Hydrogen embrittlement (HE) has been extensively studied in bulk materials. However, little is known about the role of H on the plastic deformation and fracture mechanisms of nanoscale materials such as nanowires. In this study, molecular dynamics simulations are employed to study the influence of H segregation on the behavior of intergranular cracks in bicrystalline α-Fe nanowires. The results demonstrate that segregated H atoms have weak embrittling effects on the predicted ductile cracks along the GBs, but favor the cleavage process of intergranular cracks in the theoretically brittle directions. Furthermore, it is revealed that cyclic loading can promote the H accumulation into the GB region ahead of the crack tip and overcome crack trapping, thus inducing a ductile-to-brittle transformation. This information will deepen our understanding on the experimentally-observed H-assisted brittle cleavage failure and have implications for designing new nanocrystalline materials with high resistance to HE.

It has been observed that coherent twin boundaries (CTBs) are resistant to hydrogen embrittlement (HE). However, little is known about the role of inclined twin boundaries in the H-related deformation and failure. Here we comprehensively investigate H segregation and its influence on the mechanical behaviour and deformation mechanisms of inclined Sigma 3 twin boundaries at inclination 0 degrees <=Phi <= 90 degrees using molecular dynamics simulations. Our results demonstrate that for Phi = 0 degrees CTB and Phi < 90 degrees symmetric incoherent twin boundary (SITB), the presence of H reduces the yield stress required for dislocation nucleation under uniaxial tension, while for inclined twin boundaries (0 degrees<Phi < 90 degrees), the yield stress increases with increasing H concentration. Under shear deformation, solute H increases the critical shear stress for the SITB and inclined twin boundaries ((0 degrees<Phi < 90 degrees). The underlying deformation mechanisms are directly associated with H-modified atomic structure and GB motion. These findings deepen our understanding of the HE mechanisms of inclined twin boundaries, and provide a pathway for designing materials with high HE resistance. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The three-dimensional (3D) graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties, while the preparation of uniform and stable 3D graphene structures remains a challenge. In this paper, using molecular dynamics simulations, we found that the nanotwinned copper (nt-Cu) matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression. The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation, resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity. The newly formed twin boundaries (TBs) in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range. These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains. Furthermore, we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage. This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene, which may expand the application of graphene in energy storage and catalysts.

Dislocation plasticity in the vicinity of grain boundaries (GBs) plays a critical role in H-induced intergranular failure. Their interaction mechanisms under H environment, however, remain largely unexplored. Here, the underlying interaction of a screw dislocation with [11¯0] symmetric tilt GBs was studied by using molecular dynamics simulation, with special concerns on the role of solute H in it. Our results show several interaction mechanisms including dislocation dissociation, transmission, nucleation and reflection, depending on different glide planes and GB structures. The presence of H tends to transform these reactions into ones involving dislocation absorption due to H-hindered GB migration and H-enhanced localised plasticity. Furthermore, it is quantified that solute H leads to an increase in energy barrier for dislocation-grain-boundary interaction. After dislocation absorption, the GB segregated with H atoms is activated to a more disordered atomic structure, which can be correlated to the crack nucleation and hence the ultimate fracture. These findings advance a mechanistic understanding on H-induced plasticity-mediated intergranular failure.

The embrittlement of metallic materials by hydrogen (H) segregation is widely observed, but not understood well on an atomic scale. In the present study, an atomistic investigation of H embrittlement of various grain boundaries (GBs) has been performed by mapping H segregation energy of trapping sites and examining the effect of H segregation on the decohesion of GBs. The simulation results show that under the equilibrium concentration of H atoms typical of embrittlement in Ni, in conjunction with local H diffusion process, the maximum reduction of tensile strength and fracture energy is 6.60% and 15.75% for Σ5 (210) ⟨100⟩ and Σ17 (530) ⟨100⟩ GBs, respectively. Inspired by experimental observations of the dislocation structures beneath intergranular failure features, further calculations reveal that the embrittling effect of H atoms in metallic materials can be largely facilitated by the boundary disruption and local stress state concentrated on the GB through the plasticity process. The findings directly provide a picture of H embrittlement arising from the cooperative action of H-induced plasticity and GB decohesion.

