TiAl alloy material

Abstract

To provide a TiAl alloy material that can achieve exceptionally high creep strength at high temperatures.SOLUTION: A TiAl alloy material comprises Al: 42.0 atom% or more and 44.0 atom% or less, Cu: 0.5 atom% or more and 2.5 atom% or less, and Nb: 3.0 atom% or more and 7.0 atom% or less, with the balance being Ti and inevitable impurities. The average grain size of lamellar grains is 20 μm or more and 200 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a TiAl alloy material having particularly excellent high-temperature creep strength and a TiAl intermediate alloy material for obtaining said TiAl alloy material. [Background technology]

[0002] In the field of TiAl alloys, forging TiAl alloys with high strength and excellent hot workability are attracting attention as replacements for conventional casting alloys with many casting defects and low material yields. In particular, in the field of transportation aircraft, there are increasing cases of TiAl alloys being used as materials for aircraft engines in order to reduce weight and improve fuel efficiency.

[0003] Among various TiAl alloys, conventional TiAl alloys such as casting TiAl alloys are composed of a γ phase whose crystal structure is close to a face-centered cubic lattice (FCC) structure and an α phase whose crystal structure is a hexagonal close-packed lattice (HCP) structure. The material structure of such TiAl alloys is characterized by the precipitation of thin plate-like γ phases in the α phase during the cooling process after heat treatment, forming a lamellar structure.

[0004] Since hot-forged products have superior strength and toughness compared to cast products, TiAl alloys for hot forging are being developed for use in components that require these properties. TiAl alloys for hot forging contain components that stabilize the β phase of a body-centered cubic lattice (BCC) structure, which is prone to deformation at high temperatures. The β phase is responsible for most of the deformation, making hot forging possible.

[0005] Examples of elements that stabilize the β-phase include Mn, Cr, V, and Nb, and adding these β-phase stabilizing elements to TiAl alloys is effective in improving hot forgeability. For example, Patent Document 1 discloses a forgable TiAl alloy in which Nb, V, and B are added to Ti and Al, and the boride grain size is specified. Patent Document 2 also discloses a TiAl-based alloy containing Al and Nb and in which the relative amounts of the added elements are adjusted, and a TiAl-based alloy in which Nb, V, Cr, and Mo are co-added to Ti and Al.

[0006] Furthermore, Patent Document 3 discloses a titanium-aluminum alloy material containing Ti, Al, Nb, Mo, and / or Mn, and B and / or C and / or Si, and having a specified β / B2-Ti phase ratio. Patent Document 4 discloses a titanium aluminide alloy containing titanium, aluminum, and niobium, and further composed of Cr, Zr, Mo, Fe, La, Sc, Y, Mn, Ta, V, and W, and having a specified lamellar structure. All of these alloys are intended to be hot forged.

[0007] As shown in Patent Documents 1 to 4, Nb is an important element for improving the oxidation resistance of TiAl alloys, and Nb is added to many TiAl alloys for hot forging. However, if Nb is added alone to a material with a high Al concentration, which is intended to form a gamma phase including a lamellar structure, it is not possible to sufficiently stabilize the beta phase in a practical forging temperature range below 1300°C. Therefore, many conventional techniques use "isothermal forging," a processing process with low manufacturability, or "co-add" Nb with other beta-stabilizing elements to form a sufficient beta phase in the forging temperature range and ensure good hot forgeability.

[0008] Furthermore, forged TiAl alloys require heat treatment to adjust the material structure after forging. Examples of such heat treatment methods include a first heat treatment, which involves a high-temperature heat treatment to recrystallize the α phase of the forged material and promote the formation of a single α phase, followed by a second heat treatment at a lower temperature to precipitate γ plates in the α phase and introduce a lamellar structure. The material structure after the second heat treatment then becomes the structure of the TiAl alloy material. In this case, the size and shape of the α phase are determined by the first heat treatment, and γ plates precipitate in the α phase to form a lamellar structure, so the size of the lamellar grains is essentially determined by the first heat treatment.

