order to enable those skilled in the art to better understand the solution of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0026] As described in the technical background above, nano-TiB₂ particles are considered a potential reinforcement phase due to their excellent mechanical properties. However, to date, no patents have reported the use of laser additive manufacturing (LAM) to reinforce CrCoNi medium-entropy alloys with nano-TiB₂ particles. Prior art includes a Chinese invention patent (publication number CN116727686B) that discloses a TiC ceramic powder-reinforced laser additive manufacturing method for CoCrNi medium-entropy alloys. This method utilizes micron-sized TiC ceramic particles, and the strengthening effect relies primarily on the load transfer effect and second-phase strengthening mechanism brought about by the particles' high hardness. However, micron-sized particles have the following technical limitations: they can easily lead to poor interfacial bonding between the particles and the matrix; their large size difference with the matrix induces stress concentration, which can become a source of microcrack initiation; and they are difficult to fully fuse in rapid solidification processes (such as SLM), reducing the density and overall performance of the composite material. Therefore, nano-sized ceramic particles can be used to address the key issues of poor interfacial bonding and limited mechanical property improvement associated with micron-sized particle reinforcement. However, when micron-sized TiC ceramic particles are replaced with nano-sized reinforcement phase ceramic particles, the nanoparticles have a significantly higher surface energy due to their extremely high specific surface area and surface activity. This makes it easy for van der Waals forces or electrostatic adsorption to form between the particles, which can easily lead to severe agglomeration. This agglomeration directly affects the fluidity and packing density of the powders, leading to melt pool instability, increased porosity, and structural defects during the additive manufacturing process, ultimately causing a decrease in material density and a significant deterioration in mechanical properties.

[0027] To this end, the inventors conducted a large number of experiments and provided a method for preparing a fine-grained, high-density and ultra-high-strength nano-TiB2-reinforced CoCrNi medium-entropy alloy, which comprises: Mixing CrCoNi medium entropy alloy powder with nano-TiB2 particles to obtain a mixed powder; ball milling the mixed powder; The ball-milled composite powder is subjected to additive manufacturing; Wherein, in step (1), before mixing, the CrCoNi medium entropy alloy powder and the nano-TiB2 particles are both dried.

[0028] This invention not only uses nano-scale ceramic particles but also nano-TiB2 as a reinforcing phase. Although both TiC and TiB2 are ceramic materials, TiB2 possesses higher hardness (25-30 GPa) and superior thermal stability and electrical conductivity. When combined with the metal matrix CrCoNi, TiB2 more easily forms a clean and stable interface structure, effectively improving the overall mechanical properties and application reliability of the composite material.

[0029] Prior to powder mixing, the present invention first dries both the CrCoNi medium-entropy alloy powder and the nano-TiB2 particles (drying temperature: 75-85°C, drying time: 0.8-1.2 hours). The mixture is then thoroughly mixed in a powder mixer and ball-milled. This improved additive pretreatment process not only reduces drying temperature and time but also ensures uniform distribution of the TiB2 particles within the powder, effectively preventing nanoparticle agglomeration, improving density and performance consistency, and providing a high-quality raw material foundation for subsequent processing. The more uniform distribution of TiB2 particles within the CrCoNi medium-entropy alloy effectively activates Orowan strengthening, dislocation pinning, and grain refinement mechanisms. This results in a stronger interfacial bond with the matrix alloy, significantly inhibiting particle agglomeration and pore formation. Furthermore, TiB2's high melting point and thermal stability provide enhanced structural stability during high-energy beam fabrication processes such as SLM, reducing the formation of reactive phases and brittle products, further improving the quality and consistency of the finished product. The composite material obtained by the present invention has an average grain size of less than 3 μm, a density of up to 99.6%, an ultimate tensile strength of more than 1200 MPa at room temperature, a yield strength of more than 1000 MPa, and an ultimate tensile strength of up to 1262 MPa, which is significantly better than the existing level, reflecting the obvious advantages of the present invention in balancing performance and process.

