Low-expansion magnesium alloy and preparation method and application thereof

Abstract

The invention relates to the field of alloys, in particular to a low-expansion magnesium alloy and a preparation method and application thereof. The low-expansion magnesium alloy comprises the following components in percentage by mass: 2.5%-4.5% of Sn, 0.8%-2.0% of Ge, 3.5%-6.5% of Zn, 0.2%-0.6% of Ce, 0.1%-0.5% of In, 0.8%-2.0% of Zr, 2.0%-5.0% of TiB2, 6.0%-10.0% of La2 (MoO4) 3 and the balance of Mg and inevitable impurities. According to the low-expansion magnesium alloy, through accurate compatibility of multiple elements, two functional particles of TiB2 and La2 (MoO4) 3 are further introduced, a fundamental breakthrough is achieved in component design, a unique multi-element low-expansion phase / negative-expansion particle composite reinforcing system is constructed, and low expansion, high strength and high toughness in a wide temperature range are synergistically achieved from the material essence.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of alloys, in particular to a low-expansion magnesium alloy and a preparation method and application thereof. BACKGROUND

[0002] Magnesium alloys have the advantage of lightweight, and are in urgent demand in the fields of aerospace, automobiles, extreme environment exploration, etc., but the existing low-expansion magnesium alloys have the following technical bottlenecks: (1) Poor synergy of low-expansion phases: mostly relying on a single Mg2Si phase or simple particle compounding, such as patent CN108486446A, which only reduces expansion through the Mg2Si phase, resulting in difficulty in balancing mechanical properties and expansion coefficient, and the tensile strength is generally lower than 400 MPa; (2) Limited process innovation: traditional multi-field processing only uses a combination of ultrasonic and magnetic fields, lacks deep coupling of energy fields, and the grain refinement effect is limited, with a grain size generally ≥ 30 μm; (3) Poor adaptability to extreme environments: in low-temperature environments below -100℃, the impact toughness of existing alloys decreases by more than 50% due to increased brittleness of the second phase, which cannot meet the needs of multiple scenarios; (4) Weak particle interface bonding: using single pressurization, which easily leads to agglomeration of reinforcing particles, low interface bonding strength, and easy cracking under temperature cycling.

[0003] In view of this, the present application is proposed. SUMMARY

[0004] To overcome the above-mentioned defects in the prior art, the present application realizes a magnesium alloy with wide temperature range, low expansion and high strength through the synergistic effect of five-element low-expansion phases and the innovative process of microwave-assisted multi-field coupling, and balances low expansion coefficient, high strength and high toughness in a wide temperature range.

[0005] In order to achieve the above-mentioned purpose of the present application, the following technical solutions are adopted: In one aspect of the present application, a low-expansion magnesium alloy includes the following components in terms of mass percentage: Sn 2.5%~4.5%, Ge 0.8%~2.0%, Zn 3.5%~6.5%, Ce 0.2%~0.6%, In 0.1%~0.5%, Zr 0.8%~2.0%, TiB2 2.0%~5.0%, and La2(MoO4)3 6.0%~10.0%, with the balance being Mg and impurities.

[0006] The low-expansion magnesium alloy realizes a fundamental breakthrough in component design by accurate matching of multiple components of Sn, Ge, Zn, In, Ce, Zr, and innovative introduction of two functional particles of TiB2 and La2(MoO4)3. The core beneficial effect is to construct a unique "multi-component low-expansion phase / negative-expansion particle composite enhancement system", which realizes low expansion, high strength and high toughness in a wide temperature range.

[0007] Another aspect of the present application also relates to a preparation method of the low-expansion magnesium alloy, comprising the following steps: (a) mixing and then performing heat preservation treatment on molten Mg, Sn, Ge, Zn, In, Mg-Ce intermediate alloy and Mg-Zr intermediate alloy to obtain a magnesium alloy melt; (b) performing casting and heating treatment on the magnesium alloy melt after simultaneously applying microwave radiation, pulse current, rotating magnetic field and ultrasonic vibration to obtain a semi-solid casting blank; (c) performing stirring treatment and pressurizing treatment on the semi-solid casting blank after adding TiB2 and La2(MoO4)3 to obtain a casting blank; (d) performing solid solution treatment, water quenching, first aging heat treatment, cooling and second aging heat treatment on the casting blank.

[0008] The preparation method of the low-expansion magnesium alloy realizes fine control of the whole process from the melt to the finished product by the innovative process combination of "microwave-assisted multi-field cooperation", "stepwise pressurizing in-situ compounding" and "double-temperature-zone aging", which ensures the extreme homogenization of alloy components, the refinement of microstructure and the maximization of interface bonding strength, so that the component design potential of the alloy is fully converted into stable and excellent macroscopic performance.

[0009] Another aspect of the present application also relates to the application of the low-expansion magnesium alloy or the low-expansion magnesium alloy prepared by the preparation method of the low-expansion magnesium alloy in any one of the following: (1) preparing a chip packaging substrate; (2) preparing an automobile low-temperature precision sensor; (3) preparing a lightweight new energy vehicle structural part.

[0010] Compared with the prior art, the beneficial effects of the present application are: (1) The present application realizes a breakthrough in component design by accurate proportioning of Sn, Ge, Zn, In, Ce, Zr and other multi-components, and synergistic compounding with TiB2, La2(MoO4)3 functional particles, and the core beneficial effect brought is to construct a synergistic reinforcing system of "multi-component low-expansion phase-negative expansion particle-interface strengthening phase". Specifically, first, by introducing Mg2Sn phase, Mg2Ge phase, (Mg, Zn, In) 12 Ce phase, Mg7Zr phase and Mg3In phase, five intermetallic compounds with significantly lower thermal expansion coefficient than the magnesium matrix, the "dilution effect" is used to essentially reduce the overall thermal expansion of the alloy. Second, the La2(MoO4)3 particles with negative thermal expansion characteristics are innovatively introduced, and the TiB2 particles with low expansion are compounded in a specific proportion to form a "positive-negative expansion compensation mechanism", which effectively offsets the thermal expansion of the matrix in a wide temperature range, so that the thermal expansion coefficient of the alloy is stable at ≤10×10 -6 / K, with minimal fluctuations. Finally, the nanoscale Mg3In flexible interface transition layer formed by the In element and the grain boundary pinning effect of the Ce element significantly enhance the bonding strength of the phase boundary and the grain boundary, effectively relieving the stress concentration caused by thermal mismatch, so that the alloy realizes low expansion while ensuring high strength and excellent low-temperature toughness, successfully solving the inherent contradiction that the traditional low-expansion magnesium alloy cannot simultaneously achieve high strength and toughness.

