TiAl alloy, TiAl alloy powder, TiAl alloy parts and manufacturing method thereof

Abstract

This TiAl alloy contains 47–50 at% of Al, 1–2 at% of Nb, 2–5 at% of Zr, and 0.05–0.3 at% of B, with the remainder comprising Ti and unavoidable impurities.

Description

[Technical Field]

[0001] The present disclosure relates to TiAl alloys, TiAl alloy powders, TiAl alloy parts, and methods of manufacturing the same. [Background technology]

[0002] TiAl (titanium aluminide) alloys are alloys formed from an intermetallic compound of Ti and Al. TiAl alloys have excellent heat resistance, are lighter than Ni-based alloys, and have a higher specific strength, so they are used in aircraft engine parts such as turbine blades. TiAl alloys containing Cr and Nb are used in such TiAl alloys (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-209750 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to reduce the weight of TiAl alloy parts such as turbine blades, it is necessary to increase the strength of TiAl alloys and increase their specific strength. However, with conventional TiAl alloys, it is difficult to achieve a good balance between mechanical strength and ductility, and increasing ductility can result in a decrease in mechanical strength.

[0005] Therefore, an object of the present disclosure is to provide a TiAl alloy, TiAl alloy powder, TiAl alloy part, and method for manufacturing the same, which are capable of improving the mechanical strength and ductility of the TiAl alloy in a well-balanced manner. [Means for solving the problem]

[0016] The TiAl alloy part according to the present disclosure is formed from the TiAl alloy described above.

[0017] The method for manufacturing a TiAl alloy part according to the present disclosure is to produce a TiAl alloy part containing 47 atomic % to 49 atomic % of Al, 2 atomic % to 3 atomic % of Zr, 2 atomic % of Nb, 0.05 atomic % to 0.3 atomic % of B, and the remainder being Ti and unavoidable impurities. or a TiAl alloy containing 47 atomic % to 48 atomic % Al, 2 atomic % to 4 atomic % Zr, 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and the balance being Ti and unavoidable impurities. and a hot isostatic pressing process in which the TiAl alloy powder sealed in the metal sheath is hot isostatically pressed at 1200°C to 1300°C and 150 MPa or more. [Effects of the Invention]

[0018] According to the above-mentioned configuration, it is possible to improve the mechanical strength and ductility of the TiAl alloy in a well-balanced manner. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % in an embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % in an embodiment of the present disclosure. [Figure 3] FIG. 2 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 2 atomic % in an embodiment of the present disclosure. [Figure 4] FIG. 2 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 2 atomic % in an embodiment of the present disclosure. [Figure 5] FIG. 3 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % or more and 2 atomic % or less in an embodiment of the present disclosure. [Figure 6] FIG. 3 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % or more and 2 atomic % or less in an embodiment of the present disclosure. [Figure 7] 1 is a diagram showing the configuration of a TiAl alloy part made of a turbine blade in an embodiment of the present disclosure. [Figure 8] 1 is a flowchart showing the configuration of a method for manufacturing a TiAl alloy part in an embodiment of the present disclosure. [Figure 9] 1 is a photograph showing the results of metallographic observation of TiAl alloys of Examples 1 to 9 in an embodiment of the present disclosure. [Figure 10] 1 is a photograph showing the metallographic structure observation results of TiAl alloys of Examples 10 to 18 in an embodiment of the present disclosure. [Figure 11] 1 is a graph showing the solidification morphology of TiAl alloys of Examples 1 to 9 in an embodiment of the present disclosure. [Figure 12] 1 is a graph showing the solidification morphology of TiAl alloys of Examples 10 to 18 in an embodiment of the present disclosure. [Figure 13] 1 is a photograph showing the metal structure observation results of the specimens of Examples A and B under an optical microscope in an embodiment of the present disclosure. [Figure 14] 1 is a graph showing tensile test results in an embodiment of the present disclosure. [Figure 15] 1 is a graph showing creep test results in an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0020]

[0010] The following describes in detail embodiments of the present disclosure with reference to the drawings. A TiAl (titanium aluminide) alloy according to an embodiment of the present disclosure contains 47 atomic % to 50 atomic % Al (aluminum), 1 atomic % to 2 atomic % Nb (niobium), 2 atomic % to 5 atomic % Zr (zirconium), and 0.05 atomic % to 0.3 atomic % B (boron), with the remainder consisting of Ti (titanium) and unavoidable impurities.

[0011] Next, the reasons for limiting the composition ranges of each alloy component constituting the TiAl alloy will be described.

[0021] Al (aluminum) has the function of improving mechanical strength and ductility such as room temperature ductility. The Al content is 47 atomic % or more and 50 atomic % or less. If the Al content is less than 47 atomic %, the content of Ti, which has a higher density than Al, becomes larger, resulting in a decrease in specific strength. If the Al content is more than 50 atomic %, ductility decreases. The Al content may be 47 atomic % or more and 49 atomic % or less. This can further improve the mechanical strength and ductility of the TiAl alloy.

