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Based on the method of continuous distribution of dislocations and relevant theories of fracture mechanics, the effect of the distribution of hydrogen atoms at the crack tip (CT) on the emission of dislocations from kinked cracks under far-field uniform tensile stress is studied. The results show that hydrogen atoms can promote the emission and migration of CT dislocation, which leads to the increase of dislocation density (severe dislocation plugging), the increase of energy release rate, and the decrease of elastic zone size (dislocation-free zone) at CT. In addition, the sequence of initiation of grain boundary (GB) microcracks caused by dislocation emission at both ends of the whole crack (horizontal section and kinked section) and the propagation mechanism of a crack in a kinked section are analyzed. The results show that the critical tensile stress of GB microcrack initiation decreases with the increase of hydrogen atom radius rh and the decrease of hydrogen atom position xh , indicating that hydrogen atoms promote the initiation of GB microcrack initiation. At the same time, the existence of hydrogen atoms not only promotes the initiation of GB microcrack, but also accelerates the expansion of kinked cracks, which may eventually lead to the convergence of the main crack and the microcrack, resulting in the failure and destruction of materials. Therefore, the theoretical research in this paper has made a new understanding of the mechanism of hydrogen fracture of crystal materials. Practical Applications: In this paper, the effects of hydrogen atoms at CT on dislocation emission at CT, crack initiation at GB, and the size of the dislocation-free zone were studied by establishing a crack dislocation model and using the distributed dislocation method. The results show that the accumulation of hydrogen atoms at the CT promotes the emission of dislocation at the CT, leads to the serious plugging of dislocation near the GB, promotes the initiation of microcracks and the expansion of kinked cracks, and leads to the reduction of the size of the dislocation-free zone at the CT. This work provides new information on the microscopic fracture mechanics of hydrogen on materials. On the one hand, it can better predict and prevent the fracture of materials, so as to improve the safety of engineering structures. On the other hand, it can help explain some microscopic experimental phenomena. [ABSTRACT FROM AUTHOR]

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]

The influence of material micro-defects on the main crack growth under pure shear loading is studied theoretically. The mechanism behind the initiation of micro-cracks and crack propagation induced by dislocation accumulation near the grain boundary (GB) is mainly considered, and the influence of dislocation accumulation on the main crack propagation is analyzed. The research results reveal that the initiation of micro-cracks near the GB is prior to the propagation of the main crack. In a hydrogen environment, hydrogen can cause serious embrittlement of the crack tip and promote crack growth. The energy release rate in the main crack growth direction in the dislocation emission direction is the highest. Therefore, the main crack will eventually merge with the micro-cracks at the GB along the direction of the slip band, resulting in fracture of the crystal material. The research presented in this paper provides some new information for the first stage of crack propagation and contributes to the analysis of the mechanism of crystal metal fracture. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations have unique advantages in capturing the details of the crack tip deformation at the atomic scale. As the accuracy of simulation results is heavily dependent on the selected interatomic potential, a comprehensive study of the anisotropic crack tip behavior of hcp titanium under plane strain mode-I loading with the commonly utilized potentials is presented in this paper. The typical crystallographic planes in hcp materials including {0001}, {101¯0}, {112¯0}, {101¯1} and {112¯2} are considered as the pre-existing crack planes. The intrinsic brittleness and ductility of different crack systems are discussed from the perspective of linear elastic fracture criteria. Depending on the selected potential, the dominant crack tip deformation mechanism may be basal a slip, prismatic a, pyramidal c+a slip or twinning modes in different crack systems. Finally, in comparison with experimental results, the different potentials are evaluated in the light of their capability to reproduce the experimentally observed crack tip deformation. Overall, the recent developed Mendelev-II potential is recommended for MD simulation of fracture behavior under mode-I loading, as this potential gives a more realistic crack tip response than other studied potentials.

