A high-toughness multi-stage heterogeneous middle-entropy alloy and a preparation method thereof

Abstract

This invention relates to the field of medium-high entropy alloy technology, specifically to a high-strength and high-toughness multi-level heterogeneous medium-entropy alloy and its preparation method. This invention discloses a CoNiVAlTa-based medium-entropy alloy, which, through compositional design and process adjustment, utilizes low-temperature pre-aging to induce metastable phase transformation, followed by high-temperature recrystallization to achieve grain refinement and precipitation of multi-level strengthening phases. By introducing multiple strengthening mechanisms—grain refinement strengthening, precipitation strengthening, and back stress strengthening—the material possesses both high strength and high plasticity. This series of alloys exhibits a tensile strength of 1460 MPa to 1615 MPa at room temperature and a fracture elongation of 15% to 27%. This high-strength and high-toughness multi-level heterogeneous medium-entropy alloy not only boasts excellent comprehensive mechanical properties but also features a simple and feasible process, demonstrating great promise for engineering applications.

Description

Technical Field

[0001] This invention belongs to the technical field of medium-high entropy alloy materials, specifically relating to a high-strength and tough multi-level heterogeneous medium-entropy alloy and its preparation method. Background Technology

[0002] Traditional alloys typically consist of one or two main elements, resulting in relatively simple microstructures. Strengthening mechanisms generally come at the cost of sacrificing plasticity or toughness, and the potential for performance improvement is nearing its theoretical limit. To overcome this bottleneck, high-entropy alloys employ a multi-principal-element, near-atomic ratio design strategy. Leveraging the high mixing entropy effect, they stabilize the solid solution phase structure, forming novel alloy systems with flexible composition design and controllable performance. In recent years, medium- and high-entropy alloys, as advanced structural materials possessing both high strength and high plasticity, have become a research hotspot in materials science.

[0003] Heterogeneous high-entropy alloys, possessing both the excellent plasticity of the FCC phase and the high strength of the BCC phase, can achieve synergistic optimization of strong plasticity through strain distribution and interfacial interactions, overcoming the performance limitations of traditional alloys. Specifically, the FCC phase acts as a plastic buffer zone, dispersing stress concentration during deformation, while the BCC phase acts as a reinforcing skeleton, significantly improving the overall strength of the material. However, despite the good synergistic effect exhibited by heterogeneous high-entropy alloys, they still face some key challenges in practical applications, especially the fracture toughness of the BCC phase. During deformation, the mismatch in stress-strain response between the FCC and BCC phases easily leads to stress concentration at the phase interface. When the local stress exceeds the fracture strength of the BCC phase, crack initiation can occur, leading to material damage and failure during deformation. To address these challenges, the academic community has proposed a variety of solutions, such as adding trace amounts of rare earth elements to improve the bonding performance of the phase interface, using a two-phase layered interlocking structure to optimize stress distribution, or using phase selective recrystallization to alleviate stress concentration problems through incomplete recrystallization of the FCC phase. However, these methods still face challenges in terms of cost control and process simplification, and some technologies may even lead to a decrease in the plasticity of the FCC phase.

[0004] Therefore, it remains difficult to completely solve the problem of BCC phase failure and fracture during deformation in heterogeneous high-entropy alloys. How to promote synergistic deformation of the two phases through innovative structural design and microstructure control strategies remains a key challenge in achieving the synergistic development of high strength and high plasticity. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a high-strength, high-toughness, multi-level heterogeneous medium-entropy alloy and its preparation method. Through the synergistic control of compositional design and stepwise heat treatment processes, this invention utilizes low-temperature pre-aging to induce metastable phase transformations, creating a high number of recrystallization nucleation sites and obtaining refined FCC grains. High-temperature recrystallization treatment yields multi-level precipitates and enhances the deformability of the BCC phase, resulting in an alloy possessing both high strength and high plasticity.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a high-strength and high-toughness multi-level heterogeneous medium-entropy alloy, wherein the atomic percentage expression of the alloy is (CoNiV). 94-a Al6Ta a Where 0≤a≤2, and the atomic ratio of Co, Ni, and V is 1:1:1;

[0008] The alloy comprises an FCC matrix phase, a banded BCC phase, fine BCC precipitates distributed within the FCC matrix, and fine FCC precipitates distributed within the banded BCC phase.

[0009] Furthermore, in the aforementioned high-strength and high-toughness multi-level heterogeneous medium-entropy alloy, when a > 1, the alloy also contains the Laves phase TaV2.

[0010] Furthermore, in the aforementioned high-strength and high-toughness multi-level heterogeneous medium-entropy alloy, the average grain size of the FCC matrix phase is ≤10.7μm, the average grain size of the banded BCC phase is ≤3.5μm, and the FCC matrix phase contains dispersed BCC precipitates with an average diameter of 700-750nm; the banded BCC phase contains dispersed FCC precipitates with an average diameter of 800-950nm.

