TiAl ALLOY MATERIAL AND TiAl INTERMEDIATE ALLOY MATERIAL

A TiAl alloy material containing: 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, with the balance being Ti and unavoidable impurities, and lamellar grains have an average grain size of 20 μm or more and 200 μm or less.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC § 119 from Japanese Patent Application No. 2022-165610 filed on Oct. 14, 2022, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a TiAl alloy material having particularly excellent high-temperature creep strength and a TiAl intermediate alloy material for obtaining the TiAl alloy material.

BACKGROUND ART

In the field of TiAl alloys, forging TiAl alloys having high strength and excellent hot workability are attracting attention in place of related-art cast alloys having many casting defects and low material yields. In particular, in the field of transport planes and the like, cases of adopting TiAl alloy materials as materials for aircraft engines are increasing in order to reduce the weight and improve the fuel efficiency.

Among various TiAl alloys, a related-art TiAl alloy such as a casting TiAl alloy is composed of a γ phase having a crystal structure close to a face centered cubic lattice (FCC) structure and an α phase having a hexagonal close-packed lattice (HCP) structure as a crystal structure. The material structure of such a TiAl alloy is characterized in that a thin plate-like γ phase precipitates in the α phase during a cooling process after a heat treatment, and a lamellar structure is formed.

Since hot forged products are superior in both strength and toughness compared to cast products, in particular, the development of hot forging TiAl alloys is underway for the purpose of applying them to members requiring these properties. The hot forging TiAl alloy is added with a component that stabilizes a β phase having a body centered cubic lattice (BCC) structure that is easily deformed at a high temperature, and the β phase is responsible for most of the deformation, so that hot forging is possible.

Examples of the component that stabilizes the β phase include Mn, Cr, V, and Nb. Addition of these β phase stabilizing elements to the TiAl alloy is effective in improving the hot forgeability. For example, Patent Literature 1 discloses a forging TiAl alloy in which Nb, V, and B are added to Ti and Al and a grain size of a boride is specified. In addition, Patent Literature 2 discloses a TiAl-based alloy containing Al and Nb in which relative amounts of additive components are adjusted, and a TiAl-based alloy in which Nb, V, Cr, and Mo are co-added to Ti and Al.

Further, Patent Literature 3 discloses a titanium-aluminum-based 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. Further, Patent Literature 4 discloses a titanium aluminide alloy containing titanium, aluminum, and niobium, further containing Cr, Zr, Mo, Fe, La, Sc, Y, Mn, Ta, V, or W and having a predetermined lamellar structure. All of these alloys are assumed to be subjected to hot forging.

As shown in Patent Literatures 1 to 4, Nb is an important element for improving oxidation resistance of the TiAl alloy, and Nb is added to many hot forging TiAl alloys. However, assuming the formation of a γ phase including a lamellar structure, when Nb is added alone to a material set to a high concentration of Al, the β phase cannot be sufficiently stabilized in a practical forging heating temperature range of lower than 1300° C. Therefore, in most of the related art, either “isothermal forging”, which is a low-manufacturability working process, or “co-addition” of Nb and other β-stabilizing elements is used to form a sufficient p phase in the forging heating temperature range, and to ensure good hot forgeability.

In addition, the forging TiAl alloy requires a heat treatment for adjusting the material structure after forging. Examples of a method of such a heat treatment include a method in which a high-temperature heat treatment is performed as a first heat treatment to recrystallize an α phase of a forged material and to promote a generation of an α single phase and thereafter, a lower-temperature heat treatment is performed as a second heat treatment to precipitate a γ plate in the α phase and to introduce a lamellar structure. Then, the material structure after the second heat treatment becomes the structure of the TiAl alloy material. In this case, since the first heat treatment determines the size and the form of the α phase structure and the γ plate precipitates in the α phase to form a lamellar structure, the size of the lamellar grains is substantially determined by the first heat treatment.

The α phase maintains the α phase at a high temperature, but becomes more ordered at around room temperature, and is sometimes referred to as a “α2 phase”. In addition, the β phase becomes a “B2 structure” at around room temperature. However, the description of the α phase and the β phase in the description of the present application does not particularly limit the temperature.

