Heat-resistant titanium-aluminium alloy with a high fracture toughness at room temperature and with good oxidation resistance and strength at high temperatures

Abstract: A heat-resistant TiAl alloy excellent in a room-temperature fracture toughness, a high-­ temperature oxidation resistance and a high-­ temperature strength, consisting essentially of: aluminum : from 29 to 35 wt.%, niobium : from 0.5 to 20 wt.%, at least one element selected from the group consisting of: silicon : from 0.1 to 1.8 wt.%, and zirconium : from 0.3 to 5.5 wt.%, and the balance being titanium and incidental impurities.

Description: The present invention relates to a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength. A TiAl alloy, which is an intermetallic compound, has the following features: (1) It is light in weight. More specifically, the TiAl alloy has a specific gravity of about 3.7, equal to, or smaller than, a half that of the nickel superalloy. (2) It has an excellent high-temperature strength. More specifically, the TiAl alloy has a yield strength and a Young's modulus of the same order as that in the room temperature in a temperature region near 800°C. Research is now carried out for the purpose of practically applying the TiAl alloy light in weight and having an excellent high-temperature strength in place, for example, of the nickel superalloy or the ceramics, which are used as materials for a turbine blade. However, the conventional TiAl alloy has not as yet been practically applied as a material for high-temperature uses for the following reasons: (1) Room-temperature fracture toughness is not satisfactory. More specifically, at the "International Gas Turbine Congress" held in Tokyo in 1987, Mr. Y. Nishiyama et al. reported their finding that the TiAl alloy had a room-temperature fracture toughness (KIC) of 13 MPa√ m . While this value of room-temperature fracture toughness is higher than that of Si₃N₄ and other structural ceramics of 5 MPa√ m , there is a demand for a further higher value of the room-temperature fracture toughness. (2) High-temperature oxidation resistance is not satisfactory. More specifically, high-temperature oxidation resistance of the TiAl alloy, while being superior to that of the ordinary titanium alloy, is not always higher than that of the nickel superalloy. It is known that, particularly in the temperature region of at least 900°C, the high-temperature oxidation resistance of the TiAl alloy seriously decreases, and that the high-temperature oxidation resistance of the TiAl alloy is considerably improved by adding niobium. However, the addition of niobium does not improve high-temperature strength of the TiAl alloy. (3) High-temperature strength is not very high. More specifically, while the TiAl alloy shows, as described above, a yield strength of the same order as that in the room temperature in the temperature region near 800°C, this value is not very high as about 390 MPa at the highest. Comparison of the TiAl alloy with the nickel superalloy such as the Inconel 713 alloy in terms of specific strength as represented by the value obtained by dividing, by specific gravity, such a strength characteristic as a tensile strength, a compressive strength or a creep rupture strength within the temperature range of from 700 to 1,100°C, shows almost no difference between these alloys and it is little probable that the conventional TiAl alloy substitutes for the nickel superalloy, when taking account of the fact that the nickel superalloy is superior in ductility and toughness at the room temperature. It would however be possible to use the TiAl alloy in place of the nickel superalloy as a material for a member requiring reasonably high ductility and toughness by improving high-temperature strength of the TiAl alloy to increase the specific strength thereof. Considering the fact that the TiAl alloy is superior to the ceramics in ductility and toughness, it would be possible to use the TiAl alloy in place of the structural ceramics used within the temperature range of from 700 to 1,000°C. With regard to the effect of the alloy elements on the high-temperature strength of the TiAl alloy, the following finding is disclosed in the U.S Patent No. 4,294,615 dated October 13, 1981: A Ti-31 to 36wt.% Al-0.1 to 4wt.%V TiAl alloy is excellent in high-temperature strength and room-temperature ductility, and the addition of 0.1 wt.