TiAl alloy rich in large-size alpha2 phase twins and a method of manufacturing the same

Abstract

The application provides a TiAl alloy rich in large-size alpha2 phase twins, in particular, second-type twins, which contains 34-36 at.% of Al, 7-8 at.% of Nb, and the balance of Ti and inevitable impurities. The application also provides a method for preparing the TiAl alloy rich in large-size alpha2 phase second-type twins, which comprises the following steps: melting the alloy by a vacuum arc furnace to obtain a cast ingot, carrying out homogenization treatment at 1200 DEG C for 9-12 hours, and then quenching after being kept at 900-1000 DEG C for 3-6 hours to obtain the TiAl alloy rich in large-size alpha2 phase second-type twins. A large number of second-type twins with large size can be generated in the alloy, which provides suitable research materials for the research on deformation twins, solves the problem that the deformation twins are difficult to be characterized and researched, and provides a basis for further strengthening of the alloy performance.

Description

Technical Field

[0001] This invention relates to a novel TiAl alloy and a method for preparing the same, particularly to a TiAl alloy rich in large-size α2 phase twins and a method for preparing the same, and especially to a TiAl alloy rich in large-size α2 phase second-type twins and a method for preparing the same. Background Technology

[0002] TiAl alloys possess a series of advantages, including low density, high specific strength, high specific stiffness, good oxidation resistance, and good high-temperature creep resistance. They have broad application prospects in the aerospace field, especially in the 600-800℃ range. Due to their weight reduction effect, TiAl alloys are expected to replace high-temperature titanium alloys as materials for low-pressure turbine blades and become the next generation of lightweight high-temperature structural materials.

[0003] Therefore, scholars have conducted research on improving the mechanical properties of TiAl alloys from various perspectives. Among these, deformation behavior has become a research focus due to its significant impact on the mechanical properties of the alloys. Slip and twinning are the two major plastic deformation mechanisms in metallic materials. For TiAl alloys, the α2 phase is one of the main constituent phases. Especially for the hexagonal α2 phase, due to the limited slip system, deformation twinning plays a more important role in plastic deformation.

[0004] It is generally believed that the formation of deformation twins in the α2 phase is extremely difficult due to its ordered structure, which is also considered one of the reasons for the poor plasticity of the α2 phase. Previous studies suggested that deformation twins would not form in the α2 phase of Ti3Al alloys with stoichiometric Al content (25 at%). In recent years, {202(-)1}α2 compression twins have been found in γ-TiAl alloys with a higher Al content (44 at%) after deformation under specific conditions. However, these twins are very small, only at the nanometer scale, and characterizing their nanoscale structure is very difficult. Generally, only transmission electron microscopy (TEM) can be used, which is costly and challenging. As a result, the scientific community still knows very little about the mechanism related to α2 deformation twins.

[0005] Therefore, to deepen our understanding of the mechanisms related to α2 deformation twinning, we need to reduce the difficulty of characterization and increase the size of the twins. However, in γ-TiAl alloys, α2 exists as a lamellar structure, which is inherently small, limiting the size of the twins. Therefore, to obtain large-sized twins for related research, the alloy composition needs to be redesigned. Twenty years ago, Kishida et al. discovered that in non-stoichiometric Ti3Al single crystals, under high-temperature deformation above 1000℃, Ti3Al exhibits a second type of twin, '{2(-)121(-)0(-)3}'. This type of twin has a high content and large size, which is beneficial for reducing the difficulty of characterization and deepening our understanding of the mechanisms related to α2 deformation twinning. However, due to the difficulty and high cost of preparing single crystals, and their poor mechanical properties, it has not yet been truly put into practice.

