Method for preparing high-toughness mixed-crystal structure magnesium alloy

Abstract

The application relates to a preparation method of a high-toughness mixed-crystal structure magnesium alloy, and belongs to the technical field of magnesium alloy material preparation. In view of the technical problem that the strength and plasticity of an existing magnesium alloy are difficult to be considered, the application provides a preparation method for inhibiting dynamic recrystallization by regulating a second phase. A magnesium alloy ingot containing dendritic or network eutectic second phases is quenched after being kept at 500 DEG C to 520 DEG C for 16 hours to 20 hours, so that the second phases are fully decomposed and dissolved in a magnesium matrix, and then hot extrusion deformation is carried out at 430 DEG C to 450 DEG C with an extrusion ratio of 10 to 14. The method significantly reduces particle stimulated nucleation points and pins grain boundaries through solid solution atoms, inhibits dynamic recrystallization, and forms an incomplete dynamic recrystallization mixed-crystal structure. The obtained extruded alloy has a yield strength of greater than or equal to 260 MPa, a tensile strength of greater than or equal to 330 MPa, and an elongation after fracture of greater than or equal to 20 %, and excellent synergy of high strength and good plasticity is achieved.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy material preparation technology, and relates to a method for preparing high-strength and high-toughness mixed-crystal magnesium alloys. Background Technology

[0002] As application environments place increasingly stringent demands on the performance of metallic structural materials, the development of metallic materials that combine high strength and high ductility has become crucial. Magnesium alloys, as one of the most competitive lightweight metallic materials, possess advantages such as low density, high specific strength, and good electromagnetic shielding performance, making them promising for applications in high-tech fields like aerospace and electronic communications, as well as civilian sectors like automobiles and consumer electronics. However, compared to other metallic structural materials, their lower absolute strength and poorer ductility limit the further application of magnesium alloys.

[0003] For a long time, researchers have conducted extensive studies on the strengthening mechanism of magnesium alloys. Classical strengthening theories have been widely applied to the design of high-performance magnesium alloys, but they still cannot effectively resolve the contradiction between strength and plasticity. In recent years, heterogeneous deformation-induced (HDI) hardening, unique to heterostructured materials, has attracted widespread attention. The interaction between hard and soft regions in heterostructured materials generates HDI stress at the region boundaries. HDI stress leads to strengthening and strain hardening, and this mechanism achieves a good balance between the strength and plasticity of metallic materials. With the development of high-performance heterostructured metallic materials, heterostructure theory has also been gradually applied to the design and preparation of high-performance magnesium alloys. There are two main types of heterostructured magnesium alloys that have been widely reported: one is a gradient structure prepared by surface treatment technology; the other is a bimodal grain structure (also known as a mixed grain structure) obtained by intense plastic deformation combined with incomplete recrystallization, which is mainly composed of fine recrystallized grains and coarse deformed grains. Currently, many beneficial studies have been carried out on the performance optimization and deformation mechanism of bimodal grain structure magnesium alloys.

