Effect of Cr additions on a γ-γ′ microstructure and creep behavior of a Co-based superalloy with low W content

Nickel- and cobalt-based superalloys derive their excellent oxidation and corrosion resistance from the surface oxidation of Al and Cr, which form protective oxide layers. Starting from an existing Cr-free cobalt-based superalloy (Co-30Ni-7Al-4Ti-3Mo-2W-1Nb-1Ta-0.1B at. %) with a gamma-gamma' microstructure, we create three alloys with 4, 8 and 12 at. % Cr additions. Chromium lowers the gamma' solvus temperature (from 1129 degrees C for 0% Cr to 1075 degrees C for 12 at. % Cr) and alters the gamma' precipitate morphology (from cuboidal-to-spherical), but does not affect the coarsening kinetics (which follow the Lifshitz-Slyozov-Wagner model). The alloys with 8 and 12 at. % Cr exhibit minor amounts of a Mo-rich intermetallic phase at grain boundaries after homogenization, and the 12 at. % Cr alloy displays an additional similar to 3 vol % intragranular refractory-rich secondary precipitates upon aging at 850 degrees C for 168 h. Atom-probe tomography reveals that Cr partitions strongly to the.-matrix, with a partitioning coefficient of kappa(gamma'/gamma) = 0.33 and 0.26 for Cr additions of 4 and 8%, respectively. These additions halve the creep rate of the original alloy at stresses between 275 and 400 MPa, reflecting significant changes in gamma' precipitate composition, volume fraction, morphology, and rafting tendency.