Effects of impurities on an Al grain boundary

The atomic and electronic structures of the Al Σ = 9 tilt grain boundary with segregated impurity atoms have been calculated by the first-principles pseudopotential method based on the local density functional theory. Effects of impurities of group I (Na), group II (Ca), group IV (Si) and group VI (S) have been examined. For the Na and Ca segregation cases, the impurity-Al interactions seem to have metallic characters. However, the boundary expands substantially and the charge density decreases significantly over the boundary. Thus these impurities should cause weaker intergranular adhesion. For both the Si and S segregation cases, the charge density increases around the impurity atom. The Si atom forms the covalent-metallic character mixing bonds with neighboring Al atoms. Such strong and directional bonds should prevent the rearrangement of atoms under stresses. However, the S atom forms such a strong bond with only one neighbor, differently from the Si case. It can be said that each impurity has various effects on the local atomic and electronic structure of an Al grain boundary according to the nature of each species, which seems to dominate the mechanism of embrittlement.