Grain boundary electronic structure and materials design

The relative contributions from directional and nondirectional bonding within a given alloy system can be understood to arise from differing types of atomic orbital overlap. Transition-metal alloys which are inherently brittle are argued to be the result of directional d-d overlap between the constituent atoms. On the other hand, transition-metal alloys which are inherently ductile show isotropic s-orbital overlap between constituent atoms. We report electronic structure calculations upon polyhedral models for regions of bond misorientation to show that the appearance of localized states are characteristic of directional overlap and indicative of brittle behavior. These result are summarized by plotting A vs. B s-orbital electronegativity for AB intermetallics which will indicate the degree of isotropic character to the bonding and thus serve as a predictor of mechanical behavior. (author) 44 refs., 15 figs