High-toughness rare earth magnesium alloy suitable for extremely low temperature and preparation method thereof

Abstract

The invention discloses a high strength and toughness rare earth magnesium alloy suitable for extremely low temperature and a preparation method thereof, the magnesium alloy is Mg-Gd-Y-Al-Ca, the application temperature range of the Mg-Gd-Y-Al-Ca magnesium alloy is-100 DEG C to-196 DEG C, and the Mg-Gd-Y-Al-Ca magnesium alloy is composed of the following components: 0-5 wt.% of Gd, 0-5 wt.% of Y, 0.2 wt.%-1.5 wt.% of Al, 0.1 wt.%-0.5 wt.% of Ca, less than or equal to 0.1 wt.% of impurity elements, and the balance of Mg. The magnesium alloy can still keep high strength and high toughness under the extremely low temperature condition of-100 DEG C to-196 DEG C, the rare earth content is low (less than or equal to 5wt.%), the cost is controllable, the excellent die-casting forming performance is achieved, and the problem that a traditional magnesium alloy is insufficient in strength and toughness under the extremely low temperature environment is solved by regulating and controlling the high-density nanoscale long-period ordered stacking structure.

Description

Technical Field

[0001] This invention belongs to the field of metal structural materials technology, specifically relating to a high-strength and high-toughness rare earth magnesium alloy suitable for extremely low temperatures and its preparation method. Background Technology

[0002] With the increasing demand for lightweighting, magnesium alloys, due to their low density, high specific strength, and excellent damping properties, are widely used in aerospace, transportation, and defense equipment. However, in practical engineering applications, magnesium alloys often face the severe challenges of cryogenic service environments. For example, the temperature in China's deep-space lunar exploration environment can drop to -180℃, close to liquid nitrogen temperature; temperatures in polar expeditions or the frigid regions of Northern Europe can reach -70℃; and extreme low temperatures in Northeast China can drop to -50℃. Existing commercially available AZ31 and ZK61 magnesium alloys have a tensile strength of only about 250MPa to 300MPa and an elongation of about 1% to 5% in environments below -70℃. Key components made from these magnesium alloys are highly susceptible to catastrophic fracture due to increased brittleness during cryogenic service, seriously threatening the safe and reliable operation of equipment.

[0003] The main existing strategy for improving low-temperature performance is to add large amounts of rare earth elements (such as Y and Gd) to reduce the difference in critical shear stress of the slip system and improve plasticity, while using solid solution strengthening to improve alloy strength. Xixi Qi et al. [Xixi Qi etat. J Mater Sci Technol 166(2023) 123-122] disclosed a GW83K alloy with a yield strength and elongation of 272 MPa and 10.7% at -70℃, respectively. The good plasticity is mainly attributed to the formation of a large number of cylindrical surfaces in the later stages of deformation at -70℃. Small-angle grain boundaries formed by dislocation pile-up can act as dislocation sources to emit dislocations and assist in later deformation. However, at temperatures below -100°C, dislocations in high-rare-earth magnesium alloys are difficult to thermally activate, and non-basal slip systems are difficult to initiate, resulting in limited plastic deformation and severe low-temperature embrittlement. For example, the elongation after fracture of GW83K magnesium alloy at -196°C is only 3%–5%, which is still significantly lower than the ductility and toughness requirements of equipment used in extreme low-temperature operations. In addition, magnesium alloys with high rare-earth content have problems such as high cost, easy oxidation and inclusion in the melt, and poor casting filling and forming performance, making it difficult to meet the needs of engineering applications.

[0004] Therefore, there is an urgent need to develop a magnesium alloy system with low rare earth content that can maintain high strength and toughness under extremely low temperature conditions of -100℃ to -196℃. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-strength and high-toughness rare earth magnesium alloy suitable for extremely low temperatures and its preparation method. It can maintain high strength and high toughness under extremely low temperature conditions of -100℃ to -196℃, and has low rare earth content (≤5wt.%), controllable cost and excellent die casting performance. The problem of insufficient strength and toughness of traditional magnesium alloys in extremely low temperature environment is solved by controlling the high-density nano-sized long-period ordered stacking structure phase.

[0006] In a first aspect, embodiments of the present invention provide a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures. The magnesium alloy is Mg-Gd-Y-Al-Ca, and the temperature range for application of the Mg-Gd-Y-Al-Ca magnesium alloy is -100℃ to -196℃. The Mg-Gd-Y-Al-Ca magnesium alloy is composed of the following components: Gd: 0~5wt.%, Y: 0~5wt.%, Al: 0.2wt.%~1.5wt.%, Ca: 0.1wt.%~0.5wt.%, impurity elements ≤0.1wt.%, and the balance is Mg.