CITATION LIST Patent Literature

    • Patent Literature 1: JP6687118B
    • Patent Literature 2: JP2009-215631A
    • Patent Literature 3: JP5926886B
    • Patent Literature 4: JP5512964B

SUMMARY OF INVENTION Technical Problem

However, even when Nb and other β-stabilizing elements are co-added using the methods described in Patent Literatures 1 to 4, there are problems in which a large amount of β phase remains after the heat treatment for the TiAl alloy, and even when the hot forgeability can be imparted, high-temperature creep strength, which is one of mechanical properties of the TiAl alloy material, decreases.

Examples of the hot forging for the TiAl alloy include general die forging in which a die is preheated to room temperature or several hundred Celsius degrees, and isothermal forging in which a die is heated to a temperature same as that of a forging material, such as 1200° C., and forging is performed taking time under conditions where a strain rate is slow, such as 10−3/sec. Of these hot forging methods, when isothermal forging is used, it is often possible to work even when the β phase is insufficient, but since the isothermal forging takes a very long time per stroke, the productivity decreases when industrial products are to be obtained.

Addition of the β-stabilizing element to the TiAl alloy can impart the hot forgeability, but the β phase remains even after the heat treatment, resulting in a decrease in creep strength of the TiAl alloy material. However, in order to apply the TiAl alloy material to various uses in the future, particularly excellent high-temperature creep strength is required.

The present invention has been made in view of such problems, and an object thereof is to provide a TiAl alloy material capable of obtaining particularly excellent high-temperature creep strength and a TiAl intermediate alloy material for obtaining the TiAl alloy material.

Solution to Problem

The above object of the present invention is achieved by the following configuration (1) related to a TiAl alloy material.

    • (1) A TiAl alloy material containing:
      • 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,
      • with the balance being Ti and unavoidable impurities, in which
      • lamellar grains have an average grain size of 20 μm or more and 200 μm or less.

In addition, the above object of the present invention is achieved by the following configuration (2) related to a TiAl intermediate alloy material.

    • (2) A TiAl intermediate alloy material for obtaining a TiAl alloy material, the TiAl intermediate alloy material containing:
      • 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,
      • with the balance being Ti and unavoidable impurities, in which
      • α grains have an average grain size of 20 μm or more and 200 μm or less.

Advantageous Effects of Invention

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 DESCRIPTION OF DRAWINGS

The FIG. 1s a graph showing a relationship between Invention Examples and Comparative Examples, where the vertical axis represents a minimum creep rate and the horizontal axis represents an average grain size of lamellar grains.

 DESCRIPTION OF EMBODIMENTS

The inventors of the present disclosure have found that by co-adding Nb and Cu at a specified concentration, deformability of an α phase during hot forging can be improved, and the amount of a β phase after a heat treatment can be reduced while sufficiently ensuring the β phase during forging. In addition, the inventors of the present disclosure have found that in a TiAl alloy material containing a β-stabilizing element in an appropriate content, excellent high-temperature creep strength can be obtained by appropriately controlling a concentration of Cu in particular and specifying a grain size of lamellar grains.

Hereinafter, embodiments for carrying out the present disclosure (hereinafter, referred to as “the present embodiment”) will be described in detail. The present disclosure is not limited to the embodiments described below, and can be optionally modified and implemented without departing from the gist of the present disclosure.

[TiAl Alloy Material]

A TiAl alloy material according to the present embodiment is obtained by forging a TiAl alloy and performing a heat treatment. Hereinafter, components contained in the TiAl alloy material according to the present disclosure, reasons for limiting upper limit values and lower limit values of concentrations of the components, and a grain size of lamellar grains and a grain size of a grains will be described in detail.

<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 a concentration of Al in the TiAl alloy material, baseline of oxidation resistance is improved, a γ phase is stabilized, and a large amount of γ plate is formed in the α phase to form a lamellar structure. Accordingly, creep strength of the TiAl alloy material can be improved.

When the concentration of Al in the TiAl alloy material is less than 42.0 atomic %, the desired creep strength cannot be obtained. Therefore, the concentration of Al in the TiAl alloy material is 42.0 atomic % or more, and preferably 42.5 atomic % or more.

On the other hand, when the concentration of Al in the TiAl alloy material is more than 44.0 atomic %, the γ phase is excessively stabilized and γ grains are formed, making it impossible to obtain the desired creep strength. In addition, hot forgeability of a TiAl alloy ingot for obtaining the TiAl alloy material also decreases. Therefore, the concentration of Al in the TiAl alloy material is 44.0 atomic % or less, and preferably 43.6 atomic % or less.

