Increasing γ’ volume fraction in Co–Nb–V- and Co–Ta–V-based superalloys

Abstract

Recently discovered Co–Nb–V- and Co–Ta–V-based superalloys exhibit, with additions of Ni, Cr, Ti and Al, a stable γ+γ′ microstructure leading to excellent creep resistance. Here, to obtain a γ′-volume fraction higher than in the best current alloys, the concentration of all γ′-formers is increased by 25 or 50% (for Al, Ti and V) and by 10% (for Nb or Ta). The enriched Co–Nb–V-based alloys show γ+γ′ grains, with a γ′-volume fraction of 70–90% after aging at 850 °C and with a minority γ+C15 eutectic phase at grain boundaries. The 25%-enriched Co–Ta–V-based alloy (Co–10Ni-6.25Al–4Cr-3.75V-3.3Ta-2.5Ti-0.04B, at.%) exhibits a uniform γ+γ′ microstructure with a γ′-volume fraction of ~70% after aging at 850 °C, without any eutectic phase. For all enriched Co–Nb–V- and Co–Ta–V-based alloys, no other phases form in the γ+γ′ regions upon aging at 850 °C for up to 1000 h, indicating that the γ′ phase has high stability.