[0009] Although the α phase remains α at high temperatures, it becomes more ordered near room temperature and is sometimes referred to as the "α2 phase." The β phase becomes a "B2 structure" near room temperature. However, the terms α and β in this specification do not specifically limit the temperature. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 6687118 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-215631 [Patent Document 3] Patent No. 5926886 [Patent Document 4] Patent No. 5512964 Summary of the Invention [Problem to be solved by the invention]

[0011] However, even if Nb and other β-stabilizing elements are co-added using the methods described in Patent Documents 1 to 4, a large amount of β phase remains after heat treatment of the TiAl alloy. Although hot forgeability can be imparted, the high-temperature creep strength, which is one of the mechanical properties of the TiAl alloy material, is reduced.

[0012] Hot forging of TiAl alloys can be performed in two ways: normal die forging, in which the die is preheated to room temperature or several hundred degrees, or in which the die is heated to the same temperature as the forging material, such as 1200°C, for example. -3 and isothermal forging, which involves forging over a long period of time under conditions of a slow strain rate, such as 1 / sec. Among these hot forging methods, when isothermal forging is used, it is often possible to process even if there is a shortage of β phase, but isothermal forging requires an extremely long time per stroke, which reduces productivity when used for industrial products.

[0013] Adding β-stabilizing elements to TiAl alloys can improve hot forgeability, but the β phase remains after heat treatment, reducing the creep strength of the TiAl alloy. However, in order to apply TiAl alloys to a variety of future applications, particularly excellent high-temperature creep strength is required.

[0014] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a TiAl alloy material that can obtain particularly excellent high-temperature creep strength, and a TiAl intermediate alloy material for obtaining the TiAl alloy material. [Means for solving the problem]

[0015] The above object of the present invention is achieved by the following constitution [1] relating to a TiAl alloy material. [1] Al: 42.0 at% or more and 44.0 at% or less, Cu: 0.5 atomic % or more and 2.5 atomic % or less, and Nb: 3.0 atomic % or more and 7.0 atomic % or less, the balance being Ti and unavoidable impurities, A TiAl alloy material characterized in that the average grain size of the lamellar grains is 20 μm or more and 200 μm or less.

[0016] The above object of the present invention is also achieved by the following configuration [2] relating to the TiAl alloy intermediate material. [2] A TiAl intermediate alloy material for obtaining a TiAl alloy material, Al: 42.0 atomic% or more and 44.0 atomic% or less, Cu: 0.5 atomic % or more and 2.5 atomic % or less, and Nb: 3.0 atomic % or more and 7.0 atomic % or less, the balance being Ti and unavoidable impurities, A TiAl intermediate alloy material characterized in that the average grain size of α grains is 20 μm or more and 200 μm or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a TiAl alloy material having particularly excellent high-temperature creep strength and a TiAl intermediate alloy material for obtaining the TiAl alloy material. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a graph showing the relationship between the invention examples and the comparative examples, with the vertical axis representing the minimum creep rate and the horizontal axis representing the average particle size of the lamellar grains. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present inventors have found that by adding Nb and Cu at specified concentrations, the deformability of the α phase during hot forging can be improved, and the β phase can be reduced after heat treatment while ensuring sufficient β phase during forging. Furthermore, the present inventors have found that in TiAl alloy materials containing appropriate amounts of β-stabilizing elements, excellent high-temperature creep strength can be obtained by appropriately controlling the Cu concentration and specifying the grain size of the lamellar grains.

[0020] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present invention is not limited to the embodiment described below, and can be carried out with any modifications within the scope of the gist of the present invention.

[0021] [TiAl alloy material] The TiAl alloy material according to this embodiment is obtained by forging a TiAl alloy and performing heat treatment. Hereinafter, the components contained in the TiAl alloy material according to the present invention, the reasons for limiting the upper and lower limit values of their concentrations, and the particle sizes of lamellar grains and α grains will be described in detail.

