Magnesium-based polycrystalline composite material and preparation method thereof

Abstract

The present invention relates to a magnesium-based polycrystalline composite material, comprising a magnesium-based polycrystalline material having a room-temperature cast fracture elongation greater than 10% as a matrix material, and alloying elements Si and Sr contained in the matrix material. The alloying elements Si and Sr cause a 2μm to 10μm-sized, rounded, and uniformly distributed Mg2Si second phase to be generated in the magnesium-based polycrystalline composite material formed with the matrix material. The invention also provides a method for preparing the magnesium-based polycrystalline composite material using a master alloy rolling and crushing process combined with a one-step mixing and addition method. The present invention successfully introduces a small, rounded, high-hardness, and high-thermal-stability Mg2Si phase into a high-plasticity magnesium-based composite material, overcoming the bottleneck problem of the inverted relationship between strength and plasticity at room and high temperatures in magnesium-based composite materials.

Description

Technical Field

[0001] The 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 properties, and therefore have broad application prospects in the aerospace and automotive industries. However, their low room temperature strength, poor plasticity, and unsatisfactory heat resistance greatly limit their application range.

[0003] Particle-reinforced composites, created by in-situ or externally introducing hard second-phase particles into alloys, can effectively improve the alloy's strength and heat resistance. However, due to issues with the size and distribution of the reinforcing phase, as well as inherent elastic-plastic and thermal deformation mismatches with the magnesium matrix and interfacial issues, conventional magnesium-based composites exhibit an inverted relationship between strength and plasticity at room and high temperatures. This performance bottleneck has significantly hindered the widespread application of magnesium-based composites in lightweight applications.

[0004] Studies have shown that Mg2Si particles have a high melting point (1087 ℃), high hardness (460 HV), high elastic modulus (120 GPa) and low density (1.99 g·cm -3 ), low thermal expansion coefficient (7.5×10 -6 K -1 ) and other advantages, making it an ideal room-temperature and high-temperature particle reinforcement phase for Mg alloys. However, under conventional casting technology, the primary Mg2Si particles formed in the solidification structure are coarse and dendritic, and the formed eutectic Mg2Si phase is usually in the shape of Chinese characters with sharp edges. During the stress and deformation process, stress concentration is easily generated to form crack sources, which has a splitting effect on the matrix material and seriously damages 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 technology, the improvement of the material's plasticity is limited, which restricts its application as a high-strength reinforcement in heat-resistant magnesium-based composites. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a metal magnesium-based polycrystalline composite material and its preparation method that can meet the application needs of lightweight, high-strength, high-plasticity, and heat-resistant magnesium-based composite materials in key fields such as aerospace, rail transportation, and the automotive industry, and break through the bottleneck problem of the inverted relationship between the strength and plasticity of magnesium-based composite materials at room temperature and high temperature.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a magnesium-based polycrystalline composite material, comprising a magnesium-based polycrystalline material having a room temperature cast fracture elongation higher than 10% as a matrix material, and alloying elements Si and Sr contained in the matrix material;

[0008] The alloying elements Si and Sr generate a Mg2Si second phase with a size of 2μm to 10μm, a rounded shape and uniform distribution in the magnesium-based polycrystalline composite material composed of the matrix material;

[0009] The mass fraction of element Si in the magnesium-based polycrystalline composite material is 0.3-3%, and the mass fraction of element Sr in the magnesium-based polycrystalline composite material is 0.05-0.5%.

[0010] The Mg2Si second phase makes the magnesium-based polycrystalline composite material have the following mechanical properties at a temperature of 150-200°C: yield strength of 103-112MPa, tensile strength of 138-192MPa, and elongation at break of 18-26%;

[0011] The room temperature mechanical properties are: yield strength of 110-122 MPa, tensile strength of 207-217 MPa, and elongation at break of 6-10%.

[0012] Furthermore, the magnesium-based polycrystalline material is AlN / AZ91D, and the obtained magnesium-based polycrystalline composite material is (Mg2Si+AlN) / AZ91D;

[0013] At this time, the Mg2Si second phase in (Mg2Si+AlN) / AZ91D is used to strengthen the matrix together with the in-situ AlN particles in the magnesium-based polycrystalline material, hindering dislocation slip and climb as well as grain boundary sliding at high temperature, and inhibiting the Mg2Si in the magnesium-based polycrystalline composite material. 17 Al 12 The discontinuous precipitation of phases improves the room temperature and high temperature strength of the material while also maintaining good plasticity.

