A bulk titanium aluminum single crystal alloy and a preparation method thereof

Abstract

The present invention discloses a bulk titanium aluminum single crystal alloy and a preparation method thereof, wherein the size of the bulk titanium aluminum single crystal alloy block is: a diameter between 7.0 mm and 20 mm, and a length of 25.0 mm and 200.0 mm. The present invention comprises the following preparation steps: 1. preparing a master alloy according to the nominal composition ratio, and melting and pouring the master alloy multiple times to obtain a polycrystalline titanium aluminum master alloy ingot block with a certain shape and uniform composition; 2. subjecting the polycrystalline titanium aluminum block cast in step 1 to high temperature heat treatment; 3. subjecting the titanium aluminum block after high temperature heat treatment in step 2 to low temperature annealing heat treatment; 4. repeating the above high and low temperature cycle heat treatment several times to obtain a bulk single crystal titanium aluminum. The beneficial effects of the present invention are: the method adopts a non-traditional high temperature annealing process, does not need to be heated above the melting point, and the obtained bulk titanium aluminum single crystal alloy block can take into account the characteristics of high purity, uniform composition, and controllable shape and size.

Description

Technical Field

[0001] The present invention relates to the technical field of single crystal preparation of metals and alloys, and in particular to a bulk titanium-aluminum single crystal alloy and a preparation method thereof. Background Art

[0002] Titanium-aluminum alloy has become a new generation of star materials that have attracted much attention in the fields of aerospace, weapons manufacturing and automobile industry due to its many excellent properties, and is expected to be applied in large-scale engineering. Among them, the density of titanium-aluminum alloy is only 3.85-4.20 g / cm 3 , lower than pure titanium, and less than 50% of the density of nickel-based high-temperature alloys, with significant weight reduction effects, it is a new type of high-temperature structural material that is expected to replace nickel-based alloys. It has a series of advantages such as light weight, high specific strength, wear resistance, corrosion resistance, and good high-temperature performance, making it used as low-pressure turbine blades of aircraft engines, exhaust valves of automobile engines, connecting rods and other high-temperature resistant parts, as well as some non-stressed or non-rotating parts such as housings and wings, and has broad application prospects.

[0003] At present, most titanium aluminum alloy materials are polycrystalline structures, that is, the material contains a large number of grain boundaries. At the grain boundaries, due to the transition state of atomic arrangement, high energy and stress concentration, high temperature creep and other properties deteriorate, thus limiting the further improvement of high temperature properties of titanium aluminum alloys. Single crystal titanium aluminum alloys have no grain boundaries, thus avoiding the occurrence of unfavorable factors such as grain boundary sliding and cracks. Therefore, they have excellent mechanical properties that polycrystalline materials cannot match and have received more attention. The traditional preparation method of titanium aluminum single crystals is based on melt directional solidification technology, which requires strict control of melt solidification front temperature gradient, solid-liquid two-phase zone length and interface position, casting (pulling) speed, atmosphere protection and other parameters. The process is complex and the cost is high, and large-scale production cannot be achieved. Moreover, due to the large difference in melting points of various alloy components and the different non-equilibrium solidification segregation coefficients and vapor pressures, the smelted titanium aluminum single crystal alloys often have problems such as component segregation, which causes a significant decrease in alloy performance. In addition, titanium aluminum alloy has high hardness but poor plasticity and is difficult to process. Although adding a large amount of Nb element can effectively improve the room temperature plasticity and high temperature oxidation resistance of titanium aluminum, the high melting point of Nb element makes it more difficult to prepare titanium aluminum alloy, and it is at the expense of specific strength. Therefore, it becomes difficult to prepare titanium aluminum single crystal alloy by inducing grain growth through hot working plastic deformation, thus limiting the further application of high temperature titanium aluminum alloy. In summary, the existing technology for preparing bulk titanium aluminum single crystal alloy has the following disadvantages: 1) uneven composition and small size; 2) complex preparation process and high cost.