[0004] Given the significant potential of bimodal grain structures in synergistically optimizing the strength and plasticity of magnesium alloys, effectively obtaining such structures has become a crucial issue. Currently, powder metallurgy and bulk material post-processing are the mainstream methods for constructing bimodal grain structures. For magnesium alloys, the latter is more widely used. Its main principle is to utilize incomplete recrystallization during plastic deformation or to induce recrystallization of the coarse grains after deformation combined with heat treatment, thereby obtaining a bimodal grain structure. Therefore, precisely controlling the recrystallization behavior of the alloy becomes key to preparing a bimodal grain structure. Li et al. induced non-uniform recrystallization by adding 1% Y (wt.%) to AZ91 magnesium alloy to generate coarse Al2Y particles, increasing the volume fraction of fine grains in the bimodal grain structure and achieving simultaneous improvement in strength and plasticity. However, increasing the alloy element content leads to increased costs. Tang et al. prepared an AZ31B magnesium alloy with a bimodal grain structure by reducing the extrusion temperature to suppress dynamic recrystallization during extrusion, showing a significant improvement in mechanical properties compared to the uniform equiaxed AZ31B alloy. However, excessively low extrusion temperatures can easily lead to material cracking and poor surface quality. In summary, the current methods for obtaining bimodal grain structure magnesium alloys mainly rely on compositional control and deformation processes influencing the alloy's recrystallization behavior. These methods inherently have limitations, restricting the application of bimodal grain structures in magnesium alloys, necessitating the development of new and feasible strategies. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a method for preparing a high-strength and high-toughness mixed-crystal magnesium alloy, aiming to solve the problem of the difficulty in preparing existing high-strength and high-toughness mixed-crystal magnesium alloys.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention develops a novel design and preparation method for mixed-crystalline magnesium alloys, aiming to achieve an excellent synergy between strength and plasticity. By controlling the second phase through heat treatment and influencing the dynamic recrystallization mechanism during hot deformation, a simple and efficient preparation method for mixed-crystalline magnesium alloys is achieved, while simultaneously avoiding material cracking. This method has significant scientific and engineering value for promoting the application of magnesium alloys as structural materials in high-end equipment. This invention is proposed based on this objective. Specifically, it is achieved through the following technical solution: 1) Ingot preparation: Weigh out pure magnesium and magnesium according to the specific composition ratio (by mass percentage: Gd: 8.5~9.5%, Zn: 0.8~1.2%, Zr: 0.4~0.6%, balance being Mg and unavoidable impurities). Gadolinium master alloy, pure zinc and magnesium Zirconium master alloy. The raw material is placed in a steel crucible and heated to melt under a protective atmosphere (such as a mixture of argon and sulfur hexafluoride). After complete melting, the mixture is stirred and slag is removed at a specific temperature (e.g., 740 °C). After standing and holding at this temperature, it is cooled to a semi-solid temperature (e.g., approximately 630 °C) and then water-cooled and cast to obtain a magnesium alloy ingot. The ingot contains a dendritic eutectic phase.

[0007] 2) Second Phase Regulation (Key Homogenization Treatment): The magnesium alloy ingot obtained in step (1) is subjected to a long-term heat treatment at 500~520 ℃ (preferably 510 ℃) (e.g., 16~20 hours, preferably 18 hours). The purpose of this high-temperature, long-term homogenization is to almost completely decompose the dendritic eutectic phase originally present in the ingot, thereby significantly reducing the large amount of blocky second phase that can serve as dynamic recrystallization nucleation sites during subsequent hot working, while ensuring that alloying elements (e.g., Gd, Zn) exist in the form of solid solution atoms or a small amount of fine layered long-period stacked ordered phase. After the heat treatment is completed, the ingot is rapidly cooled (e.g., placed in 90 ℃ hot water) to obtain the pretreated ingot.

[0008] 3) Hot extrusion forming: The pretreated ingot after step 2) is heated to a certain temperature (e.g., 430~450 ℃, preferably 440 ℃) and held at that temperature, and then hot extrusion deformation is performed. The extrusion process parameters are: extrusion ratio 10~14 (preferably 12), extrusion rate 0.5~1.0m / min (preferably 0.7m / min). Through this hot deformation process, an extruded alloy material with a specific cross-sectional size (e.g., 30mm×16mm) is finally obtained.

[0009] The core of this invention lies in the second-phase control step. By fully dissolving the second phase, the nucleation point of dynamic recrystallization during hot extrusion is reduced, and dynamic recrystallization is suppressed to obtain a mixed-crystal structure with incomplete dynamic recrystallization. The specific mechanism and effects are as follows: Microstructure characteristics: The high-temperature homogenization treatment at 500~520 °C successfully decomposed a large number of dendritic eutectic phases in the as-cast microstructure. During the subsequent hot extrusion process, the dynamic recrystallization process was significantly suppressed due to the significant reduction of the bulk second phase, which serves as a nucleation site for grain stimulation and drives dynamic recrystallization, and the strong pinning and inhibition effect of a large number of solid solution atoms on grain boundary migration and bowing. Ultimately, the alloy forms a mixed-grain microstructure with incomplete dynamic recrystallization, namely, a microstructure composed of unrecrystallized deformed grains and fine dynamically recrystallized grains.

