A superelastic Mg-Sc-based shape memory alloy, its preparation method and application

Abstract

A superelastic Mg-Sc based shape memory alloy, its preparation method, and its application. This invention belongs to the field of shape memory alloys. The purpose of this invention is to solve the technical problem that existing Mg-Sc based shape memory alloys cannot simultaneously achieve high performance and low manufacturing cost. The method of this invention is as follows: First, raw materials are weighed according to the following ratio: Mg - xat.%Sc - yat.%Gd, x = 17-22, y = 1-2; then, the materials are smelted in stages under high temperature and closed conditions, and the ingots are obtained after cooling; next, the ingots are homogenized; finally, the ingots are subjected to hot extrusion, cold rolling, and cyclic heat treatment. This invention achieves the goal of maximizing the strength of the alloy while reducing costs by strictly controlling the doping amount of the low-cost rare earth element Gd. At the same time, the combination of hot extrusion, cold rolling, and cyclic heat treatment comprehensively improves the superelasticity and mechanical properties of the Mg-Sc based shape memory alloy.

Description

Technical Field

[0001] This invention belongs to the field of shape memory alloys, specifically relating to a superelastic Mg-Sc-based shape memory alloy, its preparation method, and its applications. Background Technology

[0002] With the increasing demand for low-energy consumption in aerospace, automotive, and other industries, lightweight smart materials have become a research focus. Shape memory alloys, as a typical type of smart material, are widely used in aerospace, biomedicine, artificial intelligence, and automotive engineering, but their average density is only 6–9 g / cm³. 3 However, this method cannot meet the demand for lightweight materials. Although researchers have reduced the density to some extent by preparing porous shape memory alloys, problems such as low strength and poor mechanical properties remain. By replacing heavy elements in traditional NiTi alloys with lightweight elements, NiAl-based and β-Ti-based shape memory alloys have been formed. However, NiAl-based alloys have poor processing performance, while the density of β-Ti-based alloys, although reduced, still reaches 4.5 g / cm³. 3 Therefore, there is an urgent need to find a new type of lightweight shape memory alloy in order to fundamentally solve the problem.

[0003] Magnesium-scandium alloys are Mg-rare earth alloys with a BCC structure over a relatively high temperature range. Within certain compositional ranges, Mg-Sc alloys exhibit shape memory effect and superelasticity, making them the only lightweight magnesium-based shape memory alloy discovered to date. The density of Mg-Sc alloys is approximately 2 g / cm³. 3 With a density one-third that of NiTi shape memory alloys, Mg-Sc shows promise as a novel lightweight shape memory alloy, meeting the growing demand for lightweight functional materials in drive and sensing products. However, the low phase transformation temperature and poor superelasticity of Mg-Sc binary alloys, coupled with the high cost of Sc, limit their application in aerospace and other fields. Therefore, improving the superelasticity of Mg-Sc binary alloys and addressing their high production costs are particularly important. Summary of the Invention

[0004] To overcome the above-mentioned technical problems, the present invention provides a superelastic Mg-Sc-based shape memory alloy, its preparation method and application.

[0005] The technical solution of the present invention is as follows:

[0006] One objective of this invention is to provide a method for preparing a superelastic Mg-Sc-based shape memory alloy, the method comprising the following steps:

[0007] S1: Weigh out pure magnesium, pure scandium and Mg-Gd master alloy according to Mg-xat.%Sc-yat.%Gd, x=17~22, y=1~2;

[0008] S2: The raw materials are placed into a stainless steel crucible in sequence. After the air in the crucible is removed by introducing a mixture of SF6 and CO2, the materials are smelted in stages under high temperature and closed conditions. After cooling, the ingot is obtained.

[0009] S3: Homogenize the ingot;

[0010] S4: After homogenization, the ingot is first hot-extruded, then cold-rolled, and finally subjected to cyclic heat treatment to obtain a superelastic Mg-Sc-based shape memory alloy.

[0011] Further specifying, the raw materials in the S2 crucible, from bottom to top, are 1 / 3 pure magnesium, 1 / 3 pure scandium, 1 / 2 master alloy, 1 / 3 pure scandium, 1 / 3 pure magnesium, 1 / 3 pure scandium, 1 / 2 master alloy, and 1 / 3 pure magnesium.

[0012] Further specifying, the smelting in S2 is divided into three stages: Stage 1: Heating to 800-850℃ at a rate of 15-20℃ / min and holding for 60-90min; Stage 2: Heating to 1000-1050℃ at a rate of 2-4℃ / min and holding for 60-90min, turning the crucible 5-6 times within 1 minute, continuing to hold for 30-40min, then removing and turning again, repeating the Stage 2 operation 3-5 times, and continuing to hold for 1-2h; Stage 3: Cooling to 900-950℃ at a rate of 7-10℃ / min and holding for 30-60min.

