Method for obtaining high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using strip-shaped quasicrystal structure

Abstract

The present invention discloses a method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure, which relates to the technical field of directional solidification of magnesium alloys. This method uses directional solidification technology to prepare columnar polycrystalline Mg with a yield strength of 178 MPa, a tensile strength of 314 MPa, an elongation rate of up to 42%, and a high strength-ductility product of up to 11128 MPa·%. 97.85 Zn 1.85 Y 0.30 Alloy. The organizational structure characteristics of this alloy are as follows: the growth orientation of columnar crystals mostly concentrates in the #imgabs0# direction; strip-shaped (long island-shaped) quasicrystalline I-Mg3Zn6Y phases are distributed at the longitudinal grain boundaries of columnar crystals. This preparation method is simple, the alloy cost is low, and it is environmentally friendly throughout the entire preparation process.

Description

Technical Field

[0001] The invention relates to the technical field of directional solidification of magnesium alloys, in particular to a method for obtaining high-strength and plastic directional solidified magnesium alloys by inducing compression twins with a strip-like quasi-crystal structure. Background Art

[0002] Designing and regulating the grain morphology and crystal orientation of cast magnesium alloys and fully improving the grain boundary strain coordination are basic issues that need to be urgently addressed in the development of high-performance cast magnesium alloys. Directional solidification can obtain a columnar crystal structure with a preferred orientation and grain boundary constraints reduced to 3 by regulating the alloy composition and process parameters. Therefore, the columnar crystal structure is more meaningful for improving the grain boundary strain coordination of HCP structure magnesium alloys.

[0003] Precisely controlling the preferred growth orientation of columnar crystals is the key to obtaining high-strength and high-ductility directionally solidified magnesium alloys. The competition between basal plane a-slip and twinning is the main mechanism of room temperature deformation of magnesium alloys. Twinning is polar and Stretch twin and The CRSS of compression twinning is 2-3 MPa and 76-153 MPa respectively. Therefore, the columnar crystal orientation greatly affects the twinning mode and thus the mechanical behavior of magnesium alloys. Compression twin start geometry Oriented columnar crystals have higher yield strength and are expected to become the preferred orientation for directionally solidified magnesium alloys. When the oriented columnar crystal is deformed, firstly, the basal plane a slip is in hard orientation and there are few slip dislocations; secondly, there are few activated compression twin variants, which cannot fully coordinate the grain boundary strain. Therefore, activating more compression twin variants and non-basal slip systems to fully coordinate the grain boundary strain becomes Orientation is the key to the simultaneous high strength and high plasticity of directionally solidified magnesium alloys.

[0004] In view of this, this patent proposes: ① By adjusting the process parameters and alloy composition, a strip-shaped quasicrystal phase is "pre-set" on the longitudinal grain boundary of the columnar crystal, and the concentrated stress generated by the high incompatibility of elastic-plastic deformation between the strip-shaped quasicrystal structure and the matrix is ​​used as the driving force to activate a large number of ② Compression twins are formed, and the strip-shaped quasicrystal phase coordinates the grain boundary strain by forming a vertebrae, which triggers the grain boundary second phase strengthening mechanism that is beneficial to improving plasticity. ② Compression twins are used to form compression twins with a certain orientation relationship on the other side of the grain boundary through cross-grain boundary catalysis, that is, twin pairs (ATPs). Using twin pairs and their evolution under tensile strain, as well as the activated non-basal slip system, coordinate Oriented columnar grain boundary strain. That is, the design and preparation of compression twins induced by strip quasicrystal phase, the compression twins transfer to form compression twin pairs to coordinate grain boundary strain, and the strip quasicrystal reinforces the plasticization of directional solidified magnesium alloys. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure, so as to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] A method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure, specifically including the following steps:

[0008] S1: Weigh a certain amount of Mg, Zn and Mg-30Y master alloy for batching, and put the prepared metal charge into a vacuum drying oven for drying before melting.

[0009] S2: Fix the graphite sleeve on the chill table, put the metal charge into the melting crucible, close the furnace door, turn on the circulating water and the mechanical pump, evacuate to 6 Pa, then turn on the molecular pump, continue to evacuate to 0.02 Pa and then turn off the molecular pump. After the molecular pump speed drops to 1000 r / min, turn off its power supply.