[0010] Performance advantages: Compared with alloys that do not undergo the aforementioned high-temperature homogenization treatment (Comparative Example 1), or those that undergo homogenization treatment at a lower temperature (e.g., 410 °C, Comparative Example 2) or a slightly higher but insufficient temperature (e.g., 460 °C, Comparative Example 3) to ultimately obtain a homogeneous equiaxed crystal structure with complete dynamic recrystallization, the alloys prepared by the method of this invention exhibit significantly superior comprehensive mechanical properties. For example, while maintaining a good elongation at break (approximately 21%), their yield strength (approximately 268 MPa) and tensile strength (approximately 332 MPa) are significantly improved, achieving an excellent balance between high strength and high plasticity.

[0011] This invention achieves effective control over the morphology and content of the second phase in magnesium alloys through a specific high-temperature homogenization process window. This control not only reduces dynamic recrystallization nucleation sites but also strengthens the inhibition of dynamic recrystallization nucleation and growth by increasing the concentration of solid solution atoms, thereby successfully preparing a mixed-crystal magnesium alloy with a superior combination of strength and plasticity.

[0012] In summary, this invention provides a process-controllable and effective method for preparing high-strength and high-toughness magnesium alloys, offering an effective way to solve the problem of the inversion of strength and plasticity in magnesium alloys.

[0013] Based on the above technical concepts and protection priorities, it needs to be clearly pointed out that: Exemplary Composition: The specific Mg-Gd-Zn-Zr alloy composition (Gd: 9%, Zn: 1%, Zr: 0.5%) listed in this specification is merely a preferred embodiment for implementing and verifying the technical solution of this invention. This composition is designed to complement the second phase control process to best demonstrate its technical effects. The scope of protection of this invention is not limited to this specific alloy system or ratio.

[0014] Universality of the Principle: The technical principle and process concept disclosed in this invention, which involves "homogenizing the second phase at high temperature to suppress particle-induced nucleation and dynamic recrystallization, thereby obtaining a strong and tough mixed-grained structure," has universal guiding significance and application potential for other wrought magnesium alloy systems where the morphology and content of the second phase can be altered through heat treatment (e.g., other magnesium alloys containing rare earth elements, those without rare earth elements, or those containing other alloying elements). Any technical solution that adopts the core concept of this invention—namely, controlling the degree of dynamic recrystallization through similar second-phase solution treatment to prepare mixed-grained magnesium alloys—may fall within the protection scope of this invention, or constitute an equivalent substitution and extension of the technical concept of this invention.

[0015] This invention primarily protects the principles and process steps of a method for achieving second-phase regulation to suppress dynamic recrystallization and thus obtain a mixed-crystalline structure. The specific alloy compositions described in the specification are merely examples and embodiments of this method, not limitations on the scope of protection. Any method based on the same inventive concept that uses second-phase regulation to suppress dynamic recrystallization to prepare high-strength, high-toughness mixed-crystalline magnesium alloys should be considered within the overall protection framework of this invention.

[0016] This invention involves subjecting magnesium alloy ingots to a specific high-temperature homogenization heat treatment (500 ℃~520 ℃ for 16 to 20 hours) to fully decompose and dissolve the dendritic or network-like eutectic second phase in the as-cast microstructure into the magnesium matrix. This significantly reduces the number of nucleation sites stimulated by particles during subsequent hot extrusion deformation. Furthermore, the strong pinning effect of dissolved atoms on grain boundary migration and bowing effectively inhibits the nucleation and growth of dynamic recrystallization, ultimately resulting in an incomplete dynamic recrystallization mixed-crystal microstructure composed of unrecrystallized deformed grains and fine dynamic recrystallized grains.

[0017] Compared with the prior art, the present invention has the following beneficial effects: An excellent synergistic match between strength and plasticity is achieved. The extruded magnesium alloy prepared by the method of this invention has a yield strength of not less than 260 MPa, a tensile strength of not less than 330 MPa, and an elongation after fracture of not less than 20%, which is significantly better than traditional magnesium alloys with a fully dynamic recrystallized uniform equiaxed grain structure (typical yield strength of about 190~210 MPa and tensile strength of about 280~290 MPa). The strength is greatly improved while maintaining good plasticity, breaking through the problem of the inverse strength-plasticity of magnesium alloys.

