WO2015182454A1

WO2015182454A1 - TiAl-BASED CASTING ALLOY AND METHOD FOR PRODUCING SAME

Info

Publication number: WO2015182454A1
Application number: PCT/JP2015/064491
Authority: WO
Inventors: 鉄井　利光
Original assignee: 国立研究開発法人物質・材料研究機構
Priority date: 2014-05-28
Filing date: 2015-05-20
Publication date: 2015-12-03
Also published as: JP2015224372A; JP6284232B2

Abstract

Provided is a TiAl-based casting alloy in which the crystal grain size of the TiAl-based casting alloy is reduced without relying on deposits, and that is provided with excellent high-temperature creep strength and excellent ductility and toughness at room temperature by executing a heat treatment under appropriate conditions. This TiAl-based casting alloy comprises 42-44 at.% Al, 6.0-9.0 at.% Nb, 0-3.5 at.% Cr, 0.3-1.0 at.% Si, 0.3-1.0 at.% C, and Ti and inevitable impurities as the remainder, wherein: the alloy has a composition wherein the alloy element index P found by the following formula is within a range from -0.9 to 1.5; the alloy includes a microstructure in which lamellar particles wherein an α2 phase and a γ phase are layered are tightly arranged, the lamellar particles having an average crystal grain size of from 30 to 200 µm; and the alloy has a structure in which no β phase is present. P=(41-Al)/3+0.25Nb+0.8Cr-0.8Si-1.7C.

Description

TiAl-based cast alloy and method for producing the same

The present invention relates to a TiAl base alloy suitable for use in turbine rotor blades such as power generation gas turbines and jet engines, and more particularly to a TiAl base cast alloy having a fine crystal grain size and excellent in high temperature creep strength and room temperature ductility.

In recent years, a TiAl-based alloy that is lightweight and excellent in heat resistance has attracted attention as a material used for turbine blades of power generation gas turbines and jet engines. In particular, in the case of large rotating blades, the lighter the components of the moving blade, the smaller the centrifugal stress, so the maximum number of revolutions can be improved, the moving blade can be increased in area, and the disk can be attached to the moving blade. The load stress can be reduced.

This TiAl-based alloy is an alloy mainly composed of a TiAl phase or a Ti ₃ Al phase, which is an intermetallic compound phase excellent in high-temperature strength, and is excellent in heat resistance as described above. When a casting method is used as a method for producing a TiAl-based alloy member, in a conventional TiAl-based cast alloy, in order to improve high-temperature creep strength, the composition and heat treatment conditions are adjusted to obtain an α2 / γ complete lamellar structure. It is common to do. In this case, the high temperature creep strength is improved, but there is a problem that ductility and toughness at room temperature (for example, 25 ° C.) are lowered. The biggest reason is that the crystal grain size (colony size of α2 / γ complete lamellar structure) becomes coarse. Unlike a forged material, a cast material does not have a strain effect or recrystallization due to plastic working, and thus the crystal grain size is inevitably increased.

In order to improve the problem of the TiAl-based cast alloy having poor ductility and toughness at room temperature, it is essential to refine the crystal grain size. There are 1-3 proposals. Patent Document 1 proposes a method in which Al ₂ O ₃ particles formed by intentionally oxidizing the inside of an alloy in the course of a manufacturing process are used as a pinning effect for preventing grain size coarsening. Patent Document 2 proposes a method in which silicide precipitated by adding Si is similarly used as a pinning effect. Patent Document 3 proposes a method in which a boride precipitated by adding B is similarly used as a pinning effect.

Japanese Patent No. 3694341 Japanese Patent Laid-Open No. 7-252562 Japanese Patent Publication No. 7-76399

However, since all of the techniques of Patent Documents 1-3 are methods in which precipitates are forcibly mixed in a TiAl-based cast alloy, if the amount of precipitates is small, the effect of refining the crystal grain size (pinning) (Effect) does not occur, and when the amount of precipitates increases, the crystal grain size becomes finer, but the influence as precipitates increases, and there is a problem of lowering ductility and toughness at room temperature.

The present invention solves the above-mentioned problems in the TiAl-based cast alloy, and by reducing the crystal grain size of the TiAl-based cast alloy without using precipitates, it is excellent in high-temperature creep strength and at room temperature. An object is to provide a TiAl-based cast alloy having good ductility and toughness and a method for producing the same.

