TiAl ALLOY, TiAl ALLOY POWDER, TiAl ALLOY COMPONENT, AND PRODUCTION METHOD OF THE SAME

Abstract

A TiAl alloy is provided with 47 at % or more and 50 at % or less of Al, 1 at % or more and 2 at % or less of Nb, 2 at % or more and 5 at % or less of Zr, 0.05 at % or more and 0.3 at % or less of B, and the balance being Ti and inevitable impurities.

Description

BACKGROUND

Technical Field

The present disclosure relates to a TiAl alloy, TiAl alloy powder, a TiAl alloy component and a production method of the same.

Description of the Related Art

TiAl (titanium aluminide) alloys are alloys formed of intermetallic compounds of Ti and Al. As the TiAl alloys show excellent heat resistance and are lighter in weight and greater in specific strength than Ni alloys, these alloys are applied to aeroplane engine components such as turbine vanes or blades. TiAl alloys including Cr and Nb are used as these TiAl alloys. Japanese Patent Application Laid-open No. 2013-209750 discloses the related art.

SUMMARY

By the way, to reduce weight of TiAl alloy components such as turbine vanes or blades, it is required to further strengthen the TiAl alloys so as to obtain larger specific strength. The mechanical strength and the ductility of the conventional TiAl alloys are, however, difficult to be improved if the balance of these properties is considered and therefore the mechanical strength would be reduced if the ductility were improved.

An object of the present disclosure is thus to provide a TiAl alloy, TiAl alloy powder, a TiAl alloy component and a production method of the same, which enables improvement of the mechanical strength and the ductility with balance.

A TiAl alloy related to the present disclosure is provided with: 47 at % or more and 50 at % or less of Al; 1 at % or more and 2 at % or less of Nb; 2 at % or more and 5 at % or less of Zr; 0.05 at % or more and 0.3 at % or less of B; and the balance being Ti and inevitable impurities.

In the TiAl alloy related to the present disclosure, a content of Al may be 47 at % or more and 49 at % or less.

In the TiAl alloy related to the present disclosure, a content of Nb may be 1 at %, a content of Al may be 47 at % or more and 48 at % or less, and a content of Zr may be 2 at % or more and 4 at % or less.

In the TiAl alloy related to the present disclosure, a content of Nb may be 1 at %, a content of Al may be 47 at % or more and 48 at % or less, and a content of Zr may be 2 at % or more and 3 at % or less.

In the TiAl alloy related to the present disclosure, a content of Nb may be 2 at %, a content of Al may be 47 at % or more and 49 at % or less, and a content of Zr may be 2 at % or more and 3 at % or less.

In the TiAl alloy related to the present disclosure, a content of Nb may be 2 at %, a content of Al may be 47 at % or more and 48 at % or less, and a content of Zr may be 2 at % or more and 4 at % or less.

In the TiAl alloy related to the present disclosure, a content of Al may be 47 at % or more and 48 at % or less, and a content of Zr may be 2 at % or more and 4 at % or less.

In the TiAl alloy related to the present disclosure, a content of Al may be 47 at % or more and 48 at % or less, and a content of Zr may be 2 at % or more and 3 at % or less.

In the TiAl alloy related to the present disclosure, a room temperature ultimate tensile strength is 600 MPa or more, and a room temperature tensile fracture strain is 1.2% or more.

A TiAl alloy powder related to the present disclosure is formed of the TiAl alloy as described above.

A TiAl alloy component related to the present disclosure is formed of the TiAl alloy as described above.

A production method of a TiAl alloy component related to the present disclosure is provided with: a sealing step of filling a metal sheath with a TiAl alloy powder formed of the TiAl alloy as described above; and a hot isostatic pressure step of treating the TiAl alloy powder sealed in the metal sheath with a hot isostatic pressure treatment under 1200 degrees C. or higher and 1300 degrees C. or lower and 150 MPa.

Effects of the Invention

The aforementioned constitution enables improvement of the mechanical strength and the ductility of TiAl alloys with balance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 1 at %.

FIG. 2 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 1 at %.

FIG. 3 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 2 at %.

FIG. 4 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 2 at %.

FIG. 5 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 1 at, or more and 2 at % or less.

FIG. 6 is a drawing showing a relation between content percentages of Al and Zr in an embodiment presently disclosed when a content percentage of Nb is 1 at % or more and 2 at % or less.

FIG. 7 is a drawing showing a constitution of a TiAl alloy component formed of a turbine blade in an embodiment presently disclosed.

FIG. 8 is a flowchart showing a constitution of a production method of the TiAl alloy component in the embodiment presently disclosed.

FIG. 9 is photographs showing results of metallographic observation of TiAl alloys of examples 1 through 9 in an embodiment presently disclosed.

FIG. 10 is photographs showing results of metallographic observation of TiAl alloys of examples 10 through 18 in an embodiment presently disclosed.

FIG. 11 is a graph showing forms of solidification of the TiAl alloys of the examples 1 through 9 in the embodiment presently disclosed.

FIG. 12 is a graph showing forms of solidification of the TiAl alloys of the examples 10 through 18 in the embodiment presently disclosed.

FIGS. 13A and 13B are photographs showing results of metallographic observation by an optical microscope about samples of the examples A, B in an embodiment presently disclosed.

FIG. 14 is a graph showing tensile test results in an embodiment presently disclosed.

FIG. 15 is a graph showing creep test results in an embodiment presently disclosed.

