CoCrNi series high-toughness high-density twin boundary medium-entropy alloy and preparation method thereof

Abstract

The invention belongs to the technical field of high-entropy alloys, and particularly relates to a CoCrNi series high-toughness high-density twin boundary medium-entropy alloy and a preparation method thereof. Aiming at the situation that the traditional alloy component design field lacks mature and reliable design principles, the invention provides three component design principles: high elastic modulus, low stacking fault energy and proper phase stability. Based on the three principles, the general formula of the non-equal-atomic-ratio medium-entropy alloy is CoaCrbNic, 30 < = a < = 37, 30 < = b < = 35, and 28 < = c < = 40. A metal raw material with the purity being 99.5% or above is polished and cleaned and then smelted into a cast ingot in a vacuum arc furnace, and then the medium-entropy alloy with high strength and toughness and the high-proportion twin boundary can be obtained after solid solution, hot rolling and recrystallization annealing processes are conducted. The design principle can also be popularized to other alloy systems, operation is easy, the production period is short, safety and reliability are achieved, and wide application prospects are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of high entropy alloys, and in particular relates to a CoCrNi series high-strength, high-toughness, high-density twin boundary medium entropy alloy and a preparation method thereof. Background Art

[0002] After years of development, metal materials represented by steel, aluminum, and titanium have reached saturation in performance and are difficult to achieve major breakthroughs. The emergence of new medium / high entropy alloys has brought more possibilities for alloy design. According to the classification of mixed entropy, those with mixed entropy greater than 1.5R are high entropy alloys, and those with mixed entropy less than 1.5R and greater than 1R are medium entropy alloys; in theory, high entropy alloys have a series of advantages such as high entropy effect, lattice distortion effect, hysteresis diffusion effect, and cocktail effect; experiments have proved that high entropy alloys have high strength and toughness, excellent fracture toughness, high fatigue resistance, corrosion resistance, and radiation resistance, which make them have broad application prospects. However, the strength and plasticity of mature equiatomic ratio CoCrFeMnNi, CoCrNi and other components are still slightly insufficient. Researchers usually improve performance by increasing lattice distortion, introducing nano-precipitation phases, and large plastic deformation. However, traditional thermomechanical processing methods cannot break through the inherent strength-plasticity inversion problem of alloys. Another way to improve alloy performance is composition design. Non-equiatomic ratio high entropy alloys have a larger design range than equiatomic ratio high entropy alloys, which also means broader application prospects. However, improving performance by changing composition lacks mature theoretical support. Summary of the invention

[0003] In view of the situation that strength and plasticity cannot be taken into account at the same time in alloy design, the present invention aims to provide a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium-entropy alloy and a preparation method thereof. By adopting the high-strength and high-toughness metal design concept and preparation method, a non-equiatomic ratio medium-entropy alloy with high strength, high toughness and high-density annealed twins can be obtained through a simple thermomechanical processing process.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy, with the general formula Co a Cr b Ni c , a, b, and c in the chemical expression represent the molar percentage of the corresponding elements, where: 30≤a≤37, 30≤b≤35, 28≤c≤40.

[0006] The CoCrNi series high-strength, high-toughness and high-density twin boundary medium-entropy alloy includes but is not limited to CoCrNi series medium-entropy alloy.

[0007] The preparation method of the CoCrNi series high-strength, high-toughness and high-density twin boundary medium-entropy alloy comprises the following steps:

[0008] (1) pure metal raw materials are cast into ingots by vacuum induction melting;

[0009] (2) subjecting the ingot to a solution treatment at a solution temperature of ≥1100°C and a holding time of ≥1h;

[0010] (3) hot rolling the solutionized ingot;

[0011] (4) The rolled ingot is subjected to vacuum recrystallization annealing at a temperature of 700 to 950° C. for a holding time of 5 to 30 min.

[0012] In step (1), the purity of the metal raw material is greater than 99.5wt%, the surface oxide scale is polished before smelting, and the material is ultrasonically cleaned for standby use; the vacuum arc furnace is filled with protective gas argon after evacuation, the sponge titanium is first melted to absorb the air in the furnace chamber, and then the Co, Cr, and Ni materials are melted, and the smelting is repeated five times, the surface oxide scale is polished, the material is ultrasonically cleaned, and the medium entropy alloy ingot is obtained after drying, and the final size of the alloy ingot is 225×90×35mm.

[0013] In step (2), the ingot is cut into metal plates with a thickness of 10 mm using an electric spark wire, preferably subjected to solution treatment at a temperature above 1100 to 1250° C. for a holding time of 1 to 5 h to ensure composition uniformity, and then water quenched to room temperature.

