Metallic magnesium-based polycrystalline composite material and preparation method thereof

Abstract

The invention relates to a metal magnesium-based polycrystalline composite material which comprises a magnesium-based polycrystalline material with the room-temperature as-cast fracture elongation higher than 10% as a base material and alloying elements Si and Sr contained in the base material. The alloying elements Si and Sr enable an Mg2Si second phase which is 2-10 microns in size, round in shape and uniform in distribution to be generated in the metal magnesium-based polycrystalline composite material composed of the alloying elements Si and Sr and the base material; meanwhile, the invention provides a method for preparing the metal magnesium-based polycrystalline composite material by combining an intermediate alloy rolling and crushing process with a one-step mixing and adding method. According to the method, the Mg2Si phase which is small in size, round in shape, high in hardness and high in thermal stability is successfully introduced into the high-plasticity magnesium-based composite material, and the bottleneck problem that the strength and plasticity of the magnesium-based composite material at the room temperature and the high temperature are in an inverted relation is solved.

Description

Technical Field

[0001] The present invention relates to a polycrystalline composite material and a preparation method thereof. Background Art

[0002] Magnesium alloys have advantages such as low density, high specific strength, high specific elastic modulus, and good shock absorption performance. Therefore, they have broad application prospects in fields such as aerospace and the automotive industry. However, the low room-temperature strength, poor plasticity, and unsatisfactory heat resistance of magnesium alloys greatly limit their application scope.

[0003] Introducing hard second-phase particles into the alloy in-situ or by external addition to obtain particle-reinforced composite materials can effectively improve the strength and heat resistance of the alloy. However, due to problems such as the size and distribution of the reinforcement phase, and the intrinsic elastic-plastic deformation and thermal deformation mismatch and interface problems between the reinforcement phase and the magnesium matrix, there is an inverted relationship between the room-temperature and high-temperature strength and plasticity of traditional magnesium-based composite materials. This performance bottleneck greatly hinders the large-scale application of magnesium-based composite materials in the lightweight field.

[0004] Research shows that Mg2Si particles have high melting point (1087 °C), high hardness (460 HV), high elastic modulus (120 GPa), low density (1.99 g·cm -3 ) and low thermal expansion coefficient (7.5×10 -6 K -1 ) and other advantages. They are ideal room-temperature and high-temperature particle reinforcements for Mg alloys. However, under conventional casting techniques, the primary Mg2Si particles formed in the solidification structure are coarse and dendritic in shape, and the eutectic Mg2Si phase usually presents a Chinese character shape with sharp edges. During the stress deformation process, stress concentration is likely to occur to form crack sources, which have a splitting effect on the matrix material and seriously damage the plasticity of the composite material.

[0005] Although the size, morphology, and distribution of the Mg2Si phase can be improved to a certain extent through corresponding modification treatment techniques, the improvement of the material's plasticity is limited, restricting its application as a high-strength reinforcement in heat-resistant magnesium-based composite materials. Summary of the Invention

[0006] The purpose of the present invention is to avoid the deficiencies of the prior art and provide a metallic magnesium-based polycrystalline composite material and a preparation method thereof that can meet the application requirements of lightweight, high strength, high plasticity, and heat resistance of magnesium-based composite materials in key fields such as aerospace, rail transit, and the automotive industry, and break through the bottleneck problem of the inverted relationship between the room-temperature and high-temperature strength and plasticity of magnesium-based composite materials.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: A metal magnesium-based polycrystalline composite material includes a magnesium-based polycrystalline material with a room-temperature as-cast fracture elongation higher than 10% as the matrix material, and alloying elements Si and Sr contained in the matrix material; The alloying elements Si and Sr cause the formation of Mg2Si second-phase particles with a size of 2 μm to 10 μm, a round shape, and a uniform distribution in the metal magnesium-based polycrystalline composite material composed of the matrix material; Among them, the mass fraction of element Si in the metal magnesium-based polycrystalline composite material is 0.3 to 3%, and the mass fraction of element Sr in the metal magnesium-based polycrystalline composite material is 0.05 to 0.5%; The Mg2Si second-phase particles endow the metal magnesium-based polycrystalline composite material with mechanical properties at a temperature of 150 to 200 °C as follows: yield strength of 103 to 112 MPa, tensile strength of 138 to 192 MPa, and fracture elongation of 18 to 26%; The room-temperature mechanical properties are: yield strength of 110 - 122 MPa, tensile strength of 207 - 217 MPa, and fracture elongation of 6 to 10%.