[0010] S3: Then fill the melting chamber with high-purity Ar gas to reduce the burning loss rate during the melting process of the magnesium alloy and prevent the metal liquid from boiling and splashing; when the vacuum degree reaches 400 - 600 Pa, stop inflating, turn on the cooling water refrigeration system, and reduce the temperature of the circulating cooling water to 5 °C.

[0011] S4: Set the pulling-down rate to 1 mm / min - 12 mm / min.

[0012] S5: Turn on the induction melting power supply and the holding power supply respectively to heat and melt the metal charge and preheat the graphite sleeve.

[0013] S6: When the temperature of the alloy melt in the melting crucible reaches the set melting temperature, hold it. Gently shake the melting crucible during the later stage of holding to make the metal liquid composition mix evenly, and then pour the melted alloy liquid into the graphite sleeve fixed on the chill table; at the same time, turn off the melting power supply and only turn on the holding power supply to keep the alloy liquid at the pouring temperature and establish a temperature gradient from the chill table to the solidification zone to be solidified.

[0014] S7: Then pull out the graphite sleeve from the holding furnace at the set pulling-down rate. When the graphite sleeve is completely pulled out of the holding furnace, stop pulling down and turn off the holding power supply to cool the test bar. When the temperature of the directionally solidified alloy test bar drops to 50 °C, turn off the cooling water refrigeration system, open the vacuum valve, and after the vacuum degree in the furnace returns to one atmosphere, open the furnace door to take out the directionally solidified alloy, and turn off the power supply of the directional solidification control cabinet.

[0015] Furthermore, the orientation is Striped quasicrystal I-Mg3Zn6Y phases with a striped structure are distributed on the longitudinal grain boundaries of columnar crystals.

[0016] Furthermore, under tensile strain, the local concentrated stress generated by the highly incompatible elastic-plastic deformation at the interface between the striped quasicrystal phase and the matrix serves as a driving force to activate compressive twins, causing a large number of compressive twin variants to initiate on one side of the longitudinal grain boundary.

[0017] Furthermore, the compressive twin variants that initiate first on one side of the longitudinal grain boundary catalyze across the grain boundary, causing compressive twins parallel to the twin plane to form on the other side of the longitudinal grain boundary, that is, compressive twin pairs with a certain orientation relationship are formed on both sides of the longitudinal grain boundary.

[0018] Furthermore, under tensile stress, the striped quasicrystal I-Mg3Zn6Y phases distributed on the longitudinal grain boundary form "vertebra-shaped" structures through self-"breaking" to accommodate (coordinate) the grain boundary strain.

[0019] Furthermore, the magnesium alloy composition (atomic percentage, %) consists of 1.85Zn, 0.30Y, and the remainder is Mg. In the directional solidification process parameters, the temperature gradient is 70 - 100 k / min, and the pulling rate is 5 - 10 mm / min.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. Compared with traditional preparation methods, this preparation method is simple, the alloy cost is low, and it is green and environmentally friendly throughout the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the morphology of the striped quasicrystal phase of the present invention;

[0023] Figure 2 is the evolution of the quasicrystal phase during the tensile deformation process of the present invention;

[0024] Figure 3 is the formation process of the compressive twin "pair" on both sides of the longitudinal grain boundary of the columnar crystal during the tensile deformation process of the present invention;

[0025] Figure 4 is the morphology of the experimental alloy structure in the uniform plastic deformation stage of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Please refer to Figures 1 to 4 , in the embodiment of the present invention, by regulating process parameters and alloy composition, strip-shaped quasicrystalline phases are "pre-set" on the longitudinal grain boundaries of columnar grains. Using the concentrated stress generated by the highly incompatible elastic-plastic deformation between the strip-shaped quasicrystalline structure and the matrix as the driving force, a large number of compression twins are activated. Moreover, the strip-shaped quasicrystalline phases form a spinal cord-like coordinated grain boundary strain, that is, trigger the grain boundary second-phase strengthening mechanism which is beneficial to improving plasticity. ② Utilize the compression twins to catalyze across the grain boundary and form compression twins with a certain orientation relationship on the other side of the grain boundary, namely twin pairs (ATPs). Utilize the evolution of twin pairs and their tensile strain, as well as the activated non-basal slip system, to coordinate the strain of the grain boundaries of the oriented columnar grains. That is to say, design and prepare a directionally solidified magnesium alloy based on strip-shaped quasicrystalline phases inducing compression twins, the compression twins are transmitted to form compression twin pairs to coordinate the grain boundary strain, and the strip-shaped quasicrystals enhance and plasticize