[0011] Another aspect of the present invention provides a method for preparing the above-mentioned high-strength and high-toughness multi-level heterojunction medium-entropy alloy, comprising the following steps:

[0012] Step 1: Weigh the ingredients according to the alloy composition;

[0013] The raw materials for the formulation are cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules, with a purity of ≥99.5 wt.%. After removing the oxide scale from the high-purity cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules, the metal blocks are placed in a beaker and ultrasonically cleaned with industrial ethanol to remove dust and oil from their surfaces. They are then dried to obtain pre-treated cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. The atomic percentage of the alloy is converted to a mass percentage, and the raw materials are weighed using a high-precision electronic balance.

[0014] Step 2: Melt the ingredients in a vacuum non-consumable arc furnace to obtain alloy ingots;

[0015] Before melting, the vacuum chamber is pre-evacuated to 5×10⁻⁶. -3 Pa, then high-purity argon gas is introduced as a protective gas until the pressure inside the cavity reaches 4 × 10 Pa. 4At the start of the smelting process, pure titanium ingots, acting as getters, are first smelted to absorb residual oxygen and nitrogen from the air. Then, the elemental metallic raw materials in the copper crucible are smelted. After all the raw materials have melted to form an alloy ingot, it is cooled, the ingot is flipped, and smelted again with magnetic stirring activated. This process is repeated six times to obtain the final alloy ingot. The smelting current is 400A.

[0016] Step 3: Prepare the alloy ingot into plates using the copper mold casting method;

[0017] The smelted alloy ingot is placed in the copper crucible of the casting system, with a copper mold placed below. The furnace chamber is evacuated to 5 × 10⁻⁶. -3 After Pa, high-purity argon gas is introduced to a concentration of 4 × 10⁻⁶. 4 Pa. First, the ingot is melted at 200A, then the alloy ingot is completely melted at 400A, and the copper crucible is inverted to allow it to flow into the copper mold.

[0018] Step 4: Homogenize the board material;

[0019] The surface of the plate is polished to a bright and clean finish, sealed in a vacuum quartz tube, and placed in a box-type resistance furnace in a temperature-controlled manner. The homogenization temperature is 1150℃, the homogenization time is 12 hours, and the cooling method is water cooling.

[0020] Step 5: Cold-roll the homogenized sheet material.

[0021] The homogenized sheet surface is polished to a bright and clean finish, and then cold-rolled at room temperature. The total reduction in cold rolling is 81%, and multiple rolling passes are used, with a reduction of 0.1 mm per pass. The rolling direction is along the length.

[0022] Step 6: Perform step-by-step heat treatment on the cold-rolled sheet material;

[0023] The cold-rolled metal sheet is sealed in a vacuum quartz tube and placed in a box-type resistance furnace using a step-by-step heat treatment method: low-temperature pre-aging followed by high-temperature recrystallization. The low-temperature pre-aging temperature is 600℃~800℃, the pre-aging time is 12~48h, and the cooling method is air cooling. After low-temperature pre-aging, the metal sheet is sealed in a vacuum quartz tube and placed in the box-type resistance furnace using a step-by-step heat treatment method. The high-temperature recrystallization temperature is 950~1000℃, the recrystallization time is 30~120min, and the cooling method is water cooling.

[0024] It should be noted that the stepwise heat treatment process (pre-aging at 600℃~800℃ and recrystallization at 950℃~1000℃) developed in this invention has a wide process window and compositional adaptability, which is the common basis for ensuring the formation of multi-level heterogeneous structures in alloys with different Ta contents. The optimization criteria for specific Ta contents are as follows: as the Ta content increases, the pre-aging and recrystallization temperatures need to be increased (or the holding time needs to be extended) to provide sufficient thermodynamic driving force to overcome the high activation energy barrier caused by the drag of high-concentration Ta solute and the precipitation of nano-phases. However, the selection of parameters should not blindly pursue high temperatures, but should be limited to avoid excessive coarsening of the microstructure. The core of this process lies in the precise coupling of phase transformation kinetics and recrystallization driving force to achieve fine control of the microstructure in the complex phase transformation path, and finally obtain a multi-level heterogeneous structure with optimal matching of strength and plasticity.

[0025] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0026] This invention introduces multiple strengthening mechanisms through the synergistic combination of homogenization, cold rolling, and stepwise heat treatment, enabling the material to possess both high strength and high plasticity. At room temperature, the tensile strength of the material can reach 1460–1615 MPa, and the elongation at break is 15%–27%. Cold rolling leads to matrix instability, generating numerous defects such as dislocations and vacancies. Low-temperature pre-aging causes a metastable phase transformation in the matrix, FCC→κ+σ+BCC, increasing the nucleation site density for subsequent recrystallization. During high-temperature recrystallization, the κ and σ phases are thermodynamically unstable, undergoing metastable phase re-dissolution and reversing back to the stable FCC phase. This not only preserves the fine BCC phase but also yields a fine-grained FCC matrix, avoiding the problem of rapid FCC grain growth and coarsening caused by direct high-temperature recrystallization. Simultaneously, during high-temperature recrystallization, the banded BCC phase precipitates a high-density fine FCC phase within it. This invention ingeniously uses the metastable κ and σ phases as temporary nucleating agents and grain boundary pinning agents, achieving re-dissolution at high temperatures after refining FCC grains, thereby achieving high grain refinement without sacrificing plasticity.