<Cu: 0.5 Atomic % or More and 2.5 Atomic % or Less>

Cu is an element that has an effect of stabilizing the 0 phase at a high temperature, and appropriately controlling a content of Cu in the TiAl alloy material is the most important requirement in the present embodiment. In a related-art TiAl alloy, when the formation of the β phase at a high temperature is promoted in order to improve the hot forgeability, there are problems in which the β phase remains even after a heat treatment and the high-temperature creep strength of the TiAl alloy material decreases. In the present embodiment, by co-adding Nb and Cu to the TiAl alloy at a specified concentration, the deformability of the α phase during hot forging can be improved, the amount of the β phase after the heat treatment can be reduced while sufficiently ensuring the β phase during forging, and the hot forgeability of the TiAl alloy ingot can be improved. In addition, the β phase remaining after the heat treatment can be minimized, excellent high-temperature creep strength can be obtained, and both the hot forgeability of the TiAl alloy ingot and the high-temperature creep strength of the TiAl alloy material can be achieved.

When the concentration of Cu in the TiAl alloy material is less than 0.5 atomic %, it is not possible to obtain the effect of stabilizing the β phase at the temperature during hot forging of the TiAl alloy ingot, and it is not possible to improve the deformability of the α phase during hot forging and form the β phase only at a hot forging temperature. Therefore, the hot forgeability of the TiAl alloy ingot decreases. Further, when the concentration of Cu is less than 0.5 atomic %, the average grain size of the lamellar grains is too large, and the minimum creep rate decreases. Therefore, the concentration of Cu in the TiAl alloy material is 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 concentration of Cu in the TiAl alloy material is more than 2.5 atomic %, the β phase remains after the heat treatment, and the average grain size of the lamellar grains is too small to obtain the desired high-temperature creep strength. Therefore, the concentration of Cu in the TiAl alloy material is 2.5 atomic % or less, preferably 2.0 atomic % or less, more preferably 1.5 atomic % or less, and still more preferably 1.2 atomic % or less.

<Nb: 3.0 Atomic % or More and 7.0 Atomic % or Less>

Nb is an element that has an effect of improving the oxidation resistance of the TiAl alloy material.

When a concentration of Nb in the TiAl alloy material is less than 3.0 atomic %, the oxidation resistance of the TiAl alloy material decreases. In addition, when the concentration of Nb is less than 3.0 atomic %, the average grain size of the lamellar grains is too large, and the minimum creep rate decreases. Therefore, the concentration of Nb in the TiAl alloy material is 3.0 atomic % or more, and preferably 4.5 atomic % or more.

On the other hand, when the concentration of Nb in the TiAl alloy material is more than 7.0 atomic %, the α phase is unstable, the formation of lamellar grains cannot be ensured, the average grain size of the lamellar grains is too small, and the high-temperature creep strength decreases. Therefore, the concentration of Nb in the TiAl alloy material is 7.0 atomic % or less, and preferably 6.0 atomic % or less.

<Balance>

The balance of the TiAl alloy material according to the present embodiment, excluding the above components, is Ti and unavoidable impurities. Examples of the unavoidable impurities include C, N, O, H, Cl, Fe, Mg, Ca, Mn, Cr, V, Mo, Sn, Bi, Co, Ni, Zr, Na, Be, and Zn.

<Average Grain Size of Lamellar Grains: 20 μm or More and 200 μm or Less>

The grain size of the lamellar grains is a factor that influences creep properties of the TiAl alloy material.

In the TiAl alloy material, when the average grain size of the lamellar grains is less than 20 μm, the ratio of grain boundaries in the entire structure increases, and structure deterioration during creep deformation, which tends to progress in the vicinity of grain boundaries, occurs at a higher ratio in the entire structure, resulting in poor creep properties. Therefore, the average grain size of the lamellar grains in the TiAl alloy material is 20 μm or more, and preferably 30 μm or more.

In general, for metal materials, the higher the heat treatment temperature during forging, the higher the fracture toughness. However, when the average grain size of the lamellar grains is more than 200 μm, an extremely brittle behavior is exhibited even when the heat treatment temperature is high. As a result, the creep properties are remarkably decreased, such as rupture caused by stress concentration immediately after the start of a creep test. Therefore, the average grain size of the lamellar grains in the TiAl alloy material is 200 μm or less, preferably 150 μm or less, more preferably 100 μm or less, and still more preferably 75 μm or less.

