Method for preparing B2 phase and nano oxide co-reinforced ferrite-based alloy by utilizing stress-induced martensite phase transformation

Abstract

The invention discloses a method for preparing a B2 phase and nano oxide co-reinforced ferrite-based alloy through stress-induced martensite phase transformation, and belongs to the technical field of high-performance high-temperature alloy materials. The method comprises the following steps: preparing a master alloy ingot by adopting a vacuum induction melting and electroslag remelting duplex process, carrying out gas atomization powdering, high-temperature austenitizing treatment, mechanical alloying and hot isostatic pressing forming, then applying plastic deformation at a relatively low temperature to induce austenite to be converted into martensite, and combining with subsequent aging treatment to obtain the high-strength and high-toughness alloy. And synergistic precipitation and distribution of the B2 phase and the nano oxide are realized. The volume fraction of the B2 phase in the prepared alloy exceeds 30%, and the alloy has excellent high-temperature mechanical properties and thermal stability. The process path of the method can be industrially amplified, the heat treatment time is remarkably shortened, and the method is suitable for manufacturing advanced energy equipment and high-temperature structural parts.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-performance iron-based alloy preparation, and particularly provides a method for preparing a B2 phase and nano-oxide co-strengthened ferritic matrix alloy by using stress-induced martensitic transformation. Background Art

[0002] Oxide Dispersion Strengthened (ODS) ferritic matrix alloys have become important candidate structural materials in harsh service environments such as nuclear energy, aerospace, and transportation due to their low density, low thermal expansion coefficient, high thermal conductivity, and excellent high-temperature mechanical properties. The core strengthening mechanism of such materials comes from the uniform dispersion of nano-oxide particles in the matrix. Especially when the oxide particle size is fine, the distribution is dense, and the stability is good, it can significantly inhibit creep deformation and tissue coarsening at high temperatures, thereby improving the comprehensive service performance of the material. However, the strengthening effect brought by solely relying on nano-oxides still has certain limitations. On the one hand, due to the design requirements of material toughness and damage tolerance, the oxide addition amount is usually limited to within 2 wt.%. On the other hand, when the oxide content is too high, particle agglomeration is likely to occur, which instead weakens the overall strengthening effect and even increases the brittleness of the material at room temperature.

[0003] To break through the bottleneck of the strengthening ability of traditional ODS alloys, researchers have proposed introducing an ordered B2-NiAl (β′) precipitate phase in the ferritic matrix as a second strengthening mechanism. The B2 phase has an ordered body-centered cubic (BCC) structure and has good thermal stability and strengthening potential. By precisely controlling the alloy composition and heat treatment process, a large number of B2 precipitates coherent with the matrix can be introduced in the ferritic matrix to form a microstructure system similar to the γ / γ′ phase structure in nickel-based superalloys, effectively enhancing the high-temperature mechanical properties of the material. On this basis, by introducing Co elements, the thermodynamic stability and precipitate volume fraction of the B2 phase can be further optimized. Co can partially replace Ni atoms in the B2 structure, increase the precipitate phase concentration, and at the same time reduce the interface formation energy between the precipitate and the matrix, thereby enhancing the interface bonding strength and thermal stability. In addition, the regulation of ferromagnetic properties by Co elements also helps to slow down the diffusion of alloy elements and improve the high-temperature creep resistance of the material.

[0004] Although the introduction of the B2 phase significantly improves the alloy properties, there are still certain technical difficulties in the synergistic strengthening with nano-oxides. Since the precipitation temperature of nano-oxides is generally higher than that of the B2 phase, during the traditional aging process, the oxide particles often become the heterogeneous nucleation cores of the B2 phase, resulting in uneven precipitate particle size distribution and limited nucleation mechanism, thus weakening the strengthening effect. A feasible strategy is to transform the matrix into a high-density dislocation martensite structure through solution treatment before aging. The dislocation martensite is rich in various defect structures, which can significantly promote the precipitation of the B2 phase at multiple defect sites, thereby weakening the dominant role of oxide particles in the nucleation of the B2 phase. However, under the design requirement of a high volume fraction of the B2 phase, more alloying elements usually need to be added, and these elements often significantly inhibit the martensitic transformation. Especially in large-sized specimens, it is extremely difficult to obtain a dislocation martensite structure with uniform microstructure, limiting the application scope of this method.

