A low-temperature phase-change second-phase reinforced NiCoV series entropy alloy and a preparation method thereof

Abstract

This invention discloses a low-temperature phase-transformable second-phase strengthened NiCoV-based medium-entropy alloy and its preparation method, belonging to the field of metallic materials. The alloy is mainly composed of Ni, Co, and V elements, with appropriate amounts of Al and Ta added. Through multi-pass large deformation and annealing aging processes, a multi-level structure of L21 phase and local chemical order is constructed in an FCC matrix. This invention utilizes the large lattice distortion and multi-level response of the local chemical order of the L21 phase to endow the alloy with extremely high continuous work hardening capability, achieving a synergistic effect of yield strength and fracture elongation at room temperature and liquid nitrogen temperature. It is suitable for armoring materials of cryogenic structural components in deep-sea aerospace and other applications.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, specifically relating to a low-temperature phase-transformable second-phase reinforced NiCoV-based medium-entropy alloy and its preparation method. Background Technology

[0002] Structural materials used in extreme cryogenic environments (such as superconducting engineering and liquid hydrogen storage and transportation) must possess both ultra-high yield strength and fracture toughness at extremely low temperatures. NiCoV-based medium-entropy alloys exhibit excellent corrosion resistance and wide-temperature-range mechanical properties, especially at low temperatures, demonstrating significant application potential. Therefore, finding the mechanical property limits of NiCoV-based medium-entropy alloy systems has become an urgent materials science problem to be solved.

[0003] Second-phase strengthening is a traditional strengthening method for metallic materials. Existing NiCoV-based or NiCoVAl-based entropy alloys can improve strength by precipitating traditional κ or B2 phases. However, these hard and brittle second phases usually have very small lattice distortions, making it difficult to coordinate deformation with the matrix, thus improving strength but sacrificing ductility. Although controlling local chemical order can achieve a synergistic improvement in strength and ductility, its strength-ductility product is still insufficient for future development and has room for improvement. Summary of the Invention

[0004] To address the aforementioned technical problems, or some of them, this invention proposes a low-temperature phase-transformable second-phase strengthened NiCoV-based medium-entropy alloy and its preparation method. Based on the NiCoV system, this invention incorporates Al and Ta alloying design and mechanical-thermal treatment processes, promoting the formation of an L21 phase with extremely high lattice strain and high-density localized chemical order in a face-centered cubic matrix, potentially achieving a comprehensive improvement in low-temperature performance.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A low-temperature phase-change-capacitance (LCC)-based NiCoV-based medium-entropy alloy, wherein the constituent elemental composition of the NiCoV-based medium-entropy alloy, by atomic percentage, is Ni: 35-45 at%, Co: 19-29 at%, V: 19-29 at%, Al: 5-10 at%, Ta: 1-5 at%. The low-temperature LCC-based LCC-based medium-entropy alloy has a multi-level structure, including a face-centered cubic solid solution matrix, an L21 phase dispersed within the matrix and at grain boundaries, and a locally ordered chemical structure.

[0006] Furthermore, the low-temperature phase-change-reinforced NiCoV-based entropy alloy is permitted to contain trace amounts of elements such as B, C, W, Zr, La, and Ce as modifiers, up to ≤1 at %.

[0007] A method for preparing a low-temperature phase-transformable second-phase reinforced NiCoV-based medium-entropy alloy, specifically including the following steps: (1) Clean the metal raw materials according to the elements of the low-temperature phase-change-enhanced NiCoV system entropy alloy, and then weigh the elements according to atomic percentage; (2) The metal raw materials are smelted in a non-consumable argon protective vacuum arc furnace. After the smelting is completed, the alloy is cast into a multi-principal alloy ingot using a non-consumable vacuum arc furnace. (3) The multi-principal alloy ingot is subjected to homogenization heat treatment under argon protection; (4) Cut the core plate from the homogenized heat-treated ingot and roll it at room temperature; (5) The rolled plate was subjected to recrystallization annealing heat treatment and aging heat treatment to obtain NiCoV-based medium entropy alloy.

[0008] Further, in step (1), the purity of the metal raw material is ≥99.95 wt%, wherein the purity of the vanadium metal raw material is ≥99.99 wt%, the oxide scale is removed and the material is cleaned, and then the elements are weighed according to atomic percentage.

[0009] Furthermore, in step (2), the vacuum level inside the vacuum arc melting furnace needs to be controlled at 1×10 before melting. -4 The pressure is below Pa, and argon inert gas is introduced as a protective gas. During smelting, the titanium ingot must be smelted first to absorb the residual oxygen in the vacuum arc melting furnace, and then the raw material is smelted repeatedly at least 4 times. After smelting, the alloy is cast into a rectangular ingot with a final size of 80×10×10 mm.