[0007] A further improvement of the technical solution of the present invention is that the preferred composition of the Mg-Gd-Y-Al-Ca magnesium alloy is: Gd: 1wt.%~3wt.%, Y: 1wt.%~3wt.%, Al: 0.8wt.%~1.2wt.%, Ca: 0.2wt.%~0.4wt.%, impurity elements ≤0.1wt.%, and the balance is Mg.

[0008] A further improvement of the technical solution of the present invention is that the total content of Gd+Y is 3wt.% to 5wt.%.

[0009] A further improvement of the technical solution of the present invention is that the Mg-Gd-Y-Al-Ca magnesium alloy is formed by vacuum die casting.

[0010] Secondly, embodiments of the present invention provide a method for preparing a high-strength and high-toughness rare-earth magnesium alloy suitable for extremely low temperatures, using the above-mentioned Mg-Gd-Y-Al-Ca magnesium alloy, and the specific preparation method includes the following steps: Step S1: Use pure magnesium, magnesium-yttrium master alloy, magnesium-gadolinium master alloy, magnesium-calcium master alloy and pure aluminum as raw materials, and mix them according to the mass percentage of each element in the Mg-Gd-Y-Al-Ca magnesium alloy. Preheat and dry the vacuum die-casting mold and raw materials. Step S2: Melt the raw materials in a resistance furnace under a protective atmosphere; Step S3: Transfer the molten material to the holding furnace of the die casting machine for die casting; Step S4: The die-cast alloy is subjected to aging heat treatment to regulate the precipitation of high-density nano-sized long-period ordered stacked structure phases.

[0011] A further improvement of the technical solution of the present invention is that the preheating and drying temperature in step S1 is 200℃~250℃ and the drying time is 3h~10h.

[0012] A further improvement to the technical solution of the present invention is that the smelting step in step S2 is as follows: Step S21: Place the prepared pure magnesium ingot in the crucible, heat it to 720℃~740℃ until it is completely melted, and then raise the temperature to 750℃~760℃. Add the prepared magnesium yttrium master alloy and magnesium gadolinium master alloy. The entire melting process must be carried out under a CO2+SF6 protective atmosphere. Step S22: After the intermediate alloy has completely melted, keep it at 740℃~760℃ for 20 minutes to allow yttrium and gadolinium to be fully dissolved. Step S23: Cool down to 700℃~730℃ and add the prepared pure aluminum and magnesium-calcium intermediate alloy; Step S24: After all the metal has melted, stir thoroughly for 5 to 8 minutes, add RJ-5 magnesium alloy special refining agent and refining for 15 to 20 minutes to remove surface slag, and let stand at 730℃ to 750℃ for 20 to 30 minutes.

[0013] A further improvement to the technical solution of the present invention is that the die-casting parameters in step S3 are as follows: vacuum degree: 10Pa~30Pa; melt temperature in the pressure chamber: 650℃~750℃; mold temperature: 200℃~300℃; pressure: 0.03MPa~0.1MPa.

[0014] A further improvement of the technical solution of the present invention is that: in step S3, the cooling rate of the melt in the die-casting cavity is 800k / s to 1200k / s, thereby obtaining a fine and uniform microstructure.

[0015] A further improvement of the technical solution of the present invention is that the aging heat treatment process in step S4 is: the temperature is 150℃~250℃, the time is 10h~24h, and the volume fraction of nano-sized long-period ordered stacked structure phase precipitated in the crystal is 1%~5%.