[0005] Therefore, the present invention introduces the Stress-Induced Martensitic Transformation (SIMT) mechanism. This mechanism is a rapid phase transformation path dominated by shear and without diffusion. By reasonably designing the alloy composition and reducing the stacking fault energy of austenite, the austenite (γ) can be rapidly transformed into martensite (α′) by applying stress during the deformation process. It should be noted that due to the extremely low content of interstitial elements C and N in the alloy used in the present invention, the transformed martensite has a body-centered cubic structure and has good adaptability for subsequent processing. In the subsequent heat treatment stage, the high-density defects enriched in the α′ dislocation martensite significantly accelerate the nucleation and precipitation rate of the B2 phase, shorten the aging cycle of the alloy strengthening process, and improve the overall preparation efficiency. Based on the above mechanism, the inventor proposes a new process method that combines the SIMT mechanism, alloy composition regulation, and phase transformation kinetics control to achieve the synergistic strengthening of the B2 phase and nano-oxides, and promote the rapid and efficient preparation of high-performance ODS ferritic matrix alloys. Summary of the Invention

[0006] The present invention discloses a method for preparing a B2 phase and nano-oxide co-strengthened ferritic matrix alloy by stress-induced martensitic transformation to solve any of the above technical problems and other potential problems in the prior art. The method specifically includes the following steps: a. Select high-purity raw materials with a purity of not less than 99.9 wt.%, and obtain a ferritic matrix alloy master ingot by a double process of vacuum induction melting + electroslag remelting; b. Atomize the ferritic matrix alloy master ingot obtained in step a to produce powder; c. Screen the powder obtained in step b, and select powders within a specific particle size range for standby; d. Perform high-temperature heat treatment on the screened powder, and adopt furnace cooling to mainly transform the powder matrix into austenite structure; e. Mix the powder processed in step d with a nano-oxide source and then perform mechanical alloying; f. Perform hot isostatic pressing (HIP) on the mechanically alloyed powder to obtain a dense bulk material; g. Perform plastic deformation treatment such as rolling, stretching or drawing on the hot isostatically pressed sample at a relatively low temperature, and transform austenite into martensite through stress-induced martensitic transformation; h. Perform aging treatment on the deformed alloy to finally obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

[0007] In a specific embodiment, in step a, the composition of the ferritic matrix master ingot is Fe-(10-14 wt.%)Cr-(1-4.5 wt.%)Al-(15-30 wt.%)Co-(13-16 wt.%)Ni-(1-2 wt.%)Mo-(1-2 wt.%)W-(0.1-0.2 wt.%)Zr-(0.002-0.02 wt.%)B;

[0008] In a specific embodiment, in step b, the medium for atomizing the powder is argon;

[0009] In a specific embodiment, in step c, the particle size of the screened powder is selected to be 15-53 μm;

[0010] In a specific embodiment, in step d, the temperature of the high-temperature treatment is 1100-1250 °C, the time is 0.5-1 hour, the protective atmosphere is hydrogen, and the cooling method is furnace cooling;

[0011] In a specific embodiment, in step e, the nano-oxide source is Y2O3 particles with an average particle size of 50 nm;

[0012] In a specific embodiment, in step e, the addition amount of the nano Y2O3 particles is 0.3%-0.6% (mass percentage) of the final total amount;

[0013] In a specific embodiment, in step f, the temperature for hot isostatic pressing is 1100 - 1200 °C, the time is 1 - 3 hours, it is cooled with the furnace above 1000 °C, and air cooling is carried out at 1000 - 400 °C, and the air cooling rate is 60 °C / minute, finally ensuring that no B2 precipitate phase precipitates in the hot isostatic pressing sample, and more than 98% of the matrix is austenite;

[0014] In a specific embodiment, in step g, the temperature for deformation is from room temperature to 200 °C, preferably from room temperature to 150 °C, and the relative deformation amount is 2% - 8%;

[0015] In a specific embodiment, in step h, the temperature for aging is 500 - 700 °C, the time is 1 - 8 hours, preferably the time is 2 - 6 hours.