The TiAl-based cast alloy of the present invention solves the above problems, Al: 42 to 44 atomic%, Nb: 6.0 to 9.0 atomic%, Cr: 0 to 3.5 atomic%, Si: 0 .3 to 1.0 atomic%, C: 0.3 to 1.0 atomic%, balance: Ti and inevitable impurities, and an alloying element index P calculated by the following formula is −0.9 to 1.5, Preferably, it has a composition within a range of −0.9 to 1.0, and is a fine structure in which lamella grains having an average crystal grain size of 30 to 200 μm and laminated with an α2 phase and a γ phase are densely arranged. And it consists of a structure | tissue which does not have (beta) phase.
P = (41-Al) /3+0.25Nb+0.8Cr-0.8Si-1.7C

The method for producing a TiAl-based cast alloy according to the present invention solves the above-mentioned problem, and has a fine structure in which lamella grains having an average crystal grain size of 30 to 200 μm in which α2 phase and γ phase are laminated are densely arranged. And a TiAl-based cast alloy having a structure having no β phase, Al: 42 to 44 atom%, Nb: 6.0 to 9.0 atom%, Cr: 0 to 3.5 atom %, Si: 0.3 to 1.0 atomic%, C: 0.3 to 1.0 atomic%, balance: Ti and inevitable impurities, and an alloying element index P determined by the following formula is −0. A raw material having a composition in the range of 9 to 1.5, preferably −0.9 to 1.0 is melted and cast, and then held at a temperature range of 1250 to 1300 ° C. for 1 to 30 hours, and 3 to 20 It comprises a step of cooling and heat-treating at a cooling rate of [° C./min].
P = (41-Al) /3+0.25Nb+0.8Cr-0.8Si-1.7C

In the TiAl-based cast alloy of the present invention, Ti is a basic constituent element of the alloy.
In the TiAl-based cast alloy of the present invention, when Al is in the range of 42 to 44 atomic%, the β phase exists in the high temperature range immediately after solidification after casting, but the β phase does not exist in the final state after heat treatment. , Α2 / γ complete lamellar structure, and high temperature creep strength is improved. When Al is less than 42 atomic%, the ratio of the α2 phase in the lamellar grains is excessively increased, so that the room temperature ductility is lowered. When Al exceeds 44 atomic%, the ratio of α2 phase in the lamellar grains becomes too small, so that the high temperature creep strength is lowered.

Nb contributes to improving the oxidation resistance of the TiAl-based cast alloy and is preferably in the range of 6.0 to 9.0 atomic%. When Nb is less than 6.0 atomic%, the oxidation resistance cannot be improved. When Nb exceeds 9.0 atomic%, the β phase may remain in the final state after heat treatment, and the weight increases, which is not preferable particularly for aircraft parts and rotating parts.
Cr contributes to β-phase formation in a high temperature state immediately after solidification after casting, and is preferably in the range of 0 to 3.5 atomic%. Cr is an optional composition component, but if Cr exceeds 3.5 atomic%, the β phase may remain in the final state after heat treatment, which is not desirable.

Si contributes to the improvement of the high temperature creep strength of the TiAl-based cast alloy and is preferably in the range of 0.3 to 1.0 atomic%. When Si is less than 0.3 atomic%, the effect of improving the high temperature creep strength cannot be obtained. When Si exceeds 1.0 atomic%, the room temperature ductility decreases.
C contributes to the improvement of the high temperature creep strength of the TiAl base cast alloy, and is preferably in the range of 0.3 to 1.0 atomic%. When C is less than 0.3 atomic%, the effect of improving the high temperature creep strength cannot be obtained. When C exceeds 1.0 atomic%, the room temperature ductility decreases.

In the TiAl-based cast alloy of the present invention, the alloy element index P is in the range of −0.9 to 1.5, preferably −0.9 to 1.0. When the alloy element index P is less than −0.9, the effect of the β phase existing in the high temperature region immediately after solidification after casting is small, and the crystal grain size becomes coarse, so that the room temperature ductility is lowered. When the alloy element index P exceeds 1.5 atomic%, the β phase may remain after the heat treatment, so that the high temperature creep strength is low and the usable temperature is low.
In the TiAl-based cast alloy of the present invention, it is preferable that the average crystal grain size of lamella grains is 200 μm or less because room temperature ductility is ensured. In order to make the average crystal grain size of lamella grains less than 30 μm, a great production cost is generated, which is not practical as an industrial product. On the other hand, when the average crystal grain size exceeds 200 μm, the ductility and toughness at room temperature decrease.

