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A theoretical model was established to investigate the interaction between hydrogen clusters and edge dislocations emitted from a semi-elliptical surface crack tip in deformed nanometallic materials. The model's solution was obtained by using the complex method, and the influence of the concentration and location of hydrogen clusters, temperature, crack shape, material constants, and the dislocation emission angle on the critical stress intensity factor (SIFs) corresponding to the first dislocation emission from crack tips was investigated through numerical analysis. The results show that dislocations are easily emitted from the crack tip at high hydrogen concentration, and hydrogen clusters close to the crack tip will hinder the emission of dislocations from its crack tip. When considering the influence of hydrogen cluster, an increase in temperature, an extension of crack length or an increase in crack tip curvature radius can all make the emission of dislocations at the crack tip difficult, thereby reducing the toughness of the material caused by dislocation emission. [ABSTRACT FROM AUTHOR]

The metal surface exposed to hydrogen-containing environments can adsorb hydrogen atoms, which can affect the properties of the material and, in some cases, lead to hydrogen embrittlement. In this paper, the process of hydrogen diffusion from α-Fe free surface to the crack surface and the effect of surface hydrogen on crack propagation are mainly studied by molecular dynamics. The results show that at room temperature, it is difficult for the hydrogen atoms adsorbed on the surface to diffuse directly from the free surface to the interior to become dissolved hydrogen; the hydrogen atoms on the free surface will gradually diffuse to the crack surface, and finally concentrate on the crack surface and the free surface. When the surface hydrogen concentration is low, hydrogen has little effect on crack propagation, but when the hydrogen concentration on the crack surface reaches a certain concentration, it will lead to unstable crack propagation, resulting in serious hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

• Hydrogen has a strong pinning effect on dislocations near the crack tip. • The effect of the interaction between hydrogen and dislocations on the fracture mode transition of nanomaterials is investigated. • The presence of hydrogen reduces the fracture toughness of nanocrystalline materials and promotes brittle fracture of materials. • Hydrogen inhibits crack passivation and promotes crack growth. A lot of experimental and theoretical work has been done on the traditional macroscopic mechanism of hydrogen embrittlement (HE). There are a lot of theoretical studies on the micro-mechanism of HE recently published. In this article, based on the discrete dislocation method, taking into account the interaction between hydrogen atoms fixed in the defects of crystal materials and dislocation, a theoretical calculation method for the fracture of hydrogen-induced nanocrystalline (NC) materials is proposed. Considering the influence of hydrogen in the dislocation emission (DE) model, the theory of hydrogen inhibiting dislocation emission is put forward under the stable equilibrium state, which restrains the passivation of cracks and intensifies brittle fracture. At the same time, the pinning effect of hydrogen on dislocation is studied by theoretical method. In this paper, the relationship between the grain size and dislocation number in hydrogen free environment and hydrogen environment is described. The results show that the critical stress intensity factor (SIF) of crystal materials in hydrogen environment decreases by about 18.6%, indicating that hydrogen promotes the fracture of crystal materials. This paper provides a theoretical study for understanding the fracture behavior of hydrogen-induced materials. [ABSTRACT FROM AUTHOR]

Based on the theory of fracture mechanics, a hydrogen brittleness model of microcrystalline Al materials is proposed by using the distributed dislocation method. The distinguishing feature of this study is that the model can be used to clarify the dependence of hydrogen atoms at different positions and different numbers of hydrogen atoms on dislocation emission in the initial stage of corrosion fracture. At the same time, the mechanism of crack nucleation at the grain boundary in the hydrogen environment is studied. The results indicate that hydrogen can facilitate the emission, proliferation and motion of dislocations at the crack tip, and increase the accumulation energy of dislocations at the grain boundary, making it easier to generate micro-cracks. In the corrosive environment with hydrogen, a large number of hydrogen atom segregation at the grain boundary reduces the surface energy, and cracks are more likely to occur, and the length of microcracks is longer than that in the absence of hydrogen. These studies are helpful to deepen the understanding of the hydrogen embrittlement of materials. • The grain boundary has a certain inhibitory impact on the dislocation emission at the crack tip. The smaller the grain size, the less likely the dislocation is emitted. • In the hydrogen environment, the distance between the hydrogen atom cluster and the crack tip has a more remarkable effect on dislocation emission and crack nucleation than the radius of the hydrogen atom cluster. • In hydrogen environment, hydrogen can reduce the surface energy of metal materials, and it is easier to produce stable and balanced cracks at grain boundaries. [ABSTRACT FROM AUTHOR]