[0022] Nb (niobium) has the function of improving oxidation resistance and mechanical strength. The Nb content is 1 atomic % or more and 2 atomic % or less. If the Nb content is less than 1 atomic %, oxidation resistance and high-temperature strength may decrease. If the Nb content is more than 2 atomic %, the density of Nb is greater than the densities of Al and Ti, so the specific strength decreases.

[0023] Zr (zirconium) has the function of improving oxidation resistance and mechanical strength. Zr is an element that stabilizes the gamma phase and contributes to improving ductility, such as room temperature ductility. Zr also contributes to improving creep strength by reducing the diffusion rate. The Zr content is 2 atomic % or more and 5 atomic % or less. If the Zr content is less than 2 atomic %, oxidation resistance, ductility, such as room temperature ductility, and mechanical strength, such as high temperature strength, may decrease. If the Zr content is more than 5 atomic %, segregation may occur. If Zr segregation occurs, mechanical strength and ductility may decrease.

[0024] B (boron) has the function of increasing ductility, such as room temperature ductility, by refining crystal grains. The B content is 0.05 atomic % or more and 0.3 atomic % or less. If the B content is less than 0.05 atomic %, the crystal grains may become coarse and ductility may decrease. If the B content is more than 0.3 atomic %, impact resistance may decrease. By setting the B content to 0.05 atomic % or more and 0.3 atomic % or less, the steel is composed of fine crystal grains with a grain size of 100 μm or less, which improves ductility.

[0025] B has the function of improving mechanical strength by precipitating fine borides within the crystal grains through heat treatment, etc. The fine borides include those with a particle size of 0.1 μm or less. The fine borides are composed of TiB, TiB2, etc. The precipitation of fine borides within the crystal grains can improve mechanical strength such as tensile strength, fatigue strength, and creep strength.

[0026] The balance of the TiAl alloy is composed of Ti and inevitable impurities. Inevitable impurities are impurities that may be mixed in even if not intentionally added. Since the TiAl alloy does not contain Cr (chromium), it is possible to suppress a decrease in mechanical strength. Since the TiAl alloy does not contain V (vanadium), it is possible to suppress a decrease in mechanical strength and oxidation resistance. Since the TiAl alloy does not contain Mo (molybdenum), it is possible to suppress a decrease in specific strength.

[0027] Next, we will explain the solidification morphology of TiAl alloys. The solidification morphology of TiAl alloys is related to the Al, Zr, and Nb contents. By changing the Al, Zr, and Nb contents, the solidification morphology of TiAl alloys changes to α solidification, β solidification, γ solidification, or α solidification + γ solidification. α solidification is a solidification morphology in which the solidification process of TiAl alloys passes through the α single-phase region. β solidification is a solidification morphology in which the solidification process of TiAl alloys passes through the β single-phase region. γ solidification is a solidification morphology in which the solidification process of TiAl alloys passes through the γ single-phase region. α solidification + γ solidification is a solidification morphology in which the solidification process of TiAl alloys passes through the α + γ two-phase region. In the case of γ solidification, coarse columnar crystal grains are formed, resulting in strong anisotropy of the metal structure. On the other hand, in the cases of α solidification and β solidification, equiaxed crystal grains are formed, resulting in strong isotropy of the metal structure and weak anisotropy of the metal structure. In the case of α solidification + γ solidification, equiaxed crystal grains and columnar crystal grains are formed, resulting in a metal structure intermediate between the metal structure of α solidification and the metal structure of γ solidification. Note that B precipitates fine borides within the crystal grains, so it has almost no effect on the solidification morphology of the TiAl alloy.

[0028] As the Al content increases, the solidification morphology of the TiAl alloy tends to be γ solidification. As the Al content decreases, the solidification morphology of the TiAl alloy tends to be α solidification + γ solidification, or α solidification or β solidification. As the Zr content increases, the solidification morphology of the TiAl alloy tends to be γ solidification. As the Zr content decreases, the solidification morphology of the TiAl alloy tends to be α solidification + γ solidification, or α solidification or β solidification. As the Nb content increases, the solidification morphology of the TiAl alloy tends to be α solidification + γ solidification, or α solidification or β solidification. As the Nb content decreases, the solidification morphology of the TiAl alloy tends to be γ solidification.

[0029] FIG. 1 shows the relationship between the Al and Zr contents when the Nb content is 1 atomic %. The TiAl alloy may have an Nb content of 1 atomic % and Al and Zr contents within the composition range surrounded by four points shown in FIG. 1 : R1 point (Al: 47 atomic %, Zr: 2 atomic %), R2 point (Al: 48 atomic %, Zr: 2 atomic %), R3 point (Al: 48 atomic %, Zr: 4 atomic %), and R4 point (Al: 47 atomic %, Zr: 5 atomic %). That is, the TiAl alloy may contain 1 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and Al and Zr within the composition range surrounded by four points shown in FIG. 1 : R1 point, R2 point, R3 point, and R4 point, with the balance being Ti and unavoidable impurities. When the TiAl alloy has this alloy composition, the solidification morphology can be α solidification only or α solidification + γ solidification. This suppresses the anisotropy of the metal structure compared to when the solidification mode is gamma solidification only. Furthermore, the suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 1 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 4 atomic % Zr, with the remainder being Ti and unavoidable impurities.