[0004] In order to solve the above problems, the present invention proposes a new method for preparing large-size titanium-aluminum single crystal alloy by high-temperature annealing. This method not only does not need to heat the alloy above the melting point, thereby avoiding a series of complex preliminary processes such as pre-deformation treatment involved in conventional solid-phase single crystal preparation methods, but also the titanium-aluminum single crystal alloy obtained can also take into account the advantages of high purity, uniform composition, controllable shape and size, low cost, easy promotion, etc., and has huge market advantages. Summary of the invention

[0005] The present invention provides a bulk titanium aluminum single crystal alloy and a preparation method thereof, wherein the method adopts a non-traditional high temperature annealing process and does not need to be heated above the melting point;

[0006] Moreover, the obtained bulk titanium aluminum single crystal alloy can have the characteristics of high purity, uniform composition, controllable shape and size, and low cost. The size of the bulk titanium aluminum single crystal alloy is: diameter between 7.0mm-20mm, length 25.0mm-200.0mm.

[0007] Among them, the titanium aluminum single crystal alloy is expressed in atomic percentage as follows: 41~56%Ti, 42-52%Al, 2~9%(Nb, Cr, Mn).

[0008] To achieve the above object, the present invention is implemented by the following technical solutions:

[0009] A method for preparing a bulk titanium-aluminum single crystal alloy.

[0010] The steps include:

[0011] (1) preparing a master alloy according to the nominal composition ratio, melting and pouring the master alloy multiple times to obtain a polycrystalline titanium-aluminum master alloy ingot block with a certain shape and uniform composition;

[0012] (2) subjecting the polycrystalline titanium aluminum block cast in step (1) to high temperature heat treatment;

[0013] (3) subjecting the titanium aluminum block subjected to high temperature heat treatment in step (2) to low temperature annealing heat treatment;

[0014] (4) Repeat the above high and low temperature cyclic heat treatment several times to obtain bulk titanium aluminum single crystal.

[0015] As a further improvement of the present scheme, in step (1), the constituent elements such as titanium and aluminum are mixed in proportion to form a master alloy, and are smelted at high temperature in a vacuum electromagnetic induction suspension melting furnace. The melt is cast in a regular shell to obtain a titanium-aluminum alloy ingot in the shape of a rod or sheet.

[0016] As a further improvement of the present solution, in step (1), the master alloy is subjected to vacuum induction melting 2-4 times to make the composition uniform.

[0017] As a further improvement of the present scheme, in step (2), the surface of the polycrystalline titanium aluminum block is first cleaned and dried, and then annealed in a reducing atmosphere protective environment, wherein the hydrogen gas volume fraction is 2.5%-5%.

[0018] As a further improvement of the present solution, in step (2), a high temperature annealing treatment is first performed on the polycrystalline titanium aluminum block at a temperature of 1350°C-1450°C using a tubular furnace for 2-10 hours.

[0019] As a further improvement of the present scheme, in step (3), a tube furnace is used to perform a low temperature heat treatment process at a temperature of 1200-1300° C. on the polycrystalline titanium aluminum block, and the annealing time is 2-10 hours.

[0020] As a further improvement of the present scheme, in step (4), a tubular furnace is used to repeatedly perform high-low cycle heat treatment on the polycrystalline titanium aluminum block 20-30 times, and the rate of decreasing from high temperature to low temperature or increasing from low temperature to high temperature is controlled at 2-10°C / min.

[0021] As a further improvement of the present solution, in step (4), the titanium aluminum block is cut and surface polished after annealing.

[0022] A bulk titanium-aluminum single crystal alloy block is provided. The bulk titanium-aluminum single crystal alloy is prepared by a method for preparing a bulk titanium-aluminum alloy single crystal block.

[0023] As a further improvement of the present solution, the size of the bulk titanium aluminum single crystal alloy is: a diameter of 7.0 mm-20 mm, and a length of 25.0 mm-200.0 mm.

[0024] The method for preparing the bulk titanium aluminum single crystal alloy of the present invention has the following beneficial effects:

[0025] 1) In the present invention, a bulk titanium-aluminum single crystal alloy rod is prepared by a specially designed high temperature annealing process, and the size of the bulk titanium-aluminum single crystal alloy rod is: a diameter of 7.0 mm-20 mm, and a length of 25.0 mm-200.0 mm;

[0026] 2) In the present invention, the titanium-aluminum alloy rod obtained by annealing through multiple high and low temperature cycle heat treatments is a single crystal structure, and the titanium-aluminum single crystal alloy has uniform composition and no component segregation.

