Tailoring the γ-γ′-γʺ Dual Superlattice Microstructure of INCONEL^® 725 by High Temperature Aging and Nb/Ta Additions for Superior Creep Properties

Next-generation energy systems require superior resistance to creep deformation due to the considerably prolonged exposure times at operating stress and temperature. To improve the elevated temperature properties of INCONEL^® 725 (IN725), a corrosion-resistant alloy, several variants with different Ti/Al ratios and judicious amounts of Nb and Ta were made. Furthermore, a high temperature aging (HTA) heat treatment, designed to promote favorable precipitate phase formation, was explored. These adjustments allowed to tailor the amount and type of precipitate strengthening which led to significant increases in time to failure. The Ti/Al ratio was used to favor the formation of γ′ or γ″ precipitates. Compact morphology precipitates, consisting of γ′ precipitates surrounded by a γ″ shell, were formed in alloys with a low Ti/Al ratio. The HTA increased the creep life of various alloy formulations up to a maximum improvement of 371% as compared to the standard aging heat treatment. The Nb and Ta additions had a similar effect on increasing creep life by promoting and stabilizing γ″ precipitation. The positive effect of the additions was even more pronounced when coupled with the HTA. A phase stability study with up to 10,000 h exposure at 700 °C revealed that the compact morphology helped in slightly reducing coarsening of the γ′ precipitates, although the effect on creep was not significant. The findings of this study enable design of dual superlattice alloys through microstructural engineering that yields superior performance and can be applied to a wide range of alloys in the IN718 and derivatives family.