**4. Conclusions**

This article presents an ab initio study of the cohesive strength of selected types of special grain boundaries in hydrogen-free and hydrogen-charged nickel crystals under uniaxial and triaxial loading. The main motivation was to find out if not only the dislocation HELP mechanism, but also the HEDE might have been responsible for experimentally observed high susceptibility of Σ3 coherent twin boundaries to crack initiation. The results indeed revealed that the presence of hydrogen reduces the cohesive strength of Σ3 boundaries to become closer to strength levels of higher-energy GBs which, in contrary, remain hydrogen-unaffected. Thus, this HEDE (decohesion) mechanism makes, along with

the previously reported dislocation (HELP) mechanism, the Σ3 GB plane a comprehensible preferential site for nucleation of microcracks. The results of this study also brought an additional HEDE-based explanation of a small resistance of higher-energy GBs to crack propagation. The highly-triaxial stress state at the tips of microcracks (initiated at Σ3 boundaries) caused an extreme reduction of cohesive strength of adjacent high-energy grain boundaries, especially those of a general kind.

**Author Contributions:** Conceptualization, J.P. and Z.Z.; methodology, P.Š. and M.C.; ab initio calculations, P.Š.; ˇ formal analysis, M.C. and J.P.; writing—original draft preparation, P.Š.; writing—review and editing, J.P., M. ˇ C., ˇ and Z.Z.; visualization, P.Š. and M.C. ; supervision, J.P.; funding acquisition, J.P. and M. ˇ C. All authors have read ˇ and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Czech Science Foundation (projects No. 17-18566S and No. 20-08130S) and by the Ministry of Education, Youths and Sports of the Czech Republic within special support paid from the National Programme for Sustainability II funds within CEITEC 2020 (project No. LQ1601). Z.Z. would like to thank the Research Council of Norway for the support via the M-HEAT (294689) and HyLine project. Computational resources were provided by the Ministry of Education, Youths and Sports of the Czech Republic under the Project IT4Innovations National Supercomputer Center (Project No. LM2015070) within the program Projects of Large Research, Development and Innovations Infrastructures.

**Conflicts of Interest:** The authors declare no conflict of interest.
