First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms

The electronic structure of an aluminium grain boundary with segregated sulphur impurity atoms has been calculated by a first-principles pseudopotential method. It is found that a sulphur atom bonds to only one of the neighbouring aluminium atoms. This bond is a mixed-character metallic-covalent bond which is stronger than the metallic Al-Al bonds. Electrons that participate in forming this bond are 3p electrons of sulphur but not its 3s electrons. Other Al-S bonds in the boundary contain no covalent character. From the nature of Al-S bonds in the boundary it cannot be decided whether the embrittlement promotion mechanism by sulphur segregation should be classified as a 'bond mobility model' or a 'decohesion model'.