Kinetics of hydrogen-enhanced decohesion at the Σ5 symmetric tilt boundary in α-Fe

Hydrogen embrittlement (HE) is a reduction in the ductility and fracture toughness of a metal due to absorbed hydrogen. One of the widely accepted mechanism of HE is hydrogen-enhanced decohesion (HEDE), where the interatomic bonding of steel is weakened due to the donation of the 1s electrons of the H atoms to the unfilled 3d shell of the Fe atoms. Although it is anticipated that HEDE would occur at grain boundaries (GBs) due to the strong segregation of H solute atoms at these interfaces, computational studies at atomic scale have observed contradicting results regarding the cohesive properties of the structures.

In this thesis, first-principle cohesive zone models are developed to investigate the decohesion effect of H at the Σ5(012)[100] symmetric tilt grain boundary (STGB) in α-Fe. The ab initio tensile tests are first performed on the GB at several separations, and at each step the binding energy is calculated using spin-polarized density functional theory (SDFT) calculation. The binding energies are then fitted using the universal binding energy relationship (UBER), which provides information about the cohesive properties of the GB. Two variants of the tensile tests are considered: energy is calculated right after shifting the grain (RGS) or the atomic positions of the structure are allowed to relax after shifting (RGSrel). In the latter case, a thermodynamic framework is formulated using excess quantities to eliminate the residual elastic energy exhibited after relaxation.

Using the RGS tensile test model, the embrittlement effect is not observed as both the work of separation and critical stress of the GB increase at higher concentrations of H atoms. This is because the H diffusion happens at a time scale much faster than the separation speed of the grains, and the positions of the solute atoms strongly redistribute at the free surface (FS) when the GB is completely decohered. Therefore, the energy calculated using RGS model does not account for H redistribution, leading to a more positive segregation energy and causing strengthening effect. On the other hand, since RGSrel tensile test model performs relaxation before measuring the energy, the positions of solute atoms are allowed to redistribute into the FS and this shifts the segregation energy towards negative direction. This is the sufficient condition for embrittlement as proposed in Rice-Wang theory, and the results confirm the claim as both the work of separation and critical stress of the GB reduce with increasing H concentrations. From the results of this work, future research can be carried out focusing on the more complex interactions between H and alloy elements, as well as incorporating the atomic scale information with continuum level macroscopic modeling of HE in structural failures.