Distinct nitriding behaviors of γ and γ′ phases in Ni-based superalloys: A first-principles study

The diffusion of nitrogen through γ(Ni)/γ'(L12-Ni3Al) interface in Ni-based superalloys was investigated based on first-principles calculations and the different nitriding behaviors of γ matrix and γ' precipitate were determined. At the interface, nitrogen atoms prefer to occupy the 6-Ni octahedral interstitial site with a large physical volume rather than the octahedral site constituted by Ni and Al with good chemical affinity. This indicates that γ matrix can provide more trapping sites for nitrogen atom than γ' precipitate. Moreover, the migration energy barrier of N crossing the (002)γ/γ' coherent plane is higher than that in γ phase. Nitrogen atoms can move into γ' phase unless the diffusion front has higher nitrogen concentration and O4 * (5Ni-Al octahedral site) is occupied. Inside the γ' matrix, nitrogen atoms are also easily trapped by the 6-Ni octahedral interstitial site, but their escape rates are much lower. It is the potential barrier role of the (002)γ/γ’ coherent plane and the low diffusion ability of N in γ' that are responsible for the unfavorable nitriding behavior of γ' phase in Ni-based superalloys.