[0027] Regarding the microscopic deformation mechanism, the stepwise heat treatment process results in fine FCC grains and the precipitation of high-density fine BCC phases. This creates a low-stress state and a high-density FCC phase microstructure within the banded BCC phase, constructing a phase-in-phase structure. This effectively alleviates stress concentration in the BCC phase and activates its internal slip, thus solving the problem of easy fracture of the BCC phase in traditional heterostructures. Based on these characteristics, the alloy of this invention has significant competitive advantages and great engineering application prospects in the field of medium-high entropy alloys. Attached Figure Description

[0028] Figure 1 This is a flowchart of the stepwise heat treatment process of the present invention.

[0029] Figure 2The X-ray spectra of the present invention are as follows: (a) is the spectra of Example 1 after pre-aging at 600℃; (b) is the spectra of Example 2 after pre-aging at 700℃; (c) is the spectra of Example 3 after pre-aging at 800℃; (d) is the spectra of Example 4 after pre-aging at 800℃; and (e) is the spectra of Example 3 after pre-aging and high-temperature recrystallization at 1000℃.

[0030] Figure 3 The microstructures of the multi-level heterogeneous medium-entropy alloys of Examples 1-4 of the present invention are shown below: (a) is the microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 1 after stepwise heat treatment; (b) is the microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 2 after stepwise heat treatment; (c) is the microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 3 after stepwise heat treatment; and (d) is the microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 4 after stepwise heat treatment.

[0031] Figure 4 The transmission electron microscopy morphology and selected area electron diffraction pattern of the alloy in Example 2 of this invention under low-temperature pre-aging are shown.

[0032] Figure 5 This is the room temperature tensile stress-strain curve of the present invention. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. In the following specific embodiments, the step-by-step heat treatment process flow diagram of the present invention is as follows: Figure 1 As shown, the phase composition of each embodiment is as follows: Figure 2 As shown, the evolution of microstructure is as follows Figures 3 to 4 As shown, the mechanical properties are as follows Figure 5 As shown. The following embodiments are for illustrative purposes only and not for limiting the scope of the invention.

[0034] Example 1

[0035] This embodiment provides a high-strength and tough multi-level heterogeneous medium-entropy alloy, whose chemical composition is (CoNiV). 94 Al6 is prepared by the following steps:

[0036] Step 1, Batching: Convert the alloy atomic percentage to mass percentage: Co:Ni:V:Al = 33.9:33.8:29.4:2.9. All raw materials are of industrial grade purity. Remove the oxide scale from high-purity cobalt ingots, nickel ingots, crystalline vanadium, and aluminum particles. Place the metal block in a beaker, add industrial ethanol, and ultrasonically clean for 10 minutes to remove dust and oil from the surface of the metal block. Replace with industrial ethanol and ultrasonically clean for 5 minutes, then dry to obtain pretreated cobalt ingots, nickel ingots, crystalline vanadium, and aluminum particles. Convert the alloy atomic percentage to mass percentage and weigh the raw materials using a high-precision electronic balance.

[0037] Step 2, Vacuum Non-Consumable Arc Melting: Place the weighed raw materials of each element in a copper crucible in the order of lowest melting point to highest melting point, and evacuate to 5×10⁻⁶. -3 Pa, then high-purity argon gas is introduced as a protective gas until the pressure inside the furnace reaches 4 × 10 Pa. 4 Pa stops charging; when smelting begins, the pure titanium ingot, which acts as a getter, is smelted first to absorb the oxygen and nitrogen in the residual air, and then the elemental metal raw material in the copper crucible is smelted; after all the raw material has melted to form an alloy ingot, it is cooled, the alloy ingot is turned over, smelted again and the magnetic stirring is turned on, the smelting current is 400A, and the smelting is repeated 6 times to obtain the final alloy ingot.

[0038] Step 3, Copper Mold Casting: Place the smelted alloy ingot in the copper crucible of the gating system, and place a 50×30×11mm copper mold underneath. Evacuate the furnace chamber to 5×10... -3 After Pa, high-purity argon gas is introduced to a concentration of 4 × 10⁻⁶. 4 Pa; First, melt the ingot at 200A, then completely melt the alloy ingot at 400A, and invert the copper crucible to let it flow into the copper mold.

[0039] Step 4, Homogenization treatment: Grind the surface of the alloy plate until it is bright and clean, seal it in a vacuum quartz tube, and place it in a box-type resistance furnace by means of heating to the furnace. After holding at 1150℃ for 12 hours, take it out and cool it with water to ensure uniform distribution of components.

