A method for improving high-temperature performance of TiAl alloy by using twin-grid gamma phase

Abstract

The application relates to a method for improving the high-temperature performance of a TiAl alloy, and relates to a method for improving the high-temperature performance of a TiAl alloy.The application aims at solving the problem of insufficient high-temperature strength of the existing cast polycrystalline TiAl alloy.Method: I, weighing raw materials;II, according to the order from high melting point to low melting point, laying the raw materials in a mold;III, removing oxygen in the furnace body, and then introducing argon;IV, smelting;V, repeating smelting.The application is used for improving the high-temperature performance of a TiAl alloy by using a twin-grid gamma phase.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for improving high-temperature performance of TiAl alloy. BACKGROUND

[0002] With the rapid development of aerospace, the high-temperature service environment of the aero-engine blade requires higher material performance. TiAl alloy, as an important lightweight high-strength structural material, is considered as a candidate material for low-pressure turbine blades, and its microstructure and mechanical properties have attracted widespread attention.

[0003] TiAl alloy blades are usually made of polycrystalline TiAl alloy due to their large size and complex structure. However, the existing polycrystalline TiAl alloy has a high-temperature tensile strength of 400-500 MPa at 900℃. In practical applications, especially in higher temperature environments such as turbine blades in high-pressure combustion chambers, it is still limited. Therefore, it is necessary to develop a TiAl alloy with excellent 900℃ high-temperature performance to promote the application of TiAl alloy in high-temperature environments. SUMMARY

[0004] The present application solves the problem of insufficient high-temperature strength of existing cast polycrystalline TiAl alloy, and further provides a method for improving the high-temperature performance of TiAl alloy by using twin-grid γ phase.

[0005] A method for improving the high-temperature performance of TiAl alloy by using twin-grid γ phase, which is carried out according to the following steps:

[0006] I. The raw materials are weighed according to the atomic percentage of 35%-60% Al, 2%-7% Nb, 0.1%-1% C, 0.8%-3.2% Ta, 0.8%-3.2% Hf and the balance of Ti;

[0007] II. According to the order of melting point from high to low, the raw materials are sequentially placed in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace from bottom to top;

[0008] III. Remove the oxygen in the furnace body, and then introduce argon;

[0009] IV. Under the argon atmosphere, increase the input current by 40A-60A per minute until 900A-1000A is reached, then melt for 60s-100s under the condition of 900A-1000A current, and then instantaneously disconnect the power. The molten alloy is cooled to room temperature by the water-cooled copper crucible to obtain the alloy ingot after the first melting;

[0010] V. Repeat the melting of the alloy ingot after the first melting according to step IV, i.e. complete the method for improving the high-temperature performance of TiAl alloy by using twin-grid γ phase.

[0011] The beneficial effects of the present application are:

[0012] 1. The present application is designed by elements, which does not cause the formation of a large amount of B2 phase. Since C element is an alpha stable element, it narrows the beta phase region, resulting in only a small amount of B2 phase in the alloy, and Ta coupled Hf addition as a weak beta stable element does not produce excess B2 phase. B2 phase is prone to stress concentration during stress process due to its hardness and brittleness, thus causing rapid failure of the alloy as a crack source. Therefore, the present application reduces the content of B2 phase to effectively avoid the reduction of room temperature plasticity. The TiAl alloy prepared by the present application has a room temperature compressive strength of 2300MPa to 2600MPa, a compression rate of 38% to 42%, and good plasticity.

[0013] 2. The present application obtains intergranular gamma phase containing reticular twin crystals. Since Hf element is a slow diffusion element, the region rich in Hf and Al beta stable elements in the alpha phase during solidification process is preferentially transformed into gamma phase, and the alpha grain boundary migration makes the gamma possibly inside or at the boundary of the alpha parent phase, finally forming intergranular gamma phase. The residual alpha phase around the gamma occurs multiple twinning on the close-packed plane due to the effect of Ta dislocation energy reduction, and finally forms a gamma phase with a grid of twin crystals. During deformation, dislocation movement is hindered at the twin crystal, and the grid-shaped twin crystal further restricts the dislocation in the grid, which effectively stores and hinders the dislocation, thereby improving the high temperature performance of the TiAl alloy. Benefiting from the special element combination addition and the rapid power-off in the melting process of the present alloy composition, this unique organization, i.e. intergranular gamma phase containing reticular twin crystals, is obtained.

