Method for improving thermal stability of gradient nanocrystalline structure

Abstract

The invention discloses a method for improving the thermal stability of a gradient nanocrystalline structure, and belongs to the technical field of metal material processing. According to the method, Mg-RE series alloy is used as a matrix, a matrix alloy ingot is heated for solution treatment, surface strengthening is carried out after water quenching and cooling, a gradient nanocrystalline layer is formed on the surface layer of an alloy sample, finally, the alloy sample is transferred into an aging oven for low-temperature aging, and cooling is carried out, so that the magnesium alloy is obtained. According to the method, the thermal stability of the magnesium alloy gradient nanocrystalline is improved through a synergistic strategy of component design and low-temperature aging, so that the grain growth rate of Mg-RE alloy gradient nanocrystalline structures treated by various different surface strengthening methods in a high-temperature environment can be remarkably reduced; the core bottleneck that a traditional magnesium alloy gradient nanocrystalline material is poor in thermal stability and single in applicable surface treatment method is solved; and the process does not need special equipment, can be seamlessly connected with various existing magnesium alloy surface strengthening production lines, and is low in process cost, simple and convenient to operate and easy to industrially popularize.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, and specifically relates to a method for improving the thermal stability of gradient nanocrystalline structure microstructure, and more particularly to a method for improving the thermal stability of the surface microstructure of gradient nanocrystalline structure through the synergistic effect of alloy composition design and low temperature aging process. Background Technology

[0002] Gradient nanocrystalline structures are a special microstructure in which the grain size continuously transitions from nanoscale at the surface to microscale at the core along the thickness direction of the material. Their core advantage lies in the fact that the surface nanocrystalline region significantly enhances the material's hardness, strength, and wear resistance through a fine-grain strengthening effect, while the microscale grains at the core retain the material's original good plasticity and toughness, achieving a synergistic optimization of strength and plasticity. Compared to the bottleneck of traditional homogeneous nanocrystalline materials being "strong but brittle," gradient nanocrystalline structures have irreplaceable application prospects in fields with stringent requirements for material mechanical properties, such as aerospace, automotive manufacturing, and precision machinery. Currently, the mainstream methods for preparing gradient nanocrystalline structures in magnesium alloys include ultrasonic rolling (USRP), ultrasonic shot peening (USSP), and surface mechanical polishing (SMAT).

[0003] However, regardless of the surface strengthening technology used, the nanocrystalline regions formed on the metal surface have significant problems: during subsequent processing (such as heat treatment) or service (such as high temperature environment, cyclic load), the nanocrystalline boundaries have high energy and are prone to grain boundary migration, which leads to abnormal grain growth and directly causes a decrease in the surface hardness of the material, which seriously restricts its application in medium and high temperature environments.

[0004] Therefore, developing a technical solution that can retain the mechanical property advantages of gradient nanocrystalline structures, significantly improve their thermal stability, and be applicable to various surface treatment methods has become a key problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention is based on the synergistic mechanism of "defects" and "solute atoms" to improve the thermal stability of gradient nanocrystals in magnesium alloys through a two-step innovation. This mechanism does not depend on specific surface strengthening methods. The method is applicable to various age-strengthable magnesium alloys that have undergone multiple surface strengthening treatments, including Mg-Gd and Mg-Y alloys. It has a particularly significant stabilizing effect on gradient nanocrystal structures of magnesium alloys formed by ultrasonic surface treatment and mechanical grinding.

[0006] To achieve the above objectives, the present invention specifically adopts the following technical solution: This invention provides a method for improving the thermal stability of gradient nanocrystalline structures, comprising the following steps: S1. Using Mg-RE alloy as the matrix, the matrix alloy ingot is heated and solution treated. S2. After water quenching and cooling, the surface of the solution-treated sample is strengthened to form a gradient nanocrystalline layer on the surface of the alloy sample. S3. After surface strengthening treatment, the alloy sample is transferred to an aging furnace for low-temperature aging and then cooled to obtain the final product.

[0007] In a preferred embodiment, the Mg-RE alloy described in S1 is composed of the following elements by mass percentage: 5 wt.%-15 wt.% RE, 0-2 wt.% Zn, 0-2 wt.% Zr, with the balance being Mg and unavoidable impurities.