% carbon to the above-mentioned TiAl alloy improves a creep rupture strength thereof (hereinafter referred to as the "prior art"). However, specific strength of the TiAl alloy of the prior art as described above is insufficient, being almost equal to that of the nickel superalloy. US-A-2 880 087 discloses titanium-aluminum alloys containing from more than 8 to up to 34% aluminum the hot workability of which as well as the strength and ductility at both room and elevated temperatures are greatly increased by the addition of one or more selected β-stablizing elements. Under such circumstances, there is a strong demand for the development of a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, which exhibits a room-temperature fracture toughness of at least 13 MPa√ m , a 100-hour creep rupture strength at a temperature of 820°C higher than that of the conventional TiAl alloy, and a decrease in thickness of up to 0.1 mm per side after heating to a temperature of 900°C in the open air for 500 hours, but a TiAl alloy having such characteristics has not as yet been proposed. An object of the present invention is therefore to provide a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, which exhibits a room-temperature fracture toughness of at least 13 MPa√ m , a 100-hour creep rupture strength at a temperature of 820°C higher than that of the conventional TiAl alloy, and a decrease in thickness of up to 0.1 mm per side after heating to a temperature of 900°C in the open air for 500 hours. In accordance with one of the features of the present invention, there is provided a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, characterized by containing: aluminum : from 29 to 35 wt.%, niobium : from 0.5 to 20 wt.%, at least one element selected from the group consisting of: silicon : from 0.1 to 1.8 wt.%, and zirconium : from 0.3 to 5.5 wt.%, and the balance being titanium and incidental impurities. Fig. 1 is a graph illustrating the relationship between aluminum content and room-temperature fracture toughness in a TiAl alloy; Fig. 2 is a graph illustrating the relationship between niobium content and room-temperature fracture toughness in a TiAl alloy; Fig. 3 is a graph illustrating the relationship between silicon content and room-temperature fracture toughness in a TiAl alloy; Fig. 4 is a graph illustrating the relationship between zirconium content and room-temperature fracture toughness in a TiAl alloy; Fig. 5 is a graph illustrating the relationship between applied stress and creep rupture time in a TiAl alloy; Fig. 6 is a graph illustrating the relationship between room-temperature fracture toughness and 100-hour creep rupture strength in a TiAl alloy; and Fig. 7 is a graph illustrating the relationship between a decrease in thickness and 100-hour creep rupture strength in a TiAl alloy. From the above-mentioned point of view, extensive studies were carried out with a view to developing a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength. As a result, the following finding was obtained: it is possible to obtain a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, by adding niobium in a prescribed amount and at least one of silicon in a prescribed amount and zirconium in a prescribed amount. The present invention was developed on the basis of the above-mentioned finding, and the heat-resistant TiAl alloy of the present invention excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength containing: aluminum : from 29 to 35 wt.%, niobium : from 0.5 to 20 wt.%, at least one element selected from the group consisting of: silicon : from 0.1 to 1.8 wt.%, and zirconium : from 0.3 to 5.5 wt.%, and the balance being titanium and incidental impurities. The chemical composition of the heat-resistant TiAl alloy of the present invention excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength is limited within the range as described above for the following reasons: (1) Aluminum: Aluminum has the function of improving room-temperature fracture toughness and high-temperature strength of the TiAl alloy. With an aluminum content of under 29 wt.%, however, a desired effect as described above cannot be obtained. Even with an aluminum content of over 35 wt.%, on the other hand, a particular improvement in the above-mentioned effect described above is not available. In order to use a TiAl alloy poor in room-temperature fracture toughness and high-temperature strength as a structural material, it is necessary to consume much labor for ensuring a high reliability, and in addition, advantages over structural ceramics such as Si₃N₄ are too slight to achieve the object of the present invention. The aluminum content should therefore be limited within the range of from 29 to 35 wt.