Effects of Ni and Cr additions on γ + γ’ microstructure and mechanical properties of W-free Co–Al–V–Nb–Ta-based superalloys

The effect of Ni and Cr additions on microstructure, lattice misfit, oxidation, and creep properties are investigated for six W- and Mo-free cobalt-based superalloys with compositions Co-xNi-5Al-yCr-3V–2Ti-1.5Nb-1.5Ta-0.08B (at.%), where x = 10, 20, or 30 and y = 4 or 8. In all alloys, the γ + γ′ microstructure is stable for up to 1000 h upon aging at 850 °C. As Ni increases from 10 to 30 at.%, the γ′ area fraction increases from 32 to 49% in the three low-Cr alloys, while remaining constant (45%) for the high-Cr alloys. All alloys show positive γ/γ′ lattice misfits of 0.8 ± 0.2%, consistent with γ’ rafting observed after creep. Both Cr and Ni additions increase oxidation resistance, more so for 8% Cr than for 30% Ni, as expected from the high stability of chromium oxide. Increasing Cr from 4 to 8 at.% slightly increases creep resistance at 850 °C and increasing Ni from 10 to 30% has a similar effect.