For more than thirty years there has been a continuing need for improved alloys to enable engine components such as turbine discs to be operated under more rigorous conditions. The nickel base superalloy known as Waspaloy was introduced in 1967, and is still used today despite its limitations of strength and maximum temperature of use. UDIMET 720, an alloy with improved strength, was introduced in 1986 (UDIMET is a Registered Trade Mark of Special Metals Corporation). However, UDIMET 720 was found to be unstable (with respect to the formation of deleterious Topologically Close Packed (TCP) phases) and was superseded in 1990 by powder processed UDIMET 720Li (low interstitial), an alloy with reduced chromium, carbon and boron. Improvements in cast and wrought (C+W) processing led to the introduction of C+W UDIMET 720Li in 1994. Cast and wrought UDIMET 720Li exhibits near equivalent properties to those of the powder variant. Although UDIMET 720Li has adequate strength, its resistance to fatigue crack propagation is somewhat lower than Waspaloy, and its maximum operating temperature is limited to approximately 650.degree. C.
There is a continuing need to define an alloy composition, microstructure, heat treatment and process route to meet the increasing demands of future civil and military turbine discs. It is an object of the present invention to meet that need. Nickel base superalloys are so complex, with generally about ten alloying components present, that optimisation of alloy composition is extremely difficult. Phase diagram modelling has been used extensively during the development of the invention to predict the component phases and their proportions.