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 γ-γ′ microstructure, we create three alloys with 4, 8 and 12 at. % Cr additions. Chromium lowers the γ′ solvus temperature (from 1129 °C for 0% Cr to 1075 °C for 12 at. % Cr) and alters the γ′ 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 ~3 vol % intragranular refractory-rich secondary precipitates upon aging at 850 °C for 168 h. Atom-probe tomography reveals that Cr partitions strongly to the γ-matrix, with a partitioning coefficient of κ ^γ'/γ = 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 γ’ precipitate composition, volume fraction, morphology, and rafting tendency.