An impact analysis model is built to describe the effect of nanoscale amorphization on dislocation emission from a surface semi-elliptical crack tip in nanocrystalline materials. The nanoscale amorphization is formed by the splitting transformation of grain boundary(GB)disclinations caused by the motion of GBs. The analytical solution of the model is obtained by using the complex method, and the influence of nanoscale amorphization, dislocation emission angle, crack length, and curvature radius of surface crack tip on the critical stress intensity factor (SIF) of the first dislocation emission is investigated through numerical analysis. The numerical analysis shows that the impact of nanoscale amorphization on the critical SIF corresponding to dislocation emission depends on the dislocation emission angle, the position and the size of the nanoscale amorphous, the curvature radius, and the length of surface crack. As the curvature radius of surface crack tip and the crack length increase, the normalized critical SIF increases. When the nanoscale amorphization size is small, it has a great impact on the critical SIF for dislocation, but when the size is relatively large, the effect becomes small. The effect of the increasing strength of the nanoscale amorphization on dislocation emission from the surface crack tip is related to the distance between the nanoscale amorphization and the crack tip, and there is a critical crack-junction for which the increase of dislocation strength has little effect on dislocation emission. [ABSTRACT FROM AUTHOR]

An exploration of dislocation microstructure evolution with different misorientation angles was performed using phase field crystal method (PFC). The microcosmic evolution process of grain boundaries under external stress, as well as the corresponding energy curve and stress–strain curve, are analyzed. The relationship between the misorientation angle and the dislocations emission frequency is discussed. Three forms of dislocations reaction on the evolution process of 6° and 10° are analyzed in detail, which are respectively type I semi-annihilation, type II semi-annihilationand full-annihilation. Among them, the nature of type I semi-annihilation is a combination of dislocation and a single edge dislocation reaction with a single edge dislocation left. The essence of type II semi-annihilation is a pair of dislocation and the other pair of dislocation reaction leaving two edge dislocations. The essence of full-annihilation is that two pairs of dislocations or single edge dislocations with opposite Burger vectors react with each other and the distortion area disappears. When the misorientation angle is 10°, the dislocation reaction and the dislocation motion ability of the system are stronger than 6°. The peak of the energy curve is related to the number of dislocation proliferations in the evolution process. An emission frequency and average density of dislocations of 10° is greater than 6°. The causes of plastic deformation are revealed to a certain extent by stress–strain curves. [ABSTRACT FROM AUTHOR]

The study on the interaction between defects and dislocation emission is of significance to predict the behaviors of fracture and plastic deformation in crystalline metals; however, the multiscale interaction mechanism has not been well understood. In this work, based on molecular dynamics simulation and distributed dislocation technique, the interaction mechanism between dislocation emission and a crack or a void is studied at the atomic and continuum scales. The results show that dislocation emission can change the mode of crack growth, i.e., from brittle to ductile, and the emitted dislocations cause crack‐tip blunt and decrease stress concentration near the defects. The increment of void growth is completely contributed by dislocation emission, and it can be predicted by the characteristics of dislocation emission. This work reveals the influence mechanism of dislocation emission on failure behaviors and deepens the understanding of the interaction between dislocation emission and defects. Highlights: Interaction between defects and dislocation emission is analyzed at different scales.Dislocation emission changes crack growth mode and improves the resistance of fracture.Mechanism of void growth by dislocation emission is proposed based on MD simulations.Dislocation density and stress field are obtained by distributed dislocation modeling. [ABSTRACT FROM AUTHOR]

The onset of dynamic friction plays an important role in the study of sliding interfaces. Previously, the sliding precursors in the form of crack-like defects have been detected in experiments and their strain fields have been measured to be comparable to those of moving cracks. In the present work, we considered the dynamics of sliding precursors by solving the elastic problem due to a moving dislocation in a half-plane and the transient emission of a dislocation at the edge. It has been found that both the strain field of a moving dislocation and the spatiotemporal evolution agree well with those of a sliding precursor detected in experiments. The results may cast new light to the dynamics of sliding onset. [ABSTRACT FROM AUTHOR]

