Low-density, W-free Co–Nb–V–Al-based superalloys with γ/γ’ microstructure

Abstract

Previously-reported metastable γ′-L12 Co3(Nb0.65V0.35) precipitates in the Co–6Nb–6V (at. %) ternary alloy are stabilized by additions of Al, Ti, Ni and Cr to the alloy. We identify two such multinary γ/γ′ alloys - with compositions of Co–10Ni–6Ti–5Al-xCr-3Nb–3V-0.04B at.% (with x = 0 and 4% Cr) - with γ′-precipitates remaining stable for up to 1000 h at 850 °C, with no additional phases present. Decreasing the Ti concentration from 6 to 2%, two more γ/γ′ superalloys - Co–10Ni–5Al-xCr-3Nb–3V–2Ti-0.04B (with x = 4 and 8% Cr) – are created with stable γ′-precipitates (measured for 168 h at 850 °C) with morphologies more cuboidal than for the first two alloys with 6%Ti. These submicron cuboidal γ′ precipitates are arranged into crystallographically-oriented sheets with small (<50 nm) γ′-spacing within sheets and larger (~100 nm) γ′-spacing between sheets. The alloy with the highest 8% Cr concentration shows, after aging at 850 °C for 24 h, γ′-nanoprecipitates with (Co0.85Ni0.15)3(Ti0.13Al0.25 Nb0.24Cr0.21V0.16B0.01) composition (assuming full segregation onto the two sublattices), with Al and Ti replacing at a similar rate both Nb and V in the ternary Co3(Nb0.65V0.35). This high-Cr alloy exhibits the best oxidation resistance, as seen by a reduction in the parabolic growth rate constant and surface oxide thickness. Also, the present W- and Ta-free, Cr-containing superalloys show good creep resistance at 850 °C, which is comparable to other recent Cr-containing Co-base γ/γ′ superalloys with higher densities: (i) W-containing Co–9W–9Al–8Cr (at.%), and (ii) Ta-containing Co–10Ni–5Al–4Cr–3Ta–3V–2Ti-0.04B (at.%). This is the first report of a family of Co–Nb–V–Al-based γ/γ′-superalloys with low density (<8.0 g/cm3).