[0011] (2) The preparation method of the present application combines the process of "microwave-assisted multi-field synergy" and "stepwise pressurized aging regulation", and the core beneficial effect brought is to realize the extreme uniformity of the alloy composition, the fine controllability of the microstructure, and the significant improvement of the interface bonding strength. Specifically, the microwave-electric-magnetic-ultrasonic multi-field coupling treatment technology uses internal microwave heating to reduce the temperature gradient, combines the electromigration effect of pulsed current, the forced convection of rotating magnetic field and the cavitation breaking effect of variable frequency ultrasonic, and synergistically eliminates element segregation and refines the grain size to 15-30 μm and the low-expansion phase size to 0.5-2.0 μm, providing an excellent mechanical property basis for the alloy. The subsequent stepwise pressurized in-situ compounding process realizes three-stage pressure control of "low-pressure dispersion-high-pressure combination-pressure relief", and under the assistance of semi-solid melt thixotropy, ensures the uniform dispersion of the reinforcing particles and the formation of firm metallurgical bonding with the matrix, reduces the porosity to below 0.1%, and avoids early cracking caused by weak interface. The final double-temperature-zone aging treatment accurately matches the kinetic conditions of different precipitates to induce high and low activation energy phases to precipitate in stages, and finally forms a three-dimensional interwoven network structure with a node spacing of 4-8 μm, which can effectively pin dislocations and is an important guarantee for the alloy to simultaneously obtain high strength and low expansion coefficient. The whole process is closely linked to ensure the stability and reliability of the performance of the final product. DETAILED DESCRIPTION

[0012] The technical solutions of the present application will be described clearly and completely in combination with the specific embodiments below, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application. The specific conditions are not specified in the embodiments, and are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are all conventional products that can be purchased on the market.

[0013] In one aspect of the present application, a low-expansion magnesium alloy includes the following components in terms of mass percentage: Sn 2.5%~4.5% (for example, it can be but not limited to any one of 2.5%, 3%, 3.5%, 4% or 4.5% point value or range value between any two), Ge 0.8%~2.0% (for example, it can be but not limited to any one of 0.8%, 1.0%, 1.6%, 1.8% or 2.0% point value or range value between any two), Zn 3.5%~6.5% (for example, it can be but not limited to any one of 3.5%, 4.5%, 5.5% or 6.5% point value or range value between any two), Ce 0.2%~0.6% (for example, it can be but not limited to any one of 0.2%, 0.3%, 0.4%, 0.5% or 0.6% point value or range value between any two), In 0.1%~0.5% (for example, it can be but not limited to any one of 0.1%, 0.2%, 0.3%, 0.4% or 0.5% point value or range value between any two), Zr 0.8%~2.0% (for example, it can be but not limited to any one of 0.8%, 1.0%, 1.6%, 1.8% or 2.0% point value or range value between any two), TiB2 2.0%~5.0% (for example, it can be but not limited to any one of 2.0%, 3.0%, 4.0% or 5.0% point value or range value between any two), and La2(MoO4)3 6.0%~10.0% (for example, it can be but not limited to any one of 6.0%, 7.0%, 8.0%, 9.0% or 10.0% point value or range value between any two), the balance being Mg and unavoidable impurities.

[0014] Sn reacts with Mg to form Mg2Sn phase, which is a cubic C1 type crystal structure (lattice constant a=0.631 nm), and the thermal expansion coefficient is 5.8×10 -6 / K. By lattice matching with the α-Mg matrix, Mg2Sn phase can deform cooperatively with the matrix, avoiding interface stress concentration. When Sn content is 2.5%-4.5%, the volume fraction of Mg2Sn phase is stabilized at 9%-13%, and the overall thermal expansion coefficient of the alloy is reduced by 18%-22% through dilution effect. If Sn content is less than 2.5%, the volume fraction of Mg2Sn phase is less than 8%, which cannot achieve effective expansion inhibition; if it is higher than 4.5%, blocky Mg2Sn phase with size >5 μm is easily precipitated, resulting in the room temperature impact toughness of the alloy decreased to 20 J / cm 2 Below.

[0015] Ge reacts with Mg to form Mg2Ge phase, which is of anti-fluorite structure and has a thermal expansion coefficient of 4.7×10 -6 / K. When Ge content is 0.8%-2.0%, the volume fraction of Mg2Ge phase is 3%-6%, and a semi-coherent interface (3.8% mismatch) is formed with the matrix, generating a compressive stress of 200-250 MPa to constrain the matrix expansion. If Ge content exceeds 2.0%, not only the production cost will increase significantly, but also a brittle Ge-rich segregated phase will be formed, resulting in a decrease of 10%-15% in the tensile strength of the alloy.

[0016] Zn and Mg, Ce, In cooperatively precipitate (Mg,Zn,In) 12 Ce phase (ThMn 12 type tetragonal structure, with a thermal expansion coefficient of 9.5×10 -6 / K), in which In atoms replace Zn sites at a ratio of 10%-15%, increasing the interface binding energy. When Zn content is 3.5%-6.5% and In content is 0.1%-0.5% (mass ratio 15:1-20:1), the volume fraction of the phase is 4%-7%, and the grain boundary migration activation energy is further increased by the pinning effect of Ce atoms, effectively inhibiting grain coarsening. Deviating from this ratio range, the interface binding energy or pinning effect will decrease significantly.

