Blocky nano twin crystal light metal material and preparation method thereof

Abstract

The invention discloses a blocky nano twin crystal light metal material and a preparation method thereof, the chemical composition expression of the blocky nano twin crystal light metal material is Mg-xN-yM, x and y are mass fractions, x is more than or equal to 6 and less than or equal to 20, and y is more than 0 and less than or equal to 3; n is one or more of Zn, Li, Al and Sc, and M is one or more of Ti, Cu, Mo, Si, Ni, Ce and Zr; the blocky nano-twin crystal light metal material is obtained through pressure electric arc melting, low-temperature high-energy ball milling and high-temperature high-pressure sintering. The preparation method is simple, the process is easy to control, the twin crystal type of the prepared blocky nano twin crystal light metal material is {10-11} compressed twin crystals, the maximum number of the twin crystals can account for 90% of the total volume of the material, and the blocky nano twin crystal light metal material has the beneficial effects of being low in density, high in specific strength and excellent in comprehensive mechanical property; the method can be used in the fields of aerospace, new energy vehicles, biomedical treatment, national defense and military industry and the like.

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation technology, and relates to a bulk nanotwinned lightweight metal material and its preparation method. Background Technology

[0002] Currently, achieving energy conservation and emission reduction goals remains a challenging task. Therefore, vigorously promoting the application of lightweight alloy structural materials is an effective way to alleviate environmental pressure, especially in the automotive industry. Magnesium alloys are among the lightest metallic structural materials, possessing high specific strength and good machinability, electromagnetic shielding, and damping properties. However, their relatively low strength and difficulty in room temperature deformation processing severely limit their application prospects as potential structural materials. Over the past few decades, alloying, grain refinement, precipitation strengthening, and heat treatment have been widely used to improve the strength of magnesium alloys. Due to the limited independent slip systems provided by the hexagonal close-packed (HCP) crystal structure during deformation, the deformability of magnesium alloys is inevitably closely related to the formation and growth of twins. Therefore, in-depth research on the types of twins in magnesium alloys and their related deformation mechanisms is key to improving their cold working performance and mechanical properties.

[0003] Similar to grain boundaries, twin boundaries play a crucial role in improving the mechanical properties of HCP-structured magnesium alloys with finite slip systems through their interaction with dislocations. Furthermore, compared to traditional high-angle grain boundaries, twin boundaries typically exhibit higher thermal and mechanical stability, especially at the nanoscale. Therefore, twinning strengthening is considered a possible strengthening mechanism in high-performance magnesium alloys, similar to grain refinement strengthening and precipitation strengthening. However, although the strengthening effect of twin boundaries is comparable to that of grain boundaries in magnitude, when the twin boundary spacing is at the micrometer scale, its strengthening effect is not significant compared to grain refinement strengthening. The most common twin types in magnesium alloys are {10⁻¹¹} compression twins and {10⁻¹²} tensile twins. {10⁻¹²} tensile twins have poor interfacial stability and are prone to migration under stress, potentially even expanding to occupy the entire grain during deformation; therefore, their width is typically maintained between several and tens of micrometers. In contrast, {10⁻¹¹} compression twins are generally smaller and more stable. Furthermore, the critical shear stress required to activate compressive twins is typically much higher than that required for tensile twins. This indicates that a greater driving force (higher local stress) is needed to trigger the nucleation of compressive twins during plastic deformation. However, common plastic deformation methods often struggle to achieve such high stress levels. Therefore, how to prepare high-density and stable {10-11} compressive twins has become a significant technical challenge in the current field of magnesium alloy processing.

[0004] The literature “Nature Communications, 2021, 12(1): 4616” discloses a method for preparing nanotwinned magnesium alloys. Specifically, hot-rolled AZ80 plates are cut into blocks, then solution-treated at 400℃ for 24 hours, followed by cold water quenching. The solution-treated samples are subjected to 1, 3, 6, and 12 cycles of multi-directional compression, using a low-strain (3%) followed by a high-strain (6.5%) cycle, to obtain high-performance nanotwinned magnesium alloys. Although the multi-directional compression method used in the literature successfully prepared magnesium alloys with an average twin thickness of 200 nm, in-depth research revealed that the twin types were all {10-12} tensile twins. This type of twin has limited strengthening effect on magnesium alloys (even less effective than nanoscale precipitates), and the strategy of introducing a large number of {10-12} tensile twins into magnesium alloys has been widely applied and studied, lacking innovation.