[0022] <Al: 42.0 atomic % or more and 44.0 atomic % or less> Al is an element that promotes the formation of an Al2O3 protective film on the surface of the TiAl alloy material. By appropriately controlling the Al concentration in the TiAl alloy material, the base of oxidation resistance is improved, the γ phase is stabilized, a large amount of γ plates are formed into the α phase, and a lamellar structure is formed, whereby the creep strength of the TiAl alloy material can be improved. If the Al concentration in the TiAl alloy material is less than 42.0 atomic %, the desired creep strength cannot be obtained. Therefore, the Al concentration in the TiAl alloy material is set to 42.0 atomic % or more, preferably 42.5 atomic % or more. On the other hand, if the Al concentration in the TiAl alloy material exceeds 44.0 atomic %, the γ phase is excessively stabilized and γ grains are formed, so that the desired creep strength cannot be obtained. Also, the hot forging property of the TiAl alloy ingot for obtaining the TiAl alloy material is reduced. Therefore, the Al concentration in the TiAl alloy material is set to 44.0 atomic % or less, preferably 43.6 atomic % or less.

[0023] <Cu: 0.5 atomic % or more and 2.5 atomic % or less> Cu is an element that has the effect of stabilizing the β phase at high temperatures. Appropriately controlling the Cu content in the TiAl alloy material is the most important requirement in this embodiment. In conventional TiAl alloys, in order to improve hot forging properties, promoting the formation of the β phase at high temperatures led to the problem that the β phase remained even after heat treatment, resulting in a decrease in the high-temperature creep strength of the TiAl alloy material. In this embodiment, by co-adding Nb and Cu to the TiAl alloy at specified concentrations, the deformability of the α phase during hot forging can be improved. During forging, while sufficiently ensuring the β phase, the β phase after heat treatment can be reduced, and the hot forging properties of the TiAl alloy ingot can be improved. Also, the residual β phase after heat treatment can be minimized, excellent high-temperature creep strength can be obtained, and both the hot forging properties of the TiAl alloy ingot and the high-temperature creep strength of the TiAl alloy material can be achieved simultaneously.

[0024] When the Cu concentration in the TiAl alloy material is less than 0.5 atomic%, the effect of stabilizing the β phase at the temperature during hot forging of the TiAl alloy ingot cannot be obtained, and the effect of improving the deformability of the α phase during hot forging and forming the β phase only at the hot forging temperature cannot be obtained. For this reason, the hot forging properties of the TiAl alloy ingot decrease. Furthermore, when the Cu concentration is less than 0.5 atomic%, the average grain size of the lamellar grains becomes too large, resulting in a decrease in the minimum creep strength. Therefore, the Cu concentration in the TiAl alloy material should be 0.5 atomic% or more, preferably 0.7 atomic% or more, and more preferably 0.9 atomic% or more. On the other hand, when the Cu concentration in the TiAl alloy material exceeds 2.5 atomic%, the β phase remains after heat treatment, and the average grain size of the lamellar grains becomes too small, making it impossible to obtain the desired high-temperature creep strength. Therefore, the Cu concentration in the TiAl alloy material should be 2.5 atomic% or less, preferably 2.0 atomic% or less, more preferably 1.5 atomic% or less, and even more preferably 1.2 atomic% or less.

[0025] <Nb: 3.0 atomic% or more and 7.0 atomic% or less> Nb is an element that has the effect of improving the oxidation resistance of the TiAl alloy material. If the Nb concentration in the TiAl alloy material is less than 3.0 atomic %, the oxidation resistance of the TiAl alloy material will decrease. Also, if the Nb concentration is less than 3.0 atomic %, the average grain size of the lamellar grains will become too large, resulting in a decrease in the minimum creep strength. Therefore, the Nb concentration in the TiAl alloy material is set to 3.0 atomic % or more, and preferably 4.5 atomic % or more. On the other hand, if the Nb concentration in the TiAl alloy material exceeds 7.0 atomic %, the α phase becomes unstable, the formation of lamellar grains cannot be ensured, the average grain size of the lamellar grains becomes too small, and the high-temperature creep strength decreases. Therefore, the Nb concentration in the TiAl alloy material is set to 7.0 atomic % or less, and preferably 6.0 atomic % or less.