[0008] Furthermore, the magnesium-based polycrystalline material is AlN / AZ91D, and the obtained metal magnesium-based polycrystalline composite material is (Mg2Si + AlN) / AZ91D; At this time, the Mg2Si second-phase particles in (Mg2Si + AlN) / AZ91D are used to jointly strengthen the matrix with the in-situ AlN particles in the magnesium-based polycrystalline material, hinder dislocation slip and climb as well as grain boundary sliding at high temperatures, and inhibit the discontinuous precipitation of the Mg 17 Al 12 phase, improving the room-temperature and high-temperature strength of the material while taking into account good plasticity.

[0009] Furthermore, the elements Si and Sr are added to the matrix material in the form of Mg-xSi and Mg-ySr master alloys respectively.

[0010] Furthermore, in the Mg-xSi master alloy, the mass fraction of element Si is 20% to 40%; In the Mg-ySr master alloy, the mass fraction of element Sr is 10% to 25%.

[0011] The present invention also provides a preparation method of the above-mentioned metal magnesium-based polycrystalline composite material, including the following steps: First, through a rolling and crushing process, the Mg-xSi and Mg-ySr master alloys are respectively crushed into master alloy blocks with a thickness of 2 to 3 mm and a diameter of 0.5 to 2 cm; Then, in an inert gas atmosphere, the magnesium-based polycrystalline material ingot is heated and melted to obtain a melt; When the melt reaches the casting temperature of 680 °C to 730 °C, the master alloy blocks are added to the melt in multiple times, and the added mass of each master alloy block accounts for 20 - 30% of the total mass of the master alloy blocks. At the same time, after each addition of the master alloy block, the melt is stirred at a rotational speed of 30 - 60 r / min for at least 3 min. After stirring, the temperature is raised to the casting temperature and held for 5 - 20 min to improve the uniformity of the melt. Then, the melt is heated to the casting temperature again for the subsequent addition process until all the master alloy blocks are added to the melt; Finally, the melt is heated to the casting temperature and cast to obtain the metal magnesium-based polycrystalline composite material.

[0012] Further, the rolling and crushing process is specifically as follows: Using a wire electrical discharge machining machine, the Mg-xSi and Mg-ySr master alloy ingots are respectively cut into thin plates with a length, width, and height of 100×80×2 - 3 mm; Then, in a heat treatment furnace with an argon protection atmosphere, the thin plates are heat-treated at a preheating temperature of 50 - 200 °C for a holding time of 10 - 30 min; Subsequently, the heat-treated thin plates are rolled and crushed on a two-high rolling mill at a roll speed of 4 - 10 m / min to obtain the master alloy blocks.

[0013] Further, before the master alloy blocks are added to the melt in multiple times, it also includes the steps of ultrasonic mixing, cleaning, and drying the master alloy blocks, specifically: All the Mg-xSi and Mg-ySr master alloy blocks are placed in a container filled with absolute ethanol for ultrasonic mixing and cleaning, and then dried in an oven; Subsequently, the master alloy blocks are divided into multiple portions for multiple additions to the melt, so that the master alloy blocks added each time can be completely melted in the melt within 10 - 15 min, thereby reducing the smelting temperature, shortening the melting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the generation of element segregation in the melt; At the same time, each portion of the master alloy block is tightly wrapped with aluminum foil to isolate it from direct contact with the melt when the master alloy block is added to the melt each time, reducing the burning loss of the master alloy block.