[0005] Chinese invention patent application No. 202110271202.X discloses a method for preparing an ultra-high strength and high toughness nano-gradient twinned magnesium alloy. This invention first performs homogenization annealing on a magnesium alloy ingot after semi-continuous casting, then cuts off the middle portion of the ingot and processes it into bars using hot extrusion. Disc-shaped samples are cut from the resulting bars and subjected to solution treatment. Subsequently, under room temperature conditions, a high-pressure torsion process is used to achieve intense plastic deformation, followed by subsequent aging optimization treatment, ultimately obtaining a magnesium alloy with excellent comprehensive mechanical properties. However, the high performance of this alloy is mainly attributed to the intense plastic deformation process (fine-grain strengthening, texture strengthening, etc.) such as high-pressure torsion, while the contribution of its twinned structure is relatively limited, manifested in a low twinning integral number. Furthermore, this invention uses a magnesium rare-earth alloy with a total rare-earth content exceeding 10 wt.% as raw material, resulting in high costs and making it difficult to meet the needs of large-scale industrial production.

[0006] Chinese invention patent application No. 202211414290.5 discloses a method for preparing TB8 titanium alloy with a nanotwinned structure. This invention modifies heat-treated single-phase TB8 titanium alloy using high-voltage pulse treatment, successfully preparing a TB8 titanium alloy with a nanotwinned structure. The resulting titanium alloy's microstructure mainly consists of a β-titanium matrix and α-titanium precipitates containing nanotwins. Although the proposed method has significant advantages such as low cost, simple operation, and straightforward process, the nanotwin content in the prepared alloy is relatively low, resulting in limited strengthening effect. Furthermore, with increasing applied pressure, although the twin density increases, twin coarsening occurs simultaneously. This microstructure evolution weakens the strengthening effect of the material to some extent, thus limiting further improvement in mechanical properties.

[0007] In summary, the main drawbacks of current lightweight metal materials are:

[0008] First, the twin type is the {10-12} tensile twin, which has a poor strengthening effect and lacks innovation;

[0009] Second, the preparation process is complex and not suitable for large-scale production;

[0010] Third, the twin density is low, resulting in poor overall mechanical properties; while strength is increased, plasticity decreases. Summary of the Invention

[0011] To address the aforementioned technical problems, this invention aims to provide a bulk nanotwinned lightweight metal material and its preparation method. Its chemical composition is expressed as: Mg-xN-yM, where x and y are mass fractions, 6≤x≤20, 0<y≤3; N is one or more of Zn, Li, Al, and Sc; and M is one or more of Ti, Cu, Mo, Si, Ni, Ce, and Zr. This bulk nanotwinned lightweight metal material is obtained through pressure arc melting, low-temperature high-energy ball milling, and high-temperature high-pressure sintering. The preparation method of this invention is simple and the process is easy to control. The resulting bulk nanotwinned lightweight metal material exhibits {10-11} compressed twins, with the twin integral reaching up to 90% of the total material volume. It possesses characteristics of low density, high specific strength, and excellent comprehensive mechanical properties, making it suitable for applications in aerospace, new energy vehicles, biomedicine, and national defense.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] A bulk nanotwinned lightweight metallic material has the chemical composition formula: Mg-xN-yM, where x and y are mass fractions (wt.%), 6≤x≤20, 0<y≤3; N is one or more of Zn, Li, Al, and Sc, and M is one or more of Ti, Cu, Mo, Si, Ni, Ce, and Zr.

[0014] As a limitation of the present invention, the microstructure of the bulk nanotwin lightweight metal material includes nanotwins and recrystallized grains; stacking faults exist inside the nanotwins, and the average thickness of the nanotwins is 94-144 nm; the recrystallized grains are equiaxed crystals with an average grain size of 109-153 μm; the volume fraction of the nanotwins accounts for 58%-90% of the total volume of the material.

[0015] As another limitation of the present invention, the nanotwins are {10-11} compressed twins.

[0016] This invention also provides a method for preparing bulk nanotwinned lightweight metallic materials, which is carried out in the following order:

[0017] S1. Prepare the alloy according to the alloy composition, add it to a vacuum melting furnace, and melt it under an argon atmosphere at 10MPa and 300rpm to obtain the Mg-xN-yM as-cast alloy.

