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Abstract

Hydrogen embrittlement severely affects structural materials such as iron, steel, and other metallic alloys. It comprises several mechanisms at the atomic level. One of them is hydrogen enhanced decohesion (HEDE), the phenomenon of H accumulation between cleavage planes, where it reduces the interplanar cohesion. Simulation of HEDE requires a coupled approach, which describes hydrogen re-distribution during deformation, as well as the effect of H on the material failure, i.e. on crack initiation and growth. A common way to realise such a coupling is the combination of a diffusion equation or transport model with a cohesive zone model. In this contribution, we focus on the latter and explain and discuss the use of traction separation (TS) laws derived from first-principle density functional theory calculations. Grain boundaries are expected to play a significant role for HEDE, since they act as strong trapping sites for hydrogen. To elucidate this mechanism, we present the results of first-principles studies of the H solubility at and the effect of H on the cohesive strength of α-Fe single crystal (001) and (111) cleavage planes, as well as on the Σ5(310)[001] and Σ3(112)[110] symmetrical tilt grain boundaries with and without additional alloying elements (C, V, Cr, Mn) [1, 2]. 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 [3]. At higher concentrations, however, the picture can change. Ultimately, the effect of H on inter- and transgranular decohesion depends on the H chemical potential, which is in turn affected by the presence of strain and alloying elements in the microstructure. We will discuss how the different influencing factors can be captured by the parametrisation of TS laws. References [1] Tahir, A. M., Janisch, R., Hartmaier A., “Hydrogen embrittlement of a carbon segregated Σ5(310)[001] symmetrical tilt grain boundary in α-Fe, Materials Science & Engineering A, 612, 462467 (2014). [2] Subramanyam, A. P. A, Guzmán, A. A. , Vincent, S., Hartmaier, A. Janisch, R., “Ab Initio Study of the Combined Effects of Alloying Elements and H on Grain Boundary Cohesion in Ferritic Steels”, Metals, 9, 291 (2019). [3] Guzmán, A. A., Jeon, J., Hartmaier, A., Janisch, R., “Hydrogen Embrittlement at Cleavage Planes and Grain Boundaries in Bcc IronRevisiting the First-Principles Cohesive Zone Model”, Materials, 13, 5785 (2020).