[0010] Furthermore, in step (3), the ingot is sealed in a quartz tube, and the raw material is placed in the quartz tube. The vacuum degree is controlled at 1×10⁻⁶. -4 After the pressure drops below Pa, a small amount of argon gas is introduced and the quartz tube is sealed. Then, the sealed ingot is subjected to homogenization heat treatment at a temperature of 1000-1200℃ for 4-24 hours, followed by water cooling.

[0011] Further, in step (4), the homogenized heat-treated ingot is cut into plates using electrical discharge wire cutting. The plates need to be cut away from the ingot riser, the central loose area and the surface oxidation area, and rolled at room temperature. The rolling reduction is controlled at 50%-80%, and finally a plate with a thickness of 1mm-2mm is obtained.

[0012] Further, in step (5), the rolled plate is subjected to recrystallization annealing heat treatment at a temperature of 1000-1150℃ for 1-10 min, then water-cooled to room temperature, and then subjected to aging heat treatment at a temperature of 400-600℃ for 0.5-48 h, and then air-cooled to room temperature.

[0013] A low-temperature phase-change-capacitance-reinforced NiCoV-based medium-entropy alloy is prepared according to the above-mentioned preparation method of the low-temperature phase-change-capacitance-reinforced NiCoV-based medium-entropy alloy.

[0014] Compared with the prior art, the present invention exhibits the following advantages and technical effects: This invention relates to a low-temperature phase-transformable second-phase strengthened NiCoV-based medium-entropy alloy with a dense and uniform internal structure and no surface defects. Through composition design and mechanical heat treatment process control, the brittle second phase L21 is toughened, and a high strength-plasticity synergy is obtained in a low-temperature environment. It has industrial transformation potential in extreme application fields such as deep space exploration and cryogenic engineering. Attached Figure Description

[0015] Figure 1 The results of high-energy synchrotron X-ray diffraction of the medium-entropy alloy prepared in this invention represent the phase structure characteristics of the alloy. Figure 2 The results are scanning electron microscopy of the medium-entropy alloy prepared in this invention, showing the microstructure and morphology of the alloy. Figure 3 These are transmission electron microscopy results of the medium-entropy alloy prepared in this invention, representing the local chemical order structure characteristics of the alloy; Figure 4 These are tensile engineering stress-strain diagrams of the medium-entropy alloy prepared by this invention under different heat treatment conditions in room temperature / liquid nitrogen environments. Figure 5 The results are in-situ tensile measurements of the medium-entropy alloy prepared by high-energy synchrotron X-ray diffraction. Detailed Implementation

[0016] The application and implementation of the present invention will be further illustrated below through specific embodiments: Example 1: This embodiment provides a method for preparing a low-temperature phase-transformable second-phase reinforced NiCoV-based medium-entropy alloy. The medium-entropy alloy has the following atomic percentages: Ni: 42 at%, Co: 24 at%, V: 24 at%, Al: 7 at%, Ta: 3 at%. The specific preparation process is as follows: Step (1): Wash the elemental raw materials with anhydrous ethanol for 30 minutes and then dry them. Then weigh the elements according to atomic percentage, with each button ingot weighing about 50g. Step (2): Multi-principal alloys are smelted using a non-consumable argon-protected vacuum arc furnace. Before smelting, the vacuum level inside the vacuum arc furnace must be controlled at 1×10⁻⁶. -4 The pressure is below Pa, and argon inert gas is introduced as a protective gas. During the melting process, the titanium ingot needs to be melted first to absorb the residual oxygen in the vacuum arc melting furnace. The melting is repeated 4 times and electromagnetic stirring is used to ensure uniform composition. After the melting is completed, it is cast into a multi-principal element alloy ingot with an ingot size of 80×10×10 mm. In step (3), the ingot is sealed in a quartz tube, and the raw material is placed in the quartz tube. The vacuum degree is controlled at 1×10 -4 After the pressure drops below Pa, a small amount of argon gas is introduced and the quartz tube is sealed. Then, the sealed ingot is subjected to homogenization heat treatment at a temperature of 1180℃ for 12 hours, followed by water cooling. Step (4): Cut the ingot into 5mm thick plates using electrical discharge wire cutting. The plates should be cut away from the ingot riser, the central loose area and the surface oxidation area. Then, roll the plates in multiple passes at room temperature with a reduction of about 80% to a thickness of 1mm. Step (5): The cold-rolled sheet is subjected to recrystallization annealing heat treatment at a temperature of 1050℃ for 2 minutes. After annealing, it is water-cooled. The annealed filament is then subjected to aging heat treatment at a temperature of 500℃ for 24 hours. After aging, it is air-cooled.