[0040] Step 5, Cold Rolling: Polish the surface of the homogenized alloy sheet to a bright and clean finish, controlling the sheet thickness to 8mm. Cold roll the 8mm thick sheet at room temperature with a total reduction of 81%, rolling along the length direction, using multiple passes with small reductions, each pass being 0.1mm, ultimately producing a 1.5mm thick metal sheet.

[0041] Step 6, Step-by-Step Heat Treatment: The cold-rolled metal sheet is sealed in a vacuum quartz tube and subjected to a step-by-step heat treatment method, consisting of low-temperature pre-aging followed by high-temperature recrystallization. The sheet is placed in a box-type resistance furnace at 600℃ for 12 hours, then removed and air-cooled to room temperature. The pre-aged metal sheet is then sealed in a vacuum quartz tube and placed in a box-type resistance furnace at 950℃ for 30 minutes for recrystallization, followed by water cooling to room temperature.

[0042] The phase composition of the alloy was characterized by X-ray diffraction. The relevant test parameters were 40 kV and 15 mA. Kα rays were excited by Cu target, the scanning angle 2θ was 30~100°, the scanning speed was 2º / min, and the step size was 0.01°. The XRD pattern was obtained. Figure 2 (a) The XRD pattern of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 1 after pre-aging at 600℃ shows that the phase composition is κ phase and σ phase. The fine BCC phase did not show obvious diffraction peaks in the X-ray diffractometer due to its low volume fraction. In the subsequent transmission electron microscopy... Figure 4 This has been definitively confirmed.

[0043] Figure 3 (a) is a microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 1 after stepwise heat treatment. The microstructure mainly consists of an FCC matrix (continuous region) and a BCC phase (banded region). At this time, the BCC phase is mostly distributed in the FCC matrix as slender bands, with a high phase interface density. Fine BCC phases precipitate in the FCC matrix, with an average diameter of about 700 nm. Fine FCC phases precipitate inside the banded BCC, with an average diameter of about 800 nm. This microstructure provides a good balance of strength and plasticity. The average grain size of the FCC phase is about 10.7 μm, and the BCC phase has a high degree of recrystallization, with an average grain size of about 1.8 μm. The alloy sheet was cut into tensile specimens using an EDM wire cutter, and the alloy was subjected to tensile mechanical testing at room temperature using a universal testing machine with a strain rate of 5 × 10⁻⁶. -4 s -1 .like Figure 5 As shown, the room temperature tensile strength of the alloy in this embodiment is 1461 MPa, and the elongation at break is 27%.

[0044] Example 2

[0045] This embodiment provides a high-strength and tough multi-level heterogeneous medium-entropy alloy with the chemical composition (CoNiV). 93 Al6Ta1 is prepared by the following steps:

[0046] Step 1, Batching: Convert the alloy atomic percentage to mass percentage: Co:Ni:V:Al:Ta = 32.8:32.7:28.4:2.9:3.2. All raw materials are of industrial grade purity. Remove the oxide scale from high-purity cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Place the metal block in a beaker, add industrial ethanol, and ultrasonically clean for 10 minutes to remove dust and oil from the surface of the metal block. Replace with industrial ethanol and ultrasonically clean for 5 minutes, then dry to obtain pretreated cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Convert the alloy atomic percentage to mass percentage and weigh the raw materials using a high-precision electronic balance.

[0047] Step 2, Vacuum Non-Consumable Arc Melting: Place the weighed raw materials of each element in a copper crucible in the order of lowest melting point to highest melting point, and evacuate to 5×10⁻⁶. -3 Pa, then high-purity argon gas is introduced as a protective gas until the pressure inside the furnace reaches 4 × 10 Pa. 4 Pa stops charging; when smelting begins, the pure titanium ingot, which acts as a getter, is smelted first to absorb the oxygen and nitrogen in the residual air, and then the elemental metal raw material in the copper crucible is smelted; after all the raw material has melted to form an alloy ingot, it is cooled, the alloy ingot is turned over, smelted again and the magnetic stirring is turned on, the smelting current is 400A, and the smelting is repeated 6 times to obtain the final alloy ingot.

[0048] Step 3, Copper Mold Casting: Place the smelted alloy ingot in the copper crucible of the gating system, and place a 50×30×11mm copper mold underneath. Evacuate the furnace chamber to 5×10... -3 After Pa, high-purity argon gas is introduced to a concentration of 4 × 10⁻⁶. 4 Pa; First, melt the ingot at 200A, then completely melt the alloy ingot at 400A, and invert the copper crucible to let it flow into the copper mold.

[0049] Step 4, Homogenization treatment: Grind the surface of the alloy plate until it is bright and clean, seal it in a vacuum quartz tube, and place it in a box-type resistance furnace by means of heating to the furnace. After holding at 1150℃ for 12 hours, take it out and cool it with water to ensure uniform distribution of components.