[0014] Further, the time for ultrasonic mixing is 3 - 15 min.

[0015] Further, the master alloy blocks are added to the melt in 2 - 5 times.

[0016] Furthermore, when the magnesium-based polycrystalline material ingot is heated and melted to obtain a melt, it is carried out under a protective atmosphere of a mixed gas of CO2 and SF6, wherein the volume ratio of CO2 to SF6 is 100:1 to 2.

[0017] The beneficial effects of the present invention are as follows: On the one hand, the present invention provides a metal magnesium-based polycrystalline composite material. A high-plasticity magnesium-based material is selected as the matrix material. In the art, a magnesium-based material with a room-temperature as-cast fracture elongation higher than 10% is generally considered a high-plasticity magnesium-based material. Thus, the prepared metal magnesium-based polycrystalline composite material has high strength and plasticity at 150-200 °C, and at the same time has high plasticity at room temperature of 25 °C, while general metal magnesium-based composite materials cannot achieve both. The high-plasticity magnesium-based material of the present invention is preferably an AlN / AZ91D magnesium-based composite material. This material is a high-strength and high-plasticity as-cast AlN / AZ91D magnesium-based composite material prepared by in-situ particle strengthening by the research team in the early stage, with an application number of 201510882938.5 and a publication date of April 6, 2016. Its room-temperature fracture elongation is as high as 20%, far higher than that of traditional as-cast magnesium-based composite materials, and it is an ideal matrix material for preparing high-strength, high-plasticity, and heat-resistant magnesium-based composite materials. In addition, due to the high melting point of Mg2Si, whether adding Si element by elemental Si or master alloy during alloy melting will cause problems such as an increase in smelting temperature and easy element segregation. The present invention improves the uniform dispersion degree of Si element in the melt by reasonably adding active modification elements and optimizing the addition method of Si element, effectively improves the morphology, size, and distribution of Mg2Si, and further reduces the stress concentration during the deformation process, thereby enhancing the plasticity of the composite material. On the other hand, the present invention combines the rolling and crushing process with the one-step mixing addition method to add alloy elements Si and Sr to the melt, thereby successfully introducing high-hardness and high-thermal-stability Mg2Si phases with small size and round shape into the high-plasticity magnesium-based composite material, breaking through the performance bottleneck of the inversion of strength and plasticity of traditional magnesium-based composite materials at room temperature and high temperature, and preparing a metal magnesium-based polycrystalline composite material with comprehensive room-temperature and high-temperature strength and plasticity. In addition, the method of combining the master alloy rolling and crushing process with the one-step mixing addition method proposed by the present invention can effectively solve the problems of high smelting temperature, easy segregation, large size of Mg2Si, and sharp corners that are prone to cause stress concentration and damage plasticity during the addition of Si element, and has a short process flow and low cost, suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a cast plate of (Mg2Si + AlN) / AZ91D magnesium-based polycrystalline composite material prepared according to a specific example of the present invention; Figure 2 The microstructure of the magnesium-based polycrystalline composite material prepared in the specific example of the present invention; Figure 3 The room-temperature tensile stress-strain curve of the magnesium-based polycrystalline composite material prepared in the specific example of the present invention; Figure 4 The high-temperature tensile stress-strain curve at 200 °C of the magnesium-based polycrystalline composite material prepared in the specific example of the present invention. Specific embodiments