%. (2) Niobium: Niobium, which is not very high in the function of improving strength of the TiAl alloy, has the function of largely improving high-temperature oxidation resistance of the TiAl alloy. With a niobium content of under 0.5 wt.%, however, a desired effect as described above cannot be obtained. With a niobium content of over 20 wt.%, on the other hand, specific gravity of the TiAl alloy becomes larger, thus preventing achievement of a smaller weight, and creep rupture strength of the TiAl alloy decreases. The niobium content should therefore be limited within the range of from 0.5 to 20 wt.%. (3) Silicon: Silicon has the function of improving high-temperature strength of the TiAl alloy. With a silicon content of under 0.1 wt.%, however, a desired effect as described above cannot be obtained. A silicon content of over 1.8 wt.%, on the other hand, largely reduces room-temperature fracture toughness of the TiAl alloy. The silicon content should therefore be limited within the range of from 0.1 to 1.8 wt.%. (4) Zirconium: Zirconium has, like silicon, the function of improving high-temperature strength of the TiAl alloy. With a zirconium content of under 0.3 wt.%, however, a desired effect as described above, cannot be obtained. With a zirconium content of over 5.5 wt.%, on the other hand, room-temperature fracture toughness of the TiAl alloy decreases considerably, and specific gravity of the TiAl alloy increases thus preventing achievement of a smaller weight. The zirconium content should therefore be limited within the range of from 0.3 to 5.5 wt.%. In the present invention, the respective contents of oxygen, nitrogen and hydrogen as incidental impurities in the TiAl alloy should preferably be limited as follows with a view to preventing room-temperature fracture toughness of the TiAl alloy from decreasing: up to 0.6 wt.% for oxygen, up to 0.1 wt.% for nitrogen, and up to 0.05 wt.% for hydrogen. Now, the heat-resistant TiAl alloy of the present invention excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, is described further in detail by means of an example. EXAMPLE TiAl alloys each having a chemical composition within the scope of the present invention as shown in Table 1 and TiAl alloys each having a chemical composition outside the scope of the present invention as shown also in Table 1, were melted in a melting furnace, and then cast into ingots. Then, fracture toughness test pieces of the TiAl alloys within the scope of the present invention based on "ASTM E399" (hereinafter referred to as the "test pieces of the invention") Nos. 13 to 32, and fracture toughness test pieces of the TiAl alloys outside the scope of the present invention also based on "ASTM E399" (hereinafter referred to as the "test pieces for comparison") Nos. 1 to 12, were cut from the respective ingots thus cast. Room-temperature fracture toughness was then measured in accordance with "ASTM E 399" for each of these test pieces. From among the results of measurement, those for the test pieces of the invention Nos. 13 to 31 and those for the test pieces for comparison Nos. 4, 5 and 7 to 12 are shown in Table 2. For the purpose of demonstrating the effect of the respective contents of aluminum, niobium, silicon and zirconium on room-temperature fracture toughness of the TiAl alloy, the relationship between aluminum content and room-temperature fracture toughness is shown in Fig. 1 for the test pieces of the invention Nos. 13 to 17 and 20 and the test pieces for comparison Nos. 7 to 9, which are the Ti-Al-4wt.% Nb-1wt.% Si TiAl alloys; the relationship between niobium content and room-temperature fracture toughness is shown in Fig. 2 for the test pieces of the invention Nos. 15 and 27 to 31 and the test pieces for comparison Nos. 5 and 12, which are the Ti-33wt.% Al-Nb-1wt.% Si TiAl alloys; the relationship between silicon content and room-temperature fracture toughness is shown in Fig. 3 for the test pieces of the invention Nos. 18 to 20 and the test pieces for comparison Nos. 4 and 10, which are the Ti-33 wt.% Al-4wt.% Nb-Si TiAl alloys; and the relationship between zirconium content and room-temperature fracture toughness is shown in Fig. 4 for the test pieces of the invention Nos. 21 to 26 and the test pieces for comparison Nos. 4 to 11,which are the Ti-33 wt.% Al-2wt.% Nb-Zr TiAl alloys. As is clear from Fig. 1, the room-temperature fracture toughness of the TiAl alloy largely depends upon the aluminum content. More specifically, within the range of aluminum content of from 29 to 35 wt.