[0016] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows: The Mg-Gd-Y-Al-Ca magnesium alloy of the present invention can still maintain high strength and high toughness (tensile strength greater than 400MPa and elongation greater than 15%) under conditions of -100℃ to -196℃, which can meet the basic design requirements of material strength and toughness for lightweight equipment in ultra-low temperature service. The Mg-Gd-Y-Al-Ca magnesium alloy melt of the present invention has good fluidity and can be stably formed by vacuum die casting process, with excellent die casting filling capacity and forming performance; at the same time, the die casting process adopted can effectively inhibit the growth of Al2(Y, Gd) and Al2Ca phases, enhance their dispersion strengthening effect, and avoid the adverse effects caused by the low-temperature plastic deformation of coarse second phase. The Mg-Gd-Y-Al-Ca magnesium alloy of the present invention can precipitate a high-density nanoscale long-period ordered stacked structure phase, which promotes the activation and interaction of a large number of shear deformations, making up for the defects of insufficient plasticity caused by insufficient dislocation activation and difficulty in slippage during low-temperature deformation of magnesium alloys prepared by existing technologies. In addition, the high-density shear deformation network formed by subsequent deformation can effectively improve strength, achieve strain homogenization, and synergistically improve the low-temperature strength and toughness of magnesium alloys. By regulating the high-density nanoscale long-period ordered stacked structure phase, the problem of insufficient strength and toughness of traditional magnesium alloys in extremely low-temperature environments is solved. The Mg-Gd-Y-Al-Ca magnesium alloy of the present invention contains only a small amount of rare earth elements yttrium and gadolinium, and is combined with inexpensive calcium and aluminum elements. While giving full play to the toughening effect of rare earth elements, it significantly reduces the total amount of rare earth elements used, resulting in a low rare earth content (≤5wt.%), thereby reducing material costs and making it suitable for large-scale industrial application. Attached Figure Description

[0017] Figure 1 The images show the backscattered secondary electron microstructure of vacuum die-cast Mg-2Gd-2Y-1Al-0.3Ca alloy, with (a) being the low-magnification structure and (b) being the high-magnification structure. Figure 2 The tensile stress-strain curves of vacuum die-cast Mg-2Gd-2Y-1Al-0.3Ca alloy at -100℃ and -196℃ are shown. Figure 3 High-magnification scanning electron microscopy backscattered secondary electron microscopy morphology of vacuum die-cast Mg-1Gd-3Y-0.8Al-0.2Ca alloy; Figure 4 High-magnification scanning electron microscopy backscattered secondary electron microscopy morphology of gravity-cast Mg-2Gd-2Y-1Al-0.3Ca alloy; Figure 5 High-magnification scanning electron microscopy backscattered secondary electron microscopy morphology of vacuum die-cast Mg-1Gd-3Y-3Al-0.2Ca alloy; Figure 6 The microstructure of vacuum die-cast AZ91 alloy is shown. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to embodiments: A high-strength and high-toughness rare-earth magnesium alloy suitable for extremely low temperatures is disclosed. The magnesium alloy is Mg-Gd-Y-Al-Ca, and the temperature range for application of the Mg-Gd-Y-Al-Ca magnesium alloy is -100℃ to -196℃. The Mg-Gd-Y-Al-Ca magnesium alloy is composed of the following components: Gd: 0 to 5 wt.%, Y: 0 to 5 wt.%, Al: 0.2 wt.% to 1.5 wt.%, Ca: 0.1 wt.% to 0.5 wt.%, impurity elements ≤ 0.1 wt.%, and the balance is Mg.

[0019] In the above composition design, Gd and Y, as rare earth elements, can form solid solution strengthening in the magnesium matrix, improve low-temperature yield strength and dislocation slip behavior. On the other hand, they can synergistically promote the precipitation of nanoscale long-period ordered stacked structure phases with Al, thereby achieving stable precipitation strengthening and interface pinning effect under the conditions of -100℃ to -196℃, taking into account both strength and toughness. Ca has a strong ability to deoxidize / adsorb impurities, which can reduce oxidative inclusions in the melt, improve metallurgical purity and casting process stability. At the same time, Ca can segregate around the grain boundaries, which can purify and pin the grain boundaries, further improving the low-temperature toughening effect and anti-brittle fracture ability of the magnesium matrix.

[0020] In a preferred embodiment, the Mg-Gd-Y-Al-Ca magnesium alloy composition is preferably: Gd: 1wt.%~3wt.%, Y: 1wt.%~3wt.%, Al: 0.8wt.%~1.2wt.%, Ca: 0.2wt.%~0.4wt.%, impurity elements ≤0.1wt.%, and the balance is Mg.

[0021] The total content of Gd+Y is 3wt.% to 5wt.%.

[0022] Magnesium alloys with high rare earth content have problems such as high cost, easy oxidation and inclusion in the melt, and poor casting filling and forming performance, which make it difficult to meet the needs of engineering applications. Therefore, considering cost control, melt metallurgical purity and die casting filling and forming stability, it is preferable to limit the total content of Gd and Y in the alloy to 3wt.%~5wt.%.