Graphene-based materials with a negative Poisson's ratio have numerous potential applications in various fields. However, the modification of graphene is cumbersome and may worsen the mechanical properties. The scale limitation and structural instabilities of suspended graphene are also unfavorable for practical applications. In the present study, we design several nanolayered graphene/Cu composites and investigate their tensile behavior using molecular dynamics simulations. The nanolayered composites exhibit an apparent auxetic behavior without any modification of graphene, as the graphene/Cu interface can significantly enhance the surface effect and lead to an earlier phase transformation of the Cu component. A simultaneous occurrence of a positive and negative Poisson's ratio can be achieved in an asymmetric composite due to the good blocking effect of graphene on two separated Cu films. Materials with simultaneous negative/positive Poisson's ratio have potential applications in scaffold design, where it is necessary to tune the magnitude and polarity of the Poisson's ratio in tissue engineering. Furthermore, we propose a composite consisting of alternating multilayer graphene and thin Cu films to overcome the scale limitation, whose negative Poisson's ratio persists when the total thickness exceeds 100 nm. It is found that the change in the absolute value of Poisson's ratio becomes smaller with an increase in total thickness. Graphene/Cu composites with only a slight deformation under external loading may be suitable for the fabrication of telecommunication cables, whose dimensions should remain unchanged when subjected to high hydrostatic pressure in the deep ocean.

The incorporation of graphene into nanotwinned copper is a promising approach to obtain a material with superior mechanical properties, while related research is rarely reported. In this study, we design several graphene/nanotwinned copper nanocomposites and investigate their compressive behaviors by molecular dynamics simulation. An unusual structural rearrangement is observed under compression at an atomic level. More specifically, graphene provides a supporting skeleton for the lattice rotation of nanotwinned copper under certain conditions, resulting in the annihilation of dislocations with the recovery of elasticity. Such a rearrangement process improves the strength and durability of the nanocomposites because of the intensive support provided by periodic graphene wrinkles and new twin boundaries. The interaction between graphene and nanotwinned copper matrix can be enhanced by decreasing twin spacing and introducing multilayer graphene. The symmetrical lattice orientation in the matrix, parallel graphene-twin boundaries, and a suitable compression direction collectively contribute to a perfect structural rearrangement.

The influence of solute H on the interaction between screw dislocations and vicinal twin boundaries in Ni bicrystals is investigated using molecular dynamics (MD) simulations. Several interaction mechanisms such as dislocation transmission, nucleation and reflection are reported for different glide planes and grain boundary (GB) structures. The presence of H tends to transform these interactions into ones involving dislocation absorption. This disorders the atomic structure of GBs and establishes a local stress state, which promotes the ultimate failure of GBs due to the formation of vacancies. These findings will deepen our understanding on the experimentally-observed H embrittlement in metallic materials.

By using large-scale molecular dynamics (MD) simulations, the influence of solute hydrogen (H) segregation into several typical [11¯0] symmetric tilt grain boundaries (STGBs) on the shear response and coupled grain boundary (GB) migration of bicrystals in α-Fe is systematically examined. Depending on our geometric model of coupling for [11¯0] STGBs in body-centred cubic (BCC) metals, two different coupling branches (〈100〉 and 〈111〉) are predicted and further validated by the MD results. Our atomistic simulations show that solute H atoms impede coupled GB motion irrespective of the GB structure, which mainly stems from the fact that H considerably modifies the local atomic structures of the GBs. At high H concentrations, the response of GBs to shear deformation changes from GB coupling to pure GB sliding. In addition, it is found that H can facilitate the vacancy generation via enhancing the interaction of GB dislocations within the framework of the H-enhanced localized plasticity (HELP) mechanism. Although H-vacancy clusters are formed by solute H combining with nucleated vacancies, they cannot grow larger owing to the migration of GBs with extensive dislocation plasticity. This directly prevents the occurrence of H embrittlement (HE). These findings deepen our overall understanding of the role of solute H in GB-mediated plasticity process (GB migration) of metallic materials and provide a possible path to designing new materials with high resistance to HE.

Real materials have structural defects that are normally brought in during the processes of manufacturing and storage and often have a structure with abundant grains, as well as being subjected to multi-directional force conditions. The study of temperature's effect on plastic deformation mechanisms in polycrystalline materials bathed by a multi-axial force is still very rare and not clear. Therefore, we conducted very large-scale molecular dynamics simulations to study the deformation and fracture behaviour of nanostructured polycrystalline Ni under a pre-existing external tensile hydrostatic stress with various temperatures. By characterizing the deformation and fracture mechanisms at an atomic scale, our results elucidate the effect of temperature on brittle versus ductile fracture behaviour by analysing the local stresses for void nucleation and crack propagation and the associated interplays of grain boundary, dislocation/twin and void/crack activities. The lower temperature results in a more brittle fracture manner. This is because the decreasing temperatures contribute to more sources of local stress concentrators for void/crack nucleation and propagation, and suppress the plastic deformation achieved by the activities of grain boundary, twin and dislocation. Our findings shed a light on a fundamental understanding of polycrystalline Ni metals subjected to complex working environments.