[0030] FIG. 2 shows the relationship between the Al and Zr contents when the Nb content is 1 atomic %. The TiAl alloy may have an Nb content of 1 atomic % and Al and Zr contents within the composition range surrounded by four points shown in FIG. 2 : S1 point (Al: 47 atomic %, Zr: 2 atomic %), S2 point (Al: 48 atomic %, Zr: 2 atomic %), S3 point (Al: 48 atomic %, Zr: 3 atomic %), and S4 point (Al: 47 atomic %, Zr: 5 atomic %). That is, the TiAl alloy may contain 1 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and Al and Zr within the composition range surrounded by four points shown in FIG. 2 : S1 point, S2 point, S3 point, and S4 point, with the balance being Ti and unavoidable impurities. When the TiAl alloy has this alloy composition, the solidification mode can be α-solidification only. This eliminates the need for gamma solidification, further suppressing the anisotropy of the metal structure. Furthermore, the further suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 1 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 3 atomic % Zr, with the remainder being Ti and unavoidable impurities.

[0031] Fig. 3 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 2 atomic %. The TiAl alloy may have an Nb content of 2 atomic %, and the Al and Zr contents may be in the composition range surrounded by five points shown in Fig. 3: T1 point (Al: 47 atomic %, Zr: 2 atomic %), T2 point (Al: 49 atomic %, Zr: 2 atomic %), T3 point (Al: 49 atomic %, Zr: 3 atomic %), T4 point (Al: 48 atomic %, Zr: 4 atomic %), and T5 point (Al: 47 atomic %, Zr: 4 atomic %). That is, the TiAl alloy may contain 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and Al and Zr in the composition range surrounded by five points shown in Fig. 3: T1 point, T2 point, T3 point, T4 point, and T5 point, with the balance being Ti and unavoidable impurities. When a TiAl alloy is composed of this alloy composition, the solidification morphology can be α solidification only or α solidification + γ solidification. This suppresses the anisotropy of the metal structure compared to when the solidification morphology is γ solidification only. Furthermore, the suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 49 atomic % Al, and 2 atomic % to 3 atomic % Zr, with the balance being Ti and unavoidable impurities. Alternatively, such a TiAl alloy may contain 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 4 atomic % Zr, with the balance being Ti and unavoidable impurities.

[0032] FIG. 4 shows the relationship between the Al and Zr contents when the Nb content is 2 atomic %. The TiAl alloy may have an Nb content of 2 atomic % and Al and Zr contents within the composition range surrounded by four points shown in FIG. 4 : W1 point (Al: 47 atomic %, Zr: 2 atomic %), W2 point (Al: 49 atomic %, Zr: 2 atomic %), W3 point (Al: 48 atomic %, Zr: 4 atomic %), and W4 point (Al: 47 atomic %, Zr: 4 atomic %). That is, the TiAl alloy may contain 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and Al and Zr within the composition range surrounded by four points shown in FIG. 4 : W1 point, W2 point, W3 point, and W4 point, with the balance being Ti and unavoidable impurities. When the TiAl alloy has this alloy composition, the solidification mode can be limited to α solidification. This eliminates the need for gamma solidification, further suppressing the anisotropy of the metal structure. Furthermore, the further suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 4 atomic % Zr, with the balance being Ti and unavoidable impurities.

[0033] 5 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % or more and 2 atomic % or less. When the Nb content of the TiAl alloy is 1 atomic % or more and 2 atomic % or less, the Al and Zr contents may be in the composition range surrounded by four points shown in FIG. 5 : X1 point (Al: 47 atomic %, Zr: 2 atomic %), X2 point (Al: 48 atomic %, Zr: 2 atomic %), X3 point (Al: 48 atomic %, Zr: 4 atomic %), and X4 point (Al: 47 atomic %, Zr: 4 atomic %). That is, the TiAl alloy may contain 1 atomic % or more and 2 atomic % or less Nb, 0.05 atomic % or more and 0.3 atomic % or less B, and Al and Zr in the composition range surrounded by the four points shown in FIG. 5 : X1 point, X2 point, X3 point, and X4 point, with the balance being Ti and unavoidable impurities.

[0034] The composition range enclosed by four points X1, X2, X3, and X4 in FIG. 5 overlaps with the composition range enclosed by four points R1, R2, R3, and R4 in FIG. 1 and the composition range enclosed by five points T1, T2, T3, T4, and T5 in FIG. 3. When a TiAl alloy is composed of this alloy composition, the solidification morphology can be α solidification only or α solidification and γ solidification. This reduces the anisotropy of the metal structure compared to when the solidification morphology is γ solidification only. Furthermore, the suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 1 atomic % to 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 4 atomic % Zr, with the remainder being Ti and unavoidable impurities.

