A method for controlling heat treatment of high-temperature alloys to form near-core-shell structure silicides and nano-precipitates

Abstract

A high-temperature alloy heat treatment control method for forming a near-core-shell structure silicide and a nano-precipitate phase, the present invention relates to a high-temperature alloy heat treatment control method for forming a near-core-shell structure silicide and a nano-precipitate phase, the purpose of the present invention is to solve the problem that the existing heat treatment method of Nb-Si alloy cannot improve toughness and strength at the same time, the present invention combines cryogenic treatment with short-time heat treatment, controls the distribution of silicide and modifies the two-phase interface, and simultaneously improves toughness and strength through core-shell structure toughening and two-phase nano-precipitation strengthening, enriches the organizational control means of Nb-Si alloy, and reduces cost. The present invention is applied to the field of alloy heat treatment.

Description

Technical Field

[0001] The invention relates to a heat treatment control method for forming a high-temperature alloy with a near-core-shell structure silicide and a nano-precipitation phase. Background Art

[0002] Thanks to the incorporation of silicide reinforcements into a tough matrix, Nb-Si-Ti in-situ composites are considered as alternative materials to Ni-based alloys for aerospace applications due to their excellent high-temperature performance. Current research focuses on the effects of alloying elements and preparation methods on the microstructure and properties of alloys, with optimizing and controlling the two-phase structure being a key issue in alloy preparation. Although silicides (Nb5Si3 and / or Nb3Si) can alleviate the inherent brittleness of niobium-based solid solution (Nbss) by externally toughening it, thereby improving its damage tolerance, the plasticity of Nbss is conversely limited and degraded. The matching state of the two phases (morphology, proportion, and interfacial interaction) cannot fully compensate for each other's inherent defects, resulting in a strength-toughness mismatch.

[0003] Solid-state phase transformations via heat treatment are widely used to engineer metal alloy microstructures with desired properties. Heat-treated Nb-Si alloys exhibit more uniform composition and improved high-temperature stability compared to cast alloys, making them more suitable for service in high-temperature environments. Heat treatments for Nb-Si alloys typically involve long-term, high-temperature homogenization (>1400°C, >50 hours), but this stabilizes the microstructure and improves toughness at the expense of strength and is costly. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem that the existing heat treatment method of Nb-Si alloy cannot improve toughness and strength at the same time, and to provide a high-temperature alloy heat treatment control method for forming near-core-shell structure silicide and nano-precipitation phase.

[0005] The present invention provides a method for controlling heat treatment of a high-temperature alloy to form a near-core-shell structure silicide and a nano-precipitated phase, which is carried out by the following steps:

[0006] 1. Preparation of NbTiSiAlVFe alloy;

[0007] 2. Place the NbTiSiAlVFe alloy sample in a heat treatment furnace at 500℃~1000℃ and keep it warm with air as the insulation medium;

[0008] 3. Place the NbTiSiAlVFe alloy sample after the heat preservation treatment in step 2 into liquid nitrogen for deep cold quenching, and then take it out and return it to room temperature;

[0009] 4. Repeat steps 2 and 3 to perform a second deep cold quenching operation on the NbTiSiAlVFe alloy sample;

[0010] 5. Place the NbTiSiAlVFe alloy sample after deep cold quenching in a heat treatment furnace at 1200℃~1450℃ for 1h~24h, and then cool it to room temperature to complete.

[0011] Unlike conventional high-temperature homogenization heat treatment, deep cold quenching has greater thermal shock energy, which can leave greater quenching stresses in the sample, especially internal stresses at the phase interfaces. The high-temperature annealing in step five is intended to eliminate quenching stresses while inducing complex phase transformations under high temperature conditions. The precipitation of Nb4FeSi around the Nb5Si3 grain boundaries leads to the formation of a near-core-shell structure. The precipitation of silicides in the Nb-based solid solution reduces the solid solution content of Si, thereby purifying the Nb-based solid solution.

[0012] The present invention has the following beneficial effects:

[0013] 1. The present invention regulates the proportion, morphology and distribution of silicide phases (Nb5Si3 and Nb4FeSi) through a heat treatment process to prepare a special Nb5Si3 / Nb4FeSi silicide near-core-shell structure. At the same time, nano-precipitated phases are obtained within the niobium-based solid solution and the silicide, thereby improving the comprehensive mechanical properties of the alloy.