[0018] S2. Place the Mg-xN-yM as-cast alloy in a quenched steel ball mill jar, add 10 mL of n-hexane as a grinding aid, immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20 min for ball milling, then place it in a vacuum drying oven for drying, and finally press the ball-milled powder into cylinders with a diameter of 30 mm and a height of 20 mm under a pressure of 5 MPa.

[0019] S3. The pre-pressed sample is placed in a six-sided top press for high-temperature and high-pressure sintering to obtain a bulk nanotwinned lightweight metal material.

[0020] As a limitation of the preparation method of the present invention, in step S1, the melting process is as follows: first, melting at a current of 200A for 30s, then melting at a current of 600A for 120s, and finally reducing the current to 400A for 60s, and completing 3 inversions within the 60s of melting at 400A; after melting, extinguishing the arc and maintaining the pressure for 5 to 10 minutes.

[0021] As another limitation of the preparation method of the present invention, in step S2, the ball milling process is as follows: quenched steel grinding balls with a ball-to-material ratio of 20:1 and the material are placed in a quenched steel ball milling jar, and ball milled at 600-1000 rpm for 4-6 cycles. One cycle is: ball milling for 30 minutes, resting for 10 minutes. During the resting period, the ball milling jar is immersed in liquid nitrogen at -196℃ to maintain the low temperature.

[0022] The low-temperature, high-energy ball milling process in this invention is crucial for preparing precursor alloy powders rich in stacking fault structures. Twin formation primarily stems from the shearing behavior of atoms along specific crystal orientations. In stacking fault regions, the atomic arrangement changes and they are in a higher energy state, making stacking faults more likely to become nucleation sites for twins.

[0023] As a third limitation of the preparation method of the present invention, in step S2, the drying temperature is 25°C and the time is 1 hour.

[0024] As a fourth limitation of the preparation method of the present invention, in step S3, the high temperature and high pressure sintering process is as follows: the temperature is raised from room temperature to 700-1100℃ at a heating rate of 6℃ / s under 6GPa, and the temperature and pressure are maintained for 1h.

[0025] In this invention, the temperature and pressure during high-temperature and high-pressure sintering affect the densification behavior, grain growth kinetics, and stability of the twin structure of the material. When sintering at 700–1100℃ under 6 GPa pressure, sufficient dynamic recrystallization and twin nucleation occur, resulting in a highly dense, uniform, and fine nanotwin structure. If the pressure is less than 6 GPa and the temperature is less than 700℃ during this stage, insufficient plastic flow and a low diffusion rate will lead to incomplete densification, insufficient twin formation, and deterioration of mechanical properties. If the pressure is greater than 6 GPa and the temperature exceeds 1100℃ during this stage, abnormal growth of grains and nanotwins will occur, and even twin annihilation may occur, leading to softening and decreased strength of the material. Holding at high temperature and pressure for 1 hour ensures sufficient diffusion, eliminates internal stress, and stabilizes the nanotwin structure.

[0026] As is well known, {10-12} tensile twins can be activated under low critical shear stresses of 2-8 MPa and are easily expanded, even occupying the entire grain during deformation. In contrast, {10-11} compressive twins have more stable interfaces and higher critical shear stresses, approximately 30-100 MPa, which can effectively pin dislocation movement, thus producing a stronger strengthening effect. This invention employs a low-temperature high-energy ball milling combined with a high-temperature high-pressure sintering process to successfully prepare lightweight metallic materials with high-density {10-11} compressive nanotwins. The core of this invention lies in utilizing refractory metal atoms as particles to induce stacking faults and preparing precursor alloy powders rich in stacking fault structures through a low-temperature high-energy ball milling process. The formation of twins mainly originates from the shearing behavior of atoms along specific crystal directions, and the atomic arrangement in the stacking fault region changes and is in a higher energy state, thus stacking faults are more likely to become nucleation sites for twins. In addition, the six-sided press has six-directional compression characteristics, which can effectively suppress {10-12} tensile twins generated by stretching along the crystallographic c-axis. Therefore, after undergoing a high-temperature and high-pressure sintering process using a six-sided top press, a lightweight metallic material with high-density {10-11} compressed nanotwins was finally obtained.

[0027] The above-mentioned technical solution of the present invention is a whole in which each step is closely related and mutually influential, and together they determine the morphological characteristics and performance of the product.