[0016] The method proposed by the present invention effectively realizes the controllable transformation of the austenite matrix into martensite for the first time by introducing a low-temperature plastic deformation process after hot isostatic pressing and utilizing the stress-induced martensite phase transformation mechanism, without relying on traditional rapid cooling or complex heat treatment processes, and provides favorable phase interface conditions for the subsequent directional precipitation of the B2 phase. At the same time, combined with the nano-oxides (such as YO) introduced during the mechanical alloying process, the co-precipitation and spatial distribution regulation of the B2 ordered phase and nano-oxide particles are realized during the subsequent aging treatment, avoiding the problem of mutual interference between strengthening phases in traditional ODS alloys.

[0017] The innovation of the present invention is not only reflected in the process path design of significantly improving the B2 phase precipitation rate and shortening the heat treatment time through stress-induced martensite phase transformation, but also in the collaborative design of the overall thermodynamic stability of the alloy composition system and the phase transformation / precipitation kinetics, ensuring the synergistic effect of each strengthening mechanism in the entire process flow and the controllability of the structure evolution path. Through the coupled regulation of the above-mentioned microstructure and strengthening mechanisms, the present invention significantly improves the high-temperature strength, thermal stability and creep resistance of the material, and is particularly suitable for the manufacturing of complex structural parts with high requirements for high-temperature strength and toughness and tissue stability in advanced energy equipment. This method has the advantages of a wide heat treatment window, strong processing adaptability, high synergistic effect of strengthening mechanisms, etc., showing good engineering application prospects and industrial promotion value.

[0018] Advantages of the present invention:

[0019] 1. Significantly improve mechanical properties and thermal stability: Promote the rapid precipitation of the B2 phase through stress-induced martensite phase transformation, and combine with the dispersion strengthening effect of nano-oxides to achieve the synergistic improvement of high strength, high plasticity and high-temperature thermal stability. This alloy exhibits excellent comprehensive mechanical properties at both room temperature and high temperature, meeting the performance requirements of advanced energy equipment and high-temperature service structures.

[0020] 2. Achieving the stable precipitation of a high volume fraction of B2 phase: Through the control of the thermodynamic stability region and the regulation of stress-induced phase transformation in the present invention, the volume fraction of the B2 ordered phase can be stably reached above 30%. Although the size of the B2 phase is in the order of hundreds of nanometers, it is uniformly distributed in the microstructure with clear interfaces. The precipitation phase and the matrix have a significant synergistic effect, fully exerting its strengthening potential while maintaining the balance between the plasticity and thermal stability of the alloy.

[0021] 3. Synergistic optimization design of strengthening mechanisms: The present invention innovatively constructs a dual strengthening mechanism of B2 ordered phase and nano-oxides. The B2 phase itself has good high-temperature mechanical properties and creep resistance, while the nano-YO oxides are dispersed in the matrix, effectively hindering dislocation movement and grain boundary slip, and synergistically enhancing the matrix strength and microstructure stability of the material, providing a reliable guarantee for high-temperature service.

[0022] 4. Clear process route with good potential for industrial scale-up: The alloy preparation process adopted in the present invention, including vacuum induction melting-electroslag remelting, gas atomization powder making, mechanical alloying, hot isostatic pressing, and stress-induced deformation, etc., are all mature and controllable conventional processes in the existing powder metallurgy and hot processing industries, with strong replicability and industrial application basis, facilitating large-scale popularization and application in the manufacture of large-sized components.

[0023] 5. High heat treatment efficiency, shortening the cycle and reducing energy consumption: By regulating the synergistic mechanism of martensitic phase transformation behavior and B2 phase precipitation kinetics, the present invention significantly improves the precipitation rate and effectively shortens the long-time aging treatment time required for traditional strengthened alloys. The optimized aging process significantly reduces the heat treatment cost and energy consumption, improves the preparation efficiency and the overall process robustness. Brief Description of the Drawings

[0024] Figure 1 It is the process flow chart of the present invention.

[0025] Figure 2 It is the SEM morphology schematic diagram of the B2 precipitation phase in the typical alloy prepared by the present invention.