[0017] Zr reacts with Mg to form Mg7Zr phase (thermal expansion coefficient 7.8×10 -6 / K), which has a lattice mismatch of only 0.8% with the α-Mg matrix (hcp structure). When Zr content is 0.8%-2.0%, the volume fraction of Mg7Zr phase is 4%-7%, which serves as a heterogeneous nucleation core to increase the nucleation efficiency to 55%-65% and refine the grain size to 15-30 μm. If Zr content exceeds 2.0%, Zr-rich agglomerates with size >8 μm are formed, resulting in fluctuations in the mechanical properties of the alloy, with a deviation of ≥15 MPa in the tensile strength.

[0018] In addition to participating in the formation of (Mg,Zn,In) 12 In addition to participating in the formation of (Mg,Zn,In) Ce phase, In also precipitates Mg3In phase (thermal expansion coefficient 8.2×10-6 When the In content is 0.1% to 0.5%, the volume fraction of the Mg3In phase is 1% to 3%, which is uniformly distributed at the interface of other phases to form a flexible transition layer with a thickness of 10 to 20 nm, so that the interface stress concentration is reduced by 30% to 40%, and the impact toughness at -196 ℃ is increased to ≥25 J / cm 2 If the In content is less than 0.1%, the transition layer is discontinuous; if the content is excessive, a brittle phase is formed, which deteriorates the high-temperature stability.

[0019] TiB2 particles, particle size 2-6 μm, content 2.0%-5.0%. The thermal expansion coefficient of the particles is 3.6×10 -6 / K, a larger compressive stress is generated at the interface through thermal mismatch with the matrix; the shear modulus (450 GPa) of the hexagonal structure is significantly different from that of the Mg matrix (45 GPa), which can effectively inhibit dislocation slip. Compared with traditional SiC particles, TiB2 has no interface reaction with the Mg matrix (avoiding the formation of brittle Mg2Si phase), and when the addition amount is 2.0% to 5.0%, the tensile strength of the alloy can be increased by 30 to 50 MPa.

[0020] La2(MoO4)3 particles, particle size 5-12 μm, content 6.0%-10.0%. The particles have a negative thermal expansion characteristic (expansion coefficient -1.5×10 -6 / K) in the range of 20-250 ℃, and the La-O-Mo flexible skeleton can buffer temperature stress through bond angle adjustment. When compounded with TiB2 particles at a mass ratio of 2:1 to 3:1, the positive expansion of the matrix can be offset, so that the thermal expansion coefficient in the range of -50-250 ℃ fluctuates <±0.3×10 -6 / K. If the addition amount is less than 6.0%, the negative expansion compensation is insufficient; if the addition amount is more than 10.0%, the particles agglomerate, resulting in a decrease in the plasticity of the alloy.

[0021] The above-mentioned five-element low-expansion phase and the two kinds of particles form an "intermetallic compound-ceramic particle" synergistic system, which realizes the balance of low expansion (≤10×10 -6 / K) and high strength (≥400 MPa) in a wide temperature range through the "low-expansion dilution-interface constraint-stress buffering-three-dimensional network support" mechanism.

[0022] Further, the total content of the unavoidable impurities is ≤0.12wt%, wherein Fe≤0.003wt%, Ni≤0.0008wt%.

[0023] Further, the particle size of the TiB2 is 2-6 μm.

[0024] Further, the particle size of the La2(MoO4)3 is 5-12 μm.

[0025] Furthermore, based on volume percentage, the low-expansion magnesium alloy comprises the following low-expansion phases: Mg2Sn phase 9%~13% (e.g., any point value or range between any two of 9%, 10%, 11%, 12% or 13%), Mg2Ge phase 3%~6% (e.g., any point value or range between any two of 3%, 4%, 5% or 6%), (Mg,Zn,In) 12 Ce phase 4%~7% (e.g., it can be any point value of 4%, 5%, 6% or 7% or any range between any two), Mg7Zr phase 4%~7% (e.g., it can be any point value of 4%, 5%, 6% or 7% or any range between any two), and Mg3In phase 1%~3% (e.g., it can be any point value of 1%, 2% or 3% or any range between any two).

[0026] Furthermore, the coefficient of thermal expansion of the Mg2Sn phase is 5.8 × 10⁻⁶. -6 / K.

[0027] Furthermore, the coefficient of thermal expansion of the Mg2Ge phase is 4.7 × 10⁻⁶. -6 / K.

[0028] Furthermore, (Mg, Zn, In) 12 The thermal expansion coefficient of the Ce phase is 9.5 × 10⁻⁶. -6 / K.

[0029] Furthermore, the coefficient of thermal expansion of the Mg7Zr phase is 7.8 × 10⁻⁶. -6 / K.

[0030] Furthermore, the coefficient of thermal expansion of the Mg3In phase is 8.2 × 10⁻⁶. -6 / K.

[0031] Another aspect of the present invention relates to a method for preparing the aforementioned low-expansion magnesium alloy, comprising the following steps: (a) After mixing the molten Mg, Sn, Ge, Zn, In, Mg-Ce master alloy and Mg-Zr master alloy, the mixture is subjected to heat treatment to obtain a magnesium alloy melt; (b) The magnesium alloy melt is simultaneously subjected to microwave radiation, pulsed current, rotating magnetic field and ultrasonic vibration, and then cast and heated to obtain a semi-solid billet; (c) After adding TiB2 and La2(MoO4)3 to the semi-solid casting billet, the billet is stirred and pressurized to obtain the casting billet; (d) The billet is subjected to solution treatment, water quenching, first aging heat treatment, cooling and second aging heat treatment.

[0032] The method for preparing the low-expansion magnesium alloy described above utilizes the synergistic effect of each step, resulting in an alloy with a thermal expansion coefficient ≤10×10⁻⁶ in the temperature range of -50 to 250°C. -6 / K, tensile strength ≥400MPa, impact toughness at -196℃ ≥25J / cm 2 To meet the requirements for service in extreme environments.

