(1) Field of the Invention
The present invention relates to a Ni-based alloy having better high-temperature strength and more excellent ductility as an ordinary casting material, and a turbine rotor and stator blade for a gas turbine using the Ni-based alloy.
(2) Description of Related Art
In recent years, due to increase in environmental awareness such as saving of fossil fuels, reduction in the emission amount of carbon dioxide, and prevention of global warming, thermal efficiency of an internal combustion engine has been improved. It is known that thermal engines such as gas turbines and jet engines can enhance thermal efficiency most effectively by being operated with the high temperature side of the Carnot's cycle set at a higher temperature. With elevation of the temperature of the turbine inlet, importance of improvement and development of the material for use in high-temperature components of gas turbines, that is, combustors and turbine rotor or stator blades has been increased.
As for the material, in order to take measures to the elevation of the temperature, a Ni-based heat-resistant alloy which is more excellent in high-temperature strength is applied to rotor blades, and many Ni-based alloys are used for the rotor blades at present. However, a Co-based alloy which is excellent in corrosion resistance and weldability is used for the heat-resistant material used in gas turbine stators.
Due to elevation of combustion temperature (turbine inlet temperature) along with improvement in efficiency of recent years, use of a Ni-based alloy which is more excellent in high-temperature strength than a Co-based alloy has been considered. In order to enhance the strength of a Ni-based alloy, large amounts of W, Mo, Ta, Co and the like which are solid solution strengthening elements are added. In addition, by adding Al, Ti and the like to utilize precipitation strength of a γ′Ni3(Al, Ti) phase which is a reinforcement phase, the Ni-based alloy has excellent high-temperature strength and thermal fatigue characteristics.
However, since a Ni-based alloy is inferior to a Co-based alloy in workability such as weldability and ductility, it is difficult at present to apply a Ni-based alloy to gas turbine stator blades.
For example, an alloy which has a large precipitation amount of a γ′ phase and is excellent in strength characteristics is excellent in the high-temperature strength and creep characteristics, but is significantly reduced in ductility. In contrast with this, an alloy (U.S. Pat. No. 3,720,509) which is improved in workability such as ductility by reducing the precipitation amount of a γ′ phase is required to improve in strength corresponding to elevation of temperature in order to be applied to a rotor blade.