[0008] In a preferred embodiment, the Mg-RE alloy in S1 is any one of the Mg-Gd alloy, Mg-Y alloy, and Mg-Gd-Y alloy.

[0009] As a preferred embodiment, the process parameters for the solution treatment in S1 are: heating rate of 5-10℃ / min, holding temperature of 500±5℃, and holding time of 24±1 h.

[0010] As a preferred embodiment, the surface strengthening method described in S2 is any one of ultrasonic rolling, ultrasonic surface shot peening, surface mechanical polishing, and laser shock peening.

[0011] As a preferred embodiment, the surface strengthening method described in S2 employs ultrasonic rolling, with the following process parameters: rolling force 80-170 N, rolling times 10-30 times.

[0012] In a preferred embodiment, the thickness of the gradient nanocrystalline layer in S2 is 80-500 μm, and the grain size is 50-100 nm.

[0013] As a preferred embodiment, the process parameters for low-temperature aging in S3 are: aging temperature 80-200℃, preferably 100℃, and holding time 20-30 h.

[0014] This invention involves adding rare earth elements (such as Gd and Y) to a magnesium alloy matrix that readily segregate at grain boundaries or form a second phase. These elements have atomic radii significantly different from magnesium atoms, easily forming low-energy segregation regions at grain boundaries and reducing the driving force for grain boundary migration. The amount of rare earth element added must meet two requirements: firstly, ensuring the formation of a supersaturated solid solution after solution treatment to provide a solute source for subsequent precipitation; and secondly, avoiding the formation of coarse as-cast second phases that would affect surface strengthening. The preferred addition amount of rare earth elements in this invention is 5 wt.%-15 wt.%, a range that ensures sufficient solute content while providing good casting and plastic processing properties. The method proposed in this invention is effective for age-strengthable magnesium alloys treated with surface strengthening methods such as ultrasonic rolling, ultrasonic shot peening, surface mechanical grinding, and laser shock peening (specific surface strengthening process parameters can be adaptively adjusted according to the actual material and treatment method). A low-temperature aging process is introduced after surface strengthening treatment: the low-temperature environment can prevent the premature growth of nanocrystals, while promoting the diffusion of rare earth solute atoms along the grain boundaries and forming nanoscale precipitates. These nanoscale precipitates interact with the grain boundaries, further hindering grain boundary migration, thereby achieving a dual stabilizing effect on the gradient nanocrystal structure prepared by the surface strengthening method.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention, through a synergistic strategy of "composition design + low-temperature aging", enables the grain growth rate of gradient nanocrystalline structures of Mg-RE alloys (Mg-Gd alloys, Mg-Y alloys, Mg-Gd-Y alloys, etc.) treated by various surface strengthening methods (ultrasonic rolling, ultrasonic surface shot peening, surface mechanical grinding, laser shock strengthening, etc.) to be reduced by more than 10%-30% at a high temperature of 400℃. This solves the core bottleneck of traditional magnesium alloy gradient nanocrystalline materials being "poor thermal stability and limited applicable surface treatment methods", and can be extended to other age-strengthened magnesium alloys.

[0016] 2. The low-temperature aging treatment of this invention requires no special equipment and can be seamlessly integrated with existing magnesium alloy surface strengthening production lines. It features low cost, simple operation, and ease of industrialization. For different surface strengthening methods, only minor adjustments to the aging parameters are needed to achieve optimal stability, avoiding complex process modifications and further lowering the application threshold. Attached Figure Description

[0017] Figure 1 TEM image of the surface microstructure of the Mg-15Gd alloy control group.

[0018] Figure 2 TEM image of the surface microstructure of the Mg-15Gd alloy experimental group. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with embodiments. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] Example 1 A method for improving the thermal stability of gradient nanocrystalline structures, comprising the following steps: 1. Alloy composition design Using Mg-Gd magnesium alloys as the base alloy, the target composition is designed to be 15 wt.% Gd, with the balance being Mg and unavoidable impurities.

[0022] Calculate the amount of raw materials according to the design composition, place the magnesium ingot in a graphite crucible, heat to 710℃ and melt it completely. After the temperature of the magnesium liquid stabilizes, add Mg-20Gd master alloy, stir for 10 min until completely dissolved, refine and degas, let stand for 10 min, and then pour and cool to obtain the product; the composition was tested and found to be 14.97 wt.%, and the composition deviation met the requirements.