In the method for producing a TiAl-based cast alloy of the present invention, the holding temperature in the heat treatment step after casting is in the equilibrium temperature region in the α single phase region, and the temperature range is 1250 to 1300 ° C. When the temperature is lower than 1250 ° C., the complete lamellar structure is not formed because of the α + γ region. When the temperature exceeds 1300 ° C., the β phase may remain depending on the cooling rate because of the α + β region, and the high-temperature creep strength decreases.

In the method for producing a TiAl-based cast alloy of the present invention, the time for maintaining in the equilibrium temperature region in the α single phase region in the heat treatment step after casting is 1 to 30 hours. If the holding time is less than 1 hour, the time may be too short and the α single phase may not be obtained. When holding time exceeds 30 hours, time is too long and the crystal grain size of the lamella grain of the final cast alloy after heat processing will coarsen.

In the method for producing a TiAl-based cast alloy of the present invention, the cooling rate after maintaining for a predetermined time in the equilibrium temperature region in the α single phase region in the heat treatment step after casting is preferably 3 to 20 [° C./min]. When the cooling rate is less than 3 [° C./min], it is too slow and the interval between the α2 phase and the γ phase in the lamellar grains is coarsened, so that the high temperature creep strength is lowered. When the cooling rate exceeds 20 [° C./min], the ratio of the α2 phase in the lamellar grains is too high, and the room temperature ductility is lowered.

In the present invention, the phase change process (L → L + β → β →) that did not occur in the conventional TiAl base cast alloy is achieved by significantly changing the composition from that of the conventional TiAl base cast alloy and performing heat treatment under appropriate conditions. α + β → α → α + γ → α2 + γ). Due to this effect, the crystal grain size of the TiAl-based cast alloy can be made finer without relying on the precipitates required in the prior art. Specific effects are as follows. First, in the process of melting the raw material and cooling from the liquid phase after casting, the particle size is refined by the effect of the β phase existing in the high temperature region. (If two phases, a liquid phase and a β phase, or an α phase and a β phase coexist, the particle size is necessarily reduced). In addition, in the heat treatment step after casting, the β phase disappears and the high temperature creep strength is high after cooling by maintaining at a predetermined time in the equilibrium temperature region within the α single phase region and cooling at a predetermined cooling rate. An α2 / γ complete lamellar tissue is obtained. In other words, since the final state after heat treatment becomes an α2 / γ complete lamellar structure with a fine crystal grain size (colony size), a TiAl base cast alloy excellent in high temperature creep strength, ductility at room temperature, and toughness can be obtained. Can do.
In other words, the present invention makes it possible to achieve both room temperature ductility and high temperature creep strength, which was difficult with conventional TiAl-based cast alloys.