[0014] 3. The ductile-brittle transition temperature of the TiAl alloy is usually between 750℃ and 850℃, and the strength of the alloy decreases rapidly above the ductile-brittle transition temperature. The tensile strength of the TiAl alloy prepared by the present application at high temperature of 900℃ can reach 550MPa to 650MPa, and the elongation rate is 6% to 8%, which is higher than the reported polycrystalline TiAl alloy, and the use temperature of the TiAl alloy is increased by nearly 100℃, providing a theoretical component reference for the preparation of high-performance TiAl alloy. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 The structure schematic diagram of the vacuum non-consumable arc furnace melting system used in Example 1, 1 is a water-cooled copper crucible, 2 is a material turning device, 3 is a tungsten electrode gun, 4 is a power control system, 5 is a circulating water cooling system, 6 is an argon protection gas inlet system, 7 is a vacuum control system, 8 is a mold, 9 is raw material;

[0016] Figure 2 The microstructure diagram of the TiAl alloy prepared by Example 1 for improving high temperature performance by using twin crystal grid gamma phase;

[0017] Figure 3 The image shows the microstructure of the TiAl alloy prepared in Example 2, which utilizes a twinned γ-phase to improve high-temperature performance.

[0018] Figure 4 Transmission microstructure of the network twinned γ phase formed in the TiAl alloy with improved high-temperature performance using a twinned network γ phase prepared in Example 2;

[0019] Figure 5 The figures show the room temperature compressive properties of TiAl alloys prepared in Examples 1 and 2, which utilize the twinned γ-phase to improve room temperature performance. 1 represents Example 1, and 2 represents Example 2.

[0020] Figure 6 The figures show the tensile properties at 900℃ of the TiAl alloys prepared in Examples 1 and 2, which utilize the twinned γ-phase to improve high-temperature performance. 1 represents Example 1, and 2 represents Example 2. Detailed Implementation

[0021] Specific Implementation Method 1: This implementation method is a method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase, which is carried out according to the following steps:

[0022] 1. Weigh the raw materials according to the atomic percentages of Al (35%–60%), Nb (2%–7%), C (0.1%–1%), Ta (0.8%–3.2%), Hf (0.8%–3.2%), and the balance Ti.

[0023] 2. Following the order of melting point from high to low, the raw materials are laid from bottom to top in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace.

[0024] 3. Remove the oxygen from the furnace and then introduce argon gas;

[0025] IV. Under an argon atmosphere, the input current is increased by 40A to 60A per minute until it reaches 900A to 1000A. Then, the alloy is melted for 60s to 100s under a current of 900A to 1000A. After that, the power is cut off instantly, and the molten alloy is cooled to room temperature using a water-cooled copper crucible to obtain an alloy ingot after one melting.

[0026] Fifth, the alloy ingot after one melting is melted again according to step four, thus completing the method of improving the high-temperature performance of TiAl alloy by using the γ phase containing twinned grids.

[0027] In step one of this specific implementation method, the alloy composition has a gradient of melting points. During the smelting process, the low-melting-point Al and Ti melts can effectively encapsulate the high-melting-point Ta and Hf elements, which is conducive to orderly heat transfer and thus produces a uniformly composed ingot.

[0028] In step three of this specific implementation method, by repeatedly removing oxygen from the furnace, the oxygen content in the alloy ingot can be reduced, resulting in excellent alloy performance.

[0029] In step four of this specific implementation method, the fixed heating rate ensures that the alloy melting process is stable and controllable. The rapid power-off causes the alloy to cool rapidly, limiting element diffusion and making it easier to form the intergranular γ phase.

[0030] In step five of this specific implementation method, repeated melting avoids severe element segregation caused by factors such as gravity, resulting in a more stable ingot structure and properties.

[0031] The beneficial effects of this embodiment are:

[0032] 1. This embodiment, through elemental design, avoids the formation of a large amount of B2 phase. Since carbon (C), as an α-stabilizing element, shrinks the β-phase region, only a small amount of B2 phase exists in the alloy. Furthermore, the addition of Ta coupled with Hf, as a weak β-stabilizing element, does not generate excess B2 phase. Due to its high hardness and brittleness, the B2 phase is prone to stress concentration under stress, thus acting as a crack initiation and causing rapid alloy failure. Therefore, reducing the B2 phase content in this embodiment effectively avoids the reduction in room temperature plasticity. The TiAl alloy prepared in this embodiment achieves a room temperature compressive strength of 2300 MPa to 2600 MPa and a compression ratio of 38% to 42%, exhibiting good plasticity.

