Hydrogen embrittlement in nickel, visited by first principles modeling, cohesive zone simulation and nanomechanical testing

Hydrogen (H) can have dramatic consequences on material properties, especially by reducing the fracture toughness. Degradation by H initiates through mechanisms at the nano-scale, and is normally not detectable prior to the final leakage or component fracture. Computational techniques are therefore being developed in order to provide both a wider understanding of the phenomenon and engineering tools for prediction of materials susceptibility toward hydrogen embrittlement. This work presents a preliminary study for the development of a novel computational approach in which density functional theory (DFT), nanoscale experiments and finite element (FE) modeling are combined and interrelated in order to improve the understanding of hydrogen induced intergranular cracking. Two low angle and low coincidence grain boundaries types have been considered: Σ3 and Σ5. Density Functional Theory has been applied to investigate the influence of an increasing number of H atoms on the cohesive strength of these grain boundaries in pure Nickel. This provided relations between the number of H atoms (coverage of the grain boundary) and the cohesive strength, which is further applied in cohesive zone FE modeling of a triangular nanometer sized fracture mechanics cantilever beam. For verification of the model such specimens will be tested experimentally both with and without in-situ electrochemical charging.

The simulation results show that the model is suitable for describing the combined effect of grain boundary misorientation and the reduced cohesive energy due to hydrogen on the grain boundary propensity to cleave.