[0050] Step 5, Cold Rolling: Polish the surface of the homogenized alloy sheet to a bright and clean finish, controlling the sheet thickness to 8mm. Cold roll the 8mm thick sheet at room temperature with a total reduction of 81%, rolling along the length direction, using multiple passes with small reductions, each pass being 0.1mm, ultimately producing a 1.5mm thick metal sheet.

[0051] Step 6, Step-by-Step Heat Treatment: The cold-rolled metal sheet is sealed in a vacuum quartz tube and subjected to a step-by-step heat treatment method, consisting of low-temperature pre-aging followed by high-temperature recrystallization. The sheet is placed in a box-type resistance furnace at 700℃ for 30 hours, then removed and air-cooled to room temperature. The pre-aged metal sheet is then sealed in a vacuum quartz tube and placed in a box-type resistance furnace at 970℃ for 75 minutes for recrystallization, followed by water cooling to room temperature.

[0052] Figure 2 (b) is (CoNiV) 93 X-ray diffraction pattern of Al6Ta1 alloy after low-temperature pre-aging at 700℃. After pre-aging at 700℃, the phase composition is κ phase and σ phase. Although the fine BCC phase did not show obvious diffraction peaks in the X-ray diffractometer due to its low volume fraction, Figure 4 This has been definitively confirmed. Figure 4 The images show the transmission electron microscopy (TEM) morphology and corresponding diffraction spots of the precipitated phase after low-temperature pre-aging. The κ phase is (Co,Ni)3V with a trigonal crystal structure in space group R-3m, while the σ phase is Co. 1.9 V 3.1 The structure exhibits a tetragonal crystal system with space group P42 / mnm, while the fine BCC phase is NiAl with a cubic crystal system of space group Pm-3m. This indicates that low-temperature pre-aging successfully induced the formation of metastable κ and σ phases, laying the structural foundation for grain refinement and hierarchical precipitation during subsequent high-temperature recrystallization. After high-temperature recrystallization at 970℃, the... Figure 2 (e) The XRD pattern shows that the metastable κ and σ phases disappeared, and the diffraction peaks of the FCC phase reappeared, proving that the low-temperature metastable phases underwent re-dissolution during high-temperature recrystallization.

[0053] Figure 3 (b) Shows the microstructure of the high-strength and high-toughness multi-level heterogeneous medium-entropy alloy prepared in Example 2 after stepwise heat treatment. With the change of alloy composition, due to the addition of the high-melting-point element Ta, the morphology of the banded BCC phase evolved, gradually agglomerating and widening. The average grain size of the recrystallized FCC phase is about 3.5 μm, and the recrystallization degree of the banded BCC phase is slightly reduced, with an average grain size of about 2.5 μm. At the same time, a small amount of Laves phase TaV2 (granular precipitates indicated by the arrows in the figure) begins to precipitate in the microstructure, with a volume fraction of about 2%. The elemental composition of the Laves phase is TaV2, space group is Fd-3m(227), C15 type cubic structure, PDF card number is 04-001-2120, a=b=c=7.11Å, α=β=γ=90°. The appropriate introduction of this hard and brittle phase can further improve the strength of the alloy while adjusting the stress distribution through the deformation coordination mechanism with the matrix. Figure 5As shown, the alloy in this embodiment has a room temperature tensile strength of 1615 MPa and a fracture elongation of 22%. During deformation, the fine BCC phase, as a precipitate phase, hinders dislocation movement in the FCC matrix, thus playing an Orovan strengthening role. The banded BCC phase itself has poor deformability, but its internal fine FCC phase, due to its high crystal symmetry, can activate multiple slip systems. When stress is concentrated at the FCC / BCC interface, the intervention of the internal fine FCC phase induces the activation of slip systems in the banded BCC that were originally difficult to initiate, dispersing the stress over a larger volume and thus delaying fracture.

[0054] Example 3

[0055] This embodiment provides a high-strength and tough multi-level heterogeneous medium-entropy alloy with the chemical composition (CoNiV). 93 Al6Ta1 is prepared by the following steps:

[0056] Step 1, Batching: Convert the alloy atomic percentage to mass percentage: Co:Ni:V:Al:Ta = 32.8:32.7:28.4:2.9:3.2. All raw materials are of industrial grade purity. Remove the oxide scale from high-purity cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Place the metal block in a beaker, add industrial ethanol, and ultrasonically clean for 10 minutes to remove dust and oil from the surface of the metal block. Replace with industrial ethanol and ultrasonically clean for 5 minutes, then dry to obtain pretreated cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Convert the alloy atomic percentage to mass percentage and weigh the raw materials using a high-precision electronic balance.