Hydrogen (H) embrittlement of metals is a common phenomenon but the exact atomic mechanisms responsible for H-induced plasticity process and ultimate failure are not clarified. In this work, the impacts of H on tensile deformation behaviour of different types of twist grain boundaries (TGBs) have been systematically studied by molecular dynamics (MD) simulations. Different deformation mechanisms are reported, depending on grain boundary types and bulk H concentrations, including easier dislocation nucleation due to the presence of H, H-enhanced dislocation dissociation, and H-induced slip planarity. The simulations indicate that the interactions between H-enhanced dislocation plasticity and TGBs play a crucial role in the ultimate fracture path. The decohesion of the TGBs is considerably promoted by the presence of H under conditions where dislocation accommodation and emission process on the TGBs causes the changes of grain boundary structures and local stress state. Our results advance a mechanistic understanding for experimentally-observed H embrittlement and provide a viable path to engineering microstructure with high resistance to H embrittlement in new materials.

The segregation of solute H has a profound effect on mechanical properties of grain boundaries (GBs). In this work, systematic molecular dynamics (MD) simulations have been performed to elucidate the influence of H environment on dislocation nucleation process and fracture response of different Ni GBs. The results show that the effect of H on dislocation nucleation depends strongly on GB structures. The presence of H decreases the yield stress required to nucleate dislocations from GBs with C and D structural units (SUs), while increases the yield stress of GBs with E SUs. This difference is attributed to different deformation mechanisms related to the dislocation nucleation from various GBs with H. By analysing the decohesion results, it is found that H has stronger embrittling effect on GBs with E SUs than C and D SUs, originating from the fact that H significantly elongates the Ni-Ni bonds around the open E SUs while slightly expands the compact C and D SUs. These findings present fundamental understanding on H embrittlement mechanism and provide mechanistic insights for engineering microstructure against H embrittlement in metallic materials.

Ten different simulation models are used to investigate the effects of content and chirality of graphene on the mechanical properties of nanolayered graphene/Cu composites. The increasing volume fraction of graphene can significantly enhance the Young's modulus and tensile strength of composites, but it leads to a lower yield strain. The negative Poisson's ratio of composites (NPRC) under uniaxial tension is observed at an atomic level. The corresponding mechanism is revealed by enhanced surface effect and inhomogeneous distribution of stress due to graphene-Cu interface. The strengthening mechanism of graphene is investigated by constraining effect in elastic region and blocking effect in plastic region. In a multi-layer structure, the middle copper film shows better tensile properties as its movement is constrained between two graphene layers. The blocking effect plays an important role in the interface interaction between graphene and dislocations. Graphene effectively blocks dislocations from penetrating through and restricts the synergistic movement of copper atoms.

Deformation mechanisms in nanotwinned face-centered cubic (fcc) materials have been extensively studied due to the successful fabrication of nanotwinned fcc materials and the advance in experimental techniques and atomistic simulations. However, less attention has been paid to nanotwinned body-centered cubic (bcc) materials despite that deformation twinning has been widely observed in bcc materials both in experiments and computer simulations. Here we investigate the mechanical behaviour of nanotwinned Fe with various twin spacing under tensile deformation as a function of the inclination angle between the twin boundaries (TBs) and the loading direction using large scale molecular dynamics simulations. Our simulations reveal that the twin orientation determines the deformation mechanisms of nanotwinned Fe. When the TBs are parallel or inclined by an angle smaller than 20° to the loading direction, the samples fracture in an almost brittle manner. When the TBs are inclined by a medium angle between 20° and 70°, TB migration takes over the role of the main deformation mechanism and two possible pathways accounting for the disappearance of TBs, namely twinning or detwinning are distinguished. When the TBs are inclined by an angle larger than 70° or nearly perpendicular to the loading axis, plastic deformation is dominated by abundant slip-twin interactions. The dynamic transition in deformation mechanisms is discussed based on Schmid factor analysis and generalised planar fault energy. Moreover, we systematically summarise the plausible slip-twin interactions in bcc materials and determine the energy barriers according the Frank rule. The dislocation reactions at the TBs are compared with experimental observations.