• In hydrogen environment, the interaction between hydrogen and internal defects of materials intensifies the transformation of fracture mode of metal materials. In particular, when hydrogen accumulates near the crack tip, the grain size increases from 10 nm to 100 nm, and the critical strength factor decreases by about 11%. • In hydrogen environment, hydrogen atoms enter into the crystal material and accumulate at the crack tip. As the number of hydrogen atoms increases, the interaction force between them and dislocation becomes larger, thus increasing the effective stress on dislocation source, further promoting the emission of dislocation at the crack tip, and promoting the proliferation and movement of dislocation in the grain. • In hydrogen environment, hydrogen will enlarge the plastic zone before crack nucleation. In this study, when the distance between hydrogen atom and dislocation source is >6 nm, the plastic zone change in hydrogen environment becomes smaller and smaller, even can be ignored. • In the hydrogen environment, the distance between the dislocation source and the hydrogen cluster has a greater effect on the dislocation emission than the change of the radius of the hydrogen cluster. A new hydrogen embrittlement fracture model is proposed based on the discrete distributed dislocation method. The effect of hydrogen atoms distribution at the crack tip on dislocation emission before micro-crack nucleation in the early stage of corrosion fracture is investigated. The results show that hydrogen can promote the emission, diffusion and movement of dislocations, increasing the plastic zone at the crack tip. In the hydrogen environment, the critical stress intensity factor of the crystalline material decreases by a maximum of about 11%. Within the theoretical framework of this model, it is believed that hydrogen atoms accumulated closest to the crack tip will promote the emission of dislocations from the crack tip, thereby suppressing passivation of the crack while promoting brittle fracture of metallic materials. These results are helpful to analyze the hydrogen embrittlement fracture behavior of the material. [ABSTRACT FROM AUTHOR]

Abstract: The embrittlement of conventional metallic systems by hydrogen is a well documented phenomenon. However, the precise role of hydrogen in this process for nanocrystalline materials is poorly informed and comprehensive theoretical models are not available yet. Here, a new model is proposed wherein hydrogen atoms accumulate before a nanocrack tip and interact with piled up dislocations ahead of dislocation free zone (DFZ) actively. The interaction between hydrogen atoms and dislocations can prevent dislocations emitting from nanocrack tip, and thus suppressing nanocrack tip blunting and ductile fracture while promoting brittle failure. In addition, the size of DFZ in nanograins was analyzed from the macro–micro fracture mechanics point of view and the relative computing method was derived. The dependence of maximum number of dislocations emitted from nanocrack tip on grain size with and without hydrogen in nanocrystalline Ni is clarified and compared. The results show that the introduction of hydrogen into nanocrystalline materials gives rise to a reduction in critical crack intensity factor more than 30% in contrast with hydrogen free case, and this special fracture behavior driven by hydrogen atoms is especially remarkable with the reduction of grain size. [Copyright &y& Elsevier]

Abstract: Experimental and analytical studies have shown that the presence of hydrogen enhances dislocation emission. However, the effect of hydrogen on dislocation emission is not well understood. In this study, we investigated the effect of hydrogen on dislocation emission from a mode II crack in alpha iron using an atomistic model. We found that hydrogen decreases the stacking fault energy of alpha iron, resulting in an enhancement of dislocation emission. It is clarified that hydrogen atoms existing within a few angstroms of the crack tip only enhance the dislocation emission. We also confirmed that dislocation emission enhancement can occur at realistic hydrogen concentrations of hydrogen gaseous condition under thermal equilibrium conditions. [Copyright &y& Elsevier]

The history of “unified” mechanisms for liquid–metal embrittlement, hydrogen embrittlement and stress-corrosion cracking is outlined, and issues and concerns regarding a recent paper [H.W. Liu, Acta Materialia 56 (2008) 4339–4348] on a unified model of environmentally assisted cracking (EAC) are discussed. [Copyright &y& Elsevier]