The present invention relates to a TiAl base cast alloy material prepared by melting a raw material as a component of the TiAl base cast alloy of the present invention in a high frequency melting furnace and then casting it with a cast iron mold, and (A) is a front view of the cast alloy material. (B) is a side view of the cast alloy material, and (C) is an appearance photograph of the cast iron mold mold used. In the reflection electron image structure photograph of the TiAl base cast alloy of the present invention, (A) shows alloy 5 and (B) shows alloy 7. In the reflection electron image structure photograph of the TiAl base cast alloy of the present invention, (C) shows the alloy 10 and (D) shows the alloy 13. In the reflected electron image structure photograph of the TiAl base cast alloy of the present invention, (E) shows the alloy 16. As a comparative example, a reflection electron image structure photograph of a TiAl base cast alloy having an alloy element index P of less than −0.9, (A) shows Alloy 3 and (B) shows Alloy 8. As a comparative example, a reflection electron image structure photograph of a TiAl-based cast alloy having an alloy element index P of less than −0.9, (C) shows alloy 14 and (D) shows alloy 20. As a comparative example, a reflection electron image structure photograph of a TiAl-based cast alloy having an alloy element index P exceeding 1.5, (A) shows Alloy 1 and (B) shows Alloy 4. As a comparative example, a reflection electron image structure photograph of a TiAl-based cast alloy having an alloy element index P exceeding 1.5, (C) shows the alloy 11 and (D) shows the alloy 17. As a comparative example, a reflection electron image structure photograph of a TiAl-based cast alloy having an alloy element index P exceeding 1.5, (E) shows the alloy 21. As a comparative example, a reflection electron image structure of a TiAl-based cast alloy having an alloy element index P in the range of −0.9 to 1.5 but an Al component ratio of less than 42.0 at% or more than 44.0 at% In the photograph, (A) shows Alloy 2 and (B) shows Alloy 19. As a comparative example, the reflected electron image structure of a TiAl-based cast alloy in which the alloy element index P is in the range of −0.9 to 1.5, but the component ratio of C is less than 0.3 at% or more than 1.0 at% In the photograph, (A) shows alloy 6 and (B) shows alloy 9. As a comparative example, an alloy element index P is in a range of −0.9 to 1.5, but a reflection electron image structure photograph of a TiAl based cast alloy having a C component ratio exceeding 1.0 at%. Alloy 18 is shown. As a comparative example, the reflected electron image structure of a TiAl-based cast alloy in which the alloy element index P is in the range of −0.9 to 1.5 but the Si component ratio is less than 0.3 at% or more than 1.0 at% In the photograph, (A) shows alloy 15 and (B) shows alloy 12. As a comparative example, in the TiAl base cast alloy of the alloy 10 of the invention alloy, a reflection electron image photograph of the TiAl base cast alloy in the case where the holding temperature of the heat treatment condition is less than 1250 ° C. or more than 1300 ° C., (A) is the holding temperature. Is 1230 ° C., and (B) shows a holding temperature of 1320 ° C. As a comparative example, in the TiAl base cast alloy of the alloy 10 of the invention alloy, a reflection electron image photograph of the TiAl base cast alloy when the heat treatment condition holding time is less than 1 hour or more than 30 hours, (A) is the holding time. Is 0.5 hour, and (B) shows a retention time of 40 hours. As a comparative example, in a TiAl base cast alloy of the alloy 10 of the invention alloy, a reflected electron image structure photograph of the TiAl base cast alloy when the cooling rate of the heat treatment condition is less than 3 [° C./min] or exceeds 20 [° C./min] (A) shows a cooling rate of 2 [° C./min], and (B) shows a cooling rate of 30 [° C./min].

[Example 1]
FIG. 1 is a photograph of the appearance of a TiAl base cast alloy material produced using raw materials as components of the TiAl base cast alloy of the present invention. (A) is a cast alloy material front surface, (B) is a cast alloy material side surface, and (C) is an appearance photograph of the cast iron mold mold used. The raw materials were melted in a high-frequency melting furnace using an yttria crucible for the compositions shown in Table 1. A cast alloy material having a similar oxygen concentration can be obtained even if a calcia crucible is used. In addition to sponge Ti, the raw materials used were Al, Nb, Cr, and Si granular materials as other composition components, and C was TiC powder. The dissolution atmosphere is in argon gas. Three minutes after each raw material was completely dissolved, the molten metal was poured into a cast iron mold mold shown in (C) and solidified therein to produce a TiAl-based cast alloy material. Although the weight of the cast alloy material in the photograph is about 700 g, it becomes about 350 g after cutting the hot water.

Table 1 shows the component composition of the TiAl-based cast alloy material produced by the above method, the result of the structure observation after the heat treatment test, and the result of the strength test after the heat treatment under appropriate conditions.