The average grain size of the lamellar grains can be obtained, for example, as follows. First, the surface of the TiAl alloy material is mirror-finished by mechanical and chemical polishing to prepare a test piece. Next, a backscattered electron image is taken using a scanning electron microscope (SEM) near a center of a plate thickness of the test piece. Thereafter, for each SEM photograph, lamellar grains are drawn using image processing software, and circle equivalent diameters of the drawn lamellar grains are calculated using image analysis software “Image Pro Plus” (manufactured by Media Cybernetics). When the β phase remains, contours of the lamellar grains are drawn while removing the β phase. In addition, the lamellar grains in contact with four sides of the SEM photograph are excluded from the analysis. By calculating an average circle equivalent diameter from the circle equivalent diameters of a plurality of lamellar grains in this way, the average grain size of the lamellar grains can be obtained.

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 content of Cu in the TiAl alloy material can be mentioned. In the present embodiment, even when the amount of Cu added 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.

[TiAl Intermediate Alloy Material]

The TiAl intermediate alloy material according to the present embodiment is an intermediate for obtaining the above TiAl alloy material. Therefore, the composition of the TiAl intermediate alloy material is the same as the composition of the above TiAl alloy material, and if the content of each element in the TiAl intermediate alloy material is controlled as described above, the effect of each element can be obtained. In addition, the TiAl intermediate alloy material according to the present embodiment represents an alloy material after forging a TiAl alloy ingot and subjecting the obtained forged material to a first heat treatment.

<Average Grain Size of a Grains: 20 μm or More and 200 μm or Less>

Similar to the grain size of the lamellar grains, the grain size of the α grains is a factor that influences the creep properties of the TiAl alloy material. As described above, since a first heat treatment step for obtaining the TiAl alloy material (first heat treatment step) determines the size and the form of the α phase structure and the γ plate precipitates in the α phase to form a lamellar structure, the size of the lamellar grains is substantially determined by the first heat treatment. Therefore, by specifying 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. An upper limit value and a lower limit value of the average grain size of the α grains and reasons for limiting them are the same as those described for the average grain size of the lamellar grains. The average grain size of the α grains is influenced by conditions such as a temperature and a time in the first heat treatment step and the amounts of Al, Nb and Cu added.

[Method for Manufacturing TiAl Alloy Material]

The TiAl alloy material according to the present 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 subjecting the forged material to a heat treatment at a predetermined temperature. Hereinafter, the method for manufacturing a TiAl alloy material according to the present embodiment will be described in detail.

<Step of Forging TiAl Alloy Ingot>

In the present embodiment, forging conditions for forging the TiAl alloy ingot are not particularly limited. The forging step includes, for example, a step of heating the TiAl alloy ingot to a predetermined temperature and a step of applying a pressure to the heated ingot.

It is preferable to select an appropriate range for the heating temperature, the pressure, and the like depending on the desired shape and the like.

<Step of Subjecting Forged Material to Heat Treatment>

In the present embodiment, the forged material obtained by the above forging step is subjected to two heat treatments. In the first heat treatment step, a high-temperature heat treatment is performed to recrystallize the α phase of the forged material to bring it closer to an a single phase structure, thereby obtaining the TiAl intermediate alloy material according to the present embodiment. Thereafter, the TiAl intermediate alloy material is subjected to a second heat treatment step at a temperature lower than that in the first heat treatment step. Accordingly, the γ plate can be precipitated in the α phase to form a lamellar structure of [α2+γ]. In the present 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 conditions such as the temperature and the time in the first heat treatment step and the amounts of Al, Nb and Cu added, and accordingly the average grain size of the lamellar grains in the TiAl alloy material is determined.

When the heat treatment time in the first heat treatment step is too short, the recrystallized α grains do not grow sufficiently and therefore the average grain size of the α grains (lamellar grains) is too small. When the heat treatment time is too long, the average grain size of the α grains (lamellar grains) is too large. In addition, Nb and Cu influence the formation amount of the β phase that inhibits the growth of the recrystallized α grains. Therefore, when the concentrations of Nb and Cu are low, the average grain size of the α grains (lamellar grains) tends to increase, and when the concentrations are high, the average grain size of the α grains (lamellar grains) tends to decrease. Therefore, by appropriately adjusting the time conditions in the first heat treatment step according to the alloy composition, particularly the concentrations of Nb and Cu, the average grain sizes of the α grains and the lamellar grains can be controlled, and the desired creep strength can be ensured.