DESCRIPTION OF EMBODIMENTS

Certain embodiments of the present disclosure will be described hereinafter with reference to the appended drawings. TiAl (titanium aluminide) alloys related to the embodiments of the present disclosure are constituted of 47 at % or more and 50 at % or less of Al (aluminum), 1 at % or more and 2 at % % or less of Nb (niobium), 2 at % or more and 5 at % or less of Zr (zirconium), 0.05 at % or more and 0.3 at % or less of B (boron), and the balance being Ti (titanium) and inevitable impurities. Reasons for limiting the composition ranges of the respective alloy components constituting the TiAl alloy will be described next.

Al (aluminum) has a function to improve mechanical strength and ductility such as room temperature ductility. The content percentage of Al is 47 at % or more and 50 at % or less. In a case where the content percentage of Al is less than 47 at %, specific strength decreases because the content percentages of Ti or such that are larger in density becomes greater. In a case where the content percentage of Al is larger than 50 at %, ductility decreases. The content percentage of Al may be set to be 47 at % or more and 49 at % or less. This leads to improvement of mechanical strength and ductility of TiAl alloys.

Nb (niobium) has a function to improve oxidation resistance and mechanical strength. The content percentage of Nb is 1 at % or more and 2 at % or less. In a case where the content percentage of Nb is less than 1 at %, there may be potential for reduction of oxidation resistance and high-temperature strength. In a case where the content percentage of Nb is more than 2 at %, specific strength is reduced as the density of Nb is larger than the densities of Al and Ti.

Zr (zirconium) has a function to improve oxidation resistance and mechanical strength. Zr is a chemical element that stabilizes a γ phase and contributes to improvement of ductility such as room temperature ductility. Zr further reduces diffusion speed, thereby contributing to improvement of creep strength. The content percentage of Zr is 2 at % or more and 5 at % or less. In a case where the content percentage of Zr is less than 2 at %, there may be potential for reduction of oxidation resistance, ductility such as room temperature ductility, and mechanical strength such as high-temperature strength. In a case where the content percentage of Zr is more than 5 at %, it can cause segregation. If the segregation of Zr occurs, it gives rise to reduction of mechanical strength or ductility.

B (boron) has a function to refine crystal grains so as to increase ductility such as room temperature ductility. The content percentage of B is 0.05 at % or more and 0.3 at % or less. When the content percentage of B becomes less than 0.05 at %, the crystal grains become coarsened and thus it gives rise to reduction of ductility. When the content percentage of B becomes more than 0.3 at %, there may be potential for reduction of impact resistance properties. By making the content percentage of B to be 0.05 at % or more and 0.3 at % or less, as it will be constituted of crystal grains of 100 micrometers or less in crystal grain diameter, ductility can be improved.

B has a function to cause precipitation of borides in each crystal grain by heat treatment, thereby improving mechanical strength. Fine borides with those of 0.1 micrometers in grain diameter are formed. The fine borides are constituted of TiB, TiB₂ and such. As the fine borides precipitate out in each crystal grain, mechanical strength such as tensile strength, fatigue strength, creep strength or such can be improved.

The balance of the TiAl alloy is constituted of Ti and inevitable impurities. The term “inevitable impurity” means an impurity that has possibility of being mixed in any substance although it is not intentionally added. As the TiAl alloy does not contain Cr (chromium), reduction of mechanical strength can be suppressed. As the TiAl alloy does not contain V (vanadium), reduction of mechanical strength and reduction of oxidation resistance can be suppressed. As the TiAl alloy does not contain Mo (molybdenum), reduction of specific strength can be suppressed.

A form of solidification of the TiAl alloy will be described next. The form of solidification of the TiAl alloy relates to the content percentages of Al, Zr and Nb. By changing the content percentages of Al, Zr and Nb, the form of solidification of the TiAl alloy changes among a solidification, β solidification, γβ solidification, and a solidification+γ solidification. The α solidification is a form of solidification in which a solidification process of the TiAl alloy passes through an α single phase region. The β solidification is a form of solidification in which a solidification process of the TiAl alloy passes through a β single phase region. The γ solidification is a form of solidification in which a solidification process of the TiAl alloy passes through a γ single phase region. The α solidification+γ solidification is a form of solidification in which a solidification process of the TiAl alloy passes through an α+γ dual phase region. In a case of the γ solidification, anisotropy of the metallographic structure becomes stronger because coarse columnar crystal grains grow. On the other hand, in a case of the α solidification or the β solidification, isotropy of the metallographic structure becomes stronger and anisotropy of the metallographic structure becomes weaker because isometric crystal grains grow. In a case of the αβ solidification+γ solidification, a metallographic structure lying halfway between the metallographic structure by the α solidification and the metallographic structure by the γ solidification forms because both the isometric crystal grains and the columnar crystal grains grow. Meanwhile, B hardly influences the form of solidification of the TiAl alloy because fine borides precipitate in each crystal grain.

As the content percentage of Al becomes larger, the form of solidification of the TiAl alloy tends to be the γβ solidification. As the content percentage of Al becomes smaller, the form of solidification of the TiAl alloy tends to be the α solidification+γ solidification, the α solidification or the (solidification. As the content percentage of Zr becomes larger, the form of solidification of the TiAl alloy tends to be the γ solidification. As the content percentage of Zr becomes smaller, the form of solidification of the TiAl alloy tends to be the α solidification+γ solidification, the Ca solidification or the β solidification. As the content percentage of Nb becomes larger, the form of solidification of the TiAl alloy tends to be the α solidification+γ solidification, the α solidification or the β solidification. As the content percentage of Nb becomes smaller, the form of solidification of the TiAl alloy tends to be the γ solidification.

