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The strength, plasticity and ductility-brittleness transition of metals are often governed by various planar fault energies. Due to interactions between the solute hydrogen (H) and the planar faults, the planar fault energies are heavily affected in the H-charged metals. An accurate quantitative description of the H-affected planar fault energies is essential for us to understand correctly the H-induced plasticity mechanism in metals. In this paper, a reliable atomistic modeling method is s2uggested to calculate quantitatively the effect of solute H on four frequently-used fault energies. The computed results show that solute H can increase the unstable stacking fault energy but decrease the other three, and the fault energies are linearly related to the equilibrium hydrogen concentration up to 0.020. In addition, the generalized stacking fault energy curves of the H-charged Ni in the <112> and <110> directions are also computed to construct the γ -surfaces. Finally, the influence of pre-stress on the unstable and stable stacking fault energies is discussed in detail. These quantitative results are helpful to understand the dislocation/twin-dominated plastic mechanisms in the H-charged metals and to develop the H-affected discrete dislocation dynamic (DDD) and crystal plasticity (CP) algorithms. Image 1 • The effects of solute hydrogen on various planar fault energies are investigated. • The distribution and migration of solute H atoms play key role in fault energy calculation. • The relationship of fault energy and the H concentration is depicted, quantitatively. • The unstable and stable stacking fault energies can be influenced heavily by the pre-stress. [ABSTRACT FROM AUTHOR]

Highlights • The long-range interaction between solute hydrogen and crack is analytically studied. • The stress field of a dilatation line near an elliptical void in an infinite matrix is analytically obtained. • The solute atoms trapped ahead of crack tips have a total anti-shielding effect on crack tips. • A new possible mechanism of hydrogen embrittlement referred to as "hydrogen induced anti-shielding crack mechanism" is proposed. Abstract A two-dimensional elastic solution for the stress field of a dilatation line near an elliptical void in an infinite matrix is obtained. The dilatation line is used to represent a row of equally spaced solute atoms. This elastic solution is employed to study the shielding or anti-shielding effect of solute atoms near a finite length crack. The stress intensity factor of stress field induced by a dilatation line is analytically obtained. It is shown that whether solute atoms have a shielding or anti-shielding effect on the crack tips depends on the position of solute atoms. An equaling zero curve is defined, on which the solute dilatation line induces zero Mode I stress intensity factor at the crack-tip. It is found that the shape of equaling zero curve is independent of any model parameters. The shielding and anti-shielding effects of a lot of solute hydrogen (hydrogen atmospheres) are studied through an integral scheme. The hydrogen concentration field is assumed to equilibrate with external or internal stress fields. We consider two cases. One case is that an externally uniaxial load in the direction vertical to crack is applied. Another case is that there exists an edge dislocation near crack. Based on these studies, a new possible mechanism of hydrogen embrittlement is proposed, which can be referred to as hydrogen induced anti-shielding crack mechanism. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H) on the interfacial damage in Ni-based single crystal superalloy is investigated by utilizing the molecular dynamics (MD) method. Accompanying the motion of misfit dislocation networks on the γ / γ ' interphase, more vacancies can form on the γ / γ ' interface with higher pre-charged H concentration. With the same H concentration, hydrogen-enhanced vacancies can more easily form at a low temperature than a high temperature. Meanwhile, hydrogen can facilitate the reaction and dissociation of interfacial dislocation segments, aggravating subsequent damage of the interfacial dislocation networks. These results shall enrich our understanding on the hydrogen embrittlement of the Ni-based single crystal superalloy. [ABSTRACT FROM AUTHOR]

• A novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance, in which both the contributions of plastic strain and plastic strain gradient to hydrogen transport are considered. • The hydrogen transport flux driven by plastic strain gradient results in a significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. • The roles of grain refinement in hydrogen transport are clarified with the present model. Hydrogen transport by moving dislocations is one of the key mechanisms for hydrogen embrittlement, which is considered responsible for local hydrogen accumulation at preferential crack initiation sites. Although numerous experiments corroborate this mechanism, there are few theoretical models for it available. In the present work, a novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance. The hydrogen transport flux in the present model is divided into two parts: the first-order hydrogen transport flux driven by plastic strain and the second-order hydrogen transport flux driven by plastic strain gradient. With the calibrated parameters by fitting experimental results, the first-order hydrogen transport flux is negligible even for nickel with a low hydrogen diffusivity. In polycrystals, due to the intrinsic plastic heterogeneity induced by grain orientation mismatch, the second-order hydrogen transport flux results in significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. This dynamic segregation behavior is related to the evolution rate of geometrically necessary dislocations, depending on the grain boundary characters. The roles of grain refinement in hydrogen transport are clarified with the present model. This study shows the vital role of plastic strain gradient induced by grain boundary constraint in hydrogen transport, which is essential in understanding hydrogen migration and accumulation in deformed polycrystalline metals. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Two dislocation nucleation mechanisms, HDN and DDN, are observed during tensile deformation of bicrystal along different loading direction with and without H segregation. • When the DDN mechanism dominates, H segregation at the GB inhibits dislocation nucleation from GB. • When the HDN mechanism dominates, H segregation at the GB enhances or barely affects dislocation nucleation from GBs, depending on the loading direction. It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress. [ABSTRACT FROM AUTHOR]

