A medium-entropy alloy and a preparation method thereof

Abstract

The application belongs to the alloy field, and specifically discloses a medium-entropy alloy and a preparation method thereof. 94 The Al3Ti3 medium-entropy alloy powder is used as a raw material, and an alloy block is prepared by using an electron beam selective melting additive manufacturing technology, and the additive manufacturing process parameters are optimized to obtain the alloy block with few defects such as holes and cracks, so that the medium-entropy alloy with high strength and toughness and high density under room temperature and low temperature conditions can be prepared, the medium-entropy alloy is suitable for rapidly forming metal components containing easily-oxidized elements, high performance or complex shape structures, and has great potential in low-temperature complex application.

Description

Technical Field

[0001] This invention belongs to the field of alloys, specifically relating to a medium-entropy alloy and its preparation method. Background Technology

[0002] Among numerous alloys, NiCoCr-based medium-entropy alloys exhibit low stacking fault energy, good fracture toughness, and a certain level of strength, making them suitable for manufacturing high-end equipment operating under extreme conditions such as low or ultra-low temperatures. The alloy preparation process and its composition significantly influence the resulting alloy's strength, toughness, density, and other properties. Currently, most simple medium-entropy alloy parts are manufactured using traditional processes such as casting and forging. However, novel processes such as selective electron beam melting (SEBM), laser cladding, cold spraying, thermal spraying, electrical discharge deposition, plasma spraying-physical vapor deposition, and magnetron sputtering can be used to prepare medium-entropy alloy parts with alloy coatings or thin layers. Additive manufacturing and other novel processes offer higher production efficiency compared to traditional methods and can directly and precisely construct more complex parts. However, for NiCoCr-based medium-entropy alloys, whether using traditional preparation processes or the aforementioned additive manufacturing processes, the problem of not being able to simultaneously achieve high strength and toughness remains. Furthermore, due to the significant differences in the physical and chemical properties of the various metal atoms in medium-entropy alloys, defects such as compositional segregation, residual stress, and voids often occur during the forming process. This not only hinders the forming process but also results in a significant gap between the strength, toughness, density, and other properties after forming and the desired performance.

[0003] In addition, the application environment of the alloy, such as extreme environments like room temperature or low temperature, also has a significant impact on the performance of the alloy material. Even if some alloys maintain good high strength and toughness at room temperature, they cannot maintain the high strength and toughness at room temperature when applied at low temperatures such as around 77K, and defects such as brittleness occur, causing the alloys that are used at room temperature to be unable to serve in low-temperature environments.

[0004] Therefore, it is still necessary to continue developing an additive manufacturing method for medium-entropy alloys that combine high strength, toughness, and high density under both room temperature and low temperature conditions. Summary of the Invention

[0005] The existing technologies for preparing high-strength and high-toughness NiCoCr-based medium-entropy alloys mainly rely on smelting, rolling, heat treatment, and machining, which leads to problems such as a large waste of alloy materials, long processing cycles, and difficulty in processing complex-shaped components. In addition, additive manufacturing of NiCoCr-based medium-entropy alloys also suffers from problems such as uneven composition and easy formation of micro-defects, and cannot achieve both high strength and high toughness and high density under extreme conditions such as room temperature, low temperature, or ultra-low temperature. This invention will provide a medium-entropy alloy and its preparation method, so as to achieve a medium-entropy alloy that can achieve both high strength and high toughness and high density under extreme conditions such as room temperature and low temperature.

[0006] To achieve the above objectives, the following technical solutions are specifically included:

[0007] A method for preparing a medium-entropy alloy includes the following steps: (NiCoCr) in atomic stoichiometric proportions... 94 Al3Ti3 medium-entropy alloy powder is formed on a substrate using an electron beam selective melting process to obtain the medium-entropy alloy.