[0028] Example 1:

[0029] Specific preparation method: Directional solidification process parameters: The temperature gradient is 100 k / min, and the pulling rate is 5 mm / min. Alloy composition (atomic percentage, %): 1.85Zn, 0.30Y, and the rest is Mg.

[0030] The yield strength of the obtained magnesium alloy is 178 MPa, the tensile strength is 314 MPa, and the elongation is 42%. Characteristics of the microstructure: Quasicrystalline I-Mg3Zn6Y phases with fine strip-shaped structures are relatively evenly distributed on the longitudinal grain boundaries of the oriented columnar grains ( Figure 1 as shown), the area fraction of the quasicrystalline phase is 13%, and the average grain size of the columnar grains is about 52 μm.

[0031] Under tensile strain, uniform and relatively distributed local stress concentration is generated between the strip-shaped quasicrystalline structure and the matrix interface. This concentrated stress serves as the driving force to induce a large number of compression twin variants to be activated on the interface (such as Figure 2 shown), and the activated compression twin variants catalyze across the grain boundary and form compression twins parallel to the twin plane on the other side of the grain boundary, that is, form compression twin pairs. The compression twin pairs can relieve the stress concentration at the grain boundary, thereby improving the grain boundary strain coordination ability. In addition, under the action of the tensile force, the continuous strip-shaped quasicrystalline phases form a spinal cord-like shape to adapt to (coordinate) the grain boundary strain.

[0032] It can be seen that the strip-shaped quasicrystalline phases at the grain boundary utilize their own formation of spinal cord-like to coordinate the grain boundary strain and utilize the stress concentration generated by the high inconsistency of elastic-plastic deformation with the matrix as the driving force to induce compression twins to coordinate the grain boundary stress, that is, enhance and plasticize simultaneously.

[0033] Example 2:

[0034] Specific preparation method: Directional solidification process parameters: temperature gradient is 100 k / min, pulling rate is 10 mm / min. Alloy composition (atomic percentage, %): 1.85 Zn, 0.30 Y, and the balance is Mg.

[0035] The yield strength of the obtained magnesium alloy is 182 MPa, the tensile strength is 322 MPa, and the elongation is 18%. Characteristics of the microstructure: Quasicrystal I-Mg3Zn6Y phases with strip structures are also distributed on the longitudinal grain boundaries of the oriented columnar crystals, but mostly rod-shaped eutectic structures. At the same time, due to well-developed dendrites, a large number of granular quasicrystal phases are distributed on its matrix. The area fraction of the quasicrystal phase is about 20%, and the average grain size of the columnar crystals is about 47 μm.

[0036] Example 3:

[0037] Specific preparation method: Directional solidification process parameters: temperature gradient is 70 k / min, pulling rate is 5 mm / min. Alloy composition (atomic percentage, %): 1.85 Zn, 0.30 Y, and the balance is Mg.

[0038] The yield strength of the obtained magnesium alloy is 176 MPa, the tensile strength is 268 MPa, and the elongation is 22%. Characteristics of the microstructure: Quasicrystal I-Mg3Zn6Y phases with strip structures are distributed on the longitudinal grain boundaries of the oriented columnar crystals. The area fraction of the quasicrystal phase is about 10%, and the average grain size of the columnar crystals is about 88 μm.

[0039] Example 4:

[0040] Specific preparation method: Directional solidification process parameters: temperature gradient is 70 k / min, pulling rate is 1 mm / min. Alloy composition (atomic percentage, %): 1.85 Zn, 0.30 Y, and the balance is Mg.