[0014] Furthermore, the elements Si and Sr are added to the matrix material in the form of Mg-xSi and Mg-ySr master alloys, respectively.

[0015] Furthermore, in the Mg-xSi master alloy, the mass fraction of element Si is 20% to 40%;

[0016] In the Mg-ySr master alloy, the mass fraction of element Sr is 10% to 25%.

[0017] The present invention also provides a method for preparing the magnesium-based polycrystalline composite material, comprising the following steps:

[0018] First, the Mg-xSi and Mg-ySr master alloys were crushed into master alloy blocks with a thickness of 2 to 3 mm and a diameter of 0.5 to 2 cm respectively through a rolling crushing process;

[0019] Then, in a protective gas atmosphere, the magnesium-based polycrystalline material ingot is heated and melted to obtain a melt;

[0020] When the melt reaches a casting temperature of 680°C to 730°C, the master alloy blocks are added to the melt in multiple batches, with the mass of the master alloy blocks added each time accounting for 20-30% of the total mass of the master alloy blocks. At the same time, after each addition of the master alloy blocks, the melt is stirred at a speed of 30-60 r / min for at least 3 minutes. After stirring, the temperature is raised to the casting temperature and kept at this temperature for 5-20 minutes to improve the uniformity of the melt. Then, the melt is heated to the casting temperature again and the subsequent addition process is carried out until all the master alloy blocks are added to the melt.

[0021] Finally, the melt is heated to a casting temperature and cast to obtain a metallic magnesium-based polycrystalline composite material.

[0022] Furthermore, the rolling crushing process is specifically as follows:

[0023] Mg-xSi and Mg-ySr master alloy ingots were cut into thin plates with length, width and height of 100×80×2~3mm respectively using wire-cut electric discharge machine;

[0024] Then, the sheet is heat treated in a heat treatment furnace under an argon protective atmosphere with a preheating temperature of 50-200°C and a holding time of 10-30 minutes;

[0025] Subsequently, the heat-treated thin plate is rolled and crushed on a twin-roll mill at a roll speed of 4-10 m / min to obtain the intermediate alloy block.

[0026] Furthermore, before the master alloy block is added to the melt in multiple batches, the master alloy block is subjected to ultrasonic mixing, cleaning and drying steps, specifically:

[0027] All the master alloy blocks of Mg-xSi and Mg-ySr were placed in a container filled with anhydrous ethanol, ultrasonically mixed and cleaned, and then dried in an oven;

[0028] Subsequently, the master alloy block is divided into multiple portions for multiple additions to the melt, so that the master alloy block added once is completely melted in the melt within 10 to 15 minutes, thereby reducing the smelting temperature, shortening the smelting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the occurrence of element segregation in the melt;

[0029] At the same time, each intermediate alloy block is tightly wrapped with aluminum foil to isolate the intermediate alloy block from direct contact with the melt when it is added to the melt at a single time, thereby reducing the burning loss of the intermediate alloy block.

[0030] Furthermore, the ultrasonic mixing time is 3 to 15 minutes.

[0031] Furthermore, the intermediate alloy block is added to the melt in 2 to 5 batches.

[0032] Furthermore, the heating and melting of the magnesium-based polycrystalline material ingot to obtain the melt is carried out under a protective atmosphere of a mixed gas of CO2 and SF6, wherein the volume ratio of CO2 to SF6 is 100:1-2.

[0033] The beneficial effects of the present invention are:

[0034] On one hand, the present invention provides a magnesium-based polycrystalline composite material, which selects a high-plasticity magnesium-based material as the matrix material. In the art, it is generally believed that a magnesium-based material with a room temperature cast fracture elongation higher than 10% is a high-plasticity magnesium-based material. As a result, the prepared magnesium-based polycrystalline composite material has high strength and plasticity at 150-200°C, and also has high plasticity at room temperature of 25°C. Conventional magnesium-based composite materials cannot achieve both.