[0013] Further specified, the homogenization temperature in S3 is 600–630℃, and the time is 24–48h.

[0014] Further specified, the hot extrusion temperature in S4 is 550-600℃, the extrusion ratio is 25:1, and the extrusion speed is 0.3-0.5mm / s.

[0015] Further specified, in S4, the single-pass cold rolling deformation is 4-8%, the cold rolling rate is 8-10 mm / s, the total cold rolling deformation is 10-25%, and intermediate annealing is performed every two passes at 600-630℃ for 30-60 min.

[0016] Further specified, the cyclic heat treatment temperature in S4 is 660-690℃, the time is 30-60min, and then it is removed and air-cooled.

[0017] Furthermore, the cyclic heat treatment is limited to 5 to 10 cycles.

[0018] The second objective of this invention is to provide a superelastic Mg-Sc-Gd shape memory alloy prepared by the above method, wherein the alloy exhibits superelasticity in the range of -120℃ to 50℃.

[0019] The third objective of this invention is to provide an application of the superelastic Mg-Sc-Gd shape memory alloy prepared by the above method in the fields of aerospace, biomedicine, artificial intelligence, and automotive engineering.

[0020] The advantages of this invention compared to existing technologies are:

[0021] This invention uses a high-temperature closed melting method to prepare Mg-Sc-based alloys with uniform composition. By strictly controlling the doping amount of the low-cost rare earth element Gd, the strength of the alloy is maximized while reducing costs. At the same time, the superelasticity and mechanical properties of the Mg-Sc-based shape memory alloy are comprehensively improved by combining hot extrusion, cold rolling and cyclic heat treatment. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the 316 stainless steel high-temperature closed melting crucible used in the preparation method of the present invention.

[0023] Figure 2 The images show the physical specimen and elemental distribution of the as-cast Mg-19.1at%Sc alloy in Comparative Example 1; where (a) is a photograph of the specimen and (b) shows the elemental distribution.

[0024] Figure 3 The images show the metallographic and mechanical properties of the homogenized Mg-19.1at%Sc alloy after solution treatment in Comparative Example 1; where (a) is a metallographic photograph and (b) is a mechanical property.

[0025] Figure 4 EBSD results and hyperelastic properties of the Mg-19.1at%Sc alloy in Comparative Example 1 at different temperatures; (a) - EBSD, (b) - hyperelasticity;

[0026] Figure 5 The images show the physical composition and elemental distribution of the as-cast Mg-17.8at%Sc-1.3at%Gd alloy in Example 1; where (a) is a photograph of the physical composition and (b) shows the elemental distribution.

[0027] Figure 6 XRD patterns of Mg-17.8at%Sc-1.3at%Gd alloys after solution treatment at different temperatures;

[0028] Figure 7The images show the metallographic and mechanical properties of the homogenized Mg-17.8at%Sc-1.3at%Gd alloy after solution treatment in Example 1; where (a) is a metallographic photograph and (b) shows the mechanical properties.

[0029] Figure 8 The image shows the hot-rolled Mg-17.8at%Sc-1.3at%Gd alloy from Comparative Example 2.

[0030] Figure 9 The graph shows the superelastic properties of the Mg-17.8at%Sc-1.3at%Gd alloy in Comparative Example 3 at -120℃.

[0031] Figure 10 The EBSD results of the Mg-17.8at%Sc-1.3at%Gd alloy in Example 1 and its superelastic properties at different temperatures are shown in the graphs; (a) - EBSD, (b) - superelasticity;

[0032] Figure 11 The graph shows the superelastic properties of the Mg-17.8at%Sc-1.3at%Gd alloy in Comparative Example 4 at different temperatures; where (a) represents superelasticity at -120℃ and (b) represents superelasticity at different temperatures. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0034] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0035] Example 1, Combination Figure 1 The preparation method of the superelastic Mg-Sc based shape memory alloy in this embodiment is carried out according to the following steps:

[0036] (1) Cut pure magnesium (99.99% purity) and Mg-6.3at%.Gd master alloy (99.99% purity) into rectangular blocks 40mm long, 13mm wide, and 12mm high. Use 200# coarse sandpaper to remove the surface oxide scale, ultrasonically clean them in alcohol for 5 minutes, and then dry them with cold air. Cut pure scandium (99.99% purity) into strips 5mm long and 2mm wide, ultrasonically clean them in alcohol for 5 minutes, and then dry them with cold air. Weigh the corresponding masses of pure magnesium, scandium strips, and Mg-6.3at%.Gd master alloy according to the atomic percentages of Mg-17.8at%Sc-1.3at%Gd.