[0008] The medium-entropy alloy composition of this invention is (NiCoCr). 94 Al3Ti3, in which NiCoCr-based medium-entropy alloys exhibit good fracture toughness, is prepared by Al and Ti elements promoting the formation of L12 precipitates. These nano-L12 precipitates strengthen the NiCoCr matrix and also prevent grain coarsening and dislocation movement, thus playing a role in precipitation strengthening, grain refinement strengthening, and dislocation strengthening, thereby improving the room temperature and low temperature mechanical strength and toughness of the alloy. Especially under low temperature conditions, the medium-entropy alloy of this invention still maintains high strength and toughness. The main reasons are: (i) the nano-L12 precipitates hinder moving dislocations, improving mechanical strength; (ii) due to the low stacking fault energy characteristics, full dislocation slip easily decomposes into stacking faults, or even network stacking fault structures, which strongly hinder moving dislocations, improving both mechanical strength and work hardening rate, thereby enhancing the alloy's stable plastic deformation capability; (iii) the coexistence of dislocation slip and mechanical twinning-induced plasticity ensures excellent plasticity.

[0009] Preferably, the electron beam selective melting process includes the following process parameters: the substrate is also preheated at a temperature of 700–900°C, and the current during forming is 10–21 mA, the scanning speed is 2.5–7.5 m / s, and the scanning spacing is 50–100 μm; or, the current is 16–18 mA, the scanning speed is 4–5 m / s, and the scanning spacing is 50–75 μm.

[0010] The parameters of the electron beam selective melting process control the alloy powder forming process on the substrate. The current, scanning rate, and scanning spacing of the electron beam selective melting process have mutual influence and constraints on the forming process. This invention can be carried out under the above process parameters. Further preferred electron beam selective melting process includes the following process parameters: current of 16.8mA, scanning speed of 4.2m / s, and scanning spacing of 50μm, which results in better strength, toughness, and density of the medium-entropy alloy. In addition, substrate preheating provides in-situ aging heat treatment, inducing precipitates, which in turn inhibits grain growth, providing excellent grain refinement strengthening for the medium-entropy alloy, further improving its strength and toughness. Preheating can also reduce the stress of the additive medium-entropy alloy and prevent alloy cracking, resulting in low residual stress in the final alloy, suitable for forming highly active and high-performance metal components.

[0011] Preferably, the substrate is made of molybdenum. Using a molybdenum substrate can reduce the warpage of the medium-entropy alloy. Using stainless steel substrates, titanium alloy substrates, or nickel-based high-temperature alloy substrates can easily lead to warpage of the additive sample in the early or late stages of additive manufacturing of the medium-entropy alloy, making it impossible to obtain a large additive sample with a height of 20mm.

[0012] Preferably, the thickness of the medium-entropy alloy is 20-50 mm.

[0013] Preferably, the (NiCoCr) 94 The average particle size of Al3Ti3 alloy powder is 50-110 μm.

[0014] Preferably, the composition of the medium-entropy alloy is (NiCoCr). 94 The Al3Ti3 medium-entropy alloy has a pore area fraction of 1-20% and a pore number density of 8×10⁻⁶. -5 -2×10 -4 per μm 2 .

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention uses (NiCoCr) 94 Using Al3Ti3 medium-entropy alloy powder as raw material, alloy bulk materials are prepared using electron beam selective melting additive manufacturing technology. By optimizing the additive manufacturing process parameters, alloy bulk materials with fewer defects such as pores and cracks can be obtained. This allows for the production of high-strength, high-toughness, and high-density medium-entropy alloys that can withstand both room temperature and low temperature conditions. These alloys are suitable for rapid prototyping of metal components containing easily oxidized elements, high-performance components, or complex shapes and structures, and have great potential for application in complex low-temperature conditions. Specifically, the yield strength of the medium-entropy alloy at room temperature is 600-800 MPa, the tensile strength is 800-1000 MPa, and the elongation is 30-40%. At 77 K, the yield strength of the medium-entropy alloy is 800-1000 MPa, the tensile strength is 1300-1500 MPa, and the elongation is 40-50%. Attached Figure Description

[0016] Figure 1 Crack defects in the additive entropy alloy prepared in Example 3.

