The Site Preferences of Transition Elements and Their Synergistic Effects on the Bonding Strengthening and Structural Stability of γ′-Ni3Al Precipitates in Ni-Based Superalloys: A First-Principles Investigation

Advanced mechanical properties of Ni-based superalloys strongly depend on the site preferences of alloying X elements in γ′-Ni₃Al-X precipitates, which are associated with the partial bonding characteristics between Ni, Al, and X atoms. Therefore, in the current work, the site occupancy tendencies of transition X metals were revealed via first-principles ab initio calculations at 0 K. Bonding features of Ni-Al, Ni-X, and Al-X pairs were simulated by using the charge density difference (CDD), electron localization function (ELF), and density of states (DOS) methods, respectively. According to simulations, higher atomic size X elements preferably occupy Al sites of γ′-Ni₃Al-X intermetallics and lead to strong covalent-like directional bondings between themselves and their nearest neighbor (NN) Ni atoms along 〈110〉 directions. However, if these larger X metals substituted for Ni sites, the bonding properties would differ by plane due to the nature of the L1₂-type crystal structure of γ′-Ni₃Al-X precipitates. Considering all transition elements, refractory metals (i.e., X = Re, W, Mo, Ta, or Nb) appear as the most effective strength inducers, improving the structural stability of γ′ phase, even if Ni site substitution of X = Re atoms would start to increase structural instability. On the other hand, relatively small alloying X elements having electron configuration similarities with Ni (i.e., X = Co, Cu, Rh, Pd, Ag, Ir, Pt, or Au) are more likely to worsen bonding strengthening. Instead, these transition X metals creating metallic bondings with NN Ni atoms would contribute to ductility and malleability of Ni-based superalloys. Furthermore, depending on the relative atomic size of γ′-former and refractory elements, the phase and site preferences of refractory atoms would alter in multicomponent systems. As a result of the attractive or weak repulsive forces between Re-Re, Re-Mo, and Re-W pairs, the structural stability of the constituent phases would deteriorate and harmful topologically close-packed (TCP) phases would precipitate.