FIG. 1 is a drawing showing a relation between content percentages of Al and Zr when a content percentage of Nb is 1 at %. The TiAl alloy may be constituted of a composition range of the content percentages of Al and Zr, which is enclosed by four points of an R1 point (Al: 47 at %, Zr: 2 at %), an R2 point (Al: 48 at %, Zr: 2 at %), an R3 point (Al: 48 at %, Zr: 4 at %), and an R4 point (Al: 47 at %, Zr: 5 at %) shown in FIG. 1. More specifically, the TiAl alloy may contain Al and Zr of the composition range enclosed by the four points of the R1 point, the R2 point, the R3 point and the R4 point shown in FIG. 1 and the balance may be constituted of Ti and inevitable impurities. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the α solidification or the α solidification+γ solidification. Thereby anisotropy of the metallographic structure is suppressed as compared with the case where the form of solidification is of only the γ solidification. Further, by suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes more isotropic. For example, such a TiAl alloy may contain 1 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % or less of Al and 2 at % or more and 4 at % or less of Zr and the balance may be constituted of Ti and inevitable impurities.

FIG. 2 is a drawing showing a relation between content percentages of Al. and Zr when a content percentage of Nb is 1 at %. The TiAl alloy may be constituted of a composition range of the content percentages of Al. and Zr, which is enclosed by four points of an S1 point (Al: 47 at %, Zr: 2 at %), an S2 point (Al: 48 at %, Zr: 2 at %), an S3 point (Al: 48 at %, Zr: 3 at %), and an S4 point (Al: 47 at %, Zr: 5 at %) shown in FIG. 2. More specifically, the TiAl alloy may contain 1 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, Al and Zr of the composition range enclosed by the four points of the S1 point, the S2 point, the S3 point and the S4 point shown in FIG. 2 and the balance may be constituted of Ti and inevitable impurities. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the α solidification. Thereby, as the γ solidification is not contained in the form of solidification, anisotropy of the metallographic structure is further suppressed. As well, by further suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes further more isotropic. For example, such a TiAl alloy may contain 1 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % % or less of Al and 2 at % or more and 3 at % or less of Zr and the balance may be constituted of Ti. and inevitable impurities.

FIG. 3 is a drawing showing a relation between content percentages of Al and Zr when a content percentage of Nb is 2 at %. The TiAl alloy may be constituted of a composition range of the content percentages of Al. and Zr, which is enclosed by five points of a T1 point (Al: 47 at %, Zr: 2 at %), a T2 point (Al: 49 at %, Zr: 2 at %), a T3 point (Al: 49 at %, Zr: 3 at %), a T4 point (Al: 48 at %, Zr: 4 at %), and a T5 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 3. More specifically, the TiAl alloy may contain Al and Zr of the composition range enclosed by the five points of the T point, the T2 point, the T3 point, the T4 point, and the T5 point shown in FIG. 3 and the balance may be constituted of Ti and inevitable impurities. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the α solidification or the α solidification+γ solidification. Thereby, as compared with a case where the form of solidification is formed of only the γ solidification, anisotropy of the metallographic structure is suppressed. Further, by suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes more isotropic. For example, such a TiAl alloy may contain 2 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 49 at % or less of Al and 2 at % or more and 3 at % or less of Zr and the balance may be constituted of Ti and inevitable impurities. Further such a TiAl alloy may contain 2 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % or less of Al and 2 at % or more and 4 at % % or less of Zr and the balance may be constituted of Ti and inevitable impurities.

FIG. 4 is a drawing showing a relation between content percentages of Al and Zr when a content percentage of Nb is 2 at %. The TiAl alloy may be constituted of a composition range of the content percentages of Al and Zr, which is enclosed by four points of a W1 point (Al: 47 at %, Zr: 2 at %), a W2 point (Al: 49 at %, Zr: 2 at %), a W3 point (Al: 48 at %, Zr: 4 at %), and a W4 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 4. More specifically, the TiAl alloy may contain 2 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, Al and Zr of the composition range enclosed by the four points of the W1 point, the W2 point, the W3 point and the W4 point shown in FIG. 4 and the balance may be constituted of Ti and inevitable impurities. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the r solidification. Thereby, as the γ solidification is not contained in the form of solidification, anisotropy of the metallographic structure is further suppressed. As well, by further suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes further more isotropic. For example, such a TiAl alloy may contain 2 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % or less of Al and 2 at % or more and 4 at % or less of Zr and the balance may be constituted of Ti and inevitable impurities.

FIG. 5 is a drawing showing a relation between content percentages of Al and Zr when a content percentage of Nb is 1 at % or more and 2 at % or less. The TiAl alloy may be constituted of a composition range of the content percentages of Al and Zr, which is enclosed by four points of an X1 point (Al: 47 at %, Zr: 2 at %), an X2 point (Al: 48 at %, Zr: 2 at %), an X3 point (Al: 48 at %, Zr: 4 at %), and an X4 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 4. More specifically, the TiAl alloy may contain 2 at % of Nb, 0.05 at % or more and 0.3 at % or less of B, Al and Zr of the composition range enclosed by the four points of the X1 point, the X2 point, the X3 point and the X4 point shown in FIG. 5 and the balance may be constituted of Ti and inevitable impurities.

Meanwhile, the composition range enclosed by the four points of the X1 point, the X2 point, the X3 point, and the X4 point shown in FIG. 5 shows a composition range in which the composition range enclosed by the four points of the R1 point, the P2 point, the R3 point and the R4 point shown in FIG. 1 overlaps with the composition range enclosed by the five points of the T1 point, the T2 point, the T3 point, the T4 point and the T5 point shown in FIG. 3. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the α solidification or the α solidification+γ solidification. Thereby, as compared with a case where the form of solidification is formed of only the γ solidification, anisotropy of the metallographic structure is suppressed. Further, by suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes more isotropic. For example, such a TiAl alloy may contain 1 at % or more and 2 at % or less of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % or less of Al and 2 at % or more and 4 at % or less of Zr and the balance may be constituted of Ti and inevitable impurities.

