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 at room temperature in a temperature region near 800.degree. 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 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.sqroot.m. While this value of room-temperature fracture toughness is higher than that of Si.sub.3 N.sub.4 and other structural ceramics of 5 MPa.sqroot.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.degree. 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 the 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.degree. C., this value is not very high. Its about 390 MPa at the highest. Comparison of the TiAl alloy with the nickel superalloy such as the Inconel 713 alloy in terms of the specific strength as represented by the value obtained by dividing, by specific gravity, such a strength characteristic as tensile strength, compressive strength or creep rupture strength within the temperature range of from 700.degree. to 1,100.degree. C., shows almost no difference between these alloys and it is improbable that the conventional TiAl alloy will substitute for the nickel superalloy, when taking account of the fact that the nickel superalloy is superior in ductility and toughness at 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 the 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.degree. to 1,000.degree. 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. Pat. No. 4,294,615 dated Oct. 13, 1981: A Ti-31 to 36 wt. % Al-0.1 to 4 wt. % 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, the specific strength of the TiAl alloy of the prior art as described above is insufficient, being almost equal to that of the nickel superalloy.
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, one which exhibits a room-temperature fracture toughness of at least 13 MPa.sqroot.m, a 100-hour creep rupture strength at a temperature of 820.degree. 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.degree. C. in the open air for 500 hours, but a TiAl alloy having such characteristics has not as yet been proposed.