[0023] 2. Solution pretreatment and surface strengthening process The prepared Mg-15Gd matrix alloy ingot was placed in a box-type resistance furnace for solution treatment. The process parameters were: heating rate 8℃ / min, holding temperature 500℃, holding time 24 h, followed by water quenching to obtain a uniform supersaturated solid solution structure, eliminate casting stress and provide conditions for solute atom diffusion.

[0024] Surface strengthening was performed using ultrasonic rolling equipment on the solution-treated samples. The process parameters were: rolling pressure 120 N, rolling cycles 20 times. Through the above ultrasonic rolling, a gradient nanocrystalline layer with a thickness of 290-330 μm and a grain size of 60-75 nm was formed on the surface of the Mg-15Gd matrix alloy sample.

[0025] Example 2 A method for improving the thermal stability of gradient nanocrystalline structures, comprising the following steps: 1. Alloy composition design Using Mg-Y series magnesium alloy as the base alloy, the target composition is designed as follows: 10 wt.% Y, 2 wt.% Zn, 1.5 wt.% Zr, with the balance being Mg and unavoidable impurities.

[0026] Calculate the raw material dosage according to the design composition. Place the magnesium ingot in a graphite crucible and heat it to 720℃ until completely melted. After the magnesium liquid temperature stabilizes, add Mg-30Y master alloy and Mg-30Zr master alloy, stir for 10 min until completely dissolved, then add zinc granules and continue stirring for 5 min until the alloy elements are evenly distributed. Refine and degas, let stand for 10 min, and then pour and cool to obtain the final product. The composition was tested and found to be: Y content 10.03 wt.%, Zn content 1.99 wt.%, and Zn content 1.48 wt.%, and the composition deviation met the requirements.

[0027] 2. Pretreatment and surface strengthening processes The prepared Mg-10Y-2Zn-1.5Zr matrix alloy ingot was placed in a box-type resistance furnace for solution treatment. The process parameters were: heating rate 6℃ / min, holding temperature 500℃, holding time 24 h, followed by water quenching to obtain a uniform supersaturated solid solution structure, eliminate casting stress and provide conditions for solute atom diffusion.

[0028] Surface strengthening was performed using ultrasonic rolling equipment on the solution-treated samples. The process parameters were: rolling pressure 170 N, rolling times 15 times. Through the above ultrasonic rolling, a gradient nanocrystalline layer with a thickness of 240-280 μm and a grain size of 65-80 nm was formed on the surface of the Mg-10Y-2Zn-1.5Zr matrix alloy sample.

[0029] Example 3 A method for improving the thermal stability of gradient nanocrystalline structures, comprising the following steps: 1. Alloy composition design Using Mg-Gd-Y series magnesium alloys as the base alloy, the target composition is designed as follows: 10 wt.% Gd, 5 wt.% Y, 0.5 wt.% Zn, with the balance being Mg and unavoidable impurities.

[0030] Calculate the raw material dosage according to the design composition. Place the magnesium ingot in a graphite crucible and heat it to 710℃ until completely melted. After the magnesium liquid temperature stabilizes, add Mg-20Gd master alloy and Mg-30Y master alloy, stir for 10 min until completely dissolved, then add zinc granules and continue stirring for 5 min until the alloy elements are evenly distributed. Refine and degas, let stand for 10 min, and then pour and cool to obtain the final product. The composition was tested and found to be: Gd content 9.98 wt.%, Y content 5.01 wt.%, and Zn content 0.50 wt.%, and the composition deviation met the requirements.

[0031] 2. Pretreatment and surface strengthening processes The prepared Mg-10Gd-5Y-0.5Zn matrix alloy ingot was placed in a box-type resistance furnace for solution treatment. The process parameters were: heating rate 5℃ / min, holding temperature 500℃, holding time 24 h, followed by water quenching to obtain a uniform supersaturated solid solution structure, eliminate casting stress and provide conditions for solute atom diffusion.

