A high-strength, ductile, pentagonal, lightweight, medium-entropy alloy and its preparation method

Abstract

This invention discloses a high-strength, ductile, pentagonal, lightweight, medium-entropy alloy and its preparation method. The alloy, by atomic percentage, comprises: Mg 45-48 at.%, Li 35-39 at.%, Al 15 at.%, Ca 0.5-1 at.%, and Si 0.5-1.5 at.%. The microstructure of the alloy is based on a continuous BCC solid solution matrix, with the BCC solid solution metallographic area accounting for 56%-77%. AlLi and Li2MgAl intermetallic compounds are discontinuous precipitates distributed in the matrix as dispersed particles. This invention proposes a Mg-Li-Al-Ca-Si pentagonal, medium-entropy, lightweight magnesium-lithium alloy system with a mixing entropy of 8.9–9.1 J / (mol·K), which falls within the standard medium-entropy alloy category and is completely different from existing traditional low-entropy magnesium alloys and magnesium-lithium alloys, which are typically below 5.0 J / (mol·K).

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation, and particularly relates to a high-strength, ductile, pentagonal, lightweight, medium-entropy alloy and its preparation method. Background Technology

[0002] With the increasing demand for lightweight, high-strength materials in aerospace, rail transportation, automotive lightweighting, and portable electronic devices, the development of metallic structural materials that combine low density, high specific strength, and good plasticity is of great significance. While traditional magnesium and aluminum alloys have low densities, their strength and plasticity ratios still fall short of the requirements for some load-bearing structural components and complex formed structures. For example, commercially available AZ91D magnesium alloys typically have low yield strengths and limited room-temperature plasticity.

[0003] Magnesium-lithium alloys exhibit lower density due to the addition of Li, with high-Li content alloys capable of forming a β-Li phase with good deformability, thus attracting attention in ultralight structural materials. However, traditional Mg-Li binary or Mg-Li-Al ternary alloys generally suffer from insufficient strength, limited microstructural stability, and easy volatilization and oxidation of Li during smelting. To improve the strength of magnesium-lithium alloys, existing technologies often incorporate rare earth elements, transition metals, or noble metals to form a second phase, with rare earth elements (such as Y and Gd) being common, but this leads to high costs and increased density. Furthermore, while there are reports on Mg-Li-Si or Mg-Li-Ca systems, these are mostly for flame retardancy or corrosion resistance, with low Li content, and have not formed a stable medium-entropy multiphase structure under high Li content. In high-Li content systems, precisely controlling the Li / (Mg+Al) ratio to stabilize the BCC / HCP phase ratio and utilizing non-rare earth elements to achieve a dispersed distribution of the strengthening phase remains a challenge in the field of ultralight high-strength alloys.

[0004] Existing Mg-Al-Ca-Si magnesium alloys are primarily designed for improved corrosion resistance or casting performance, with extremely low or no Li content, and do not possess the BCC / HCP multiphase structure formed by high-Li-content Mg-Li matrices. Some magnesium-lithium alloys are strengthened by adding elements such as Nd, Er, Y, Zr, Ag, and Zn, but typically do not contain both Ca and Si simultaneously. High-Li-content alloys are prone to Li volatilization, oxidation, and compositional segregation during melting, leading to deviations from the design composition and affecting the repeatability of microstructure and properties. Current conventional vacuum melting processes lack sufficient targeted control over crucible materials, sealing methods, and atmosphere replacement processes, making it difficult to stably prepare high-Li-content Mg-Li-Al-Ca-Si pentagonal alloy systems.

[0005] Traditional commercial magnesium alloys and conventional magnesium-lithium alloys are mostly binary or ternary low-entropy alloys, with mixing entropy generally below 6.0 J / (mol·K). They have low element solid solution limits, are prone to microstructure coarsening, and exhibit poor stability under high temperatures and long-term service. Currently, there are few cases of using a five-element principal component design to construct a medium-entropy system in existing magnesium-lithium alloy systems. The advantages of medium entropy, such as high solid solubility, refined microstructure, and stable phase structure, have not been applied in lightweight magnesium-lithium alloys. In addition, existing rare-earth-reinforced magnesium-lithium alloys typically keep the Li content at a low level (mass fraction below 5%) to control costs and process difficulty, resulting in an alloy density that is still close to that of pure magnesium systems (1.70–1.75 g / cm³). 3 The potential for lightweight design has not been fully realized. Summary of the Invention

[0006] The purpose of this invention is to provide a high-strength and ductile pentagonal lightweight medium-entropy alloy and its preparation method, so as to solve the problems of poor strength and ductility, poor microstructure stability, high cost of rare earth strengthening, lack of high-lithium pentagonal medium-entropy magnesium-lithium system, and easy deviation of the composition of high-Li alloys.

