The role of Ti addition on the evolution and stability of γ/γ′ microstructure in a Co-30Ni-10Al-5Mo-2Ta alloy

Abstract

The paper explores the influence of Ti addition to the Ta stabilized γ/γ′ two-phase Co-based superalloys having the compositions of Co-30Ni-10Al-5Mo-2Ta-xTi (x = 0, 1, 2 and 4 at.%). The microstructure of the alloys after solutionizing (1300 °C for 25 h) and quenching show fine distribution of γ′ precipitates with spherical shape within the γ matrix. Aging of these alloys at 900 °C results in a morphological transition from spheroidal to cuboidal with a concomitant increase in γ/γ′ lattice misfit as compared to the solutionized alloys. However, the γ/γ′ lattice misfit of cuboidal particles decreases with an increase in Ti in the alloys. Atomic-scale compositional analysis by atom probe tomography across γ/γ′ interface for the aged alloys indicates a preferential partitioning of Ti to the γ′. This can be correlated with the observed increase in the γ′ solvus temperature up to 1166 °C for alloys with 4 at.% Ti addition. The increase in Ti content also decreases the partitioning of Mo to the γ′. Despite the rise in solvus temperature, the coarsening study reveals an escalation in the γ′ coarsening rate at 900 and 950 °C with a maximum for the alloy containing the highest Ti content. The initial stages of γ′ coarsening follow diffusion-controlled LSW kinetics. However, during later stages, a propensity for particle coalescence could be observed. An addition of 3 and 4 at.% Ti is shown to improve the high-temperature strength with 0.2% proof stress at 1000 °C of 360 and 400 MPa, respectively.