[0035] 6 is a diagram showing the relationship between the Al and Zr contents when the Nb content is 1 atomic % or more and 2 atomic % or less. When the Nb content of a TiAl alloy is 1 atomic % or more and 2 atomic % or less, the Al and Zr contents may be in the composition range surrounded by five points shown in FIG. 6: Y1 point (Al: 47 atomic %, Zr: 2 atomic %), Y2 point (Al: 48 atomic %, Zr: 2 atomic %), Y3 point (Al: 48 atomic %, Zr: 3 atomic %), Y4 point (Al: 47.5 atomic %, Zr: 4 atomic %), and Y5 point (Al: 47 atomic %, Zr: 4 atomic %). That is, the TiAl alloy may contain 1 atomic % or more and 2 atomic % or less of Nb, 0.05 atomic % or more and 0.3 atomic % or less of B, Al and Zr having a composition range surrounded by five points Y1, Y2, Y3, Y4, and Y5 shown in Figure 6, with the remainder being Ti and unavoidable impurities.

[0036] The composition range enclosed by five points Y1, Y2, Y3, Y4, and Y5 in FIG. 6 overlaps with the composition range enclosed by four points S1, S2, S3, and S4 in FIG. 2 and the composition range enclosed by four points W1, W2, W3, and W4 in FIG. 4. When a TiAl alloy is composed of this alloy composition, the solidification mode can be limited to α solidification. Since the solidification mode does not include γ solidification, the anisotropy of the metal structure is further suppressed. Furthermore, the further suppression of the anisotropy of the metal structure makes the mechanical properties of the TiAl alloy more isotropic. For example, such a TiAl alloy may contain 1 atomic % to 2 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, 47 atomic % to 48 atomic % Al, and 2 atomic % to 3 atomic % Zr, with the remainder being Ti and unavoidable impurities.

[0037] Next, the metal structure of the TiAl alloy will be described. The metal structure of the TiAl alloy is composed of fine crystal grains with a crystal grain size of 100 μm or less. This can improve the ductility of the TiAl alloy. Furthermore, the metal structure of the TiAl alloy is composed of lamellar grains and γ grains, and there is no Zr segregation. The lamellar grains are formed by regularly arranging an α2 phase made of Ti3Al and a γ phase made of TiAl in layers. The γ grains are made of TiAl. The γ grains are, for example, equiaxed γ grains. The γ grains contain borides with a grain size of 0.1 μm or less. The borides are composed of TiB, TiB2, etc., and are acicular, etc.

[0038] Lamellar grains can improve mechanical strength such as tensile strength, fatigue strength, and creep strength. γ grains can improve ductility and high-temperature strength. Fine borides with a grain size of 0.1 μm or less can improve mechanical strength. The metal structure of a TiAl alloy should preferably have a volume fraction of γ grains of 80% or more by volume, with the remainder being lamellar grains, when the total volume fraction of lamellar grains and γ grains is 100% by volume. Since the metal structure of a TiAl alloy is mainly composed of γ grains, it can improve mechanical strength and ductility in a balanced manner. Furthermore, since the metal structure of a TiAl alloy does not have Zr segregation, it can suppress deterioration of mechanical strength and ductility.

[0039] Next, the mechanical properties of the TiAl alloy according to the embodiment of the present disclosure will be described. When a tensile test is performed in accordance with JIS, ASTM, or the like, the mechanical properties of the TiAl alloy at room temperature can be such that the room temperature tensile strength at break is 600 MPa or more and the room temperature tensile strain at break is 1.2% or more. Thus, the TiAl alloy according to the embodiment of the present disclosure can improve mechanical strength and ductility in a well-balanced manner.

[0040] Next, a TiAl alloy part using a TiAl alloy according to an embodiment of the present disclosure will be described. TiAl alloy parts can be applied to aircraft engine parts, turbine blades for power-generating gas turbines, and the like. FIG. 7 is a diagram showing the configuration of a TiAl alloy part 10 made of a turbine blade. The TiAl alloy has high mechanical strength, such as high-temperature strength, and can improve the heat resistance of the TiAl alloy part 10. Furthermore, the TiAl alloy has excellent ductility, such as room-temperature ductility, and can therefore suppress damage to the TiAl alloy part 10 during assembly or installation. Note that TiAl alloy parts are not limited to aircraft engine parts, and may also be, for example, supercharger parts, such as supercharger turbine wheels, or vehicle parts, such as automobile engine valves.

[0041] TiAl alloy parts can be produced by melting and casting the above TiAl alloy. TiAl alloy parts can be produced by melting and casting the above TiAl alloy in a vacuum induction furnace or the like. Casting can be carried out using casting equipment commonly used for casting general metallic materials.