[0014] In step (3), the rolling temperature is 1200±50°C, the total pressing amount is 50% to 60%, and then the rolling is cooled to room temperature by water quenching.

[0015] In order to prevent cracking during rolling, use as few passes as possible, preferably less than 5 passes.

[0016] In step (4), after cutting the tensile specimen from the rolled plate, a vacuum recrystallization annealing treatment is performed, preferably at an annealing temperature of 900°C ± 20°C and a holding time of 5 minutes, followed by water quenching and cooling to room temperature.

[0017] The design principle and beneficial effects of the present invention are as follows:

[0018] 1. Design principle: The composition design of the present invention is based on three principles of composition design: (1) high elastic modulus, which helps to delay the occurrence of plastic deformation; (2) low stacking fault energy: reducing the stacking fault energy can improve the uniformity of dislocation slip during plastic deformation, and can also promote the occurrence of twinning and phase transformation, and simultaneously improve the alloy's toughening performance. (3) appropriate phase stability: the phase transformation produced during plastic deformation helps to improve the work hardening ability, but the premature generation of brittle intermetallic compounds may lead to catastrophic fracture, so a suitable balance point should be found [reference: JXYan, ZJZhang, P.Zhang, JHLiu, H.Yu, QMHu, JBYang, ZFZhang.J.Mater.Sci.Technol.139(2023)232-244.P.Zhang, ZFZhang.Science 378(2022)947-948.]. Combining the results of previous simulations and experiments, we know that the three elements that make up the CoCrNi medium entropy alloy have different effects. Among them, Co and Cr can reduce the stacking fault energy, but excessive content will produce brittle intermetallic compounds. Cr can increase the elastic modulus, and Ni will increase the stacking fault energy but improve the phase stability of the face-centered cubic system. Therefore, based on the three principles of composition design, achieving precise control of alloy composition is an effective means to simultaneously improve the strength and plasticity of the alloy.

[0019] Combined with the atomic model of annealing twins proposed by H. Gleitert, the calculation formula for annealing twin density is as follows:

[0020]

[0021] Where p is the twin density (1 / m 2 ), Q is the activation energy of grain boundary migration (J / m 2 ), ε is the step energy (J / m 2 ), h is the nucleation height (m), σ z is the surface energy of the coherent twin boundary (J / m 2 ), ΔG 0 is the Gibbs free energy difference between the growing grain and the shrinking grain (J / m 2 ), k is the Boltzmann constant (J / K), and T is the absolute temperature (K). It can be seen that the twin density is mainly related to σ z The surface energy of the coherent twin boundary is about half of the stacking fault energy, and low stacking fault energy materials usually have a higher proportion of twin boundaries. During the formation process of annealing twins, grain refinement can be achieved inside the austenite grains. Grain refinement can effectively improve the performance of metal materials and make them have a wider range of applications.

[0022] 2. Beneficial effects: The present invention aims to provide a design idea and preparation method for high-strength and toughness metal materials. Based on the three principles of composition design, a medium-entropy alloy composition with non-equiatomic ratio, high strength and toughness, and high density twin boundary can be obtained, and then a high-performance medium-entropy alloy can be obtained through simple thermomechanical processing. The medium-entropy alloy provided by the present invention has excellent mechanical properties, a simple preparation process, safety and reliability, and has a wide range of engineering application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1-Figure 2 The grain observation results of the non-equiatomic ratio medium entropy alloy prepared by the present invention are shown in FIG. Figure 1 is the grain distribution; Figure 2 It is the statistics of twin boundary ratio. The horizontal axis Misorientation represents the mismatch angle of grain boundary (°), and the vertical axis Frequency represents the frequency of twin boundary occurrence at different mismatch angles (%).

[0024] Figure 3 This is the tensile engineering stress-strain curve result of the non-equiatomic ratio medium entropy alloy prepared by the present invention. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other examples improved or modified by ordinary technicians in this field belong to the scope of protection of the present invention. It should be understood that the embodiments of the present invention are only used to illustrate the technical effects of the present invention, rather than to limit the scope of protection of the present invention. In the embodiments, the methods used are conventional methods unless otherwise specified.

[0026] Example 1

[0027] A medium entropy alloy with high strength, toughness and high density twin boundaries, made of high purity metal with a purity of more than 99.5wt%, composed of the following atomic percentages of metal elements: Co37%, Cr35%, Ni28%, which can be expressed as Co 37 Cr 35 Ni 28 , prepared according to the following steps:

[0028] Step 1: Before smelting, the surface oxide scale of the material should be polished and ultrasonically cleaned for use. The vacuum arc furnace is filled with protective gas argon after evacuation. Sponge titanium should be melted first to absorb the air in the furnace cavity, and then Co, Cr, and Ni materials should be melted. The smelting is repeated five times, the surface oxide scale is polished, ultrasonically cleaned, and dried to obtain a medium-entropy alloy ingot. The final size of the metal ingot is 225×90×35mm.

