Effects of alloying additions on the microstructure, lattice misfit, and solvus temperature of a novel Co–Ni-based superalloy

Abstract

In this study, the effects of Ti, Nb, Mo and W addition on the microstructure, lattice misfit, and solvus temperature of a novel Co–Ni–Al–V–Ta-based superalloy are presented. Only a γ/γ′ two-phase microstructure was detected in the studied alloys after equilibrating heat treatment. Ti, Nb, Mo and W are confirmed as γ′-phase-forming elements, the addition of which increases the solvus temperature. In addition, the addition of strong γ′ formers like Ti and Nb, which have high partition coefficients, significantly increases lattice misfit, whereas the lattice misfit in Mo- and W-containing alloys is at the same level as that of the base alloy. The γ′ phase coarsening behaviour of the studied alloys follows LSW theory, and all alloying elements increase the coarsening rate. The activation energy of the 2Ti alloy is 355.9 ± 12.5 kJmol-1 at 900–1000 °C, which is higher than the activation energy of most solutes in the γ-Co matrix. In particular, the 2Mo and 2W alloys have lower densities than the Co–Ni–Al–Mo-based and Co–Ni–Al–W-based alloys, respectively, and the γ′ phase solvus temperature of the 2Mo alloy is higher than that of the Co–Ni–Al–Mo-based alloys. Moreover, the specific yield strengths of 2Ti and 2Nb alloys at 700 °C are higher than those of Co–Al–W-based and traditional Co-based (Mar-M-302) superalloys.