JP7732507B2 - TiAl alloy, TiAl alloy powder, TiAl alloy part, and manufacturing method thereof - Google Patents

Description

This disclosure relates to TiAl alloys, TiAl alloy powders, TiAl alloy parts, and methods for manufacturing the same.

TiAl (titanium aluminide) alloys are alloys formed from intermetallic compounds of Ti and Al. TiAl alloys have excellent heat resistance, are lighter than Ni-based alloys, and have a higher specific strength, making them suitable for use in aircraft engine parts such as turbine blades. Among such TiAl alloys, TiAl alloys containing Cr and Nb are used (see Patent Document 1).

JP 2013-209750 A

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 balanced improvement in mechanical strength and ductility, and increasing ductility can result in a decrease in mechanical strength.

The object of this disclosure is to provide a TiAl alloy, TiAl alloy powder, TiAl alloy parts, and a manufacturing method thereof that can improve the mechanical strength and ductility of TiAl alloys in a balanced manner.

The TiAl alloy parts disclosed herein are formed from the TiAl alloy described above.

The method for producing a TiAl alloy part according to the present disclosure includes a sealing step of filling a metal sheath with TiAl alloy powder formed of a TiAl alloy containing 47 to 49 atomic % Al, 2 to 3 atomic % Zr, 2 atomic % Nb, 0.05 to 0.3 atomic % B, with the balance being Ti and unavoidable impurities, or a TiAl alloy containing 47 to 48 atomic % Al, 2 to 4 atomic % Zr, 2 atomic % Nb, 0.05 to 0.3 atomic % B, with the balance being Ti and unavoidable impurities, and a hot isostatic pressing step of hot isostatically pressing the TiAl alloy powder sealed in the metal sheath at 1200°C to 1300°C and 150 MPa or more.

The above configuration makes it possible to improve the mechanical strength and ductility of the TiAl alloy in a balanced manner.

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. 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. FIG. 1 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. FIG. 1 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. FIG. 1 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. FIG. 1 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. 1 is a diagram showing the configuration of a TiAl alloy part made of a turbine blade in an embodiment of the present disclosure. 1 is a flowchart illustrating a method for manufacturing a TiAl alloy part according to an embodiment of the present disclosure. 1 is a photograph showing the metallographic structure observation results of TiAl alloys of Examples 1 to 9 in an embodiment of the present disclosure. 1 is a photograph showing the metal structure observation results of TiAl alloys of Examples 10 to 18 in an embodiment of the present disclosure. 1 is a graph showing the solidification morphology of TiAl alloys of Examples 1 to 9 in an embodiment of the present disclosure. 1 is a graph showing the solidification morphology of TiAl alloys of Examples 10 to 18 in an embodiment of the present disclosure. 1 is a photograph showing the results of metal structure observation of specimens of Examples A and B using an optical microscope in an embodiment of the present disclosure. 1 is a graph showing tensile test results in an embodiment of the present disclosure. 1 is a graph showing creep test results in an embodiment of the present disclosure.

Embodiments of the present disclosure are described in detail below with reference to the drawings. The 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 balance consisting of Ti (titanium) and unavoidable impurities. Next, the reasons for limiting the composition ranges of each alloy component that makes up the TiAl alloy will be explained.

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 greater, resulting in a decrease in specific strength. If the Al content is greater than 50 atomic %, ductility decreases. The Al content may also be 47 atomic % or more and 49 atomic % or less. This can further improve the mechanical strength and ductility of the TiAl alloy.

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.

Zr (zirconium) has the function of improving oxidation resistance and mechanical strength. Zr is an element that stabilizes the gamma phase, contributing to improved ductility, such as room temperature ductility. Zr also contributes to improved 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 greater than 5 atomic %, segregation may occur. Zr segregation may result in reduced mechanical strength and ductility.

B (boron) has the function of increasing ductility, such as room temperature ductility, by refining the 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 properties may decrease. By setting the B content to 0.05 atomic % or more and 0.3 atomic % or less, the steel will be composed of fine crystal grains with a grain size of 100 μm or less, thereby improving ductility.