In addition, in the TiAl alloy containing Cu and Nb, the amount of the β phase, which inhibits the growth of the recrystallized α grains, tends to reduce most in a certain temperature range, and by setting the heat treatment temperature in this temperature range, the amount of the β phase can be reduced and the generation of the α single phase can be promoted. Since the remaining β phase, which has low strength at a high temperature, has the effect of decreasing the baseline of creep strength, it is desirable to reduce the formation amount of the β phase as much as possible. Therefore, by appropriately adjusting the temperature conditions in the first heat treatment step to the temperature at which the amount of the β phase is most reduced according to the alloy composition, particularly the concentration of Al, the average grain size of the lamellar grains can be controlled, and the desired creep strength can be ensured.

The TiAl alloy material according to the present embodiment has excellent high-temperature resistance, is lightweight, and has excellent creep strength, and is thus preferably used as a member that requires such material properties. Therefore, the TiAl alloy material according to the present embodiment can be suitably used, for example, as a member for an internal combustion engine such as a turbine for transport planes and industrial machines.

EXAMPLES

Hereinafter, Examples and Comparative Examples of the TiAl alloy material according to the present disclosure will be described.

[Creep Strength Evaluation] (Preparation of Forged Material)

First, TiAl alloy raw materials with various concentrations of Al, Nb, and Cu were prepared, and TiAl alloy ingots each weighing about 9 kg were prepared by a cold crucible induction melting (CCIM) method. Each of the TiAl alloy ingots had a tapered cylinder shape, and had a diameter of 110 mm at one end face in an axial direction, a diameter of 85 mm at the other end face in the axial direction, and a length in the axial direction of 300 mm.

Next, a cylindrical test piece having a diameter of 80 mm and a length in an axial direction of 120 mm was prepared from the obtained TiAl alloy ingot and was held at a temperature of 1250° C. or higher for 0.5 hours or longer, and then uniaxial compression processing was performed with respect to the axial direction of the test piece by using a pressing machine at a rolling reduction of 70% to 80%, to prepare a disc-shaped forged material. Even when the rolling reduction is changed by about 10%, it is considered that the evaluation results for the hot forgeability and the creep strength are not greatly influenced.

(Preparation of Test Material)

Six rectangular materials each having a width of 14 mm, a length of 130 mm, and a thickness of 24 mm were taken from an approximate center position in the radial direction of the disc-shaped forged material obtained as described above, and any one of these was used as a test material for a creep strength evaluation test.

(Heat Treatment)

The above test material was subjected to two heat treatments under various conditions shown in Table 1 below. The purpose of the first heat treatment step was to bring the structure closer to the α single phase structure, and a TiAl intermediate alloy material was obtained by the first heat treatment step. The purpose of the second heat treatment step was to form a γ phase to form a lamellar structure of [α2+γ], 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.

(Creep Strength Evaluation Test)

A flanged creep test piece having a total length of 80 mm, a parallel portion diameter of 6 mm, a parallel portion length of 30 mm, and screw portions (both ends of the test piece in the longitudinal direction) of M12 was prepared from the obtained heat-treated material, and a creep rate test was performed with α single type creep tester. The test conditions were a temperature of 800° C. and a stress of 150 MPa, and a minimum creep rate was obtained based on test data. The minimum creep rate is an index of how much deformation occurs at a high temperature, and is a value generally used as one of indices for the creep strength.

As evaluation criteria for the creep strength, those with a minimum creep rate of 3.0×10−7 (sec−1) or less were acceptable, and those with a minimum creep rate of more than 3.0×10−7 (sec−1) were unacceptable.

[Structure Observation on TiAl Alloy Material] (Preparation of Test Piece)

A creep test piece was taken from the heat-treated material obtained by the above heat treatment, and then the surface of the remaining material at the end in the longitudinal direction was mirror-finished by mechanical and chemical polishing to prepare a test piece for structure observation.