[0017] Organizational observation: The sample was tested using high-energy synchrotron radiation diffraction (λ=0.1259 Å, spot size 500 μm×500 μm). The results showed that the medium-entropy alloy in this embodiment possesses FCC and L21 phases, such as... Figure 1 As shown. Furthermore, the microstructure of the alloy was observed using scanning electron microscopy and transmission electron microscopy. The results showed that the alloy in this embodiment has a uniform internal microstructure, possessing FCC, L21 phase, and a localized chemical order structure with L12 spots, as shown. Figure 2 and Figure 3 As shown. The multiphase microstructure obtained in this embodiment can optimize stress distribution during tensile testing, which helps to achieve a balance between strength and toughness in the alloy. Local Chemical Order (LCO) refers to the unique microstructure of the alloy resulting from fluctuations in the nanoscale chemical composition formed by atomic pair bonding. It is widely present in multi-principal element alloys and has a positive effect on the mechanical properties of the alloy.

[0018] Mechanical property testing: Standard plate-shaped tensile specimens were cut from the heat-treated alloy and subjected to tensile tests at room temperature and in liquid nitrogen environments, with a tensile rate of 1×10⁻⁶. -3 / s. The room temperature yield strength of the medium-entropy alloy in this embodiment is 1238.3±12 MPa, the room temperature ultimate tensile strength is 1618.9±16 MPa, and the room temperature elongation at break is 36.4±1.8% (e.g., ...). Figure 4 (As shown). In this embodiment, the yield strength of the medium-entropy alloy in a liquid nitrogen environment is 1302.1±13 MPa, the cryogenic tensile strength in a liquid nitrogen environment is 1815.5±18 MPa, and the cryogenic elongation at break in a liquid nitrogen environment is 39.4±1.9% (as shown). Figure 4 (As shown).

[0019] In-situ tensile testing: In-situ tensile tests were performed on the samples using high-energy synchrotron radiation diffraction (λ=0.1239 Å, spot size 500 μm × 500 μm). Figure 5 As shown, the results indicate that the L21 phase of the medium-entropy alloy in this embodiment undergoes large lattice distortion during plastic deformation, proving that a phase transformation has occurred and instability has not occurred.

[0020] Example 2: This embodiment provides a method for preparing a low-temperature phase-change-capacitance-enhanced NiCoV-based medium-entropy alloy. The medium-entropy alloy has the following atomic percentages: Ni: 44 at%, Co: 23 at%, V: 24 at%, Al: 6 at%, Ta: 3 at%. The specific preparation process is as follows: Step (1): Wash the elemental raw materials with anhydrous ethanol for 30 minutes and then dry them. Then weigh the elements according to atomic percentage, with each button ingot weighing about 50g. Step (2): Multi-principal alloys are smelted using a non-consumable argon-protected vacuum arc furnace. Before smelting, the vacuum level inside the vacuum arc furnace must be controlled at 1×10⁻⁶. -4 The pressure is below Pa, and argon inert gas is introduced as a protective gas. During the melting process, the titanium ingot needs to be melted first to absorb the residual oxygen in the vacuum arc melting furnace. The melting is repeated 4 times and electromagnetic stirring is used to ensure uniform composition. After the melting is completed, it is cast into a multi-principal element alloy ingot with an ingot size of 80×10×10 mm. In step (3), the ingot is sealed in a quartz tube, and the raw material is placed in the quartz tube. The vacuum degree is controlled at 1×10 -4 After the pressure drops below Pa, a small amount of argon gas is introduced and the quartz tube is sealed. Then, the sealed ingot is subjected to homogenization heat treatment at a temperature of 1180℃ for 12 hours, followed by water cooling. Step (4): Cut the ingot into 5mm thick plates using electrical discharge wire cutting. The plates should be cut away from the ingot riser, the central loose area and the surface oxidation area. Then, roll the plates in multiple passes at room temperature with a reduction of about 80% to a thickness of 1mm. Step (5): The cold-rolled sheet is subjected to recrystallization annealing heat treatment at a temperature of 1050℃ for 2 minutes. After annealing, it is water-cooled. The annealed filament is then subjected to aging heat treatment at a temperature of 500℃ for 24 hours. After aging, it is air-cooled.

[0021] Organizational observation: The sample was tested using high-energy synchrotron radiation diffraction (λ=0.1259 Å, spot size 500 μm×500 μm). The results showed that the medium-entropy alloy in this embodiment possesses FCC and L21 phases, such as... Figure 1 As shown.

[0022] Mechanical property testing: Standard plate-shaped tensile specimens were cut from the heat-treated alloy and subjected to tensile tests at room temperature and in liquid nitrogen environments, with a tensile rate of 1×10⁻⁶. -3 / s. The room temperature yield strength of the medium-entropy alloy in this embodiment is 1003.5±10 MPa, the room temperature ultimate tensile strength is 1445.8±14 MPa, and the room temperature elongation at break is 47.4±2.3% (e.g., ...). Figure 4 (As shown). The yield strength of the medium-entropy alloy in this embodiment in a liquid nitrogen environment is 1133.7±11 MPa, the cryogenic tensile strength in a liquid nitrogen environment is 1653.6±17 MPa, and the cryogenic elongation at break in a liquid nitrogen environment is 44.4±2.2% (as shown). Figure 4 (As shown).