[0014] Second, the present invention promotes the precipitation of silicides within the Nb-based solid solution through the combined effects of cryogenic quenching and high-temperature annealing, reducing the Si content in the Nb-based solid solution, purifying the Nb-based solid solution and improving the room-temperature toughness of the alloy. This method is applicable to the purification of Nb-based solid solutions in various Nb-Si-based alloy systems.

[0015] 3. The heat / cold treatment process of the present invention is carried out in a common heat treatment furnace and liquid nitrogen. The required equipment is simple, the operation is simple, the cost is low, and it is applicable to various systems of Nb-Si based alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The as-cast structure and conventional heat-treated structure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in Examples 1 to 3;

[0017] Figure 2 This is the microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after secondary 1000°C deep cold quenching in Example 1;

[0018] Figure 3 The microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in Example 1 after 1000°C deep cold quenching and 1200°C annealing;

[0019] Figure 4This is the EBSD pattern of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Example 1;

[0020] Figure 5 This is the microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in Example 2 after 600°C deep cold quenching and 1200°C annealing;

[0021] Figure 6 The room temperature fracture toughness and compression properties of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Examples 1 to 3;

[0022] Figure 7 This is the high-temperature performance of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Example 1. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is not limited to the specific implementation methods listed below, but also includes any combination of the specific implementation methods.

[0024] Specific embodiment 1: This embodiment is a method for controlling heat treatment of a high-temperature alloy to form a near-core-shell structure silicide and a nano-precipitate phase, which is carried out in the following steps:

[0025] 1. Preparation of NbTiSiAlVFe alloy;

[0026] 2. Place the NbTiSiAlVFe alloy sample in a heat treatment furnace at 500℃~1000℃ and keep it warm with air as the insulation medium;

[0027] 3. Place the NbTiSiAlVFe alloy sample after the heat preservation treatment in step 2 into liquid nitrogen for deep cold quenching, and then take it out and return it to room temperature;

[0028] 4. Repeat steps 2 and 3 to perform a second deep cold quenching operation on the NbTiSiAlVFe alloy sample;

[0029] 5. Place the NbTiSiAlVFe alloy sample after deep cold quenching in a heat treatment furnace at 1200℃~1450℃ for 1h~24h, and then cool it to room temperature to complete.

[0030] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that the chemical formula of the NbTiSiAlVFe alloy is Nb-(16-24)Ti-(14-20)Si-(2-10)Fe-(2-5)Al-(0-10)V. Other aspects are the same as specific embodiment 1.

[0031] Specific embodiment 3: This embodiment differs from specific embodiments 1 or 2 in that a NbTiSiAlVFe alloy is composed, by atomic percentage, of 49% Nb, 24% Ti, 16% Si, 2% Al, 4% Fe, and 5% V, with a chemical formula of Nb-24Ti-16Si-2Al-4Fe-5V. Otherwise, this embodiment is the same as specific embodiments 1 or 2.

[0032] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: in step 1, a water-cooled copper crucible arc melting is used to prepare the Nb-24Ti-16Si-2Al-5V-4Fe alloy. Other aspects are the same as specific embodiments 1 to 3.

[0033] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the heating rate in step 2 is 10° C. / min. Other aspects are the same as specific embodiments 1 to 4.

[0034] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the heat preservation in step 2 is 30 minutes. Other aspects are the same as specific embodiments 1 to 5.

[0035] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that in step 3, after deep-cold quenching, the product is taken out and placed in air to return to room temperature. Other aspects are the same as specific embodiments 1 to 6.

[0036] Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that in step 5, the NbTiSiAlVFe alloy sample is placed in a heat treatment furnace at 1200° C. for 1 hour. Other aspects are the same as specific embodiments 1 to 7.

[0037] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that in step 5, the NbTiSiAlVFe alloy sample is placed in a heat treatment furnace at 1200° C. for 24 hours. Other aspects are the same as specific embodiments 1 to 8.

[0038] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that during the heat treatment process, the NbTiSiAlVFe sample is placed in the uniform temperature zone of the heat treatment furnace, and the holding time is counted after the furnace temperature reaches the target temperature. Otherwise, this embodiment is the same as specific embodiments 1 to 9.

[0039] The following examples are used to verify the beneficial effects of the present invention:

[0040] Example 1: This example prepares a Nb-24Ti-16Si-2Al-5V-4Fe alloy by arc melting in a water-cooled copper crucible. The alloy is composed of 49% Nb, 24% Ti, 16% Si, 2% Al, 4% Fe, and 5% V in atomic percentage. The Nb-24Ti-16Si-2Al-5V-4Fe alloy structure consists of a Nb-based solid solution and silicides (Nb5Si3 and Nb4FeSi). The proportion and distribution of silicides are controlled by combining cryogenic quenching and heat treatment as follows:

[0041] (1) Using the electric spark cutting technology, a sample with a size of 20 mm × 20 mm × 10 mm was prepared. The front and back of the sample were marked and a high-temperature anti-oxidation coating was applied on the surface.

