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

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 gamma-(fcc) matrix with gamma'-(L1(2) structure) precipitates is studied using scanning electron microscopy and atom-probe tomography. Ruthenium partitions to the gamma'-precipitates in the Co-9.4A1-7.5W-2.1Ru at.% alloy with a partitioning coefficient, K-Ru(gamma'/gamma) = 1.27, after aging at 900 degrees 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 gamma-phase. The addition of ruthenium does not significantly affect the gamma' volume-fraction or the coarsening kinetics of the gamma' precipitates compared to the base ternary alloy. The addition of Ru also leads, however, to a rapid discontinuous transformation of (gamma plus gamma'), which initiates at the grain boundaries after 128 h aging at 900 degrees C; (gamma plus gamma') is transformed into a lamellar phase mixture containing Co3W (D0(19)), fcc solid-solution (gamma), and Co(Al,W) (B2). After 256 h aging at 900 degrees C in the Ru-containing alloy, some grains have completely transformed, although regions of gamma plus gamma' persist. The base ternary Co Al W alloy does not exhibit a discontinuous transformation and contains a (gamma plus gamma') microstructure up to 1024 h of aging at 900 degrees C. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.