• Rice model is used to predict intrinsic cleavage crack resistance in BCC Fe. • Cube, goss and rotated goss are detrimental for low temperature fracture toughness. • Presence of γ-fibre texture effectively enhances the intrinsic crack resistance. • Selective components of α-fibre and ε-fibre are also beneficial for crack resistance. Based on the classical Rice model of crack tip dislocation emission, the present article provides a theoretical prediction of suitable grain orientations (texture components) in a rolled polycrystalline BCC Fe plate which can improve the overall resistance to cleavage crack propagation. Besides, the beneficial effect of alloying additions on further improvement in the intrinsic crack resistance of the steel is also discussed pertaining to the role of unstable stacking fault energy. [ABSTRACT FROM AUTHOR]

An exploration of dislocation microstructure evolution with different misorientation angles was performed using phase field crystal method (PFC). The microcosmic evolution process of grain boundaries under external stress, as well as the corresponding energy curve and stress–strain curve, are analyzed. The relationship between the misorientation angle and the dislocations emission frequency is discussed. Three forms of dislocations reaction on the evolution process of 6° and 10° are analyzed in detail, which are respectively type I semi-annihilation, type II semi-annihilationand full-annihilation. Among them, the nature of type I semi-annihilation is a combination of dislocation and a single edge dislocation reaction with a single edge dislocation left. The essence of type II semi-annihilation is a pair of dislocation and the other pair of dislocation reaction leaving two edge dislocations. The essence of full-annihilation is that two pairs of dislocations or single edge dislocations with opposite Burger vectors react with each other and the distortion area disappears. When the misorientation angle is 10°, the dislocation reaction and the dislocation motion ability of the system are stronger than 6°. The peak of the energy curve is related to the number of dislocation proliferations in the evolution process. An emission frequency and average density of dislocations of 10° is greater than 6°. The causes of plastic deformation are revealed to a certain extent by stress–strain curves.

Void growth, as a critical stage in ductile fracture, could play some important role in the ductility of high entropy alloys (HEAs). Under a high-strain-rate tension (strain rate: 0.1~0.4/ns), our atomistic simulations reveal a severely suppressed growth of void (initial diameter: 4.0~9.1 nm) in HEAs at a low temperature, compared with that in Ni metal or Ni-based binary alloys. The emission of unclosed glide loops is found critical to void growth. Void growth is suppressed, once dislocation emission is suppressed due to the solute drag effect. Regarding the alloying elements, we find the void suppression relevance as Fe>Co and Cr>Fe>Mn. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The interaction between nanotwin and dislocation pileup at twin boundary and interface cracks are of vital importance in studying the toughening mechanism of nanocrystalline materials. We have developed a theoretical model to discuss the effects of nanotwin and dislocation pileup on dislocation emission from an interfacial collinear crack tip in nanocrystalline bimaterials. The nanotwin as a stress source can be descripted by two disclination dipoles, and some dislocations produced by the grain boundary pile up at twin boundary. Using the complex potential method, the complex form of dislocation force and critical stress intensity factors (SIFs) corresponding to dislocation emission under remote plane loadings are deduced. Through numerical analysis, it is discussed that the effects of twin strength, twin orientation, twin size, twin position, dislocation pileup, crack length, and material constant on the critical SIF corresponding to dislocation emission in detail. The results show that the nanoscale twin and the dislocation pileup at the twin boundary will hinder dislocation emitted from the collinear crack tip and reduce toughness caused by dislocation emission. There is a critical nanotwin size making the critical SIF equal to zero and a best twin position minimizing the value of the critical SIF making dislocations emission easiest. The shear modulus of the two half plane have great influence on the critical SIF. [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]