[0023] The Mg-Gd-Y-Al-Ca magnesium alloy is produced by vacuum die casting, and the specific preparation method includes the following steps: Step S1: Use pure magnesium, magnesium-yttrium master alloy, magnesium-gadolinium master alloy, magnesium-calcium master alloy and pure aluminum as raw materials, and mix them according to the mass percentage of each element in the Mg-Gd-Y-Al-Ca magnesium alloy. Preheat and dry the vacuum die-casting mold and raw materials at a temperature of 200℃~250℃ for 3h~10h.

[0024] Step S2: Melt the raw materials in a resistance furnace under a protective atmosphere; The smelting steps are as follows: Step S21: Place the prepared pure magnesium ingot in the crucible, heat it to 720℃~740℃ until it is completely melted, and then raise the temperature to 750℃~760℃. Add the prepared magnesium yttrium master alloy and magnesium gadolinium master alloy. The entire melting process must be carried out under a CO2+SF6 protective atmosphere. Step S22: After the intermediate alloy has completely melted, keep it at 740℃~760℃ for 20 minutes to allow yttrium and gadolinium to be fully dissolved. Step S23: Cool down to 700℃~730℃ and add the prepared pure aluminum and magnesium-calcium intermediate alloy; Step S24: After all the metal has melted, stir thoroughly for 5 to 8 minutes, add RJ-5 magnesium alloy special refining agent and refining for 15 to 20 minutes to remove surface slag, and let stand at 730℃ to 750℃ for 20 to 30 minutes.

[0025] Step S3: The molten material is transferred to the holding furnace of the die-casting machine for die casting. The die-casting parameters are as follows: vacuum degree: 10Pa~30Pa; melt temperature in the pressure chamber: 650℃~750℃; mold temperature: 200℃~300℃; pressure: 0.03MPa~0.1MPa. The cooling rate of the melt in the die-casting cavity is 800k / s~1200k / s, resulting in a fine and uniform microstructure.

[0026] Step S4: The die-cast alloy is subjected to aging heat treatment to regulate the precipitation of high-density nano-sized long-period ordered stacked phase. The aging heat treatment process is as follows: temperature is 150℃~250℃, time is 10h~24h, and the volume fraction of nano-sized long-period ordered stacked phase precipitated within the grains is 1%~5%.

[0027] The feed composition is Mg-2Gd-2Y-1Al-0.3Ca according to the design mass fraction, with a total feed weight of 30kg. The loss of magnesium is calculated as 5%. The feed composition includes: pure magnesium: 25.618kg; pure aluminum: 0.286kg; magnesium-gadolinium (gadolinium mass fraction of 30%) master alloy: 1.905kg; magnesium-yttrium (yttrium mass fraction of 30%) master alloy: 1.905kg; and magnesium-calcium (calcium mass fraction of 30%) master alloy: 0.286kg.

[0028] Before smelting, all molds and raw materials were preheated and dried at 220℃ for 6 hours. The entire smelting process was carried out in a resistance furnace under a CO2+SF6 protective atmosphere. The first step involved placing the prepared pure magnesium ingot in a crucible and heating it to 730℃ until completely melted. Then, the temperature was raised to 755℃, and the prepared magnesium-yttrium (yttrium mass fraction 30%) and magnesium-gadolinium (gadolinium mass fraction 30%) master alloys were added. After the master alloys were completely melted, the temperature was held at 755℃ for 20 minutes to ensure complete dissolution of yttrium and gadolinium. The temperature was then lowered to 720℃, and the prepared pure aluminum and magnesium-calcium (calcium mass fraction 30%) master alloys were added. After all the metals had melted, the mixture was stirred thoroughly for 6 minutes, and then RJ-5 magnesium alloy refining agent was added. The mixture was then refining for 15 minutes to remove surface slag, and finally held at 740℃ for 25 minutes. Finally, the mixture was transferred to a vacuum die-casting machine for die casting. Die-casting process parameters: vacuum degree 20 Pa, melt temperature in the pressure cavity 720℃, die-casting mold temperature 240℃, pressure 0.05MPa, melt cooling rate in the die-casting cavity 1000k / s. Aging process parameters: aging temperature 240℃, aging time 20h, resulting in the precipitation of 3% by volume of nano-sized long-period ordered stacked phases within the grains, with a microstructure as follows: Figure 1 As shown, in addition to the nanoscale long-period ordered stacking structure phase, the alloy also contains a dispersed Al2(Gd, Y) phase with a size of 1μm to 3μm, but no Al2(Gd, Y) phase with a size exceeding 10μm.