%, the room-temperature fracture toughness (KIC) of the TiAl alloy becomes at least 13 MPa√ m which is the target value of the present invention. Then, as is clear from Fig. 2, the room-temperature fracture toughness of the TiAl alloy is hardly affected by the niobium content. Then, as is clear from Fig. 3, the room-temperature fracture toughness of the TiAl alloy becomes lower along with the increase in the silicon content. In order to obtain a room-temperature fracture toughness of at least 13 MPa√ m , therefore, it is necessary to limit the silicon content to up to 1.8 wt.%. Then, as is clear from Fig. 4, the room-temperature fracture toughness of the TiAl alloy becomes lower along with the increase in the zirconium content. In order to obtain a room-temperature fracture toughness of at least 13 MPa√ m , therefore, it is necessary to limit the zirconium content to up to 5.5 wt.%. Then, TiAl alloys each having a chemical composition within the scope of the present invention as shown in Table 1 and TiAl alloys each having a chemical composition outside the scope of the present invention as shown also in Table 1, were melted in a melting furnace, and then cast into ingots. Then, test pieces of the TiAl alloys within the scope of the present invention (hereinafter referred to as the "test pieces of the invention") Nos. 13 to 32, each having a parallel portion with a diameter of 6 mm and a length of 30 mm, and test pieces of the TiAl alloys outside the scope of the present invention (hereinafter referred to as the "test pieces for comparison") Nos. 1 to 12, also each having a parallel portion with a diameter of 6 mm and a length of 30 mm, were cut from the respective ingots thus cast. A creep rupture strength at 820°C was then measured for each of these test pieces. The relationship between stress applied to the test piece and creep rupture time is shown in Fig. 5. As is clear from Fig. 5, the test pieces are classified into several groups. More specifically, the test pieces for comparison Nos. 1 to 4 and 9 come under the lowest group in Fig. 5, having an applied stress at which the test piece ruptures after the lapse of 100 hours, i.e., a 100-hour creep rupture strength, of about 150 MPa. In contrast, the test pieces of the invention Nos. 14 to 16, 20 and 32 have a 100-hour creep rupture strength of about 350 MPa, a very high value. Table 3 shows niobium content, 100-hour creep rupture strength at a temperature of 820°C, specific gravity and specific strength which is a value obtained by dividing the 100-hour creep rupture strength by the specific gravity, for each of the test pieces of the invention Nos. 15 and 27 to 31 and the test pieces for comparison Nos. 2, 5 and 12, which are the Ti-33wt.%Al-Nb-1wt.%Si TiAl alloy. As is clear from Table 3, the addition of niobium causes almost no change in a 100-hour creep rupture strength, which rather shows a tendency toward decreasing, while specific gravity is increasing. Also as is evident from Table 3, in order to achieve a specific strength of over that for the test piece for comparison No. 2, which is the alloy of the prior art, of 39.5 x 10⁷ cm²/s² , it is necessary to limit the niobium content of the TiAl alloy to up to 20 wt.%. Table 4 shows aluminum content and 100-hour creep rupture strength at a temperature of 820°C for each of the test pieces of the invention Nos. 13 to 17 and 20 and the test pieces for comparison Nos. 7 to 9, which are the Ti-Al-4wt.%Nb-1wt.%Si TiAl alloy; Table 5 shows silicon content and 100-hour creep rupture strength at a temperature of 820°C for each of the test pieces of the invention Nos. 15 and 18 to 20 and the test pieces for comparison Nos. 4 and 10, which are the Ti-33wt.%Al-4wt.%Nb-Si TiAl alloy; and Table 6 shows zirconium content and 100-hour creep rupture strength at a temperature of 820°C for each of the test pieces of the invention Nos. 21 to 26 and the test pieces for comparison Nos. 4 and 11, which are the Ti-33wt.%Al-2wt.%Nb-Zr TiAl alloy. As is clear from Tables 4, 5 and 6, it is possible to improve high-temperature strength of the TiAl alloy by limiting the aluminium content within the range of from 29 to 35 wt.%, and limiting the lower limit of the silicon content to 0.1 wt.%, and limiting the lower limit of the zirconium content to 0.3 wt.