First-principles thermodynamics and experimental study of interface oxidation in Ni/NiAl structures.

Anti-oxidation is one of the significant properties of nickel-based superalloys useful for their potential applications in industry. However, previous research mainly focused on single-phase compounds of NiAl or Ni3Al. In the present study, first-principles density functional theory coupled with thermodynamics analysis are employed to investigate the atomistic oxidation behaviours of the Ni/Ni3Al composites systematically. An oxidation experiment with a DD6 alloy is conducted as well to further confirm the theoretical prediction. Initial surface formation energy analysis shows that the systems composed of Ni(111) and Ni3Al(100)/(111) surfaces are more stable and therefore are selected for further investigation. Thermodynamics calculations indicate that the Ni3Al phase is oxidized first, accompanied by Al-segregation on the top surfaces. This is followed by subsequent oxidation of the Ni phase. Surface oxidation diagrams with respect to the surface formation energies show that oxygen adsorption could enhance Al-segregation to the surface and Ni3Al(111) surfaces tend to be oxidized completely with slightly lower oxygen coverage. Oxidation at the interface is also investigated and the results show that oxygen atoms bind with the upper layers of the Ni3Al phase from the point of view of binding energy. The experimental results provide a reasonable explanation for the selective oxidation of Al atoms at the atomic-scale so as to form a dense anti-oxidation membrane. The present work could serve as a beneficial reference for subsequent investigations of oxidation or adsorption processes of two-phase composites.