[0007] The present invention adopts the following technical solution: by atomic percentage: Mg 45-48 at.%, Li 35-39 at.%, Al 15 at.%, Ca 0.5-1 at.%, Si 0.5-1.5 at.%; The microstructure of the alloy is based on a continuous BCC solid solution matrix, with the BCC solid solution metallographic area accounting for 56%-77%. AlLi and Li2MgAl intermetallic compounds are discontinuous precipitates distributed in the matrix in the form of dispersed particles. The alloy has a density of less than 1.7 g / cm³. 3 .

[0008] Furthermore, the proportion of AlLi intermetallic compounds is 15%-28%, and the volume fraction of Li2MgAl intermetallic compounds and HCP structure solid solutions is ≤10%.

[0009] A method for preparing a high-strength, high-ductility, pentagonal, lightweight, medium-entropy alloy, comprising: Step 1: Proportion the raw materials according to the target atomic percentage and perform vacuum induction melting under an inert atmosphere; Step 2: During the melting process, argon gas is introduced and a positive pressure protective atmosphere of 1.8-2.2 atm is maintained; Step 3: Melt the raw materials completely at 700-750℃ and hold for 10-20 minutes to obtain a uniform alloy melt. Step 4: After casting the alloy melt into shape, perform homogenization heat treatment at 300-400℃ for 3-5 h to obtain an alloy with a stable continuous BCC matrix structure and dispersed intermetallic compound distribution. In step 2, the atmospheric conditions are used to maintain the stability of the alloy melt composition and ensure that the Li / (Mg+Al) composition window does not shift significantly, thereby achieving controllable BCC / HCP phase ratio.