[0018] This invention provides a universally applicable and simple method for microstructure control. The core of this invention lies in precisely controlling the dynamic recrystallization behavior during hot deformation by regulating the degree of solid solution of the second phase. The principle of this method is not limited to specific alloy compositions. It has good guiding significance and adaptability adjustment potential for other deformed magnesium alloy systems (such as magnesium alloys with or without rare earth elements) whose second phase morphology and content can be changed through heat treatment. This avoids the increased costs and process risks caused by relying on the addition of high alloying elements or extreme deformation processes (such as extrusion at extremely low temperatures, which can easily lead to cracking).

[0019] This improves the overall service performance of the material. The mixed-grain structure fully utilizes the back stress strengthening (HDI strengthening) and non-uniform deformation mechanism induced by the heterostructure, generating significant strain hardening capacity during deformation, effectively improving the work hardening rate and damage resistance of the alloy. It is suitable for fields such as aerospace, automotive lightweighting, and electronic equipment where there is an urgent need for lightweight, high-strength, and tough structural materials.

[0020] The process window is stable and highly repeatable. Optimized parameters (such as 510 ℃ × 18h homogenization, 440 ℃ × extrusion ratio 12, and extrusion rate 0.7m / min) have been experimentally verified to be reliable. The quenching process uses gentle hot water cooling (80 ℃~95 ℃), avoiding the cracking risk that may be caused by traditional water cooling. At the same time, the entire preparation process does not require complex equipment or multiple intermediate heat treatments, resulting in high production efficiency and controllable costs.

[0021] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 The images are scanning electron microscope (SEM) images of alloy A before extrusion and alloys B and C after extrusion in Comparative Example 1. Figure 2 The images are scanning electron microscope (SEM) images of alloy A before extrusion and alloys B and C after extrusion in Comparative Example 2. Figure 3 The images are scanning electron microscope (SEM) images of alloy A before extrusion and alloys B and C after extrusion in Comparative Example 3. Figure 4 The images are scanning electron microscope (SEM) images of alloy A before extrusion and alloy B and C after extrusion in Example 1. Figure 5 Engineering stress-strain curves of alloys prepared for Comparative Examples 1, 2, 3 and 1; Figure 6 Microstructure evolution during the extrusion process of Comparative Example 2 and Example 1, characterized by electron backscatter diffraction. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0024] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0025] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0026] Example 1 This embodiment provides a method for preparing a high-strength and high-toughness mixed-grain magnesium alloy, including the following steps: Step 1: Ingot Preparation The formulation was designed by mass percentage according to Gd: 9%, Zn: 1%, Zr: 0.5%, with the balance being Mg and unavoidable impurities. Pure magnesium, magnesium-gadolinium master alloy, pure zinc, and magnesium-zirconium master alloy were placed in a steel crucible and melted in an electric resistance furnace according to the aforementioned formulation requirements. The raw materials were placed in the steel crucible and melted under the protection of a mixture of argon and sulfur hexafluoride gas. After the raw materials were fully melted, the mixture was stirred at 740 °C for 6 minutes, surface slag was removed, and the mixture was held at that temperature for 10 minutes. The crucible was then removed and allowed to cool naturally to approximately 630 °C while continuing to circulate the protective gas. Finally, it was cooled to room temperature using circulating water to obtain a magnesium alloy ingot. Step 2: Second Phase Regulation Magnesium alloy ingots were kept at 510 °C for 18 hours, and then removed and cooled in hot water at 90 °C to obtain pretreated ingots. Step 3: Hot extrusion The pretreated ingot was held at 440 ℃ for 30 minutes, and then extruded at an extrusion ratio of 12 and an extrusion rate of 0.7 m / min to obtain an extruded alloy with a cross-sectional size of 30 mm × 16 mm. The following comparative examples differ from Example 1 only in the homogenization process or whether homogenization is performed; all other steps and parameters are the same.

[0027] Comparative Example 1 Without performing the second phase regulation in step two, the ingot obtained in step one is directly subjected to hot extrusion in step three.

