The site occupancy, structure, and bonding properties of O in an NiAl grain boundary were investigated by employing a first-principles total energy method based upon density functional theory with the generalized gradient approximation and ultra-soft pseudopotential. The Σ5(310)/[001] tilt grain boundary of NiAl was chosen because (i) the Σ = 5 grain boundary was experimentally observed to be of a higher fraction in NiAl, and (ii) the Σ5(310)/[001] was energetically favorable in comparison with the Σ5(210)/[001]. The NiAl grain boundary was shown to favour O segregation with a segregation energy of -1.75eV; indicating that most of the O impurity would distribute in the NiAl grain boundary thermodynamically. Moreover, O was shown to prefer occupying the interstitial sites rather than the substitutional sites in the grain boundary according to the calculated formation energies. The O–Al bond was energetically favorable as compared with the O–Ni bond due to different electronegativity of Al and Ni in reference to O. Charge distribution and the density of states further indicated the intrinsic bonding properties of O–Al that contained obvious covalent characteristics. It was interesting to find that O was coplanar with the surrounding Al atoms in both interstitial and substitutional cases with lower formation energies, forming stronger coplanar O–Al bonding clusters. Such stronger bonding clusters in the grain boundary could embrittle the NiAl intermetallics and thus were not beneficial to the plasticity of NiAl. These results were expected to provide a useful reference for improving the mechanical properties and for understanding the oxidation effect of the NiAl intermetallics.

Formation of a Coplanar O–Al Bonding Cluster - the Effect of O Impurity on a Σ = 5 NiAl Grain Boundary from First-Principles. L.H.Liu, Y.Zhang, X.L.Hu, G.H.Lu: Journal of Physics - Condensed Matter, 2009, 21[1], 015002 (7pp)