[0026] <Remainder> The remainder of the TiAl alloy material according to this embodiment, excluding the above-mentioned components, is Ti and unavoidable impurities, such as C, N, O, H, Cl, Fe, Mg, Ca, Mn, Cr, V, Mo, Sn, Bi, Co, Ni, Zr, Na, Be, and Zn.

[0027] <Average particle size of lamellar particles: 20 μm to 200 μm> The grain size of the lamellar grains is a factor that influences the creep properties of TiAl alloy materials. In TiAl alloy materials, if the average diameter of lamellar grains is less than 20 μm, the proportion of grain boundaries in the entire structure increases, and structural deterioration during creep deformation, which tends to progress near grain boundaries, occurs at a higher rate throughout the entire structure, resulting in poor creep properties. Therefore, the average diameter of lamellar grains in TiAl alloy materials is set to 20 μm or more, and preferably 30 μm or more.

[0028] Generally, in metallic materials, the fracture toughness improves with increasing heat treatment temperature during forging. However, when the average diameter of lamellar grains exceeds 200 μm, even high heat treatment temperatures can result in extremely brittle behavior. As a result, creep properties are significantly reduced, with fractures occurring due to stress concentration immediately after the start of creep testing. Therefore, the average diameter of lamellar grains in TiAl alloy materials should be 200 μm or less, preferably 150 μm or less, more preferably 100 μm or less, and even more preferably 75 μm or less.

[0029] The average diameter of lamellar grains can be determined, for example, as follows. First, a test specimen is prepared by mirror-finishing the surface of a TiAl alloy material using mechanical chemical polishing. Next, a backscattered electron image is taken of the test specimen near the center of the plate thickness using a scanning electron microscope (SEM). Lamellar grains are then drawn for each SEM photograph using image processing software, and the circle-equivalent diameter of the drawn lamellar grains is calculated using image analysis software "Image Pro Plus" (manufactured by Media Cybernetics). Note that if a β phase remains, the outline of the lamellar grain is drawn excluding the β phase. Furthermore, lamellar grains that are in contact with the four sides of the SEM photograph are excluded from the analysis. In this way, the average circle-equivalent diameter can be calculated from the circle-equivalent diameters of multiple lamellar grains to determine the average diameter of the lamellar grains.

[0030] In order to obtain excellent high-temperature creep strength without controlling the average grain size of the lamellar grains, for example, a method of more strictly controlling the Cu content in the TiAl alloy material can be mentioned. In this embodiment, even if the Cu content is increased to 2.5 atomic %, excellent creep properties can be obtained by controlling the average grain size of the lamellar grains in the TiAl alloy material.

[0031] [TiAl intermediate alloy material] The TiAl intermediate alloy material according to this embodiment is an intermediate for obtaining the above TiAl alloy material. Therefore, the composition of the TiAl intermediate alloy material is the same as that of the above TiAl alloy material. If the content of each element of the TiAl intermediate alloy material is controlled as described above, the effects of the above elements can be obtained. Further, the TiAl intermediate alloy material according to this embodiment represents an alloy material after forging a TiAl alloy ingot and performing the first heat treatment on the obtained forged material.