[Display omitted] Hydrogen-affected dislocation motion and hydrogen-induced intergranular fracture play key roles in hydrogen embrittlement. The quantitative characterization of H-affected interaction between dislocation and grain boundary (GB) is of great significance to understand the underlying physics of hydrogen embrittlement, which is systematically studied here by hybrid Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The structural features of GBs are depicted through the structural unit model (SUM) and grain boundary dislocation model (GBDM). The mechanisms of interactions between dislocation and H-free/H-segregated GB are thoroughly investigated and classified. For the cases without H segregation at GB, dislocation nucleation from GB and dislocation gliding on GB are the fundamental mechanisms governing the dislocation-GB interaction. In contrast, for the cases with H segregation, the dislocation-GB interaction mechanism is changed owing to H-inhibited GB dislocation emission, dislocation transmission across GB and dislocation gliding on GB. Due to dislocation pile-up promoted by H segregation, crack initiation is facilitated at the H-segregated GB. These results can provide essential information not only for understanding H-induced intergranular fracture but also for developing an up-scaled discrete dislocation dynamics (DDD) simulation framework. [ABSTRACT FROM AUTHOR]

• A new dislocation-density based crystal plasticity framework with dislocation line energy incorporated explicitly is proposed. • The reduction of dislocation line energy due to hydrogen is depicted by Gibbs theory of absorption isotherm. • The experimentally observed hydrogen-reduced activation volume and total activation free energy in thermally activated forest intersection in face-centered cubic (FCC) metals can be attributed to the reduction of dislocation line energy in hydrogen environment. • Hydrogen reduced dislocation line energy also contributes to hydrogen-increased flow stress, hydrogen-enhanced dislocation multiplication, hydrogen-promoted heterogeneity of plastic strain and hydrogen-delayed exhaustion of mobile dislocations, according to FE simulations. Hydrogen-enhanced localized plasticity (HELP) has been recognized as an important mechanism for hydrogen embrittlement. Two recognized explinations have been proposed for HELP. The first one is the elastic shielding theory, in which the elastic stress field of solute hydrogen weakens the interactions between dislocations. Another one is the Gibbs theory of absorption isotherm: the lowered dislocation line energy by segregated hydrogen is considered as the main reason. In this work, a dislocation-density based crystal plasticity framework concerning the explicit incorporation of the dislocation line energy is proposed. The explicit consideration of dislocation line energy leads the way to the modelling of HELP mechanism according to the Gibbs theory of absorption isotherm. It is shown that the experimentally observed hydrogen-reduced activation volume and total activation free energy in thermally activated forest intersection in face-centered cubic (FCC) metals can be easily attributed to the reduction of dislocation line energy in the hydrogen environment. Finite element calculations of tensile tests for polycrystalline Pd-H and Ni-H alloys capture several important hydrogen-affected plasticity behaviors: hydrogen-increased flow stress, hydrogen-enhanced dislocation multiplication, hydrogen-promoted heterogeneity of plastic strain and hydrogen-delayed exhaustion of mobile dislocations, which can be all attributed to hydrogen reduced dislocation line energy in the present model. Besides, the influences of hydrogen-reduced vacancy formation free energy and stacking fault energy on thermal annihilation are also considered. However, the simulation results show that they are less important compared with hydrogen-reduced dislocation line energy. The present work is essential for further physically-based modeling of hydrogen distribution in polycrystals and hydrogen-induced damage and failure. Image, graphical abstract [ABSTRACT FROM AUTHOR]