[0057] Step 2, Vacuum Non-Consumable Arc Melting: Place the weighed raw materials of each element in a copper crucible in the order of lowest melting point to highest melting point, and evacuate to 5×10⁻⁶. -3 Pa, then high-purity argon gas is introduced as a protective gas until the pressure inside the furnace reaches 4 × 10 Pa. 4 Pa stops charging; when smelting begins, the pure titanium ingot, which acts as a getter, is smelted first to absorb the oxygen and nitrogen in the residual air, and then the elemental metal raw material in the copper crucible is smelted; after all the raw material has melted to form an alloy ingot, it is cooled, the alloy ingot is turned over, smelted again and the magnetic stirring is turned on, the smelting current is 400A, and the smelting is repeated 6 times to obtain the final alloy ingot.

[0058] Step 3, Copper Mold Casting: Place the smelted alloy ingot in the copper crucible of the gating system, and place a 50×30×11mm copper mold underneath. Evacuate the furnace chamber to 5×10... -3 After Pa, high-purity argon gas is introduced to a concentration of 4 × 10⁻⁶. 4 Pa; First, melt the ingot at 200A, then completely melt the alloy ingot at 400A, and invert the copper crucible to let it flow into the copper mold.

[0059] Step 4, Homogenization treatment: Grind the surface of the alloy plate until it is bright and clean, seal it in a vacuum quartz tube, and place it in a box-type resistance furnace by means of heating to the furnace. After holding at 1150℃ for 12 hours, take it out and cool it with water to ensure uniform distribution of components.

[0060] Step 5, Cold Rolling: Polish the surface of the homogenized alloy sheet to a bright and clean finish, controlling the sheet thickness to 8mm. Cold roll the 8mm thick sheet at room temperature with a total reduction of 81%, rolling along the length direction, using multiple passes with small reductions, each pass being 0.1mm, ultimately producing a 1.5mm thick metal sheet.

[0061] Step 6, Step-by-Step Heat Treatment: The cold-rolled metal sheet is sealed in a vacuum quartz tube and subjected to a step-by-step heat treatment method, consisting of low-temperature pre-aging followed by high-temperature recrystallization. The sheet is placed in a box-type resistance furnace at 800℃ for 48 hours, then removed and air-cooled to room temperature. The pre-aged metal sheet is then sealed in a vacuum quartz tube and placed in a box-type resistance furnace at 1000℃ for 120 minutes for recrystallization, followed by water cooling to room temperature.

[0062] Figure 2 (c) is (CoNiV) 93 X-ray diffraction pattern of Al6Ta1 alloy under 800℃ low-temperature pre-aging: phase composition consists of κ and σ phases, and the phase composition remains unchanged. Figure 3 (c) The alloy of Example 3, after high-temperature recrystallization, shows an average grain size of approximately 4.2 μm for the FCC phase and approximately 3.1 μm for the banded BCC phase. Compared to Example 2, the grain sizes of the FCC and banded BCC phases are slightly increased, while the size and volume fraction of the Laves phase TaV2 remain largely unchanged. The alloy of this example exhibits a room-temperature tensile strength of 1554 MPa and an elongation at break of 24%.

[0063] Example 4

[0064] This embodiment provides a high-strength and tough multi-level heterogeneous medium-entropy alloy, whose chemical composition is (CoNiV). 92 Al6Ta2 is prepared by the following steps:

[0065] Step 1, Batching: Convert the alloy atomic percentage to mass percentage: Co:Ni:V:Al:Ta = 31.7:31.6:27.5:2.8:6.4. All raw materials are of industrial grade purity. Remove the oxide scale from high-purity cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Place the metal block in a beaker, add industrial ethanol, and ultrasonically clean for 10 minutes to remove dust and oil from the surface of the metal block. Replace with industrial ethanol and ultrasonically clean for 5 minutes, then dry to obtain pretreated cobalt ingots, nickel ingots, crystalline vanadium, aluminum granules, and tantalum granules. Convert the alloy atomic percentage to mass percentage and weigh the raw materials using a high-precision electronic balance.

[0066] Step 2, Vacuum Non-Consumable Arc Melting: Place the weighed raw materials of each element in a copper crucible in the order of lowest melting point to highest melting point, and evacuate to 5×10⁻⁶. -3 Pa, then high-purity argon gas is introduced as a protective gas until the pressure inside the furnace reaches 4 × 10 Pa. 4 Pa stops charging; when smelting begins, the pure titanium ingot, which acts as a getter, is smelted first to absorb the oxygen and nitrogen in the residual air, and then the elemental metal raw material in the copper crucible is smelted; after all the raw material has melted to form an alloy ingot, it is cooled, the alloy ingot is turned over, smelted again and the magnetic stirring is turned on, the smelting current is 400A, and the smelting is repeated 6 times to obtain the final alloy ingot.

[0067] Step 3, Copper Mold Casting: Place the smelted alloy ingot in the copper crucible of the gating system, and place a 50×30×11mm copper mold underneath. Evacuate the furnace chamber to 5×10... -3 After Pa, high-purity argon gas is introduced to a concentration of 4 × 10⁻⁶. 4 Pa; First, melt the ingot at 200A, then completely melt the alloy ingot at 400A, and invert the copper crucible to let it flow into the copper mold.

