Abstract
The ideas of the electron theory of chemisorption are applied to the behaviour of H, B, C, N, and 0 atoms segregated on and in iron. The general theory can be divided naturally into two branches depending on whether the valence levels of the embedded atoms ‘float’ at the Fermi level, as in the case of B, C, and N; or ‘sink’ to a level below the bottom of the d band, as with Hand O. In the former, the valence states take on the cationic role when they hybridise with nearby d states and a predominantly covalent bond is formed, which increases grain boundary cohesion. In the latter, the atoms form screened negative ions, with little covalent interaction, and thereby decrease cohesion, so promoting intergranular brittleness.
MST/1267
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