[0028] The above technical solution has the following advantages or beneficial effects:

[0029] 1. The bulk nanotwin lightweight metal material prepared by this invention has {10-11} compressed twins as its twin type and has a high twin density. The volume fraction of nanotwins can account for up to 90% of the total volume of the material.

[0030] 2. The bulk nanotwinned lightweight metal material prepared by this invention has excellent mechanical properties, with a yield strength of up to 412 MPa and a tensile strength of up to 455 MPa.

[0031] 3. The preparation method of this invention is simple, the process is easy to control, and it is suitable for large-scale industrial production.

[0032] This invention is applicable to the preparation of bulk nanotwinned lightweight metal materials.

[0033] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0034] Figure 1 This is an optical microscope image of the bulk nanotwinned lightweight metal material prepared in Example 1 of the present invention;

[0035] Figure 2 This is a transmission electron microscope image of the bulk nanotwinned lightweight metallic material prepared in Example 2 of the present invention;

[0036] Figure 3 This is a Vickers hardness variation curve of the bulk nanotwinned lightweight metal material prepared in Example 2 of the present invention;

[0037] Figure 4 This is a high-resolution image of the nanotwin structure of the bulk nanotwin lightweight metal material prepared in Example 3 of the present invention. The upper right corner of the image is a Fast Fourier Transform (FFT) image. Detailed Implementation

[0038] The following embodiments are merely some, not all, of the embodiments of the present invention. Therefore, the detailed descriptions of the embodiments provided below are not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0039] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified.

[0040] Example 1

[0041] This embodiment prepares a Mg-6Li-1Cu bulk nanotwinned lightweight metallic material (Li: 6wt.%, Cu: 1wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0042] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 5 min to obtain the Mg-6Li-1Cu as-cast alloy;

[0043] S2. Place the Mg-6Li-1Cu as-cast alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20min before ball milling. Place the quenched steel grinding balls and the material with a ball-to-material ratio of 20:1 in the quenched steel ball mill jar and ball mill at 800rpm for 4 cycles. One cycle consists of ball milling for 30min, followed by a 10min rest. During the rest period, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature. Then dry at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere of 5MPa.

[0044] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 700℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1 hour to obtain a bulk nanotwinned lightweight metal material. The average thickness of the nanotwins is 135nm, and the average grain size of the equiaxed crystals is 122μm. The nanotwins account for 61% of the total volume of the material.

[0045] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing. Its tensile strength was 370 MPa, yield strength was 322 MPa, and fracture strain was 25%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were tested. The test results showed that the Mg-6Li-1Cu as-cast alloy obtained only through pressure arc melting had a tensile strength of only 151 MPa and a yield strength of only 90 MPa.

[0046] like Figure 1 The image shows an optical microscope image of the bulk nanotwinned lightweight metal material prepared in this embodiment. As can be seen from the image, the grains of the sample after high temperature and high pressure are equiaxed crystals.

[0047] Example 2

[0048] This embodiment prepares a Mg-9Li-1.5Zr bulk nanotwinned lightweight metallic material (Li: 9wt.%, Zr: 1.5wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0049] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 10 min to obtain the Mg-9Li-1.5Zr as-cast alloy;

[0050] S2. Place the as-cast Mg-9Li-1.5Zr alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for 20min to pre-cool it before ball milling. Place the quenched steel grinding balls and the material with a ball-to-material ratio of 20:1 in the quenched steel ball mill jar, and ball mill at 1000rpm for 6 cycles. One cycle consists of 30min of ball milling followed by a 10min break. During the break, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature. Then dry it at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere of 5MPa.

[0051] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 900℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1h to obtain a bulk nanotwin lightweight metal material with an average nanotwin thickness of 94nm, an average equiaxed grain size of 109μm, and a nanotwin integral number accounting for 90% of the total volume of the material.

[0052] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing. Its tensile strength was 455 MPa, yield strength was 412 MPa, and fracture strain was 16%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were tested. The test results showed that the as-cast alloy of Mg-9Li-1.5Zr obtained only through pressure arc melting had a tensile strength of only 132 MPa and a yield strength of only 85 MPa.