• In a first effort, dislocation structure and mass transfer of a macroporous metal is studied. • The mass transfer is due to emitted multiple shear loops. • The critical stress for dislocation emission is in good agreement with the Lubarda model. • The critical stress of void with a short ILD is found insensitive to strain rate. In a first effort, the dislocation structure and related mass transfer in a macroporous FCC metal is studied via atomistic simulation. The interaction between the void and its periodic image has been intense ever since the onset of dislocation emission. The accumulated mass transfer is due to the emission of multiple shear loops, which shapes the morphology of the macroporous metal. The removal of mass from the intervoid ligament triggers its breakdown, connecting the voids. The strengthening of intervoid interaction is shown by running multiple cases with atom count ranging from 7.1 million to 95.4 million. The critical stress for dislocation emission versus the size of void is in a good agreement to the Lubarda model adjusted by the Gibson-Ashby scaling law. Nevertheless, the critical stress of void with a short intervoid ligament distance (ILD) demonstrates a surprisingly insensitivity of strain rate compared with the void that can be deemed standalone. [ABSTRACT FROM AUTHOR]

• Effect of nanoscale amorphization on dislocation emission from a blunt crack is studied. • Nanoscale amorphization occurs through splitting transformation of GB disclinations. • The criterion for the first dislocation emission form the elliptical blunt crack is derived. • Nanoscale amorphization can either enhance or weaken the critical applied SIFs. • There is a critical crack-junction distance making the dislocation emission most difficult. A theoretical model is established to describe the effect of nanoscale amorphization on the lattice dislocation emission from an elliptical blunt crack tip in deformed nanocrystalline and ultrafine-grained materials. Within the description, the nanoscale amorphization occurs through the splitting transformation of grain boundary disclinations which are produced by grain boundary sliding. The criterion for the first dislocation emission form the elliptical blunt crack is derived. The influence of the nanoscale amorphization and the features of the elliptical blunt crack on the critical stress intensity factors for the dislocation emission are discussed in detail. The obtained results show that the nanoscale amorphization can either enhance or weaken the critical applied SIFs for dislocation emission, depending on the emission angle, the radius of curvature of the elliptical blunt crack and the distance between the nanoscale amorphization and the crack tip. There is a critical crack-junction distance making the dislocation emission most difficult and at this critical crack-junction distance, the stress-release effect of the nanoscale amorphization is strongest. [ABSTRACT FROM AUTHOR]

Extensive experiments and molecular dynamics simulations have shown that stress-driven grain growth can greatly contribute to enhanced dislocation emission and tensile ductility in nanocrystalline metals. However, the underlying mechanism behind this correlation remains unclear. In this work a theoretical model based on the cooperative nano-grain rotation and grain-boundary migration for grain growth is proposed to explain the enhanced dislocation emission from grain boundaries. Two partial dislocation dipoles are taken to emit from opposite grain boundaries. It is demonstrated that the joint, coupled growth behavior can indeed be achieved. The energetic characteristics and the critical stress of the emission process are analyzed. We found that under some circumstances dislocation emission is energetically unfavorable when pure nano-grain rotation or migration is operating. However, the dislocation emission process can be turned to become energetically favorable when their cooperative grain growth dominates the deformation. Moreover, the critical stress required to initiate the emission can be considerably reduced by enhancing the level of rotation, and minimized by tuning the coupling factor of the migration process. As a result, the cooperative grain growth by nano-grain rotation and grain-boundary migration can serve as a new route to enhanced dislocation emission and improved tensile ductility in nanocrystalline materials. [ABSTRACT FROM AUTHOR]

A theoretical model is established to investigate the effect of martensitic transformation particle on the dislocation emission from a crack tip in ceramic-matrix nanocomposites. Using the model of dislocation-based strain nucleus and the Green's function method, the expressions of complex potentials and stress fields are derived in closed form. The critical stress intensity factors for the first-lattice dislocation emission and the maximum number of emitted dislocations are well calculated. The effects of important parameters such as the size of transformation particle, the dislocation emission angle and the distance from the crack tip to the transformation particle on dislocation emission are discussed in detail. The results reveal that the transformation particle shows a significant shielding effect on the dislocation emission from the crack tip, and the shielding effect enhances with an increase in the size of transformation particle. On the other hand, the results also imply that the emission of edge dislocations is closely related with the dislocation emission angle, and there exists a probable angle | θ | ≈ 74 ∘ making the dislocation emission easiest. Besides, the remarkable crack blunting induced by the dislocation emission is quite difficult for small grain size but easy for the growth of crack. [ABSTRACT FROM AUTHOR]