[0023] In-situ tensile testing: In-situ tensile tests were performed on the samples using high-energy synchrotron radiation diffraction (λ=0.1239 Å, spot size 500 μm × 500 μm). Figure 5 As shown, the results indicate that the L21 phase of the medium-entropy alloy in this embodiment undergoes large lattice distortion during plastic deformation, accompanied by broadening of diffraction peaks, proving that a phase transformation occurred without instability. However, the lattice distortion of the L21 phase in this embodiment is larger than that in Example 1, and the peak broadening is increased.

[0024] The results show that the low-temperature phase-transformable second-phase reinforced NiCoV-based medium-entropy alloy, through reasonable annealing and aging process control, achieves the design of a phase-transformable second phase, and can obtain a high strength-plasticity match at room temperature and low temperature, proving the rationality and feasibility of the design principle of this invention.

[0025] The low-temperature phase-change-enhanced NiCoV-based medium-entropy alloy of the present invention has a dense and uniform internal structure, a short preparation process, and has the potential for industrial application.

[0026] The above embodiments are merely several specific implementations of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A low-temperature phase-transformable second-phase strengthened NiCoV-based medium-entropy alloy, characterized in that, The compositional element range of the NiCoV-based medium entropy alloy, in atomic percentage, is Ni: 35-45 at%, Co: 19-29 at%, V: 19-29 at%, Al: 5-10 at%, Ta: 1-5 at%. The low-temperature phase-transformable second-phase strengthened NiCoV-based medium entropy alloy has a multi-level structure, including a face-centered cubic solid solution matrix, an L21 phase dispersed in the matrix and at grain boundaries, and a localized chemical order structure.

2. The low-temperature phase-transformable second-phase strengthened NiCoV-based medium-entropy alloy according to claim 1, characterized in that, It contains ≤1 at % of a regulator, wherein the regulator comprises at least one of the elements B, C, W, Zr, La, and Ce.

3. A method for preparing a low-temperature phase-transformable second-phase reinforced NiCoV-based medium-entropy alloy, characterized in that, Includes the following steps: The elements of the low-temperature phase-change second-phase enhanced NiCoV system entropy alloy according to claim 1 or 2 are used to clean the metal raw materials, and then the elements are weighed according to atomic percentage; The metal raw materials are smelted using a non-consumable argon-protected vacuum arc furnace. After smelting, the alloy is cast into a multi-principal element alloy ingot using the non-consumable vacuum arc furnace. The ingot is subjected to homogenization heat treatment under argon protection conditions; Core plates are cut from the homogenized heat-treated ingot and rolled at room temperature; The rolled sheet was subjected to recrystallization annealing and aging heat treatment to obtain NiCoV-based medium entropy alloys.

4. The preparation method according to claim 3, characterized in that, The purity of the metal raw material is ≥99.95 wt%, of which the purity of vanadium is ≥99.99 wt%. The oxide scale is removed and the material is cleaned, and then the elements are weighed according to atomic percentage.

5. The preparation method according to claim 3, characterized in that, Before the melting process, the vacuum level inside the vacuum arc melting furnace is controlled at 1×10⁻⁶. -4 The pressure is below Pa, and argon inert gas is introduced as a protective gas. During smelting, the titanium ingot is first smelted to absorb the residual oxygen in the vacuum arc melting furnace, and then the raw material is smelted repeatedly at least 4 times. After smelting, the alloy is cast into a rectangular ingot with a final size of 80×10×10 mm.

6. The preparation method according to claim 3, characterized in that, The homogenization heat treatment of the ingot under argon protection includes: placing the ingot in a quartz tube, and controlling the vacuum degree at 1×10⁻⁶. -4 After the pressure drops below Pa, a small amount of argon gas is introduced, and the quartz tube is sealed. Then, a homogenization heat treatment is performed at a temperature of 1000-1200℃ for 4-24 hours, followed by water cooling.

7. The preparation method according to claim 3, characterized in that, After homogenization and heat treatment, the ingot is cut into plates using electrical discharge wire cutting. During cutting, the riser, the central loose area and the surface oxidation area of ​​the ingot are avoided. The plates are then rolled at room temperature with a rolling reduction of 50%-80% to obtain plates with a thickness of 1mm-2mm.

8. The preparation method according to claim 3, characterized in that, The recrystallization annealing heat treatment has an annealing temperature of 1000-1150℃ and an annealing time of 1-10 min, followed by water cooling to room temperature; the aging heat treatment has an aging temperature of 400-600℃ and an aging time of 0.5-48 h, followed by air cooling to room temperature.

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