Effects of Cr on the properties of multicomponent cobalt-based superalloys with ultra high γ’ volume fraction

Abstract

Abstract Chromium is known to confer oxidation resistance on Co-based superalloys. Herein we studied the influence of Cr concentration on the properties of a Co-based superalloy, Co–30Ni–11Al–2Ti-5.5W-2.5Ta-0.1B (at.%), with high γ ’ -volume fractions (up to 98%). Additions of 4, 8, or 12 at.% Cr decreased the γ ’ -solvus temperature and the γ ’ -volume fraction, while also reducing the mean radius of γ ’ -particles. Synchrotron X-ray diffraction and atom-probe tomography confirmed that Cr reduces the lattice parameter misfit by altering the partitioning behavior of W between the γ- and γ ’ -phases. Concomitantly, an increase in Cr content significantly improved the oxidation resistance of the alloy. The results indicate that 4 at.% Cr is sufficient to decrease the oxidation rate by two orders of magnitude. A creep study at 850 ∘C revealed that Cr does not affect adversely the creep performance in high γ ’ -volume fraction Co-based superalloys, but the tendency toward γ ’ rafting changes with Cr concentration. Compared to Ni3Al intermetallic compounds, the alloys with high γ ’ -volume fraction studied exhibited a significantly higher creep resistance. This may be attributed to the increase in antiphase boundary energy caused by the addition of Cr, Ti, or Ta, and significant solid-solution hardening in the Co-based intermetallic γ ’ -phase.