[0006] In view of this, the inventors of this invention aim to design and invent a novel alloy that is easy to generate α2 deformation twins, especially type II twins. It can be prepared by melting, which is simpler and easier to obtain than single crystals. After deformation, the alloy generates a large number of large-sized type II twins, thus providing suitable research material for the study of deformation twins, solving the problem of the difficulty in characterizing deformation twins, and providing a basis for further strengthening the alloy properties. Summary of the Invention

[0007] Therefore, in order to further study the role of α2 phase deformation twins in the plastic deformation of Ti3Al alloys and overcome the problems of difficult preparation, high cost, poor mechanical properties, and lack of application value of existing Ti3Al '{2(-)12 1(-)0(-)3}' type II twin single crystals, the inventors of this invention have made great strides and designed and invented a new alloy that easily produces type II twins. Its particularly preferred composition is Ti-(34-36)Al-(7-8)Nb (atomic percentage, at.%). Compared with the α2 phase in traditional TiAl alloys where deformation twins are difficult to produce, the new alloy designed in this invention can produce a large number of large-sized micron-sized '{2(-)121(-)0(-)3}' type II α2 twins after deformation. This provides a foundation for subsequent research on α2 phase deformation twins and the improvement of alloy properties, provides suitable research material for the study of deformation twins, solves the problem of difficult characterization of deformation twins, and provides a basis for further strengthening of alloy properties. Moreover, the alloy of the present invention can be prepared by melting, which is much simpler and easier to obtain than the α2 phase single crystal of the existing Ti3Al alloy.

[0008] Specifically, the inventors of this invention considered that the conditions for the formation of ordered α2-phase twins are quite demanding. Existing research shows that a higher aluminum content is beneficial for twin precipitation, but when the Al content is too high, the γ phase will precipitate in the α2 phase, forming lamellar clusters, which will limit the twin size. Nb is beneficial for improving the brittleness of TiAl alloys, and dissolved Nb helps reduce the order of the α2 phase, thereby promoting twin formation. Nb is a β-stabilizing element, which helps increase the β-phase content in the alloy, forming a two-phase structure and reducing the alloy's brittleness. Therefore, the inventors of this invention carefully selected the Al and Nb contents.

[0009] The inventors of this invention also considered the influence of heat treatment and hot working processes on the phase transformation of TiAl alloys. When the Al content is 34-36 at%, and the Nb content is 7-9 at%, the alloy is less likely to generate the γ phase at high temperatures, while a higher content of the α2 phase will be generated. Ordered α2 phase twins only appear at higher temperatures. Therefore, the inventors of this invention believe that the deformation treatment temperature needs to be 900℃ or higher. Since the recrystallization phenomenon of the alloy is very active at this temperature, in order to reduce the influence of dynamic recrystallization on the microstructure, the deformation time needs to be reduced. Therefore, hot deformation requires a high deformation rate and a small deformation amount. Therefore, the inventors of this invention ultimately believe that a feasible solution is to perform the deformation at 900-1000℃ with a deformation rate of 0.01 s. -1 The deformation rate is 10-20%.

[0010] Specifically, the present invention achieves the above-mentioned objectives through the following means.

[0011] According to a specific embodiment of the present invention, a TiAl alloy rich in large-size twins, particularly α2 phase type II twins, is provided, which contains 34-36 at.% Al, 7-8 at.% Nb, and the balance being Ti and unavoidable impurities.

[0012] According to one embodiment of the present invention, the TiAl alloy rich in large-size α2 phase type II twins contains 35 at.% Al, 7.5 at.% Nb, and the balance being Ti and unavoidable impurities.

[0013] According to a specific embodiment of the present invention, a method for preparing a TiAl alloy rich in large-size α2 phase type II twins as described above is provided, wherein after obtaining the alloy ingot by melting in a vacuum arc furnace, it needs to be homogenized by holding at 1100℃~1300℃ for 9~12 hours, and then quenched after holding at 900-1000℃ for 3~6 hours.

[0014] According to an embodiment of the present invention, a method for preparing TiAl alloy rich in large-size α2 phase type II twins is further preferably wherein, after obtaining the alloy ingot by vacuum arc furnace melting, it is homogenized by holding at 1200°C for 10 hours, and then quenched after holding at 900-1000°C for 5 hours.

