Preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy

Abstract

Preparation method of nano-spherical B2 phase ultrafine-grained TiAl alloy. The present invention relates to a preparation method of nano-spherical B2 phase ultrafine-grained TiAl alloy. The purpose of the present invention is to solve the problem that a large amount of reticular B2 phase continuously distributes inside and on the boundaries of the lamellae in the existing TiAl alloy containing β-phase stabilizing elements, which will exacerbate the brittle fracture of the alloy as a crack source. The present invention weighs Al, Cr, Re and Ti according to mass percentage, and then uses a vacuum arc melting furnace to prepare an alloy ingot; the alloy ingot is subjected to isothermal axial compression, the strain rate is set to 0.01 s-1, the compression temperature is 1000-1150 °C, and the deformation amount is 80%. After hot compression, the alloy does not occur side cracking and shear fracture, has excellent plastic deformation ability, and at the same time improves the strength and deformation ability of the TiAl alloy. The present invention is applied to the technical field of hot processing preparation of high-temperature light alloys.

Description

Technical Field

[0001] The invention relates to a method for preparing a nano-spherical B2 phase ultrafine grain TiAl alloy. Background Art

[0002] The rapid development of the aerospace industry in recent years has put forward more stringent requirements for high-temperature alloys. Engine structural materials with lighter weight, higher strength and service temperature and faster operating speed are the unremitting pursuit of scientific researchers, and are also an important driving force for improving fuel efficiency and promoting environmental protection and energy conservation. Due to its high specific strength, good creep resistance and high-temperature oxidation resistance, γ-TiAl alloy is considered to be a new structural material that is expected to replace nickel-based high-temperature alloys in the temperature range of 800-900℃. With the addition of various alloying elements, the mechanical properties of γ-TiAl-based alloys have been significantly improved, and they have been applied in turbine blades and thermal protection systems. However, the intrinsic brittleness of this alloy still restricts its further development, and it is difficult to obtain good strength and plasticity at the same time. At the same time, the significant plastic anisotropy, delayed dynamic recovery and low dislocation slip rate of γ-TiAl alloy reduce hot workability.

[0003] In order to improve the hot working ability and use temperature of γ-TiAl alloy, researchers added a variety of strong β-phase stabilizing elements to make the alloy contain a certain amount of β / B2 phase, which has a bcc crystal structure at high temperature and can activate a large number of slip systems to improve the deformation ability of the alloy. However, the large amount of network B2 phase formed by adding more β-phase stabilizing elements is continuously distributed inside the lamellae and on the boundaries as a crack source, which aggravates the brittle fracture of the alloy. For TiAl alloys containing β-phase stabilizing elements, regulating the morphology and size of the β / B2 phase is currently an effective way to solve the adverse effects of the B2 phase on the microstructure. Usually, the hot deformation of γ-TiAl alloys is mostly carried out in the (α+β), (β+γ) two-phase region or the (α+β+γ) three-phase region, with higher deformation temperature (>1150℃) and lower deformation rate (~0.001s -1 ) is conducive to the nucleation and growth of dynamic recrystallization, but the flow stress is reduced, the deformation time is increased, and even the dynamic recrystallization grows abnormally, reducing the uniformity of the microstructure of the TiAl alloy. Although the lower deformation amount ensures that the alloy does not crack during the deformation process, the resulting reduced strain energy leads to incomplete dynamic recrystallization, which is not conducive to grain refinement and thermal stability of the microstructure.

[0004] In summary, how to control the microstructure with spherical B2 phase and fine equiaxed crystals is the key issue to significantly improve the strength and plasticity of TiAl alloy. Summary of the invention

[0005] The object of the present invention is to solve the problem that a large amount of reticular B2 phase continuously distributes inside and at the boundaries of the lamellae in the existing TiAl alloy containing β-phase stabilizing elements, which will exacerbate the brittle fracture of the alloy as a crack source, and a preparation method of a nanospherical B2-phase ultrafine-grained TiAl alloy is proposed.

[0006] The preparation method of a nanospherical B2-phase ultrafine-grained TiAl alloy of the present invention is carried out according to the following steps: First, raw materials are weighed according to the proportion of 32.73-34.16% of Al, 2.63-2.74% of Cr, Re≤2.89%, and the balance is Ti, and then an alloy ingot is prepared by a vacuum arc melting furnace;

[0007] Second, the alloy ingot is isothermally axially compressed on a thermo-mechanical simulation testing machine, the strain rate is set to 0.01 s -1 , the compression temperature is 1000-1150 °C, the deformation amount is 80%, and then the sample after hot compression is immediately water quenched to obtain a nanospherical B2-phase ultrafine-grained TiAl alloy, that is, it is completed.