[0019] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0020] In order to achieve the above object, the present invention provides the following specific embodiments: Example 1: A magnesium-based polycrystalline composite material includes a magnesium-based polycrystalline material with a room-temperature as-cast fracture elongation higher than 10% as the matrix material, and alloying elements Si and Sr contained in the matrix material; The alloying elements Si and Sr cause the formation of Mg2Si secondary phases with a size of 2 μm to 10 μm, a round morphology and a uniform distribution in the magnesium-based polycrystalline composite material composed of the matrix material; Among them, the mass fraction of element Si in the magnesium-based polycrystalline composite material is 0.3 to 3%, and the mass fraction of element Sr in the magnesium-based polycrystalline composite material is 0.05 to 0.5%; The Mg2Si secondary phase enables the magnesium-based polycrystalline composite material to have mechanical properties at a temperature of 150 to 200 °C as follows: yield strength of 103 to 112 MPa, tensile strength of 138 to 192 MPa, and fracture elongation of 18 to 26%; The room-temperature mechanical properties are: yield strength of 110 - 122 MPa, tensile strength of 207 - 217 MPa, and fracture elongation of 6 to 10%.

[0021] Example 2: The same as Example 1, except that the magnesium-based polycrystalline material is AlN / AZ91D, and the obtained magnesium-based polycrystalline composite material is (Mg2Si + AlN) / AZ91D; At this time, the Mg2Si secondary phase in (Mg2Si + AlN) / AZ91D is used to jointly strengthen the matrix with the in-situ AlN particles in the magnesium-based polycrystalline material, hinder dislocation slip and climb as well as grain boundary sliding at high temperatures, and inhibit the discontinuous precipitation of the Mg 17 Al 12 phase, improve the room-temperature and high-temperature strength of the material while taking into account good plasticity.

[0022] Example 3: It is the same as Example 1, except that elements Si and Sr are added to the matrix material in the form of Mg-xSi and Mg-ySr master alloys respectively.

[0023] In the Mg-xSi master alloy, the mass fraction of element Si is 20% - 40%; In the Mg-ySr master alloy, the mass fraction of element Sr is 10% - 25%.

[0024] Example 4: The present invention also provides a method for preparing a magnesium-based polycrystalline composite material as described in Examples 1 - 3, including the following steps: Step 1, through the rolling and crushing process, specifically using a wire electrical discharge machine, cut the Mg-xSi and Mg-ySr master alloy ingots into thin plates with a length, width, and height of 100×80×2 - 3 mm respectively; Then, in a heat treatment furnace with an argon protection atmosphere, heat-treat the thin plates at a preheating temperature of 50 - 200 °C and a holding time of 10 - 30 min; Subsequently, roll and crush the heat-treated thin plates on a two-roll mill at a roll speed of 4 - 10 m / min to obtain master alloy blocks with a thickness of 2 - 3 mm and a diameter of 0.5 - 2 cm.

[0025] Step 2, perform ultrasonic mixing, cleaning, and drying on the master alloy blocks, specifically as follows: Place all the master alloy blocks of Mg-xSi and Mg-ySr in a container filled with anhydrous ethanol for ultrasonic mixing and cleaning for 3 - 15 min, and then dry them in an oven; Subsequently, divide the master alloy blocks into multiple portions for multiple additions to the melt, and ensure that the master alloy blocks added each time are completely melted in the melt within 10 - 15 min, thereby reducing the smelting temperature, shortening the melting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the generation of element segregation in the melt; At the same time, tightly wrap each portion of the master alloy blocks with aluminum foil to isolate them from direct contact with the melt when adding the master alloy blocks to the melt each time, reducing the burning loss of the master alloy blocks; Among them, the burning loss refers to the oxidation loss that occurs when the master alloy floats on the surface of the melt without quickly melting and mixing with the melt after being added to the melt.

[0026] Step 3, under the protection atmosphere of a mixed gas of CO2 and SF6, heat and melt the magnesium-based polycrystalline material ingot to obtain a melt; Among them, the volume ratio of CO2 and SF6 is 100:1 - 2.