[0026] Figure 3 It is the room temperature tensile stress-strain curve of the typical alloy prepared by the present invention, with a yield strength of 1109 MPa, a tensile strength of 1705 MPa, and an elongation before fracture of 16%. Detailed Description of the Invention

[0027] Example 1:

[0028] In this embodiment, high-purity raw materials with a purity of 99.95 wt.% are first selected, and a ferritic matrix alloy master alloy ingot is prepared by a double process of vacuum induction melting and electroslag remelting. The alloy composition is Fe-12 wt.% Cr-3 wt.% Al-18 wt.% Co-14 wt.% Ni-1 wt.% Mo-0.15 wt.% W-0.01 wt.% Zr-0.015 wt.% B. The alloy ingot is atomized into powder under an argon protection atmosphere by gas atomization method. The obtained powder is screened, and the powder with a particle size of 15-53 μm is selected for standby. Subsequently, the powder is heat-treated at 1100 °C for 1 hour under a hydrogen protection atmosphere and cooled in the furnace to ensure that the matrix phase of the powder is austenite. Then, the powder is mixed with Y2O3 particles with an average particle size of 50 nm at a mass ratio of 0.5% and subjected to mechanical alloying treatment. The mechanically alloyed powder is hot isostatically pressed at a temperature of 1150 °C for 2 hours, cooled in the furnace to 1000 °C, and then air-cooled at an air-cooling rate of 60 °C / minute. Finally, after 3% relative deformation at room temperature, the alloy is transformed into martensite through stress-induced martensitic transformation. Finally, the alloy is aged at 600 °C for 3 hours to obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

[0029] Example 2:

[0030] In this embodiment, high-purity raw materials with a purity of 99.98 wt.% are selected, and a ferritic matrix alloy master alloy ingot is prepared by a double process of vacuum induction melting and electroslag remelting. The alloy composition is Fe-13 wt.% Cr-2 wt.% Al-19 wt.% Co-15 wt.% Ni-1.5 wt.% Mo-0.12 wt.% W-0.01 wt.% Zr-0.02 wt.% B. The alloy ingot is atomized into powder under an argon protection atmosphere by gas atomization method. The obtained powder is screened, and the powder with a particle size of 15-53 μm is selected for standby. The powder is heat-treated at 1200 °C for 0.5 hour under a hydrogen protection atmosphere and cooled in the furnace. Then, the powder is mechanically alloyed with Y2O3 particles with an average particle size of 40 nm at a mass ratio of 0.4%. The mechanically alloyed powder is hot isostatically pressed at a temperature of 1100 °C for 2 hours, cooled in the furnace to 1000 °C, and then air-cooled at an air-cooling rate of 60 °C / minute to ensure that no B2 precipitate phase precipitates in the sample and more than 98% of the matrix is austenite. Then, the sample is subjected to tensile deformation in the range of room temperature to 150 °C with a deformation amount of 6%, and is transformed into martensite through stress-induced martensitic transformation. Finally, it is aged at 550 °C for 5 hours to obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

[0031] Example 3:

[0032] In this embodiment, high-purity raw materials with a purity of 99.9 wt.% are selected, and a ferritic matrix alloy master alloy ingot is prepared by a combined process of vacuum induction melting and electroslag remelting. The alloy composition is Fe-11 wt.% Cr-2.5 wt.% Al-16 wt.% Co-13 wt.% Ni-1 wt.% Mo-0.15 wt.% W-0.01 wt.% Zr-0.015 wt.% B. The alloy ingot is atomized into powder under an argon protection atmosphere by gas atomization. The obtained powder is screened, and the powder with a particle size of 15 - 53 μm is selected for standby. The powder is heat-treated at 1150 °C for 1 hour under a hydrogen protection atmosphere and cooled in the furnace. Then, the powder and Y2O3 particles with an average particle size of 50 nm are mechanically alloyed at a mass ratio of 0.5%. The mechanically alloyed powder is subjected to hot isostatic pressing at a temperature of 1125 °C for 2 hours, cooled in the furnace to 1000 °C, and then air-cooled at a rate of 60 °C / minute to ensure that no B2 precipitate phase precipitates in the hot isostatic pressing sample and more than 98% of the matrix is austenite. Then, the sample is subjected to drawing deformation in the range from room temperature to 150 °C with a deformation amount of 5%, and is transformed into martensite through stress-induced martensite transformation. Finally, the sample is subjected to aging treatment at 600 °C for 3 hours to finally obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