[0010] The beneficial effects of this invention are: (1) This invention proposes a pentagonal medium-entropy lightweight magnesium-lithium alloy system with an alloy mixing entropy of 8.9–9.1 J / (mol·K), which falls within the standard medium-entropy alloy category and is completely different from existing traditional low-entropy magnesium alloys and magnesium-lithium alloys, which are usually below 5.0 J / (mol·K). Relying on the medium-entropy thermodynamic effect, it significantly improves the solid solution capacity of multiple elements, suppresses component segregation, refines the microstructure, and enhances the thermal stability of the microstructure; (2) This invention proposes a Mg-Li-Al-Ca-Si pentagonal multi-principal-element lightweight alloy system. Ca and Si are introduced into the high-Li-content Mg-Li-Al matrix at the same time, so that the alloy has a lower density than traditional magnesium alloys (less than 1.7 g / cm³). 3 (), but with higher strength; (3) The alloy of this invention does not contain rare earth elements or precious metal elements, but only uses five major industrial materials: Mg, Li, Al, Ca, and Si. Taking the typical high-performance rare earth magnesium alloy Mg-10Gd-3Y-0.5Zr as an example, its total rare earth content reaches 13.5 wt.%. Based on the current market price (approximately RMB 180,000 / ton for metallic Gd and approximately RMB 450,000 / ton for metallic Y), the raw material cost per ton of alloy is approximately RMB 345,000. In contrast, the raw material cost per ton of the alloy of this invention (taking Example 1 as an example) is approximately RMB 81,000, which is only about 23% of the former. More importantly, heavy rare earth elements such as Gd and Y have large atomic weights, and their addition in large quantities increases the alloy density to approximately 1.85–1.90 g / cm³. 3 The present invention, through its high Li content design, achieves a density reduction to approximately 1.62 g / cm³. 3 The cost of raw materials per unit density (yuan / ton alloy·g / cm³) 3 By measurement, the alloy of this invention is only about 27% of Mg-10Gd-3Y-0.5Zr; (4) The present invention achieves a composite effect of matrix plasticization, dispersion strengthening and grain boundary pinning through the synergistic effect of BCC structure solid solution and HCP structure solid solution matrix and in-situ self-generated AlLi and L2MgAl intermetallic compounds, thereby improving the matching between strength and room temperature compressive plasticity. (5) This invention achieves a performance gradient of 320-360 MPa room temperature compressive yield strength and about 45% room temperature compressive fracture strain by precisely controlling the content of Ca and Si to form different reinforcing phase contents; (6) In this invention, argon gas is introduced during the smelting process and a positive pressure protective atmosphere of 1.8-2.2 atm is maintained, which can effectively suppress the volatilization and oxidation of Li during the smelting process of high Li content alloys, and improve the compositional uniformity and preparation repeatability of the five-element system; (7) This invention achieves synergistic stability of the multiphase structure by controlling the BCC / HCP phase ratio through Li / (Mg+Al) and controlling the nucleation density and volume fraction of intermetallic compounds through Ca and Si. Ca and Si regulate the nucleation behavior and volume fraction of AlLi and Li2MgAl intermetallic compounds to adjust the precipitation strengthening degree. That is: when Ca+Si=1.8-2.0 at.% and Ca / Si≈1:1, the volume fraction of intermetallic compounds is relatively high; when Ca / Si is biased towards Si (Ca:Si=0.3-0.7:1.5), the uniformity of precipitated phase distribution is improved; when Ca+Si≤1.0 at.%, the continuity of the BCC matrix is ​​enhanced. (8) This invention controls the composition range of the five main components Mg-Li-Al-Ca-Si to form a multiphase microstructure with BCC structure solid solution as the continuous matrix phase. By adjusting the Li / (Mg+Al) ratio, the volume fraction of BCC structure solid solution is significantly increased, thereby obtaining good continuous plastic deformation ability. At the same time, AlLi and Li2MgAl intermetallic compounds are dispersed in the BCC matrix, accompanied by a small amount of HCP structure solid solution, thereby achieving a synergistic match between strength and plasticity. In order to ensure the stability of this specific composition window and phase structure, this invention further supports a high-pressure back-pressure melting process, which effectively solves the technical problem of easy deviation of the composition of high Li content alloys. (9) This invention achieves increased strength while reducing alloy density to 1.7 g / cm³ through the synergistic design of high Li content (35-39 at.%) and medium entropy effect. 3 The invention achieves a true integration of "ultra-lightweight" and "high strength and plasticity." It simultaneously introduces Ca and Si into a high-Li-content Mg-Li-Al matrix, forming a multiphase structure with a body-centered cubic (BCC) solid solution phase and / or a hexagonal close-packed (HCP) solid solution phase as the matrix, and dispersed high-content AlLi intermetallic compounds, L2MgAl intermetallic compounds, etc., thereby achieving a synergistic improvement in strength and plasticity without introducing rare earth and noble metal elements. Attached Figure Description

[0011] Figure 1 This is a comparison chart of the room temperature compressive stress-strain curves of Examples 1-3 and Comparative Example AZ91D of the present invention; Figure 2 The X-ray diffraction patterns and quantitative phase content analysis results of the alloys in Examples 1-3 of this invention are shown below. Figure 3 These are scanning electron microscope (SEM) images of the alloys in Examples 1-3 of this invention. Detailed Implementation

[0012] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0013] This invention discloses a high-strength, ductile, pentagonal, lightweight, medium-entropy alloy, with the following atomic percentages: Mg 45-48 at.%, Li 35-39 at.%, Al 15 at.%, Ca 0.5-1 at.%, Si 0.5-1.5 at.%. The alloy has a density of less than 1.7 g / cm³. 3 .

[0014] The microstructure of the alloy is based on a continuous BCC solid solution matrix, with the BCC solid solution metallographic area accounting for 56%-77%. AlLi and Li2MgAl intermetallic compounds are discontinuous precipitates distributed in the matrix as dispersed particles.

[0015] Preferably, the AlLi intermetallic compound accounts for 15%-28%, and the volume fraction of both the Li2MgAl intermetallic compound and the HCP structure solid solution is ≤10%.

[0016] This invention also discloses a method for preparing the high-strength, high-ductility, pentagonal, lightweight, medium-entropy alloy of claim 1, comprising: Step 1: Proportion the raw materials according to the target atomic percentage and perform vacuum induction melting under an inert atmosphere; Step 2: During the melting process, argon gas is introduced and a positive pressure protective atmosphere of 1.8-2.2 atm is maintained; Step 3: Melt the raw materials completely at 700-750℃ and hold for 10-20 minutes to obtain a uniform alloy melt. Step 4: After casting the alloy melt into shape, perform homogenization heat treatment at 300-400℃ for 3-5 h to obtain an alloy with a stable continuous BCC matrix structure and dispersed intermetallic compound distribution. In step 2, the atmospheric conditions are used to maintain the stability of the alloy melt composition and ensure that the Li / (Mg+Al) composition window does not shift significantly, thereby achieving controllable BCC / HCP phase ratio.