[0029] The strength and toughness of this alloy at -100℃ and -196℃ were characterized by tensile stress-strain curves (e.g.) Figure 2 (As shown in the figure), the analysis results are shown in Table 1. The yield strength of this alloy under the test condition of -100℃ is 250±8MPa, the tensile strength is 420±12MPa, and the elongation is 21±3%; under the test condition of -196℃, the yield strength is 270±8MPa, the tensile strength is 450±12MPa, and the elongation is 17±4%.

[0030] Example 2 The materials are formulated according to the design mass fraction Mg-1Gd-3Y-0.8Al-0.2Ca, with a total feed weight of 30 kg. The magnesium burn-off is calculated as 5%. The materials include: pure magnesium: 25.77 kg; pure aluminum: 0.23 kg; magnesium-gadolinium (gadolinium mass fraction of 30%) master alloy: 0.95 kg; magnesium-yttrium (yttrium mass fraction of 30%) master alloy: 2.86 kg; and magnesium-calcium (calcium mass fraction of 30%) master alloy: 0.23 kg.

[0031] Before smelting, all molds and raw materials were preheated and dried at 220℃ for 6 hours. The entire smelting process was carried out in a resistance furnace under a CO2+SF6 protective atmosphere. The first step involved placing the prepared pure magnesium ingot in a crucible and heating it to 730℃ until completely melted. Then, the temperature was increased to 755℃, and prepared magnesium-yttrium (yttrium mass fraction 30%) and magnesium-gadolinium (gadolinium mass fraction 30%) master alloys were added. After the master alloys were completely melted, the temperature was maintained at 750℃ for 20 minutes to ensure complete dissolution of yttrium and gadolinium. The temperature was then lowered to 720℃, and prepared pure aluminum and magnesium-calcium (calcium mass fraction 30%) master alloys were added. After all the metals had melted, the mixture was stirred thoroughly for 4 minutes, and then RJ-5 magnesium alloy refining agent was added. The mixture was then statically refining for 15 minutes to remove surface slag, and finally statically set at 740℃ for 25 minutes. Finally, the mixture was transferred to a vacuum die-casting machine for die casting. Die-casting process parameters: vacuum degree 20 Pa, melt temperature in the pressure cavity 718℃, die-casting mold temperature 235℃, pressure 0.05MPa, melt cooling rate in the die-casting cavity 900k / s. Aging heat treatment process parameters: aging temperature 230℃, time 18h, resulting in the precipitation of 3.5% by volume of nano-sized long-period ordered stacked phases within the grains, with a microstructure as follows: Figure 3 As shown. In addition, the alloy also contains dispersed Al2(Gd, Y) phase and Al2Ca phase with a size of 1μm to 3μm, but no Al2(Gd, Y) phase with a size exceeding 10μm.

[0032] The strength and toughness of the alloy at -100℃ and -196℃ were characterized by tensile stress-strain curves, and the results are shown in Table 1. Under the test condition of -100℃, the alloy exhibits a yield strength of 230±6 MPa, a tensile strength of 410±8 MPa, and an elongation of 22±2%; under the test condition of -196℃, the alloy exhibits a yield strength of 260±6 MPa, a tensile strength of 440±8 MPa, and an elongation of 19±2%.

[0033] Comparative Example 1 The raw materials were prepared according to the designed mass fraction Mg-2Gd-2Y-1Al-0.3Ca, with a total feed weight of 30 kg. Magnesium loss was calculated at 5%. The composition included: pure magnesium: 25.618 kg; pure aluminum: 0.286 kg; magnesium-gadolinium (gadolinium mass fraction 30%) master alloy: 1.905 kg; magnesium-yttrium (yttrium mass fraction 30%) master alloy: 1.905 kg; and magnesium-calcium (calcium mass fraction 30%) master alloy: 0.286 kg. The main difference from Example 1 was the use of gravity casting. Before smelting, all molds and raw materials were preheated and dried at 220℃ for 6 hours. The entire smelting process was carried out in a resistance furnace under a CO2+SF6 protective atmosphere. The first step involved placing the prepared pure magnesium ingot in a crucible and heating it to 730℃ until completely melted. Then, the temperature was raised to 755℃, and the prepared magnesium-yttrium (yttrium mass fraction 30%) and magnesium-gadolinium (gadolinium mass fraction 30%) master alloys were added. After the master alloys were completely melted, the temperature was held at 755℃ for 20 minutes to allow the yttrium and gadolinium to fully dissolve. Then, the temperature was lowered to 720℃, and the prepared pure aluminum and magnesium-calcium (calcium mass fraction 30%) master alloys were added. After all the metals had melted, the mixture was stirred thoroughly for 6 minutes, and then RJ-5 magnesium alloy refining agent was added and the mixture was refining for 15 minutes to remove surface slag. The mixture was then allowed to stand at 740℃ for 25 minutes before being cast into a steel mold. The microstructure was as follows: Figure 4 As shown. Aging process parameters: aging temperature 240℃, time 20h. No nanoscale long-period ordered stacked phases precipitate within the crystals, and the microstructure is as follows. Figure 4 As shown. The main difference between the microstructure and Example 1 is that there is no nanoscale long-period ordered stacking structure phase precipitation in the crystal, and there is a primary Al2(Gd, Y) phase with a size of 10μm-15μm.