[0015] A method for preparing a TiAl alloy rich in large-size α2 phase type II twins according to an embodiment of the present invention, wherein, more preferably, after the homogenization treatment and the quenching, the TiAl alloy is subjected to hot compression deformation with a deformation amount of 10-15%.

[0016] A method for preparing a TiAl alloy rich in large-size α2 phase type II twins according to an embodiment of the present invention, wherein, more preferably, the hot compression deformation is performed at 900-1000°C for 0.01 s⁻¹ after the homogenization treatment and the quenching. -1 The deformation rate was determined.

[0017] A method for preparing a TiAl alloy rich in large-size α2 phase type II twins according to an embodiment of the present invention includes the following steps:

[0018] Step (1) Ingredient preparation

[0019] Using grade zero sponge titanium, high-purity aluminum, and niobium aluminum alloy smelting and casting ingots as raw materials, they were mixed evenly according to the component ratio to prepare a certain mass of sample for use. In order to prevent a large amount of Al from volatilizing during smelting, an additional 3 wt.% of Al was added during the batching to compensate for the element loss that may occur during smelting.

[0020] Step (2) Melting and casting ingots

[0021] The raw materials in step (1) are melted into ingots using a vacuum arc melting furnace (VAR) or a vacuum solidification furnace, and the melting process is repeated 5 times.

[0022] Step (3) Heat treatment

[0023] The smelted TiAl alloy ingot is first homogenized and then heat-treated to adjust the microstructure to a two-phase structure.

[0024] Step (4) Wire cutting

[0025] A cylinder is cut from the center of the above-mentioned ingot using a CNC wire cutting machine.

[0026] Step (5) Thermal deformation

[0027] The cut, heat-treated compressed sample is subjected to hot compression deformation, with a deformation amount of 10%-20%.

[0028] According to an embodiment of the present invention, a method for preparing TiAl alloy rich in large-size α2 phase type II twins is further preferably described in step (2), wherein, when the raw materials in step (1) are melted into ingots using a vacuum arc melting furnace, the melting process is repeated 5 times, and from the second time onwards, the ingot needs to be flipped over each time to obtain an ingot with uniform composition, the melting vacuum degree is controlled to be lower than 0.1 Pa, and the melting current is controlled in the range of 0.2 kA to 0.3 kA.

[0029] According to an embodiment of the present invention, a method for preparing TiAl alloy rich in large-size α2 phase type II twins is provided, wherein, more preferably, in the heat treatment of step (3), the homogenization treatment is to place the ingot in a heat treatment furnace, hold it at 1200°C for 10 hours, and then cool it in the furnace.

[0030] According to an embodiment of the present invention, a method for preparing TiAl alloy rich in large-size α2 phase second twins is further preferably described in step (3) heat treatment, wherein the microstructure is adjusted to a two-phase microstructure by means of placing the sample after homogenization treatment into a heat treatment furnace, holding it at 1000°C for 5 hours, and then taking it out and quickly quenching it in room temperature water.

[0031] Through observation and analysis, the microstructure of the alloy of the present invention at room temperature is a two-phase structure of βo+α2. After hot deformation, the α2 phase contains a large number of deformed twins with a size of tens of micrometers. The twin type is mainly the second type of twin '{2(-)12 1(-)0(-)3}', and part of it is {101(-)1} twin. The above-mentioned invention objective has been achieved. Compared with the prior art, the advantages and beneficial effects of the present invention are as follows.

[0032] 1. The Al content in traditional Ti3Al alloys prevents the formation of twins in the α2 phase. In TiAl alloys, the α2 phase twins are very small, only nanometer-sized, due to the influence of phase size, making them difficult to analyze. The alloy of this invention can produce a large number of large α2 phase twins, reaching the micrometer scale, facilitating related characterization studies.

[0033] 2. Compared with Ti3Al single crystal with high Al content, the present invention can be prepared by melting method, which is simple and low cost. Moreover, the alloy is a two-phase βo+α2, which is less prone to brittle fracture and has better deformation performance than the single phase α2.