[0033] Furthermore, the purity of Mg is ≥99.95%, and the purity of Sn is ≥99.9%.

[0034] Furthermore, Ce is provided in the form of a Mg-Ce master alloy, in which the mass fraction of Ce is 20% to 30%.

[0035] Furthermore, Zr is provided in the form of a Mg-Zr master alloy, in which the mass fraction of Zr is 40% to 50%.

[0036] Furthermore, under a protective atmosphere of Ar gas and C2F6 mixed (volume ratio 92:8, flow rate 1.0~1.5L / min), Mg was melted at 650~730℃ by microwave heating at 2.45GHz (power 3~5kW).

[0037] In some specific embodiments, Mg ingots are placed at the bottom of a corundum crucible, and other alloying elements are layered according to their melting points from high to low, with Ge and Zr in the upper layer, and Sn, Zn, In, and Ce in the middle layer. The crucible is then placed in a microwave melting furnace and evacuated to a vacuum of ≤5×10⁻⁶. -3 Pa, introduce Ar gas with a purity ≥99.99% to 0.1~0.12MPa to form a protective atmosphere.

[0038] In some specific implementations, a 2.45 GHz microwave generator is started, and the power is gradually increased from 1 kW to 3-5 kW at a heating rate of 5-8 °C / s. After the Mg ingot is completely melted, it is held at that temperature for 10-15 minutes to allow the alloying elements to dissolve fully. The mixture is then stirred for 3-5 minutes using a graphite stirrer at a speed of 300-500 r / min. Microwave heating is then stopped, and the melt is poured into a metal mold preheated to 200-250 °C to obtain a magnesium alloy melt.

[0039] Microwave heating ensures uniform heating within the melt, with a temperature gradient of <5℃ / s, preventing macroscopic agglomeration of high-density elements such as Sn and Ge due to gravitational segregation. Microwave energy intensifies the thermal motion of melt atoms, increasing the diffusion coefficient of alloying elements by 1.5 to 2 times, which can significantly improve the uniformity of Ce element distribution.

[0040] Furthermore, the temperature of the heat preservation treatment is 650~730℃ (for example, it can be any one of 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, 710℃, 720℃ or 730℃ or any range between two), and the time is 10~25min (for example, it can be any one of 10min, 13min, 15min, 18min, 20min, 23min or 25min or any range between two).

[0041] Furthermore, the heat preservation treatment employs microwave heating.

[0042] Furthermore, the power of the microwave radiation is 1~2kW (for example, it can be any one of 1kW, 1.2kW, 1.4kW, 1.6kW, 1.8kW or 2kW or any range between two), and it is continuously subjected to a 2.45GHz alternating electromagnetic field for 10~15min (for example, it can be any one of 10min, 11min, 12min, 13min, 14min or 15min or any range between two), the waveguide insertion depth is 15~30mm, and the grain surface energy is reduced by 15%~20%.

[0043] Furthermore, the current density of the pulse current is 20~30 A / mm. 2 (For example, it can be, but is not limited to, 20A / mm) 2 22A / mm 2 24A / mm 2 26A / mm 2 28A / mm 2 Or 30A / mm 2 The tungsten electrodes are inserted into the melt at a spacing of 50-80 mm. Atoms migrate directionally under the influence of the electric field at a migration rate of 8 × 10⁻⁶. The frequency is 40-70 Hz (e.g., any value of 40 Hz, 50 Hz, 60 Hz, or 70 Hz, or any range between two values), and the application is sustained for 8-12 minutes (e.g., any value of 8 min, 9 min, 10 min, 11 min, or 12 min, or any range between two values). -8 cm 2 / (V•s), utilizing the electromigration effect to further homogenize the composition.

[0044] Furthermore, the rotating magnetic field has a rotational speed of 300~500 r / min (e.g., any value or range between any two of 300 r / min, 350 r / min, 400 r / min, 450 r / min, or 500 r / min), and an intensity of 0.4~0.6 T (e.g., any value or range between any two of 0.4 T, 0.5 T, or 0.6 T), and is continuously applied for 10~15 min (e.g., any value or range between any two of 10 min, 11 min, 12 min, 13 min, 14 min, or 15 min). A rotating magnetic field device is installed outside the crucible, driving the melt to form a three-dimensional helical convection through the Lorentz force, thus avoiding edge overheating caused by the skin effect of the magnetic field.

[0045] Furthermore, the frequency of the ultrasonic vibration is 25~35kHz (for example, it can be any one of 25kHz, 27kHz, 29kHz, 31kHz, 33kHz or 35kHz or any range between any two), the power is 3~5kW (for example, it can be any one of 3kW, 4kW or 5kW or any range between any two), the frequency change rate is 100~800 times / second (for example, it can be any one of 100 times / second, 200 times / second, 300 times / second, 400 times / second, 500 times / second, 600 times / second, 700 times / second or 800 times / second or any range between any two), the titanium alloy ultrasonic radiation rod is inserted 40~60mm below the surface of the molten metal, and the action is continuous for 5~8min (for example, it can be any one of 5min, 6min, 7min or 8min or any range between any two). The cavitation effect is used to generate microjets with a velocity of 150~200m / s to break up the primary phase.

[0046] Microwave radiation reduces the critical nucleation work of grains (ΔG*=16πσ) through electromagnetic energy conversion. 3 / (3ΔGv 2(σ represents surface energy), promoting an increased nucleation rate; the electromigration effect of pulsed current causes high-potential elements (such as Ce and Zr) to migrate to low-potential regions, counteracting gravitational segregation and ensuring compositional homogeneity; the Lorentz force convection of the rotating magnetic field breaks the laminar flow state of the melt, making the temperature and concentration fields more uniform and avoiding the precipitation of coarse phases caused by excessive local composition; the cavitation effect of frequency-converted ultrasound (periodic bubble generation and rupture) generates instantaneous high pressure (100~500MPa) and microjets, breaking the dendrite arms of primary phases such as Mg7Zr, with a grain refinement effect 50% higher than that of single ultrasound. The synergistic effect of multiple fields ultimately refines the grain size to 15~30μm, with an average size of 0.5~2.0μm for the five-element low-expansion phase (1.0~1.8μm for the Mg7Zr phase), which is 50%~60% finer than traditional processes.

