The effects of solutes like ruthenium on precipitation of σ phase and σ/γ interface stability in Ni-based superalloys

Abstract

σ and μ phases are two of the most common topological close-packed (TCP) phases in nickel-based single crystal superalloys, which are detrimental to the comprehensive properties of superalloys. It is universally acknowledged that Ru plays an important role in the precipitation of TCP, and its inhibiting effects on the nucleation and precipitation of μ phases have already been confirmed, but its effects on σ phases remain vague. In this work, the types and area fractions of the TCP phases precipitated from 4 alloys with different compositions have been characterized. It is found that Ru has different effects on the precipitation of different types of TCP. With the help of density functional theory (DFT), the interface structures between σ and matrix γ are further studied, and on this basis, the effects of different solutes such as Ru on interface stability and electronic structure are discussed in detail. Ru, Re, W, Cr and Ta are able to enhance the interface stability, boosting the nucleation of σ phase. Co is different, for it can reduce the interface stability and elevate the nucleation difficulty of σ. Additionally, the distribution behaviors of solutes between σ and γ phases have been calculated, from which it can be found that the energies of solutes like Cr and Ru distributing in σ are more stable than distributing inside γ, and these solutes have the tendency to diffuse from γ to σ. These results reveal the effects of solutes like Ru on the precipitation of σ phase in terms of interfacial stability and atomic diffusion, which provide a new understanding of Ru effects.