FIG. 6 is a drawing showing a relation between content percentages of Al and Zr when a content percentage of Nb is 1 at % or more and 2 at % or less. The TiAl alloy may be constituted of a composition range of the content percentages of A and Zr, which is enclosed by four points of a Y1 point (Al: 47 at %, Zr: 2 at %), a Y2 point (Al: 48 at %, Zr: 2 at %), a Y3 point (Al: 48 at %, Zr: 3 at %), a Y4 point (Al: 47.5 at %, Zr: 4 at %), and a Y5 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 6. More specifically, the TiAl alloy may contain 1 at % or more and 2 at % or less of Nb, 0.05 at % or more and 0.3 at % or less of B, and Al and Zr of the composition range enclosed by the five points of the Y1 point, the Y2 point, the Y3 point, the Y4 point and the Y5 point shown in FIG. 6 and the balance may be constituted of Ti and inevitable impurities.

Meanwhile, the composition range enclosed by the five points of the Y1 point, the Y2 point, the Y3 point, the Y4 point and the Y5 point shown in FIG. 6 shows a composition range in which the composition range enclosed by the four points of the S1 point, the S2 point, the S3 point and the S4 point shown in FIG. 2 overlaps with the composition range enclosed by the four points of the W1 point, the W2 point, the W3 point and the W4 point shown in FIG. 4. In a case where the TiAl alloy is constituted of this alloy composition, the form of solidification can be only the α solidification. Thereby, as the γ solidification is not contained in the form of solidification, anisotropy of the metallographic structure is further suppressed. Further, by further suppressing anisotropy of the metallographic structure, the mechanical property or such of the TiAl alloy becomes more isotropic. For example, such a TiAl alloy may contain 1 at % or more and 2 at % or less of Nb, 0.05 at % or more and 0.3 at % or less of B, 47 at % or more and 48 at % or less of Al and 2 at % or more and 3 at % or less of Zr and the balance may be constituted of Ti and inevitable impurities.

The metallographic structure of the TiAl alloy will be described next. The metallographic structure of the TiAl alloy is constituted of fine crystal grains with a grain diameter of 100 micrometers or less. Thereby ductility of the TiAl alloy is improved. The metallographic structure of the TiAl alloy is constituted of lamellar grains and γ grains and is free from segregation. Each lamella is formed in which α₂ phases formed of TiAl and γ phases formed of TiAl are regularly arranged in a layered form. The γ grains are formed of TiAl. The γ grains are for example isometric γ grains. In each grain of the γ grains, borides with a grain diameter of 0.1 micrometers or less are contained. The borides are constituted of TiB, TiB₂ or such in an acicular shape or such.

The lamellar grains can improve the mechanical strength such as the tensile strength, the fatigue strength, the creep strength or such. The γ grains can improve the ductility and the high-temperature strength. The borides with a grain diameter of 0.1 micrometers or less can improve the mechanical strength. The metallographic structure of the TiAl alloy preferably exhibits a volume fraction of the γ grains that is 80 vol % or more given that the total volume fraction of the lamellar grains and the γ grains is 100 vol %, and the remainder is preferably the lamellar grains. As the metallographic structure of the TiAl alloy is constituted mainly of the γ grains, the mechanical strength and the ductility can be improved with balance. As the metallographic structure of the TiAl alloy is free from segregation of Zr, reduction of the mechanical strength and the ductility can be suppressed.

Mechanical properties of the TiAl alloy in accordance with the present disclosure will be next described. The mechanical properties of the TiAl alloy at room temperature are such that, if tensile tests are carried out in conformity with JIS, ASTM or such, a room temperature ultimate tensile strength can be 600 MPa or more, and a room temperature tensile fracture strain can be 1.2% or more. In accordance with the TiAl alloy according to the present disclosure, the mechanical strength and the ductility can be improved with balance.

TiAl alloy components that utilize the TiAl alloy in accordance with the present disclosure will be next described. The TiAl alloy components are applicable to a turbine blade or vane, or such, of an aeroplane engine or a gas turbine for power generation. FIG. 7 is a drawing showing a constitution of a TiAl alloy component 10 formed of a turbine blade. As the TiAl alloy as described above is superior in the mechanical Strength such as the high temperature strength, it can improve heat resistance of the TiAl alloy component 10. Further, as the TiAl alloy as described above shows excellent ductility such as room temperature ductility, damage of the TiAl alloy component 10 would be suppressed even in a case when the TiAl alloy component 10 is under assembly or installation. Meanwhile the TiAl alloy component is not limited to an aeroplane engine component but may be a supercharger component such as a turbine wheel for a supercharger or a vehicle component such as an engine valve for a vehicle.

The TiAl alloy component can be produced by melting and casting the TiAl alloy as described above. The TiAl alloy component can be produced by melting the TiAl alloy as described above in a vacuum induction heater furnace or such and then casting the same. For casting, any casting machine used for casting a general metal material can be used.

The TiAl alloy component may be powder molded by using the TiAl alloy powder formed of the TiAl alloy as described above as an ingredient powder and by means of a metal powder injection molding (MIM) method or a hot isostatic pressing (HIP) method. The TiAl alloy powder is formed of the TiAl alloy as described above and may be produced through a sinter synthesis method, a mechanical alloying method, a plasma rotary electrode method, an atomizing method (water atomizing or gas atomizing) or such. The TiAl alloy powder is preferably made as a rapid solidified powder. Because the rapid solidified powder is produced by rapidly solidifying alloy liquid droplets, segregation of Zr contained in the TiAl alloy can be further suppressed.

