Hydrogen enhanced decohesion at grain boundaries - insights from ab-initio calculations

Grain boundaries in ferritic microstructures play a dual role in the context of hydrogen embrittlement: on the one hand, they act as H traps and thus reduce the amount of mobile H in the system. On the other hand, exactly this trapping is expected to promote hydrogen enhanced decohesion at the grain boundaries. In order to understand and ultimately influence the segregation process, as well as the cohesive properties of interfaces in ferrite, a detailed understanding is required, between the local composition and residual strain on the one hand and the solubility of hydrogen and its impact on the cohesive strength on the other.

This talk summarises some recent results of ab-initio studies of H segregation at Fe single crystal cleavage planes, as well as at special symmetrical tilt grain boundaries. The solution energy of hydrogen was investigated in the presence of different alloying elements [1,2] as well as a function of tractions normal to the interface [3].

The qualitative findings are, that in relaxed as well as strained microstructures, H tends to accumulate at the grain boundaries, and the influence of alloying elements on this process is comparatively weak. For low to medium H concentrations, H clearly reduces the surface energies, and hence the work of separation, but it has only a mild effect on the transgranular or intergranular fracture stress. The quantitative results, however, depend significantly on the loading and relaxation scheme. Whether this is only a technical issue, or also has some physical background, will be discussed in the presentation. Finally, an outlook on ongoing work on the effect of increasing the H concentration will be given.

References

[1] X. Huang and R. Janisch. Partitioning of interstitial segregants during decohesion: A DFT case study of the S3 symmetric tilt grain boundary in ferritic steel. Materials 12, 2019. DOI: 10.3390/ma12182971

[2] A. P. A. Subramanyam, A. Azócar Guzmán, S. Vincent, A. Hartmaier, and R. Janisch. Ab initio study of the combined effects of alloying elements and H on grain boundary cohesion in ferritic steels. Metals 9, 2019. DOI: 10.3390/met9030291

[3] A. Azócar Guzmán, J. Jeon, A. Hartmaier, and R. Janisch. Hydrogen embrittlement at cleavage planes and grain boundaries in bcc iron - revisiting the first principles cohesive zone model. Materials 13, 2020. DOI: 10.3390/ma13245785