[0034] 3. Unlike traditional alloys, the twins produced in this invention are mainly of the second type, α2 twins, '{2(-)12 1(-)0(-)3}'. This type of twin is difficult to find in traditional alloys, and related research is also difficult. Attached Figure Description

[0035] Figure 1The images show the microstructure and α2 phase IPF distribution of the Ti-35Al-8Nb alloy after heat treatment and hot deformation in Example 1 of this invention, obtained by scanning electron microscopy (BSE-SEM). (a) is the BSE-SEM image of the microstructure; (b) is the IPF image of the α2 phase; and (c) is the diffraction contrast and special interface color diagram, where the black lines represent the second type of twin boundaries '{2(-)12 1(-)0(-)3}' and the white lines represent the {101(-)1} twin boundaries.

[0036] Figure 2 The images show the microstructure and α2 phase IPF distribution of the Ti-35Al-8Nb alloy after heat treatment and hot deformation in Example 2 of this invention, obtained by scanning electron microscopy (BSE-SEM). (a) is the BSE-SEM image of the microstructure; (b) is the IPF image of the α2 phase; and (c) is the diffraction contrast and special interface color diagram, where the black lines represent the second type of twin boundaries '{2(-)12 1(-)0(-)3}' and the white lines represent the {101(-)1} twin boundaries. Implementation

[0037] This invention prepares a TiAl alloy that can generate a large number of α2 phase type II twins and explores its preparation method. The atomic percentage content of the TiAl alloy is: Ti-(34-36)Al-(7-8)Nb, with the balance being Ti, and contains a small amount of unavoidable impurities such as H, O, and N.

[0038] The preparation method involves melting the alloy in a vacuum electric arc furnace, then homogenizing it by holding it at 1200℃ for 10 hours, followed by heat treatment, specifically holding it at 900-1000℃ for 5 hours and then quenching. After this heat treatment, the alloy is deformed at a relatively high rate (0.01 s⁻¹) at 900-1000℃. -1 Perform hot compression deformation with a small amount of deformation (10-15%).

[0039] The alloy obtained by this invention has a two-phase structure of βo+α2 at room temperature. After hot deformation, the α2 phase contains a large number of deformed twins with a size of tens of micrometers. The twin type is mainly the second type of twin '{2(-)12 1(-)0(-)3}', and part of it is the {101(-)1} twin.

[0040] The alloy can be prepared by vacuum arc melting. The prepared alloy must undergo a suitable heat treatment process to achieve an α2+β two-phase microstructure. A feasible process is as follows: the alloy is held at 900-1000℃ for 5 hours and then quenched. After heat treatment, the alloy is cut into compressible dimensions using wire cutting. Subsequently, it is deformed at 900-1000℃ at a high deformation rate (0.01 s⁻¹). -1Compressive deformation with a small amount of deformation (10-15%) is performed. Finally, a large amount of α2 phase deformation twins can be generated in the alloy.

[0041] Example

[0042] The present invention will be further illustrated below with reference to the preparation of button ingots and the characterization of material properties, but the present invention is not limited to the following embodiments.

[0043] Example 1

[0044] The TiAl alloy prepared in this embodiment has a composition of Ti-35Al-8Nb (all numbers represent atomic percentages), that is, it contains 35 at.% Al, 8 at.% Nb, and the balance is Ti and unavoidable impurities. The specific steps include:

[0045] (1) Ingredients:

[0046] The raw materials used for smelting and casting the ingot were sponge titanium (99.7 wt.%), high-purity aluminum particles (99.9 wt.%), and niobium-aluminum alloy (containing 72.63 wt.% niobium and 27.3 wt.% aluminum). These materials were mixed uniformly according to the component ratio to prepare a 40g sample for use. To prevent excessive volatilization of Al during smelting, which could deviate from the predetermined ratio, an additional 3 wt.% Al was added during the batching process, meaning the actual added Al mass was 1.03 times the predetermined mass, to compensate for potential element losses during smelting.