[0037] (2) This embodiment uses a 316 stainless steel crucible. The crucible structure diagram is shown below. Figure 1 As shown. The upper and lower covers are sealed to the crucible wall by welding, and the thickness of the upper / lower covers is twice the wall thickness. θ is 85°, H = 6d, the diameter Φ5 is 1 / 4 * Φ3, and the bolt height h = 2 * d. The stainless steel crucible and bolts are placed in a muffle furnace and heat-treated at 850℃ for 30 minutes, then quenched. After quenching, the inside of the crucible and the bolts are immersed in 1.8wt.% dilute sulfuric acid for 24 hours to remove surface oxide scale and oil stains, then rinsed with alcohol and placed in an oven to dry at 150℃, ensuring that the inside of the crucible and the bolts are completely dry and free of moisture.

[0038] (3) The metal and alloy raw materials are placed into the crucible through the feeding port at the top of the crucible. To ensure the uniform distribution of Sc element during the smelting process, 1 / 3 of the pure magnesium is placed at the bottom of the crucible, followed by 1 / 3 of the scandium bar, 1 / 2 Mg-6.3at%.Gd master alloy, 1 / 3 of the scandium bar, 1 / 3 of the pure magnesium, 1 / 3 of the scandium bar, 1 / 2 Mg-6.3at%.Gd master alloy, and finally 1 / 3 of the pure magnesium is placed at the top. Then, a mixture of sulfur hexafluoride and carbon dioxide (content of 6 vol.%: 94 vol.%) is introduced through the feeding port at the top of the crucible at a flow rate of 1.0 mL / min to remove the air inside the crucible. Since the density of the mixed gas is higher than that of air, after the mixed gas has completely removed the air inside the crucible, the vent pipe is immediately pulled out, the bolts are tightened, and the connection between the bolts and the crucible is welded to complete the sealing of the crucible, thereby ensuring that only the mixed gas exists inside the crucible.

[0039] (4) Place the crucible in the high-temperature furnace. The melting process is divided into three stages: ① In the first stage, heat the furnace to 850°C at a heating rate of 15°C / min and hold for 60 min; ② In the second stage, heat the furnace to 1050°C at a heating rate of 4°C / min and hold for 60 min. Then, take out the crucible and rotate it 360° six times within one minute to fully mix the molten alloy inside the crucible. Then, put the crucible back into the 1050°C high-temperature furnace and hold for 30 min. Then, take it out and rotate it again. Repeat step ② three times. Then, put the crucible back into the 1050°C high-temperature furnace and hold for 2 hours; ③ In the third stage, cool the furnace to 900°C at a rate of 7°C / min and hold for 30 min. Then, take out the crucible and first touch the bottom of the crucible to the water surface in the water tank. Then, slowly sink the crucible into the water tank at a rate of 1 mm / s. After it is completely submerged in the water tank, hold for 6 min and then take out the crucible. Use a lathe to cut off the top cover and then take out the ingot to complete the melting process.

[0040] (5) Homogenize the Mg-17.8at.%Sc-1.3at.%Gd alloy ingot by holding it at 600℃ for 24h.

[0041] (6) A circular piece with a diameter of 59 mm and a thickness of 11 mm was cut from the homogenized Mg-17.8at.%Sc-1.3at.%Gd alloy ingot for hot extrusion. Before extrusion, the alloy and extrusion die were heated to 550℃ in a resistance furnace and held for 120 min. Then, hot extrusion deformation was carried out at an extrusion ratio of 25:1 and an extrusion rate of 0.5 mm / s. The hot-extruded alloy was then subjected to cold rolling. The deformation amount per pass during cold rolling was 5%, the cold rolling rate was 10 mm / s, and intermediate annealing was performed at 600℃ for 30 min every two passes. The total cold rolling deformation amount was 15%. After cold rolling deformation, the alloy was subjected to cyclic heat treatment, that is, heat treatment at 690℃ for 30 min, and then air cooling. This cycle was repeated 10 times.

[0042] Comparative Example 1: Mg-19.1 at.%Sc binary alloy

[0043] The difference between this comparative example and Example 1 is that:

[0044] In step (1): Weigh the corresponding mass of pure magnesium and scandium bars according to the atomic percentage of Mg-19.1at%Sc to prepare Mg-19.1at.%Sc binary alloy.

