Effect of Al, Ti and Cr additions on the γ-γ’ microstructure of W-free Co-Ta-V-Based superalloys

The recently-discovered metastable γ′-Co₃(Ta_0.76V_0.24) phase formed on aging in a Co-6Ta-6V (at.%) ternary alloy is stabilized by partial replacement of Ta and V with Al and Ti. In two alloys with composition Co-6Al-3Ta-3V and Co-5Al-3Ta-3V-1Ti with γ+γ′ microstructure, the γ′-precipitates remain stable for up to 168 h at 850 and 900 °C, with no precipitation of additional phases. Adding Ni and B and doubling the Ti concentration produces a γ/γ′ superalloy, Co-10Ni-5Al-3Ta-3V-2Ti-0.04B (at.%), with γ′ precipitates which are stable up to six weeks of aging at 850 °C, while slowly coarsening and coalescing from cuboidal to elongated shapes. After one day of aging at 850 °C, the γ′ nanoprecipitates have a (Co_0.83Ni_0.17)₃(Ta_0.42Al_0.23Ti_0.19V_0.15B_0.01) composition, with Al and Ti replacing at the same rate both Ta and V in the original metastable Co₃(Ta_0.76V_0.24) phase. To improve oxidation resistance, 4% Cr is added to the new superalloy, resulting in a somewhat higher volume fraction of finer cuboidal γ′ precipitates after one week of aging at 850 °C, but no other deleterious phases. These W- and Mo-free γ/γ′ superalloys show good creep resistance at 850 °C, on par with two other recent Co-base γ/γ′ superalloys: (i) Co-9W-9Al-8Cr (at.%) which has higher density due to its high W content, and (ii) Co-30Ni-10Al-5Mo-2Nb (at.%) which has lower density (as it is W-free) but contains triple the Ni concentration.