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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 deformation mechanisms in the α-Fe twist bi-crystals (TBCs) containing differently angled twist grain boundaries (TGBs) are investigated carefully using the molecular dynamics modeling, with especial concerns on how solute hydrogen affects them. The results show that there are three main deformations in the TBCs, i.e. the dislocation glide-dominated mechanism, the twining-dominated mechanism, the dislocation glide and twining co-dominated mechanism, depending upon both the twist angle and the loading direction. In the dislocation glide-dominated TBCs, solute hydrogen increases the dislocation nucleation strength, dislocation mobility and dislocation density, further increases the vacancies concentration due to frequent interactions of solute hydrogen atoms with dislocations. In the dislocation glide and twining co-dominated TBCs, the solute hydrogen has weaker effect on the increase of dislocations density and the decrease of twins fraction with increasing tensile strain. However, in the twining-dominated TBCs, solute hydrogen assists the deformation twinning but doesn't increase significantly the vacancies concentration. So, it seems that twinning deformation is beneficial to resist hydrogen embrittlement (HE). These knowledge is helpful for us to understand the HE mechanism and develop new hydrogen-resistant high-strength materials. [ABSTRACT FROM AUTHOR]

Informed by the current understanding of hydrogen-dislocations/vacancies interactions, a coupled diffusional-mechanical model accounting for hydrogen-enhanced strain-induced dislocations and vacancies is developed. This model is applied to capture hydrogen diffusion and trapping in the uniaxially tensioned column and type-I loaded crack specimens of nickel alloy, with special attentions paid to the influence of hydrogen-vacancies interaction on them, which is seldom discussed in previous studies. The results show that the hydrogen-enhanced hardening rate and the hydrogen-promoted vacancy enrichment for the nickel alloys observed in experiments can be addressed well by the present model. The strain-induced vacancies rather than dislocations dominate the hydrogen trapping behavior, due to their higher hydrogen binding energies for the nickel alloy. The hydrogen-saturated state of vacancies can heavily impact hydrogen diffusion/trapping, which could be characterized by the effective trap concentration. The supersaturated vacancies caused by hydrogen-enhanced strain-induced vacancies can promote void growth through vacancy condensation, thus accelerating plastic localization or interface rupture. For the type-I loaded blunt crack specimen, the hydrogen-enhanced dislocation multiplication contributes to intergranular cracking in the pressure valley, while hydrogen-enhanced vacancy generation facilitates void growth in the plastic zone, both of which can exacerbate plastic localization ahead of crack tip. A quantitative model for plasticity localization that accounts for the influences of hydrogen-induced IG cracking and void growth would be helpful in physically based modeling of hydrogen embrittlement. [Display omitted] • A coupled diffusional-mechanical model accounting for hydrogen enhanced strain-induced dislocations and vacancies is developed. • The strain-induced vacancies rather than dislocations dominate the hydrogen trapping behavior in nickel alloys. • For type I loading blunt crack, the hydrogen-enhanced dislocation multiplication and vacancy generation exacerbate plastic localization ahead of the crack-tip. [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]

Due to trapping of hydrogen atoms to vacancy, vacancy-Hydrogen complex exists dispersively in materials working in hydrogen environment and influences significantly the dislocation mobility, rendering so-called hydrogen embrittlement. The interactions between a moving edge dislocation and vacancy-Hydrogen complexes or clusters in α - Fe are studied carefully in this paper by atomistic modelling. Our results show that vacancy and vacancy cluster are stable and easier to grow due to their lower formation energy and binding energy with the help of trapped H atoms. When approaching a vacancy-H complex, the edge dislocation is first attracted and in most cases pinned by the complex. The pinning strength of a vacancy-H complex or cluster on the edge dislocation increases with the increasing number of hydrogen atoms trapped in vacancy or vacancy cluster. The critical shear stress for an edge dislocation de-pinning from the vacancy-H complex can be described by a de-pinning equilibrium equation. The inherent pinning mechanism mainly originates from the migration of H atoms in vacancy or vacancy cluster when it is cut by the moving edge dislocation. These results will be helpful for further understanding of the hydrogen induced deformation and failure. [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]

Abstract A two-dimensional elastic solution for the stress field of a line dilatation near a circular inhomogeneity is obtained. A line dilatation is used to represent a row of solute atoms, referred to as a solute rod. The image effect is investigated with an especial attention. It is shown that a soft inhomogeneity attracts solute rods and a hard inhomogeneity repels solute rods. Based on this, the interactions of solute atoms with inhomogeneity-edge dislocation are studied by inserting more solute atoms into the matrix one-by-one, with an especial focus on the distribution of solute atoms (rods) around inhomogeneity and edge dislocation. As two typical applications of the present method, two simple cases are analyzed in details, with the inhomogeneity considered as a nano-void and a hard nano-fiber, respectively. It is shown that the equilibrium concentration distribution of solutes near an inhomogeneity is non-local, which not only depends on the local hydrostatic stress but also on the hydrostatic stress in the adjacent region. Besides, the shear stress on an edge dislocation, generated by hydrogen atmospheres between a circular nano-void/fiber and the dislocation, is calculated numerically. The results show that the hydrogen atmosphere around dislocations can shield the long-range elastic interaction between dislocations and other internal stress sources, supporting the so-called hydrogen enhanced localized plasticity. Graphical abstract Image 1 Highlights • A 2-D elastic solution for stress field of a line dilatation near internal stress sources is given. • The elastic inhomogeneity-dislocation-solute interaction is studied. • The solute atmospheres influences heavily the inhomogeneity-edge dislocation interaction. • The hydrogen enhanced localized plasticity mechanism is supported. [ABSTRACT FROM AUTHOR]