[0033] 2. This embodiment yields an intergranular γ-phase containing network twins. Since Hf is a slow-diffusion element, regions rich in Hf and β-stable elements such as Al in the α-phase preferentially transform into the γ-phase during solidification. α-grain boundary migration allows γ to potentially reside within or at the boundaries of the α-parent phase, ultimately forming an intergranular γ-phase. The residual α-phase around γ undergoes multiple twinning on close-packed surfaces due to the stacking fault energy reduction effect of Ta, ultimately forming a γ-phase with a twinned network. During deformation, dislocation movement to the twins is hindered, and the network-like twins further confine dislocations within the network, effectively storing and hindering dislocations, thereby improving the high-temperature performance of the TiAl alloy. This unique microstructure, namely the intergranular γ-phase containing network twins, is achieved thanks to the special elemental combination added to the alloy composition and the rapid power-off during the melting process.

[0034] 3. Typically, the ductile-brittle transition temperature of TiAl alloys is between 750℃ and 850℃. Above this temperature, the strength of the alloy decreases rapidly. However, the TiAl alloy prepared in this embodiment can achieve a tensile strength of 550MPa to 650MPa and an elongation of 6% to 8% at a high temperature of 900℃, which is higher than that of polycrystalline TiAl alloys reported in the past. This increases the service temperature of TiAl alloys by nearly 100℃ and provides a theoretical compositional reference for the preparation of high-performance TiAl alloys.

[0035] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the raw materials mentioned in step one are Al particles, AlNb master alloy blocks, C powder, Ta powder, Hf powder, and Ti particles. Everything else is the same as in Specific Implementation Method One.

[0036] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that: the Al particles are regular square prisms with a base length of 2mm-6mm and a height of 5mm-9mm; the AlNb intermediate alloy block is a regular square prism with a base length of 4mm-7mm and a height of 6mm-8mm; the C powder has a particle size of 100-200 mesh; the Ta powder has a particle size of 300-400 mesh; the Hf powder has a particle size of 200-500 mesh; and the Ti particles are cylinders with a diameter of φ2mm-φ4mm and a height of 3mm×5mm. Everything else is the same as in Specific Implementation Method One or Two.

[0037] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that, in step two, Ta powder, C powder, Hf powder, AlNb intermediate alloy block, Ti particles, and Al particles are sequentially laid from bottom to top in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace. Everything else is the same as in Specific Implementation Methods One to Three.

[0038] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the removal of oxygen from the furnace body in step three is specifically carried out as follows: First, argon gas is introduced into the furnace body for 30 to 50 seconds, then a vacuum is drawn until the pressure is 10 Pa to 20 Pa. This process of introducing argon gas and drawing a vacuum is repeated 3 to 5 times. Everything else is the same as in Specific Implementation Methods One to Four.

[0039] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that: in step five, the alloy ingot after one melting is repeatedly melted 5 to 10 times as in step four. Everything else is the same as in Specific Implementation Methods One to Five.

[0040] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that: before each repeated melting in step five, the alloy ingot is flipped using a turning device. Everything else is the same as in Specific Implementation Methods One to Six.

[0041] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: in step one, the raw materials are weighed according to an atomic percentage of 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 0.8% Hf, and the balance Ti. Everything else is the same as in Specific Implementation Methods One to Seven.

[0042] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: in step one, the raw materials are weighed according to an atomic percentage of 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 1.6% Hf, and the balance Ti. Everything else is the same as in Specific Implementation Methods One to Eight.

[0043] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One through Nine in that: in step four, under an argon atmosphere, the input current increases by 50A per minute until it reaches 1000A. Then, melting is performed for 60 seconds at a current of 1000A, followed by an instantaneous power cut-off. The molten alloy is then cooled to room temperature using a water-cooled copper crucible, yielding an alloy ingot after one melting process. Everything else is the same as in Specific Implementation Methods One through Nine.

[0044] The beneficial effects of the present invention are verified using the following embodiments:

[0045] Example 1:

[0046] A method for improving the high-temperature properties of TiAl alloys using a twinned γ-phase comprises the following steps:

[0047] 1. Weigh out Al particles, AlNb master alloy block, C powder, Ta powder, Hf powder and Ti particles according to the atomic percentages of 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 0.8% Hf and balance Ti.

[0048] 2. Following the order of melting point from high to low, Ta powder, C powder, Hf powder, AlNb intermediate alloy block, Ti particles, and Al particles are sequentially laid from bottom to top in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace.