[0035] 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 cast AlN / AZ91D magnesium-based composite material prepared by the research team in the early stage through in-situ particle strengthening. The application number is 201510882938.5, and the publication date is April 6, 2016. Its room temperature elongation at break is as high as 20%, which is much higher than that of traditional cast magnesium-based composite materials. It is an ideal matrix material for preparing high-strength, high-plasticity, and heat-resistant magnesium-based composite materials.

[0036] In addition, due to the high melting point of Mg2Si, adding Si elements during alloy smelting, whether through elemental Si or intermediate alloying, will cause the smelting temperature to increase and easily cause element segregation. The present invention improves the uniform dispersion of Si elements in the melt by rationally adding active metamorphic elements and optimizing the addition method of Si elements, effectively improving the morphology, size and distribution of Mg2Si, thereby reducing stress concentration during deformation and improving the plasticity of the composite material.

[0037] On the other hand, the present invention uses a rolling and crushing process combined with a one-step mixing and addition method to add the alloying elements Si and Sr to the melt, thereby successfully introducing a small, rounded, high-hardness, high-thermal-stable Mg2Si phase into a high-plasticity magnesium-based composite material. This breaks through the performance bottleneck of the inversion of strength and plasticity at room temperature and high temperature of traditional magnesium-based composite materials, and prepares a metal magnesium-based polycrystalline composite material with comprehensive room temperature and high temperature strength and plasticity.

[0038] In addition, the intermediate alloy rolling and crushing process proposed in the present invention combined with the one-step mixed addition method can effectively solve the problems of high smelting temperature, easy segregation, coarse Mg2Si size and sharp corners that easily cause stress concentration and damage to plasticity when Si elements are added. The process flow is short and the cost is low, which is suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 A (Mg2Si+AlN) / AZ91D magnesium-based polycrystalline composite material casting plate prepared in a specific example of the present invention;

[0040] Figure 2 The microstructure of the magnesium-based polycrystalline composite material prepared in a specific embodiment of the present invention;

[0041] Figure 3 The room temperature tensile stress-strain curve of the magnesium-based polycrystalline composite material prepared in a specific example of the present invention;

[0042] Figure 4 This is the stress-strain curve of the magnesium-based polycrystalline composite material prepared in a specific example of the present invention under high temperature tensile test at 200°C. DETAILED DESCRIPTION

[0043] 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 used to limit the scope of the present invention.

[0044] In order to achieve the above object, the present invention provides the following specific implementation methods:

[0045] Example 1: A magnesium-based polycrystalline composite material, comprising a magnesium-based polycrystalline material having a room temperature cast fracture elongation higher than 10% as a matrix material, and alloying elements Si and Sr contained in the matrix material;

[0046] The alloying elements Si and Sr generate a Mg2Si second phase with a size of 2μm to 10μm, a rounded shape and uniform distribution in the magnesium-based polycrystalline composite material composed of the matrix material;

[0047] The mass fraction of element Si in the magnesium-based polycrystalline composite material is 0.3-3%, and the mass fraction of element Sr in the magnesium-based polycrystalline composite material is 0.05-0.5%.

[0048] The Mg2Si second phase makes the magnesium-based polycrystalline composite material have the following mechanical properties at a temperature of 150-200°C: yield strength of 103-112MPa, tensile strength of 138-192MPa, and elongation at break of 18-26%;

[0049] The room temperature mechanical properties are: yield strength of 110-122 MPa, tensile strength of 207-217 MPa, and elongation at break of 6-10%.

[0050] 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;

[0051] At this time, the Mg2Si second phase in (Mg2Si+AlN) / AZ91D is used to strengthen the matrix together with the in-situ AlN particles in the magnesium-based polycrystalline material, hindering dislocation slip and climb as well as grain boundary sliding at high temperature, and inhibiting the Mg2Si in the magnesium-based polycrystalline composite material. 17 Al 12 The discontinuous precipitation of phases improves the room temperature and high temperature strength of the material while also maintaining good plasticity.

[0052] Example 3: The same as Example 1, except that the elements Si and Sr are added to the base material in the form of Mg-xSi and Mg-ySr master alloys, respectively.

[0053] In Mg-xSi master alloys, the mass fraction of element Si is 20%~40%;

[0054] In the Mg-ySr master alloy, the mass fraction of element Sr is 10%~25%.