[0042] The TiAl alloy part may be powder molded by metal powder injection molding (MIM) or hot isostatic pressing (HIP) using TiAl alloy powder formed from the above TiAl alloy as raw material powder. The TiAl alloy powder is formed from the above TiAl alloy and can be produced by sintering synthesis, mechanical alloying, plasma rotating electrode processing, atomization (water atomization, gas atomization), etc. The TiAl alloy powder is preferably a rapidly solidified powder. Since the rapidly solidified powder is produced by rapidly solidifying alloy droplets, the segregation of Zr contained in the TiAl alloy can be further suppressed.

[0043] Next, a method for manufacturing a TiAl alloy part by hot isostatic pressing (HIP) will be described as an example. Fig. 8 is a flowchart showing the configuration of the manufacturing method for a TiAl alloy part. The manufacturing method for a TiAl alloy part includes a sealing step (S10) and a hot isostatic pressing step (S12).

[0044] The sealing step (S10) is a step of filling a metal sheath with TiAl alloy powder made of the above-mentioned TiAl alloy and sealing it. The raw material powder is TiAl alloy powder made of the above-mentioned TiAl alloy. The TiAl alloy powder may be rapidly solidified powder produced by a gas atomization method or the like. The TiAl alloy powder is filled into the metal sheath and sealed. The metal sheath may be a titanium sheath made of pure titanium. The thickness of the titanium sheath may be, for example, 1 mm. The TiAl alloy powder filled into the metal sheath is vacuum degassed and then sealed by electron beam welding or the like.

[0045] The hot isostatic pressing step (S12) is a step of hot isostatically pressing the TiAl alloy powder filled in the metal sheath at 1200°C to 1300°C and 150 MPa or more. The hot isostatic pressing of the TiAl alloy powder filled in the metal sheath is performed to form a TiAl alloy part. The hot isostatic pressing can be performed at a heating temperature of 1200°C to 1300°C and a pressure of 150 MPa or more. The pressure can be, for example, 150 MPa to 200 MPa. The holding time at the heating temperature can be 3 hours or more. The holding time at the heating temperature can be, for example, 3 hours to 5 hours. After the hot isostatic pressing, the pressure can be released, the product can be furnace-cooled to 900°C, and then rapidly cooled to 900°C or less. This cooling method can prevent cracking of the TiAl alloy part. Rapid cooling from 900°C should be performed at a cooling rate faster than air cooling, and can be performed by gas fan cooling or the like.

[0046] The method for producing a TiAl alloy part may further include a stress relief step of holding the TiAl alloy part at 800°C to 950°C for 1 hour to 5 hours after the hot isostatic pressing step (S12), thereby removing residual stress and improving the ductility of the TiAl alloy part.

[0047] To prevent oxidation, the hot isostatic pressing treatment and stress relief are preferably performed in a vacuum atmosphere or an inert gas atmosphere such as argon gas. For the hot isostatic pressing treatment, a HIP apparatus or the like used for hot isostatic pressing of general metal materials can be used. For stress relief, an atmospheric furnace or the like used for stress relief annealing of general metal materials can be used. Note that a heat treatment process for adjusting the metal structure may be performed after the hot isostatic pressing process (S12) or the stress relief process.

[0048] As described above, the TiAl alloy of the above composition contains 47 atomic % to 50 atomic % Al, 1 atomic % to 2 atomic % Nb, 2 atomic % to 5 atomic % Zr, 0.05 atomic % to 0.3 atomic % B, and the remainder is Ti and unavoidable impurities, which allows the TiAl alloy to have a well-balanced improvement in mechanical strength and ductility. [Example]

[0049] The solidification morphology of the TiAl alloys was evaluated. The TiAl alloys of Examples 1 to 18 are described below. The TiAl alloys of Examples 1 to 18 contain Al, Nb, Zr, and B, with the balance being Ti and unavoidable impurities. The alloy composition of each TiAl alloy is shown in Table 1.

[0050] [Table 1]

[0051] The TiAl alloys of Examples 1 to 9 had an Nb content of 1 atomic %, a B content of 0.2 atomic %, an Al content of 47 atomic % to 50 atomic %, and a Zr content of 3 atomic % to 5 atomic %. The TiAl alloys of Examples 10 to 18 had an Nb content of 2 atomic %, a B content of 0.2 atomic %, an Al content of 48 atomic % to 50 atomic %, and a Zr content of 2 atomic % to 4 atomic %.

[0052] Each TiAl alloy raw material having the alloy composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and cast to form a TiAl alloy ingot having each alloy composition. The TiAl alloy was then subjected to metallographic observation to evaluate the solidification morphology. Fig. 9 is a photograph showing the metallographic observation results of the TiAl alloys of Examples 1 to 9. Fig. 10 is a photograph showing the metallographic observation results of the TiAl alloys of Examples 10 to 18.

[0053] The solidification morphology of the TiAl alloys of Examples 1 and 2 was only α solidification. The solidification morphology of the TiAl alloy of Example 3 was α solidification + γ solidification. The solidification morphology of the TiAl alloys of Examples 4 to 9 was only γ solidification. The solidification morphology of the TiAl alloys of Examples 10 to 13 was only α solidification. The solidification morphology of the TiAl alloy of Example 14 was α solidification + γ solidification. The solidification morphology of the TiAl alloys of Examples 15 to 18 was only γ solidification.