[0068] Step 4, Homogenization treatment: Grind the surface of the alloy plate until it is bright and clean, seal it in a vacuum quartz tube, and place it in a box-type resistance furnace by means of heating to the furnace. After holding at 1150℃ for 12 hours, take it out and cool it with water to ensure uniform distribution of components.

[0069] Step 5, Cold Rolling: Polish the surface of the homogenized alloy sheet to a bright and clean finish, controlling the sheet thickness to 8mm. Cold roll the 8mm thick sheet at room temperature with a total reduction of 81%, rolling along the length direction, using multiple passes with small reductions, each pass being 0.1mm, ultimately producing a 1.5mm thick metal sheet.

[0070] Step 6, Step-by-Step Heat Treatment: The cold-rolled metal sheet is sealed in a vacuum quartz tube and subjected to a step-by-step heat treatment method, consisting of low-temperature pre-aging followed by high-temperature recrystallization. The sheet is placed in a box-type resistance furnace at 800℃ for 48 hours, then removed and air-cooled to room temperature. The pre-aged metal sheet is then sealed in a vacuum quartz tube and placed in a box-type resistance furnace at 1000℃ for 120 minutes for recrystallization, followed by water cooling to room temperature.

[0071] Figure 2 (d) is (CoNiV) 92 X-ray diffraction pattern of Al6Ta2 alloy under low-temperature pre-aging at 800℃, showing phase composition of κ phase and σ phase. Figure 3 (d) This shows that after high-temperature recrystallization, the average grain size of the FCC phase in the alloy of Example 4 is approximately 5.7 μm, and the average grain size of the banded BCC phase is approximately 3.5 μm. Furthermore, the volume fraction of the Laves phase TaV2 increases to approximately 8%. The alloy sheet was cut into tensile specimens using an electrical discharge wire cutter, and tensile mechanical tests were performed on the alloy at room temperature using a universal testing machine with a strain rate of 5 × 10⁻⁶. -4 s -1 The room temperature stress-strain curve of the alloy in this embodiment is as follows: Figure 5 As shown, the tensile strength is 1500 MPa and the elongation at break is 15%.

[0072] A comparison of Examples 2 and 3 shows that, for alloys with the same Ta content (a=1), within the parameter range protected by this invention, changing the heat treatment temperature and time affects the final microstructure and mechanical properties. When the heat treatment temperature is too high and the time is too long (such as the 800℃ pre-aging and 1000℃ recrystallization used in Example 3), the excessive thermodynamic driving force leads to a certain degree of coarsening of the microstructure, causing the strength-ductility matching to deviate from the optimal state of Example 2. This indicates that for the same alloy composition, the specific parameters cannot be arbitrarily combined blindly, but require targeted optimization.

[0073] Further comparison of Examples 3 and 4 reveals that when the Ta content increases to 2 at.%, the addition of Ta, a high-melting-point element, significantly reduces the atomic diffusion coefficient, resulting in a significant solute dragging effect on grain boundary migration and dislocation movement, and greatly increasing the activation energy for phase transformation and recrystallization. At this point, to ensure sufficient phase transformation and recrystallization to achieve excellent strength and plasticity, a higher heat treatment temperature and a longer treatment time (i.e., using the same 800°C pre-aging and 1000°C recrystallization process as in Example 3) are required. Example 4 ultimately achieved excellent properties with a tensile strength of 1500 MPa and an elongation of 15%.

[0074] Table 1. Size and content of precipitated phases in this invention

[0075] Average diameter of BCC precipitates in FCC matrix / nm Volume fraction of BCC precipitates in FCC matrix / % Average diameter of FCC precipitates in banded BCC (nm) Volume fraction of FCC precipitates in strips / % Example 1 700 13 800 31.2 Example 2 710 14 830 27.5 Example 3 740 15 900 28.3 Example 4 750 17 950 29.9

[0076] As shown in the detailed data in Table 1, in Examples 1 to 4 of this invention, the average diameter of the fine BCC precipitates in the FCC matrix was controlled within a narrow range of 700 nm to 750 nm, while their volume fraction remained stable between 13% and 17%. The hard, fine BCC precipitates dispersed within this size range strongly pinned and effectively hindered dislocation movement within the FCC matrix, resulting in significant precipitation strengthening. The average diameter of the banded FCC precipitates ranged from 800 nm to 950 nm, and their overall volume fraction remained at a high level of 27.5% to 31.2%. This portion of the FCC phase functioned as a plastic buffer zone. When the alloy underwent macroscopic deformation under external loads, this portion of FCC precipitates with specific sizes and proportions could not only uniformly accommodate a large amount of plastic strain within itself, but also effectively coordinate the deformation incompatibility between adjacent heterogeneous phase interfaces. The geometric dimensions and volume fractions of the BCC and FCC precipitates listed in Table 1 are key technical features of this invention that solve the problem of easy fracture of the BCC phase in traditional heterostructures and promote the synergistic deformation of the two phases.

