First-principles study of multiple-site substitutions of alloying elements in Ni-based single crystal superalloys

The development of Ni-based single crystal superalloys relies heavily on the composition design with the addition of critical alloying elements, e.g., Re and Ru. Understanding the role of alloying effects require to know the configurations of the alloying element distribution between γ-Ni and γ′-Ni₃Al phases and among various non-equivalent sites. This work employed first-principles density functional theory calculations to study the preference of phase and site occupancy of 11 alloying elements including Al and transition metal elements: 3d (Ti, Cr, Co, Ni), 4d (Mo, Ru), and 5d (Hf, Ta, W, Re) in Ni and Ni₃Al. We calculated the substitution energies of 1298 triple-site doping configurations including 286 NiNiNi site doping of Ni, 726 AlNiNi site doping, and 286 NiNiNi site doping of Ni₃Al with alloying elements Ni, Co, Ru, Cr, Re, Mo, W, Al, Ti, Ta, and Hf. In the dual-site and triple-site doping of Ni and Ni₃Al, all studied alloying elements preferred to occupy Ni phase rather than Ni₃Al phase. We found that the most stable defect complexes often contained the favorable substitutions of Al, Ti, Ta, and Hf for the Ni sites that stabilized the alloying elements doping at the other one or two nearest neighbor sites. The co-substitutions of various alloying elements at multiple sites are critical to understanding the strengthening mechanism of alloying elements in Ni-based single crystal superalloys.