[0017] In this invention, Li serves as a key element for lightweighting and phase structure regulation, primarily by reducing alloy density and controlling the BCC / HCP phase ratio. As the Li content increases to 35-39 at.%, the volume fraction of the BCC solid solution significantly increases and can be stably raised to over 56%, while the alloy density decreases to 1.7 g / cm³. 3The density is significantly lower than that of traditional rare earth magnesium alloy systems (such as Mg-10Gd-3Y-0.5Zr, with a density of approximately 1.88 g / cm³). 3 Mg and Al together form a lightweight matrix and provide a basis for solid solution strengthening. Al preferentially forms AlLi intermetallic compound with Li, and Mg, Al and Li further synergistically form Li2MgAl intermetallic compound, thus forming a multi-scale multiphase strengthening system.

[0018] In this invention, the atomic ratio of Li / (Mg+Al) is controlled within the range of 0.55–0.65. When this ratio is below 0.55, the proportion of the HCP phase increases significantly, leading to a decrease in plasticity; when this ratio is above 0.62, the proportion of the BCC phase further increases, but the strength support is insufficient, resulting in a decrease in yield strength. Within this range, the volume fraction of the BCC solid solution can be stably maintained above 56%, forming a continuous plastic deformation channel, while retaining an appropriate amount of HCP phase and intermetallic compounds to provide strength support, achieving a synergistic match between strength and plasticity.

[0019] In this system, Ca and Si primarily control the precipitation strengthening by regulating the nucleation behavior and volume fraction of the AlLi and Li2MgAl intermetallic compounds. Specifically, when the synergistic content of Ca and Si is high (Ca+Si close to 2 at.%) and the Ca / Si ratio is close to 1:1, Ca preferentially segregates at grain boundaries and defect sites during solidification and subsequent heat treatment, lowering the nucleation energy barrier of the AlLi and Li2MgAl phases and increasing the nucleation density of precipitates. This results in an increase in the volume fraction of intermetallic compounds and a more compact distribution, leading to a higher strengthening level and higher yield strength in the alloy. When the Ca / Si ratio is biased towards Si (e.g., Ca:Si≈1:3), Si forms a locally short-range ordered coordination structure with Al and Mg, which to some extent inhibits the formation of continuous coarse precipitates. This slightly reduces the number of precipitates but improves their uniformity, thus achieving a relative balance between strength and plasticity. When the total content of Ca and Si decreases (Ca+Si≈1 at.%), the nucleation driving force weakens, the volume fraction of AlLi and Li2MgAl precipitates decreases, and the continuity of the BCC matrix is ​​enhanced, thereby improving the plastic deformation capacity. In summary, the role of Ca and Si is essentially to achieve synergistic regulation of the continuity of the strengthening phase and the matrix by controlling the nucleation density and volume fraction of the precipitates, thereby obtaining different levels of strength and plasticity matching.

[0020] The alloy preparation of this invention uses a graphite crucible sealed with a graphite lid. The crucible is placed in a vacuum induction melting furnace, and a vacuum of 5 × 10⁻⁶ is first applied. -3The pressure is below Pa, and high-purity argon is repeatedly purged three times to reduce the residual oxygen and moisture content in the furnace. Then, high-purity argon is introduced again as a protective atmosphere, and melting is carried out under an inert atmosphere of 1.8–2.2 atmospheres. The alloy melt is then completely melted at 700–750℃ and held for 10–20 min to allow for thorough diffusion and mixing of the components. Afterward, it is cast and homogenized at 350–400℃ for 3–5 h, and finally air-cooled to room temperature to obtain a stable BCC matrix and a diffusely distributed intermetallic compound microstructure.

[0021] In the high Li content (35–42 at.%) Mg-Li-Al-Ca-Si pentagonal system of this invention, Li is highly sensitive to oxygen and interfacial reactions in its high-temperature molten state. Furthermore, the compositional coupling effect between components in a multi-principal system easily leads to localized compositional fluctuations during melting, which in turn affects the subsequent BCC / HCP phase ratio and the precipitation behavior of intermetallic compounds. Based on these system characteristics, a process combination of "high-vacuum pretreatment – ​​inert atmosphere protection – moderate gas pressure control" can reduce the oxygen partial pressure in the furnace and improve the stability of the melt-gas interface to a certain extent. This helps to mitigate the volatilization loss of Li during melting and improves the uniformity and repeatability of the pentagonal system composition.