[0047] (2) Melting and casting ingots:

[0048] The raw materials from step (1) were melted into button ingots using a vacuum arc melting furnace (VAR). The melting process was repeated 5 times to obtain ingots with uniform composition. The melting vacuum was below 0.1 Pa, and the melting current was controlled within the range of 0.2 kA to 0.3 kA.

[0049] (3) Heat treatment:

[0050] The ingot was placed in a heat treatment furnace at 1200℃ and held for 10 hours before being cooled in the furnace for homogenization. Then the ingot was placed in a heat treatment furnace at 1000℃ and held for 5 hours before being taken out and quenched with water.

[0051] (4) Wire EDM:

[0052] A compression test specimen was cut from the center of the heat-treated ingot in step three using a CNC wire cutting machine. The specimen was a cylinder with dimensions of Φ5×10mm.

[0053] (5) Hot compression deformation:

[0054] The cut compression sample was subjected to high-temperature compression deformation on a Gleeble thermal simulation testing machine. The deformation temperature was 900℃, the deformation amount was 10%, and the deformation rate was 0.01s. -1 .

[0055] (6) Microscopic tissue observation:

[0056] The heat-treated specimen was cut parallel to the compression direction and mechanically polished until the surface was mirror-like and free of scratches. The tissue was observed using a scanning electron microscope, followed by electropolishing. After electropolishing, the sample was observed using electron backscatter diffraction (EBSD).

[0057] like Figure 1 As shown, EBSD observation of the Ti-35Al-8Nb alloy after high-temperature compression revealed that the microstructure mainly consists of the βo(ωo) phase and the α2 phase. The IPF image of the α2 phase after deformation showed a large number of large deformation twins within the α2 phase. Analysis of the phase difference between the twins and the parent phase indicated that the main type of twin is '{2(-)121(-)0(-)3}' type II twins, with a small number being {101(-)1} twins, such as... Figure 1 As shown in (c).

[0058] Example 2

[0059] The TiAl alloy prepared in this embodiment has a composition of Ti-35Al-8Nb (all numbers represent atomic percentages), that is, it contains 35 at.% Al, 8 at.% Nb, and the balance is Ti and unavoidable impurities. The preparation method includes the following steps:

[0060] (1) Ingredients:

[0061] The raw materials used for smelting and casting the ingot were sponge titanium (99.7 wt.%), high-purity aluminum particles (99.9 wt.%), and niobium-aluminum alloy (containing 72.63 wt.% niobium and 27.3 wt.% aluminum). These materials were mixed uniformly according to the component ratio to prepare a 40g sample for use. To prevent excessive volatilization of Al during smelting, which would cause the ingot composition to deviate from the predetermined composition, an additional 3 wt.% Al was added during batching, meaning the actual added Al mass was 1.03 times the predetermined mass, to compensate for potential element losses during smelting.

[0062] (2) Melting and casting ingots:

[0063] The raw materials from step one were melted into button ingots using a vacuum arc melting furnace (VAR). The melting process was repeated five times to obtain ingots with uniform composition. The melting vacuum was below 0.1 Pa, and the melting current was controlled within the range of 0.2 kA to 0.3 kA.

[0064] (3) Heat treatment:

[0065] The ingot was placed in a heat treatment furnace at 1200℃ and held for 10 hours before being cooled in the furnace for homogenization. Then the ingot was placed in a heat treatment furnace at 1000℃ and held for 5 hours before being taken out and quenched with water.

[0066] (4) Wire EDM:

[0067] A compression test specimen was cut from the center of the heat-treated ingot in step three using a CNC wire cutting machine. The specimen was a cylindrical specimen with dimensions of Φ5×10mm.

[0068] (5) Hot compression deformation:

[0069] The cut compression sample was subjected to high-temperature compression deformation on a Gleeble thermal simulation testing machine. The deformation temperature was 1000℃, the deformation amount was 10%, and the deformation rate was 0.01s. -1 .

