Effect of Co and Cr on the Stability of Strengthening Phases in Nickelbase Superalloys

Nickel-base superalloys such as VDM 780 may possess a high content of Cr and Co. This influences solution energies of phase-forming elements such as Al and Ta (γ′-phase), Nb (γ′′- and δ-phase), and Ti (η-phase). We perform density functional theory studies of a nickel matrix at 0 K with high concentrations of either Co and Cr and calculate the influence of these elements on solution energies. In the case of Co, the solution energy can be predicted well by the nearest-neighbor interaction in the Co-rich matrix. For Cr, the effect is more complicated because Cr has a larger ionic radius and changes the magnetic state of the material. The effect of a Cr-rich matrix on the energy of Co is dominated by magnetic effects and interactions with the other elements by elastic deformation of the lattice. A high content of Co or Cr will thus increase the solvus temperature of the strengthening phase in nickel-base superalloys, in agreement with the literature and thermodynamic calculations.