Molecular dynamic simulation was used to study the brittle versus ductile fracture behaviour in nanotwinned Ni at various temperatures. The simulation results show that three temperature regimes correspond to three different fracture behaviours: brittle, brittle-to-ductile transition and ductile. A dual fracture transition mechanism in nanotwinned Ni within a small temperature interval was observed: (1) local phase transformation and (2) ledge formation ahead of the crack tip induced a sharp fracture transition from brittle mode to ductile mode. Our simulation results reveal that the very rare double fracture transition mechanisms could be transformed quickly in nanotwinned Ni within a narrow temperature interval, suggesting a new interpretation of fracture and deformation of nanotwinned Face Centred Cubic metals. (C) 2017 Elsevier B.V. All rights reserved.

Nanotwinned materials exhibit a combination of high strength and good ductility which is attributed to the interactions between dislocations and twin boundaries. But no attempt has been made to explore the possibility for deformation twinning in nanotwinned face-centered cubic materials. Here we use large scale molecular dynamics simulations to elucidate the mechanical behaviour of nanotwinned Cu. We demonstrate that deformation twinning plays an important role in the deformation of nanotwinned Cu with specific twin orientations, in addition to conventional dislocation slip. Deformation twins are formed through the glide of Shockley partials on adjacent {1 1 1} slip planes and two twinning mechanisms are identified based on the arrangement of Shockley partials. The first mechanism involves the successive motion of Shockley partials of different types, named as double-Shockley partials, which forms unstable thin twin plates. The second process involves the successive passage of the same twinning dislocations on neighbouring slip planes, which forms stable deformation twins along one primary twinning system or symmetric twinning systems. The dislocation processes involved in the dislocation-twin reactions are analysed at atomic level. The orientation dependence of deformation twinning is discussed and compared with available experimental results. (C) 2017 Elsevier B.V. All rights reserved.

Grain boundary (GB) can serve as an efficient sink for radiation-induced defects, and therefore nanocrystalline materials containing a large fraction of grain boundaries have been shown to have improved radiation resistance compared with their polycrystalline counterparts. However, the mechanical properties of grain boundaries containing radiation-induced defects such as interstitials and vacancies are not well understood. In this study, we carried out molecular dynamics simulations with embedded-atom method (EAM) potential to investigate the interaction of Σ5(210)/[001] symmetric tilt GB in Cu with various amounts of self-interstitial atoms. The mechanical properties of the grain boundary were evaluated using a bicrystal model by applying shear deformation and uniaxial tension. Simulation results showed that GB migration and GB sliding were observed under shear deformation depending on the number of interstitial atoms that segregated on the boundary plane. Under uniaxial tension, the grain boundary became a weak place after absorbing self-interstitial atoms where dislocations and cracks were prone to nucleate.

To meet the spraying demand of personalized customized product,an automated spraying system was pro posed.To achieve the function of coordinate transformation module in system,a coordinate transformation method based on point cloud and image matching was built.The 2D image and 3D point cloud of a spraying object were acquired by camera and 3D scanner respectively,and the 2D point cloud image was obtained by projecting 3D point cloud.Through improving the canny operator,the closed edges of 2D image and point cloud projection image were obtained.The curvatures of each point in the closed edges were calculated and expanded to the form of curvature chain code to match images,and then the transformation of point cloud coordinate system and the robot coordinate system was achieved.The feasibility of the proposed method was verified with experiments.

Increasing demands for high-performance handling of fluids in oil and gas as well as other applications require improvements of efficiency and reliability of screw pumps. Rotor profile plays the key role in the performance of such machines. This paper analyses difference in performance of 2–3 lobe combination of twin-screw pumps with different rotor profiles. A-type profile formed of involute–cycloid curves and D-type formed of cycloid curves are typical representatives for 2–3 type screw pumps. The investigation is performed by use of a full 3-D computational fluid dynamics analysis based on a single-domain structured moving mesh obtained by novel grid generation procedure. The real-time mass flow rate, rotor torque, pressure distribution and velocity field were obtained from 3D computational fluid dynamics calculations. The performance curves were produced for variable rotation speeds and variable discharge pressures. The computational fluid dynamics model was validated by comparing the simulation results of the A-type pump with the experimental data. In order to get the performance characteristics of D-type profile, two rotors with D-type profile were designed. The first has the same displacement volume as A-type while the second has the same lead and rotor length as A-type but different displacement volume. The comparison of results obtained with two rotor profiles gave an insight on the advantages and disadvantages of each of them.