[0032] Surface strengthening was performed using ultrasonic rolling equipment on the solution-treated samples. The process parameters were: rolling pressure 100 N, rolling cycles 30 times. Through the above ultrasonic rolling, a gradient nanocrystalline layer with a thickness of 210-260 μm and a grain size of 60-85 nm was formed on the surface of the Mg-10Gd-5Y-0.5Zn matrix alloy sample.

[0033] Test case The Mg-15Gd, Mg-10Y-2Zn-1.5Zr, and Mg-10Gd-5Y-0.5Zn samples obtained after ultrasonic rolling surface strengthening in Examples 1-3 were divided into two groups, with the untreated samples serving as control groups 1-3 (corresponding to Examples 1-3).

[0034] The remaining alloy samples were transferred to a low-temperature aging furnace for low-temperature aging treatment. The process parameters were: aging temperature 100℃, holding time 28 hours (Example 1); aging temperature 170℃, holding time 20 hours (Example 2); and aging temperature 140℃, holding time 26 hours (Example 3). After treatment, the samples were water-cooled, and the aged samples were used as experimental groups 1-3 (corresponding to Examples 1-3).

[0035] control group 1 surface tissue Figure 1 It can be seen that it contains a large dislocation density and fine grain structure. The surface structure of experimental group 1 is shown below. Figure 2 It can be seen that the Gd elements on the two white spots on the surface are segregated at the grain boundaries.

[0036] All samples were simultaneously placed in a resistance furnace at 400℃ and held for different times to simulate medium- and high-temperature service environments. A thin slice with a thickness of 0.5 mm was cut along the cross-section of the sample by wire cutting. After sanding and electrolytic polishing, the morphology of the grain boundary precipitates was observed by TEM. The grain size within a 100 μm range of the surface layer was statistically analyzed by EBSD and the average value was taken as the final grain size. The results are shown in Table 1.

[0037] Table 1

[0038] It can be seen that the surface grain size of the experimental group samples treated with the low temperature aging method of the present invention is reduced by 10%-30% compared with that of the control group, indicating that the method of the present invention has a significant stabilizing effect on the magnesium alloy gradient nanocrystalline structure prepared by surface strengthening means.

[0039] The embodiments described above are merely preferred embodiments of the present invention, and while the descriptions are specific and detailed, they are not intended to limit the present invention. It should be noted that various changes and modifications can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the concept and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving thermal stability of a gradient nanocrystalline structure, comprising: The method comprises the following steps: S1, taking Mg-RE alloy as a matrix, heating a matrix alloy ingot for solid solution treatment; S2, after water quenching, surface strengthening is performed on the sample after the solid solution treatment to form a gradient nanocrystalline layer on the surface of the alloy sample; S3, after the surface strengthening treatment, the alloy sample is transferred to an aging furnace for low-temperature aging, and cooling is performed to obtain the alloy sample.

2. The method of claim 1, wherein the method is characterized by: The Mg-RE alloy in S1 is composed of the following elements in percentage by mass: 5 wt.%-15 wt.% of RE, 0-2 wt.% of Zn, 0-2 wt.% of Zr, and the balance of Mg and inevitable impurities.

3. The method of claim 1, wherein the method is characterized by: The Mg-RE alloy in S1 is any one of Mg-Gd alloy, Mg-Y alloy, and Mg-Gd-Y alloy.

4. The method of claim 1, wherein the method is characterized by: The process parameters of the solid solution treatment in S1 are as follows: a heating rate of 5-10 ℃ / min, a holding temperature of 500±5 ℃, and a holding time of 24±1 h.

5. The method of claim 1, wherein the method is characterized by: The surface strengthening method in S2 is any one of ultrasonic rolling, ultrasonic surface shot peening, surface mechanical grinding, and laser shock peening.

6. The method of claim 1, wherein the method is characterized by: The surface strengthening method in S2 adopts ultrasonic rolling, and the process parameters are as follows: a rolling force of 80-170 N and a rolling frequency of 10-30 times.

7. The method of claim 1, wherein the method is characterized by: The thickness of the gradient nanocrystalline layer in S2 is 80-500 μm, and the grain size is 50-100 nm.

8. The method of claim 1, wherein the method is characterized by: The process parameters of the low-temperature aging in S3 are as follows: an aging temperature of 80-200 ℃ and a holding time of 20-30 h.

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