Precipitate phase stability and compositional dependence on alloying additions in γ–γ′–δ–η Ni-base superalloys

Abstract Ternary eutectic γ–γ′–δ Ni-base superalloys have been the subject of a number of recent investigations due to the relatively high volume fraction of δ phase providing composite strengthening. However, an additional intermetallic precipitate phase, η, has been observed in Ni-base superalloys containing additions of both Nb and Ti. As both precipitate phases δ and η appear similar and are often mistaken for one another, a better fundamental understanding of their formation is required for developing innovative new classes of Ni-base superalloys. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored in this investigation. Additionally, compositional relationships were developed to facilitate the design of γ–γ′–(δ/η) Ni-base superalloys. When the alloy chemistry was observed to exhibit a compositional ratio of Al/(Nb + Ta + Ti) less than 0.85, δ and/or η phase precipitates formed while a ratio greater than 0.85 resulted in conventional γ–γ′ microstructures. For alloys in which δ and/or η phase precipitates were formed, the prevalent phase could be determined by evaluating the compositional ratio for (Nb + Ta)/(Al + Ti). Alloys that had ratios greater than 1 were largely comprised of δ phase precipitates, whereas a ratio less than 1 resulted in the predominance of the η phase precipitates.