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Abstract

Hydrogen embrittlement (HE) is a collective term for several phenomena associated with the deleterious effects of hydrogen on metallic microstructures. Understanding and preventing HE, e.g. of iron and steel, is a long standing challenge for materials science, and it is currently gaining even more urgency as H is among the top candidates for renewable energies. The beginning of the end, which is common to all mechanisms of hydrogen embrittlement, is the high mobility of H in metallic microstructures. Therefore, the most promising approach to attempt to prevent HE so far is to influence the diffusivity of H in microstructures. Sufficiently strongly trapped hydrogen might not harm steel, as long as the accumulated hydrogen does not trigger an embrittlement mechanism of its own. In this sense, grain boundaries in steel play a dual role. They represent likely trapping sites for H, but are also prone to suffer from hydrogen enhanced decohesion (HEDE). Both, the segregation tendency of H to grain boundaries and the effect of H on GB cohesion depend on the local atomic structure of the grain boundary and can be influenced by alloying elements. To arrive at alloying recipes against HEDE, it is therefore desirable to identify the structural parameters of grain boundaries, which promote H segregation, and at the same time unravel the relation of these parameters to the cohesive strength. To this end we performed studies of hydrogen solubility and grain boundary embrittlement in ferritic iron and iron alloys using ab initio density functional theory calculations. We present the results of such first-principles studies of the H solubility cohesive strength of α-Fe single crystal cleavage planes, as well as selected symmetrical tilt grain boundaries as limiting cases of open and close-packed atomistic structures. We also search for trends with and without additional alloying elements (B, C, V, Cr, Mn). The calculated results show that at low to medium H concentrations, the single crystal cleavage planes are much more sensitive to a change in H concentration than the grain boundaries. At higher concentrations, however, the picture can change. Ultimately, the effect of H on interand transgranular decohesion depends on the H generalized chemical potential. This chemical potential is again a function of strain, composition, and structure. The challenges and the results on the way to disentangle this complexity will be discussed in the presentation.