[0070] (6) Microscopic tissue observation:

[0071] The heat-treated specimen was cut parallel to the compression direction and mechanically polished until the surface was mirror-like and free of scratches. The tissue was observed using a scanning electron microscope, followed by electropolishing. After electropolishing, the sample was observed using electron backscatter diffraction (EBSD).

[0072] like Figure 2 As described above, EBSD observation of the Ti-35Al-8Nb alloy after high-temperature compression revealed that the microstructure of the alloy mainly consists of the βo(ωo) phase and the α2 phase. The IPF image of the α2 phase after deformation showed that a large number of large deformation twins were generated within the α2 phase. Analysis of the phase difference between the twins and the parent phase indicated that the main type of twins was '{2(-)121(-)0(-)3}' type II twins, with some being {101(-)1} twins, such as... Figure 2 As shown in (c).

Claims

1. A method for preparing TiAl alloys rich in large-size α2 phase twins, characterized in that, The TiAl alloy rich in large-size α2 phase twins contains 34-36 at.% Al, 7-8 at.% Nb, with the balance being Ti and unavoidable impurities. After obtaining the alloy ingot through vacuum arc furnace melting, it is homogenized by holding at 1100-1300℃ for 9-12 hours, followed by quenching after holding at 900-1000℃ for 3-6 hours. After the homogenization treatment and the quenching, the TiAl alloy is subjected to hot compression deformation with a deformation amount of 10-15%, wherein the hot compression deformation is carried out at 900-1000°C at a rate of 0.01 s⁻¹. -1 The deformation rate was determined.

2. The method for preparing TiAl alloys rich in large-size α2 phase type-two twins according to claim 1, characterized in that, After obtaining the alloy ingot by melting in a vacuum electric arc furnace, it is homogenized by holding at 1200℃ for 10 hours, and then quenched after holding at 900-1000℃ for 5 hours.

3. The method for producing TiAl alloys rich in large-size α2 phase second-type twins according to claim 1 or 2, characterized in that, Includes the following steps: Step (1) Ingredient preparation Using grade zero sponge titanium, high-purity aluminum, and niobium aluminum alloy smelting and casting ingots as raw materials, they were mixed evenly according to the component ratio to prepare a certain mass of test sample. In order to prevent a large amount of Al from volatilizing during smelting, an additional 3 wt.% of Al was added during the batching to compensate for the element loss that may occur during smelting. Step (2) Melting and casting ingots The raw materials in step (1) are melted into ingots using a vacuum arc melting furnace (VAR) or a vacuum solidification furnace, and the melting process is repeated 5 times. Step (3) Heat treatment The TiAl alloy ingot obtained from smelting is first homogenized, and then heat-treated to adjust the microstructure to a two-phase structure. Step (4) Wire cutting A cylinder is cut from the center of the above-mentioned ingot using a CNC wire cutting machine. Step (5) Thermal deformation The cut, heat-treated compressed sample is subjected to hot compression deformation, with a deformation amount of 10%-20%.

4. The method for producing TiAl alloys rich in large-size α2 phase second-type twins according to claim 3, characterized in that, In step (2), when the raw materials in step (1) are melted into ingots using a vacuum arc melting furnace, the melting process is repeated 5 times. From the second time onwards, the ingot needs to be flipped over each time to obtain an ingot with uniform composition. The melting vacuum degree is controlled to be lower than 0.1 Pa, and the melting current is controlled within the range of 0.2 kA to 0.3 kA.

5. The method for producing TiAl alloys rich in large-size α2 phase second-type twins according to claim 3, characterized in that, In the heat treatment of step (3), the homogenization process involves placing the ingot in a heat treatment furnace, holding it at 1200°C for 10 hours, and then cooling it in the furnace.

6. The method for producing TiAl alloys rich in large-size α2 phase second-type twins according to claim 3, characterized in that, In the heat treatment of step (3), the structure is adjusted to a two-phase structure by the heat treatment. Specifically, the sample after the homogenization treatment is placed in a heat treatment furnace, held at 1000°C for 5 hours, and then taken out and quickly placed in room temperature water for quenching.

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