[0008] The service temperature selected in the present invention is in the α 2 +γ+B2 three-phase region. As the deformation amount increases, the strain energy stored in the lamellae is released, which promotes the slip and pile-up of mobile dislocations inside the lamellae, and the γ lamellae gradually transform into equiaxed grains. The formation of local stress concentration at the lamellar boundaries not only causes the strip-shaped B2 phase to twist and bend, but also releases a large number of dislocations at the stress tip, providing nucleation sites for dynamic recrystallization. The grain boundaries of the deformed B2 phase lose thermodynamic stability. Driven by the local concentration gradient, a groove appears on one side of the self-bending boundary. When the tip of the groove penetrates the B2 phase with atomic diffusion, the original reticular B2 phase is divided into small blocks. During the subsequent deformation process, the B2-phase grains gradually spheroidize to reduce the interface energy and are distributed on the recrystallized grain boundaries, restricting each other's growth. The method used in the present invention can provide sufficient deformation storage energy, promote the transformation of the reticularly distributed B2 phase into fine spheres, and induce the nucleation and growth of dynamic recrystallization. The deformed microstructure of the TiAl alloy contains nano-spherical B2 phases, sub-micron recrystallized grains and a very small amount of residual lamellae, greatly reducing the average size of the as-cast grains, which is beneficial to improving strength and plasticity simultaneously. The selected deformation temperature for isothermal compression is relatively low (1000-1150 °C, and most of the existing isothermal compression temperatures are above 1150 °C), and the deformation amount can reach more than 80% (the deformation limit of the thermo-mechanical simulation testing machine is 80%, and the deformation amount of the current TiAl alloy during hot compression has not reached 80%). The refinement degree of the microstructure is large, which is of great significance for optimizing the hot processing process parameters of the TiAl alloy, broadening the hot processing window, and realizing the industrial production of high-quality large-sized forgings and plates.

[0009] The selected TiAl alloy of the present invention can undergo 80% deformation at temperatures below 1150°C, with an average grain size of 0.60 - 0.80 μm, and the size of the spherical B2 phase is only 10 - 100 nm. Due to the significant refinement of grains, both the deformation resistance and deformation ability of the alloy are improved. Therefore, the Vickers hardness of the TiAl alloy after hot compression is increased from 393 HV of the as-cast alloy to 440 HV, and no shear fracture occurs during the deformation process. The deformation control method proposed by the present invention can obtain fine and uniform grains, and this ideal microstructure improves both the strength and plasticity of the alloy. The prepared TiAl alloy of the present invention contains spherical B2 (β-Ti) phase (volume fraction of B2 phase ≤ 6.5%) at room temperature, equiaxed L1 2 structured γ-TiAl phase, D0 19 structured α 2 -Ti 3 Al phase, where the average size of the spherical B2 phase is 10 - 100 nm, the average grain size of the ultrafine grains is 0.60 - 0.80 μm, and the Vickers hardness is 440 HV, which is 1.4 times that of the as-cast alloy. The room-temperature compression performance of the alloy is the best (2607 MPa, 39.9%) under the deformation conditions of 1000°C - 80% - 0.01 s -1 The overall operation process is simple, the preparation cycle is short, effectively broadening the hot processing window of the TiAl alloy, which is beneficial to the multi-directional application of Re-containing TiAl alloys.

[0010] The present invention has the following beneficial effects:

[0011] First, in the preparation technology proposed by the present invention, the deformation temperature can be reduced to 1000°C (currently, the selected hot compression temperature of TiAl alloy under high deformation conditions is at least above 1200°C), and the deformation amount can reach more than 80% (the deformation limit of the thermo-mechanical simulation testing machine is 80%, and currently the deformation amount of hot compression of TiAl alloy has not reached 80%). No edge cracking or shear fracture occurs in the alloy after hot compression, and it has excellent plastic deformation ability.

[0012] Second, the morphology and size of the B2 phase in the TiAl alloy prepared by the present invention are controllable. The average size of the spherical B2 phase is 10 - 100 nm. Compared with the network B2 phase, the nano-spherical B2 phase reduces the degree of splitting of the matrix and the formation and propagation of cracks during low-temperature deformation, which is beneficial to improving the plastic deformation ability of the alloy.

[0013] Third, the hot-compressed alloy prepared by the present invention has an ultrafine grain structure. When the temperature is raised from 1000 to 1150°C, the average grain size is 0.60 - 0.80 μm, and it is less affected by temperature changes, having good thermal stability.

[0014] IV. Under the deformation conditions of 1050°C - 80% - 0.01s -1 The total Vickers hardness of TiAl obtained under the deformation conditions reaches 440 HV, which is 1.4 times that of the as-cast alloy. Under the deformation conditions of 1000°C - 80% - 0.01s -1 The room-temperature compression properties of the TiAl alloy obtained under the deformation conditions are the best (2607 MPa, 39.9%). Description of the Drawings

[0015] Figure 1 It is a deformed specimen of the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention;

[0016] Figure 2 It is the true stress-strain curve of the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention;

[0017] Figure 3 It is the SEM image of the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention; where a is 1000°C - 80%, b is 1050°C - 80%, c is 1100°C - 80%, and d is 1150°C - 80%;

[0018] Figure 4 It is the recrystallization size and relative content diagram of the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention; where a is the relative content diagram and b is the γ-phase grain size diagram;

[0019] Figure 5 It is the XRD pattern of the as-cast alloy and the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention;

[0020] Figure 6 It is the TEM image of the B2 phase in the as-cast alloy and the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention; where a is the as-cast alloy, and b and c are the nano-spherical B2-phase ultrafine-grained TiAl alloy at different magnifications;

[0021] Figure 7 It is the room-temperature compression strength-strain curve diagram of the as-cast alloy and the nano-spherical B2-phase ultrafine-grained TiAl alloy prepared in the embodiment of the present invention. Detailed Embodiments

[0022] The technical solution of the present invention is not limited to the following specific embodiments listed, and also includes any combination between the specific embodiments.