[0027] Step 4: After the melt reaches the casting temperature of 680°C to 730°C, add the master alloy blocks to the melt in multiple times, and the mass of the master alloy blocks added each time accounts for 20 - 30% of the total mass of the master alloy blocks. Meanwhile, after adding the master alloy blocks each time, stir the melt at a rotational speed of 30 - 60 r / min for at least 3 minutes. After stirring, heat up the melt to the casting temperature and keep it warm for 5 - 20 minutes to improve the uniformity of the melt. Then, heat up the melt to the casting temperature again to carry out the subsequent addition process until all the master alloy blocks are added to the melt. Step 4: Heat up the melt to the casting temperature of 680°C to 730°C, and cast to obtain the metal magnesium-based polycrystalline composite material.

[0028] Example 5: The same as Example 4, except that in Step 2, the master alloy blocks are divided into 2 - 5 parts, and further in Step 4, the master alloy blocks are added to the melt in 2 - 5 times.

[0029] As Figures 1-4 shown, in order to further illustrate the technical solution and effect of the present invention, the following specific examples are provided: (1) The matrix material is selected as AlN / AZ91D magnesium-based polycrystalline material.

[0030] (2) Addition amounts of alloying elements Si and Sr: In this specific example, the mass fraction of the added Si element relative to the metal magnesium-based polycrystalline composite material is 0.3% - 3%, and the mass fraction of the added Sr element relative to the metal magnesium-based polycrystalline composite material is 0.05% - 0.5%. Among them, the Si or Sr element is added to the matrix in the form of Mg-xSi master alloy and Mg-ySr master alloy, and the morphology of the Mg2Si second phase formed after adding Si and Sr is round and evenly distributed, and the size of the Mg2Si second phase particles is 2μm - 10μm.

[0031] (3) Rolling and crushing process of the master alloy: Use a wire electrical discharge machine to cut the Mg-xSi and Mg-ySr master alloy ingots into thin plates with a size of 100×80×2 - 3 mm respectively. Polish the upper and lower surfaces of the thin plates with a wire brush until they are shiny, then put them into an aqueous solution with a temperature of 100°C, and the concentrations of NaOH and Na2CO3 are 0.2 mol / L and 0.4 mol / L respectively, and carry out alkali washing for 10 minutes, and dry with a hair dryer.

[0032] Then, place the thin plate into a heat treatment furnace, keep it warm for 10 minutes under a preheating temperature of 50°C to 200°C (in this example, it is selected as 180°C) and an argon inert atmosphere, and then roll and crush it into irregular master alloy blocks with a thickness of 2 to 3 mm and a diameter of 0.5 to 2 cm through a rolling mill.

[0033] Put the two types of master alloy blocks into a beaker containing absolute ethanol according to the addition ratio and addition amount, carry out ultrasonic mixing and cleaning for a period of time, and then dry them in an oven.

[0034] Subsequently, divide it into multiple portions by weight and wrap each portion tightly with aluminum foil, which is convenient for the master alloy blocks to be added to the melt and melt quickly, thereby reducing the smelting temperature, reducing burning loss, and improving the modification efficiency; at the same time, it promotes the uniform distribution of elements in the melt and avoids the generation of element segregation in the melt.

[0035] Among them, the number of rolling passes is 1 pass, the roll temperature is room temperature, the roll speed is 4 to 10 m / min (in this example, it is 8.4 m / min), and the time for ultrasonic mixing and cleaning is 3 to 15 min (in this example, it is 10 min).

[0036] (4) One-step mixing and adding master alloy and casting: Cut a certain amount of AlN / AZ91D composite material ingot and put it into a stainless steel crucible. At this time, the selection of the ingot mass should ensure that the mass fractions of added Si and Sr elements are 0.3% to 3% and 0.05% to 0.5% of the total weight of the final metal magnesium-based polycrystalline composite material respectively; Carry out smelting in a resistance melting furnace under a (SF6 + CO2) protective atmosphere. When the melt temperature reaches the pouring temperature of 720°C, take 1 portion of the master alloy wrapped with aluminum foil and press it into the melt with a bell jar and keep it warm for 10 min; then carry out mechanical stirring on the melt, where the rotation speed of the mechanical stirring is 30 to 60 r / min and the stirring time is 3 min. After the stirring ends, raise the temperature of the melt to 720°C, continue to add the master alloy, repeat the above process until the addition is completed, and finally raise the temperature of the melt to 720°C, and cast the (Mg2Si + AlN) / AZ91D composite material into a plate, as shown in Figure 1 shown.

