Development of new γ′-strengthened Co-based superalloys with low mass density, high solvus temperature and high temperature strength

Abstract

In this work we report new γ′ strengthened cobalt base superalloys that have low mass densities in the range of ~ 8.3–8.5 gm/cm3, high solvus up to ~ 1128 °C and high specific 0.2% proof strength of ~ 90 MPa.gm-1.cm3 at temperatures up to 770 °C. The basic alloy composition is Co–10Al–5Cr–2Ta that exhibits γ/γ′ microstructure on heat treatment. Electron diffraction and elemental partitioning across γ/γ′ indicate the γ′ has an ordered L12 structure with a stoichiometry Co3(Al,Ta,Cr) coherently embedded in disordered face-centered-cubic (fcc) γ-Co matrix. The γ′ structure is primarily stabilized by Ta addition. The γ′ volume fraction and the solvus temperature of the base alloy was measured to be 14% and 870 °C respectively. These values are significantly enhanced by the addition of 30at.% Ni (35% and 975 °C) and together with 4at.% Ti attaining the values of 63% and 1128 °C, respectively. Atom Probe measurements reveal both Ni and Ti strongly partition to γ′ precipitates enhancing the stability of the γ' while Cr partitions to the γ phase. At 900 °C after 200 h of aging, the 30at.% Ni added alloy without Ti reveals formation of coarsened γ′ at the grain boundaries, directionally coarsened γ′ with wavy γ/γ′ interface and coagulated rod shape γ′ inside the grains. Addition of Ti (2 and 4 at. %), however, suppresses the formation of these features up to 1000 h of aging at 900 °C. The feasibility of tuning the properties of the present alloys by alloying additions makes them attractive for their further development.