[0045] In step (3): the raw materials are placed in the following order: 1 / 3 pure magnesium is placed at the bottom, followed by 1 / 3 scandium bar, 1 / 3 pure magnesium, 1 / 3 scandium bar, 1 / 3 pure magnesium, and 1 / 3 scandium bar.

[0046] In step (6): the homogenized Mg-19.1at.%Sc binary alloy is hot-rolled at a temperature of 660℃, with a total deformation of 85%, a deformation of 8% per pass, and a hot rolling speed of 1m / s. Then, the alloy is subjected to cyclic heat treatment, i.e., heat-treated at 690℃ for 30min, then removed and air-cooled, cyclic treatment is repeated 10 times, and finally heat-treated at 690℃ for 30min before being removed and quenched.

[0047] The other steps and parameters are the same as in Example 1.

[0048] Comparative Example 2:

[0049] The difference between this comparative example and Example 1 is that:

[0050] In step (6): the homogenized Mg-17.8at%Sc-1.3at%Gd alloy is hot-rolled at a temperature of 660℃, with a total deformation of 85%, a deformation of 8% per pass, and a hot rolling speed of 1m / s. Then, the alloy is subjected to cyclic heat treatment, i.e., heat-treated at 690℃ for 30min, then removed and air-cooled, cyclic treatment is repeated 10 times, and finally heat-treated at 690℃ for 30min before being removed and quenched.

[0051] The other steps and parameters are the same as in Example 1.

[0052] Comparative Example 3:

[0053] The difference between this comparative example and Example 1 is that in step (6), cold rolling is omitted, and only hot extrusion and cyclic heat treatment are performed. Other steps and parameters are the same as in Example 1.

[0054] Comparative Example 4:

[0055] The difference between this comparative example and Example 1 is that in step (6), the total cold rolling deformation is 30%. Other steps and parameters are the same as in Example 1.

[0056] Figure 2 The image shows the physical composition and elemental distribution of the as-cast Mg-19.1at%Sc alloy. The alloy, which was melted in a high-temperature closed environment, has a lower degree of surface oxidation and a more uniform distribution of Mg and Sc elements.

[0057] The homogenized Mg-19.1at%Sc alloy from Comparative Example 1 was solution treated at 690℃ for 30 min. Subsequently, metallographic structure and mechanical properties were tested, and the results are as follows: Figure 3 As shown, from Figure 3 Metallographic results show that the alloy has an equiaxed grain morphology with an average grain size of ~100μm; the compressive stress-strain curve shows that the alloy's compressive yield strength σ sThe pressure was approximately 200 MPa, and the alloy did not exhibit the typical double yield phenomenon of shape memory alloys, indicating that the alloy that was only treated with solid solution does not have shape memory effect or superelasticity.

[0058] Figure 4 The EBSD results and hyperelastic properties at different temperatures are shown for Mg-19.1at%Sc alloy after hot rolling and cyclic heat treatment. The results show that the average grain size of the alloy is about 163 μm. It has hyperelasticity in the temperature range of -120℃ to -20℃, but the hyperelasticity is poor and there is a large residual strain (about 1%).

[0059] Figure 5 The image shows the physical composition and elemental distribution of the as-cast Mg-17.8at%Sc-1.3at%Gd alloy. Similar to the Mg-19.1at%Sc binary alloy, it has a lower degree of surface oxidation and a more uniform distribution of Mg, Sc, and Gd elements.

[0060] The homogenized Mg-17.8at%Sc-1.3at%Gd alloy from Example 1 was subjected to solution treatment at 540℃, 600℃, 660℃, and 690℃ for 30 min each. XRD analysis was then performed, and the results are as follows: Figure 6 As shown, a β single phase was obtained by solution treatment at 690℃.

[0061] The homogenized Mg-17.8at%Sc-1.3at%Gd alloy from Example 1 was solution treated at 690°C for 30 min. Subsequently, metallographic structure and mechanical properties were tested, and the results are as follows: Figure 7 As shown, from Figure 7 Metallographic results show that the alloy grains are also equiaxed, with an average grain size of about 50 μm, smaller than that of the Mg-19.1at.%Sc binary alloy; the yield strength σ s The strength was 292 MPa, higher than that of the Mg-19.1 at.%Sc alloy, indicating an increase in the strength of the 1Mg-Sc alloy. This increase in strength is likely due to grain refinement and solid solution strengthening. However, it also did not exhibit the typical double yield phenomenon of shape memory alloys, indicating that the alloy does not possess shape memory effect or superelasticity.