[0037] (5) Metallographic structure observation and performance testing: Cut a metallographic specimen from the (Mg2Si + AlN) / AZ91D composite material casting plate and observe its metallographic structure, as shown in Figure 2 shown, where the formed Mg2Si secondary phase particles have a round shape, are evenly distributed, and have a size of 2 to 10 μm.

[0038] Carry out room temperature tensile property and high temperature tensile property testing at 200°C on the (Mg2Si + AlN) / AZ91D composite material casting plate. The test results are as follows Figure 3 , Figure 4 and shown in Table 1. Among them, Table 1 shows the room-temperature tensile and high-temperature tensile properties of the (Mg2Si + AlN) / AZ91D composite material at 200 °C: Table 1

[0039] In this specific example, Si and Sr elements were added to the high-plasticity AlN / AZ91D magnesium matrix composite material through a rolling and crushing process combined with a one-step mixing addition method, breaking through the performance bottleneck of the inversion of room-temperature and high-temperature strength and plasticity of traditional magnesium matrix composites, and preparing a high-strength ((yield strength and tensile strength at 200 °C are about 103 / 138 MPa)) and high-plasticity ((room-temperature fracture elongation is about 10%)) (Mg2Si + AlN) / AZ91D heat-resistant magnesium matrix composite material. Its room-temperature plasticity is about 156% higher than that of commercial AZ91D magnesium alloy ((room-temperature fracture elongation is about 3.9%)). At the same time, its high-temperature tensile strength at 200 °C is 27% and 14% higher than the performance of commercial AE42 at 175 °C ((yield strength and tensile strength at 175 °C are about 81 / 121 MPa)). The method and technical route of the present invention can be extended to the preparation of Mg2Si and other reinforcing body hybrid-reinforced metal magnesium matrix polycrystalline composite materials, such as whisker-reinforced magnesium matrix composite materials, graphene-reinforced magnesium matrix composite materials, etc.

[0040] The above are only the preferred specific examples of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A magnesium-based polycrystalline composite material, characterized in that, It includes a magnesium-based polycrystalline material with an as-cast fracture elongation rate higher than 10% at room temperature as the matrix material, and alloying elements Si and Sr contained in the matrix material; The alloying elements Si and Sr cause the formation of Mg2Si secondary phases with a size of 2 μm to 10 μm, a round morphology and a uniform distribution in the metal magnesium-based polycrystalline composite material composed of the matrix material; Among them, the mass fraction of element Si in the metal magnesium-based polycrystalline composite material is 0.3 to 3%, and the mass fraction of element Sr in the metal magnesium-based polycrystalline composite material is 0.05 to 0.5%; The Mg2Si secondary phase makes the mechanical properties of the metal magnesium-based polycrystalline composite material at a temperature of 150 to 200 °C be: yield strength of 103 to 112 MPa, tensile strength of 138 to 192 MPa, and fracture elongation of 18 to 26%; The mechanical properties at room temperature are: yield strength of 110 - 122 MPa, tensile strength of 207 - 217 MPa, and fracture elongation of 6 to 10%.

2. The magnesium-based polycrystalline composite material according to claim 1, wherein The magnesium-based polycrystalline material is AlN / AZ91D, and the obtained metal magnesium-based polycrystalline composite material is (Mg2Si + AlN) / AZ91D; At this time, the Mg2Si secondary phase in (Mg2Si + AlN) / AZ91D is used to jointly strengthen the matrix with the in-situ AlN particles in the magnesium-based polycrystalline material, hinder dislocation slip and climb as well as grain boundary sliding at high temperatures, and inhibit the discontinuous precipitation of the Mg 17 Al 12 phase, improve the strength of the material at room temperature and high temperature, and at the same time take into account good plasticity.

