Segregation of Re at the γ/γ' boundary of Ni-based single crystal superalloys revealed by first-principles calculations based Monte-Carlo simulations

Nickel-based single crystal superalloys have been widely used in aero-engines and gas turbine engines. To improve the creep resistance, rhenium is often added to the alloys. However, it is not yet fully under-stood how the added Re elements distribute in the alloys and how the microstructure evolves with the addition of Re. Here, we performed extensive first-principles calculations based Monte-Carlo simulated annealing of Ni-Al-Re ternary alloys with different Re concentrations ranging from 0.5 at.% to 6.0 at.%. The results demonstrate that with the decreasing temperature, most of Re atoms stay in the gamma phase, while a few of Re atoms stay in the gamma' phase and tend to occupy the Al positions. At low temperatures, the Re atoms segregate at the gamma/gamma' boundary, in good agreement with experiment. We find that the disorder-order transition temperature of the Ni-Al-Re ternary alloys increases with the Re concentration due to the Re-enhanced Al-Al ordering tendency. In addition, we observe that at low temperatures the Re segregation at the gamma/gamma' boundary promotes the formation of Ni4Re-or Ni8Re-like local structures as the Re concentration is over 2 at.%. The formation of a large amount of these local structures consumes the Re atoms in solid solutions, and thus from the solid-solution strengthening point of view, this would have a negative influence on the creep resistance of the superalloys. This work provides important atomistic insights on the Re distribution and its effects on the stability of superalloys.