[0028] Comparative Example 2 This comparative example provides a method for preparing a magnesium alloy, which is basically the same as that in Example 1. The difference is that the second phase control in step two is changed to keeping the magnesium alloy ingot at 410 °C for 18 hours, and then taking it out and cooling it in hot water at 90 °C to obtain a pretreated ingot before proceeding to step three; the remaining process steps and parameters are the same as in Example 1.

[0029] Comparative Example 3 This comparative example provides a method for preparing a magnesium alloy, which is basically the same as that in Example 1. The difference is that the second phase control in step two is changed to keeping the magnesium alloy ingot at 460 °C for 18 hours, and then taking it out and cooling it in hot water at 90 °C to obtain a pretreated ingot before proceeding to step three; the remaining process steps and parameters are the same as in Example 1.

[0030] Organizational characteristics and mechanical properties Figure 1 The scanning electron microscope image of Comparative Example 1 before extrusion, shown in Figure A, reveals the presence of a large amount of eutectic phase within the alloy. After extrusion, the dendritic eutectic phase in the alloy breaks down into granular particles distributed along the extrusion direction, exhibiting typical extrusion streamline distribution characteristics, such as... Figure 1 As shown in B and C. High-magnification scanning electron microscope images show that the alloy of Comparative Example 1 exhibits a homogeneous equiaxed grain structure with complete dynamic recrystallization, as... Figure 1 As shown in C.

[0031] Figure 2 The scanning electron microscope (SEM) image of Comparative Example 2 before extrusion, shown in Figure A, reveals a large number of blocky, long-period ordered stacked phases within the alloy. After homogenization treatment at 410 °C for 18 h, the dendritic eutectic phase transforms into blocky, long-period ordered stacked phases. After extrusion, the long-period ordered stacked phases in the alloy do not break into granular forms but undergo significant plastic deformation along the extrusion direction, exhibiting typical extrusion streamline distribution characteristics, such as... Figure 2 As shown in B and C, this implies that the long-period ordered stacked phase exhibits better toughness compared to the dendritic eutectic phase. High-magnification scanning electron microscopy images reveal that Comparative Example 2 alloy also exhibits a fully dynamically recrystallized, homogeneous equiaxed grain structure, such as... Figure 2 As shown in C.

[0032] Figure 3The scanning electron microscope (SEM) image of Comparative Example 3 shown in Figure A before extrusion reveals the presence of numerous blocky, long-period ordered stacked phases within the alloy. After homogenization treatment at 460 °C for 18 h, the dendritic eutectic phase also transforms into blocky, long-period ordered stacked phases. After extrusion, the long-period ordered stacked phases in the alloy undergo significant plastic deformation along the extrusion direction, exhibiting typical extrusion streamline distribution characteristics, such as... Figure 3 As shown in B and C. High-magnification scanning electron microscope images show that Comparative Example 3 alloy also exhibits a homogeneous equiaxed grain structure with complete dynamic recrystallization, as... Figure 3 As shown in C.

[0033] Figure 4 The scanning electron microscope image of Example 1 before extrusion, shown in Figure A, indicates that only a small amount of granular second phase exists inside the alloy. After homogenization treatment at 510 °C for 18 h, the dendritic eutectic phase is almost completely decomposed. After extrusion, the alloy of Example 1 exhibits a mixed-crystal structure with incomplete dynamic recrystallization, such as... Figure 4 As shown in B and C.

[0034] Figure 5 The results show that the yield strength, tensile strength, and elongation at break of the alloy in Comparative Example 1 are 209 MPa, 279 MPa, and 22%, respectively; the yield strength, tensile strength, and elongation at break of the alloy in Comparative Example 2 are 193 MPa, 282 MPa, and 24%, respectively; the yield strength, tensile strength, and elongation at break of the alloy in Comparative Example 3 are 201 MPa, 288 MPa, and 25%, respectively; and the yield strength, tensile strength, and elongation at break of the alloy in Example 1 are 268 MPa, 332 MPa, and 21%, respectively. This indicates that the alloy in Example 1 maintains good plasticity while significantly improving strength, achieving excellent strength-plasticity synergy. Compared to the uniform equiaxed crystal structure exhibited in Comparative Examples 1, 2, and 3, the mixed crystal structure with incomplete dynamic recrystallization exhibited in Example 1 is more beneficial to the alloy's mechanical properties.

