First-principles study of the effect of boron on grain boundary in NiAl

The grain boundary (GB) is critical to the mechanical properties of NiAl. The addition of B in NiAl can lead to the increase of yield strength due to its segregation in GB. To elucidate the B-induced enhancement of GB, we investigate the segregation state and its strengthening mechanism of the additive B in the typical Sigma 5(310)[001] GB of NiAl at atomic scale using the first-principles method. Our calculations shown that the additive B atom prefers to segregate at the interstitial I1 site of GB, in which case it is surrounded by four first-nearest-neighboring (1NN) and two second-nearest-neighboring (2NN) Ni atoms. It is found the segregated B prefers to bond with its neighboring Ni instead of Al atoms. Our calculations further shown not only the intergranular cohesion is improved by the covalent bonding between the segregated B and its four 1NN Ni atoms, but also the intra-planar cohesion of GB is enhanced by the covalent bonding between the segregated B and its two 2NN Ni atoms. The covalent bonding between B and Ni atoms mainly stem from the contribution of B-p and Ni-d states, partly from the weak interactions of B-s and Ni-s states. Our first-principles tensile test shown that the yield stress and ductility of NiAl are indeed improved due to the segregation of B in GB. (C) 2016 Elsevier B.V. All rights reserved.