Next, the heat treatment test performed on the cast alloy material of alloy 1-21 shown in Table 1 will be described in detail.
For small pieces (10mm x 10mm) cut out from each cast alloy material, using an electric furnace capable of temperature and time program control, the three conditions of holding temperature, holding time and cooling rate among the heat treatment conditions are as follows. The heat treatment was performed in the air while changing.
Holding temperature: 1200-1350 ° C
Holding time: 0.5-50 hours Cooling rate: 1-30 [° C / min]
Next, the cross-section of the small piece after the heat treatment was subjected to a structure observation by a reflection electron image of a scanning electron microscope, and it was determined whether or not the heat treatment conditions were appropriate from the state of the structure. Here, the organization judged to be appropriate by the tissue observation satisfies the following two requirements.
(I) A fine structure in which lamella grains having an average crystal grain size of 30 to 200 μm in which α2 phase and γ phase are laminated are densely arranged.
(Ii) The tissue has no β phase.
Here, the “average crystal grain size” refers to a value determined in accordance with JIS G 0551. Further, “lamellar grains are densely arranged” means that the gap between the lamellar grains is less than 10 μm in the observation of the structure of the alloy sample by the reflected electron image of the scanning electron microscope.
“No β phase” means a phase having a brighter contrast than those of the α2 phase and the γ phase in addition to those identified as precipitates in the observation of the structure of the alloy sample by the reflection electron image of the scanning electron microscope. Is not confirmed.

Subsequently, the details of the evaluation of the mechanical properties of the cast alloy material of the alloy 1-21 shown in Table 1 will be described.
Regarding the cast alloy material of Alloy 1-21, the remaining material after cutting the heat treatment test piece was subjected to heat treatment under conditions appropriate for each cast alloy material obtained in the heat treatment test, and the parallel part diameter was 4 mm. Two test pieces were processed, and the following two strength tests were performed. Here, the heat treatment conditions of the alloy according to the invention were set to satisfy the requirements (i) and (ii) determined to be appropriate in the heat treatment test. The heat treatment conditions for the comparative alloy were set to appropriate conditions for the inventive alloy having a similar Al component ratio in the overall composition of the alloy. Specific heat treatment conditions are shown below.
Alloy 2, 4-7: Hold at 1265 ° C. for 10 hours, then cool at 3 [° C./min] Alloy 13-13: Hold at 1275 ° C. for 10 hours, then cool at 5 [° C./min] 14-16 18, 19: After holding at 1285 ° C. for 10 hours and cooling at 10 [° C./min], the mechanical properties of Alloys 1, 3, 8, 17, 20, and 21 were not evaluated.
The strength test performed was a rupture test at 870 ° C. × 225 MPa and a tensile test at room temperature. As for the former, the high temperature creep strength of each alloy was compared according to the length of the rupture time, and it was judged that the rupture time was good when it was 50 hours or more. Moreover, the latter was compared with the room temperature ductility of each alloy by the magnitude of elongation, and when the elongation was 0.5% or more, it was judged that it was favorable.

Subsequently, the reflected electron image structure photograph of the cast alloy material of Alloy 1-21 shown in Table 1 will be described separately for the invention alloy and the comparative alloy.
2A, FIG. 2B, and FIG. 2C are reflection electron image structure photographs of the alloys according to the present invention. (A) is alloy 5, (B) is alloy 7, (C) is alloy 10, (D) is alloy 13, (E ) Shows the alloy 16. Both have a complete lamellar structure composed of α2 phase and γ phase, and have a fine structure in which fine lamella grains having an average crystal grain size of 30 to 200 μm in which α2 phase and γ phase are laminated are densely arranged. . Further, there is no β phase (phase having a brighter contrast than α2 phase and γ phase) having low high-temperature creep strength. That is, it can be said that the TiAl base cast alloy of the present invention is excellent in both high temperature creep strength and room temperature ductility because it has the above-described structure.

[Comparative Example 1]
FIG. 3A and FIG. 3B are reflected electron image structure photographs of a TiAl-based cast alloy having an alloy element index P of less than −0.9 as a comparative example, (A) is Alloy 3, (B) is Alloy 8, and (C) Indicates an alloy 14, and (D) indicates an alloy 20. In Alloy 3, the alloy element index P is -0.96, in Alloy 8 is -0.96, in Alloy 14 is -1.01, and in Alloy 20 is -1.01. Since all of the comparative alloys shown in FIG. 3 have an alloy element index P of less than −0.9, the average crystal grain size is coarsened to exceed 200 μm.

