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Abstract

Studying crack nucleation is the first step of understanding the fracture behavior of a material. With ab-initio density-functional theory calculations of fracture strength we identify possible nucleation sites for brittle cracks. In ferritic iron potential candidates are the well known {001} cleavage planes as well as different interfaces in the microstructure. The strength of both can change considerably with impurity or alloying element content, in our case silicon and carbon. Detailed studies on the strengthening effect of C on the interface structures and the embrittling nature of Si at the grain boundaries are available. In this work, the co-doping effects of both Si and C are of special interest. To identify the weakest type of interface or cleavage plane, and to investigate the influence of alloying elements on its strength, different model structures were created. The energy of formation of interstitial C in the single crystal as well as at the interfaces in the presence or absence of Si gives an insight on the expected composition of grain boundaries in Fe-Si-C alloys. The subsequent calculations of fracture strength with one or both type of elements reveal interesting non-linear effects.