(Calculation of Average Grain Size of Lamellar Grains)

A backscattered electron image of the test piece for structure observation was taken using a scanning electron microscope (SEM). The photographing was performed near the center of the plate thickness of the test piece, and one or more photographs were obtained by appropriately adjusting the magnification such that the lamellar grains could be evaluated for each test piece. Then, the lamellar grains were drawn for each SEM photograph using image processing software. Thereafter, the average circle equivalent diameter corresponding to the average grain size of the lamellar grains was calculated using image analysis software “Image Pro Plus” (manufactured by Media Cybernetics).

Although the β phase remained in some lamellar grain boundaries, contours of the lamellar grains were drawn while removing the β phase, and the lamellar grains in contact with four sides of the SEM photograph were excluded from the analysis. For each test piece, the average circle equivalent diameter was calculated from 50 or more lamellar grains. However, in Comparative Example No. 10, the grain size of the lamellar grains was too large to fit in the SEM photograph, so that the average circle equivalent diameter was calculated from 18 lamellar grains.

Table 1 below shows the measurement results of the average grain size of the lamellar grains and the minimum creep rate. As described above, since the grain size of the lamellar grains of the TiAl alloy material is determined by the grain size of the α grains of the TiAl intermediate alloy material after the first heat treatment step, the average grain size of the lamellar grains shown in Table 1 below can be considered to have the same value as the average grain size of the α grains.

TABLE 1 Composition Condition in heat treatment Average (atomic %) Temperature × time grain Balance: Ti and Second size unavoidable heat (μm) of Minimum impurities First heat treatment lamellar creep rate No. Al Nb Cu treatment step step grains (sec−1) Example 1 43.15 4.91 1.01   1275° C. × 0.5 hr 950° C. × 1 hr 33.4   8.60 × 10−8 2 43.15 4.91 1.01 1275° C. × 1 hr 950° C. × 1 hr 42.3   5.16 × 10−8 3 43.15 4.91 1.01 1275° C. × 3 hr 950° C. × 1 hr 52.7   7.02 × 10−8 4 43.15 4.91 1.01 1275° C. × 9 hr 950° C. × 1 hr 60.0   4.26 × 10−8 5 42.56 5.00 1.42 1280° C. × 1 hr 950° C. × 1 hr 24.2   2.13 × 10−7 6 43.47 4.94 1.02 1280° C. × 1 hr 950° C. × 1 hr 50.4   6.96 × 10−8 7 43.47 4.94 1.02 1300° C. × 1 hr 900° C. × 3 hr 40.8   9.85 × 10−8 8 43.83 4.98 1.05 1280° C. × 1 hr 950° C. × 1 hr 50.4   4.59 × 10−8 Comparative 9 43.31 5.09 2.11 1280° C. × 1 hr 950° C. × 1 hr 15.4   3.71 × 10−7 Example 10 43.69 4.07 0.84 1280° C. × 1 hr 950° C. × 1 hr 375.3 >1.00 × 10−1

As shown in the above Table 1, in 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 disclosure, 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 disclosure, so that excellent high-temperature creep strength was able to be obtained.

On the other hand, in Comparative Example No. 9, the average grain sizes of the α grains of the TiAl intermediate alloy material and the lamellar grains of the TiAl alloy material are less than the lower limit value of the range of the present disclosure, so that the high-temperature creep strength decreased. Examples of the reason why the average grain sizes of the α grains and the lamellar grains are less than the lower limit value of the range of the present disclosure include that the content of Cu in Comparative Example No. 9 is within the range specified by the present disclosure, but is larger than that in Examples, and the formation amount of the β phase that inhibits the growth of the α grains is large. As shown in Examples Nos. 1 to 4, since the average grain size of the lamellar grains increases as the heat treatment time is prolonged, it is considered that the average grain sizes of the α grains and the lamellar grains fall within the specified grain size by prolonging the heat treatment time in Comparative Example No. 9.