[0033] Example 4:

[0034] In this embodiment, high-purity raw materials with a purity of 99.95 wt.% are selected, and a ferritic matrix alloy master alloy ingot is prepared by a combined process of vacuum induction melting and electroslag remelting. The alloy composition is Fe-12 wt.% Cr-2 wt.% Al-17 wt.% Co-14 wt.% Ni-1.5 wt.% Mo-0.1 wt.% W-0.02 wt.% Zr-0.015 wt.% B. The alloy ingot is prepared into powder by gas atomization under an argon protection atmosphere. The obtained powder is screened, and the powder with a particle size of 15 - 53 μm is selected for standby. The powder is heat-treated at 1200 °C for 1 hour under a hydrogen protection atmosphere and cooled in the furnace. Then, the powder and Y2O3 particles with an average particle size of 50 nm are mechanically alloyed at a mass ratio of 0.4%. The mechanically alloyed powder is subjected to hot isostatic pressing at a temperature of 1100 °C for 2 hours, cooled in the furnace to 1000 °C, and air-cooled at a rate of 60 °C / minute during the period from 1000 - 400 °C to ensure that no B2 precipitate phase precipitates in the hot isostatic pressing sample and more than 98% of the matrix is austenite. Then, the sample is subjected to tensile deformation in the range from room temperature to 150 °C with a deformation amount of 4%, and is transformed into martensite through stress-induced martensite transformation. Finally, the sample is subjected to aging treatment at 650 °C for 3 hours to finally obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

Claims

1. A method for preparing a B2 phase and nano-oxide co-strengthened ferritic matrix alloy by using stress-induced martensitic transformation, characterized in that, The method includes the following steps: a Select high-purity raw materials with a purity of not less than 99.9 wt.%, and prepare a ferritic matrix alloy master alloy ingot by a double process of vacuum induction melting and electroslag remelting; b Atomize the master alloy ingot by gas to obtain powder; c Screen the powder and select powder with a certain particle size for standby; d Perform high-temperature treatment on the screened powder and cool it in the furnace to ensure that the matrix phase of the powder is mainly austenite; e Mix the powder treated in step d with a nano-oxide source and perform mechanical alloying; f Hot isostatically press the mechanically alloyed powder obtained in step e; g Roll, stretch or draw and deform the sample after hot isostatic pressing at a lower temperature, and convert the austenite matrix into martensite through stress-induced martensitic transformation; h Perform aging treatment on the deformed alloy to finally obtain a B2 phase and nano-oxide co-strengthened ferritic matrix alloy.

2. The method according to claim 1, wherein In step a, the composition of the ferritic matrix master alloy ingot is Fe-(10-14wt.%)Cr-(1-4.5 wt.%)Al-(15-30 wt.%)Co-(13-16 wt.%)Ni-(1-2 wt.%)Mo-(1-2 wt.%)W-(0.1-0.2 wt.%)Zr-(0.002-0.02 wt.%)B.

3. The method according to claim 1, wherein In step d, the temperature of the high-temperature treatment is 1100-1250°C, the time is 0.5-1 hour, the protective atmosphere is hydrogen, and the cooling method is furnace cooling.

4. The method according to claim 1, wherein In step e, the nano-oxide source is Y2O3 particles with an average particle size of 50 nm, and the addition amount is 0.3%-0.6% of the total amount of the final alloy.

5. The method according to claim 1, wherein In step f, the temperature of the hot isostatic pressing is 1100-1200°C, the time is 1-3 hours, the cooling method is furnace cooling above 1000°C, air cooling is performed in the range of 1000-400°C, and the air cooling rate is 60°C / minute. Finally, ensure that no B2 precipitate phase precipitates in the hot isostatic pressing sample and more than 98% of the matrix is austenite.

6. The method according to claim 1, wherein In step g, the deformation temperature is from room temperature to 200°C, preferably from room temperature to 150°C, and the relative deformation amount is 2%-8%.

7. The method according to claim 1, wherein In step h, the aging temperature is 500-700°C, and the aging time is 1-8 hours, preferably 2-6 hours.

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