Abstract An XFEM-based DDD scheme is developed to study multiple-dislocation emission from the crack tip and crack propagation in the ductile fracture of a single crystal. A dislocation emission model based on Rice-Thomson theory is incorporated to capture the dynamic dislocation emission from the crack-tip and a normal traction-separation distance model to depict the propagation of mode I cohesive crack. The boundary value problem containing complex surfaces/interfaces and discrete dislocations can be solved by the present XFEM-based DDD scheme directly in a unified framework with satisfactory accuracy. A careful examination shows that the local stress field at the crack tip induced by emitted dislocations can be exactly depicted by this XFEM-based DDD scheme. This local stress field can not only shield the crack from the applied load but also inhibit the emission of subsequent dislocations from the crack-tip; therefore, it plays a crucial role in the ductile-to-brittle competition during crack propagation. After careful verification, this XFEM-based DDD scheme is used to simulate dynamic propagation of mode I cohesive crack. The ductile-to-brittle competition in the crack-tip process zone is investigated with a special attention. Both the dislocation emission-crack propagation process within the crack-tip zone and the discrete dislocation dynamics details behind it are captured and show good agreements with the previous MD simulations conducted by other researchers. Highlights • A XFEM-based 2D-DDD scheme for crack propagation was developed. • The stress fields due to the emitted dislocation was captured accurately enough. • A crack-tip dislocation emission scheme was incorporated into the XFEM-based DDD scheme. • The dislocation dynamics mechanism of ductile-to-brittle transition was captured. • The competition between crack-tip dislocation emission and crack propagation was studied. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The kinetics of dislocations emitted from a crack is studied via molecular dynamics (MD) in a 3D bcc iron crystal. The atomistic results show that edge dislocation segments in the middle of the crystal accelerate at the nearest vicinity of the free crystal surface. The dislocations in MD penetrate the surface layers in transonic or supersonic regimes. The possible sources for such behavior are discussed in the framework of continuum models and using stress calculations on the atomistic level. Acoustic emission patterns arising from the fast dislocation motion in MD are visualized via the local kinetic energies of individual atoms and further modeled as a moving source of the stress waves in the anisotropic continuum. [ABSTRACT FROM AUTHOR]

• Based on the discrete dislocation method, the corrosion fatigue crack propagation model in nanocrystal materials is established. • When the distance between the hydrogen atom group and the crack tip is 2 nm, the crack growth rate in the hydrogen environment is increased by about 45% compared with the air environment. • The grain size can affect the roughness of the material surface, leading to a significant impact on the FCP rate. The larger the grain size, the rougher the material surface, leading to an accelerated crack propagation rate. • The change in the distance x h between hydrogen clusters and crack tips has a smaller impact on fatigue crack propagation than the change in hydrogen cluster radius r h. In this article, the effect of the external corrosion environment on dislocation emission is introduced, and the microscopic mechanism of fatigue crack propagation (FCP) in a hydrogen environment is studied. Meanwhile, based on the discrete distributed dislocation method, the corrosion FCP model of nanocrystal materials is established. In the framework of our model, the FCP is mainly determined by the irreversibility of the crack tip dislocation. The research results indicate that the aggregation of hydrogen atoms at the crack tip (CT) promotes the dislocations emission and increases the FCP rate. Meanwhile, considering the application of materials in different hydrogen circumstances, the effects of changes in the distance between hydrogen atoms and the crack tip and the radius of hydrogen atoms on crack propagation rate are studied. The results show that the accumulation of hydrogen atoms at the crack tip promotes the fatigue crack growth rate in nanocrystalline materials. Compared with the air environment, the maximum increase in crack propagation rate is about 45% under the action of hydrogen environment. Meanwhile, increasing the grain size and dislocation emission angle helps to increase the crack propagation rate. The research in this paper provides a theoretical basis for evaluating the corrosion fatigue life of metal materials. [ABSTRACT FROM AUTHOR]