[0049] 3. First, introduce argon gas into the furnace body for 20 seconds, then evacuate to a pressure of 20 Pa. Repeat the process of introducing argon gas and evacuating the furnace body 4 times to remove oxygen from the furnace body. Finally, introduce argon gas.

[0050] IV. Under an argon atmosphere, the input current is increased by 50A per minute until it reaches 1000A. Then, the alloy is melted for 60 seconds at a current of 1000A. After that, the power is cut off instantly, and the molten alloy is cooled to room temperature using a water-cooled copper crucible to obtain an alloy ingot after one melting.

[0051] 5. Repeat the melting process of the alloy ingot after one melting step 4 6 times. Before each repeated melting, the alloy ingot is flipped using a turning device to obtain a TiAl alloy with improved high-temperature performance by utilizing the γ phase containing twinned grids.

[0052] The Al particles are regular square prisms with a base length of 3mm × 7mm (height); the AlNb intermediate alloy block is a regular square prism with a base length of 5mm × 7mm (height); the C powder has a particle size of 150 mesh; the Ta powder has a particle size of 300 mesh; the Hf powder has a particle size of 300 mesh; and the Ti particles are cylinders with a diameter of φ3mm × 4mm.

[0053] Example 2: This example differs from Example 1 in that the raw materials in step 1 are weighed according to the atomic percentages of 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 1.6% Hf, and the balance Ti. Everything else is the same as in Example 1.

[0054] Figure 1 This is a schematic diagram of the vacuum non-consumable arc furnace melting system used in Example 1. 1 is a water-cooled copper crucible, 2 is a turning device, 3 is a tungsten electrode gun, 4 is a power control system, 5 is a circulating water cooling system, 6 is an argon protective gas supply system, 7 is a vacuum control system, 8 is a mold, and 9 is the raw material. The raw material is placed in the mold of the water-cooled copper crucible. Before melting, a vacuum is created using the vacuum control system, and the arc furnace cavity is filled with argon as the protective gas. Power is then supplied through the power control system, and the tungsten electrode gun is used as the electrode to ignite the arc. The circulating water cooling system remains on throughout the melting process. The turning device is used to turn the alloy ingot during multiple melting operations. The water-cooled copper crucible is a water-cooled copper crucible, and it contains 10 molds measuring 80mm × 100mm, resulting in rectangular ingots with a single weight of 80g. The rectangular ingots facilitate sample processing, have a high ingot utilization rate, and the 80g weight of a single ingot is sufficient for all microstructure characterization and performance testing.

[0055] Figure 2 The image shows the microstructure of the TiAl alloy prepared in Example 1, which utilizes a twinned γ-phase to improve high-temperature performance. As can be seen from the image, the TiAl alloy, after adding 0.8 Hf, exhibits a polycrystalline morphology composed of lamellar clusters, with a γ-phase existing between the lamellar clusters. Furthermore, a small amount of B2 phase is present within the lamellar clusters. Figure 3 This is a microstructure image of the TiAl alloy prepared in Example 2, which utilizes a twinned γ-phase to improve high-temperature performance. As shown in the image, the TiAl alloy with the addition of 1.6Hf exhibits a large amount of intergranular γ-phase, which is relatively uniformly distributed. Furthermore, combined with… Figure 2 and Figure 3 It can be seen that with the increase of Hf element addition, the content of intergranular γ phase increases, the content of B2 phase in the alloy further decreases, and the size of intergranular γ phase also increases.

[0056] Figure 4The image shows the transmission electron microstructure of the network twinned γ phase formed in the TiAl alloy prepared in Example 2, which utilizes the twinned network γ phase to improve high-temperature performance. As can be seen from the image, the addition of 1.6 Hf element results in a large number of network twins in the γ phase. Hf, as a slow-diffusion element, stabilizes the β phase, causing the Hf-enriched portion to be retained during the phase transformation, forming the α phase and ultimately the γ phase. The remaining α phase around it undergoes multiple twinning processes to form the γ phase with network twins.

[0057] Room temperature compression performance was tested according to the GB / T 7314-2017 test standard; Figure 5 The figures show the room-temperature compressive properties of the TiAl alloys prepared in Examples 1 and 2, where 1 represents Example 1 and 2 represents Example 2. As shown in the figures, the alloy prepared in Example 1 has a room-temperature compressive strength of 2489 MPa and a compressibility of 38.8%; the alloy prepared in Example 2 has a room-temperature compressive strength of 2541 MPa and a compressibility of 40.3%. This is because Example 2 has a lower B2 phase content than Example 1. The B2 phase, as a brittle phase, is prone to cracking during room-temperature deformation, leading to rapid fracture. The lower B2 phase content increases the room-temperature compressibility and improves the alloy's plasticity. Furthermore, the intergranular network twins can hinder dislocation movement, thereby increasing the alloy's strength.

