Density functional theory study of the thermodynamic and elastic properties of Ni-based superalloys

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Abstract
The thermophysical properties of Ni-based single-crystal superalloys were investigated using first-principles calculations combined with the quasiharmonic approximation. The effect of alloying elements X ( X = Re, Ru, Ta, W, Mo, Cr, and Co) on the thermophysical properties of the γ -Ni and γ ′-Ni 3 Al phases was investigated. The calculations showed that alloying can effectively adjust the lattice misfit between the two phases, and Cr can suppress lattice misfit and may improve the creep resistance of alloys. At 0 K, doping with refractory elements leads to tetragonal shear softening of the γ -Ni phase. For γ -Ni, Re, Ru, Cr, and Co slightly increase c 44 , while Mo, W, and Ta decrease c 44 . Importantly, high-temperature relative hardening was found to occur close to the service temperature of the superalloy, at which Ru and Cr increase c ′ and Mo and W increase c 44 of γ -Ni. For the γ ′-Ni 3 Al phase, all of the alloying elements except Co considerably increase c ′ and c 44 . Re and W at the Al site were found to most effectively harden the γ ′-Ni 3 Al phase. The thermophysical and elastic properties were fully understood by analysis of the electronic structures and phonon spectra. It was found that the electronic density of states (DOS) can account for elastic hardening due to alloying. The phonon spectra along with electronic DOS analysis showed that alloying not only strengthens the first nearest neighbor Ni–X bond through additional d – d hybridization, but it is also important for stiffening the second nearest neighbor Al–X bonding through p -band filling.

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