The Frank-Read model, as a way of generating dislocations in metals and alloys, is widely accepted. In the early 1960s, Li proposed an alternate mechanism. Namely, grain boundary sources for dislocations, with the aim of providing a different model for the Hall-Petch relation without the need of dislocation pile-ups at grain boundaries, or Frank-Read sources inside the grain. This article provides a review of his model, and supporting evidence for grain boundaries or interfacial sources of dislocations, including direct observations using transmission electron microscopy. The Li model has acquired new interest with the recent development of nanomaterial and multilayers. It is now known that nanocrystalline metals/alloys show a behavior different from conventional polycrystalline materials. The role of grain boundary sources in nanomaterials is reviewed briefly.

Highlights • Nanotwin decrease toughness of materials produced by dislocation emission. • Crack blunting increase the critical threshold for dislocation emission. • Nanotwin size has great effect on the critical SIF for dislocation emission. • There is a best location of nanotwin for dislocation emission. Abstract The effects of the nanotwin and dislocation pileup at the twin boundary on lattice dislocation emission from a surface semi-elliptical blunt crack in deformed nanocrystalline materials (NCMs) are investigated. The nanotwin as a stress source can be described by a wedge disclination quadrupole. Using the complex variable method, complex analytic solution of stress field, the dislocation force and the critical stress intensity factors (SIFs) for the first edge dislocation emission were obtained. Then, through numerical calculation, the influence of twin size, twin orientation, twin position, twin strength and dislocation pileup, the curvature radius of blunt crack tip on the critical stress intensity factors were discussed in detail. The results show that the nanotwin and dislocation pileup has significant influence on the dislocation emission from the blunt crack tip. The presence of nanotwin and dislocation pileup will increase the most probable emission angle, and increase the critical SIFs for dislocation emission, making it harder for the dislocation to emit from the blunt crack tip and thus to decrease the toughness of materials produced by dislocation emission. [ABSTRACT FROM AUTHOR]

Experiments have shown that stress-driven grain growth is closely related to the enhanced crack growth resistance and the exceptional tensile ductility as observed in several nano-metals. However, the quantitative correlation remains unsolved. Here we developed a theoretical model to investigate the effect of nanograin rotation, one of the main modes of stress-driven grain growth, on dislocation emission from the tip of a semi-infinite crack in a nanograined solid. Our findings show that the nanograin rotation can significantly enhance the capability of the crack to emit dislocations, thus leading to strong crack blunting in nanomaterials. [ABSTRACT FROM AUTHOR]

The shock responses of crystalline Ni with two types of symmetric tilt grain boundaries (STGBs) are studied by large-scale molecular dynamics simulations, with special concerns on the effect of hydrogen segregated STGBs on them. The crystalline Ni with different kinds of H-free STGBs have significantly different plastic responses during the shock processes due to different mechanisms of dislocation emission from the STGBs. The segregation of hydrogen atoms at the STGBs significantly affects the dislocation emission from the H-segregated STGBs, and then heavily affects the shock plastic responses of crystalline Ni with H-segregated STGBs. On the one hand, dislocation emission from the H-segregated STGBs is delayed compared with that from the H-free ones at the low shock velocity, showing the so-called lagging effect. On the other hand, multi-mode dislocation emissions (or dislocation emission on the multi slip systems) are frequently activated from the H-segregated STGBs at the high shock velocity. In addition, the spallation behavior of the crystalline Ni under shock loading is also significantly affected by hydrogen segregation at the STGBs. In the targets with the H-free STGBs, spallation originates mainly from the interior of the grains due to strong interaction and reaction between dislocations there, however, it originates definitely from the H-segregated STGBs due to the reduced GB cohesive strength by hydrogen segregation. [ABSTRACT FROM AUTHOR]