[0034] The strength and toughness of the alloy at -100℃ and -196℃ were characterized by tensile stress-strain curves, and the results are shown in Table 1. Under the test condition of -100℃, the alloy exhibits a yield strength of 105±10 MPa, a tensile strength of 120±10 MPa, and an elongation of 2±1%; under the test condition of -196℃, the alloy exhibits a yield strength of 110±10 MPa, a tensile strength of 130±10 MPa, and an elongation of 3±1%.

[0035] Comparative Example 2 The materials are formulated according to the design mass fraction Mg-1Gd-3Y-3Al-0.2Ca, with a total feed weight of 30kg. The magnesium burn-off is calculated as 5%. The materials include: pure magnesium: 25.75kg; pure aluminum: 0.90kg; magnesium-gadolinium (gadolinium mass fraction of 30%) master alloy: 0.95kg; magnesium-yttrium (yttrium mass fraction of 30%) master alloy: 2.85kg; and magnesium-calcium (calcium mass fraction of 30%) master alloy: 0.2kg.

[0036] The main difference from Example 2 is that the Al content in the alloy is increased from 0.8 wt.% to 3 wt.%. Before smelting, all molds and raw materials were preheated and dried at 220℃ for 6 hours. The entire smelting process was carried out in a resistance furnace under a CO2+SF6 protective atmosphere. The first step involved placing the prepared pure magnesium ingot in a crucible and heating it to 730℃ until completely melted. Then, the temperature was increased to 755℃, and prepared magnesium-yttrium (yttrium mass fraction 30%) and magnesium-gadolinium (gadolinium mass fraction 30%) master alloys were added. After the master alloys were completely melted, the temperature was maintained at 745℃ for 15 minutes to ensure complete dissolution of yttrium and gadolinium. The temperature was then lowered to 710℃, and prepared pure aluminum and magnesium-calcium (calcium mass fraction 30%) master alloys were added. After all the metals had melted, the mixture was stirred thoroughly for 8 minutes, and then RJ-5 magnesium alloy refining agent was added. The mixture was then refining for 15 minutes to remove surface slag, and finally held at 740℃ for 25 minutes before being transferred to a vacuum die-casting machine for die casting. Die-casting process parameters: vacuum degree 20 Pa, melt temperature in the pressure cavity 720℃, die-casting mold temperature 220℃, pressure 0.05MPa, melt cooling rate in the die-casting cavity 1100k / s. Aging heat treatment process parameters: aging temperature 230℃, time 18h, microstructure as follows... Figure 5 As shown, the alloy also contains dispersed Al2(Gd, Y) phases with a size of 1–3 μm, but no Al2(Gd, Y) phases with a size exceeding 10 μm. The main difference between the microstructure and that of Example 2 is that there are no nanoscale long-period ordered stacking structure phases precipitated within the crystals.

[0037] The strength and toughness of the alloy at -100℃ and -196℃ were characterized by tensile stress-strain curves, and the results are summarized in Table 1. Under the test condition of -100℃, the alloy exhibits a yield strength of 170±10 MPa, a tensile strength of 200±8 MPa, and an elongation of 4±1%; under the test condition of -196℃, the alloy exhibits a yield strength of 190±6 MPa, a tensile strength of 210±6 MPa, and an elongation of 2±1%.

[0038] Comparative Example 3 The commercially available proportion of Mg-9Al-1Zn (AZ91 commercial grade magnesium alloy) is designed with a total feed weight of 30kg, and the magnesium burn-off is calculated as 5%; of which pure magnesium: 27.14kg; pure aluminum: 2.58kg; pure zinc: 0.29kg.