[0055] Example 4: The present invention also provides a method for preparing the magnesium-based polycrystalline composite material as described in Examples 1-3, comprising the following steps:

[0056] Step 1: Through a rolling and crushing process, specifically using a wire-cut electric discharge machine, the Mg-xSi and Mg-ySr master alloy ingots are cut into thin plates with a length, width, and height of 100×80×2~3mm respectively;

[0057] Then, the sheet is heat treated in a heat treatment furnace under an argon protective atmosphere with a preheating temperature of 50-200°C and a holding time of 10-30 minutes;

[0058] Subsequently, the heat-treated thin plate is rolled and crushed on a twin-roll mill at a roll speed of 4-10 m / min to obtain an intermediate alloy block with a thickness of 2-3 mm and a diameter of 0.5-2 cm.

[0059] Step 2: ultrasonically clean and dry the master alloy block, specifically:

[0060] Place all the master alloy blocks of Mg-xSi and Mg-ySr in a container filled with anhydrous ethanol, perform ultrasonic mixing and cleaning for 3 to 15 minutes, and then dry them in an oven.

[0061] Subsequently, the master alloy block is divided into multiple portions for multiple additions to the melt, and the master alloy block added once is ensured to be completely melted in the melt within 10 to 15 minutes, thereby reducing the smelting temperature, shortening the smelting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the occurrence of element segregation in the melt.

[0062] At the same time, each master alloy block is tightly wrapped with aluminum foil to isolate the master alloy block from direct contact with the melt when it is added to the melt at a time, thereby reducing the burning loss of the master alloy block;

[0063] Among them, burn-out refers to the failure of the intermediate alloy to melt and mix with the melt quickly after being added to the melt, but to float on the surface of the melt and undergo oxidation loss.

[0064] Step 3: In a protective atmosphere of a mixed gas of CO2 and SF6, heating and melting the magnesium-based polycrystalline material ingot to obtain a melt;

[0065] Among them, the volume ratio of CO2 and SF6 is 100:1~2.

[0066] Step 4: When the melt reaches a casting temperature of 680°C to 730°C, add the master alloy block to the melt in multiple batches, with the mass of the master alloy block added each time accounting for 20-30% of the total mass of the master alloy block;

[0067] At the same time, after each addition of the master alloy block, the melt is stirred at a speed of 30-60 r / min for at least 3 minutes, and then heated to the casting temperature and kept warm for 5-20 minutes to improve the uniformity of the melt. Then, the melt is heated to the casting temperature again and the subsequent addition process is carried out until all the master alloy blocks are added to the melt;

[0068] Step 4: heating the melt to a casting temperature of 680° C. to 730° C., and casting to obtain a magnesium-based polycrystalline composite material.

[0069] Example 5: The same as Example 4, except that in step 2, the intermediate alloy block is divided into 2 to 5 parts, and further in step 4, the intermediate alloy block is added to the melt in 2 to 5 times.

[0070] like Figure 1-4In order to further illustrate the technical solutions and effects of the present invention, the following specific examples are provided:

[0071] (1) The matrix material is selected as AlN / AZ91D magnesium-based polycrystalline material.

[0072] (2) Addition amount of alloying elements Si and Sr:

[0073] In this specific example, the mass fraction of the added Si element relative to the metal magnesium-based polycrystalline composite material is 0.3% to 3%, and the mass fraction of the added Sr element relative to the metal magnesium-based polycrystalline composite material is 0.05% to 0.5%;

[0074] 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 Mg2Si second phase generated after the addition of Si and Sr is round and evenly distributed, and the size of the Mg2Si second phase particles is 2μm to 10μm.

[0075] (3) Rolling and crushing process of master alloy:

[0076] Mg-xSi and Mg-ySr master alloy ingots were cut into thin plates of 100×80×2~3mm using a wire electric discharge machine. The upper and lower surfaces of the thin plates were polished with a wire brush and then placed in an aqueous solution at a temperature of 100℃ with NaOH and Na2CO3 concentrations of 0.2mol / L and 0.4mol / L, respectively, for alkaline cleaning for 10 minutes and then blown dry with a hair dryer.

[0077] Then, the thin plate is placed in a heat treatment furnace and kept warm for 10 minutes at a preheating temperature of 50°C~200°C (in this case, 180°C is selected) and an argon inert atmosphere. It is then rolled and crushed into irregular intermediate alloy blocks with a thickness of 2~3mm and a diameter of 0.5~2cm by a rolling mill.