The emission of dislocations from the tip of a newly transformed austenite lath, with a near Pitsch orientation relationship with the ferrite matrix, was observed at 760 °C in a duplex stainless steel, using in-situ transmission electron microscopy. The dynamics of dislocation loops with [111] b /2 Burgers vector were carefully analyzed. An estimation of stress concentration at the tip was made using dislocations as stress probes. These real-time observations verify directly for the first time that dislocation activity assists the growth of austenite precipitates, and provide quantitative data for revealing the stress field generated by interface migration. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations under different mechanical and thermal constraints are carried out with a nanovoid embedded inside a single-crystal, face-centred-cubic copper. The dislocation emission angles measured from MD plots under 0.1 K, uniaxial-strain simulation are in line with the theoretical model. The dislocation density calculated from simulation is qualitatively consistent with the experimental measurement in terms of a saturation feature. The ‘relatively farthest-travelled’ atoms are employed to reflect the correlation between the dislocation structure and the void growth. At a smaller scale, the incomplete shear dislocation loops on the slip plane contribute to the local material transport. At a larger scale, the dislocation structures formed by those incomplete shear loops further facilitate the growth of nanovoid. Compared to the uniaxial-strain case, the void growth under the uniaxial-stress is very limited. The uniaxial-strain loading results in an octahedron void shape. The uniaxial-stress loading turns the nanovoid into a prolate ellipsoid along the loading direction. In the simulation, the largest specimen contains 12 million atoms and the lowest strain rate applied is 2 × 106s−1. Under all the different thermomechanical constraints concerned, the formation of incomplete shear dislocation loops are found capable of growing the void. [ABSTRACT FROM PUBLISHER]

A theoretical model is developed that discusses the effect of nanoscale deformation twins on dislocation emission from the tip of slant edge crack in nanocrystalline materials. By combining complex variable method of Muskhelishvili and conformal mapping technique, the explicit solutions of complex potentials are obtained analytically. The critical stress intensity factors (SIFs) for the emission of first lattice edge dislocation from a slant edge crack tip are calculated. The effects of vital parameters such as the crack length, the inclined angle of the crack, the relative thickness of the nanotwin on critical SIFs for dislocation emission are evaluated in detail. The results show that the emission of lattice dislocation from the slant edge crack tip is significantly influenced by nanoscale deformation twins. The smaller the inclined angle is, the more difficult it is for dislocation emission from the slant edge crack tip. Particularly, the dislocation near the tip of slant edge crack is prone to emit when the inclined angle of the crack is about 45°. As a special case, when the inclined angle of slant edge crack is 0°, the present results will reduce to those of the problem of nanoscale deformation twins interacting with mode I straight crack. [ABSTRACT FROM AUTHOR]

We present results of 3D molecular dynamic (MD) simulations accompanied by stress calculations on the atomistic level in 3D bcc iron crystals with edge cracks ( 1 ¯ 1 0 ) [ 1 1 0 ] . Two different crack lengths in samples of SEN type loaded uniaxially in mode I were treated with negative and positive values of T-stress according to the continuum prediction by Fett for constant stress boundary condition. Atomistic results are compared with continuum predictions, including influence of T-stress on the ductile-brittle behavior of cracks. [ABSTRACT FROM AUTHOR]

A theoretical model is established to describe the effect of cooperative grain boundary (GB) sliding and migration on dislocation emission from the tip of branched crack in deformed nanocrystalline solids. The explicit solutions of complex potentials are obtained by means of complex variable method and conformal mapping technique. The critical stress intensity factors (SIFs) for the first lattice dislocation emission from the tip of branched crack are calculated. The effects of the lengths of branched crack and main crack, and the angle between their planes on the critical SIFs for dislocation emission are evaluated in detail. The results indicate that the emission of lattice dislocations from the tip of branched crack is strongly influenced by cooperative GB sliding and migration. When main crack approaches the branched crack, dislocation emission from the tip of branched crack will be suppressed. The main crack tends to propagate while shorter branched crack is prone to be blunted by emitting lattice dislocations from its tip. As a special case, when the planes of main crack and the branched crack are flattened out into one, the present results are in good agreement with previously known results. [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]