[0035] To further reveal the evolution of the alloy's microstructure, electron backscattering diffraction characterization was performed on the microstructure of the alloys in Example 1 and Comparative Example 2 during the extrusion process. The sampling locations for microstructure characterization are shown below. Figure 6 The green dots in G are labeled P1, P2, and P3, with an interval of approximately 20mm between adjacent positions. Figure 6 Figures A through C show orientation imaging at different locations in the alloy of Comparative Example 2, and the corresponding energy dispersive spectroscopy (EDS) spectra are shown below. Figure 6As shown in A1~C1, it can be seen that the proportion of dynamic recrystallization gradually increases from P1 to P3, and the dynamically recrystallized grains in P1 exhibit a typical necklace-like distribution. The energy dispersive spectroscopy (EDS) images show that a large number of bulk second phases (long-period ordered stacked phases) exist in these regions of the alloy of Comparative Example 2. Combined with the orientation imaging, it can be seen that many finer dynamically recrystallized grains exist around these bulk second phases. Figure 6 Figures D and F show orientation imaging during the extrusion process of the alloy in Example 1, where the dynamic recrystallization ratio gradually increases. The difference is that both the dynamic recrystallization ratio and the dynamic recrystallized grain size are significantly lower than those of the alloy in Comparative Example 2. Furthermore, the alloy in Example 1 contains only a small amount of micron-sized blocky second phase, such as... Figure 6 As shown in D1~F1.

[0036] The dominant dynamic recrystallization mechanism during the extrusion of the alloy in Comparative Example 2 was particle-stimulated nucleation dynamic recrystallization and discontinuous dynamic recrystallization. During the extrusion of the alloy in Example 1, the dynamically recrystallized grains were distributed in a necklace-like pattern at the grain boundaries of the deformed grains (…). Figure 6 As shown in Figures D to E), this exhibits a discontinuous dynamic recrystallization mechanism. Some dynamically recrystallized grains also exist within the deformed grains, indicating a continuous dynamic recrystallization mechanism. Furthermore, a small amount of blocky second phase in the alloy of Example 1 promotes dynamic recrystallization through particle-stimulated nucleation. Therefore, the dominant dynamic recrystallization mechanism during the extrusion of the alloy of Example 1 is discontinuous dynamic recrystallization. Meanwhile, the alloys of Comparative Example 1 and Comparative Example 3 also underwent complete dynamic recrystallization during extrusion (…). Figure 1 and Figure 3 In Comparative Example 1 and Comparative Example 3 alloys, the presence of a large amount of bulk second phase during extrusion promoted particle-stimulated nucleation and dynamic recrystallization. However, the lack of a large amount of bulk second phase in Example 1 alloy reduced the number of nucleation sites for particle-stimulated dynamic recrystallization, resulting in a lower dynamic recrystallization ratio. Furthermore, the alloying elements generated from the decomposition of a large amount of eutectic phase mainly exist in Example 1 alloy in the form of layered long-period ordered stacked phases and solute atoms. A large number of solute atoms significantly reduces the migration rate of grain boundaries, delaying the growth of dynamically recrystallized grains. Simultaneously, the pinning effect of solute atoms on grain boundaries also inhibits grain boundary bowing during discontinuous dynamic recrystallization, thereby suppressing the nucleation of discontinuous dynamic recrystallization. Therefore, the lower dynamic recrystallization ratio and smaller dynamic recrystallized grain size of Example 1 alloy are mainly attributed to the reduction in particle-stimulated dynamic recrystallization nucleation sites and the inhibitory effect of solute atoms on dynamic recrystallization nucleation and growth.

[0037] The experimental results of the above embodiments and comparative examples show that the present invention achieves effective control of the second phase through high-temperature homogenization treatment, which significantly improves the strength and toughness matching of the alloy.