%. Then TiAl alloys each having a chemical composition within the scope of the present invention as shown in Table 1, and TiAl alloys each having a chemical composition outside the scope of the present invention as shown also in Table 1, were melted in a melting furnace, and then cast into ingots. Then, test pieces of the TiAl alloys within the scope of the present invention (hereinafter referred to as the "test pieces of the invention") Nos. 13 to 32, each having a longitudinal width of 8 mm, a transverse width of 10 mm and a thickness of 2 mm, and test pieces of the TiAl alloys outside the scope of the present invention (hereinafter referred to as the "test pieces for comparison") Nos. 1 to 12, also each having a longitudinal width of 8 mm, a transverse width of 10 mm and a thickness of 2 mm, were cut from the respective ingots thus cast. To investigate high-temperature oxidation resistance, these test pieces were heated to a temperature of 900°C in the open air for 100 hours, 200 hours and 500 hours, and decrease in thickness per side of the test piece caused by oxidation after the lapse of these hours was measured. From among the results of measurement, those for the test pieces of the invention Nos. 15, 24 and 32 and the test pieces for comparison Nos. 1, 2 and 4 to 6 are shown in Table 7. As is clear from Table 7, the addition of niobium brings about a remarkable improvement of a high-temperature oxidation resistance of the TiAl alloy,whereas the addition of silicon and zirconium does not exert a remarkable effect on high-temperature oxidation resistance of the TiAl alloy. Table 8 shows niobium content and high-temperature oxidation resistance for each of the test pieces of the invention Nos. 15 and 27 to 31 and the test pieces for comparison Nos. 5 and 12. As is clear from Table 8, the addition of niobium in an amount of at least 0.5 wt.% results in improvement of high-temperature oxidation resistance of the TiAl alloy. The results of these measurements are illustrated in Figs. 6 and 7. Fig. 6 is a graph illustrating the relationship between room-temperature fracture toughness and high-temperature strength, i.e., a 100-hour creep rupture strength at a temperature of 820°C for each of the test pieces of the invention Nos. 13 to 32 and the test pieces for comparison Nos. 1 to 12. In Fig. 6, the region enclosed by hatching represents that of the present invention giving excellent room-temperature fracture toughness and high-temperature strength. Fig. 7 is a graph illustrating the relationship between high-temperature oxidation resistance, i.e., decrease in thickness per side of the test piece after heating to a temperature of 900°C in the open air for 500 hours, on the one hand, and high-temperature strength, i.e., a 100-hour creep rupture strength at a temperature of 820°C, on the other hand, for each of the test pieces of the invention Nos. 13 to 32 and the test pieces for comparison Nos. 1 to 12. In Fig. 7, the region enclosed by hatching represents that of the present invention giving excellent high-temperature oxidation resistance and high-temperature strength. As is clear from Figs. 6 and 7, the test pieces of the invention Nos. 13 to 32 are excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength in all cases. In contrast, high-temperature strength is low in the test pieces for comparison Nos. 1 to 4, 8, 9 and 12. While the test pieces for comparison Nos. 5 to 7, 10 and 11 show satisfactory high-temperature strength, the test pieces for comparison Nos. 7, 10 and 11 are poor in room-temperature fracture toughness, and the test pieces for comparison Nos. 5 and 6 are poor in high-temperature oxidation resistance. According to the present invention, as described above in detail, it is possible to obtain a heat-resistant TiAl alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, thus providing industrially useful effects.

Claims: A TiAl heat-resistant alloy excellent in room-temperature fracture toughness, high-temperature oxidation resistance and high-temperature strength, characterized by containing: aluminum : from 29 to 35 wt.%, niobium : from 0.5 to 20 wt.%, at least one element selected from the group consisting of: silicon : from 0.1 to 1.8 wt.%, and zirconium : from 0.3 to 5.5 wt.%, and the balance being titanium and incidental impurities. The TiAl heat-resistant alloy as claimed in Claim 1 wherein; the respective contents of oxygen, nitrogen and hydrogen as said incidental impurities are limited to: up to 0.6 wt.% for oxygen, up to 0.1 wt.% for nitrogen, and up to 0.05 wt.% for hydrogen.

Inventors: MINAKAWA KUNINORI, MITAO SHINJI, TSUYAMA SEISHI, MINAKAWA, KUNINORI, MITAO, SHINJI, TSUYAMA, SEISHI
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1989
Main classification: C22C14
IPC codes: C22C14