A method for producing the TiAl alloy component by the hot isostatic pressing (HIP) method will be next described as an example. FIG. 8 is a flowchart showing a constitution of the production method of the TiAl alloy component. The production method of the TiAl alloy component is provided with a sealing step (S10) and a hot isostatic pressing step (S=2).

The sealing step (S10) is a step for filling a metal sheath with the TiAl alloy powder formed of the TiAl alloy as described above and sealing it. As an ingredient powder, the TiAl alloy powder formed of the TiAl alloy is used. As the TiAl alloy powder, preferably a rapid solidified powder produced by gas atomizing or such is used. The TiAl alloy powder is filled and sealed in the metal sheath. As the metal sheath, preferably a titanium sheath formed of pure titanium is used. The thickness of the titanium sheath is preferably 1 mm for example. The TiAl alloy powder filled in the metal sheath is subjected to sealing by electron beam welding or such after vacuum evacuation.

The hot isostatic pressing step (S12) is a step for treating the TiAl alloy powder filled in the metal sheath with hot isostatic pressing at 1200 degrees C. or higher and 1300 degrees C. or lower and under a pressure of 150 MPa or higher. By treating the TiAl alloy powder filled in the metal sheath with hot isometric pressing, the TiAl alloy component is molded. The hot isostatic pressing treatment can be carried out at 1200 degrees C. or higher and 1300 degrees C. or lower and under a pressure of 150 MPa or higher. The pressure is preferably 150 MPa or higher and 200 MPa or lower for example. Duration of time for keeping the heating temperature may be 3 hours or longer. The duration of time for keeping the heating temperature is preferably 3 hours or longer and 5 hours or shorter for example. After hot isostatic pressing, preferably, the pressure is released, furnace cooling down to 900 degrees C. is carried out, and further rapid cooling below 900 degrees C. is carried out. By carrying out such a cooling method, cracking in the TiAl alloy component can be suppressed. In the rapid cooling from 900 degrees C., preferably a cooling rate faster than air cooling is used and the cooling is possibly performed by gas fan cooling or such.

The production method of the TiAl alloy component may include, after the hot isostatic pressing step (S12), a stress relieving step for relieving stress by holding the component at a temperature range from 800 degrees C. or higher to 950 degrees C. or lower, for 1 hour or more and 5 hours or less. As residual stress or such in the TiAl alloy component is thereby relieved, the ductility of the TiAl alloy component can be improved.

The hot isostatic pressing treatment and the stress relieving are preferably carried out in a vacuum atmosphere or an inert gas atmosphere such as argon gas for the purpose of oxidation prevention. For the hot isostatic pressing treatment, a HIP machine or such used for general hot isostatic pressing of metal materials may be used. For the stress relieving, a general atmosphere furnace used for stress relief annealing of metal materials may be used. After the hot isostatic pressing step (S12) and the stress relieving step, any heat treatment step for regulating the metallographic structure may be provided.

In sum, the TiAl alloy constituted as in the way described above contains 47 at % or more and 50 at % or less of Al, 1 at % or more and 2 at % or less of Nb, 2 at % or more and 5 at % or less of Zr and 0.05 at % or more and 0.3 at % or less of B, and the balance is constituted of Ti and inevitable impurities. Thereby the mechanical strength and the ductility of the TiAl alloy can be improved with balance.

Working Examples

A form of solidification of the TiAl alloy was examined. TiAl alloys of examples 1 through 18 will be described. Each TiAl alloy of the examples 1 through 18 contains Al, Nb, Zr and B and the balance is constituted of Ti and inevitable impurities. Alloy compositions of the TiAl alloys are summarized in TABLE 1.

	TABLE 1
	CHEMICAL COMPOSITION (at %)
					Ti + UNAVOIDABLE	SOLIDIFICATION
	Nb	Al	Zr	B	IMPURITIES	FORM
EXAMPLE 1	1	47	5	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 2	1	48	3	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 3	1	48	4	0.2	REMAINDER	α-SOLIDIFICATION +
						γ-SOLIDIFICATION
EXAMPLE 4	1	48	5	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 5	1	49	3	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 6	1	49	4	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 7	1	49	5	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 8	1	50	3	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 9	1	50	4	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 10	2	48	2	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 11	2	48	3	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 12	2	48	4	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 13	2	49	2	0.2	REMAINDER	α-SOLIDIFICATION
EXAMPLE 14	2	49	3	0.2	REMAINDER	α-SOLIDIFICATION +
						γ-SOLIDIFICATION
EXAMPLE 15	2	49	4	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 16	2	50	2	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 17	2	50	3	0.2	REMAINDER	γ-SOLIDIFICATION
EXAMPLE 18	2	50	4	0.2	REMAINDER	γ-SOLIDIFICATION

In regard to the TiAl alloys of the examples 1 through 9, the content percentage of Nb was set at 1 at % and the content percentage of B was set at 0.2 at % and the content percentages of Al were varied from 47 at % to 50 at % and the content percentages of Zr were varied from 3 at % to 5 at %. In regard to the TiAl alloys of the examples 10 through 18, the content percentage of Nb was set at 2 at % and the content percentage of B was set at 0.2 at %, and the content percentages of Al were varied from 48 at % to 50 at % and the content percentages of Zr were varied from 2 at % to 4 at %.

The respective TiAl alloy ingredients with the alloy compositions shown in TABLE 1 were melted in a high-frequency vacuum melting furnace and casted to form ingots of the TiAl alloys of the respective alloy compositions. Next metallographic observation was carried out on the TiAl alloys to examine the forms of solidification. FIG. 9 is photographs showing results of the metallographic observation on the TiAl alloys of the examples 1 through 9. FIG. 10 is photographs showing results of the metallographic observation on the TiAl alloys of the examples 10 through 18.