[0047] Furthermore, the casting speed is 100~140 mm / min, and the cooling water flow rate is 20~30 L / min.

[0048] Furthermore, the temperature of the heat treatment is 550~580℃ (for example, it can be any one of the values ​​or any range between the two, but not limited to the values ​​in the above), and the time is 20~30min (for example, it can be any one of the values ​​in the above, but not limited to the values ​​in the above).

[0049] Furthermore, the solid fraction of the semi-solid casting is 65% to 75%.

[0050] Furthermore, TiB2 and La2(MoO4)3 were surface-treated with a titanate coupling agent at a mass concentration of 2.0% to 3.0%.

[0051] Further, the surface treatment includes: mixing TiB2 particles and La2(MoO4)3 particles at a mass ratio of 2:1 to 3:1, adding 2.0% to 3.0% by mass of isopropyl tristearate titanate, and treating in a stirred tank at 90 to 110°C at a speed of 600 to 800 r / min for 40 to 70 min, so that the hydroxyl conversion rate on the particle surface is ≥90%, and then drying for later use.

[0052] Furthermore, the surface hydroxyl conversion rate of TiB2 particles and La2(MoO4)3 particles is ≥90%.

[0053] Furthermore, the stirring speed is 500~700 r / min (for example, it can be any one of 500 r / min, 550 r / min, 600 r / min, 650 r / min or 700 r / min or any range between two), and the time is 15~20 min (for example, it can be any one of 15 min, 16 min, 17 min, 18 min, 19 min or 20 min or any range between two).

[0054] Furthermore, the pressurization process is a stepped pressurization; the stepped pressurization includes: increasing the pressure at a rate of 5~10MPa / s to 60~90MPa (for example, it can be any one of 60MPa, 70MPa, 80MPa or 90MPa or a range between any two), and holding the pressure for 15~25s (for example, it can be any one of 15s, 17s, 19s, 21s, 23s or 25s or a range between any two). The pressure is increased to 130-170 MPa at a rate of 10-15 MPa / s (e.g., any one of 130 MPa, 140 MPa, 150 MPa, 160 MPa, or 170 MPa, or a range between any two), held for 50-80 s (e.g., any one of 50 s, 60 s, 70 s, or 80 s, or a range between any two), and decreased to 100-120 MPa at a rate of 5-8 MPa / s (e.g., any one of 100 MPa, 105 MPa, 110 MPa, 115 MPa, or 120 MPa, or a range between any two), held for 10-15 s (e.g., any one of 10 s, 11 s, 12 s, 13 s, 14 s, or 15 s, or a range between any two). The low-pressure stage utilizes the thixotropic properties of the semi-solid melt (viscosity decreases under shear force) to promote uniform particle dispersion; the high-pressure stage ensures close contact between the particles and the matrix, and a metallurgical reaction occurs at the interface; the depressurization stage slowly releases residual stress.

[0055] The hydroxyl conversion reaction of the titanate coupling agent (condensation of -OH on the particle surface with the coupling agent alkoxy-OR) forms a covalent bond (-O-Ti-), reducing the polarity of the particle surface and decreasing the size of the agglomerates. In the low-pressure stage, the thixotropic properties of the semi-solid melt are utilized, and the shear force causes the particle agglomerates to deagglomerate, resulting in uniform dispersion. In the high-pressure stage, atomic diffusion promotes the reaction between the particle surface and the matrix: a Mg-Ti-O transition layer (3~6nm) is formed at the TiB2 interface, and a La-Mg-O transition layer is formed at the La2(MoO4)3 interface, improving the interfacial bonding strength. In the decompression stage, stress release reduces the residual stress to below 80MPa (below the yield strength of Mg alloy), avoiding cooling cracking, and ultimately achieving a porosity ≤0.1%.

[0056] Furthermore, the solution treatment temperature is 440~470℃ (for example, it can be any one of 440℃, 450℃, 460℃ or 470℃ or a range between any two), and the time is 7~12h (for example, it can be any one of the above or a range between any two). The time increases by 1h for every 10mm increase in thickness.

[0057] Furthermore, the cooling rate of the water quenching is ≥50℃ / s.

[0058] Furthermore, the temperature of the first aging heat treatment is 170~200℃ (for example, it can be any one of the values ​​or any range between the two, but not limited to), the time is 10~18h (for example, it can be any one of 10h, 12h, 14h, 16h or 18h or any range between the two), and the heating rate is 3~5℃ / min.

[0059] Furthermore, the temperature of the second aging heat treatment is 230~260℃ (for example, it can be any one of 230℃, 240℃, 250℃ or 260℃ or a range between any two), the time is 8~12h (for example, it can be any one of 8h, 9h, 10h, 11h or 12h or a range between any two), and the heating rate is 3~5℃ / min.

[0060] Solution treatment, through high-temperature diffusion, allows elements such as Sn, Ge, and Zn to fully dissolve in the Mg matrix, increasing the supersaturation to ≥15%, providing a driving force for subsequent precipitation. The temperature of 440℃ is close to the eutectic temperature of Mg (451℃), but lower than the solidus of the Mg-Zr alloy (460℃), thus avoiding overheating. Low-temperature aging matches the precipitation kinetics of high-activation-energy phases (Mg₂Ge 85kJ / mol, Mg₂Sn 75kJ / mol). At 170~200℃, the atomic diffusion rate is moderate, exhibiting the highest nucleation rate, with a grain size of approximately 0.5~1.0μm, avoiding coarsening at high temperatures. High-temperature aging targets low-activation-energy phases (Mg, Zn, In). 12 Ce (65kJ / mol), Mg7Zr (70kJ / mol), and Mg3In (55kJ / mol) are used to accelerate atomic migration at 230~260℃, enabling them to grow in the low-temperature interphase gaps and form a three-dimensional network, controlling the node spacing to 4~8μm; the Mg3In phase is enriched at the interface to form a 10~20nm transition layer, which can reduce the interface energy, alleviate stress concentration, and ensure the integrity of the network structure.