[0030] The present invention optimizes the process flow. First, the CrCoNi medium-entropy alloy powder and nano-TiB2 particles are dried, and then fully mixed to ensure that the nano-TiB2 particles are evenly dispersed in the CrCoNi medium-entropy alloy powder. Subsequently, a planetary ball mill is used to ball-mill the fully mixed powder to ensure that the nano-TiB2 particles can firmly adhere to the surface of the CrCoNi medium-entropy alloy powder, thereby laying a high-quality raw material foundation for subsequent laser additive manufacturing. Finally, laser additive manufacturing technology is used to accurately prepare nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy. In this process, the uniform dispersion of nano-TiB2 particles and the in-situ regulation of the composition and microstructure of the CrCoNi medium-entropy alloy are achieved, effectively enhancing the mechanical properties of the alloy. This method provides an innovative path for the preparation of CrCoNi medium-entropy alloys with ultra-high strength at room temperature, breaking through the limitations of traditional preparation processes and laying a solid foundation for the development of high-performance alloy materials.

[0031] This invention utilizes a single laser additive manufacturing process to efficiently produce high-performance nano-TiB2 particle-reinforced CrCoNi medium-entropy alloys. Compared to traditional processes, this method significantly shortens the processing cycle and has the potential to enable the structurally integrated manufacturing of large, complex, and critical equipment, significantly improving manufacturing efficiency and its potential for industrial application.

[0032] The ball milling treatment adopts a planetary ball milling method, with a ball milling time of 110 - 130 min and a ball mill rotation speed of 250 - 350 rpm. The composite powder after ball milling is not dried before additive manufacturing forming.

[0033] The present invention adopts a more concise and efficient processing sequence: drying → powder mixing → ball milling, without introducing ultrasonic-assisted ball milling and re-drying after ball milling, effectively reducing energy consumption and process complexity, and being more conducive to industrial continuous preparation. During the ball milling treatment, the ball milling time is 110 - 130 min and the ball mill rotation speed is 250 - 350 rpm, which not only ensures the uniform attachment of nanoparticles while avoiding agglomeration, but also avoids the technical problems that a long ball milling time (6 - 10 h) is not conducive to controlling the surface state of the powder and may cause particle breakage and agglomeration.

[0034] The average particle size of the nano-TiB2 is 20 - 40 nm. Further, the mass fraction of the added nano-TiB2 is 4 wt%.

[0035] The TiB2 reinforcing particles used in the present invention are nano-scale. Compared with traditional micron-scale particles, they can achieve a more significant strengthening effect. Research shows that when the addition amount of nano-TiB2 particles is 4 wt%, the composite material can obtain the best comprehensive mechanical properties. During the rapid cooling process of laser additive manufacturing, the nano-TiB2 particles can serve as effective heterogeneous nucleation sites, significantly promoting grain refinement, thereby exerting an excellent fine grain strengthening effect. In the present invention, the TiB2 particles are used as a reinforcing phase in the CrCoNi matrix, and utilize their high hardness (30 GPa), thermal stability (melting point 3225 °C) and interface compatibility with the face-centered cubic matrix to form a unique strengthening effect; at the same time, the addition of nano-scale particles can stimulate the synergistic effect of fine grain strengthening, dislocation strengthening and Orowan strengthening, different from the single load transfer strengthening of traditional micron particles, forming a unique innovation in the strengthening mechanism.

[0036] The process parameters of additive manufacturing forming: laser power 180 - 250 W, scanning speed 600 - 1000 mm / s. More preferably, the process parameters of additive manufacturing forming: laser power 180 W, scanning speed 600 mm / s, filling line spacing 0.07 mm, layer thickness 0.03 mm.

[0037] The present invention does not require substrate preheating treatment, but directly adopts single laser beam scanning; at the same time, by optimizing the laser additive manufacturing process parameters, high-efficiency processing under low heat input is achieved, and the density and overall mechanical properties of the finished product are significantly improved. The prepared CrCoNi medium-entropy alloy matrix composite material is nearly fully dense, has ultrafine grains, a high dislocation density, and the nano-TiB2 particles are uniformly distributed in the matrix.

[0038] In addition, under different VED conditions, not only does the change in density directly affect the mechanical properties of the composite material, but the uniform dispersion degree of nano-TiB2 particles in the matrix and the formation of brittle intermetallic compounds are also significantly affected by the process parameters, thereby determining the final comprehensive performance. This systematic optimization of process parameters and meticulous control of the microstructure fully demonstrate the innovation and technical advantages of the present invention in improving material density, mechanical properties, and process stability.

[0039] The present invention will be described in detail below in conjunction with embodiments. The following embodiments are only preferred embodiments of the present invention and do not limit the present invention. Unless otherwise specified in this article, the solution concentration is the volume concentration.