[0027] 3) The titanium aluminum single crystal alloy prepared in the present invention has high purity, controllable and uniform composition, and a simple preparation process. It can realize the preparation of titanium aluminum single crystal alloys of various compositions, shapes and sizes, with low cost and easy large-scale promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1It is a flow chart of the method for preparing bulk titanium aluminum single crystal alloy of the present invention;

[0029] Figure 2 Schematic diagram of a titanium-aluminum single crystal alloy rod prepared by the present invention;

[0030] Figure 3 This is a local optical corrosion photograph of the titanium aluminum single crystal alloy rod prepared by the present invention;

[0031] Figure 4 Is a scanning electron microscope (SEM) photograph of the titanium aluminum single crystal alloy rod prepared by the present invention;

[0032] Figure 5 Is the electron backscattering (EBSD) image of the titanium aluminum single crystal alloy rod prepared by the present invention;

[0033] Figure 6 It is an energy dispersion spectrum (EDS) diagram of the titanium aluminum single crystal alloy rod prepared by the present invention;

[0034] Figure 7 This is a scanning electron microscope (SEM) image of the titanium aluminum alloy after 5 cycles of heat treatment annealing;

[0035] Figure 8 This is a scanning electron microscope (SEM) image of a titanium-aluminum alloy after conventional long-term constant temperature annealing; DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clear, the following embodiments and the accompanying drawings are combined to illustrate the present invention. Figure 1 , 2 The present invention will be further described:

[0037] Example 1

[0038] Combination Figures 1 to 6 , this embodiment 1 is described in detail.

[0039] The present embodiment relates to a method for preparing a bulk titanium aluminum single crystal alloy. The polycrystalline titanium aluminum billet is Ti with a purity of 99.999%. 48 Al 48 Cr2Nb2 (abbreviated as 4822), the preparation method includes the following steps:

[0040] (1) The experiment uses high-purity Ti (99.999%) in flake form, high-purity Al (99.999%) in cylindrical form, high-purity Nb (99.999%) in cylindrical form, and high-purity Cr (99.999%) in granular form as raw materials, and the master alloy is prepared according to the nominal composition of TiAl-4822, Ti-48Al-2Cr-2Nb (at%). Under argon protection, the master alloy is prepared in a magnetic levitation induction melting furnace, and the melt is poured into a mold shell to obtain TiAl alloy ingots of different sizes. In this embodiment, a polycrystalline titanium aluminum alloy round rod with a diameter of 1 cm is cast.

[0041] (2) The sample cast in step (1) is subjected to high temperature heat treatment.

[0042] Several cylindrical samples are cut from the alloy ingot for further analysis and heat treatment. In this embodiment, the heat-treated sample is a polycrystalline titanium-aluminum alloy round rod with a diameter of 1 cm and a length of 5 cm.

[0043] Before annealing, the high-temperature tube furnace cavity was purged with hydrogen and argon for 2-3 times, and then the temperature was raised from room temperature to the annealing temperature for 250 min, raised to 1400°C, maintained at high temperature for 2 hours, and then dropped to 1250°C at a rate of 5°C / min, with a cooling time of 50 min. The protective gas used was a 5% (vol. %) hydrogen-argon mixture with a gas flow rate of 480 sccm.

[0044] (3) The polycrystalline titanium aluminum round rod, which has been subjected to high temperature annealing and cooled to 1250° C. in step (2), is further annealed at 1250° C. for 2 hours. The inert gas used is a 5% (vol. %) hydrogen-argon mixture gas with a gas flow rate of 480 sccm.

[0045] (4) The titanium aluminum polycrystal that has undergone low-temperature heat treatment in step (3) is heated to 1400°C again and maintained for 2 hours, with a heating time of 250 min. It is then cooled to 1250°C at a rate of 5°C / min, with a cooling time of 50 min. The protective gas used is a 5% vol. hydrogen-argon mixture with a gas flow rate of 480 sccm.

[0046] (5) Repeat steps (3) and (4) for 20 times of high and low temperature cycle heat treatment.

[0047] (6) Cooling the titanium aluminum alloy sample after multiple high and low temperature heat treatments in step (5), wherein the cooling method is furnace cooling.