In the TiAl alloys of the examples 1 and 2, the forms of solidification were only the α solidification. In the TiAl alloy of the example 3 was the α solidification+γ solidification. In the TiAl alloys of the examples 4 through 9, the forms of solidification were only the γ solidification. In the TiAl alloys of the examples 10 through 13, the forms of solidification were only the α solidification. In the TiAl alloy of the example 14, the form of solidification was the α solidification+γ solidification. In the TiAl alloys of the examples 15 through 18, the forms of solidification were only the γ solidification.

FIG. 11 is a graph showing forms of solidification of the TiAl alloys of the examples 1 through 9. FIG. 12 is a graph showing forms of solidification of the TiAl alloys of the examples 10 through 18. In the graphs of FIG. 11 and FIG. 12, the Zr contents (at %) are put on the horizontal axes, the Al contents (at %) are put on the vertical axes, circles represent that the form of solidification is only the α solidification, triangles represent that the form of solidification is the α solidification+γ solidification, and squares represent that the form of solidification is only the γ solidification. Meanwhile, in FIG. 11, the four points of the E1 point, the R2 point, the R3 point and the R4 point shown in FIG. 1 as described above and the four points of the S1 point, the S2 point, the S3 point and the S4 point shown in FIG. 2 as described above are also plotted. In FIG. 12, the five points of the T1 point, the T2 point, the T3 point, the T4 point and the T5 point shown in FIG. 3 as described above and the four points of the W1 point, the W2 point, the W3 point and the W4 point shown in FIG. 4 as described above are also plotted.

It became apparent that, as the content percentage of Al is made larger, the form of solidification of the TiAl alloy tends to change from the α solidification through the α solidification+γ solidification into the γ solidification. In a case where the content percentage of Nb is 1 at % as shown in FIG. 11 for example, where the content percentage of Zr is from 3 at % to 5 at %, the form of solidification became only the γ solidification when the content percentage of Al was 49 at % or more. As well, in a case where the content percentage of Nb is 2 at % as shown in FIG. 12, where the content percentage of Zr is from 2 at % to 4 at %, the form of solidification became only the γ solidification when the content percentage of Al was 50 at % or more.

It became apparent that, as the content percentage of Zr is made larger, the form of solidification of the TiAl alloy tends to change from the or solidification or the α solidification+γ solidification into the γ solidification. In a case where the content percentage of Nb is 1 at % and the content percentage of Al is 48 at % as shown in FIG. 11 for example, the form of solidification became only the α solidification when the content percentage of Zr was 3 at %, the form of solidification became the α solidification+γ solidification when the content percentage of Zr was 4 at %, and the form of solidification became only the γ solidification when the content percentage of Zr was 5 at %. Further, in a case where the content percentage of Nb is 2 at % and the content percentage of Al is 49 at % as shown in FIG. 12, the form of solidification became only the α solidification when the content percentage of Zr was 2 at %, the form of solidification became the α solidification+γ solidification when the content percentage of Zr was 3 at %, and the form of solidification became only the γ solidification when the content percentage of Zr was 4 at %.

It became apparent that, as the content percentage of Nb is made larger, the form of solidification of the TiAl alloy tends to change from the γ solidification into the α solidification+γ solidification or the α solidification. In a case where the content percentage of Al is 49 at % and the content percentage of Zr is 3 at %, it became only the γ solidification when the content percentage of Nb was 1 at % as shown in FIG. 11, and it became the α solidification+γ solidification when the content percentage of Nb was 2 at % as shown in FIG. 12.

It was found from the graph of FIG. 11 that the form of solidification becomes either only the α solidification or the α solidification+γ solidification in a case where the content percentage of Nb is 1 at % and the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the R1 point (Al: 47 at %, Zr: 2 at %), the R2 point (Al: 48 at %, Zr: 2 at %), the R3 point (Al: 48 at %, Zr: 4 at %) and the R4 point (Al: 47 at %, Zr: 5 at %) shown in FIG. 1 as described above.

This reason will be described next. First, it is apparent that the R3 point (Al: 48 at %, Zr: 4 at %) causes the α solidification+γ solidification and the R4 point (Al: 47 at %, Zr: 5 at %) causes only the α solidification. As the content percentage of Zr at the R1 point (Al: 47 at %, Zr: 2 at %) is smaller than that at the R4 point (Al: 47 at %, Zr: 5 at %), it becomes only the α solidification. As the content percentage of Zr at the R2 point (Al: 48 at %, Zr: 2 at %) is smaller than that at the point of FIG. 11 (Al: 48 at %, Zr: 3 at %), it becomes only the α solidification. Therefore, in a case where the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the R1 point, the P2 point, the R3 point and the R4 point shown in FIG. 1 as described above, the form of solidification becomes only the α solidification or the α solidification+γ solidification.

It was found from the graph of FIG. 11 that the form of solidification becomes only the α solidification in a case where the content percentage of Nb is 1 at % and the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the S1 point (Al: 47 at %, Zr: 2 at %), the S2 point (Al: 48 at %, Zr: 2 at %), the S3 point (Al: 48 at %, Zr: 3 at %) and the S4 point (Al: 47 at %, Zr: 5 at %) shown in FIG. 2 as described above.