[0041] The yield strength of the obtained magnesium alloy is 156 MPa, the tensile strength is 248 MPa, and the elongation is 16%. Characteristics of the microstructure: A small amount of quasicrystal I-Mg3Zn6Y phases with strip structures are distributed on the longitudinal grain boundaries of the oriented columnar crystals. Most are eutectic quasicrystal phases with irregular morphologies. The area fraction of the quasicrystal phase is about 7%, and the average grain size of the columnar crystals is about 102 μm.

[0042] It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any aspect, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Accordingly, all changes that fall within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention, and any reference signs in the claims should not be construed as limiting the claims involved.

[0043] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure, characterized in that Specifically, it includes the following steps: S1: Weigh a certain amount of Mg, Zn, and Mg-30Y master alloy for batching. Before melting, put the prepared metal charge into a vacuum drying oven for drying. The composition of the magnesium alloy is calculated by atomic percentage and consists of 1.85Zn, 0.30Y, and the rest is Mg; S2: Fix the graphite sleeve on the chill table, put the metal charge into the melting crucible, close the furnace door, turn on the circulating water and mechanical pump, evacuate to 6 Pa, then turn on the molecular pump, continue to evacuate to 0.02 Pa and then turn off the molecular pump. After the molecular pump speed drops to 1000 r / min, turn off its power supply; S3: Then, fill the melting chamber with high-purity Ar gas to reduce the burning loss rate during the melting of the magnesium alloy and prevent the metal liquid from boiling and splashing; when the vacuum reaches 400 - 600 Pa, stop inflating, turn on the cooling water refrigeration system, and reduce the temperature of the circulating cooling water to 5°C; S4: Set the pulling rate to 1 mm / min - 12 mm / min; S5: Turn on the induction melting power supply and the holding power supply respectively to heat and melt the metal charge and preheat the graphite sleeve; S6: When the temperature of the alloy melt in the melting crucible reaches the set melting temperature, hold the temperature. During the later stage of holding, gently shake the melting crucible to make the metal liquid composition mix evenly, and then pour the melted alloy liquid into the graphite sleeve fixed on the chill table; at the same time, turn off the melting power supply and only turn on the holding power supply to keep the alloy liquid at the pouring temperature and establish a temperature gradient from the chill table to the solidification zone to be solidified. The temperature gradient is 70 - 100 k / min; S7: Subsequently, the graphite sleeve is pulled out of the holding furnace at a set pulling-down rate. After the graphite sleeve is completely pulled out of the holding furnace, stop pulling down and turn off the holding power supply to cool the test bar. When the temperature of the directionally solidified alloy test bar drops to 50 °C, turn off the cooling water refrigeration system, open the vacuum valve. After the vacuum degree in the furnace returns to one atmosphere, open the furnace door to take out the directionally solidified alloy, and turn off the power supply of the directionally solidified control cabinet. The strip-like quasicrystal structure is: the orientation is The strip-like structure of the quasicrystal I-Mg3Zn6Y phase is distributed on the longitudinal grain boundaries of the columnar crystals with this orientation.

2. The method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure according to claim 1, wherein Under tensile strain, the local concentrated stress generated by the highly incompatible elastic-plastic deformation at the quasicrystal phase / matrix interface of the strip structure acts as a driving force to activate compressive twins, causing a large number of compressive twin variants to be initiated on one side of the longitudinal grain boundary.

3. The method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure according to claim 1, wherein The compressive twin variants initiated first on one side of the longitudinal grain boundary catalyze across the grain boundary, causing compressive twins parallel to the twin plane to form on the other side of the longitudinal grain boundary, that is, compressive twin pairs with a certain orientation relationship are formed on both sides of the longitudinal grain boundary.

4. A method for obtaining a high-strength and high-ductility directionally solidified magnesium alloy by inducing compressive twins using a strip-shaped quasicrystalline structure, characterized in that, Under tensile stress, the strip-shaped quasicrystal I-Mg3Zn6Y phase distributed on the longitudinal grain boundary forms a "vertebra-shaped" structure by "breaking" itself to adapt to the grain boundary strain.

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