By using a first-principles plane-wave pseudopotential method, the energetics and electronic structure of the Σ5(210) grain boundary and the (210) surface of undoped, as well as B- and/or H-doped, Ni3Al were investigated. The geometrical structures of the grain boundaries and surfaces were fully relaxed by minimizing the total energy and interatomic force. The results showed that B induced a large lattice expansion, but H did not. Both B and H preferred to occupy the Ni-rich hole at the grain boundary or surface, but not the Ni-deficient one. The segregation energies of B and H, as well as the interaction energy between them at the grain boundary and surface, were calculated. The calculation indicated that B segregated more strongly to the grain boundary than to the surface. This resulted in an increase in the Griffith work of the grain boundary and therefore, in agreement with experiment, improved the ductility. Contrary to the case of B, it was found that H segregated more strongly to the surface than to the grain boundary. This resulted in a decrease in the Griffith work and confirmed that H was an embrittler of Ni3Al. Calculation of the interaction energy between B and H demonstrated that B and H repelled one another. Thus, B could block the occupation site of H at the grain boundary, and restrain H-induced embrittlement. In order to understand these energetic features, the electronic densities of states were calculated. A comparison of the total densities of states of B-doped grain boundaries, and undoped as well as H-doped ones, showed that B increased hybridization of the grain boundary. This contributed to an enhanced binding of the B-doped grain boundary as compared with undoped and H-doped ones. When the site of B changed from bulk to grain boundary to surface, the hybridization between B and Ni decreased accordingly. It was proposed that the segregation behavior of B at the grain boundary and the surface was dominated by the competition between B(p)–Ni(d) bond energy, and the strain energy induced by B. The preference of B for the Ni-rich interstices was explained by a repulsive interaction between B and Al atoms; resulting from hybridization between their electrons when they were close. The repulsion between B and H could also be explained by the same electronic structure mechanism as that for the B-Al interaction. The segregation of B to the surface shifted the densities of states of its nearest-neighbor Ni to a lower energy. This could increase the chemisorption potential energy of H2O on the Ni3Al surface and therefore decreased the reactivity of the surface; thus inhibiting the environmental embrittlement of Ni3Al.

Energetics and Electronic Structure of Grain Boundaries and Surfaces of B- and H-Doped Ni3Al. Hu, Q.M., Yang, R., Xu, D.S., Hao, Y.L., Li, D., Wu, W.T.: Physical Review B, 2003, 67[22], 224203