[0023] Specific Embodiment 1: The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy in this embodiment is carried out according to the following steps: First, weigh raw materials according to the ratio of 32.73 - 34.16% Al, 2.63 - 2.74% Cr, Re ≤ 2.89%, and the balance is Ti, and then use a vacuum arc melting furnace to prepare an alloy ingot;

[0024] Second, perform isothermal axial compression on the alloy ingot on a thermo-mechanical simulation testing machine, set the strain rate to 0.01 s -1 , the compression temperature is 1000 - 1150 °C, the deformation amount is 80%, and then immediately water-quench the sample after hot compression to obtain a nano-spherical B2-phase ultrafine-grained TiAl alloy, that is, it is completed.

[0025] Specific Embodiment 2: The difference between this embodiment and Specific Embodiment 1 is that: weigh raw materials according to the ratio of 33.4280% Al, 2.6823% Cr, 2.8817% Re, and the balance is Ti. It is the same as Specific Embodiment 1.

[0026] Specific Embodiment 3: The difference between this embodiment and Specific Embodiment 1 or 2 is that: the method of using a vacuum arc melting furnace to prepare an alloy ingot is as follows: put the raw materials into a water-cooled copper crucible of a non-consumable vacuum arc melting furnace, evacuate and then fill with argon for protective melting, after cooling, obtain an alloy ingot, and then remelt the alloy ingot 2 - 5 times, and cool to obtain an alloy ingot. Others are the same as Specific Embodiment 1 or 2.

[0027] Specific Embodiment 4: The difference between this embodiment and any one of Specific Embodiments 1 to 3 is that: evacuate to below 3×10 -3 Pa. Others are the same as any one of Specific Embodiments 1 to 3.

[0028] Specific Embodiment 5: The difference between this embodiment and any one of Specific Embodiments 1 to 4 is that: after the alloy ingot is cooled to room temperature, perform ultrasonic ethanol cleaning, and then perform isothermal axial compression. Others are the same as any one of Specific Embodiments 1 to 4.

[0029] Specific Embodiment 6: The difference between this embodiment and any one of Specific Embodiments 1 to 5 is that: in step two, after the temperature rises to 1000 °C, hold for 5 min, and then deform 80% at a temperature of 1000 °C at a strain rate of 0.01 s -1 . Others are the same as any one of Specific Embodiments 1 to 5.

[0030] Specific Embodiment 7: The difference between this embodiment and any one of Specific Embodiments 1 to 6 is that: in step two, after the temperature rises to 1050 °C, hold for 5 min, and then deform 80% at a temperature of 1050 °C at a strain rate of 0.01 s -1At a strain rate of, it is deformed by 80% at a temperature of 1050°C. The rest is the same as one of the specific embodiments one to six.

[0031] Specific Embodiment Eight: The difference between this embodiment and one of the specific embodiments one to seven is that: in step two, after the temperature rises to 1050°C, it is held for 5 minutes, and then at 0.01 s -1 At a strain rate of, it is deformed by 80% at a temperature of 1100°C. The rest is the same as one of the specific embodiments one to seven.

[0032] Specific Embodiment Nine: The difference between this embodiment and one of the specific embodiments one to eight is that: in step two, after the temperature rises to 1050°C, it is held for 5 minutes, and then at 0.01 s -1 At a strain rate of, it is deformed by 80% at a temperature of 1150°C. The rest is the same as one of the specific embodiments one to eight.

[0033] The following examples are used to verify the beneficial effects of the present invention:

[0034] Example One. The preparation method of a nano-spherical B2 phase ultra-fine grained TiAl alloy in this example is carried out according to the following steps:

[0035] Step One. Weigh the raw materials according to the proportion of 33.4280% of Al, 2.6823% of Cr, 2.8817% of Re, and the balance being Ti. Place the raw materials in a water-cooled copper crucible in a vacuum arc melting furnace. Among them, the powdery Re particles are wrapped with aluminum foil and placed at the bottom of the water-cooled copper crucible in the vacuum arc melting furnace. Due to the large difference in melting points between the Al element and the Re element, in order to prevent the burning loss of the Al element, the compensation amount of the Al element is set to 2% of the added mass. Place the Al and Cr particles above the aluminum foil, and place the large sponge Ti at the top of the water-cooled copper crucible to prevent the powder and small particle raw materials from being blown off by the arc. Evacuate the non-consumable vacuum arc melting furnace to below 3×10 -3 Pa to remove most of the oxygen. After filling with argon protective gas, carry out vacuum arc melting, and then repeat the melting 5 times. This is to make the alloy tissue composition uniform. After the ingot cools and solidifies to room temperature, take it out, clean the water-cooled copper crucible, and turn off the main power supply to obtain a as-cast alloy ingot. Use a digital wire electrical discharge machine to cut a cuboid specimen of 8×6×4 mm from the ingot for Vickers hardness testing, and take the average value of 10 tests as the final Vickers hardness value of the as-cast TiAl alloy. Cut 3 Compression specimens for room temperature compression testing. And take the average value of 3 tests as the final compression performance of the as-cast TiAl alloy.