3. The magnesium-based polycrystalline composite material according to claim 1, wherein The elements Si and Sr are added to the matrix material in the form of Mg-xSi and Mg-ySr master alloys respectively.

4. The metal magnesium-based polycrystalline composite material according to claim 3, characterized in that In the Mg-xSi master alloy, the mass fraction of element Si is 20% - 40%; In the Mg-ySr master alloy, the mass fraction of element Sr is 10% - 25%.

5. A method for preparing a magnesium-based polycrystalline composite material as described in any one of claims 1-4, characterized in that, It includes the following steps: First, through a rolling and crushing process, the Mg-xSi and Mg-ySr master alloys are respectively crushed into master alloy blocks with a thickness of 2 - 3 mm and a diameter of 0.5 - 2 cm; Then, in a protective gas atmosphere, the magnesium-based polycrystalline material ingot is heated and melted to obtain a melt; When the melt reaches the casting temperature of 680 °C - 730 °C, the master alloy blocks are added to the melt in multiple times, and the added mass of each master alloy block accounts for 20 - 30% of the total mass of the master alloy blocks. At the same time, after each addition of the master alloy block, the melt is stirred at a rotational speed of 30 - 60 r / min for at least 3 min, then heated to the casting temperature and kept warm for 5 - 20 min to improve the uniformity of the melt. Then, the melt is heated to the casting temperature again for the subsequent addition process until all the master alloy blocks are added to the melt; Finally, the melt is heated to the casting temperature and cast to obtain the metal magnesium-based polycrystalline composite material.

6. A method for preparing the magnesium-based polycrystalline composite material as described in claim 5, characterized in that, The rolling and crushing process described above is specifically: Using a wire electrical discharge machine, the Mg-xSi and Mg-ySr master alloy ingots are respectively cut into thin plates with a length, width and height of 100×80×2 - 3 mm; Then, in a heat treatment furnace with an argon protection atmosphere, the thin plates are heat-treated at a preheating temperature of 50 - 200 °C and a holding time of 10 - 30 min; Subsequently, the heat-treated thin plates are rolled and crushed on a two-roll mill at a roll rotational speed of 4 - 10 m / min to obtain the master alloy blocks described above.

7. A method for preparing the magnesium-based polycrystalline composite material as described in claim 5, characterized in that, Before adding the master alloy block to the melt in multiple times, it further includes the steps of ultrasonic mixing, cleaning and drying the master alloy block, specifically: Put all the master alloy blocks of Mg-xSi and Mg-ySr into a container filled with absolute ethanol, carry out ultrasonic mixing and cleaning, and then dry them in an oven; Subsequently, divide the master alloy block into multiple portions for adding to the melt multiple times, so that the master alloy block added each time can be completely melted in the melt within 10 - 15 minutes, thereby reducing the smelting temperature, shortening the melting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the generation of element segregation in the melt; At the same time, wrap each portion of the master alloy block tightly with aluminum foil to isolate it from direct contact with the melt when the master alloy block is added to the melt each time, reducing the loss of the master alloy block.

8. A method for preparing the magnesium-based polycrystalline composite material as described in claim 7, characterized in that, The time of the ultrasonic mixing is 3 - 15 minutes.

9. A method for preparing the magnesium-based polycrystalline composite material as described in claim 5, characterized in that, The master alloy block is added to the melt in 2 - 5 times.

10. A method for preparing a magnesium-based polycrystalline composite material as described in any one of claims 5-9, characterized in that, When heating and melting the magnesium-based polycrystalline material ingot to obtain the melt, it is carried out under the protective atmosphere of a mixed gas of CO2 and SF6, where the volume ratio of CO2 to SF6 is 100:1 - 2.

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