[0078] The two master alloy blocks were placed in a beaker containing anhydrous ethanol according to the addition ratio and amount, and ultrasonically mixed and cleaned for a period of time, and then dried in an oven.

[0079] It is then divided into several portions by weight and tightly wrapped with aluminum foil to facilitate the rapid melting of the intermediate alloy blocks added to the melt, thereby lowering the smelting temperature, reducing burn-off, and improving the modification efficiency; at the same time, it promotes the uniform distribution of elements in the melt and avoids the occurrence of element segregation in the melt.

[0080] The rolling pass is 1, the roller temperature is room temperature, the roller speed is 4-10 m / min (8.4 m / min in this example), and the ultrasonic mixing and cleaning time is 3-15 min (10 min in this example).

[0081] (4) One-step mixing, adding master alloy and casting:

[0082] A certain amount of AlN / AZ91D composite ingot is cut and placed in a stainless steel crucible. The ingot quality is selected to ensure that the mass fractions of the added Si and Sr elements are 0.3% to 3% and 0.05% to 0.5% of the total weight of the final magnesium-based polycrystalline composite material, respectively.

[0083] The smelting was carried out in a resistance melting furnace under a protective atmosphere of (SF6+CO2). When the melt temperature reached the pouring temperature of 720°C, a portion of the master alloy wrapped in aluminum foil was pressed into the melt using a bell jar and kept warm for 10 minutes. The melt was then mechanically stirred at a speed of 30-60 r / min for 3 minutes. After the stirring was completed, the melt was heated to 720°C and the master alloy was continuously added. The above process was repeated until the addition was completed. Finally, the melt was heated to 720°C and the (Mg2Si+AlN) / AZ91D composite material was cast into a plate. Figure 1 shown.

[0084] (5) Metallographic structure observation and performance testing:

[0085] Metallographic samples were cut from (Mg2Si+AlN) / AZ91D composite material casting plates and their metallographic structures were observed. Figure 2 As shown in the figure, the generated Mg2Si second phase particles are round, evenly distributed and have a size of 2~10μm.

[0086] The tensile properties of (Mg2Si+AlN) / AZ91D composite casting plates were tested at room temperature and at high temperature of 200℃.

[0087] The test results are as follows Figure 3 、 Figure 4 As shown in Table 1, Table 1 shows the room temperature tensile properties and high temperature tensile properties at 200°C of the (Mg2Si+AlN) / AZ91D composite material:

[0088] Table 1

[0089]

[0090] This specific example incorporates Si and Sr into a high-plasticity AlN / AZ91D magnesium-based composite through a rolling and crushing process combined with a one-step mixing and addition method. This overcomes the performance bottleneck of conventional magnesium-based composites, which suffer from the inverse relationship between strength and plasticity at both room and high temperatures. The resulting (Mg2Si+AlN) / AZ91D heat-resistant magnesium-based composite exhibits high strength (yield strength and tensile strength of approximately 103 / 138 MPa at 200°C) and high plasticity (elongation at break of approximately 10% at room temperature). Its room-temperature plasticity is approximately 156% higher than that of commercial AZ91D magnesium alloy (elongation at break of approximately 3.9%). Furthermore, its high-temperature tensile strength at 200°C is 27% and 14% higher than that of commercial AE42 at 175°C (yield strength and tensile strength of approximately 81 / 121 MPa at 175°C). The methods and technical approaches of this invention can be extended to the preparation of magnesium-based polycrystalline composites reinforced with Mg2Si and other reinforcements, such as whisker-reinforced magnesium-based composites and graphene-reinforced magnesium-based composites.