B has the function of precipitating fine borides within crystal grains by heat treatment or the like, thereby improving mechanical strength. The fine borides are formed including those with a particle size of 0.1 μm or less. The fine borides are composed of TiB, _TiB2 , etc. Precipitation of fine borides within crystal grains can improve mechanical strength such as tensile strength, fatigue strength, and creep strength.

The remainder 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. Because the TiAl alloy does not contain Cr (chromium), it is possible to suppress a decrease in mechanical strength. Because the TiAl alloy does not contain V (vanadium), it is possible to suppress a decrease in mechanical strength and oxidation resistance. Because the TiAl alloy does not contain Mo (molybdenum), it is possible to suppress a decrease in specific strength.

Next, we will explain the solidification morphology of TiAl alloys. The solidification morphology of TiAl alloys is related to the Al, Zr, and Nb content. By changing the Al, Zr, and Nb content, 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.

As the Al content increases, the solidification morphology of the TiAl alloy tends to be gamma solidification. As the Al content decreases, the solidification morphology of the TiAl alloy tends to be alpha solidification + gamma solidification, or alpha solidification or beta solidification. As the Zr content increases, the solidification morphology of the TiAl alloy tends to be gamma solidification. As the Zr content decreases, the solidification morphology of the TiAl alloy tends to be alpha solidification + gamma solidification, or alpha solidification or beta solidification. As the Nb content increases, the solidification morphology of the TiAl alloy tends to be alpha solidification + gamma solidification, or alpha solidification or beta solidification. As the Nb content decreases, the solidification morphology of the TiAl alloy tends to be gamma solidification.

Figure 1 shows the relationship between the Al and Zr contents when the Nb content is 1 atomic %. A TiAl alloy may have an Nb content of 1 atomic % and Al and Zr contents within the composition range bounded by four points shown in Figure 1: 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 %). That is, a TiAl alloy may contain 1 atomic % Nb, 0.05 atomic % to 0.3 atomic % B, and Al and Zr within the composition range bounded by four points shown in Figure 1: R1, R2, R3, and R4, with the balance consisting of Ti and unavoidable impurities. When a 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. 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 balance being Ti and unavoidable impurities.

Figure 2 shows the relationship between the Al and Zr contents when the Nb content is 1 atomic %. A TiAl alloy may have an Nb content of 1 atomic % and Al and Zr contents within the composition range bounded by four points shown in Figure 2: 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 %). 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 bounded by four points S1, S2, S3, and S4 shown in Figure 2, with the balance being Ti and unavoidable impurities. When a TiAl alloy has this alloy composition, the solidification mode can be alpha solidification only. This prevents the solidification from occurring due to γ 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 balance being Ti and unavoidable impurities.

Figure 3 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 enclosed by the five points shown in Figure 3: 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 %). 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 enclosed by the five points shown in Figure 3: T1, T2, T3, T4, and T5, 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.

Figure 4 shows the relationship between the Al and Zr contents when the Nb content is 2 atomic %. A TiAl alloy may have an Nb content of 2 atomic % and Al and Zr contents within the composition range bounded by four points shown in Figure 4: 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 %). 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 bounded by four points shown in Figure 4: W1, W2, W3, and W4, with the balance being Ti and unavoidable impurities. When a TiAl alloy has this alloy composition, the solidification mode can be alpha solidification only. This prevents the solidification from occurring due to γ 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.

Figure 5 shows 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 within the composition range enclosed by four points shown in Figure 5: X1 (Al: 47 atomic %, Zr: 2 atomic %), X2 (Al: 48 atomic %, Zr: 2 atomic %), X3 (Al: 48 atomic %, Zr: 4 atomic %), and X4 (Al: 47 atomic %, Zr: 4 atomic %). That is, the TiAl alloy may contain 1 atomic % or more and 2 atomic % Nb, 0.05 atomic % or more and 0.3 atomic % B, and Al and Zr within the composition range enclosed by the four points X1, X2, X3, and X4 shown in Figure 5, with the remainder being Ti and unavoidable impurities.