[0038] In summary, the embodiments and comparative examples in this specification, through comparative analysis of the microstructure evolution and mechanical properties of different alloys after homogenization treatment and hot extrusion, reveal the evolution of the second phase and its influence mechanism on dynamic recrystallization behavior. The results show that: Comparative Example 1 Alloy 3, containing a large amount of blocky second phases (dendritic eutectic phases or long-period ordered stacked phases), undergoes dynamic recrystallization during extrusion, promoting particle-stimulated nucleation, ultimately forming a homogeneous equiaxed crystalline structure with complete dynamic recrystallization, resulting in relatively low mechanical properties. In contrast, Alloy 1, after high-temperature homogenization treatment, almost completely decomposes the dendritic eutectic phase, leaving only a small amount of granular second phase. After extrusion, it forms a mixed-crystal structure with incomplete dynamic recrystallization. This structure, due to the reduction of particle-stimulated nucleation points and the inhibition of grain boundary migration and bowing by solute atoms, significantly slows down the dynamic recrystallization process. This allows the alloy to maintain good plasticity while significantly improving yield strength and tensile strength, achieving excellent synergy between strength and plasticity. These results demonstrate that controlling the morphology and content of the second phase to suppress complete dynamic recrystallization and form a mixed-crystal structure is an effective way to improve the comprehensive mechanical properties of alloys.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a high-strength and high-toughness mixed-crystalline magnesium alloy, characterized in that: Includes the following steps: A magnesium alloy ingot is provided, wherein the as-cast microstructure of the magnesium alloy ingot contains a dendritic or network-like eutectic second phase; The magnesium alloy ingot is held at 500 ℃~520 ℃ for 16 hours to 20 hours and then quenched to allow the second phase to fully decompose and dissolve in the magnesium matrix; The quenched ingot was hot extruded at 430 ℃~450 ℃ with an extrusion ratio of 10~14 to obtain an extruded magnesium alloy with an incomplete dynamic recrystallization mixed crystal structure.

2. The method for preparing high-strength and high-toughness mixed-crystal magnesium alloy according to claim 1, characterized in that: The homogenization heat treatment was performed by holding at 510 °C for 18 hours.

3. The method for preparing high-strength and high-toughness mixed-crystal magnesium alloy according to claim 1, characterized in that: The quenching process involves immersing the heat-preserved ingot in hot water at 80°C to 95°C for cooling.

4. The method for preparing a high-strength, high-toughness mixed-grain magnesium alloy according to claim 1, characterized in that: The hot extrusion deformation has an extrusion ratio of 12, an extrusion temperature of 440 ℃, and an extrusion rate of 0.5 m / min to 1.0 m / min.

5. The method for preparing a high-strength, high-toughness mixed-grain magnesium alloy according to claim 4, characterized in that: The extrusion rate is 0.7 m / min.

6. The method for preparing a high-strength, high-toughness mixed-grain magnesium alloy according to any one of claims 1 to 5, characterized in that: The chemical composition of the magnesium alloy ingot, by mass percentage, includes 8.5%~9.5% Gd, 0.8%~1.2% Zn, and 0.4%~0.6% Zr, with the balance being Mg and unavoidable impurities.

7. The method for preparing a high-strength, high-toughness mixed-grain magnesium alloy according to claim 6, characterized in that: The chemical composition of the magnesium alloy ingot, by mass percentage, is 9% Gd, 1% Zn, and 0.5% Zr, with the balance being Mg and unavoidable impurities.

8. A high-strength, high-toughness mixed-crystal magnesium alloy prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The magnesium alloy has a mixed-grain structure with incomplete dynamic recrystallization, a yield strength of not less than 260 MPa, a tensile strength of not less than 330 MPa, and an elongation after fracture of not less than 20%.

9. A method for strengthening and toughening magnesium alloy materials, characterized in that: This includes homogenizing heat treatment of magnesium alloy ingots containing dendritic or network eutectic second phases. The homogenizing heat treatment causes the second phase to fully decompose and dissolve in the magnesium matrix, thereby inhibiting the nucleation and growth of dynamic recrystallization during subsequent hot extrusion deformation, forming a mixed crystal structure with incomplete dynamic recrystallization.

10. The strengthening and toughening method according to claim 9, characterized in that: The homogenization heat treatment temperature is 500℃~520℃, the holding time is 16 hours~20 hours, the hot extrusion deformation extrusion ratio is 10~14, and the extrusion temperature is 430℃~450℃.

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