[Display omitted] • H segregation can inhibit grain boundary (GB) migration, ensuring crack propagation along the GB. • H segregation promotes dislocation emission from the GB via H-induced atomistic distortion on the GB. • H-enhanced dislocation emission from GB and H-enhanced GB decohesion can significantly reduce the critical stress intensity factor of the crack propagation along GB. • With H segregation, the process of crack propagation along GB is dominated by alternate GB dislocation emission and GB cleavage. Hydrogen ingression in metals generally causes catastrophic failure. The H-induced fracture surface mostly exhibits intergranular feature, suggesting that grain boundary (GB) is the dominant crack propagation path. In this paper, the grand canonical Monte Carlo method was first performed to obtain equilibrated H distribution around various symmetric tilt grain boundaries (STGBs) in nickel. Then, atomistic simulations were conducted to study the influence of H segregation on the Mode I crack propagation behavior. In the absence of hydrogen, significant crack tip blunting and transgranular fracture are observed, implying a tendency toward ductile transgranular fracture. However, in the case of H segregation, it is found that dislocation emission from the crack tip is enhanced but the crack tip still remains highly sharp. Moreover, crack propagation is dominated by alternate GB dislocation emission and GB cleavage, implying a tendency of ductile–brittle alternating intergranular fracture behavior. The H-inhibited GB migration and H-enhanced GB dislocation emission are believed to be the key mechanisms governing the H-assisted crack propagation along the GB. [ABSTRACT FROM AUTHOR]

Vacancies can easily capture H atoms in metals, forming hydrogen-vacancy complexes/clusters. In this work, the hydrogen-vacancy interactions in CoCrFeMnNi high-entropy alloy (HEA) were studied with first-principles calculations based on the density functional theory (DFT). The special quasi-random structure (SQS) method was used to construct a chemically disordered HEA, and the effects of solute H atoms on the formation energy of monovacancy, the formation energy and binding energy of multi-vacancy cluster were calculated. It is found that an H atom prefers to occupy an octahedral interstitial site neighboring a vacancy and attracts the charge from the surrounding first-nearest neighbor atoms (e.g. Co, Ni, Fe or Mn atom, excluding Cr atom), weakening the stability of the atoms around the vacancy and reducing the vacancy formation energy in CoCrFeMnNi HEA. After introducing H atoms, the formation energies of both vacancy and vacancy cluster decrease in CoCrFeMnNi HEA, but they are still higher than those in pure Fe and Ni. In addition, the reduction of the binding energy of vacancies in CoCrFeMnNi HEA is much lower than that in pure Fe and Ni, and the binding energy of vacancies even increases in some cases. The results of the first-principles calculations indicate that the solute hydrogen atoms, although promoting vacancies, unfavorably combining vacancies into clusters to form micro-voids. This provides a good explanation for the good resistance to hydrogen embrittlement of CoCrFeMnNi HEA observed in experiments. [Display omitted] • The vacancy formation energy reduction in HEA was revealed by the preferred site and electronic structure of a trapped H atom. • The effect of trapped H on the formation and growth process of multi-vacancy clusters in HEA was found. • The experimental phenomenon of HE in HEA was explained by first-principles calculations on H-vacancy interactions. [ABSTRACT FROM AUTHOR]

Premature failure due to hydrogen has been widely observed in different metallic materials. How does hydrogen affect the competition between brittle cleavage and ductile fracture is an important scientific question to be addressed. The key to solve this problem is to quantitatively depict the interactions between hydrogen and various defects in metals. In the present work, four atomically-informed mesoscale models which depict quantitatively four widely used hydrogen embrittlement (HE) mechanisms are established by DFT or MD simulations, and then integrated into the XFEM-based DDD framework. By this multi-scale framework, hydrogen-induced intergranular fracture in FCC Al and Ni is investigated, with typical twin boundary (TB) and high angle grain boundary (HAGB) considered for comparison. Computational results show that the dominant HE mechanism in different metals depends on the GB type when multiple HE mechanisms coexist and interact with each other. Compared with the HAGB, the TB has better resistance to hydrogen embrittlemen. The adsorption-induced dislocation emission (AIDE) mechanism dominates the crack propagation along the TB in Al and Ni, while the hydrogen-enhanced decohesion (HEDE) mechanism dominates the crack propagation along the HAGB in Al and Ni. Compared with the HEDE and AIDE mechanisms, the other two mechanisms, i.e., the elastic shielding (ES) and the hydrogen-enhanced strain-induced vacancy (HESIV), have much weaker effect on the hydrogen-induced intergranular fracture, unless the hydrogen concentration is unusually high. These results are helpful for us to understand the complex physical mechanisms behind the hydrogen embrittlement phenomenon. [Display omitted] • Four atomically-informed models were integrated in XFEM-based DDD framework to depict hydrogen embrittlement mechanisms. • The twin boundary has better resistance to hydrogen embrittlemen than high angle GB. • The AIDE and HEDE mechanisms dominate the crack propagation of twin boundary and high-angle GB,respectively. • The ES and HESIV mechanisms have weaker effect on intergranular fracture, unless hydrogen concentration is unusually high. [ABSTRACT FROM AUTHOR]