[0032] <Average grain size of α grains: 20 μm or more and 200 μm or less> The grain size of the α grains, like the grain size of the lamellar grains, is a factor that affects the creep properties of the TiAl alloy material. As described above, the structure size and morphology of the α phase are determined by the first heat treatment step (the first heat treatment step) for obtaining the TiAl alloy material, and γ plates precipitate on the α phase to form a lamellar structure. Therefore, the size of the lamellar grains is substantially determined by the first heat treatment. Therefore, by defining the average grain size of the α phase after the first heat treatment step, the average grain size of the lamellar grains can be controlled to a desired value. The upper limit value, lower limit value, and the reason for the limitation of the average grain size of the α grains are the same as the contents described for the average grain size of the lamellar grains. Note that the average grain size of the α grains is affected by conditions such as temperature and time in the first heat treatment step, and the addition amounts of Al, Nb, and Cu.

[0033] [Manufacturing method of TiAl alloy material] The TiAl alloy material according to this embodiment can be manufactured by a step of forging a TiAl alloy ingot having the same composition as the above TiAl alloy material to obtain a forged material, and a step of heat-treating this forged material at a predetermined temperature. Hereinafter, the manufacturing method of the TiAl alloy material according to this embodiment will be described in detail.

[0034] [Step of forging a TiAl alloy ingot] In this embodiment, the forging conditions when forging the TiAl alloy are not particularly limited. The forging process includes, for example, a step of heating the TiAl alloy ingot to a predetermined temperature and a step of applying pressure to the heated ingot. It is preferable to select an appropriate range for the heating temperature and pressure depending on the desired shape and the like.

[0035] <Process for heat treating forged material> In this embodiment, two heat treatments are performed on the forged material obtained by the forging process. In the first heat treatment process (first heat treatment process), a high-temperature heat treatment is performed to recrystallize the α phase of the forged material and bring it closer to a single α phase structure, thereby obtaining the TiAl intermediate alloy material according to this embodiment. Thereafter, the TiAl intermediate alloy material is subjected to a second heat treatment process (second heat treatment process) at a temperature lower than that of the first heat treatment process. This allows γ plates to precipitate in the α phase, forming an [α + γ] lamellar structure. In this embodiment, the average grain size of the lamellar grains in the TiAl alloy material and the average grain size of the α grains in the TiAl intermediate alloy material are specified. The average grain size of the α grains in the TiAl intermediate alloy material is determined by the conditions such as temperature and time in the first heat treatment process (first heat treatment process) and the amounts of Al, Nb, and Cu added, and thereby the average grain size of the lamellar grains in the TiAl alloy material is determined.

[0036] If the heat treatment time in the first heat treatment step is too short, the recrystallized α grains will not grow sufficiently, resulting in an excessively small average diameter of the α grains (lamellar grains). On the other hand, if the heat treatment time is too long, the average diameter of the α grains (lamellar grains) will be excessively large. Furthermore, since Nb and Cu affect the amount of β phase formed, which inhibits the growth of recrystallized α grains, low concentrations of these elements tend to increase the average diameter of the α grains (lamellar grains), while high concentrations of these elements tend to decrease the average diameter of the α grains (lamellar grains). Therefore, by appropriately adjusting the time conditions of the first heat treatment step to match the concentrations of Nb and Cu, in particular, in the alloy composition, the average diameter of the α grains and lamellar grains can be controlled, and the desired creep strength can be ensured.

[0037] Furthermore, in TiAl alloys containing Cu and Nb, the β phase, which inhibits the growth of recrystallized α grains, tends to be most reduced within a certain temperature range. Setting the heat treatment temperature within this temperature range can reduce the β phase and promote the formation of a single α phase. Because residual β phase, which has low strength at high temperatures, reduces the base creep strength, it is desirable to minimize the amount of β phase formed. Therefore, by appropriately adjusting the temperature conditions of the first heat treatment step to a temperature at which the β phase is most reduced in accordance with the alloy composition, particularly the Al concentration, the average grain size of lamellar grains can be controlled, ensuring the desired creep strength.