[0042] (2) The high temperature heat treatment furnace is heated to 1000°C at a heating rate of 10°C / min. After reaching the temperature, the heat treatment sample is placed in the uniform temperature zone of the heat treatment furnace and kept warm for 30 minutes.

[0043] (3) When the time is up, quickly remove the sample from the furnace chamber and immerse it with its back facing down in liquid nitrogen (-196°C) for 2 to 5 minutes for deep cold quenching. Then remove the sample and place it in the air to return to room temperature.

[0044] (4) Place the sample back in the furnace chamber at 1000℃ and keep it warm for 30 minutes. After the time is up, take it out quickly and immerse it face down in liquid nitrogen for 2 to 5 minutes for secondary deep cold quenching. Take out the sample and place it in the air to return to room temperature.

[0045] (5) Set the heat treatment furnace temperature to 1200℃, place the sample after two deep cold quenching in the heat treatment furnace, keep it warm for 1 hour, take out the sample, and air cool it to room temperature.

[0046] Example 2: A Nb-24Ti-16Si-2Al-5V-4Fe alloy was prepared by arc melting in a water-cooled copper crucible. The alloy consists of 49% Nb, 24% Ti, 16% Si, 2% Al, 4% Fe, and 5% V in atomic percentage. The microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy consists of a Nb-based solid solution and silicides (Nb5Si3 and Nb4FeSi). The proportion and distribution of silicides were controlled by a combination of cryogenic quenching and heat treatment as follows:

[0047] (1) A Nb-24Ti-16Si-2Al-5V-4Fe alloy specimen with a size of 20 mm × 20 mm × 10 mm was prepared using electrospark cutting technology. The front and back sides of the specimen were marked, and a high-temperature anti-oxidation coating was applied on the surface.

[0048] (2) Heat the high-temperature heat treatment furnace at a heating rate of 10°C / min to a medium temperature of 600°C. After reaching the temperature, place the heat treatment sample in the uniform temperature zone of the heat treatment furnace and keep it warm for 30 minutes.

[0049] (3) When the time is up, quickly remove the sample from the furnace chamber and immerse it in liquid nitrogen with its back facing down for 2 to 5 minutes for deep cold quenching. Remove the sample and place it in the air to return to room temperature.

[0050] (4) Place the sample back in the furnace chamber at 600°C and keep it warm for 30 minutes. After the time is up, take it out quickly and immerse it face down in liquid nitrogen for 2 to 5 minutes for secondary deep cold quenching. Then take out the sample and place it in the air to return to room temperature.

[0051] (5) Set the heat treatment furnace temperature to 1200℃, place the sample after two deep cold quenching in the heat treatment furnace, keep it warm for 1 hour, then take out the sample and air cool it to room temperature.

[0052] Example 3: A Nb-24Ti-16Si-2Al-5V-4Fe alloy was prepared by arc melting in a water-cooled copper crucible. The alloy consists of 49% Nb, 24% Ti, 16% Si, 2% Al, 4% Fe, and 5% V in atomic percentage. The microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy consists of a Nb-based solid solution and silicides (Nb5Si3 and Nb4FeSi). The proportion and distribution of silicides were controlled by a combination of cryogenic quenching and heat treatment as follows:

[0053] (1) Using the electric spark cutting technology, a sample with a size of 20 mm × 20 mm × 10 mm was prepared. The front and back of the sample were marked and a high-temperature anti-oxidation coating was applied on the surface.

[0054] (2) The high temperature heat treatment furnace is heated to 1000°C at a heating rate of 10°C / min. After reaching the temperature, the heat treatment sample is placed in the uniform temperature zone of the heat treatment furnace and kept warm for 30 minutes.

[0055] (3) When the time is up, quickly remove the sample from the furnace chamber and immerse it in liquid nitrogen with its back facing down. Keep it in liquid nitrogen for 12 hours for deep freezing treatment. Then take out the sample and place it in the air to return to room temperature.

[0056] (4) Place the sample back in the furnace chamber at 1000℃ and keep it warm for 30 minutes. After the time is up, take it out quickly and immerse it face down in liquid nitrogen. Keep it in liquid nitrogen for 12 hours for deep freezing treatment. Then take out the sample and place it in the air to return to room temperature.