[Comparative Example 2]
4A, FIG. 4B, and FIG. 4C are reflection electron image structure photographs of a TiAl-based cast alloy having an alloy element index P exceeding 1.5 as a comparative example, where (A) is alloy 1, (B) is alloy 4, and ( C) shows the alloy 11, (D) shows the alloy 17, and (E) shows the alloy 21. In alloy 1, the alloy element index P is 1.87, in alloy 4 is 1.62, in alloy 11 is 1.81, in alloy 17 is 1.89, and alloy 21 is 1.68. . Since all of the comparative alloys of FIG. 4 have an alloy element index P exceeding 1.5, a β phase (a phase having a brighter contrast than the α2 phase and the γ phase) exists.

[Comparative Example 3]
FIG. 5 shows a comparative example of a TiAl-based cast alloy having an alloy element index P in the range of −0.9 to 1.5, but an Al component ratio of less than 42.0 at% or more than 44.0 at%. In the reflected electron image structure photograph, (A) shows alloy 2 and (B) shows alloy 19. The alloy element index P is 0.66 for alloy 2 and 0.52 for alloy 19. The component ratio of Al is 41.0 at% for alloy 2 and 45.0 at% for alloy 19. In Alloy 2, since the component ratio of Al is low, there are too many α2 phase ratios in the lamellar grains, and even in a high magnification image, there is a portion where the laminated structure with the γ phase having a darker contrast than the α2 phase is unclear, Small elongation at room temperature. In alloy 19, since the Al component ratio is high, the ratio of α2 phase in the lamellar grains is too small, and the creep rupture time is short.

[Comparative Example 4]
6A and 6B show, as a comparative example, a TiAl group in which the alloy element index P is in the range of −0.9 to 1.5, but the component ratio of C is less than 0.3 at% or more than 1.0 at%. In the reflection electron image structure photograph of the cast alloy, (A) shows the alloy 6, (B) shows the alloy 9, and (C) shows the alloy 18. The alloying element index P is 0.60 for alloy 6, -0.31 for alloy 9, and 0.24 for alloy 18. The component ratio of C is 0.2 at% for the alloy 6, 1.2 at% for the alloy 9, and 1.2 at% for the alloy 18. The alloy 6 has an alloy element index P in the range of −0.9 to 1.5, so that the result of the structure observation is good. However, since the C component ratio is low, the creep rupture time is short. In Alloys 9 and 18, the alloy element index P is in the range of −0.9 to 1.5, so that the result of the structure observation is good. However, since the C component ratio is high, the elongation at room temperature is small.

[Comparative Example 5]
FIG. 7 shows, as a comparative example, a TiAl-based cast alloy in which the alloy element index P is in the range of −0.9 to 1.5, but the Si component ratio is less than 0.3 at% or more than 1.0 at%. In the reflected electron image structure photograph, (A) shows the alloy 15 and (B) shows the alloy 12. The alloy element index P is 0.62 for the alloy 15 and −0.03 for the alloy 12. The component ratio of Si is 0.2 at% for the alloy 15 and 1.2 at% for the alloy 12. Since the alloy element index P of the alloy 15 is in the range of −0.9 to 1.5, the result of the structure observation is good. However, since the Si component ratio is low, the creep rupture time is short. Since the alloy element index P of the alloy 12 is in the range of −0.9 to 1.5, the result of the structure observation is good, but the elongation at room temperature is small because of the high Si component ratio. Note that the fine precipitate having a bright contrast in the alloy 12 is silicide caused by Si.

[Comparative Example 6]
FIG. 8 shows a TiAl-based cast alloy having an alloy element index P in the range of −0.9 to 1.5 as a comparative example, but the reflection when the holding temperature under the heat treatment condition is less than 1250 ° C. or more than 1300 ° C. An electronic image structure photograph is shown. (A) is a reflection electron image structure photograph in the case where the holding temperature is 1230 ° C. with the alloy 10 of the invention alloy, and (B) is a reflected electron image structure photograph in the case where the holding temperature is 1320 ° C. In (A), since the holding temperature of the heat treatment conditions is as low as 1230 ° C., the α single phase is not formed and a complete lamellar structure is not formed. In (B), since the holding temperature of the heat treatment condition is as high as 1320 ° C., a β phase (a phase having a brighter contrast than the α2 phase and the γ phase) exists.