[0061] Furthermore, the grain size of the solidified melt structure in the semi-solid billet is 15~30μm, consisting of Mg2Sn phase, Mg2Ge phase, and (Mg,Zn,In) phase. 12 The Ce and Mg3In phases have sizes of 0.5–2.0 μm, while the Mg7Zr phase has sizes of 1.0–1.8 μm.

[0062] Furthermore, the first aging treatment and the second aging treatment cause the low-expansion phase to form a three-dimensional interwoven network structure with a network node spacing of 4~8μm, wherein the Mg3In phase is uniformly distributed at the interface of other phases, forming an interface transition layer with a thickness of 10~20nm.

[0063] Before feeding, each alloy raw material is polished to remove the surface oxide layer.

[0064] Another aspect of the present invention relates to the use of the aforementioned low-expansion magnesium alloy or the low-expansion magnesium alloy prepared by the method thereof in any of the following: (1) Preparation of chip packaging substrate; (2) Fabrication of automotive low-temperature precision sensors; (3) Prepare lightweight structural components for new energy vehicles.

[0065] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0066] Example 1 A magnesium alloy containing a pentagonal synergistic low-expansion phase comprises the following components by mass percentage: Sn 2.5%, Ge 0.8%, Zn 3.5%, Ce 0.2%, In 0.1%, Zr 0.8%, TiB2 particles 2.0% and La2(MoO4)3 particles 6.0%, with the balance being Mg and unavoidable impurities.

[0067] Preparation method: (1) Raw material preparation and microwave melting: Weigh the raw materials according to their composition, and heat them in a mixed atmosphere of Ar and C2F6 (92:8, 1.0L / min) with a 2.45GHz microwave (3kW) to melt pure Mg at 690℃. Add pure Sn, Ge, Zn, In and intermediate alloy, stir at 300r / min for 10min, and hold for 20min to obtain magnesium alloy melt; (2) Microwave-assisted multi-field processing: The magnesium alloy melt is transferred into the crystallizer and microwave radiation (power of 1kW, continuous for 10min, waveguide insertion depth of 15mm) and pulsed current (current density of 20A / mm) are applied. 2 The casting process involved a combination of three methods: a 40Hz frequency for 8 minutes, an alternating magnetic field (300 r / min rotation speed, 0.4 T intensity, 10 minutes), and ultrasonic vibration (25 kHz frequency, 3 kW power, frequency change rate of 100 times / second, 40 mm immersion depth in the melt for 5 minutes). The casting speed was 100 mm / min, the cooling water flow rate was 20 L / min, and the heat treatment temperature was 550℃ for 20 minutes, resulting in a semi-solid billet with a solid content of 75%. (3) Stepwise pressure composite: Add TiB2 particles (particle size of 2μm) and La2(MoO4)3 particles (particle size of 5μm) treated with titanate coupling agent, stir at 500r / min for 15min, and apply stepwise pressure: 60MPa×15s→130MPa×50s→100MPa×10s; (4) Dual temperature zone aging: after solution treatment at 440℃ for 7 hours, water quenching is performed, followed by aging at 170℃ for 10 hours → cooling to room temperature → aging at 230℃ for 8 hours → air cooling.

[0068] Performance testing: Coefficient of thermal expansion 10×10 -6 / K (-50~250℃), tensile strength 400MPa, thermal conductivity 150W / (m•K), impact toughness at -196℃ 25J / cm 2 .

[0069] Example 2 A magnesium alloy containing a pentagonal synergistic low-expansion phase comprises the following components by mass percentage: Sn 3.5%, Ge 1.4%, Zn 5.0%, Ce 0.4%, In 0.3%, Zr 1.4%, TiB2 particles 3.5% and La2(MoO4)3 particles 8.0%, with the balance being Mg and unavoidable impurities.

[0070] Preparation method: (1) Raw material preparation and microwave melting: 4kW microwave heating, 710℃ melting, 350r / min stirring for 15min, heat preservation for 22min, the rest is the same as in Example 1; (2) Microwave-assisted multi-field processing: The magnesium alloy melt is transferred into the crystallizer and microwave radiation (power of 1.5kW, continuous action for 13min, waveguide insertion depth of 25mm) and pulsed current (current density of 25A / mm) are applied. 2 The semi-solid billet was obtained by casting and heat treatment after being subjected to a combination of the following methods: a rotating magnetic field (400 r / min, 0.5 T intensity, alternating magnetic field, 12 min) and ultrasonic vibration (30 kHz frequency, 4 kW power, frequency change rate of 450 times / second, inserted 50 mm below the molten surface, 6 min). The casting speed was 120 mm / min, the cooling water flow rate was 25 L / min, and the heat treatment temperature was 570℃ for 20 min. (3) Stepwise pressure compounding: Add TiB2 particles and La2(MoO4)3 particles treated with titanate coupling agent, stir at 600r / min for 18min, stepwise pressure: 75MPa×20s→150MPa×65s→110MPa×12s; (4) Dual temperature zone aging: after solution treatment at 455℃ for 9 hours, water quenching is performed, followed by aging at 185℃ for 14 hours, cooling to room temperature, and aging at 245℃ for 10 hours.

[0071] Performance test: Coefficient of thermal expansion 8.5×10 -6 / K, tensile strength 430MPa, thermal conductivity 155W / (m•K), impact toughness at -196℃ 28J / cm 2 .

[0072] Example 3 A magnesium alloy containing a pentagonal synergistic low-expansion phase comprises the following components by mass percentage: Sn 4.5%, Ge 2.0%, Zn 6.5%, Ce 0.6%, In 0.5%, Zr 2.0%, TiB2 particles 5.0% and La2(MoO4)3 particles 10.0%, with the balance being Mg and unavoidable impurities.