This reason will be described next. First, it is apparent from FIG. 11 that the S3 point (Al: 48 at %, Zr: 3 at %) and the S4 point (Al: 47 at %, Zr: 5 at %) cause only the α solidification. As the content percentage of Zr at the S1 point (Al: 47 at %, Zr: 2 at %) is smaller than that at the S4 point (Al: 47 at %, Zr: 5 at %), it becomes only the α solidification. Further, as the content percentage of Zr at the S2 point (Al: 48 at %, Zr: 2 at %) is smaller than that at the S3 point (Al: 48 at %, Zr: 3 at %), it becomes only the α solidification. Therefore, in a case where the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the S1 point, the S2 point, the S3 point and the S4 point shown in FIG. 2 as described above, the form of solidification becomes only the α solidification.

It was found from the graph of FIG. 12 that the form of solidification becomes either only the α solidification or the α solidification+γ solidification in a case where the content percentage of Nb is 2 at % and the content percentages of Al and Zr are constituted of the composition range enclosed by the five points of the T1 point (Al: 47 at %, Zr: 2 at %), the T2 point (Al: 49 at %, Zr: 2 at %), the T3 point (Al: 49 at %, Zr: 3 at %), the T4 point (Al: 48 at %, Zr: 4 at %) and the T5 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 3 as described above.

This reason will be described next. First, it is apparent from FIG. 12 that the T2 point (Al: 49 at %, Zr: 2 at %) and the T4 point (Al: 48 at %, Zr: 4 at %) cause only the α solidification and the T3 point (Al: 49 at %, Zr: 3 at %) causes the α solidification+γ solidification. As the content percentage of Al at the Ti point (Al: 47 at %, Zr: 2 at %) is smaller than that at the T2 point (Al: 49 at %, Zr: 2 at %), it becomes only the α solidification. Further, as the content percentage of Al at the T5 point (Al: 47 at %, Zr: 4 at %) is smaller than that at the T4 point (Al: 48 at %, Zr: 4 at %), it becomes only the α solidification. Therefore, in a case where the content percentages of Al and Zr are constituted of the composition range enclosed by the five points of the T1 point, the T2 point, the T3 point, the T4 point and the T5 point shown in FIG. 3 as described above, the form of solidification becomes either only the α solidification or the α solidification+γ solidification.

It was found from the graph of FIG. 12 that the form of solidification becomes only the α solidification in a case where the content percentage of Nb is 2 at % and the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the W1 point (Al: 47 at %, Zr: 2 at %), the W2 point (Al: 49 at %, Zr: 2 at %), the W3 point (Al: 48 at %, Zr: 4 at %) and the W4 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 4 as described above.

This reason will be described next. First, it is apparent from FIG. 12 that the W2 point (Al: 49 at %, Zr: 2 at %) and the W3 point (Al: 48 at %, Zr: 4 at %) cause only the α solidification. As the content percentage of Al at the W1 point (Al: 47 at %, Zr: 2 at %) is smaller than that at the W2 point (Al: 49 at %, Zr: 2 at %), it becomes only the α solidification. As the content percentage of Al at the W4 point (Al: 47 at %, Zr: 4 at %) is smaller than that at the W3 point (Al: 48 at %, Zr: 4 at %), it becomes only the α solidification. Therefore, in a case where the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the W1 point, the W2 point, the W3 point and the W4 point shown in FIG. 4 as described above, the form of solidification becomes only the α solidification.

It was found from the graphs of FIG. 11 and FIG. 12 that the form of solidification becomes only the α solidification or the α solidification+γ solidification in a case where the content percentage of Nb is 1 at % or more and 2 at % or less and the content percentages of Al and Zr are constituted of the composition range enclosed by the four points of the X1 point (Al: 47 at %, Zr: 2 at %), the X2 point (Al: 48 at %, Zr: 2 at %), the X3 point (Al: 48 at %, Zr: 4 at %) and the X4 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 5 as described above.

It was found from the graphs of FIG. 11 and FIG. 12 that the form of solidification becomes only the <solidification in a case where the content percentage of Nb is 1 at % or more and 2 at % or less and the content percentages of Al and Zr are constituted of the composition range enclosed by the five points of the Y1 point (Al: 47 at %, Zr: 2 at %), the Y2 point (Al: 48 at %, Zr: 2 at %), the Y3 point (Al: 48 at %, Zr: 3 at %), the Y4 point (Al: 47.5 at %, Zr: 4 at %) and the Y5 point (Al: 47 at %, Zr: 4 at %) shown in FIG. 6 as described above.

Next, test pieces of the examples A, B were prepared by using the TiAl alloy powder formed of the TiAl alloy of the examples 1, 11 and then its mechanical properties were examined. First, the preparation method of the test pieces of the examples A, B will be described. The test pieces of the examples A, B were prepared through powder molding by the hot isostatic pressing method.

First, the TiAl alloy powder was filled and sealed in pure titanium sheaths. For the test piece of the example A, the TiAl alloy powder formed of the TiAl alloy of the example 1 was used. For the test piece of the example B, the TiAl alloy powder formed of the TiAl alloy of the example 11 was used. For the TiAl alloy powders formed of the TiAl alloys of the examples 1, 11, rapid solidified powders produced by the gas atomizing method were used. The TiAl alloy powders filled in the pure titanium sheaths were subjected to sealing by electron beam welding.

The TiAl alloy powders filled in the pure titanium sheaths were subjected to a hot isostatic pressing treatment at 1250 degrees C., under 172 MPa and for three hours. After the hot isostatic pressing, the pressure was relieved and furnace cooling down to 900 degrees C. was carried out, and further rapid cooling below 900 degrees C. was carried out. The rapid cooling from 900 degrees C. was carried out by gas fan cooling. Thus the test pieces of the examples A, B were prepared.

