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Abstract

The cleavage strength of a symmetrical tilt grain boundary (STGB) in body centred cubic (bcc) Fe is investigated by means of ab-initio calculations with respect to the effect of a varying number of C and H atoms at the grain boundary. Our results indicate that hydrogen enhanced decohesion of interfaces in Fe-C alloys could be understood as a co-segregation effect. The calculated segregation energy for C shows that in a bcc Fe-C system with a sufficient amount of interstitial C, the C segregated state should be considered as the ground state of this interface. The work of separation as well as the tensile strength increase significantly with increasing C content. A partial exchange of C with hydrogen changes the cohesion enhancing elastic contribution of C to an embrittling contribution, and also reduces the beneficial chemical contribution to the cohesion. The reduction in strength amounts to almost 20% in the co-segregated case, and to more than 25% if C is completely replaced by H. Compared to the strength of the STGB in pure iron, however, the influence of H is negligible. Hence, H embrittlement of this interface can only be understood in the three component Fe-C-H system.