[0039] Before smelting, all molds and raw materials were preheated and dried at 220℃ for 6 hours. The entire smelting process was carried out in a resistance furnace under a CO2+SF6 protective atmosphere. The first step involved placing the prepared pure magnesium ingot in a crucible, heating it to 730℃ until completely melted, then cooling it to 720℃ and adding the prepared high-purity aluminum and high-purity zinc. After the intermediate alloy was completely melted, it was held at 740℃ for 20 minutes to allow the aluminum and zinc elements to fully dissolve. After all the metal had melted, it was cooled to 720℃, stirred thoroughly for 6 minutes, and then RJ-5 magnesium alloy refining agent was added and the mixture was statically refined for 15 minutes to remove surface slag. It was then held at 710℃ for 25 minutes before being transferred to a vacuum die-casting machine for die casting. Die-casting process parameters: vacuum degree 20 Pa, melt temperature in the pressure chamber 720℃, die-casting mold temperature 230℃, pressure 0.05 MPa, and melt cooling rate in the die-casting cavity 1000 kJ / s. Aging heat treatment process parameters: aging temperature 200℃, time 20h, precipitating 5% Mg by volume within the crystals. 17 Al 12 Phase, microstructure such as Figure 6 As shown.

[0040] The strength and toughness of the alloy at -100℃ and -196℃ were characterized by tensile stress-strain curves, and the results are summarized in Table 1. Under the test condition of -100℃, the alloy exhibits a yield strength of 120±10 MPa, a tensile strength of 150±8 MPa, and an elongation of 5±1%; under the test condition of -196℃, the alloy exhibits a yield strength of 140±6 MPa, a tensile strength of 160±6 MPa, and an elongation of 2±1%.

[0041] Table 1 below shows the tensile yield strength, tensile strength, and elongation after fracture of the magnesium alloys obtained in the examples and comparative examples at -100°C and -196°C.

[0042] Table 1 The embodiments of the present invention, compared with the control examples, verify the superiority of the Mg-Gd-Y-Al-Ca alloy and its preparation process in terms of high strength and toughness at extremely low temperatures: Example 1 uses a Mg-2Gd-2Y-1Al-0.3Ca composition, and after vacuum die casting and aging heat treatment at 240℃ for 20h, forms a nano-sized long-period ordered stacked structure phase and finely dispersed Al2(Gd,Y) phase with a volume fraction of about 3% in the crystal, resulting in a tensile strength of about 420MPa and an elongation of about 21% at -100℃, and a tensile strength of about 450MPa and an elongation of about 17% at -196℃, exhibiting excellent strength and toughness at extremely low temperatures; Example 2 uses a Mg-1Gd-3Y-0.8Al-0.2Ca composition, and under vacuum die casting and aging heat treatment at 230℃ for 18h, a nano-sized long-period ordered stacked structure phase with a volume fraction of about 3.5% in the crystal, accompanied by dispersed Al2(Gd,Y) phase, precipitates in the crystal. The alloy contains Al2(Gd, Y) and Al2(Gd, Y) phases. At -100℃, the tensile strength is about 410 MPa and the elongation is about 22%. At -196℃, the tensile strength is about 440 MPa and the elongation is about 19%, which also achieves high strength and toughness under extremely low temperature conditions. In contrast, Comparative Example 1, with the same composition as Example 1 but using gravity casting, exhibited a significantly reduced cooling rate. After aging, no nanoscale long-period ordered stacking structure phase was found within the crystals; instead, coarse primary Al2(Gd, Y) phases with a size of 10μm to 15μm were present. Its tensile strength at -196℃ was only about 130MPa, with an elongation of about 3%, demonstrating significant low-temperature brittleness. Comparative Example 2 used a Mg-1Gd-3Y-3Al-0.2Ca composition, significantly increasing the Al content compared to Example 2. Although vacuum die casting was used, it was difficult to form a nanoscale long-period ordered stacking structure phase within the crystals, retaining only a dispersed Al2(Gd, Y) phase, resulting in a tensile strength of about 210MPa and an elongation of about 2% at -196℃. Comparative Example 3 used a commercially available AZ91 alloy, and under the same vacuum die casting and aging process, only Mg precipitated. 17 Al 12 The phase has a tensile strength of about 160 MPa and an elongation of about 2% at -196℃, and its overall strength and toughness are the worst.