[0091] The above description is only a preferred specific example of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A magnesium-based polycrystalline composite material, characterized in that: The invention comprises a magnesium-based polycrystalline material having a room temperature cast fracture elongation higher than 10% as a matrix material, and alloying elements Si and Sr contained in the matrix material; The alloying elements Si and Sr generate a Mg2Si second phase with a size of 2μm to 10μm, a rounded shape and uniform distribution in the magnesium-based polycrystalline composite material composed of the matrix material; The mass fraction of element Si in the magnesium-based polycrystalline composite material is 0.3-3%, and the mass fraction of element Sr in the magnesium-based polycrystalline composite material is 0.05-0.5%. The Mg2Si second phase makes the magnesium-based polycrystalline composite material have the following mechanical properties at a temperature of 150-200°C: yield strength of 103-112MPa, tensile strength of 138-192MPa, and elongation at break of 18-26%; The room temperature mechanical properties are: yield strength of 110-122MPa and tensile strength of 207-217MPa, and elongation at break of 6-10%; 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 second phase in (Mg2Si+AlN) / AZ91D is used to strengthen the matrix together with the in-situ AlN particles in the magnesium-based polycrystalline material, hindering dislocation slip and climb as well as grain boundary sliding at high temperature, and inhibiting the Mg2Si in the magnesium-based polycrystalline composite material. 17 Al 12 The discontinuous precipitation of phases improves the room temperature and high temperature strength of the material while also maintaining good plasticity.

2. 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.

3. The magnesium-based polycrystalline composite material according to claim 2, wherein: 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%.

4. A method for preparing the magnesium-based polycrystalline composite material according to any one of claims 1 to 3, characterized in that: The steps include: First, the Mg-xSi and Mg-ySr master alloys were crushed into master alloy blocks with a thickness of 2 to 3 mm and a diameter of 0.5 to 2 cm respectively through a rolling crushing process; Then, in a protective gas atmosphere, the magnesium-based polycrystalline material ingot is heated and melted to obtain a melt; When the melt reaches a casting temperature of 680°C to 730°C, the master alloy blocks are added to the melt in multiple batches, with the mass of the master alloy blocks added each time accounting for 20-30% of the total mass of the master alloy blocks. At the same time, after each addition of the master alloy blocks, the melt is stirred at a speed of 30-60 r / min for at least 3 minutes. After stirring, the temperature is raised to the casting temperature and kept at this temperature for 5-20 minutes to improve the uniformity of the melt. Then, the melt is heated to the casting temperature again and the subsequent addition process is carried out until all the master alloy blocks are added to the melt. Finally, the melt is heated to a casting temperature and cast to obtain a metallic magnesium-based polycrystalline composite material.

5. A method for preparing the magnesium-based polycrystalline composite material according to claim 4, characterized in that: The rolling crushing process is specifically as follows: Mg-xSi and Mg-ySr master alloy ingots were cut into thin plates with length, width and height of 100×80×2~3mm respectively using wire-cut electric discharge machine; Then, the sheet is heat treated in a heat treatment furnace under an argon protective atmosphere with a preheating temperature of 50-200°C and a holding time of 10-30 minutes; Subsequently, the heat-treated thin plate is rolled and crushed on a twin-roll mill at a roll speed of 4-10 m / min to obtain the intermediate alloy block.

6. A method for preparing the magnesium-based polycrystalline composite material according to claim 4, characterized in that: Before the master alloy blocks are added to the melt in multiple batches, the master alloy blocks are subjected to ultrasonic mixing, cleaning, and drying steps, specifically: All the master alloy blocks of Mg-xSi and Mg-ySr were placed in a container filled with anhydrous ethanol, ultrasonically mixed and cleaned, and then dried in an oven; Subsequently, the master alloy block is divided into multiple portions for multiple additions to the melt, so that the master alloy block added once is completely melted in the melt within 10 to 15 minutes, thereby reducing the smelting temperature, shortening the smelting time, improving the modification efficiency, promoting the uniform distribution of elements in the melt, and avoiding the occurrence of element segregation in the melt; At the same time, each intermediate alloy block is tightly wrapped with aluminum foil to isolate the intermediate alloy block from direct contact with the melt when it is added to the melt at a single time, thereby reducing the burning loss of the intermediate alloy block.

7. A method for preparing the magnesium-based polycrystalline composite material according to claim 6, characterized in that: The ultrasonic mixing time is 3 to 15 minutes.

8. A method for preparing the magnesium-based polycrystalline composite material as claimed in claim 4, characterized in that: The intermediate alloy block is added to the melt in 2 to 5 times.

9. A method for preparing the magnesium-based polycrystalline composite material according to any one of claims 4 to 8, characterized in that: The heating and melting of the magnesium-based polycrystalline material ingot to obtain the melt is carried out under a protective atmosphere of a mixed gas of CO2 and SF6, wherein the volume ratio of CO2 to SF6 is 100:1-2.

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