Compositional Design and Mechanical Properties of INCONEL^® Alloy 725 Variants

With a combination of high strength, toughness and resistance to corrosion, INCONEL^® alloy 725 has been widely used in marine, aerospace, and land-based power industries. Typically, the alloy presents a conventional precipitate-strengthened γ-γ′/γ″ microstructure when appropriate aging treatments are employed. Although the corrosion resistance of INCONEL^® alloy 725 is significant, its use is limited to relatively low temperatures when compared to other γ′ precipitate-strengthened Ni-based superalloys. This can limit the use of the alloy as turbine engine components or in other power-generation applications since future engine designs suggest increases in the operating temperature. This investigation aims at modifying the composition of the alloy to assess its high-temperature mechanical properties. Variations to the Ti/Al ratio were considered with respect to the precipitate phases stability as well as additions of Ta and Nb. Thermodynamic and kinetic predictions, such as phase fraction/stability and time–temperature–transformation diagrams, were used to help in the design process and were validated experimentally. The various compositions and relative aging treatments investigated produced microstructures differing in grain boundary phases and γ′ precipitate sizes and fractions. Tensile and creep testing were performed and the effect of the various compositions and microstructures on the mechanical performance of the modified INCONEL^® 725 alloys was examined.