[0017] Figure 2 Micron-sized pores in the additive medium-entropy alloy prepared in Example 1.

[0018] Figure 3 Microstructure of the additive medium-entropy alloy prepared in Example 1.

[0019] Figure 4 Nanoscale L12 phase of the additive medium-entropy alloy prepared in Example 1. Detailed Implementation

[0020] To better illustrate the purpose, technical solution, and advantages of this invention, specific embodiments will be used to further explain the invention below. Unless otherwise specified, the test methods used in the embodiments and / or comparative examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0021] Example 1

[0022] (1) The commercially available product is composed of (NiCoCr). 94 Al3Ti3 alloy powder (in atomic stoichiometry) with a particle size of 53–105 micrometers is placed in the powder hopper of an electron beam selective melting additive manufacturing equipment, and the equipment is evacuated.

[0023] (2) Preheat for 8 hours until the molybdenum substrate reaches 900°C;

[0024] (3) The beam current in the electron beam selective melting equipment is set to 16.8mA, the scanning speed is 4.2m / s, the sample scanning distance is 50μm, and the layer thickness is set to 50μm. Based on the corresponding position of each layer and the preset shape structure, the alloy powder in the selected area is melted by the focused beam. After the alloy powder is melted, it is formed on the substrate to obtain a medium-entropy alloy containing the substrate.

[0025] Example 2

[0026] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 16.8mA, the scanning speed is 4.2m / s, the sample scanning spacing is 75μm, and the layer thickness is set to 50μm.

[0027] Example 3

[0028] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 16.8mA, the scanning speed is 4.2m / s, the sample scanning spacing is 100μm, and the layer thickness is set to 50μm.

[0029] Example 4

[0030] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 10mA, the scanning speed is 2.5m / s, the sample scanning spacing is 75μm, and the layer thickness is set to 50μm.

[0031] Example 5

[0032] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 10mA, the scanning speed is 2.5m / s, the sample scanning distance is 100μm, and the layer thickness is set to 50μm.

[0033] Example 6

[0034] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 21mA, the scanning speed is 7.5m / s, the sample scanning spacing is 75μm, and the layer thickness is set to 50μm.

[0035] Example 7

[0036] The difference between this embodiment and embodiment 1 lies in the setting of electron beam selective melting parameters in step (3). In this embodiment, the electron beam selective melting current is set to 21mA, the scanning speed is 7.5m / s, the sample scanning interval is 100μm, and the layer thickness is set to 50μm.

[0037] Comparative Example 1

[0038] The difference between this comparative example and Example 1 lies in the setting of electron beam selective melting parameters in step (3). In this example, the electron beam selective melting current is set to 10mA, the scanning speed is 2.5m / s, the sample scanning spacing is 50μm, and the layer thickness is set to 50μm. Under the process parameters in step (3), the warping of this comparative example is severe, making it impossible to form.

[0039] Comparative Example 2

[0040] The difference between this comparative example and Example 1 lies in the setting of electron beam selective melting parameters in step (3). In this example, the electron beam selective melting current is set to 21 mA, the scanning speed is 7.5 m / s, and the sample scanning distance is 50 μm. This comparative example cannot be formed under the process parameters in step (3).

[0041] The medium-entropy alloys prepared in Examples 1-6 were subjected to defect testing. The specific testing method is as follows: The prepared medium-entropy alloy blocks were cut into cross-sections using wire cutting, followed by mechanical polishing. Then, they were observed using an optical microscope and a scanning electron microscope. Five images of each sample were randomly taken at 100-200x magnification. Based on the five images, the average number, size, and area ratio of holes were statistically analyzed. The test results are shown in Table 1.