[0057] (5) Set the heat treatment furnace temperature to 1200°C, place the sample after deep cold quenching in the heat treatment furnace, keep it warm for 24 hours, then take out the sample and air cool it to room temperature.

[0058] The as-cast structure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in Examples 1 to 3 is as follows: Figure 1 (a) is shown. The sample was mechanically polished and then the microstructure was observed in SEM-BSE mode. The as-cast structure consists of light gray Nb-based solid solution, gray Nb5Si3 and dark gray Nb4FeSi. The Nb-based solid solution and Nb5Si3 exist in the form of a eutectic structure, and irregular block-shaped Nb4FeSi phases are adjacent to Nb5Si3. There are two different morphologies of Nb5Si3 in the organization, large blocks and fine particles. After conventional heat treatment (1400℃, 50h), the organization is as shown Figure 1 As shown in (b), the fine silicide particles are coarse and the size difference is significantly reduced compared with the coarse bulk. There is no nano-precipitation phenomenon in the conventional heat treatment structure.

[0059] The microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in the fourth stage of secondary deep cold quenching in Example 1 is as follows: Figure 2 As shown. After the second deep cryogenic quenching, nano-scale silicide particles are precipitated in the Nb-based solid solution, which reduces the Si content in the Nb-based solid solution and plays a purification role. This is because after the initial deep cryogenic quenching, a large quenching stress remains in the constituent phase. During the heat preservation process of the second quenching, the stress promotes the precipitation of silicide particles. Except for the precipitation of nano-scale silicides in the Nb-based solid solution, deep cryogenic quenching has little effect on the morphology, size and distribution of the constituent phases. For the Nb4FeSi phase, it still exists in the form of irregular blocks. The role of deep cryogenic quenching is to purify the Nb-based solid solution and retain quenching stress in the structure, preparing for the solid-state phase transformation during the subsequent high-temperature annealing.

[0060] The core-shell structure and nano-precipitation phase of the silicide of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Example 1 are as follows: Figure 3As shown, Figure a is a low magnification, and Figure b is a high magnification. Two distinct types of fine silicide precipitation are observed: nanoscale silicide particles within the Nbss matrix and silicide particles precipitated within the Nb5Si3 phase. After cryogenic quenching at 1000°C and high-temperature annealing, the bulky Nb5Si3 phase is surrounded by Nb4FeSi, resulting in a near-core-shell structure of Nb5Si3-Nb4FeSi. Due to the wide compositional range of Nb4FeSi, elemental diffusion occurs during high-temperature annealing, resulting in deep contrast in the Fe- and Ti-rich regions, resulting in a similar contrast to the Nb5Si3 phase. The distinction is that the Nb5Si3 grains, acting as the "core," precipitate submicron-sized silicide particles under lattice distortion, while the Nb4FeSi, acting as the "shell," surround the Nb5Si3 grains without precipitated particles. Nb5Si3 and Nb4FeSi have different thermal expansion coefficients, resulting in significant residual quenching stress at the phase interface. During annealing, stress induces a phase transformation (Nb5Si3+Nbss→Nb4FeSi), resulting in a near-core-shell structure of Nb5Si3-Nb4FeSi. Furthermore, lamellar silicide precipitation is observed within the Nb-based solid solution. Compared to the microstructure as obtained by secondary cryogenic quenching, the high-temperature annealed Nb-based solid solution exhibits lamellar growth of fine silicides, reducing the solubility of Si within the solution, further purifying the Nb-based solid solution, and improving its toughness.

[0061] Figure 4 This is the EBSD mode diagram of the Nb-24Ti-16Si-2Al-5V-4Fe alloy in Example 1 after deep cold quenching + annealing. It can be seen from the figure that after deep cold quenching and high temperature annealing, the Nb4FeSi phase precipitates along the grain boundary of the Nb5Si3 phase, forming a Nb5Si3-Nb4FeSi near core-shell structure, as shown in Figure (a). Figure (b) is a phase diagram of this area, with red representing Nb, blue representing Nb5Si3, and yellow representing Nb4FeSi. The Nb5Si3 phase and Nb4FeSi phase contents account for 25.8 vol.% and 23.3 vol.%, respectively. Due to the limitations of the scanning step size (0.8 μm) and accuracy during EBSD detection, the nano-precipitated phase is not shown in the figure.