[0036] Step Two. Use a digital wire electrical discharge machine to cut 10 - 15 Cylindrical specimens, for each cylindrical specimen, the wire cutting marks are polished with sandpaper to ensure that the side end faces of each cylindrical specimen are smooth, and the upper and lower end faces are smooth and parallel. The polished cylindrical specimens are placed in an ethanol reagent for ultrasonic cleaning. After the specimens are dried, they are prepared to connect platinum-rhodium thermocouples and are subjected to isothermal axial compression on a thermo-mechanical simulation testing machine.

[0037] Step 3: Convert the engineering strain of 80% into true strain, calculate the heating time and holding time according to the set heating rate, and input the required parameters into the computer program terminal. To ensure the accurate measurement of temperature changes during the hot compression process, two platinum-rhodium thermocouples are welded to the center of the side surface of the cylindrical specimen using a spot welder to prevent the thermocouples from falling during deformation. The other ends of the two thermocouples are connected to the two electrodes on the thermo-mechanical simulation testing machine. After the temperature rises to the set temperature of 1000 °C, hold for 5 min. Place the circular specimen in the center between the two punches in the mold, and at a strain rate of 0.01 s -1 Compress 80% of the original height at a temperature of 1000 °C.

[0038] Step 4: After the isothermal compression is completed, stop the thermo-mechanical simulation testing machine, immediately perform water quenching on the deformed specimen to retain the isothermal deformation structure, take out the specimen after cooling to room temperature, turn off the computer program control terminal, and then turn off the power of the testing machine to ensure that the thermo-mechanical simulation testing machine stops running. Use a digital wire electrical discharge machining machine to cut the axial-radial cross-section center of the compressed specimen for microscopic structure observation and perform Vickers hardness testing. Take the average hardness value after 10 tests as the final Vickers hardness value; for each alloy under each deformation condition, 3 Compressed specimens are subjected to room temperature compression testing, and the average value of 3 tests is used as the final compression performance of the TiAl alloy prepared by the present invention.

[0039] Example 2: The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy in this example is carried out according to the following steps:

[0040] Step 1: Weigh raw materials according to the proportion of 33.4280% Al, 2.6823% Cr, 2.8817% Re, and the balance being Ti. Place the raw materials in a water-cooled copper crucible in a vacuum arc melting furnace, where the powdered Re particles are wrapped with aluminum foil and placed at the bottom of the water-cooled copper crucible in the vacuum arc melting furnace. Due to the large difference in melting points between Al and Re elements, to prevent the burning loss of Al elements, the compensation amount of Al elements is set to 2% of the added mass. Place the Al and Cr particles above the aluminum foil, and place the large sponge Ti at the top of the water-cooled copper crucible to prevent the powder and small particle raw materials from being blown off by the arc. Evacuate the non-consumable vacuum arc melting furnace to 3×10 -3Below Pa, most of the oxygen is removed. After filling with argon as the protective gas, vacuum arc melting is carried out, and then the melting is repeated 5 times to make the alloy composition uniform. After the ingot cools and solidifies to room temperature, it is taken out, the water-cooled copper crucible is cleaned, and the main power supply is turned off to obtain the as-cast alloy ingot. A rectangular sample with dimensions of 8×6×4 mm is cut from the ingot using a digital wire electrical discharge machine for Vickers hardness testing, and the average value of 10 tests is taken as the final Vickers hardness value of the as-cast TiAl alloy. Three compression samples are taken for room temperature compression testing. The average value of 3 tests is taken as the final compression performance of the as-cast TiAl alloy.

[0041] Step 2: Use a digital wire electrical discharge machine to cut 10 - 15 cylindrical samples from the center of the ingot. For each cylindrical sample, the wire cutting marks are polished with sandpaper to ensure that the side end faces of each cylindrical sample are smooth, and the upper and lower end faces are smooth and parallel. The polished cylindrical samples are placed in an ethanol reagent for ultrasonic cleaning. After the samples are dried, they are prepared for connecting platinum-rhodium thermocouples and are subjected to isothermal axial compression on a thermo-mechanical simulation testing machine.

[0042] Step 3: Convert the engineering strain of 80% to true strain, calculate the heating time and holding time according to the set heating rate, and input the required parameters into the computer program terminal. To ensure accurate measurement of the temperature change during the hot compression process, two platinum-rhodium thermocouples are welded to the center of the side of the cylindrical sample using a spot welder to prevent the thermocouples from falling during deformation. The other ends of the two thermocouples are connected to the two electrodes on the thermo-mechanical simulation testing machine. After the temperature rises to the set temperature of 1050 °C, hold for 5 min. Place the circular sample in the center between the two punches in the mold, and compress 80% of the original height at a temperature of 1050 °C at a strain rate of 0.01 s -1 .

