Hydrogen-enhanced or hydrogen-induced failure has been a topic of debate for the past decades. Different experimental observations and theoretical simulations proposed different models. One model is the hydrogen-enhanced decohesion (HEDE), which was proposed by Troiano in the 1960s [1], which simply postulated, that the hydrogen accumulated at the crack tip reduces the cohesive bond energy between atoms, thus decreasing the work needed for fracture to occur. The HEDE mechanism was found to agree with the observations that the crack tip opening angle decreases with rising hydrogen pressure in Fe-3Si (wt.%) and Ni single crystals [2]. Later, the HEDE mechanism has been expanded to the grain boundary and phase boundary area [3, 4] based on the experimental observations that hydrogen will weaken the boundary toughness, thus promoting intergranular fracture. The next proposed model is the hydrogen-enhanced localized plasticity (HELP), proposed mainly based on the in-situ observation of dislocation motion inside environmental TEM cells [5-8]. From these observations, the solid solution hydrogen increases the dislocation mobility or decreases the stacking fault energy, thus improving the cross-slip. These mechanisms will introduce local plasticity and strain softening, and thus explain the hydrogen-enhanced plastic failure. Another
The present study evaluates the hydrogen induced damage by in-situ hydrogen plasma charging in dual phase (DP) steel. Cold deformation of 15% is applied on the material to change microstructural defects, such as dislocation density. The susceptibility to hydrogen embrittlement is hence evaluated for two material conditions, i.e. DP 0% and DP 15%. Small scale tensile tests are done inside an ESEM chamber for which in-situ hydrogen plasma charging is compared with electrochemical hydrogen charging while uncharged samples serve as a reference. Generally, the hydrogen effect on the ductility and stress level is increased when deformation is applied, due to the hydrogen trapping ability of the deformation induced defects, as confirmed by thermal desorption spectroscopy. Complementary in-situ electrochemical nanoindentation tests verify the more pronounced hardness increase due to hydrogen when cold deformation is applied. A slightly increased ductility loss is observed when the samples are charged electrochemically, although similar tendencies are found for both hydrogen charging procedures. These observations are confirmed by the fractographic analysis, where the detrimental role of MnS inclusions in the segregation line on hydrogen induced cracking is demonstrated as well.
An electrolyte for electrochemical hydrogen charging of corrosion-susceptible alloys is developed, which preserves the surface integrity at nano-scale by minimizing the surface roughness alternation. To assure the formation and adsorption of the hydrogen from the electrolyte, permeation tests were performed on Fe 3wt.%Si ferritic steel.X-ray photoelectron spectroscopy method was used to check the effect of the glycerol-based solution on the chemical composition of the sample surface. The surface analysis revealed minimal chemical and topography alteration on the surface after different electrochemical treatments. Various types of in situ small-scale mechanical tests such as nano-indentation, micro-pillar compression, and micro-cantilever bending tests were performed inside this electrolyte while the samples being charged with hydrogen under cathodic potential. These small-scale mechanical tests showed that the solution facilitates studying hydrogen embrittlement in nano-or micro-scale.
Hydrogen (H) enhanced cracking was studied in Fe-3wt%Si by means of electrochemical microcantilever bending test. It was clearly shown that the presence of H causes hydrogen embrittlement (HE) by triggering crack initiation and propagation at the notch where stress concentration is existing. Additionally, the effect of carbon content and the presence of a grain boundary (GB) in the cantilever were studied. It was shown that in the presence of H the effect of carbon atom on pinning the dislocations is reduced. On the other hand, the presence of a GB, while the chemical composition of material kept constant, will promote the HE. Crack initiation and propagation occur in the presence of H, while the notch blunting was observed for both single and bi-crystalline beams bent in air. Post-mortem analysis of the crack propagation path showed that a transition from transgranular fracture to intragranular fracture mechanism is highly dependent on the position of the stress concentration relative to the GB.This article is part of the themed issue 'The challenges of hydrogen and metals'.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.