[0054] FIG. 11 is a graph showing the solidification morphologies of the TiAl alloys of Examples 1 to 9. FIG. 12 is a graph showing the solidification morphologies of the TiAl alloys of Examples 10 to 18. In the graphs of FIGS. 11 and 12, the horizontal axis represents the Zr content (atomic %) and the vertical axis represents the Al content (atomic %). Circles indicate α solidification only, triangles indicate α and γ solidification, and squares indicate γ solidification only. Note that FIG. 11 also shows the four points R1, R2, R3, and R4 shown in FIG. 1 and the four points S1, S2, S3, and S4 shown in FIG. 2. FIG. 12 also shows the five points T1, T2, T3, T4, and T5 shown in FIG. 3 and the four points W1, W2, W3, and W4 shown in FIG. 4.

[0055] It has been revealed that the solidification morphology of TiAl alloys tends to change from α solidification or α + γ solidification to γ ​​solidification as the Al content increases. For example, as shown in Figure 11, when the Nb content is 1 atomic %, when the Zr content is 3 atomic % to 5 atomic %, the solidification morphology becomes γ solidification only when the Al content is 49 atomic % or more. Also, as shown in Figure 12, when the Nb content is 2 atomic %, when the Zr content is 2 atomic % to 4 atomic %, the solidification morphology becomes γ solidification only when the Al content is 50 atomic % or more.

[0056] It was found that the solidification morphology of TiAl alloys tends to change from α solidification or α solidification + γ solidification to γ ​​solidification as the Zr content increases. For example, as shown in Figure 11, when the Nb content is 1 atomic % and the Al content is 48 atomic %, only α solidification occurs when the Zr content is 3 atomic %, α solidification + γ solidification occurs when the Zr content is 4 atomic %, and only γ solidification occurs when the Zr content is 5 atomic %. Also, as shown in Figure 12, when the Nb content is 2 atomic % and the Al content is 49 atomic %, only α solidification occurs when the Zr content is 2 atomic %, α solidification + γ solidification occurs when the Zr content is 3 atomic %, and only γ solidification occurs when the Zr content is 4 atomic %.

[0057] It was revealed that as the Nb content increases, the solidification morphology of TiAl alloys tends to change from gamma solidification to alpha solidification + gamma solidification or alpha solidification. For example, when the Al content is 49 atomic % and the Zr content is 3 atomic %, gamma solidification only occurs when the Nb content is 1 atomic %, as shown in Figure 11, and alpha solidification + gamma solidification occurs when the Nb content is 2 atomic %, as shown in Figure 12.

[0058] From the graph in FIG. 11, it was found that when the Nb content is 1 atomic % and the Al and Zr contents are in the composition range surrounded by the four points R1 (Al: 47 atomic %, Zr: 2 atomic %), R2 (Al: 48 atomic %, Zr: 2 atomic %), R3 (Al: 48 atomic %, Zr: 4 atomic %), and R4 (Al: 47 atomic %, Zr: 5 atomic %) shown in FIG. 1 described above, the solidification morphology is α solidification only or α solidification + γ solidification.

[0059] Next, the reason for this will be explained. First, it is clear from FIG. 11 that point R3 (Al: 48 atomic %, Zr: 4 atomic %) is α-coagulation plus γ-coagulation, and point R4 (Al: 47 atomic %, Zr: 5 atomic %) is α-coagulation only. Point R1 (Al: 47 atomic %, Zr: 2 atomic %) has a lower Zr content than point R4 (Al: 47 atomic %, Zr: 5 atomic %), so it is α-coagulation only. Point R2 (Al: 48 atomic %, Zr: 2 atomic %) has a lower Zr content than point R4 (Al: 47 atomic %, Zr: 5 atomic %), so it is α-coagulation only. Therefore, when the Al and Zr contents are within the composition range surrounded by the four points R1, R2, R3, and R4 shown in FIG. 1, the solidification morphology is α-coagulation only or α-coagulation plus γ-coagulation.

[0060] From the graph in Figure 11, it was found that when the Nb content is 1 atomic % and the Al and Zr contents are in the composition range surrounded by the four points S1 (Al: 47 atomic %, Zr: 2 atomic %), S2 (Al: 48 atomic %, Zr: 2 atomic %), S3 (Al: 48 atomic %, Zr: 3 atomic %), and S4 (Al: 47 atomic %, Zr: 5 atomic %) shown in Figure 2 above, the solidification mode is only α solidification.

