First-principles atomic cluster study of boron interactions in Ni3Al

First-principles atomic cluster calculations have been carried out in the local density approximation to understand the segregation behavior and strengthening effects of boron in Ni3Al. The binding energy of boron is calculated in lattice fragment clusters representing the perfect crystal, as well as various defect sites. The agreement between trends in energetics determined for small clusters and periodic supercells indicates the dominant role of boron's interaction with nearest-neighbors of the host. The stereochemical factor underlying boron's preferential bonding to nickel atoms in four-fold planar coordination (i.e., sp(3) hybridization) suggests a mechanism for the baron-effect in Ni3Al: increased cohesion provides a driving force for B segregation to open sites, such as at Ni-enriched grain boundary sites, and the strengthening is a result of strong localized Ni-B covalent bond formation. (C) 1997 American Institute of Physics.