[0053] like Figure 2 The image shows a transmission electron microscope image of the bulk nanotwinned lightweight metal material prepared in this embodiment. As can be seen from the image, after high-temperature and high-pressure sintering, high-density nanotwins are formed inside the grains of the bulk nanotwinned lightweight metal material. This indicates that the low-temperature high-energy ball milling combined with high-temperature and high-pressure sintering process can effectively prepare bulk alloys with high-density nanotwins.

[0054] like Figure 3The figure shows the Vickers hardness variation curve of the bulk nanotwinned lightweight metal material prepared in this embodiment. As can be seen from the figure, after a series of high-temperature and high-pressure processes, the Vickers hardness of the alloy is significantly improved compared to the as-cast state, reaching a peak at 6 GPa and 900℃. This indicates that the low-temperature high-energy ball milling combined with high-temperature and high-pressure sintering can significantly improve the mechanical properties of the alloy.

[0055] Example 3

[0056] This embodiment prepares a Mg-10Sc-1Ti bulk nanotwinned lightweight metallic material (Sc: 10 wt.%, Ti: 1 wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0057] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 7 min to obtain the Mg-10Sc-1Ti as-cast alloy;

[0058] S2. Place the as-cast Mg-10Sc-1Ti alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20min for ball milling: Place quenched steel grinding balls and materials with a ball-to-material ratio of 20:1 in a quenched steel ball mill jar, and ball mill at 900rpm for 5 cycles. One cycle consists of ball milling for 30min, resting for 10min. During the rest period, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature, and then dry at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere and a pressure of 5MPa.

[0059] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 1100℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1h to obtain a bulk nanotwin lightweight metal material. The average thickness of the nanotwins is 122nm, the average grain size of the equiaxed crystals is about 140μm, and the nanotwins account for 77% of the total volume of the material.

[0060] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing. Its tensile strength was 408 MPa, yield strength was 364 MPa, and fracture strain was 20%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were also tested. The test results showed that the as-cast Mg-10Sc-1Ti alloy obtained only through pressure arc melting had a tensile strength of only 218 MPa and a yield strength of only 147 MPa.

[0061] like Figure 4 The image shows a high-resolution image of the nanotwin structure of the bulk nanotwin lightweight metal material prepared in this embodiment. The upper right corner of the image is a Fast Fourier Transform (FFT) image. Analysis shows that the matrix and the (0001) plane of the twin are mirror-symmetric along the (10-11) twin plane, and the (10-10) plane also exhibits the same symmetry. Therefore, it can be determined that the twin type belongs to the {10-11} compression twin.

[0062] Example 4

[0063] This embodiment prepares a Mg-10Sc-1Si bulk nanotwinned lightweight metallic material (Sc: 10 wt.%, Si: 1 wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0064] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 7 min to obtain the Mg-10Sc-1Si as-cast alloy;

[0065] S2. Place the as-cast Mg-10Sc-1Si alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20min for ball milling: Place quenched steel grinding balls and materials with a ball-to-material ratio of 20:1 in a quenched steel ball mill jar, and ball mill at 900rpm for 5 cycles. One cycle consists of ball milling for 30min, followed by a 10min rest. During the rest period, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature, and then dry at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere and a pressure of 5MPa.

[0066] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 1100℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1h to obtain a bulk nanotwin lightweight metal material with an average nanotwin thickness of 130nm, an average equiaxed grain size of 134μm, and a nanotwin integral number accounting for 72% of the total volume of the material.

[0067] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing, and its tensile strength was 391 MPa, yield strength was 354 MPa, and fracture strain was 22%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were also tested. The test results showed that the tensile strength of the Mg-10Sc-1Si as-cast alloy obtained only through pressure arc melting was only 202 MPa, and its yield strength was only 142 MPa.

[0068] Example 5

[0069] This embodiment prepares a Mg-9Al-2Ce-1Ni bulk nanotwinned lightweight metallic material (Al: 9wt.%, Ce: 2wt.%, Ni: 1wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0070] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 5 min to obtain the Mg-9Al-2Ce-1Ni as-cast alloy;

[0071] S2. Place the Mg-9Al-2Ce-1Ni as-cast alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20min for ball milling: Place quenched steel grinding balls and materials with a ball-to-material ratio of 20:1 in a quenched steel ball mill jar, and ball mill at 800rpm for 4 cycles. One cycle consists of ball milling for 30min, resting for 10min. During the rest period, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature, and then dry at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere and a pressure of 5MPa.