In Comparative Example No. 10, the average grain sizes of the α grains of the TiAl intermediate alloy material and the lamellar grains of the TiAl alloy material are more than the upper limit of the range of the present disclosure, so that brittle fracture was exhibited and rupture occurred immediately after the creep test. Examples of the reason why the average grain sizes of the α grains and the lamellar grains are more than the upper limit of the range of the present disclosure include that the contents of Nb and Cu in Comparative Example No. 10 are within the range specified by the present disclosure, but are smaller than those in Examples, so that the amount of the β phase is reduced and the α grains easily grow. In such a case, it is considered that by lowering the heat treatment temperature or extremely increasing the heat treatment temperature, the β phase or the r phase is formed, which prevents the growth of the α grains, thereby preventing the grain growth. In this way, if the compositions of the TiAl intermediate alloy material and the TiAl alloy material are within the ranges specified by the present disclosure, by adjusting conditions in the heat treatment, it is possible to control the average grain sizes of the α grains and the lamellar grains, and accordingly, excellent high-temperature creep strength can be obtained.

The FIG. is a graph showing a relationship between Examples and Comparative Examples, where the vertical axis represents the minimum creep rate obtained by the creep test and the horizontal axis represents the average grain size of the lamellar grains. In the FIGURE, black diamonds (♦) indicate good creep strength, and white diamonds (⋄) indicate poor creep strength. The range indicated by the dashed line in the FIGURE is a region in which the average grain size (μm) of the lamellar grains falls within the numerical range of the present disclosure.

As shown in FIGURE, in the TiAl alloy material containing Cu and Nb, when the composition is within the range specified in the present disclosure and the average grain size of the lamellar grains is within the range specified in the present disclosure, a TiAl alloy material having particularly excellent creep strength can be obtained.

Claims

1. A TiAl alloy material comprising:

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,
with the balance being Ti and unavoidable impurities, wherein
lamellar grains have an average grain size of 20 μm or more and 200 μm or less.

2. The TiAl alloy material according to claim 1, wherein

Al is contained in an amount of 42.5 atomic % or more and 44.0 atomic % or less.

3. The TiAl alloy material according to claim 1, wherein

Cu is contained in an amount of 0.7 atomic % or more and 2.0 atomic % or less.

4. The TiAl alloy material according to claim 1, wherein

Cu is contained in an amount of 0.9 atomic % or more and 1.5 atomic % or less.

5. The TiAl alloy material according to claim 1, wherein

Cu is contained in an amount of 0.9 atomic % or more and 1.2 atomic % or less.

6. The TiAl alloy material according to claim 1, wherein

Nb is contained in an amount of 4.5 atomic % or more and 6.0 atomic % or less.

7. The TiAl alloy material according to claim 1, wherein

the average grain size of the lamellar grains is 30 μm or more and 150 μm or less.

8. The TiAl alloy material according to claim 1, wherein

the average grain size of the lamellar grains is 30 μm or more and 100 μm or less.

9. A TiAl intermediate alloy material for obtaining a TiAl alloy material, the TiAl intermediate alloy material comprising:

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,
with the balance being Ti and unavoidable impurities, wherein
α grains have an average grain size of 20 μm or more and 200 μm or less.

10. The TiAl intermediate alloy material according to claim 9, wherein

Al is contained in an amount of 42.5 atomic % or more and 44.0 atomic % or less.

11. The TiAl intermediate alloy material according to claim 9, wherein

Cu is contained in an amount of 0.7 atomic % or more and 2.0 atomic % or less.

12. The TiAl intermediate alloy material according to claim 9, wherein

Cu is contained in an amount of 0.9 atomic % or more and 1.5 atomic % or less.

13. The TiAl intermediate alloy material according to claim 9, wherein

Cu is contained in an amount of 0.9 atomic % or more and 1.2 atomic % or less.

14. The TiAl intermediate alloy material according to claim 9, wherein

Nb is contained in an amount of 4.5 atomic % or more and 6.0 atomic % or less.

15. The TiAl intermediate alloy material according to claim 9, wherein

the average grain size of α grains is 30 μm or more and 150 μm or less.

16. The TiAl intermediate alloy material according to claim 9, wherein

the average grain size of α grains is 30 μm or more and 100 μm or less.
Patent History
Publication number: 20240124956
Type: Application
Filed: Oct 10, 2023
Publication Date: Apr 18, 2024
Applicants: KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) (Kobe-shi), TOKYO INSTITUTE OF TECHNOLOGY (Tokyo)
Inventors: Takeo MIYAMURA (Kobe-shi), Masao TAKEYAMA (Tokyo), Hirotoyo NAKASHIMA (Tokyo)
Application Number: 18/484,049
Classifications
International Classification: C22C 14/00 (20060101); C22F 1/18 (20060101);