[0043] The above results show that the embodiments of the present invention achieve high strength and toughness at extremely low temperatures under low rare earth content by coupling rapid solidification of vacuum die casting with reasonable composition design and regulating the precipitation of nanoscale long-period ordered stacked structure phases. This breaks through the problem of insufficient strength and toughness of magnesium alloys under extremely low temperatures of -100℃ to -196℃ by solving the problem of insufficient strength and toughness of magnesium alloys under extremely low temperatures of -100℃ to -196℃ through single composition or process design. It helps to solve the problem of service of lightweight equipment in ultra-low temperature extreme environments.

[0044] The above is a description of preferred embodiments of the present invention. It should be noted that the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, or improvements made to the present invention, provided they satisfy the scope of the claims, the content of the invention, and the accompanying drawings, should be included within the protection scope of the present invention.

Claims

1. A high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures, characterized in that: The magnesium alloy is Mg-Gd-Y-Al-Ca, and the temperature range for application of the Mg-Gd-Y-Al-Ca magnesium alloy is -100℃ to -196℃. The Mg-Gd-Y-Al-Ca magnesium alloy is composed of the following components: Gd: 0 to 5 wt.%, Y: 0 to 5 wt.%, Al: 0.2 wt.% to 1.5 wt.%, Ca: 0.1 wt.% to 0.5 wt.%, impurity elements ≤ 0.1 wt.%, and the balance is Mg.

2. The high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 1, characterized in that: The preferred composition of the Mg-Gd-Y-Al-Ca magnesium alloy is as follows: Gd: 1wt.%~3wt.%, Y: 1wt.%~3wt.%, Al: 0.8wt.%~1.2wt.%, Ca: 0.2wt.%~0.4wt.%, impurity elements ≤0.1wt.%, and the balance is Mg.

3. A high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 1 or 2, characterized in that: The total Gd+Y content is 3wt.% to 5wt.%.

4. A high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 1 or 2, characterized in that: The Mg-Gd-Y-Al-Ca magnesium alloy is formed by vacuum die casting.

5. A method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures, characterized in that: The preparation method of the Mg-Gd-Y-Al-Ca magnesium alloy according to claim 1 includes the following steps: Step S1: Use pure magnesium, magnesium-yttrium master alloy, magnesium-gadolinium master alloy, magnesium-calcium master alloy and pure aluminum as raw materials, and mix them according to the mass percentage of each element in the Mg-Gd-Y-Al-Ca magnesium alloy. Preheat and dry the vacuum die-casting mold and raw materials. Step S2: Melt the raw materials in a resistance furnace under a protective atmosphere; Step S3: Transfer the molten material to the holding furnace of the die casting machine for die casting; Step S4: The die-cast alloy is subjected to aging heat treatment to regulate the precipitation of high-density nano-sized long-period ordered stacked structure phases.

6. The method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 5, characterized in that: The preheating and drying temperature in step S1 is 200℃~250℃, and the drying time is 3h~10h.

7. The method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 5, characterized in that: The smelting step in step S2 is as follows: Step S21: Place the prepared pure magnesium ingot in the crucible, heat it to 720℃~740℃ until it is completely melted, and then raise the temperature to 750℃~760℃. Add the prepared magnesium yttrium master alloy and magnesium gadolinium master alloy. The entire melting process must be carried out under a CO2+SF6 protective atmosphere. Step S22: After the intermediate alloy has completely melted, keep it at 740℃~760℃ for 20 minutes to allow yttrium and gadolinium to be fully dissolved. Step S23: Cool down to 700℃~730℃ and add the prepared pure aluminum and magnesium-calcium intermediate alloy; Step S24: After all the metal has melted, stir thoroughly for 5 to 8 minutes, add RJ-5 magnesium alloy special refining agent and refining for 15 to 20 minutes to remove surface slag, and let stand at 730℃ to 750℃ for 20 to 30 minutes.

8. The method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 5, characterized in that: The die-casting parameters in step S3 are as follows: vacuum degree: 10Pa~30Pa; melt temperature in the pressure chamber: 650℃~750℃; mold temperature: 200℃~300℃. Pressure: 0.03MPa~0.1MPa.

9. The method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 5, characterized in that: In step S3, the melt is cooled at a rate of 800 k / s to 1200 k / s in the die-casting cavity to obtain a fine and uniform microstructure.

10. The method for preparing a high-strength, high-toughness rare-earth magnesium alloy suitable for extremely low temperatures according to claim 5, characterized in that: The aging heat treatment process in step S4 is as follows: the temperature is 150℃~250℃, the time is 10h~24h, and the volume fraction of nano-sized long-period ordered stacked structure phases with a volume fraction of 1%~5% is precipitated in the crystal.

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