Note that the composition range enclosed by four points X1, X2, X3, and X4 in Figure 5 represents the overlapping composition range between the composition range enclosed by four points R1, R2, R3, and R4 in Figure 1 and the composition range enclosed by five points T1, T2, T3, T4, and T5 in Figure 3. When a TiAl alloy is composed of this alloy composition, the solidification morphology can be alpha solidification only or alpha solidification + gamma solidification. This suppresses the anisotropy of the metal structure compared to when the solidification morphology is gamma solidification only. Furthermore, suppressing 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, 2 atomic % to 4 atomic % Zr, and the remainder being Ti and unavoidable impurities.

Figure 6 shows 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 within the composition range surrounded by five points shown in Figure 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, and the remainder may be composed of Ti and unavoidable impurities.

The composition range bounded by five points Y1, Y2, Y3, Y4, and Y5 in Figure 6 represents the overlapping composition range between the composition range bounded by four points S1, S2, S3, and S4 in Figure 2 and the composition range bounded by four points W1, W2, W3, and W4 in Figure 4. When a TiAl alloy is composed of this alloy composition, the solidification mode can be limited to α solidification. This eliminates γ 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 % 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 consisting of Ti and unavoidable impurities.

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 consisting of _Ti3Al and a γ phase consisting of TiAl in layers. The γ grains are formed 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.

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 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. Because the metal structure of a TiAl alloy is primarily composed of γ grains, it can improve mechanical strength and ductility in a balanced manner. Furthermore, because the metal structure of a TiAl alloy does not have Zr segregation, it can suppress a decrease in mechanical strength and ductility.

Next, the mechanical properties of the TiAl alloy according to the embodiment of the present disclosure will be described. When tensile tests are conducted in accordance with JIS, ASTM, etc., the mechanical properties of the TiAl alloy at room temperature can be such that the room temperature tensile fracture strength is 600 MPa or more and the room temperature tensile fracture strain is 1.2% or more. Thus, the TiAl alloy according to the embodiment of the present disclosure can achieve a balanced improvement in mechanical strength and ductility.

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. Figure 7 shows 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 even when assembling or installing it. 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.

TiAl alloy parts can be cast by melting the above-mentioned TiAl alloy. TiAl alloy parts can be cast by melting the above-mentioned TiAl alloy in a vacuum induction furnace or the like. Casting can be done using casting equipment that is used for casting general metallic materials.

TiAl alloy parts may be powder molded using a TiAl alloy powder formed from the above-mentioned TiAl alloy as the raw material powder, using methods such as metal powder injection molding (MIM) and hot isostatic pressing (HIP). The TiAl alloy powder is formed from the above-mentioned TiAl alloy and can be produced by sintering synthesis, mechanical alloying, plasma rotating electrode processing, atomization (water atomization, gas atomization), and other methods. Rapidly solidified powder is preferably used as the TiAl alloy powder. Because rapidly solidified powder is produced by rapidly solidifying alloy droplets, it is possible to further suppress the segregation of Zr contained in the TiAl alloy.

Next, as an example, we will explain a method for manufacturing TiAl alloy parts using hot isostatic pressing (HIP). Figure 8 is a flowchart showing the configuration of the manufacturing method for TiAl alloy parts. The manufacturing method for TiAl alloy parts includes a sealing process (S10) and a hot isostatic pressing process (S12).

The sealing process (S10) is a process in which TiAl alloy powder made from the above-mentioned TiAl alloy is filled into a metal sheath and sealed. The raw material powder is TiAl alloy powder made from the above-mentioned TiAl alloy. Rapidly solidified powder produced by a gas atomization method or the like can be used as the TiAl alloy powder. The TiAl alloy powder is filled into a metal sheath and sealed. A titanium sheath made from pure titanium can be used as the metal sheath. The thickness of the titanium sheath can 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.

The hot isostatic pressing process (S12) involves hot isostatic pressing the TiAl alloy powder filled into the metal sheath at 1200°C to 1300°C and 150 MPa or higher. Hot isostatic pressing of the TiAl alloy powder filled into the metal sheath results in the formation of a TiAl alloy part. Hot isostatic pressing can be performed at a heating temperature of 1200°C to 1300°C and a pressure of 150 MPa or higher. The pressure can be, for example, 150 MPa to 200 MPa. The holding time at the heating temperature can be 3 hours or longer. The holding time at the heating temperature can be, for example, 3 hours to 5 hours. After 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 lower. 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.