[0061] Next, the reason for this will be explained. First, it is clear from FIG. 11 that only α solidification occurs at points S3 (Al: 48 atomic %, Zr: 3 atomic %) and S4 (Al: 47 atomic %, Zr: 5 atomic %). At point S1 (Al: 47 atomic %, Zr: 2 atomic %), the Zr content is lower than at point S4 (Al: 47 atomic %, Zr: 5 atomic %), so only α solidification occurs. Furthermore, at point S2 (Al: 48 atomic %, Zr: 2 atomic %), the Zr content is lower than at point S3 (Al: 48 atomic %, Zr: 3 atomic %), so only α solidification occurs. Therefore, when the Al and Zr contents are within the composition range enclosed by the four points S1, S2, S3, and S4 shown in FIG. 2, the solidification mode is only α solidification.

[0062] From the graph in FIG. 12, it was found that when the Nb content is 2 atomic % and the Al and Zr contents are in the composition range surrounded by the five points T1 (Al: 47 atomic %, Zr: 2 atomic %), T2 (Al: 49 atomic %, Zr: 2 atomic %), T3 (Al: 49 atomic %, Zr: 3 atomic %), T4 (Al: 48 atomic %, Zr: 4 atomic %), and T5 (Al: 47 atomic %, Zr: 4 atomic %) shown in FIG. 3 described above, the solidification morphology is α solidification only or α solidification + γ solidification.

[0063] Next, the reason for this will be explained. First, it is clear from FIG. 12 that points T2 (Al: 49 atomic %, Zr: 2 atomic %) and T4 (Al: 48 atomic %, Zr: 4 atomic %) are α solidification only, and point T3 (Al: 49 atomic %, Zr: 3 atomic %) is α solidification + γ solidification. Point T1 (Al: 47 atomic %, Zr: 2 atomic %) has a lower Al content than point T2 (Al: 49 atomic %, Zr: 2 atomic %), so it is α solidification only. Furthermore, point T5 (Al: 47 atomic %, Zr: 4 atomic %) has a lower Al content than point T4 (Al: 48 atomic %, Zr: 4 atomic %), so it is α solidification only. Therefore, when the Al and Zr contents are within the composition range surrounded by the five points T1, T2, T3, T4, and T5 shown in FIG. 3, the solidification morphology is α solidification only or α solidification + γ solidification.

[0064] From the graph in FIG. 12, it was found that when the Nb content is 2 atomic % and the Al and Zr contents are in the composition range surrounded by the four points W1 (Al: 47 atomic %, Zr: 2 atomic %), W2 (Al: 49 atomic %, Zr: 2 atomic %), W3 (Al: 48 atomic %, Zr: 4 atomic %), and W4 (Al: 47 atomic %, Zr: 4 atomic %) shown in FIG. 4 described above, the solidification mode is only α solidification.

[0065] Next, the reason for this will be explained. First, it is clear from FIG. 12 that only α solidification occurs at points W2 (Al: 49 atomic %, Zr: 2 atomic %) and W3 (Al: 48 atomic %, Zr: 4 atomic %). At point W1 (Al: 47 atomic %, Zr: 2 atomic %), the Al content is lower than at point W2 (Al: 49 atomic %, Zr: 2 atomic %), so only α solidification occurs. At point W4 (Al: 47 atomic %, Zr: 4 atomic %), the Al content is lower than at point W3 (Al: 48 atomic %, Zr: 4 atomic %), so only α solidification occurs. Therefore, when the Al and Zr contents are within the composition range enclosed by the four points W1, W2, W3, and W4 shown in FIG. 4, the solidification mode is only α solidification.

[0066] From the graphs of FIGS. 11 and 12, it was found that when the Nb content is 1 atomic % or more and 2 atomic % or less, and the Al and Zr contents are constituted in the composition range surrounded by four points shown in FIG. 5 described above, namely, point X1 (Al: 47 atomic %, Zr: 2 atomic %), point X2 (Al: 48 atomic %, Zr: 2 atomic %), point X3 (Al: 48 atomic %, Zr: 4 atomic %), and point X4 (Al: 47 atomic %, Zr: 4 atomic %), the solidification morphology becomes α solidification only or α solidification + γ solidification.

[0067] From the graphs of Figures 11 and 12, it was found that when the Nb content is 1 atomic % or more and 2 atomic % or less, the solidification mode is only α-solidification when the Al and Zr contents are constituted in the composition range surrounded by five points shown in Figure 6 above, namely, point Y1 (Al: 47 atomic %, Zr: 2 atomic %), point Y2 (Al: 48 atomic %, Zr: 2 atomic %), point Y3 (Al: 48 atomic %, Zr: 3 atomic %), point Y4 (Al: 47.5 atomic %, Zr: 4 atomic %), and point Y5 (Al: 47 atomic %, Zr: 4 atomic %).

[0068] Next, specimens of Examples A and B were prepared using the TiAl alloy powders formed from the TiAl alloys of Examples 1 and 11, and their mechanical properties were evaluated. First, the method for preparing the specimens of Examples A and B will be described. The specimens of Examples A and B were prepared by powder molding using hot isostatic pressing.