[0062] Figure 8 The image shows a hot-rolled Mg-17.8at%Sc-1.3at%Gd alloy. After the same hot rolling process, the addition of the third component Gd worsens the alloy's processing performance, making it unsuitable for hot rolling as a hot deformation process.

[0063] Figure 9The graph shows the superelastic properties of Mg-17.8at%Sc-1.3at%Gd alloy after hot extrusion combined with cyclic heat treatment at -120℃. Although the alloy exhibits superelasticity at -120℃, its mechanical properties are poor, and its superelastic properties are also poor.

[0064] Figure 10 The EBSD results and hyperelastic properties at different temperatures of the Mg-17.8at%Sc-1.3at%Gd alloy after hot extrusion, total cold rolling deformation (Example 1), and cyclic heat treatment are shown. The average grain size of the alloy is about 50 μm, which is smaller than that of the Mg-19.1at%Sc alloy. It exhibits hyperelasticity in a wide temperature range of -120℃ to 50℃, which is 70℃ higher than that of the Mg-19.1at%Sc alloy. Furthermore, the hyperelastic properties are improved, the residual strain is reduced, and complete hyperelasticity with zero residual strain is achieved at -120℃.

[0065] Figure 11 The graph shows the superelastic properties of Mg-17.8at%Sc-1.3at%Gd alloy at different temperatures after hot extrusion, total cold rolling deformation (Comparative Example 7), and cyclic heat treatment. The superelasticity at -120℃ is better than that of the Mg-19.1at%Sc alloy, but less than... Figure 10 It has poor superelasticity and poor mechanical properties.

[0066] Therefore, by adding a third component, Gd, combined with specific deformation and post-deformation heat treatment, the superelastic properties of Mg-Sc alloys can be improved.

[0067] The above description is merely a preferred embodiment of the present invention. These specific embodiments are different implementations based on the overall concept of the present invention, and the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a superelastic Mg-Sc-based shape memory alloy, the method comprising the following steps: S1: Weigh out pure magnesium, pure scandium and Mg-Gd master alloy according to Mg-xat.%Sc-yat.%Gd, x=17~22, y=1~2; S2: The raw materials are placed into a stainless steel crucible in sequence. After the air in the crucible is removed by introducing a mixture of SF6 and CO2, the materials are smelted in stages under high temperature and closed conditions. After cooling, the ingot is obtained. S3: Homogenize the ingot; S4: After homogenization, the ingot is first hot-extruded, then cold-rolled, and finally subjected to cyclic heat treatment to obtain a superelastic Mg-Sc-based shape memory alloy. The hot extrusion temperature is 550~600℃, the extrusion ratio is 25:1, the extrusion rate is 0.3~0.5mm / s, the single-pass cold rolling deformation is 4~8%, the cold rolling rate is 8~10mm / s, and the total cold rolling deformation is 10~25%. Intermediate annealing is performed every two passes at 600~630℃ for 30~60min. The cyclic heat treatment temperature is 660~690℃ for 30~60min. After removal, it is air-cooled. The cyclic heat treatment is repeated 5~10 times.

2. The method for preparing a superelastic Mg-Sc-based shape memory alloy according to claim 1, characterized in that, The raw materials in crucible S2, from bottom to top, are 1 / 3 pure magnesium, 1 / 3 pure scandium, 1 / 2 master alloy, 1 / 3 pure scandium, 1 / 3 pure magnesium, 1 / 3 pure scandium, 1 / 2 master alloy, and 1 / 3 pure magnesium.

3. The method for preparing a superelastic Mg-Sc-based shape memory alloy according to claim 1, characterized in that, The S2 melting process is divided into three stages: Stage 1: Heat to 800-850℃ at a rate of 15-20℃ / min and hold for 60-90min; Stage 2: Heat to 1000-1050℃ at a rate of 2-4℃ / min and hold for 60-90min, rotate the crucible 5-6 times within 1 minute, continue to hold for 30-40min, then remove and rotate again, repeat Stage 2 operation 3-5 times, and continue to hold for 1-2h; Stage 3: Cool to 900-950℃ at a rate of 7-10℃ / min and hold for 30-60min.

4. The method for preparing a superelastic Mg-Sc-based shape memory alloy according to claim 1, characterized in that, The homogenization temperature in S3 is 600~630℃, and the time is 24~48h.

5. The superelastic Mg-Sc-Gd shape memory alloy prepared by the method according to any one of claims 1-4, characterized in that, The alloy exhibits superelasticity within the temperature range of -120℃ to 50℃.

6. The application of the superelastic Mg-Sc-Gd shape memory alloy of claim 5 in aerospace, biomedicine, artificial intelligence and automotive engineering.

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