[Comparative Example 7]
FIG. 9 shows a TiAl-based cast alloy having an alloy element index P in the range of −0.9 to 1.5 as a comparative example, but the reflection when the heat treatment condition holding time is less than 1 hour or more than 30 hours. An electronic image structure photograph is shown. (A) is a reflection electron image structure photograph in the case of the alloy 10 of the invention alloy when the holding time is 0.5 hours, and (B) is a reflected electron image structure photograph in the case of the alloy 10 of the invention alloy and the holding time is 40 hours. In (A), the holding time of the heat treatment conditions is as short as 0.5 hours, the α single phase is not formed, and a complete lamellar structure is not formed. In (B), the holding time of the heat treatment conditions is too long, 40 hours, and the average crystal grain size exceeds 200 μm and is coarsened.

[Comparative Example 8]
FIG. 10 shows a TiAl-based cast alloy having an alloy element index P in the range of −0.9 to 1.5 as a comparative example, but the cooling rate under heat treatment conditions is less than 3 [° C./min] or 20 [° C. / Min] shows a reflected electron image structure photograph. (A) is a backscattered electron image of the alloy 10 of the invention alloy when the cooling rate is 2 [° C / min], and (B) is a backscattered electron image of the alloy 10 of the invention alloy and the cooling rate of 30 [° C / min]. It is an organization photograph. In (A), the cooling rate under heat treatment conditions is as slow as 2 [° C./min], the interval between lamella grains is too large, and the high temperature creep strength is lowered. (B) is a γ phase having a darker contrast than the α2 phase even in a high-magnification image because the cooling rate of the heat treatment condition is too high at 30 [° C./min] and the ratio of α2 phase in the lamellar grains is too large. There is a part where the laminated structure is unclear. This also reduces the ductility at room temperature.

As described above, in the strength test of the inventive alloy 10 shown in Table 1, heat treatment is performed under appropriate conditions.

The above embodiments are shown for the purpose of explaining the present invention, and do not limit the scope of rights of the present invention. The scope of right of the present invention includes the scope disclosed herein, as well as the scope obvious to those skilled in the art based on this disclosure.

The TiAl-based cast alloy of the present invention is suitable for use as a moving blade of a power generation gas turbine or an aircraft jet engine.
For example, when the TiAl-based cast alloy material of the present invention is used, a moving blade that is lightweight and excellent in high-temperature creep strength, room temperature ductility, and toughness can be obtained. By using these blades for power generation gas turbines and aircraft jet engines, it is possible to contribute to reducing carbon dioxide emissions and fuel consumption by improving energy efficiency while maintaining reliability. It becomes.

Claims

Al: 42 to 44 atomic%, Nb: 6.0 to 9.0 atomic%, Cr: 0 to 3.5 atomic%, Si: 0.3 to 1.0 atomic%, C: 0.3 to 1. The composition is composed of 0 atomic%, the balance: Ti and inevitable impurities, and the alloy element index P obtained by the following formula is in the range of −0.9 to 1.5, and the α2 phase and the γ phase are laminated. A TiAl-based cast alloy characterized in that it has a fine structure in which lamella grains having an average crystal grain size of 30 to 200 μm are densely arranged and has no β phase.
P = (41-Al) /3+0.25Nb+0.8Cr-0.8Si-1.7C
This is a method for producing a TiAl-based cast alloy having a fine structure in which lamella grains having an average crystal grain size of 30 to 200 μm laminated with an α2 phase and a γ phase are densely arranged and a structure in which no β phase is present. And
Al: 42 to 44 atomic%, Nb: 6.0 to 9.0 atomic%, Cr: 0 to 3.5 atomic%, Si: 0.3 to 1.0 atomic%, C: 0.3 to 1. After melting and casting a raw material having a composition of 0 atomic%, balance: Ti and inevitable impurities, and an alloying element index P determined by the following formula within a range of −0.9 to 1.5, 1250 A method for producing a TiAl-based cast alloy comprising the steps of: holding at a temperature range of ˜1300 ° C. for 1 to 30 hours, and cooling and heat-treating at a cooling rate of 3 to 20 [° C./min].
P = (41-Al) /3+0.25Nb+0.8Cr-0.8Si-1.7C
The TiAl-based cast alloy is characterized in that the L → L + β → β → α + β → α → α + γ → α2 + γ transformation occurs in the melting process of the raw material, the cooling process from the liquid phase after casting, and the heat treatment process. The manufacturing method of the TiAl base casting alloy of Claim 2.