[0073] Preparation method: (1) Raw material preparation and microwave melting: 5kW microwave, 730℃ melting, 400r / min stirring for 20min, heat preservation for 25min, the rest is the same as in Example 1; (2) Microwave-assisted multi-field processing: The magnesium alloy melt is transferred into the crystallizer and microwave radiation (power of 2kW, continuous for 15min, waveguide insertion depth of 30mm) and pulsed current (current density of 30A / mm) are applied. 2 The semi-solid billet was obtained by casting and heat treatment after being subjected to a combination of the following methods: a rotating magnetic field (500 r / min, 0.6 T intensity, alternating magnetic field, 15 min) and ultrasonic vibration (35 kHz frequency, 5 kW power, frequency change rate of 800 times / second, 60 mm below the molten surface, 8 min). The casting speed was 140 mm / min, the cooling water flow rate was 30 L / min, and the heat treatment temperature was 580℃ for 30 min. (3) Stepwise pressure compounding: Add TiB2 particles and La2(MoO4)3 particles treated with titanate coupling agent, stir at 700r / min for 20min, stepwise pressure: 90MPa×25s→170MPa×80s→120MPa×15s; (4) Dual temperature zone aging: after solution treatment at 470℃ for 12h, water quenching is performed, followed by aging at 200℃ for 18h → cooling to room temperature → aging at 260℃ for 12h.

[0074] Performance test: Coefficient of thermal expansion 7×10 -6 / K, tensile strength 460MPa, thermal conductivity 160W / (m•K), impact toughness at -196℃ 30J / cm 2 .

[0075] Comparative Example The Mg-Si-Ce alloy (6% Si, 0.8% Ce) using CN108486446A was prepared by conventional semi-continuous casting. Its properties include a thermal expansion coefficient of 18 × 10⁻⁶. -6 / K (-50~250℃), tensile strength 280MPa, impact toughness at -196℃ 5J / cm 2 The difference is significantly lower than that in the embodiments of the present invention.

[0076] Explanation of the results of the examples and comparative examples The composition and process parameters of Examples 1 to 3 exhibit gradient optimization. The systematic improvement in performance (reduction in thermal expansion coefficient, improvement in strength and toughness) stems from the dual effect of synergistic strengthening by the composition and matching of process parameters, as detailed below: (1) The contribution of component optimization to performance.

[0077] Increased content of low-expansion phases: In Example 3, the Sn (4.5%) and Ge (2.0%) contents were significantly higher than in Example 1 (Sn 2.5%, Ge 0.8%), resulting in an increase in the volume fraction of the Mg2Sn and Mg2Ge phases. According to the "dilution effect," for every 1% increase in the volume fraction of low-expansion phases, the overall thermal expansion coefficient of the alloy can be reduced by 1.2 × 10⁻⁶. -6 / K, therefore the coefficient of thermal expansion of Example 3 (7×10) -6 / K) compared to Example 1 (10×10 -6 / K) decreased by 30%.

[0078] Synergistic enhancement through particle blending: La2(MoO4)3 particles (negative expansion, -1.5×10⁻⁶) -6 The content of / K increased, which is related to the TiB2 particles (3.6×10). -6 The mass ratio of α-Mg to K is maintained at 2:1 to 3:1, forming a more effective "positive-negative expansion compensation network". Thermodynamic calculations show that this compound can offset the expansion of the matrix (α-Mg, thermal expansion coefficient 26 × 10⁻⁶). -6 The expansion amount of 60%~70% ( / K) significantly improves the expansion stability over a wide temperature range (-50~250℃).

[0079] Synergistic effect of rare earth elements and trace elements: Ce content increased from 0.2% to 0.6%, promoting the growth of (Mg, Zn, In). 12 The increased Ce phase volume fraction leads to increased grain boundary migration activation energy due to Ce atom pinning at grain boundaries, thus inhibiting high-temperature grain coarsening. The increase in In content from 0.1% to 0.5% increases the thickness of the Mg3In transition layer, reduces interfacial stress concentration, and improves the impact toughness at -196℃ from 25 J / cm². 2 Increased to 30J / cm 2 .

[0080] (2) The impact of process parameter optimization on performance.

[0081] Microwave melting strengthening: The microwave power was increased from 3kW (Example 1) to 5kW (Example 3), and the melt temperature was increased from 690℃ to 730℃. The diffusion ability of alloying elements was improved, the uniformity of Ce element distribution was increased, and the local expansion abnormality caused by segregation was avoided.

[0082] Multi-field synergistic refinement: The ultrasonic power increased from 3kW to 5kW, the cavitation microjets velocity increased from 150m / s to 200m / s, the effect of breaking up the primary phase was enhanced, and the grain size was refined from 30μm to 15μm; the rotating magnetic field strength increased from 0.4T to 0.6T, the melt convection intensity was increased by 50%, the uniformity of particle distribution was increased, and the intensity fluctuation caused by particle agglomeration was avoided.

[0083] Stepped pressurization and aging matching: The pressure in the high-pressure stage is increased from 130MPa to 170MPa, which increases the interfacial bonding strength, increases the thickness of the Mg-Ti-O transition layer, and makes the "metallurgical bond" between the particles and the matrix stronger; the dual-temperature aging time is extended (10h→18h low temperature, 8h→12h high temperature), which reduces the spacing between nodes of the five-element phase three-dimensional network, increases the resistance to dislocation slip, and increases the tensile strength from 400MPa to 460MPa.

[0084] The comparative example uses a Mg-Si-Ce alloy (6% Si, 0.8% Ce) from CN108486446A, whose properties (coefficient of thermal expansion 18 × 10⁻⁶) are described. -6 / K, tensile strength 280MPa, impact toughness 5J / cm 2 The results are significantly lower than those of the embodiments of this invention, with the core differences stemming from defects in the component system and outdated processes: (1) The inherent limitations of the component system.