[0042] The medium-entropy alloy obtained in Example 1 was subjected to mechanical property tests at room temperature and 77K, respectively. The specific testing methods are as follows: a tensile sample was prepared by wire cutting a portion of the medium-entropy alloy, and then a room temperature tensile test was performed on a tensile testing machine at a tensile rate of 5 × 10⁻⁶.-4 / s. Additionally, a tensile specimen was prepared using a portion of the medium-entropy alloy obtained by wire cutting. The specimen was then fixed on a tensile testing machine and immersed in liquid nitrogen for at least 30 minutes. After cooling to 77 K, the tensile rate was 5 × 10⁻⁶. -4 / s. Samples obtained under each process condition were tested three times at room temperature and 77K, and the average value was taken as the mechanical property index of the sample. The test results showed that the medium-entropy alloy prepared in Example 1 had a yield strength of 612MPa, a tensile strength of 941MPa, and an elongation of 37% at room temperature, and a yield strength of 829MPa, a tensile strength of 1348MPa, and an elongation of 41% at 77K.

[0043] Table 1. Influence of electron beam selective melting additive manufacturing parameters on void defects.

[0044]

[0045] As can be seen from the above embodiments, the medium-entropy alloy prepared by the present invention has a yield strength of 600-800 MPa, a tensile strength of 800-1000 MPa, and an elongation of 30-40% at room temperature; a yield strength of 800-1000 MPa, a tensile strength of 1300-1500 MPa, and an elongation of 40-50% at 77 K; a void area fraction of 1-20%; and a void number density of 8 × 10⁻⁶. -5 -2×10 -4 per μm 2 It possesses the characteristics of high strength and toughness, and high density under both room temperature and low temperature conditions. Furthermore, as shown in Example 1 and Comparative Examples 1-2, Examples 1 and 4 have the lowest void area fraction and the fewest crack defects; Examples 2, 3, 5, 6, and 7 not only have high void area fractions but also large crack defects. Cracks and void defects can severely impair the mechanical properties of high-strength and high-toughness alloys; the more cracks and void defects, the worse the material's strength and toughness, especially for high-strength materials. The current, scanning speed, and scanning spacing in the electron beam selective melting process have mutual influences and constraints on the alloy forming process. It can be carried out under the following conditions: current of 10-21 mA, scanning speed of 2.5-7.5 m / s, and scanning spacing of 75-100 μm; or current of 16-18 mA, scanning speed of 4-5 m / s, and scanning spacing of 50-75 μm. Further optimization is achieved when the current is 16.8 mA, the scanning speed is 4.2 m / s, and the scanning spacing is 50 μm, resulting in better strength, toughness, and density of the medium-entropy alloy.

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a medium-entropy alloy, characterized in that, Includes the following steps: The substrate undergoes a preheating treatment, and (NiCoCr) is added in atomic stoichiometric proportions. 94 Al3Ti3 medium-entropy alloy powder is formed on a substrate by electron beam selective melting process to obtain the medium-entropy alloy; the electron beam selective melting process includes the following process parameters: the preheating temperature is 700~900℃, the forming current is 10-21mA, the scanning speed is 2.5-7.5m / s, and the scanning spacing is 50-100μm.

2. The method for preparing the medium-entropy alloy as described in claim 1, characterized in that, The electron beam selective melting process includes the following process parameters: current of 16.8 mA, scanning speed of 4.2 m / s, and scanning spacing of 50 μm.

3. The method for preparing the medium-entropy alloy as described in claim 1, characterized in that, The substrate is made of molybdenum.

4. A medium-entropy alloy prepared by the method of any one of claims 1 to 3.

5. The medium-entropy alloy as described in claim 4, characterized in that, The thickness of the medium-entropy alloy is 20-50 mm.

6. The medium-entropy alloy as described in claim 4, characterized in that, The composition of the medium-entropy alloy is (NiCoCr). 94 Al3Ti3, the medium-entropy alloy has a pore area fraction of 1-20% and a pore number density of 8×10⁻⁶. -5 -2×10 -4 per μm 2 .

7. The medium-entropy alloy as described in claim 4, characterized in that, The medium-entropy alloy has a yield strength of 600-800 MPa, a tensile strength of 800-1000 MPa, and an elongation of 30-40% at room temperature; and a yield strength of 800-1000 MPa, a tensile strength of 1300-1500 MPa, and an elongation of 40-50% at 77 K.

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