[0072] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 1100℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1h to obtain a bulk nanotwin lightweight metal material with an average nanotwin thickness of 121nm, an average equiaxed grain size of about 162μm, and a nanotwin integral number accounting for 65% of the total volume of the material.

[0073] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing. Its tensile strength was 382 MPa, yield strength was 334 MPa, and fracture strain was 25%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were tested. The test results showed that the as-cast Mg-9Al-2Ce-1Ni alloy obtained only through pressure arc melting had a tensile strength of only 188 MPa and a yield strength of only 137 MPa.

[0074] Example 6

[0075] This embodiment prepares a Mg-20Zn-3Mo bulk nanotwinned lightweight metallic material (Zn: 20wt.%, Mo: 3wt.%, balance Mg by mass percentage). The preparation process and steps are as follows:

[0076] S1. Prepare the alloy according to the alloy composition, add it to the vacuum melting furnace, and melt it under an argon atmosphere at 10 MPa and 300 rpm: first melt at 200 A current for 30 s, then melt at 600 A current for 120 s, and finally reduce the current to 400 A for 60 s, and complete 3 flips within the 60 s of melting at 400 A current (on average once every 20 s); after the arc is extinguished, maintain the pressure for 10 min to obtain the Mg-20Zn-3Mo as-cast alloy;

[0077] S2. Place the as-cast Mg-20Zn-3Mo alloy in a quenched steel ball mill jar, add 10mL of n-hexane as a grinding aid, and immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20min for ball milling: Place quenched steel grinding balls and materials with a ball-to-material ratio of 20:1 in a quenched steel ball mill jar, and ball mill at 600rpm for 6 cycles. One cycle consists of ball milling for 30min, followed by a 10min rest. During the rest period, immerse the ball mill jar in liquid nitrogen at -196℃ to maintain the low temperature, and then dry at 25℃ for 1h. Finally, press the ball-milled powder into cylinders with a diameter of 30mm and a height of 20mm under an argon atmosphere and a pressure of 5MPa.

[0078] S3. The pre-pressed sample is placed in a six-sided press and heated from room temperature to 900℃ at a heating rate of 6℃ / s under 6GPa. The temperature is held for 1h to obtain a bulk nanotwin lightweight metal material with an average nanotwin thickness of 144nm, an average equiaxed grain size of 153μm, and a nanotwin integral number accounting for 58% of the total volume of the material.

[0079] The bulk nanotwinned lightweight metal material prepared in this embodiment was subjected to mechanical property testing. Its tensile strength was 362 MPa, yield strength was 308 MPa, and fracture strain was 20%. Furthermore, the as-cast alloy prepared in step S1 of this embodiment was used as a comparative example, and its mechanical properties were tested. The test results showed that the as-cast Mg-20Zn-3Mo alloy obtained only through pressure arc melting had a tensile strength of only 177 MPa and a yield strength of only 124 MPa.

[0080] Comparative Example

[0081] To investigate the effects of different preparation processes and parameters used in the preparation process on the performance of the product of this invention, the following comparative experiments were conducted. Different metallic materials were prepared in the following comparative examples:

[0082] Comparative Example 1

[0083] This comparative example prepares a metallic material. The preparation process is similar to that of Example 1, except that step S2 is omitted. That is, after obtaining the cast alloy through step S1, the high-temperature and high-pressure sintering of step S3 is carried out directly.

[0084] The mechanical properties of the metallic material prepared in this comparative example were tested, and its tensile strength was 301 MPa, yield strength was 277 MPa, and fracture strain was 28%. Due to the lack of low-temperature high-energy ball milling treatment, the as-cast alloy had fewer internal stacking fault structures, and thus failed to form high-density nanotwins after high-temperature and high-pressure treatment, resulting in relatively poor mechanical properties.

[0085] Comparative Example 2

[0086] This comparative example prepares a metallic material. The preparation process is similar to that of Example 1, except that the high-temperature and high-pressure sintering in step S3 is not performed.

[0087] The mechanical properties of the metallic material prepared in this comparative example were tested, and its tensile strength was 263 MPa, yield strength was 226 MPa, and fracture strain was 17%. High-temperature and high-pressure sintering can promote sufficient dynamic recrystallization and twin nucleation, thus giving the material a high density and uniform and fine nanotwin structure; while after low-temperature high-energy ball milling, the material only has a large number of stacking fault structures inside and lacks an effective strengthening mechanism, so its mechanical properties are relatively poor.