The manufacturing method for TiAl alloy parts may include a stress relief step after the hot isostatic pressing step (S12), in which stress is relieved by holding the part at 800°C or higher and 950°C or lower for 1 hour or higher and 5 hours or lower. This removes residual stress and other imperfections from the TiAl alloy part, thereby improving the ductility of the TiAl alloy part.

To prevent oxidation, hot isostatic pressing and stress relief are preferably carried out in a vacuum atmosphere or an inert gas atmosphere such as argon gas. For hot isostatic pressing, a HIP apparatus, which is used for hot isostatic pressing of general metal materials, can be used. For stress relief, an atmospheric furnace, which is used for stress relief annealing of general metal materials, can be used. After the hot isostatic pressing process (S12) or stress relief process, a heat treatment process to adjust the metal structure may be carried out.

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, and 0.05 atomic % to 0.3 atomic % B, with the remainder being Ti and unavoidable impurities. This allows for a balanced improvement in the mechanical strength and ductility of the TiAl alloy.

The solidification morphology of 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 consisting of Ti and unavoidable impurities. The alloy composition of each TiAl alloy is shown in Table 1.

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 %.

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 form. Figure 9 is a photograph showing the results of metallographic observation of the TiAl alloys of Examples 1 to 9. Figure 10 is a photograph showing the results of metallographic observation of the TiAl alloys of Examples 10 to 18.

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.

Figure 11 is a graph showing the solidification morphology of the TiAl alloys of Examples 1 to 9. Figure 12 is a graph showing the solidification morphology of the TiAl alloys of Examples 10 to 18. In the graphs of Figures 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 Figure 11 also shows the four points R1, R2, R3, and R4 shown in Figure 1, as well as the four points S1, S2, S3, and S4 shown in Figure 2. Figure 12 also shows the five points T1, T2, T3, T4, and T5 shown in Figure 3, as well as the four points W1, W2, W3, and W4 shown in Figure 4.

It has been revealed that as the Al content increases, the solidification morphology of TiAl alloys tends to change from α solidification or α + γ solidification to γ solidification. 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.

It has been revealed that as the Zr content increases, the solidification morphology of TiAl alloys tends to change from α solidification or α solidification + γ solidification to γ solidification. 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 %.

It has been 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 alone 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.

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 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 Figure 1 above, the solidification form will be α solidification only or α solidification + γ solidification.

Next, we will explain the reason for this. First, it is clear from Figure 11 that point R3 (Al: 48 atomic %, Zr: 4 atomic %) is alpha solidification + gamma solidification, and point R4 (Al: 47 atomic %, Zr: 5 atomic %) is alpha solidification 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 alpha solidification only. Point R2 (Al: 48 atomic %, Zr: 2 atomic %) has a lower Zr content than point R4 (Al: 47 atomic %, Zr: 3 atomic %), so it is alpha solidification 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 Figure 1 above, the solidification morphology will be alpha solidification only or alpha solidification + gamma solidification.

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 form is only alpha solidification.

Next, we will explain the reason for this. First, it is clear from Figure 11 that point S3 (Al: 48 atomic %, Zr: 3 atomic %) and point S4 (Al: 47 atomic %, Zr: 5 atomic %) are alpha solidification only. Point S1 (Al: 47 atomic %, Zr: 2 atomic %) has a lower Zr content than point S4 (Al: 47 atomic %, Zr: 5 atomic %), so it is alpha solidification only. Furthermore, point S2 (Al: 48 atomic %, Zr: 2 atomic %) has a lower Zr content than point S3 (Al: 48 atomic %, Zr: 3 atomic %), so it is alpha solidification only. Therefore, when the Al and Zr contents are within the composition range surrounded by the four points S1, S2, S3, and S4 shown in Figure 2 above, the solidification form is alpha solidification only.

From the graph in Figure 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 shown in Figure 3 above, namely, 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 %), the solidification form will be α solidification only or α solidification + γ solidification.