One major elementary process in ductile failure is the emission of a dislocation from the crack tip, the criterion for which is well established within the framework of fracture mechanics and involves the intensity of the stress singularity appearing near the crack tip. The continuum-media postulate of fracture mechanics, however, might prevent us from predicting the failure of nanoscale materials, such as ultrathin films and nanowires, because the continuum concept becomes less plausible as the size of the region of interest approaches the atomic scale. Here, we examine the applicability and lower size limit of continuum fracture mechanics at the nanoscale in the case of dislocation emission, by conducting computational simulations that were modeling nanometer-scale Cu specimens. We find that fracture mechanics works successfully for singular fields larger than a few nanometers, but fails for singular fields smaller than 1.5–2.0 nm, i.e., the lower size limit of fracture mechanics for dislocation emission. We further demonstrate that this lower size limit is determined by the atomistic nature of the structural instability, suggesting that the limit depends on atomic-level phenomena at the crack tip. [ABSTRACT FROM AUTHOR]

The interaction between a wedge disclination dipole and a linear crack of the surface in nanocrystalline materials is investigated. Utilizing the conformal mapping technique and the Muskhelishvili complex variable method, the explicit closed-form solutions are derived for complex potentials and stress fields. The critical stress intensity factors (SIFs) for the first dislocation emission from the crick tip are also calculated. The effects of the relative parameters on the normalized critical SIFs, such as the disclination strength, the edge dislocation emission angle, the dipole size, the distance between the crack tip and the center of the disclination dipole, the direction angle of the disclination dipole and the crack length, are discussed in detail as well. The results reveal that the shielding effect produced by disclination dipole increases with the increasing disclination strength, which means it can prevent the dislocation emission from the crack tip to decrease material toughness. [ABSTRACT FROM AUTHOR]

The effect of void interaction on the damage progression in aluminum under uniaxial tension is studied via molecular dynamics (MD) method. MD geometries containing two adjacent elliptical voids are created with different void shape combination and intervoid ligament distance (ILD). The dislocation emission and development, stress–strain relation, porosity accumulation, void shape evolution and ILD shrinkage are monitored during loading process. The critical stress required by the voids to trigger the dislocation emission is in line with the prediction of the Lubarda model under the uniaxial tension case. Our results indicate that the onset strain of dislocation emission is mainly subject to the void shape. The void shape combination is found more influential than the ILD on the stress response and the porosity accumulation. [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]

Quasi-continuum method is applied to investigate the anisotropic behavior of nanoscale adhesive contact when a Ni tip indents into and then pulls out of single crystal Cu substrates. For 1 ¯ 10 substrate, the Ni tip exhibits excellent brittle behavior. Interface fracture is mainly characterized by atomic rearrangement since few slip planes are available. For (001) substrate, Shockley partials and deformation twins are both observed during the indentation process. Interface fracture is dominated by atomic slip which leads to the ductile fracture of the interface. As for (111) substrate, the nanocontact leads to significant plastic deformation of the substrate and the piling up of Cu atoms as well as the wetting of the Ni tip. This is because the Ni tip and Cu substrate share the same crystal orientations and the misfit effect between them is less evident. These anisotropic adhesive behaviors illustrate the importance of choosing a proper crystal contact surface to prevent wearing in micro-electro-mechanical systems. Distinctive nanocontact-induced deformation mechanisms and fracture mechanisms are revealed and analyzed. We found that available slip systems should be paid enough attention to for adhesive contact at nanoscale. It is recommended that adhesive contact between the Ni tip and Cu substrate with the same orientations should be avoided in order to reduce wearing. Some of the results found in our work are also validated by similar or related experiments by other researchers. [ABSTRACT FROM AUTHOR]

Abstract: Thermal activation processes are of fundamental importance for the understanding and modeling the strength of structural materials. In this paper, the effect of thermal activation energy on dislocation emission from an elliptically blunted crack tip is researched. Critical stress intensity factors are calculated for an edge dislocation emission from an elliptically blunted crack under mode I and mode II loading conditions at high temperature. The results show that the impact of thermal activation processes is remarkable, the value of the critical stress intensity factor for dislocation emission decreases at high temperature, which means the applied loads for dislocation emission will decrease with increment of temperature. [Copyright &y& Elsevier]