[0040] Example 1 A preparation method of a nano-TiB2 reinforced CoCrNi medium-entropy alloy with fine grains, high density, and ultra-high strength, comprising the following steps: Mix the CrCoNi medium-entropy alloy powder with nano-TiB2 particles to obtain a mixed powder; Specifically, first, the nano-TiB2 particles with an average particle size of 30 nm and the CrCoNi medium-entropy alloy powder with a particle size of 15 - 53 μm (average particle size of 15 - 53 μm) are dried at a drying temperature of 80 °C for 1 h; subsequently, the dried powder is placed in a powder mixer, and the mass fraction of added TiB2 is 4 wt%, and sufficient mixing is carried out for 4 h to ensure the uniform dispersion of nano-TiB2 particles in the CrCoNi medium-entropy alloy powder; Perform ball milling on the mixed powder; Specifically, the planetary ball milling technology is used for ball milling, and its specific process parameters are: the ball milling time is 120 min, and the rotation speed of the ball mill is 300 rpm to ensure the uniform dispersion and firm attachment of nano-TiB2 particles on the surface of the CrCoNi medium-entropy alloy powder; Perform additive manufacturing on the ball-milled composite powder; Specifically, the ball-milled composite powder is placed in a laser additive manufacturing device, and a nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy is prepared by laser additive manufacturing technology. Laser additive manufacturing does not require substrate preheating treatment, but directly uses a single laser beam for scanning. The specific process parameters are: laser power 180 W, scanning speed 600 mm / s, filling line spacing 0.07 mm, layer thickness 0.03 mm; high-purity argon gas is used as the shielding gas during the additive manufacturing process to ensure the chemical purity of the material.

[0041] Figure 1In Example 1 of the present invention, the mechanical properties of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by different laser additive manufacturing volume energy densities are compared, as well as the engineering stress-strain curve of the material obtained under optimized laser additive manufacturing process parameters. Among them, Figure (a) is the engineering stress-strain curve, and Figure (b) is the comparison of the mechanical properties of the composite materials under various volume energy density conditions. The results show that when the laser volume energy density is 142.86 J / mm³, the prepared composite material exhibits the best comprehensive mechanical properties, with an ultimate tensile strength of 1262 MPa, a yield strength of 1020 MPa, and a total elongation of 4.85%.

[0042] Figure 2 In Example 1 of the present invention, it is the change trend of the relative density of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared under different laser additive manufacturing volume energy densities. The results show that when the laser volume energy density is 142.86 J / mm³, the relative density of the composite material reaches the highest value of 99.6%.

[0043] The present invention uses different laser additive manufacturing process parameters to prepare the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy. Before determining the optimal parameters, the density and tensile mechanical properties of the composite material samples under each process condition are tested. The relevant results are shown in the attached drawings of the specification Figure 1 (b) and Figure 2 . The test results show that when the laser volume energy density (VED) is 85.71 J / mm³, due to insufficient energy input and poor melt fluidity, irregularly shaped unfused pores appear in the composite material; as the VED gradually increases, the unfused pores are significantly eliminated, and only a small amount of spherical pores are retained in the sample. This is because a higher laser power or a lower scanning speed provides sufficient energy to form a stable molten pool. On the contrary, when the VED exceeds 142.86 J / mm³, excessive laser energy causes the material to over-melt and violently evaporate, the molten pool becomes unstable and a large number of pores are generated, ultimately causing a sharp drop in the density of the sample.

[0044] Figure 3 In Example 1 of the present invention, it is the grain size distribution, kernel average orientation difference distribution and geometrically necessary dislocation density of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by using optimized laser additive manufacturing process parameters. Among them, Figure (a) is the inverse pole figure, Figure (b) is the histogram of grain size frequency distribution, and Figure (c) is the histogram of kernel average orientation difference frequency distribution. The results show that the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by the present invention has a fine grain structure, with an average grain size of only 2.71 µm, and at the same time exhibits a very high dislocation density, and the geometrically necessary dislocation density is as high as 4.35×10 14m -2 。

[0045] Figure 4 This is a bright-field transmission electron microscopy image of the as-cast microstructure of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by using optimized laser additive manufacturing process parameters in Example 1 of the present invention. Among them, Figure (a) shows the cellular substructure in the as-cast microstructure of the composite material, and Figure (b) clearly shows the uniform distribution of nano-TiB2 particles in the CrCoNi medium-entropy alloy matrix. The results show that in the composite material prepared in the example of the present invention, the nano-TiB2 particles are distributed in the matrix with high uniformity.