[0062] The microstructure of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Example 2 is as follows: Figure 5As shown. After deep cold quenching at 600℃ and high temperature annealing, there is still a tendency to form a core-shell structure, and fine needle-shaped silicides are also precipitated inside the Nb-based solid solution. Compared with Example 1, precipitated particles inside the Nb5Si3 grains were not observed, and only Nb4FeSi precipitated phases appeared along the Nb5Si3 grain boundaries. This is because the low quenching temperature leads to a low degree of lattice distortion in the crystal, which is not enough to cause phase transformation precipitation inside the Nb5Si3, while the high stress at the interface still promotes the precipitation of the Nb4FeSi phase.

[0063] The properties of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Examples 1 to 3 are as follows: Figure 6 As shown, Figure a is the compression performance and Figure b is the room temperature fracture toughness. Although the conventional heat treatment process (1400℃ / 50h) can also improve the toughness of the alloy, its room temperature compressive strength is significantly reduced, from 2061MPa in the cast state to 1556MPa. After deep cold quenching at 1000℃ and annealing at 1200℃, the room temperature compressive strength of the alloy increases, among which Example 1 reaches 2193MPa. This is related to the precipitation of silicides. The nano-silicide particles precipitated together in the Nb-based solid solution and Nb5Si3 grains increase the strength of the alloy. In addition, the room temperature fracture toughness of the alloy is also improved, from 11.50MPa·am in the cast state. 1 / 2 Increased to 12.80 MPa·am 1 / 2 , indicating that although the conventional heat treatment method improves the toughness of the alloy, it loses strength compared to the cast state. However, the toughness and strength of the alloy after deep cold quenching + annealing treatment in this embodiment are improved at room temperature.

[0064] The properties of the Nb-24Ti-16Si-2Al-5V-4Fe alloy after deep cold quenching and annealing in Example 1 are as follows: Figure 7 As shown in the figure, after deep cold quenching at 1000°C and annealing at 1200°C, the alloy's high-temperature strength increases. This is related to the precipitation of silicides. The nanosilicide particles co-precipitated within the Nb-based solid solution and Nb5Si3 grains enhance the alloy's high-temperature strength, demonstrating that the heat treatment method of this embodiment improves the alloy's high-temperature performance compared to the as-cast alloy, making it more suitable for high-temperature service.

Claims

1. A method for controlling heat treatment of a high-temperature alloy to form a near-core-shell structure silicide and a nano-precipitate phase, characterized in that The method proceeds as follows:

1. Prepare a NbTiSiAlVFe alloy; wherein the NbTiSiAlVFe alloy is composed of 49% Nb, 24% Ti, 16% Si, 2% Al, 4% Fe and 5% V in atomic percentage, and has a chemical formula of Nb-24Ti-16Si-2Al-4Fe-5V; 2. Place the NbTiSiAlVFe alloy sample in a heat treatment furnace at 500℃~1000℃ and keep it warm with air as the insulation medium; 3. Place the NbTiSiAlVFe alloy sample after the heat preservation treatment in step 2 into liquid nitrogen for deep cold quenching, and then take it out and return it to room temperature; 4. Repeat steps 2 and 3 to perform a second deep cold quenching operation on the NbTiSiAlVFe alloy sample; 5. Place the NbTiSiAlVFe alloy sample after deep cold quenching in a heat treatment furnace at 1200℃~1450℃ for 1h~24h, and then cool it to room temperature to complete.

2. The method for heat treatment and control of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: Step 1: Prepare Nb-24Ti-16Si-2Al-5V-4Fe alloy by arc melting in a water-cooled copper crucible.

3. The method for heat treatment of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: The heating rate in step 2 is 10°C / min.

4. The method for heat treatment of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: In step 2, keep warm for 30 minutes.

5. The method for controlling heat treatment of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: After deep cold quenching in step 3, take it out and place it in the air to return to room temperature.

6. The method for heat treatment control of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: In step five, the NbTiSiAlVFe alloy sample is placed in a heat treatment furnace at 1200° C. and kept warm for 1 hour.

7. The method for controlling heat treatment of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: In step five, the NbTiSiAlVFe alloy sample is placed in a heat treatment furnace at 1200° C. and kept warm for 24 hours.

8. The method for heat treatment and control of a high-temperature alloy for forming a near-core-shell structure silicide and a nano-precipitated phase according to claim 1, characterized in that: During the heat treatment process, the NbTiSiAlVFe sample is placed in the uniform temperature zone of the heat treatment furnace, and the holding time is started after the temperature in the furnace reaches the target temperature.

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