[0043] Step 4: After the isothermal compression is completed, stop the thermo-mechanical simulation testing machine. Immediately quench the deformed sample in water to retain the isothermal deformed microstructure. After cooling to room temperature, take out the sample, turn off the computer program control terminal, and then turn off the power of the testing machine to ensure that the thermo-mechanical simulation testing machine stops running. Use a digital wire electrical discharge machine to cut the center of the axial-radial section of the compression sample for microstructural observation and perform Vickers hardness testing. The average hardness value after 10 tests is taken as the final Vickers hardness value; for each alloy under each deformation condition, three compression samples are taken for room temperature compression testing, and the average value of 3 tests is taken as the final compression performance of the TiAl alloy prepared by the present invention.

[0044] Example 3: The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy in this example is carried out according to the following steps:

[0045] Step 1: Weigh the raw materials according to the ratio of 33.4280% Al, 2.6823% Cr, 2.8817% Re, and the balance being Ti. Place the raw materials in a water-cooled copper crucible in a vacuum arc melting furnace. The powdered Re particles are wrapped with aluminum foil and placed at the bottom of the water-cooled copper crucible in the vacuum arc melting furnace. Due to the large difference in melting points between Al and Re elements, to prevent the burning loss of Al element, the compensation amount of Al element is set to 2% of the added mass. Place the Al and Cr particles above the aluminum foil, and place the large sponge Ti at the top of the water-cooled copper crucible to prevent the powder and small particle raw materials from being blown off by the arc. Evacuate the non-consumable vacuum arc melting furnace to below 3×10 -3 Pa to remove most of the oxygen. After filling with argon protective gas, conduct vacuum arc melting, and then repeat the melting 5 times to make the alloy tissue components uniform. After the ingot cools and solidifies to room temperature, take it out, clean the water-cooled copper crucible, and turn off the main power supply to obtain the as-cast alloy ingot. Use a digital wire electrical discharge machining cutter to cut a cuboid sample of 8×6×4 mm from the ingot for Vickers hardness testing, and take the average value of 10 tests as the final Vickers hardness value of the as-cast TiAl alloy. Cut 3 compression samples for room temperature compression testing. And take the average value of 3 tests as the final compression performance of the as-cast TiAl alloy.

[0046] Step 2: Use a digital wire electrical discharge machining cutter to cut 10 - 15 cylindrical samples from the center of the ingot. Polish the wire cutting marks on each cylindrical sample with sandpaper to ensure that the side end faces of each cylindrical sample are smooth, and the upper and lower end faces are smooth and parallel. Place the polished cylindrical samples in an ethanol reagent for ultrasonic cleaning. After the samples are dried, prepare to connect platinum-rhodium thermocouples and conduct isothermal axial compression on a thermo-mechanical simulation testing machine.

[0047] Step 3: Convert the engineering strain of 80% to true strain, calculate the heating time and holding time according to the set heating rate, and input the required parameters into the computer program terminal. To ensure the accurate measurement of temperature changes during the hot compression process, use a spot welder to weld two platinum-rhodium thermocouples to the center of the side of the cylindrical sample to prevent the thermocouples from falling during deformation. The other ends of the two thermocouples are connected to the two electrodes on the thermo-mechanical simulation testing machine. After the temperature rises to the set temperature of 1100°C, hold for 5 minutes. Place the circular sample in the center between the two indenter heads in the mold, and compress 80% of the original height at a strain rate of 0.01s -1 at a temperature of 1100°C.

[0048] Step 4. After the isothermal compression is completed, stop the thermo-mechanical simulator. Immediately water-quench the deformed specimen to retain the isothermal deformation microstructure. After cooling to room temperature, take out the specimen, close the computer program control terminal, and then turn off the power of the simulator to ensure that the thermo-mechanical simulator stops running. Use a digital wire electrical discharge machine to cut the center of the axial-radial section of the compressed specimen for microscopic structure observation, and conduct Vickers hardness tests. Take the average hardness value after 10 tests as the final Vickers hardness value; cut 3 compressed specimens under each deformation condition for room temperature compression tests, and take the average value of the 3 tests as the final compression performance of the TiAl alloy prepared by the present invention.

[0049] Example 4. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy in this example is carried out according to the following steps:

[0050] Step 1. Weigh raw materials according to the proportion of 33.4280% Al, 2.6823% Cr, 2.8817% Re, and the balance being Ti. Place the raw materials in a water-cooled copper crucible in a vacuum arc melting furnace, where the powdered Re particles are wrapped with aluminum foil and placed at the bottom of the water-cooled copper crucible in the vacuum arc melting furnace. Due to the large difference in melting points between Al and Re elements, to prevent the burning loss of Al element, set the compensation amount of Al element as 2% of the added mass. Place the Al and Cr particles above the aluminum foil, and place the large sponge Ti at the top of the water-cooled copper crucible to prevent the powder and small particle raw materials from being blown off by the arc. Evacuate the non-consumable vacuum arc melting furnace to below 3×10 -3 Pa to remove most of the oxygen. After filling with argon protective gas, conduct vacuum arc melting, and then repeat the melting 5 times to make the alloy tissue composition uniform. After the ingot cools and solidifies to room temperature, take it out, clean the water-cooled copper crucible, and turn off the main power to obtain the as-cast alloy ingot. Use a digital wire electrical discharge machine to cut a cuboid specimen of 8×6×4 mm from the ingot for Vickers hardness tests, and take the average value of 10 tests as the final Vickers hardness value of the as-cast TiAl alloy. Cut 3 compressed specimens for room temperature compression tests. And take the average value of the 3 tests as the final compression performance of the as-cast TiAl alloy.

[0051] Step 2. Use a digital wire electrical discharge machine to cut 10 - 15 cylindrical specimens from the center of the ingot. Polish the wire cutting marks on each cylindrical specimen to ensure that the side end faces of each cylindrical specimen are smooth, and the upper and lower end faces are smooth and parallel. Put the polished cylindrical specimens into ethanol reagent for ultrasonic cleaning. After the specimens are dried, prepare to connect platinum-rhodium thermocouples and conduct isothermal axial compression on a thermo-mechanical simulator.

[0052] Step 3: Convert the engineering strain of 80% into true strain, calculate the heating time and holding time according to the set heating rate, and input the required parameters into the computer program terminal. To accurately measure the temperature change during the hot compression process, use a spot welder to weld two platinum-rhodium thermocouples to the center of the side surface of the cylindrical specimen to prevent the thermocouples from falling off during deformation. The other ends of the two thermocouples are connected to the two electrodes on the thermo-mechanical simulation testing machine. After the temperature rises to the set temperature of 1150 °C, hold for 5 min. Place the circular specimen in the center between the two punches in the mold, and compress 80% of the original height at a strain rate of 0.01 s -1 at a temperature of 1150 °C.

[0053] Step 4: After the isothermal compression is completed, stop the thermo-mechanical simulation testing machine, immediately quench the deformed specimen in water to retain the isothermal deformation microstructure. After cooling to room temperature, take out the specimen, turn off the computer program control terminal, and then turn off the power of the testing machine to ensure that the thermo-mechanical simulation testing machine stops running. Use a digital wire electrical discharge machining machine to cut the axial-radial cross-section center of the compressed specimen for microstructural observation and perform Vickers hardness testing. Take the average hardness value after 10 tests as the final Vickers hardness value; cut 3 compressed specimens under each deformation condition for room temperature compression testing, and take the average value of the 3 tests as the final compression performance of the TiAl alloy prepared by the present invention.

[0054] The macroscopic morphologies of the alloys after hot compression in Examples 1 to 4 are as Figure 1 shown. After deforming 80% of the original height at temperatures of 1000 °C, 1050 °C, 1100 °C, and 1150 °C, the circular specimens are all pressed into disc shapes, and the thickness is only 20% of the original height. It can be seen that as the deformation temperature increases, the boundary of the deformed specimen becomes smoother, and the edge cracking phenomenon gradually disappears. Moreover, the upper and lower end surfaces of the original cylindrical specimen are both located in the central area after deformation, indicating that the alloy has good uniform deformation ability during compression, which is beneficial to reducing the cracking phenomenon of the TiAl alloy during isothermal compression.

[0055] The true stress-strain curves of the alloys in Examples 1 to 4 at a strain rate of 0.01 s -1 after deforming 80% at temperatures of 1000 °C, 1050 °C, 1100 °C, and 1150 °C are as Figure 2 shown. Table 1 shows the peak stresses corresponding to the Figure 2 deformation conditions. From Figure 2As can be seen from Table 1, when the deformation amount is 80% (true strain is 1.61), when the temperature increases from 1000 to 1150 °C, the peak stress decreases from 497 to 265 MPa. On the one hand, the increased deformation temperature provides higher thermal activation energy for the slip system, reducing the critical resolved shear stress and the stress concentration degree. On the other hand, the atomic diffusion barrier decreases, effectively reducing the vacancy concentration, and then reducing the pinning effect of vacancies on dislocations. Both factors reduce the flow stress of the alloy.

[0056] Table 1 Peak stress of Ti-Al-Cr-Re alloy after hot compression

[0057]

[0058] Figure 3 (a)–(d) are the microstructures at the center of the axial-radial section of the Ti-Al-Cr-Re alloy at a strain rate of 0.01 s -1 under the conditions of 1000 °C - 80%, 1050 °C - 80%, 1100 °C - 80%, and 1150 °C - 80%, respectively. Among them, RD is the radial direction of the specimen, and CD is the compression direction of the specimen. In Figure 3 (a)–(d), recrystallized grains and lamellar clusters are observed. Dynamic recrystallization has occurred at a lower temperature. The recrystallized grains nucleate at the (α 2 / γ) lamellar boundaries and precipitate in the form of chains. The lamellar clusters with different orientations gradually elongate, bend, and kink along the radial direction under the action of the axial force. With the deformation temperature increasing from 1000 to 1150 °C, under the dual action of temperature and deformation amount, the strain energy stored in the lamellae is released, and the dislocations in the lamellae can undergo long-range slip and annihilation. The recrystallized grains nucleate and grow by absorbing dislocations and energy and gradually replace the position of the original lamellae, indicating that the increase in deformation temperature is beneficial to the nucleation and growth of dynamic recrystallization of TiAl alloy. However, it is observed that Figure 3 (c) and (d), the content of the residual lamellae decreases significantly, and grain boundary migration and atomic diffusion promote the formation of recrystallization. The B2 phase is orderly distributed at the recrystallization and residual lamellar boundaries. Compared with the common B2 phase in TiAl alloy, the size of the spheroidized B2 phase is significantly reduced, restricting each other with the recrystallized grains and inhibiting each other's growth.

[0059] Figure 4 This is the recrystallized size and content diagram of the Ti-Al-Cr-Re alloy under the deformation conditions of 1000 °C - 80% and 1050 °C - 80% prepared by the present invention. From Figure 4From the statistical results of (a), it can be seen that at 1000 °C, the recrystallization (DRX) ratio is 40.5 vol.%, the ratio of deformed grains (un DRX) is 15.4 vol.%, and the sub-grain ratio is 44.1 vol.%; when the temperature rises to 1050 °C, the recrystallization ratio increases to 60.7 vol.%, and the ratios of deformed grains and sub-grains decrease to 2.6 vol.% and 36.7 vol.% respectively. The γ-phase grain size statistically analyzed by EBSD is as shown in Figure 4 (b). At 1000 °C, the average grain size is 0.69 μm, and the ratio of grains with a size less than 1 μm is 77.7%; while at 1050 °C, the average grain size is 0.71 μm, and the ratio of grains with a size less than 1 μm decreases to 75.7%. Although the grain size shows a slight increase, the average grain size is in the sub-micron range, so it exhibits good stability and uniformity during the heating process.

[0060] Figure 5 XRD patterns of the as-cast Ti-Al-Cr-Re alloy prepared in the examples at 1000 °C - 80%, 1050 °C - 80%, 1100 °C - 80% and 1150 °C - 80% are shown. Table (2) shows the results of the relative contents of each phase measured from the XRD patterns. From Figure 5 it can be seen that the phase types of the alloy do not change before and after deformation, but (200) B2 the diffraction peak intensity of the crystal plane decreases, which indicates that the increase in the deformation temperature during the hot compression process promotes the transformation from the α-phase to the γ-phase, resulting in an increase in the relative content of the γ-phase. The results in Table (2) show that compared with the as-cast alloy, with the increase of the deformation temperature, at the same deformation amount, the phase contents of the B2 phase and the α 2 phase gradually decrease, and the relative content of the B2 phase does not exceed 6.5%. This corresponds to the XRD pattern, indicating that the increase in temperature will promote the re-dissolution of the B2 phase into the matrix, which is not conducive to its precipitation and spheroidization. The prepared TiAl alloy contains equiaxed L1 2 structured γ-TiAl phase, D0 19 structured α 2 -Ti 3 Al phase, and spherical B2 (β-Ti) phase at room temperature, where the volume fraction of the B2 phase ≤ 6.5%.

[0061] Table 2 Volume fraction changes of the γ-phase, α 2 phase and B2 phase with the deformation temperature

[0062]

[0063] Figure 6TEM images of the B2 phase in the as-cast alloy obtained in Step 1 of the example and the Ti-Al-Cr-Re alloy obtained under the condition of 1000 °C - 80% deformation. From Figure 6 (a), it can be seen that the morphology of the B2 phase in the as-cast alloy is strip-shaped and continuously distributed inside and at the boundaries of the lamellae. Figure 6 (b) shows that the morphology of the B2 phase transforms into spherical after isothermal compression. The average size of the spherical B2 phase is 10 - 100 nm, and it is easy to exist near the lamellar boundaries and twins. The B2 phase also tends to be distributed in regions with relatively high energy such as dislocation tangles. This distribution phenomenon is Figure 6 (c) more obvious, indicating that the preferred distribution of the B2 phase exists throughout the deformed microstructure. After the B2 phase is deformed, the curvature radii of its kink boundaries and straight boundaries change, resulting in a solute concentration difference between the two boundaries and becoming the driving force for solute diffusion. Under the dual driving of the deformation temperature and deformation amount, the strain energy stored inside the B2 phase continuously increases, and the spheroidization driving force is greater. When solute atoms diffuse from the curved boundary to the straight boundary, a groove pointing from the curved boundary to the straight boundary will be formed. With the diffusion of solute atoms, the degree of depression intensifies, and finally the B2 phase is penetrated along the depression direction and divided into numerous small blocks, reducing the interface energy by continuous spheroidization. The lamellar clusters and twins adjacent to the B2 phase have a higher dislocation density, providing a high-speed channel for the diffusion of atoms and accelerating the spheroidization process.