[0046] Figure 5 This is a bright-field transmission electron microscopy image of the deformed microstructure of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared by using optimized laser additive manufacturing process parameters in Example 1 of the present invention. Figure (a) shows a high density of dislocation tangles and obvious dislocation slip traces in the composite material, while Figure (b) shows a large number of dislocation pile-ups around the nano-TiB2 particles. The results show that the interaction between dislocations and between dislocations and nano-TiB2 particles significantly activates the dislocation strengthening and Orowan strengthening effects, thereby further improving the overall strength of the material.

[0047] Figure 6 This is a strength comparison chart of the nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared in Example 1 of the present invention, the existing high-entropy alloy-based composite materials, and other ceramic particle-reinforced CrCoNi medium-entropy alloy-based composite materials. The nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy prepared in the example of the present invention exhibits excellent mechanical properties, and its strength is significantly higher than that of the existing high-entropy alloy-based composite materials and other ceramic particle-reinforced CrCoNi medium-entropy alloy-based composite materials.

[0048] The main functions of preheating the substrate in traditional LPBF are to reduce the thermal gradient, relieve residual stress, improve interlayer fusion, and control the microstructure to prevent crack and pore formation.

[0049] The present invention does not require substrate preheating treatment, but directly uses a single laser beam for scanning. At the same time, by optimizing the process parameters of laser additive manufacturing, efficient processing under low heat input is achieved, and the density and overall mechanical properties of the finished product are significantly improved. The prepared CrCoNi medium-entropy alloy-based composite material is nearly fully dense, has ultrafine grains, a high dislocation density, and the nano-TiB2 particles are uniformly distributed in the matrix. The present invention adopts a specific pretreatment sequence: drying → powder mixing → ball milling to obtain high-quality pretreated powder, which not only ensures the uniform attachment of nano-particles while avoiding agglomeration, but also avoids the technical problems that a long ball milling time is not conducive to controlling the surface state of the powder and may cause particle breakage and agglomeration. The addition of nano-TiB2 particles not only improves the crack resistance through particle strengthening and grain refinement, but also promotes uniform heat distribution and improves heat transfer efficiency. Further optimization of the laser parameters and high-quality powder pretreatment, that is, by adopting a specific material system and optimized laser process parameters, the two work together to ensure that even under the condition of no preheating, a stable and uniform molten pool can be formed, so as to achieve a high density of 99.6% and no cracks.

[0050] Example 2 A method for preparing a nano-TiB2-reinforced CoCrNi medium-entropy alloy with fine grains, high density and ultra-high strength, which comprises the following steps: (1) Mix the CrCoNi medium-entropy alloy powder with nano-TiB2 particles to obtain a mixed powder; Specifically, first, the nano-TiB2 particles with an average particle size of 20 nm and the CrCoNi medium-entropy alloy powder with a particle size of 15 - 53 μm are dried at a drying temperature of 85 °C for 0.8 h. Subsequently, the dried powder is placed in a powder mixer, and the mass fraction of added TiB2 is 4 wt%, and sufficient mixing is carried out for 4 h to ensure the uniform dispersion of the nano-TiB2 particles in the CrCoNi medium-entropy alloy powder; (2) Perform ball milling treatment on the mixed powder; Specifically, the ball milling treatment adopts planetary ball milling technology, and its specific process parameters are: the ball milling time is 130 min, and the rotation speed of the ball mill is 250 rpm to ensure the uniform dispersion and firm attachment of the nano-TiB2 particles on the surface of the CrCoNi medium-entropy alloy powder; (3) Perform additive manufacturing and forming on the ball-milled composite powder; Specifically, the ball-milled composite powder is placed in a laser additive manufacturing device, and a nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy is prepared by laser additive manufacturing technology. Laser additive manufacturing does not require substrate preheating treatment, but directly uses a single laser beam for scanning. The specific process parameters are as follows: laser power 200 W, scanning speed 800 mm / s, filling line spacing 0.07 mm, layer thickness 0.03 mm; high-purity argon gas is used as a protective gas during the additive manufacturing process to ensure the chemical purity of the material.