[0048] The surface of the titanium-aluminum alloy rod is cleaned by immersing the round rod in a 98% alcohol solution and ultrasonically treating it in an ultrasonic machine for 1 hour to remove impurities attached to the surface. After the ultrasonic treatment, the rod is washed in deionized water and then dried.

[0049] In this embodiment, three melting and pouring processes are performed to obtain a titanium-aluminum alloy rod ingot with uniform composition.

[0050] In this embodiment, the temperature is raised to 1400° C. within 250 min and maintained for 2 hours of annealing time.

[0051] In this embodiment, the temperature is lowered from 1400° C. to 1250° C. within 50 min and maintained for 2 hours of annealing time.

[0052] The annealed titanium aluminum alloy rod was cut to obtain a sample, and the surface was electrochemically polished using a polishing liquid, which was a 5% perchloric acid acetic acid solution. After polishing, it was washed with deionized water and then dried.

[0053] Figure 2 This is a physical picture of the titanium aluminum single crystal alloy rod after high temperature heat treatment.

[0054] Figure 3 This is an optical photograph of a cut sample of a titanium-aluminum single crystal alloy rod. No obvious grain boundaries can be seen with the naked eye.

[0055] Figure 4 This is a SEM micrograph of a titanium-aluminum single crystal alloy rod. No obvious grain boundaries are found, and the single crystal structure is layered.

[0056] Figure 5 This is the electron backscattering (EBSD) image of the titanium-aluminum single crystal alloy rod. The color is uniform, indicating that the titanium-aluminum alloy rod obtained after multiple cycles of heat treatment and annealing has a single crystal structure and uniform organization.

[0057] FIG6 is an energy dispersive spectrum (EDS) diagram of a titanium-aluminum single crystal alloy rod, which shows that the prepared single crystal titanium-aluminum alloy has uniform composition and no component segregation.

[0058] Note: If step (5) in Example 1 is omitted, that is, if the high and low temperature heat treatment is not repeated, the appearance of abnormally large grains can be observed, and the large grains are surrounded by small grains. From the above comparison, it can be seen that the high and low temperature cyclic heat treatment has a great influence on the experimental results. Each time the cyclic heat treatment is heated and cooled, internal stress and structural defects will be introduced, the grain boundary mobility will be increased, and the crystal growth will be more obvious.

[0059] Comparative Example 1

[0060] In Comparative Example 1, the number of cyclic heat treatments was reduced to 5 times, that is, the high and low temperature cyclic heat treatments in steps (3) and (4) of Example 1 were repeated 5 times, while other conditions remained unchanged.

[0061] The titanium aluminum surface after the above five cycles of heat treatment annealing is cleaned by immersing the titanium aluminum in a 98% alcohol solution and ultrasonically treating it in an ultrasonic machine for 1 hour to remove impurities attached to the surface. After the ultrasonic treatment, it is washed in deionized water and then dried. After cutting, the sample is obtained and the surface is electrochemically polished using a polishing liquid. The polishing liquid used is a 5% perchloric acid acetic acid solution. After polishing, it is washed with deionized water and then dried.

[0062] The obtained titanium aluminum alloy was characterized by microstructure, and Figure 7 is a morphology of the titanium aluminum alloy after 5 cycles of heat treatment in Comparative Example 1. It can be seen that the grain size after 5 cycles of heat treatment has increased, but only a few millimeters, and obvious grain boundaries can be observed, and it has not yet reached the level of complete single crystallization.

[0063] It can be seen that it is difficult to induce complete single crystallization of polycrystalline titanium aluminum alloy with a small number of cyclic heat treatments. Therefore, introducing multiple high and low temperature cyclic heat treatments during the heat treatment process is beneficial to increasing the grain size and the degree of single crystallization, which is very necessary for the preparation of larger sized titanium aluminum single crystals.

[0064] Comparative Example 2

[0065] In this comparative example, only the polycrystalline titanium aluminum obtained in step (1) of the embodiment was subjected to a long-term high-temperature heat treatment. The time from room temperature to the annealing temperature was 250 min, the temperature was raised to 1400°C, and the high-temperature annealing time was maintained for 48 hours. No high / low temperature cycle heat treatment was performed.