Next, we will explain the reason for this. First, it is clear from Figure 12 that points T2 (Al: 49 atomic %, Zr: 2 atomic %) and T4 (Al: 48 atomic %, Zr: 4 atomic %) are alpha solidification only, while point T3 (Al: 49 atomic %, Zr: 3 atomic %) is alpha solidification + gamma 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 alpha 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 alpha solidification only. Therefore, when the Al and Zr contents are within the composition range enclosed by the five points T1, T2, T3, T4, and T5 shown in Figure 3 above, the solidification mode will be alpha solidification only or alpha solidification + gamma solidification.

From the graph in Figure 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 shown in Figure 4 above, namely, point W1 (Al: 47 atomic %, Zr: 2 atomic %), point W2 (Al: 49 atomic %, Zr: 2 atomic %), point W3 (Al: 48 atomic %, Zr: 4 atomic %), and point W4 (Al: 47 atomic %, Zr: 4 atomic %), the solidification form is only alpha solidification.

Next, we will explain the reason for this. First, it is clear from Figure 12 that points W2 (Al: 49 atomic %, Zr: 2 atomic %) and W3 (Al: 48 atomic %, Zr: 4 atomic %) are exclusively alpha solidification. Point W1 (Al: 47 atomic %, Zr: 2 atomic %) has a lower Al content than point W2 (Al: 49 atomic %, Zr: 2 atomic %), so it is exclusively alpha solidification. Point W4 (Al: 47 atomic %, Zr: 4 atomic %) has a lower Al content than point W3 (Al: 48 atomic %, Zr: 4 atomic %), so it is exclusively alpha solidification. Therefore, when the Al and Zr contents are within the composition range enclosed by the four points W1, W2, W3, and W4 shown in Figure 4 above, the solidification mode is exclusively alpha solidification.

From the graphs in Figures 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 composed of a composition range surrounded by four points shown in Figure 5 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 form will be α coagulation only or α coagulation + γ coagulation.

From the graphs in 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 composed in the composition range surrounded by the five points shown in Figure 6 above: 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 %).

Next, specimens of Examples A and B were prepared using the TiAl alloy powder 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 explained. The specimens of Examples A and B were prepared by powder molding using the hot isostatic pressing method.

First, the TiAl alloy powder was filled into a pure titanium sheath and sealed. For the specimen of Example A, TiAl alloy powder formed from the TiAl alloy of Example 1 was used. For the specimen of Example B, 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.

The TiAl alloy powder filled 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, 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.

Metal structure observations were performed on the specimens of Examples A and B. Metal structure observations were performed using an optical microscope and an electron microscope. Figure 13 is a photograph showing the results of metal structure observations of the specimens of Examples A and B using an optical microscope. 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.

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 gamma grains, with the equiaxed gamma grains containing 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 lamellar grains and equiaxed gamma grains was taken as 100 volume percent, the volume fraction of equiaxed gamma grains was 80 volume percent or more, with the remainder being lamellar grains. The volume fraction of each grain was calculated by image processing using the contrast information of each grain in metallographic photographs taken with 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.

Next, the room temperature mechanical properties of the specimens of Examples A and B were evaluated. Room temperature tensile tests were conducted on the specimens of Examples A and B. Similarly, room temperature tensile tests were conducted 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.

The tensile test was conducted in accordance with ASTM E8. Figure 14 is a graph showing the tensile test results. Figure 14 shows the stress-strain curve for each specimen, with strain on the horizontal axis and stress on the vertical axis. The specimens of Examples A and B had greater 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 also 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 demonstrate that the specimens of Examples A and B have excellent mechanical strength and ductility, demonstrating a well-balanced improvement in both mechanical strength and ductility.

Creep tests were conducted on the specimens of Example A and Comparative Example A. The creep tests were conducted 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, the vertical axis represents the specific strength, and the specimens of Example A are represented by squares and the specimens of Comparative Example A by crosses. The Larson-Miller parameter P is a parameter expressed as P = T × log(t _r + C), where T is the absolute temperature (K), t _r is the time to rupture (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 indicates that the specimen of Example A had better high-temperature strength properties than the specimen of Comparative Example A.

This disclosure can improve the mechanical strength and ductility of TiAl alloys in a balanced manner, making it useful for aircraft engine parts, turbine blades for power generation gas turbines, etc.

Claims (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: