Abstract
The cohesive strength of Σ 3, Σ 5, and Σ 11 grain boundaries (GBs) in clean and hydrogen-segregated fcc nickel was systematically studied as a function of the superimposed transverse biaxial stresses using ab initio methods. The obtained results for H-free GBs revealed a quite different response of the coherent twinning boundary Σ 3 to the applied transverse stresses in comparison to the other GB types. While the cohesive strength of Σ 5 and Σ 11 GBs increased with increasing level of tensile transverse stresses, the strength of Σ 3 GB remained constant for any applied levels of transverse stresses. In the case of GBs with segregated hydrogen, the cohesive strength of Σ 3 was distinctly reduced for all levels of transverse stresses, while the strength reduction of Σ 5 and Σ 11 GBs was significant only for a nearly isotropic (hydrostatic) triaxial loading. This extraordinary response explains a high susceptibility of Σ 3 GBs to crack initiation, as recently reported in an experimental study. Moreover, a highly triaxial stress at the fronts of microcracks initiated at Σ 3 boundaries caused a strength reduction of adjacent high-energy grain boundaries which thus became preferential sites for further crack propagation.
Highlights
Hydrogen may cause a significant reduction of ductility of metallic materials which leads to a premature fracture of engineering components and structures
The relevance of Hydrogen-Enhanced Decohesion (HEDE) and Hydrogen-Enhanced Localized Plasticity (HELP) damage mechanisms should be identified for each particular case of the hydrogen-assisted fracture
We studied the aforementioned effect of triaxiality of the stress state on the strength response of grain boundaries (GBs) in our ab initio predictions
Summary
Hydrogen may cause a significant reduction of ductility of metallic materials which leads to a premature fracture of engineering components and structures. Decohesion processes in polycrystalline metallic materials are mostly restricted to a vicinity of planar defects like grain boundaries and, as a rule, they are affected by a presence of hydrogen and impurity atoms segregating at these defects [18,19,20] This was the case of the H-charged nickel-based superalloy that exhibited quasi-brittle fracture surfaces of a mixed intergranular and transgranular morphology in Ni matrice as reported in the experimental study [1]. Further crack propagation followed along general grain boundaries due to their highest energy (lowest separation energy and cohesive strength) and the highest hydrogen concentration Such an interpretation of the peculiar fracture behavior is certainly not exhaustive without exploring the effect of hydrogen segregation on the cohesive strength of GBs in H-charged nickel specimens—i.e., without taking the HEDE mechanism into account. The GBs included in our study have been studied theoretically and there is enough data for comparison in the literature
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