[0058] High-temperature tensile properties were tested according to the GB / T 228.2-2015 test standard. Figure 6 The figures show the tensile properties at 900℃ of the TiAl alloys prepared in Examples 1 and 2, which utilize the twinned γ-phase to improve high-temperature performance. Figure 1 represents Example 1, and Figure 2 represents Example 2. As shown in the figures, at 900℃, the alloy prepared in Example 1 has a tensile strength of 550 MPa and an elongation of 6.5%; the alloy prepared in Example 2 has a tensile strength of 590 MPa and an elongation of 7.6%, respectively. This is because Example 2 contains more intergranular γ-phase compared to Example 1. The twinned γ-phase within the intergranular γ-phase acts as a barrier to hinder dislocation movement. Simultaneously, the formation of the grid confines dislocations within the grid, preventing dislocation pile-up and crack initiation at the phase interface. This improves the high-temperature strength and plasticity of the alloy.

Claims

1. A method for improving the high-temperature properties of TiAl alloys using a twinned γ-phase, characterized in that... It is done in the following steps:

1. Weigh the raw materials according to the atomic percentages of Al (35%–60%), Nb (2%–7%), C (0.1%–1%), Ta (0.8%–3.2%), Hf (0.8%–3.2%), and the balance Ti.

2. Following the order of melting point from high to low, the raw materials are laid from bottom to top in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace.

3. Remove the oxygen from the furnace and then introduce argon gas; IV. Under an argon atmosphere, the input current is increased by 40A to 60A per minute until it reaches 900A to 1000A. Then, the alloy is melted for 60s to 100s under a current of 900A to 1000A. After that, the power is cut off instantly, and the molten alloy is cooled to room temperature using a water-cooled copper crucible to obtain an alloy ingot after one melting. Fifth, the alloy ingot after one melting is melted again according to step four, thus completing the method of improving the high-temperature performance of TiAl alloy by using the γ phase containing twinned grids.

2. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... The raw materials mentioned in step one are Al particles, AlNb master alloy blocks, C powder, Ta powder, Hf powder, and Ti particles.

3. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 2, characterized in that... The Al particles are regular square prisms with a base length of 2mm to 6mm and a height of 5mm to 9mm; the AlNb intermediate alloy block is a regular square prism with a base length of 4mm to 7mm and a height of 6mm to 8mm; the C powder has a particle size of 100 mesh to 200 mesh; the Ta powder has a particle size of 300 mesh to 400 mesh; the Hf powder has a particle size of 200 mesh to 500 mesh; and the Ti particles are cylinders with a diameter of φ2mm to φ4mm and a height of 3mm × 5mm.

4. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 2, characterized in that... In step two, Ta powder, C powder, Hf powder, AlNb master alloy block, Ti particles, and Al particles are sequentially laid from bottom to top in the water-cooled copper crucible mold of the vacuum non-consumable arc melting furnace.

5. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... The process of removing oxygen from the furnace body as described in step three is as follows: first, introduce argon gas into the furnace body for 30 to 50 seconds, then evacuate to a pressure of 10 Pa to 20 Pa, and repeat the process of introducing argon gas and evacuating the furnace body 3 to 5 times.

6. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... In step five, the alloy ingot after one melting is melted repeatedly 5 to 10 times as in step four.

7. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... Before each repeated melting in step five, the alloy ingot is flipped using a turning device.

8. The method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... In step one, the raw materials are weighed out as follows: 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 0.8% Hf, and the balance Ti.

9. A method for improving the high-temperature performance of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... In step one, the raw materials are weighed out as follows: 47% Al, 6% Nb, 0.1% C, 1.6% Ta, 1.6% Hf, and the balance Ti.

10. A method for improving the high-temperature properties of TiAl alloys using a twinned γ-phase according to claim 1, characterized in that... In step four, under an argon atmosphere, the input current increases by 50A per minute until it reaches 1000A. Then, the alloy is melted for 60 seconds at a current of 1000A. After that, the power is cut off instantly, and the molten alloy is cooled to room temperature using a water-cooled copper crucible to obtain an alloy ingot after one melting.

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