[0038] The TiAl alloy material according to the present embodiment is excellent in high-temperature resistance, lightweight, and has excellent creep strength, and is therefore preferably used as a component requiring such material properties, and therefore can be suitably used as a component for internal combustion engines such as turbines for transportation and industrial machinery. [Example]

[0039] Inventive examples and comparative examples of the TiAl alloy material according to the present invention will be described below.

[0040] [Evaluation of creep strength] (Production of forged materials) First, TiAl alloy raw materials with various concentrations of Al, Nb, and Cu were prepared, and TiAl alloy ingots weighing approximately 9 kg were fabricated by the cold crucible induction melting (CCIM) method. The TiAl alloy ingots were cylindrical with a tapered shape, with one axial end having a diameter of 110 mm, the other having a diameter of 85 mm, and an axial length of 300 mm.

[0041] Next, cylindrical test pieces with a diameter of 80 mm and an axial length of 120 mm were prepared from the TiAl alloy ingots obtained, and after holding them at a temperature of 1250°C or higher for 0.5 hours or more, the test pieces were subjected to uniaxial compression in the axial direction of the test pieces using a press at a reduction ratio of 70 to 80% to produce disc-shaped forged materials. Note that even if the reduction ratio was changed by about 10%, it is believed that this does not significantly affect the evaluation results of hot forgeability and creep strength.

[0042] (Preparation of test materials) From the disk-shaped forged material obtained as described above, six square pieces of material measuring 14 mm in width, 130 mm in length, and 24 mm in thickness were taken from approximately the center position in the radial direction, and one of these was selected as the test material for the creep strength evaluation test.

[0043] (Heat treatment) The above test material was subjected to two heat treatments under various conditions as shown in Table 1 below. The first heat treatment step was intended to approximate a single-phase α structure, and a TiAl intermediate alloy material was obtained by the first heat treatment step. The second heat treatment step was intended to form a γ phase and form an [α2 + γ] lamellar structure, and a TiAl alloy material (heat-treated material) was obtained by the second heat treatment step. The alloy composition of the obtained TiAl alloy material is also shown in Table 1 below.

[0044] (Creep strength evaluation test) From the obtained heat-treated material, a flanged creep test specimen was prepared with a total length of 80 mm, a parallel section diameter of 6 mm, a length of 30 mm, and M12 threads (at both longitudinal ends of the test specimen), and a creep plate test was carried out using a single-type creep testing machine. The test conditions were a temperature of 800°C and a stress of 150 MPa, and the minimum creep rate was determined from the test data. The minimum creep rate is an index of the degree of deformation that occurs at high temperatures, and is a value commonly used as one of the indexes of creep strength.

[0045] The creep strength evaluation criteria are: minimum creep rate of 3.0 × 10 -7 (sec -1) or less was deemed to be acceptable, and the minimum creep rate was 3.0 × 10 -7 (sec -1 ) was deemed to have failed.

[0046] [Observation of the structure of TiAl alloy material] (Creating test specimens) After taking creep test specimens from the heat-treated materials obtained by the above heat treatment, the surfaces of the remaining materials at the longitudinal ends were mirror-finished by mechanical chemical polishing to prepare test specimens for microstructure observation.

[0047] (Calculation of the average particle size of lamellar particles) A scanning electron microscope (SEM) was used to take backscattered electron images of the test specimens for microstructure observation. The images were taken near the center of the thickness of the test specimens, and one or more photographs were obtained by adjusting the magnification appropriately so that the lamellar grains could be evaluated for each test specimen. Then, lamellar grains were drawn on each SEM photograph using image processing software. Then, the average circle-equivalent diameter, which corresponds to the average grain size of the lamellar grains, was calculated using image analysis software "Image Pro Plus" (manufactured by Media Cybernetics).

[0048] Although β-phase remained at some lamellar grain boundaries, the outlines of the lamellar grains were drawn excluding the β-phase, and lamellar grains that contacted the four sides of the SEM photograph were excluded from the analysis. For each test specimen, the average circular equivalent diameter was calculated from 50 or more lamellar grains, but for Comparative Example No. 10, the lamellar grain diameter was too large to fit in the SEM photograph, so the average circular equivalent diameter was calculated from 18 lamellar grains.