[0066] The titanium aluminum surface after the above annealing is cleaned by immersing the titanium aluminum in a 98% alcohol solution, ultrasonically treating it in an ultrasonic machine for 1 hour to remove impurities attached to the surface, and then washing it in deionized water and drying it after the ultrasonic treatment. After cutting, the sample is obtained and the surface is electrochemically polished using a polishing liquid. The polishing liquid used is a 5% perchloric acid acetic acid solution. After polishing, it is washed with deionized water and then dried.

[0067] The microstructure of the titanium-aluminum alloy was analyzed. Figure 8 This is a microscopic morphology of the titanium aluminum alloy after long-term constant temperature annealing in Example 2. It can be seen that long-term constant temperature annealing leads to normal grain growth. After annealing, the titanium aluminum alloy rod is a polycrystalline structure composed of lamellar grains, the microstructure contains a large number of grain boundaries, the degree of single crystallization is low, the grain size is small, and it is difficult to form a larger size titanium aluminum single crystal alloy.

[0068] It can be seen that it is difficult to induce single crystallization of polycrystalline titanium-aluminum alloy by long-term high-temperature annealing. Therefore, introducing multiple high- and low-temperature cycle heat treatments during the heat treatment process is beneficial to the further elimination of the grain boundaries of the polycrystalline titanium-aluminum alloy, which is very necessary for the preparation of larger-sized titanium-aluminum single crystals.

[0069] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent changes made using the present invention are within the patent protection scope of the present invention.

Claims

1. A method for preparing a bulk titanium aluminum single crystal alloy, characterized in that: The steps include: (1) According to the nominal composition ratio of the master alloy, the titanium aluminum single crystal alloy is expressed in atomic percentage as follows: 41 ~ 56% Ti, 42 ~ 52% Al, 2 ~ 9% (Nb, Cr, Mn); The master alloy is melted and poured for multiple times to obtain a polycrystalline titanium-aluminum master alloy ingot block with a certain shape and uniform composition; (2) subjecting the polycrystalline titanium aluminum block cast in step (1) to high temperature heat treatment; (3) subjecting the titanium aluminum block subjected to high temperature heat treatment in step (2) to low temperature annealing heat treatment; (4) Repeat the above high and low temperature cycle heat treatment several times to obtain bulk single crystal titanium aluminum; In step (2), a tube furnace is used to perform high temperature annealing treatment on the polycrystalline titanium aluminum block at a temperature of 1200° C. to 1400° C. for 2 to 10 hours; In step (3), a tube furnace is used to perform a low temperature heat treatment process on the polycrystalline titanium aluminum block at a temperature of 1000 to 1100° C., and the annealing time is 2 to 10 hours; In step (4), the polycrystalline titanium aluminum block is repeatedly subjected to high-low cycle heat treatment 20 to 30 times using a tubular furnace, and the rate of decreasing from high temperature to low temperature or increasing from low temperature to high temperature is controlled at 2 to 10°C / min.

2. The method for preparing a bulk titanium aluminum single crystal alloy according to claim 1, characterized in that: In step (1), titanium, aluminum and other constituent elements are mixed into a master alloy in proportion, and smelted at high temperature in a vacuum electromagnetic induction suspension melting furnace. The melt is cast in a regular shell to obtain a rod-shaped or sheet-shaped titanium-aluminum alloy ingot.

3. The method for preparing a bulk titanium aluminum single crystal alloy according to claim 1, characterized in that: In step (1), the master alloy is subjected to vacuum induction melting 2 to 4 times to make the composition uniform.

4. The method for preparing a bulk titanium aluminum single crystal alloy according to claim 1, characterized in that: In step (2), the surface of the polycrystalline titanium aluminum block is first cleaned and dried, and then annealed in a reducing atmosphere, wherein the hydrogen gas volume fraction is 2.5% to 5%.

5. The method for preparing a bulk titanium aluminum single crystal alloy according to claim 1, characterized in that: In step (4), the titanium aluminum block is cut and surface polished after annealing.

6. A bulk titanium aluminum single crystal alloy, characterized in that: The bulk titanium-aluminum single crystal alloy block is prepared by the method for preparing the bulk titanium-aluminum single crystal alloy block according to any one of claims 1 to 5.

7. The bulk titanium aluminum single crystal alloy according to claim 6, characterized in that: The size of the bulk titanium aluminum single crystal alloy block is: a diameter between 7.0 mm and 20 mm, and a length between 25.0 mm and 200.0 mm.

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