[0077] In summary, Examples 1-4 fully demonstrate the flexibility and necessity of parameter control within a wide process range of the present invention. The parameter ranges provided by the present invention (600℃~800℃ and 950℃~1000℃) do not necessarily mean that a lower Ta content requires a lower heat treatment temperature. On the contrary, within this broad process window, those skilled in the art can flexibly control the balance between the phase transformation driving force and the element diffusion rate for any Ta content within the range of 0≤a≤2 by optimizing specific temperature and time combinations. As long as the sequential nature of the stepwise heat treatment is ensured and thermodynamic parameters are optimized according to the specific composition, metastable phase transformation and dissolution mechanisms can be triggered, resulting in customized multi-level heterostructures and excellent mechanical properties.

[0078] In summary, a high-strength and high-toughness multi-level heterogeneous medium-entropy alloy was prepared by homogenization, cold rolling, and stepwise heat treatment. The core of this method lies in the sequential combination of mechanical deformation and heat treatment processes, actively utilizing non-equilibrium phase transformation and reversal transformation to upgrade simple deformation and heat treatment into a customized microstructure engineering process. Cold rolling deformation pre-establishes high-density dislocation regions in the matrix, providing preferential nucleation sites for subsequent low-temperature phase transformation. The temperature and time of the stepwise heat treatment precisely control the degree and location of phase transformation, utilizing metastable phase transformation to increase the nucleation rate and refine the FCC grain size without affecting the recrystallization degree of BCC. This invention is of great significance for the development and preparation of a new generation of multi-principal element alloys with excellent comprehensive properties.

[0079] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. A high-strength and high-toughness multi-level heterogeneous medium-entropy alloy, characterized in that, The atomic percentage expression for the alloy is (CoNiV). 94-a Al6Ta a Where 0≤a≤2, and the atomic ratio of Co, Ni, and V is 1:1:1; The alloy comprises an FCC matrix phase, a banded BCC phase, fine BCC precipitates distributed within the FCC matrix, and fine FCC precipitates distributed within the banded BCC phase.

2. The high-strength and high-toughness multi-level heterogeneous medium-entropy alloy according to claim 1, characterized in that, When a > 1, the alloy also contains the Laves phase TaV2.

3. The high-strength and high-toughness multi-level heterogeneous medium-entropy alloy according to claim 1, characterized in that, The FCC matrix phase has an average grain size ≤ 10.7 μm, the banded BCC phase has an average grain size ≤ 3.5 μm, and the FCC matrix phase contains dispersed BCC precipitates with an average diameter of 700-750 nm; the banded BCC phase contains dispersed FCC precipitates with an average diameter of 800-950 nm.

4. A method for preparing the high-strength and tough multi-level heterogeneous medium-entropy alloy according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Weigh the ingredients according to the alloy composition; Step 2: Melt the ingredients in a vacuum non-consumable arc furnace to obtain alloy ingots; Step 3: Prepare the alloy ingot into plates using the copper mold casting method; Step 4: Homogenize the board material; Step 5: Cold-roll the homogenized sheet material. Step 6: Perform step-by-step heat treatment on the cold-rolled sheet. First, perform low-temperature pre-aging at 600℃~800℃ for 12h~48h to precipitate metastable κ and σ phases to pin grain boundaries and serve as recrystallization nucleation points. The cooling method is air cooling. Then, perform high-temperature recrystallization at 950℃~1000℃ for 30min~120min to dissolve the metastable κ and σ phases back into the matrix and retain the stable BCC precipitate. The cooling method is water cooling.

5. The preparation method according to claim 4, characterized in that, In step 2, the vacuum chamber is pre-evacuated to 5×10 before melting. -3 Pa, filled with high-purity argon gas to 4 × 10⁻⁶. 4 Pa, melting current of 400A, repeated melting 6 times.

6. The preparation method according to claim 4, characterized in that, In step 4, the homogenization temperature is 1150℃, the homogenization time is 12h, and the cooling method is water cooling.

7. The preparation method according to claim 4, characterized in that, In step 5, the total reduction in cold rolling is 60% to 90%, and multi-pass rolling is used, with a reduction of 0.1 mm per pass, and the rolling direction is along the length.

8. The preparation method according to claim 4, characterized in that, In step 6, metastable κ phase, σ phase and stable BCC phase are precipitated in the matrix by the low temperature pre-aging; by the high temperature recrystallization, the metastable κ phase and σ phase are re-dissolved and the fine BCC phase is retained.

9. The preparation method according to claim 4, characterized in that, The raw materials used in step 1 have a purity of 99.5 wt.% or higher, and are polished to remove oxide scale and ultrasonically cleaned before smelting.

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