On the effect of elemental partitioning to secondary phases after solution and aging heat treatments in a Co-Ni-based superalloy

•The phase stability and elemental partitioning of 3Nb alloy was shown in details.•The phases MC and μ-Co7W6 enriched in Nb and W were found in 3Nb alloy.•Both cuboidal and spheroidal-like γ′-precipitates were enriched in Ni, Al, and Nb.•The APT results showed that W, Ni, Al, and Nb contents decreased from γ′ to μ.•Most of the Nb partitions to γ′ (∼2.0 at. %) after one week at 1000 °C. This paper presents the phase stability and elemental partitioning in a Co-40Ni-10Al-7.5W-10Cr-3Nb-0.06B-0.6C (at.%) superalloy (referred to as 3Nb) after a two-step heat treatment. The resulting microstructure consisted of γ, γ′, MC carbides and µ-Co7W6 phase decorating the regions near and at the grain boundaries. Nb preferentially partitioned to the γ′-phase and other elements such as Ni, Cr and W were responsible for the μ-phase formation.