[0029] Step 2: The ingot is cut into metal plates with a thickness of 10 mm each using an electric spark wire, and solution treated at a temperature of 1200°C for 2 hours to ensure the uniformity of the composition, and then water quenched and cooled to room temperature.

[0030] Step 3: The rolling temperature is 1200℃. After two rolling passes, the final thickness is 5mm.

[0031] Step 4: After cutting the tensile specimens from the rolled plate, vacuum recrystallization annealing treatment is performed at 900°C for 5 min, followed by water quenching and cooling to room temperature.

[0032] Step 5: Perform a room temperature tensile test on an 8862 testing machine equipped with a video extensometer, with a tensile rate of 0.36 mm / min.

[0033] like Figure 1 As shown, the non-equiatomic ratio medium entropy alloy of the present invention exhibits a uniform and completely recrystallized structure after thermomechanical processing. The quantitative statistical results are as follows: Figure 2 As shown in Figure 2, the twin boundary ratio reached 73.2%. The tensile test results are shown in Figure 2. Figure 3 As shown, the yield strength, tensile strength, uniform elongation and total elongation are 329.5MPa, 829.4MPa, 56.9% and 68.6% respectively, showing excellent strong plasticity. The present invention provides a way for the preparation of high-strength and high-toughness metal materials and the production of components, which is suitable for medium entropy alloys, and is also suitable for other metal and alloy systems.

[0034] The implementation results show that, in response to the lack of mature and reliable design principles in the field of traditional alloy composition design, the present invention proposes three principles for composition design: high elastic modulus, low stacking fault energy, and appropriate phase stability. Based on these three basic principles, the present invention proposes a method for preparing a medium-entropy alloy with high strength, high toughness and high-density annealing twins. This design principle can also be extended to other alloy systems. It is simple to operate, has a short production cycle, is safe and reliable, and has broad application prospects.

Claims

1. A CoCrNi-based high-strength, high-toughness, high-density twin boundary medium-entropy alloy, characterized in that: The general formula is Co a Cr b Ni c , a, b, and c in the chemical expression represent the molar percentage of the corresponding elements, where: 30≤a≤37, 30≤b≤35, 28≤c≤40.

2. A CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy according to claim 1, characterized in that: Medium entropy alloys include but are not limited to CoCrNi-based medium entropy alloys.

3. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy according to claim 2, characterized in that: The following steps are involved: (1) pure metal raw materials are cast into ingots by vacuum induction melting; (2) subjecting the ingot to a solution treatment at a solution temperature of ≥1100°C and a holding time of ≥1h; (3) hot rolling the solutionized ingot; (4) The rolled ingot is subjected to vacuum recrystallization annealing at a temperature of 700 to 950° C. for a holding time of 5 to 30 min.

4. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium-entropy alloy according to claim 3, characterized in that: In step (1), the purity of the metal raw material is greater than 99.5wt%, the surface oxide scale is polished before smelting, and the material is ultrasonically cleaned for standby use; the vacuum arc furnace is filled with protective gas argon after evacuation, the sponge titanium is first melted to absorb the air in the furnace chamber, and then the Co, Cr, and Ni materials are melted, and the smelting is repeated five times, the surface oxide scale is polished, the material is ultrasonically cleaned, and the medium entropy alloy ingot is obtained after drying, and the final size of the alloy ingot is 225×90×35mm.

5. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy according to claim 4, characterized in that: In step (2), the ingot is cut into metal plates with a thickness of 10 mm using an electric spark wire, preferably subjected to solution treatment at a temperature above 1100 to 1250° C. for a holding time of 1 to 5 h to ensure composition uniformity, and then water quenched to room temperature.

6. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium-entropy alloy according to claim 5, characterized in that: In step (3), the rolling temperature is 1200±50°C, the total pressing amount is 50% to 60%, and then the rolling is cooled to room temperature by water quenching.

7. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy according to claim 6, characterized in that: In order to prevent cracking during rolling, use as few passes as possible, preferably less than 5 passes.

8. The method for preparing a CoCrNi-based high-strength, high-toughness, high-density twin boundary medium entropy alloy according to claim 6, characterized in that: In step (4), after cutting the tensile specimen from the rolled plate, a vacuum recrystallization annealing treatment is performed, preferably at an annealing temperature of 900°C ± 20°C and a holding time of 5 minutes, followed by water quenching and cooling to room temperature.

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