Elemental partitioning in Ru-containing nickel-base single crystal superalloys

Abstract

Single crystal nickel-base superalloys with spherical, intermediately shaped and cuboidal γ′ precipitates have been observed in Ru-containing superalloys with substantial amounts of Re and W. The γ′ precipitates experienced stress-induced directional coarsening (rafting) during creep deformation at 950 °C and 290 MPa indicative of a wide range of γ–γ′ lattice misfit, which vary from negative to near-zero to positive. The precipitate morphology, lattice misfit and compositions of the γ and the γ′ phases are interrelated, and therefore, the elemental partitioning between the phases were investigated to determine the origin of the unusual microstructural features. TEM-based EDS analysis of the γ and γ′ phases indicate that the combination of Ru and Cr influences the partitioning of Re, which displays a wide range of partitioning behavior within the investigated alloy compositions. In alloys containing 5.7–9.6 wt.% Ru and no Cr, Re partitioned to both the γ and γ′ phases. With high levels of Cr (6.7 wt.%) the Re strongly partitioned to the γ matrix, while in alloys with intermediate Cr and Ru levels, the Re displayed intermediate partitioning behavior. The Ru and Co partitioned nearly equally between the γ and the γ′ phase in all of the experimental Ru-containing alloys investigated. The degree of Cr partitioning was also similar among the alloys investigated, however, it partitioned more strongly to the γ phase. These investigations demonstrate that Ru and Cr control the lattice misfit and precipitate shape, through the associated changes in system thermodynamics that alter the γ and γ′ phase compositions.