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Abstract
The energy levels of the various physicochemical states of a hydrogen or carbon atom in iron are compared. Both types of atom segregate strongly to grain boundaries in a iron. The embedded atom theory shows that the grain boundary state of hydrogen is very similar to the chemisorbed state, so that when a fast crack passes along the boundary the atom can readily transfer from one state to the other, thus making no contribution to intergranular cohesion. By contrast, the carbon atom cannot gain its full chemisorption energy without making a jump from the grain boundary state, a process that is slow compared with that of fast cracking. As a result, the carbon atom contributes strongly to intergranular cohesion, estimated at about 2 eV per segregated atom.
MST/1166
