*3.3. Cohesive Strength of Hydrogen-Charged GBs*

The influence of hydrogen segregation on the cohesive strength was then studied using supercells charged with the hydrogen. Its amount introduced to the supercell was 4, 6, and 2 atoms for Σ3, Σ5, and Σ11 GBs, respectively. These numbers correspond to an optimum coverage (all available interstitial positions are filled with hydrogen) but also to a locally-enhanced hydrogen concentration (much greater than can be expected for the rest of the crystal.

The supercells were subjected to tensile tests with all the considered loading conditions and the results were added to Figure 6. The H-charged Σ3 GB again exhibits a quite different strength response than the other two GB types. Similarly to the results for hydrogen-free GBs, the cohesive strengths of Σ5 and Σ11 GBs increased with increasing transverse stresses while the value of the cohesive strength of Σ3 GB remained constant in the whole range of the transverse stresses. More interestingly however, the cohesive strength of Σ3 was distinctly reduced for all levels of the superimposed transverse stresses. The strength reduction for Σ5 and Σ11 GBs was significant only in the case of high biaxial stresses, i.e., for nearly isotropic (hydrostatic) loading cases.

To understand the effect of the compact Σ3 GB (with a negligible excess volume) in the H-charged crystal, we also computed the tensile strength of H-charged bulk Ni (perfect crystal) subjected to uniaxial deformation along the [111] direction. As can be seen from Figure 6, the strength values for the clean bulk and the Σ3 GB are very similar (33.9 GPa and 33.2 GPa, respectively). After introducing H to the Σ3 GB, the strength decreased to 26.3 GPa, but the same amount of H introduced to the perfect lattice (using comparable supercell) reduced the strength only to 28.6 GPa. This suggests that the higher strength reduction of Σ3 GB than that of the perfect lattice is caused by an interaction of H atoms that get closer to each other in the Σ3 GB than in the perfect lattice. The values of cohesive strength are seemingly too high in comparison with typical levels of stress applied to the specimen in the instant of first occurrence of cracks. However, one must bear in mind that, due to the presence of stress concentrators, local stress levels are much higher and can reach the computed strength values.

Thus, our study reveals strong additional HEDE-based reasons for the highest susceptibility of special Σ3 GBs to crack initiation, as well as for the lowest resistance of general GBs to crack propagation during uniaxial loading. Indeed, the presence of hydrogen reduces the cohesive strength of Σ3 GBs to become closer to strength levels of special GBs which remain hydrogen-unaffected. Therefore, along with the movement and interaction of dislocations in the Σ3 GB plane, such a decrease in the cohesive strength makes the Σ3 GB plane a comprehensible preferential site for nucleation of microcracks. Once these cracks appear in the Σ3 GB planes, the cohesive strength of other types of GBs adjacent to Σ3 GBs also becomes reduced due to a highly triaxial tensile loading induced in the vicinity of the microcrack networks. Naturally, a further crack propagation preferentially occurs along general grain boundaries which exhibit the lowest cohesive strength and the highest hydrogen concentrations. The most probable fracture scenario of nickel polycrystal in the hydrogen environment is, therefore, a result of both HELP and HEDE damage mechanisms.