[0022] Example 1 An alloy with atomic percentages of Mg47Li36Al15Ca1Si1 was prepared, where Li / (Mg+Al) = 36 / (47+15) ≈ 0.58 and Ca+Si = 2 at.%. Raw materials were weighed according to the target composition and placed in a graphite crucible. High-purity argon gas was introduced to a furnace pressure of 1.8–2.2 atm. The mixture was melted at 720℃ and held for 15 min, then remelted three times. The melt was poured into a copper mold with an inner diameter of 10 mm and allowed to cool naturally. Subsequently, it was homogenized and annealed at 380℃ for 8 h and then air-cooled. Compression specimens with a diameter of 6 mm and a height of 12 mm were machined by wire cutting at a strain rate of 1 × 10⁻⁶. -4 The room temperature compression test was performed at / s, and the test results are shown in Table 1.

[0023] In this embodiment, the Ca and Si contents are relatively high, and the number of AlLi and Li2MgAl reinforcing phases is large, resulting in a significant strengthening effect. It is suitable for lightweight structural components with high load-bearing strength requirements.

[0024] like Figure 2As shown, the strongest X-ray diffraction peak of the alloy (Mg47Li36Al15Ca1Si1) prepared in Example 1 corresponds to the BCC solid solution phase. It also contains diffraction peaks for AlLi, HCP solid solutions, and Li2MgAl phases. Through Rietveld full-spectrum refined quantitative analysis of X-ray diffraction, the volume fraction of the BCC phase is approximately 56%, AlLi approximately 28%, HCP approximately 9%, Li2MgAl approximately 9%, and approximately 3% unidentified phases. The corresponding microstructure distribution is shown below. Figure 3 As shown in (a).

[0025] Example 2 An alloy with atomic percentages of Mg48Li35Al15Ca0.5Si1.5 was prepared, wherein Li / (Mg+Al)=35 / (48+15)≈0.56, and Ca+Si=2 at.%. Except for composition adjustments, the remaining operational steps were the same as in Example 1. The results of the room temperature compression test are shown in Table 1.

[0026] This embodiment achieves a balance between the type and quantity of reinforcing phases by adjusting the Mg / Li ratio and the Ca and Si contents, resulting in a good match between strength and plasticity, and is suitable for general lightweight structural components.

[0027] like Figure 2 As shown, the alloy (Mg48Li35Al15Ca0.5Si1.5) prepared in Example 2 also contains diffraction peaks of BCC solid solution, AlLi, HCP solid solution, and Li2MgAl phase, but the diffraction intensities of each phase are different from those in Example 1, with a significant decrease in the intensity of the HCP diffraction peak. Through Rietveld full-spectrum refined quantitative analysis, the volume fraction of the BCC phase is approximately 60%, AlLi phase approximately 20%, HCP phase approximately 8%, Li2MgAl phase approximately 9%, and unlabeled phase less than 3%. The corresponding microstructure distribution is shown below. Figure 3 As shown in (b).

[0028] Example 3 An alloy with an atomic percentage of Mg45Li39Al15Ca0.5Si0.5 was prepared, wherein Li / (Mg+Al)=39 / (45+15)≈0.65, and Ca+Si=1 at.%. Except for the composition adjustment, the remaining operating steps were the same as in Example 1. The results of the room temperature compression test are shown in Table 1.

[0029] In this embodiment, the total content of Ca and Si is relatively low, the number of reinforcing phases is relatively reduced, and the continuous BCC solid solution matrix provides more sufficient deformation capability, making it suitable for lightweight parts with high requirements for formability or energy absorption deformation capability.

[0030] like Figure 2As shown, the alloy (Mg45Li39Al15Ca0.5Si0.5) obtained in Example 3 has a relatively simple phase composition, mainly consisting of BCC solid solution, AlLi intermetallic compounds, and Li2MgAl intermetallic compounds. The diffraction peak intensity of the HCP phase is extremely weak. Through Rietveld full-spectrum refined quantitative analysis, the volume fraction of the BCC phase is approximately 77%, the AlLi phase is approximately 15%, the HCP phase content is less than 5%, and the Li2MgAl phase and unidentified phases account for approximately 3%. The corresponding microstructure distribution is shown below. Figure 3 As shown in (c).