[0069] First, the TiAl alloy powder was filled into a pure titanium sheath and sealed. For the specimen of Example A, the TiAl alloy powder formed from the TiAl alloy of Example 1 was used. For the specimen of Example B, the TiAl alloy powder formed from the TiAl alloy of Example 11 was used. For the TiAl alloy powder formed from the TiAl alloy of Examples 1 and 11, rapidly solidified powder produced by gas atomization was used. The TiAl alloy powder filled into the pure titanium sheath was vacuum degassed and then sealed by electron beam welding.

[0070] The TiAl alloy powder packed into a pure titanium sheath was hot isostatically pressed at 1250°C and 172 MPa for 3 hours. After hot isostatic pressing, the pressure was released and the material was furnace cooled to 900°C, and then rapidly cooled below 900°C. Rapid cooling from 900°C was performed using a gas fan. In this way, specimens for Examples A and B were prepared.

[0071] Metallographic observation was performed on the specimens of Examples A and B. The metallographic observation was performed using an optical microscope and an electron microscope. Figure 13 shows photographs showing the results of metallographic observation of the specimens of Examples A and B using an optical microscope, where Figure 13(a) is a photograph of the specimen of Example A and Figure 13(b) is a photograph of the specimen of Example B.

[0072] The metallographic structures of the specimens of Examples A and B were composed of fine crystal grains with a grain size of 100 μm or less. The metallographic structures of the specimens of Examples A and B were composed of lamellar grains and equiaxed γ grains, and the equiaxed γ grains contained borides with a grain size of 0.1 μm or less. In the metallographic structures of Examples 1 and 11, when the total volume fraction of the lamellar grains and equiaxed γ grains was taken as 100 volume %, the volume fraction of the equiaxed γ grains was 80 volume % or more, with the remainder being lamellar grains. Regarding the volume fraction of each grain, the area fraction of each grain was calculated by image processing based on the contrast information of each grain in metallographic photographs taken by an electron microscope, and this was used as the volume fraction of each grain. Furthermore, no Zr segregation was observed in the metallographic structures of the specimens of Examples A and B.

[0073] Next, the room temperature mechanical properties of the specimens of Examples A and B were evaluated. A room temperature tensile test was performed on the specimens of Examples A and B. Similarly, a room temperature tensile test was performed on the specimen of Comparative Example A. The specimen of Comparative Example A was formed from a TiAl alloy containing 48 atomic % Al, 2 atomic % Nb, and 2 atomic % Cr, with the balance being Ti and unavoidable impurities.

[0074] The tensile test was conducted in accordance with ASTM E8. FIG. 14 is a graph showing the tensile test results. In FIG. 14, the horizontal axis represents strain and the vertical axis represents stress, showing the stress-strain curve of each specimen. The specimens of Examples A and B had larger room-temperature tensile breaking strength and room-temperature tensile breaking strain than the specimen of Comparative Example A. The specimens of Examples A and B had room-temperature tensile breaking strengths of 600 MPa or more and room-temperature tensile breaking strains of 1.2% or more. The specimen of Example A had a room-temperature tensile breaking strength of 700 MPa or more, and the specimen of Example B had a room-temperature tensile breaking strain of 1.4% or more. These results revealed that the specimens of Examples A and B had excellent mechanical strength and ductility, and that mechanical strength and ductility were improved in a balanced manner.

[0075] A creep test was carried out on the specimens of Example A and Comparative Example A. The creep test was carried out in accordance with JIS Z 2271. Fig. 15 is a graph showing the creep test results. In the graph of Fig. 15, the horizontal axis represents the Larson-Miller parameter P, and the vertical axis represents the specific strength, with the specimens of Example A indicated by a square and the specimens of Comparative Example A indicated by an x. The Larson-Miller parameter P is calculated by the following equation: P = T × log(t r + C), where T is the absolute temperature (K) and t r is the time to fracture (h), and C is a material constant. The material constant C was set to 20. As shown in FIG. 15, the specimen of Example A had better creep properties than the specimen of Comparative Example A. This result shows that the specimen of Example A had better high-temperature strength properties than the specimen of Comparative Example A. [Industrial Applicability]

[0076] The present disclosure can improve the mechanical strength and ductility of TiAl alloys in a well-balanced manner, and is therefore useful for aircraft engine parts, turbine blades for power generation gas turbines, and the like.

Claims

[Claim 1] 47 atomic % or more and 49 atomic % or less of Al and 2 atomic % or more and 3 atomic % or less of Zr; 2 atomic % Nb, and 0.05 atomic % or more and 0.3 atomic % or less of B, a TiAl alloy, the balance of which is Ti and unavoidable impurities; or 47 atomic % or more and 48 atomic % or less of Al and 2 atomic % or more and 4 atomic % or less of Zr, 2 atomic % Nb, and 0.05 atomic % or more and 0.3 atomic % or less of B, a sealing step of filling a metal sheath with TiAl alloy powder, the balance of which is Ti and unavoidable impurities, and sealing the metal sheath; a hot isostatic pressing step of hot isostatically pressing the TiAl alloy powder sealed with the metal sheath at 1200°C to 1300°C and 150 MPa or more; A method for manufacturing a TiAl alloy part, comprising:

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