To study macroscopic mechanical response and microscopic deformation mechanism of nano-polycrystalline (NPC) Ta under shock compression, massive-scale non-equilibrium molecular dynamics (NEMD) simulations were systematically performed. The effects of grain size, shock strength on the wave structure, plasticity mechanism and flow stress were unveiled. Under the weak shock compression, as the grain size increases, a transition of plastic wave structure from single to double and then to single one was found, driven by the transition of dominant deformation mechanism from grain boundary (GB)-mediated plasticity to its coexistence with twinning and slipping and then to twinning and slipping, respectively. A transition from twinning-to slipping-preferred plastic deformation as the grain size exceeds ~30 nm was captured in the simulations and explained by a simple theoretical model proposed in this work. Under the strong and ultra-strong shock compressions, twinning-detwinning and amorphization-recrystallization were revealed as the dominant deformation mechanisms, respectively, which show weak grain size dependences. The flow stresses at the Hugoniot state were calculated, which follow the Hall-Petch relation under the weak and strong shocks but show complexity under the ultra-strong shock. These results can help understand intrinsic grain size and extrinsic shock strength effects on the mechanical and microstructural responses of Ta, as a key structural and typical BCC metal. [Display omitted] • Grain size dependent plastic wave structure is observed on shock profiles. • Transition of GB sliding-twinning-slipping as grain size varies is captured by atomistic and theoretical analyses. • Grain size effects on flow strength and underlying mechanisms are revealed by MD simulations. [ABSTRACT FROM AUTHOR]

Re doping is an essential way to improve the high temperature mechanical properties of nickel-based single crystal superalloy (NBSCS). However, how the doped Re affects the mechanical properties of NBSCS is still unclear, and a quantitative description is lacking. This paper attempts to study the influence of Re doping on plastic deformation mechanisms of NBSCS at atomic scale. First, the grand-canonical Monte Carlo method was employed to determine the distribution of Re atoms within the two-phase microstructure of NBSCS. Then, the molecular dynamics simulations were carried out to study the effect of Re doping on the dislocation motion and evolution during plastic deformation, with a focus on two important deformation mechanisms in NBSCS, i.e., dislocation gliding in the hairpin-like shape in the narrow γ matrix channel and dislocation cutting into the γ′ precipitation from the γ matrix. The results show that Re atoms prefer to segregate at the γ/γ′ interface. The increase of Re concentration in NBSCS can significantly improve both the critical stress of dislocations gliding in the γ matrix channel and the critical stress of dislocation cutting into the γ′ precipitation. These simulation results are qualitatively consistent with experimental observations. Based on these atomic scale simulations, two quantitative models for screw dislocation gliding in a hairpin-like shape in the γ matrix channel and dislocation cutting into the γ′ precipitation in the super-dislocation pair that take into account the Re doping effects have been proposed. The good agreement between the molecular dynamics simulation and the model prediction suggests that these quantitative models can be used for up-scale simulations of the dislocation movement and evolution in the presence of Re doping in NBSCS. • Re segregation near the γ/γ′ interface was obtained by GCMC calculation. • The critical stresses for leading dislocation cutting into the γ′ precipitation and dislocation gliding in a hairpin-like shape in γ matrix both increase with the increasing Re concentration. • Two theoretical models were proposed to describe the critical stresses of the dislocation mechanisms. [ABSTRACT FROM AUTHOR]

[Display omitted] • The strength asymmetry of four metallic nanopillars were investigated via MD simulation. • Both Schmid factor and non-Schmid factors have the negative contribution to the asymmetry. • The different surface energy reduction is one but not the only factor that results in the asymmetry. • The non-Schmid stress has positive influence on the asymmetry by changing the ideal shear strength. Nanomaterials or structures usually exhibit characteristic performance under complex stress states. The tensile and compressive behaviors of [0 0 1]-oriented single-crystalline nanopillars were studied, by performing molecular dynamic simulations on several typical FCC metals. For all those metallic nanopillars, their yield strengths for nucleating the initial dislocation show strong loading direction dependence, i.e., the strength under tension is higher than that under compression, showing the typical T/C asymmetry. The origins of the T/C asymmetry were investigated from the new aspects of the surface energy difference under tension and compression, the large ultimate elastic deformation, and the non-Schmid stress, in detail. The results indicate that both the Schmid factor and the non-Schmid factors change considerably due to the large elastic deformation under tension or compression, which contribute negatively to the T/C asymmetry. The difference in surface energy reduction due to the large elastic deformation is one but not the only factor that results in the T/C asymmetry. Although the non-Schmid factor contributes negatively to the T/C asymmetry, the non-Schmid stress can increase the difference of unstable stacking fault energies under tension and compression, which has a significant positive influence on the T/C asymmetry by changing the ideal shear strength of the slip plane. [ABSTRACT FROM AUTHOR]