[0031] Comparative Example Commercially available AZ91D magnesium alloy was selected as a comparative example, with a nominal composition of Mg-9Al-1Zn (mass percentage). Test results under the same sample size and room temperature compression test conditions are shown in Table 1.

[0032] Table 1 Test Results The aforementioned BCC solid solution phase provides a continuous plastic deformation channel, while the HCP solid solution phase provides strength support. The AlLi and Li2MgAl intermetallic compounds effectively hinder dislocation movement and refine the microstructure through dispersion strengthening and grain boundary pinning. The synergistic effect of multiple phases enables the alloys prepared in Examples 1–3 to simultaneously achieve high room temperature compressive strength and good plasticity under low-density conditions.

[0033] As seen in Examples 1–3, within the Mg-Li-Al main component framework, the proportions and distributions of each phase can be effectively controlled by adjusting the Ca and Si contents and the Li / (Mg+Al) ratio, forming a performance gradient from high strength to high plasticity. In summary, this invention, through a high-Li-content Mg-Li-Al matrix, Ca-Si synergistic microalloying, and a high-pressure argon-protected vacuum induction melting process, has obtained a class of phases with both low density (<1.65 g / cm³) and high plasticity. 3 Mg-Li-Al-Ca-Si pentagonal lightweight medium-entropy alloy with high specific strength, good room temperature compressive plasticity and cost advantage.

[0034] The alloys prepared in Examples 1–3 do not contain rare earth elements or precious metal elements, have a density of less than 1.65 g / cm3, a room temperature compressive yield strength of 320–360 MPa, and a room temperature compressive fracture strain of up to 45%.

[0035] In the three embodiments, Li / (Mg+Al) were approximately 0.58 (36 / (47+15)), 0.56 (35 / (48+15)), and 0.65 (39 / (45+15)), respectively, all within this range. The corresponding BCC phase volume fractions were approximately 56%, 60%, and 77%, respectively, verifying the effectiveness of this ratio range.

[0036] The alloy of this invention is a five-element medium-entropy alloy with a molar mixing entropy in the medium-entropy range of 8.85-9.14 J / (mol·K). Compared with traditional low-entropy magnesium alloys (such as AZ91D with a mixing entropy of less than 3.16 J / (mol·K)), the medium-entropy effect can significantly improve the mutual solid solution ability of each component element, suppress element segregation and precipitation of harmful phases, further refine grains, strengthen the matrix, and improve the overall stability of the multiphase structure. From the thermodynamic perspective of the alloy, it can ensure the comprehensive performance of high strength and plasticity matching of the alloy.

[0037] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength, ductile, pentagonal, lightweight, medium-entropy alloy, characterized in that, On an atomic percentage basis: Mg 45-48 at.%, Li 35-39 at.%, Al 15 at.%, Ca 0.5-1 at.%, Si 0.5-1.5 at.%; The microstructure of the alloy is based on a continuous BCC solid solution matrix, with the BCC solid solution metallographic area accounting for 56%-77%. AlLi and Li2MgAl intermetallic compounds are discontinuous precipitates distributed in the matrix in the form of dispersed particles. The density of the alloy is less than 1.7 g / cm³. 3 .

2. The high-strength, ductile, pentagonal, lightweight, medium-entropy alloy according to claim 1, characterized in that, The AlLi intermetallic compound accounts for 15%-28%, and the volume fraction of both the Li2MgAl intermetallic compound and the HCP structure solid solution is ≤10%.

3. A method for preparing the high-strength, ductile, pentagonal, lightweight, medium-entropy alloy as described in claim 1, characterized in that, include: Step 1: Proportion the raw materials according to the target atomic percentage and perform vacuum induction melting under an inert atmosphere; Step 2: During the melting process, argon gas is introduced and a positive pressure protective atmosphere of 1.8-2.2 atm is maintained; Step 3: Melt the raw materials completely at 700-750℃ and hold for 10-20 minutes to obtain a uniform alloy melt. Step 4: After casting the alloy melt into shape, perform homogenization heat treatment at 300-400℃ for 3-5 h to obtain an alloy with a stable continuous BCC matrix structure and dispersed intermetallic compound distribution. In step 2, the atmospheric conditions are used to maintain the stability of the alloy melt composition and ensure that the Li / (Mg+Al) composition window does not shift significantly, thereby achieving controllable BCC / HCP phase ratio.

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