[0051] Example 3 A preparation method of a nano-TiB2-reinforced CoCrNi medium-entropy alloy with fine grains, high density and ultra-high strength, comprising the following steps: (1) Mix the CrCoNi medium-entropy alloy powder with nano-TiB2 particles to obtain a mixed powder; Specifically, first, the nano-TiB2 particles with an average particle size of 40 nm and the CrCoNi medium-entropy alloy powder with a particle size of 15 - 53 μm are dried. The drying temperature is 75 °C and the time is 1.2 h; subsequently, the dried powder is placed in a powder mixer, and the mass fraction of added TiB2 is 4 wt%, and sufficient mixing is carried out for 4 h to ensure the uniform dispersion of nano-TiB2 particles in the CrCoNi medium-entropy alloy powder; (2) Perform ball milling on the mixed powder; Specifically, the ball milling process uses planetary ball milling technology. The specific process parameters are as follows: the ball milling time is 110 min, and the rotation speed of the ball mill is 350 rpm to ensure the uniform dispersion and firm adhesion of nano-TiB2 particles on the surface of the CrCoNi medium-entropy alloy powder; (3) Perform additive manufacturing on the ball-milled composite powder; Specifically, the ball-milled composite powder is placed in a laser additive manufacturing device, and a nano-TiB2 particle-reinforced CrCoNi medium-entropy alloy is prepared by laser additive manufacturing technology. Laser additive manufacturing does not require substrate preheating treatment, but directly uses a single laser beam for scanning. The specific process parameters are as follows: laser power 250 W, scanning speed 1000 mm / s, filling line spacing 0.07 mm, layer thickness 0.03 mm; high-purity argon gas is used as a protective gas during the additive manufacturing process to ensure the chemical purity of the material.

[0052] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device.

[0053] As described above, the above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

[0054] Obviously, the embodiments described above are only a part of the embodiments of the present application, rather than all of the embodiments. The preferred embodiments of the present application are given in the drawings, but do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure content of the present application more thorough and comprehensive. Although the present application has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions described in the foregoing specific embodiments, or make equivalent substitutions for some of the technical features. Any equivalent structure directly or indirectly using the content of the specification and drawings of the present application in other related technical fields shall be similarly within the scope of the patent protection of the present application.

Claims

1. A preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy, characterized in that, It includes: Mixing CrCoNi medium-entropy alloy powder with nano-TiB2 particles to obtain mixed powder; Performing ball milling treatment on the mixed powder; Performing additive manufacturing on the ball-milled composite powder to obtain a nano-TiB2-reinforced CoCrNi medium-entropy alloy with fine grains, high density, and ultra-high strength; Among them, in step (1), before mixing, both the CrCoNi medium-entropy alloy powder and the nano-TiB2 particles are subjected to drying treatment.

2. The preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 1, characterized in that, The drying temperature of the drying treatment is 75-85 °C, and the drying time is 0.8-1.2 h.

3. The preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 1, characterized in that, Before the ball-milled composite powder is subjected to additive manufacturing, it is not dried.

4. The preparation method of a nano-TiB2 reinforced CoCrNi medium-entropy alloy according to claim 1, characterized in that, The average particle size of the nano-TiB2 is 20-40 nm; the mass fraction of the added nano-TiB2 is 4 wt%.

5. The preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 1, characterized in that, In step (3), there is no substrate preheating step before forming.

6. The preparation method of a nano-TiB2 reinforced CoCrNi medium-entropy alloy according to claim 6, characterized in that, The specific optimization range of the process parameters for additive manufacturing: laser power 180-250 W, scanning speed 600-1000 mm / s.

7. The preparation method of a nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 7, characterized in that, The process parameters for additive manufacturing: laser power 180 W, scanning speed 600 mm / s.

8. A method for preparing a nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 6 or 7, characterized in that, The range of the volumetric energy density in additive manufacturing forming is 85.71 - 174.60 J / mm 3 .

9. A nano-TiB2 reinforced CoCrNi medium entropy alloy, characterized in that, It is obtained by the preparation method of the nano-TiB2-reinforced CoCrNi medium-entropy alloy according to any one of claims 1-9.

10. A nano-TiB2 reinforced CoCrNi medium entropy alloy according to claim 1, characterized in that, Its average grain size <3 μm, the density is as high as 99.6%, the ultimate tensile strength at room temperature >1200 MPa, and the yield strength >1000 MPa.

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