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

Abstract

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 densityfunctional theory with the generalized gradient approximation and ultrasoft pseudopotential. TheΣ5(310)/[001] tilt GB of NiAl has been chosen because (i) theΣ = 5 GB has been observed to be a higher fraction in NiAl experimentally, and (ii) theΣ5(310)/[001] is energetically favorable in comparison with theΣ5(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 GBthermodynamically. Moreover, O is shown to prefer occupying the interstitialsites rather than the substitutional sites in the GB according to the calculatedformation energies. The O–Al bond is energetically favorable as compared withthe O–Ni bond due to different electronegativity of Al and Ni in reference toO. Charge distribution and the density of states further indicate the intrinsicbonding properties of O–Al that contain obvious covalent characteristics. It isinteresting to find that O is coplanar with the surrounding Al atoms in bothinterstitial and substitutional cases with lower formation energies, forming strongercoplanar O–Al bonding clusters. Such stronger bonding clusters in the GB canembrittle the NiAl intermetallics and thus are not beneficial to the plasticity ofNiAl. Our results will provide a useful reference for improving the mechanicalproperties and for understanding the oxidation effect of the NiAl intermetallics.