[0085] The strengthening phase is singular and has a high coefficient of thermal expansion: the comparative example relies solely on the Mg2Si phase (coefficient of thermal expansion 11×10). -6 / K) strengthening, without Mg2Ge (4.7×10) -6 / K), La2(MoO4)3 (-1.5×10 -6 Low / negative expansion phases such as / K cannot reduce overall expansion through "multi-component synergy," therefore the coefficient of thermal expansion (18×10) remains low. -6 / K) is 2.6 times that of Example 3.

[0086] Lack of interfacial transition phase: The absence of an In-formed Mg3In flexible transition layer prevents the reduction of Si-Mg interfacial mismatch, resulting in significant interfacial stress concentration and an impact toughness of only 5 J / cm. 2 This is 1 / 6 of Example 3.

[0087] (2) Limited technological means.

[0088] Severe casting segregation: Traditional semi-continuous casting is used without microwave heating, and the temperature gradient reaches 25℃ / cm, while in this invention it is <5℃ / cm. The Si element macroscopically segregates, with a local concentration difference >3%, forming coarse blocky Mg2Si phase (size >10μm), which becomes the source of crack initiation. The tensile strength is only 280MPa, which is 61% of that in Example 3.

[0089] Without multi-field refinement and pressure composite: without the use of ultrasonic, magnetic field and other refinement processes, the grain size reaches 100μm (15μm in this invention), the dislocation movement resistance is low; the particles are unevenly dispersed, the interfacial bonding strength is low, and it is impossible to form effective reinforcement.

[0090] Conclusion: The comparative examples, lacking a multi-component low-expansion phase system and improved processing methods, struggle to overcome the limitations of traditional magnesium alloys characterized by "high expansion, low strength, and embrittlement." The performance gradient improvements in Examples 1 to 3 validate the scientific validity of the "five-component low-expansion phase synergy + dual-particle compounding + multi-process coupling" technical solution. In terms of composition, this invention achieves dual suppression of "dilution-compensation" through a gradient increase in low / negative expansion phases; in terms of processing, it achieves synergistic effects of microstructure refinement, compositional homogenization, and interface strengthening through parameter optimization of microwave, multi-field, pressurization, and aging. The comparison between the two examples fully demonstrates the breakthrough of this invention in compositional design and process innovation.

[0091] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A low-expansion magnesium alloy, characterized in that, It includes the following components by mass percentage: Sn 2.5%~4.5%, Ge 0.8%~2.0%, Zn 3.5%~6.5%, Ce 0.2%~0.6%, In 0.1%~0.5%, Zr 0.8%~2.0%, TiB2 2.0%~5.0% and La2(MoO4)3 6.0%~10.0%, with the balance being Mg and impurities.

2. The low-expansion magnesium alloy according to claim 1, characterized in that, The low-expansion magnesium alloy comprises the following low-expansion phases by volume percentage: Mg2Sn phase 9%~13%, Mg2Ge phase 3%~6%, (Mg,Zn,In) 12 Ce phase 4%~7%, Mg7Zr phase 4%~7% and Mg3In phase 1%~3%.

3. The method for preparing the low-expansion magnesium alloy as described in claim 1 or 2, characterized in that, Includes the following steps: (a) After mixing the molten Mg, Sn, Ge, Zn, In, Mg-Ce master alloy and Mg-Zr master alloy, the mixture is subjected to heat treatment to obtain a magnesium alloy melt; (b) The magnesium alloy melt is simultaneously subjected to microwave radiation, pulsed current, rotating magnetic field and ultrasonic vibration, and then cast and heated to obtain a semi-solid billet; (c) After adding TiB2 and La2(MoO4)3 to the semi-solid casting billet, the billet is stirred and pressurized to obtain the casting billet; (d) The billet is subjected to solution treatment, water quenching, first aging heat treatment, cooling and second aging heat treatment.

4. The method for preparing low-expansion magnesium alloy according to claim 3, characterized in that, Includes at least one of the following technical features: (1) The power of the microwave radiation is 1~2kW, and the effect lasts for 10~15min; (2) The current density of the pulse current is 20~30A / mm 2 The frequency is 40~70Hz, and the effect lasts for 8~12 minutes; (3) The rotational speed of the rotating magnetic field is 300~500 r / min, the intensity is 0.4~0.6T, and the continuous action lasts for 10~15 min; (4) The frequency of the ultrasonic vibration is 25~35kHz, the power is 3~5kW, the frequency change rate is 100~800 times / second, and the duration is 5~8min.

5. The method for preparing the low-expansion magnesium alloy according to claim 3, characterized in that, The pressurization process is a stepped pressurization; the stepped pressurization includes: holding pressure at 60~90MPa for 15~25s, holding pressure at 130~170MPa for 50~80s, and holding pressure at 100~120MPa for 10~15s.

6. The method for preparing the low-expansion magnesium alloy according to claim 3, characterized in that, The temperature of the first aging heat treatment is 170~200℃, and the time is 10~18h; And / or, the temperature of the second aging heat treatment is 230~260℃, and the time is 8~12h.

7. The method for preparing the low-expansion magnesium alloy according to claim 3, characterized in that, The heat preservation treatment is performed at a temperature of 650~730℃ for a time of 10~25 minutes.

8. The method for preparing the low-expansion magnesium alloy according to claim 3, characterized in that, The heat treatment is performed at a temperature of 550~580℃ for 20~30 minutes.

9. The method for preparing low-expansion magnesium alloy according to claim 3, characterized in that, The solution treatment is performed at a temperature of 440~470℃ for 7~12 hours.

10. The use of the low-expansion magnesium alloy prepared by the method of preparation of the low-expansion magnesium alloy according to claim 1 or 2 or any one of claims 3 to 9 in any of the following: (1) Preparation of chip packaging substrate; (2) Fabrication of automotive low-temperature precision sensors; (3) Prepare lightweight structural components for new energy vehicles.