First-principles study on effects of X (Ta, Ti, and W) on the nitriding behavior of γ′-Ni3Al in Ni-based superalloys: From the surface adsorption and diffusion of nitrogen

The role of X (Ta, Ti, and W) doping in affecting the adsorption and diffusion behavior of nitrogen at Ni3Al(1 1 0) and Ni3Al(1 1 1) surfaces was studied based on first-principles calculations to understand the nitriding mechanism of γ′-Ni3Al in Ni-based superalloys. The calculated results show that the substituting of X atom on the 1st-layer (top layer) Al site could enhance the adsorption ability of N on Ni3Al surface, especially on Ni3Al(1 1 1), owing to the stronger bonding strength between N and X atoms. For comparison, the effect of the X atom on the adsorption behavior of nitrogen is not significant when it is doped in the 2nd-layer or 3rd-layer. Nitrogen diffusion into the Ni3Al surfaces as the rate-determining step of nitriding is calculated based on the climbing image nudged elastic band (CI-NEB) method. The doping of Ti could significantly decrease the energy barriers of N penetration into γ′-Ni3Al surface, regardless of surface orientation, which contributes to promoting the nitriding process of γ′ phase. The strongest strengthening effect of Ti explains the experimental results that γ′-Ni3Al contains Ti element having a great nitriding behavior in Ni-based superalloys. Our results can provide an in-depth insight into the diffusion mechanism of nitriding γ′-Ni3Al phase in Ni-based superalloys.