Formation of a coplanar O-Al bonding cluster: the effect of O impurity on a Σ = 5 NiAl grain boundary from first-principles.

The site occupancy, structure, and bonding properties of O in an NiAl grain boundary (GB) have been investigated by employing a first-principles total energy method based on density functional theory with the generalized gradient approximation and ultrasoft pseudopotential. The Σ5(310)/[001] tilt GB of NiAl has been chosen because (i) the Σ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) the Σ5(310)/[001] is energetically favorable in comparison with the Σ5(210)/[001]. The NiAl GB is shown to favor the O segregation with a segregation energy of -1.75 eV, indicating that most of the O impurity will distribute in the NiAl GB thermodynamically. Moreover, O is shown to prefer occupying the interstitial sites rather than the substitutional sites in the GB according to the calculated formation energies. The O-Al bond is 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 indicate the intrinsic bonding properties of O-Al that contain obvious covalent characteristics. It is interesting to find that O is 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 GB can embrittle the NiAl intermetallics and thus are not beneficial to the plasticity of NiAl. Our results will provide a useful reference for improving the mechanical properties and for understanding the oxidation effect of the NiAl intermetallics.