Influence of ruthenium on microstructural evolution in a model Co Al W superalloy

Abstract The effect of 2 at.% Ru addition on the elemental partitioning and microstructural evolution of a base Co-8.8Al-7.3W at.% superalloy, consisting of a γ-(fcc) matrix with γ’-(L12 structure) precipitates is studied using scanning electron microscopy and atom-probe tomography. Ruthenium partitions to the γ’-precipitates in the Co-9.4Al-7.5W-2.1Ru at.% alloy with a partitioning coefficient, K Ru γ ′ /γ = 1.27, after aging at 900 °C for 16 h, in contrast to the behavior observed in Ni-base superalloys and theoretically predicted for Co-base superalloys, for which Ru partitions preferentially to the γ-phase. The addition of ruthenium does not significantly affect the γ′ volume-fraction or the coarsening kinetics of the γ′ precipitates compared to the base ternary alloy. The addition of Ru also leads, however, to a rapid discontinuous transformation of (γ plus γ′), which initiates at the grain boundaries after 128 h aging at 900 °C; (γ plus γ′) is transformed into a lamellar phase mixture containing Co3W (D019), fcc solid-solution (γ), and Co(Al,W) (B2). After 256 h aging at 900 °C in the Ru-containing alloy, some grains have completely transformed, although regions of γ plus γ′ persist. The base ternary Co Al W alloy does not exhibit a discontinuous transformation and contains a (γ plus γ′) microstructure up to 1024 h of aging at 900 °C.