[0049] The measurement results of the average grain size of lamellar grains and the minimum creep rate are also shown in the following Table 1. As mentioned above, the grain size of lamellar grains in the TiAl alloy material is determined by the grain size of α grains in the TiAl intermediate alloy material after the first heat treatment step, so the average grain size of lamellar grains shown in the following Table 1 can be considered to be the same value as the average grain size of α grains.

[0050] [Table 1]

[0051] As shown in Table 1 above, in invention examples Nos. 1 to 8, the compositions of the TiAl intermediate alloy material and the TiAl alloy material satisfy the requirements specified in the present invention, and the average grain sizes of the α grains of the TiAl intermediate alloy material and the lamellar grains of the TiAl alloy material also satisfy the requirements specified in the present invention, thereby achieving excellent high-temperature creep strength.

[0052] On the other hand, in Comparative Example No. 9, the average grain sizes of the α grains in the TiAl intermediate alloy material and the lamellar grains in the TiAl alloy material were below the lower limit of the range of the present invention, resulting in reduced high-temperature creep strength. The reason why the average grain sizes of the α grains and lamellar grains were below the lower limit of the range of the present invention is that, although the Cu content of Comparative Example No. 9 was within the range specified by the present invention, it was higher than that of the Examples, resulting in a large amount of β phase, which inhibits the growth of α grains. As shown in Inventive Examples Nos. 1 to 4, the average grain size of the lamellar grains increased with increasing heat treatment time. Therefore, it is believed that by increasing the heat treatment time of Comparative Example No. 9, the average grain sizes of the α grains and lamellar grains fell within the specified grain size range.

[0053] Comparative Example No. 10 exhibited brittle fracture and fractured immediately after the creep test because the average grain sizes of the α grains of the TiAl intermediate alloy material and the lamellar grains of the TiAl alloy material exceeded the upper limit of the range of the present invention. The reason why the average grain sizes of the α grains and lamellar grains exceeded the upper limit of the range of the present invention is that, although the Nb and Cu contents of Comparative Example No. 10 were within the ranges specified in the present invention, they were lower than those of the Examples, resulting in a smaller amount of β phase and easily coarsening of the α grains. In such cases, it is believed that lowering or excessively raising the heat treatment temperature would form β phase or γ phase, which would suppress the coarsening of the α grains and thereby prevent the coarsening of the crystal grains. Thus, as long as the compositions of the TiAl intermediate alloy material and the TiAl alloy material are within the ranges specified in the present invention, the average grain sizes of the α grains and lamellar grains can be controlled by adjusting the heat treatment conditions, thereby obtaining excellent high-temperature creep strength.

[0054] Figure 1 is a graph showing the relationship between the inventive examples and comparative examples, with the vertical axis representing the minimum creep rate obtained in a creep test and the horizontal axis representing the average diameter of lamellar particles. In Figure 1, a filled diamond (♦) indicates good creep strength, and a hollow diamond (◇) indicates poor creep strength. The range indicated by the dashed line in the figure is the region where the average diameter (μm) of lamellar particles falls within the numerical range of the present invention.

[0055] As shown in FIG. 1, in a TiAl alloy material containing Cu and Nb, when the composition is within the range specified in the present invention and the average particle size of the lamellar grains is within the range specified in the present invention, it is possible to obtain a TiAl alloy material having particularly excellent creep strength.

Claims

[Claim 1] Al: 42.0 at% or more and 44.0 at% or less, Cu: 0.5 atomic % or more and 2.5 atomic % or less, and Nb: 3.0 atomic % or more and 7.0 atomic % or less, the balance being Ti and unavoidable impurities; A TiAl alloy material characterized in that the average grain size of the lamellar grains is 20 μm or more and 200 μm or less.

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