In regard to the test pieces of the examples A, B, metallographic observation was carried out. In the metallographic observation, an optical microscope and an electron microscope were used. FIGS. 13A and 13B are photographs showing results of the metallographic observation on the examples A, B obtained by the optical microscope and FIG. 13A is a photograph of the test piece of the example A and FIG. 13B is a photograph of the test piece of the example B.

The metallographic structures of the test pieces of the examples A, B were constituted of fine crystal grains with a grain diameter of 100 micrometers or less. The metallographic structures of the test pieces of the examples A, B were constituted of lamellar grains and isometric γ grains, and contained borides with a grain diameter of 0.1 micrometers or less in the isometric γ grains. The metallographic structures of the examples 1, 11 exhibited volume fractions of the isometric γ grains that were 80 vol % or more given that the total volume fraction of the lamellar grains and the isometric γ grains is 100 vol %, and the remainders were constituted of the lamellar grains. Meanwhile, in regard to the volume fractions of the respective grains, areal shares of the respective grains were calculated by image processing from information about contrasts of the respective grains in metallographic structural photos by the electron microscope, and were adopted as these volume fractions. Further, any segregations of Zr were not found in the metallographic structures of the test pieces of the examples A, B.

The room temperature mechanical properties of the test pieces of the examples A, B were next examined. The test pieces of the examples A, B were subjected to room temperature tensile testing. Similarly a test piece of comparative example A was subjected to room temperature tensile testing. The test piece of the comparative example A was formed of a TiAl alloy containing 48 at % of Al, 2 at % of Nb and 2 at % of Cr, and the balance was formed of Ti and inevitable impurities.

The tensile testing was carried out in accordance with ASTM E8. FIG. 14 is a graph showing tensile test results. In FIG. 14, strain is put on the horizontal axis and stress is put on the vertical axis, and it shows stress-strain curves of the respective test pieces. In regard to the test pieces of the examples A, B as compared with the test piece of the comparative example A, the room temperature ultimate tensile strengths and the room temperature tensile fracture strains were larger. The room temperature ultimate tensile strengths of the test pieces of the examples A, B were 600 MPa or more and the room temperature tensile fracture strains were 1.2% or more. Further, the room temperature ultimate tensile strength of the test piece of the example A was 700 MPa or more and the room temperature tensile fracture strain of the test piece of the example B was 1.4% or more. It became apparent from these results that the test pieces of the examples A, B show excellent mechanical properties and ductility, and the mechanical properties and the ductility are improved with balance.

Creep tests were carried out on the test pieces of the example A and the comparative example A. The creep tests accorded with JIS Z 2271. FIG. 15 is a graph showing creep test results. In FIG. 15, Larson-Miller parameter P is put on the horizontal axis, specific strength is put on the vertical axis, squares represent the test piece of the example A and Xs represent the test piece of the comparative example A. Meanwhile, the Larson-Miller parameter P is a parameter represented by P=T×log(t_Y+C). T represents absolute temperature (K), t_K represents fracture time (b), and C represents a material constant. Meanwhile, the material constant was set to be 20. As shown in FIG. 15, the test piece of the example A showed excellent creep properties as compared with the test piece of the comparative example A. It was found from these results that the test piece of the example A is superior in the high temperature strength properties to the test piece of the comparative example A.

INDUSTRIAL APPLICABILITY

The present disclosure is useful in aeroplane engine components or turbine vanes or blades for generator gas turbines.

Claims

1. A TiAl alloy comprising: 47 at % or more and 50 at % or less of Al;1 at % or more and 2 at % or less of Nb;2 at % or more and 5 at % or less of Zr;0.05 at % or more and 0.3 at % or less of B; andthe balance being Ti and inevitable impurities.

2. The TiAl alloy as recited in claim 1, wherein the content of Al is 47 at % or more and 49 at % or less.

3. The TiAl alloy as recited in claim 1, wherein the content of Nb is 1 at %,the content of Al is 47 at % or more and 48 at % or less, andthe content of Zr is 2 at % or more and 4 at % or less.

4. The TiAl alloy as recited in claim 1, wherein the content of Nb is 1 at %,the content of Al is 47 at % or more and 49 at % or less, andthe content of Zr is 2 at % or more and 3 at % or less.

5. The TiAl alloy as recited in claim 1, wherein the content of Nb is 2 at %,the content of Al is 47 at % or more and 49 at % or less, andthe content of Zr is 2 at % or more and 3 at % or less.

6. The TiAl alloy as recited in claim 1, wherein the content of Nb is 2 at %,the content of Al is 47 at % or more and 48 at % or less, andthe content of Zr is 2 at % or more and 4 at % or less.

7. The TiAl alloy as recited in claim 1, wherein the content of Al is 47 at % or more and 48 at % or less, andthe content of Zr is 2 at % or more and 4 at % or less.

8. The TiAl alloy as recited in claim 1, wherein the content of Al is 47 at % or more and 48 at % or less, andthe content of Zr is 2 at % or more and 3 at % or less.

9. The TiAl alloy as recited in claim 1, wherein a room temperature ultimate tensile strength is 600 MPa or more, and a room temperature tensile fracture strain is 1.2% or more.

10. A TiAl alloy powder formed of the TiAl alloy as recited in claim 1.

11. A TiAl alloy component formed of the TiAl alloy as recited in claim 1.

12. A production method of a TiAl alloy component, comprising: a sealing step of filling a metal sheath with a TiAl alloy powder formed of the TiAl alloy as recited in claim 1; anda hot isostatic pressure step of treating the TiAl alloy powder sealed in the metal sheath with a hot isostatic pressure treatment under 1.200 degrees C. or higher and 1300 degrees C. or lower and 150 MPa.