[0088] Comparative Example 3

[0089] This comparative example prepares a metallic material. The preparation process is similar to that of Example 1, except that the sintering temperature in step S3 is 1200℃.

[0090] The mechanical properties of the metallic material prepared in this comparative example were tested, and its tensile strength was 342 MPa, yield strength was 301 MPa, and fracture strain was 24.5%. Because temperatures exceeding 1100℃ can induce abnormal growth of grains and nanotwins, and even twin annihilation, leading to material softening and a decrease in strength.

[0091] Comparative Example 4

[0092] This comparative example prepares a metallic material. The preparation process is similar to that of Example 1, except that the sintering pressure in step S3 is 4 GPa.

[0093] The mechanical properties of the metallic material prepared in this comparative example were tested, and its tensile strength was 341 MPa, yield strength was 298 MPa, and fracture strain was 18%. Since pressure less than 6 GPa leads to insufficient plastic flow, resulting in incomplete densification and insufficient twin nucleation, the mechanical properties are relatively poor.

[0094] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A bulk nanotwinned lightweight metallic material, characterized in that, Its chemical composition formula is: Mg-xN-yM, where x and y are mass fractions, 6≤x≤20, 0<y≤3; N is one or more of Zn, Li, Al, and Sc, and M is one or more of Ti, Cu, Mo, Si, Ni, Ce, and Zr.

2. The bulk nanotwinned lightweight metal material according to claim 1, characterized in that, The microstructure of the bulk nanotwin lightweight metal material includes nanotwins and recrystallized grains; stacking faults exist inside the nanotwins, and the average thickness of the nanotwins is 94–144 nm; the recrystallized grains are equiaxed crystals with an average grain size of 109–153 μm; the volume fraction of the nanotwins accounts for 58%–90% of the total volume of the material.

3. The bulk nanotwinned lightweight metal material according to claim 2, characterized in that, The nanotwins are {10-11} compressed twins.

4. A method for preparing a bulk nanotwinned lightweight metallic material according to any one of claims 1-3, characterized in that, Follow these steps in sequence: S1. Prepare the alloy according to the alloy composition, add it to a vacuum melting furnace, and melt it under an argon atmosphere at 10MPa and 300rpm to obtain the Mg-xN-yM as-cast alloy. S2. Place the Mg-xN-yM as-cast alloy in a quenched steel ball mill jar, add 10 mL of n-hexane as a grinding aid, immerse the ball mill jar in liquid nitrogen at -196℃ for pre-cooling for 20 min for ball milling, then place it in a vacuum drying oven for drying, and finally press the ball-milled powder into cylinders with a diameter of 30 mm and a height of 20 mm under an argon atmosphere and a pressure of 5 MPa. S3. The pressed sample is placed in a six-sided press for high-temperature and high-pressure sintering to obtain a bulk nanotwinned lightweight metal material.

5. The method for preparing a bulk nanotwinned lightweight metallic material according to claim 4, characterized in that, In step S1, the melting process is as follows: first, melting at a current of 200A for 30s, then melting at a current of 600A for 120s, and finally reducing the current to 400A for 60s, and completing 3 inversions within the 60s of melting at 400A; after melting, extinguishing the arc and maintaining the pressure for 5 to 10 minutes.

6. The method for preparing a bulk nanotwinned lightweight metallic material according to claim 4, characterized in that, In step S2, the ball milling process is as follows: quenched steel grinding balls with a ball-to-material ratio of 20:1 and the material are placed in a quenched steel ball milling jar, and ball milling is performed at 600-1000 rpm for 4-6 cycles. One cycle consists of ball milling for 30 minutes and resting for 10 minutes. During the rest period, the ball milling jar is immersed in liquid nitrogen at -196℃ to maintain a low temperature.

7. The method for preparing a bulk nanotwinned lightweight metallic material according to claim 4, characterized in that, In step S2, the drying temperature is 25°C and the drying time is 1 hour.

8. The method for preparing a bulk nanotwinned lightweight metallic material according to claim 4, characterized in that, In step S3, the high-temperature and high-pressure sintering process is as follows: the temperature is increased from room temperature to 700-1100℃ at a heating rate of 6℃ / s under 6GPa, and the temperature and pressure are maintained for 1 hour.

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