[0064] Figure 7Room temperature compression test results of the as-cast alloys (As-cast) and nano-spherical B2 phase ultrafine-grained TiAl alloys prepared in Examples 1 to 4. The measured room temperature compression strength and compression strain of the Ti-Al-Cr-Re alloy in the as-cast state were 2398 MPa and 39.3% respectively. The measured room temperature compression strength and compression strain of the Ti-Al-Cr-Re alloy at 1000 °C - 80% were 2607 MPa and 39.9% respectively; the room temperature compression strength and compression strain of the Ti-Al-Cr-Re alloy at 1050 °C - 80% were 2498 MPa and 39.7% respectively; the room temperature compression strength and compression strain of the Ti-Al-Cr-Re alloy at 1100 °C - 80% were 2093 MPa and 34.1% respectively. By comparison, it can be seen that the alloy prepared under the condition of 1000 °C - 80% has the best compression performance. The spheroidized nano B2 phase in the microstructure transforms into a ductile phase at this temperature, allowing dislocations to cut through and slip, so that the crack tip is passivated near the B2 phase instead of continuing to expand, reducing the possibility of crack initiation during deformation. Therefore, compared with the as-cast state, the room temperature compression performance of the alloy under this deformation condition is significantly improved. Under the deformation conditions of 1050 °C - 80% and 1100 °C - 80%, due to the increased temperature, some of the spheroidized B2 phases grow and even dissolve into the matrix, and the spheroidization effect is reduced. Therefore, the room temperature compression performance under these two deformation conditions does not reach the best results. In addition, the formation of fine γ equiaxed grains under the conditions of 1000 °C - 80% and 1050 °C - 80% also improves the room temperature compression strength and compression strain of the alloy.

[0065] Table 3 shows the Vickers hardness statistical results of the as-cast alloys and TiAl alloys prepared in Examples 1 to 4. The measured Vickers hardness of the TiAl alloy in the as-cast state was 306 HV, while the Vickers hardness of the Ti-Al-Cr-Re alloy under the three deformation conditions of 1000 °C - 80%, 1050 °C - 80%, and 1100 °C - 80% was 438 HV, 440 HV, and 412 HV respectively.

[0066] Table 3 Vickers hardness of Ti-Al-Cr-Re alloy after hot compression

[0067]

[0068] The Vickers hardness of the nano-spherical B2 phase ultrafine-grained TiAl alloys prepared in Examples 1 to 4 is higher than that of the as-cast alloys, indicating that the nano-spherical B2 phase ultrafine-grained TiAl alloy prepared by the present invention has higher deformation resistance and work hardening ability. This method improves both the strength and deformation ability of the TiAl alloy, broadens the hot deformation process idea of the TiAl alloy, is beneficial to broadening the hot working window of the TiAl alloy, and has great significance for the multi-directional application of the TiAl alloy in military fields such as aerospace engines, thermal protection systems, aircraft skins, and panels.

Claims

1. Preparation method of nano-spherical B2-phase ultrafine-grained TiAl alloy, Characterized in that, This preparation method is carried out according to the following steps: First, weigh raw materials according to the proportion of 33.4280% of Al, 2.6823% of Cr, 2.8817% of Re, and the balance being Ti, and then use a vacuum arc melting furnace to prepare an alloy ingot; II. Isothermally axially compress the alloy ingot on a thermo-mechanical simulation testing machine, set the strain rate to 0.01 s -1 , the compression temperature is 1000, 1050 or 1100 °C, the deformation amount is 80%, and then immediately water quench the specimen after hot compression to obtain a nano-spherical B2 phase ultrafine-grained TiAl alloy, and that's it.

2. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 1, Characterized in that The method for preparing an alloy ingot by using a vacuum arc melting furnace is: put the raw materials into a water-cooled copper crucible of a non-consumable vacuum arc melting furnace, evacuate and then fill with argon for protective melting. After cooling, an alloy ingot is obtained, and then the alloy ingot is remelted 2 - 5 times and cooled to obtain an alloy ingot.

3. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 2, Characterized in that Vacuum to below 3×10 -3 Pa.

4. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 2, Characterized in that After the alloy ingot is cooled to room temperature, it is ultrasonically cleaned with ethanol and then subjected to isothermal axial compression.

5. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 1, Characterized in that In Step 2, after the temperature rises to 1000°C, keep it warm for 5 minutes, and then deform it by 80% at a temperature of 1000°C at a strain rate of 0.01 s -1 .

6. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 1, Characterized in that In Step 2, after the temperature rises to 1050°C, hold for 5 minutes, and then deform by 80% at a temperature of 1050°C at a strain rate of 0.01 s -1 .

7. The preparation method of a nano-spherical B2-phase ultrafine-grained TiAl alloy according to claim 1, Characterized in that In Step 2, after the temperature rises to 1100°C, keep it warm for 5 minutes, and then deform it by 80% at a temperature of 1100°C at a strain rate of 0.01 s -1 .

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