In the present work we have performed classical molecular dynamics modelling to investigate the effects of different types of force-fields on the stress-strain and yielding behaviours in ...semi-crystalline lamellar stacked linear polyethylene. To this end, specifically the all-atomic optimized potential for liquid simulations (OPLS-AA) and the coarse-grained united-atom (UA) force-fields are used to simulate the yielding and tensile behaviour for the lamellar separation mode. Despite that the considered samples and their topologies are identical for both approaches, the results show that they predict widely different stress-strain and yielding behaviours. For all UA simulations we obtain oscillating stress-strain curves accompanied by repetitive chain transport to the amorphous region, along with substantial chain slip and crystal reorientation. For the OPLS-AA modelling primarily cavitation formation is observed, with small amounts of chain slip to reorient the crystal such that the chains align in the tensile direction. This force-field dependence is rooted in the lack of explicit H-H and C-H repulsion in the UA approach, which gives rise to underestimated ideal critical resolved shear stress. The computed critical resolved shear stress for the OPLS-AA approach is in good agreement with density functional theory calculations and the yielding mechanisms resemble those of the lamellar separation mode. The disparate energy and shear stress barriers for chain slip of the different models can be interpreted as differently predicted intrinsic activation rates for the mechanism, which ultimately are responsible for the observed diverse responses of the two modelling approaches.
Display omitted
•All-atomic and coarse-grained atomistic modelling of lamellar separation in semi-crystalline polyethylene was performed.•A comparison of the yielding and stress-strain behaviours of explicit and united-atom modelling approaches was made.•The two approaches were found to predict to very different yielding and stress-strain behaviours.•The disparate response is explained by the widely different critical resolved shear stresses for chain slip of the models.
In the present work, we have used classical molecular dynamics and quantum mechanical density functional theory modeling to investigate the grain size-dependent thermal expansion coefficient (CTE) of ...nanocrystalline Cu. We find that the CTE increases by up to 20% with a gradually decreasing grain size. This behavior emerges as a result of the increased population of occupied anti-bonding states and bond order variation in the grain boundary regions, which contribute to the reduced resistance against thermally-induced bond stretching and dictate the thermal expansion behavior in the small grain size limit. As a part of the present work, we have established a procedure to produce ab initio thermal expansion maps that can be used for the prediction of the grain size-dependent CTE. This can serve as a modeling tool, e.g., to explore the impact of grain boundary impurity segregation on the CTE.
In the present work, we have evaluated the performance of different embedded atom method (EAM) and second-nearest neighbour modified embedded atom method (2NN-MEAM) potentials based on their ...predictive capabilities for modelling fracture in single- and bicrystalline tungsten. As part of the study, a new 2NN-MEAM was fitted with emphasis on reproducing surface, unstable stacking fault and twinning energies as derived from density functional theory (DFT) modelling. The investigation showed a systematic underestimation of surface energies by most EAM potentials, and a significant variation in unstable stacking and twinning fault energies. Moreover, the EAM potentials in general lack the ability to reproduce the DFT traction–separation (TS) curves. The shorter interaction length and higher peak stress of the EAM TS curves compared to the 2NN-MEAM and DFT TS curves result in one order of magnitude higher lattice trapping than for cracks studied with 2NN-MEAM. These differences in lattice trapping can lead to significant qualitative differences in the fracture behaviour. Overall, the new 2NN-MEAM potential best reproduced fracture-relevant material properties and its results were consistent with fracture experiments. Finally, the results of fracture simulations were compared with analytical predictions based on Griffith and Rice theories, for which emerging discrepancies were discussed.
Display omitted
•The performance of EAM and MEAM potentials is explored for fracture studies in tungsten.•EAM potentials systematically underestimate surface energies.•Notable effects of lattice/bond trapping are observed, particularly for EAM potentials.•Lattice trapping is found to correlate with inherent features of the traction–separation curves.
In the present work, the impact of phosphorus impurities on the grain boundary strength of nickel has been investigated by means of density functional theory (DFT) modelling. Owing to different ...outcomes and trends previously reported in the literature, it is unclear whether P is strengthening or weakening the Ni grain boundary. To address this issue, we utilize three different DFT based methods: the excess-energy approach, rigid grain separation, and Rice–Wang’s thermodynamic approach. The results show that the commonly used rigid model predicts P to have an increasing effect on the peak stress of Ni of up to 14%, as opposed to a reduction, which is indicated by the excess-energy approach. Employment of the Rice–Wang approach, on the other hand, displays a slight reduction in work of separation. The results show that the discrepancies between previous works can be attributed not so much to the physics of the system, but to the applied model, the partition scheme and the interpretation of the outcomes. This underlines the importance of a proper description of the fracture process, and shows that common simplifications can have a decisive impact on the observed trends.
•Ab initio fracture modelling of clean and P inhabited Ni grain boundaries.•The effect of P is either strengthening or weakening depending on applied model.•Rigid model is shown to have a strong impact on observed trends.
We study the stability of twin boundaries and slip in crystalline orthorhombic polyethylene by means of density functional theory (DFT), using a nonempirical, truly nonlocal density function, and by ...means of classical molecular dynamics (MD). The results show that, in accordance with experimental observations, there is a clear preference to chain slip over transverse slip for all considered slip planes. The activation energy for pure chain slip lies in the range 10–20 mJ/m2 while that for transverse slip corresponds to 40–280 mJ/m2. For the (11¯0)-slip plane the energy landscape is non-convex with multiple potential energy minima, indicating the presence of stable stacking faults. This suggests that dissociation of perfect dislocations into partials may occur. For the two low-energy twin boundaries considered in this work, {110} and {310}, we find that the former is more stable than the latter, with ground state energies corresponding to 8.9 and 28 mJ/m2, respectively. We have also evaluated how well the empirical MD simulations with the all-atom optimized potential for liquid MD simulations (OPLS-AA) and the coarse-grained united atom (UA) potential concur with the DFT results. It is found that an all-atom potential is necessary to partially capture the γ-surface energy landscapes obtained from the DFT calculations. The OPLS-AA predicts chain slip activation energies comparable with DFT data, while the transverse slip energy thresholds are low in comparison, which is attributed to weak close ranged monomer repulsion. Finally, we find that the H-H interaction dominates the slip activation. While not explicitly represented in the UA potential, its key role is revealed by correlating the DFT energy landscape with changes in the electron distributions and by MD simulations in which components of the OPLS-AA intermolecular potential are selectively silenced.
•Ab initio and classical atomistic modelling of twin boundaries and slip interfaces of crystalline polyethylene was performed.•A detailed comparison of the performances of vdW-DFT, all-atom and united atom modelling approaches was conducted.•γ-surfaces and activation energies for chain and transverse slip were computed and preferential slip mechanisms were identified.•Preferential dissociation of dislocations into partials was observed for the {110}-slip interface.•Charge density distributions reveal the significance of explicit H-H repulsion on slip activation energies.
We investigated the role of phosphorus (P) impurities on the fracture toughness and underlying failure mechanisms by means of classical atomistic modeling for a set of 〈110〉 symmetric tilt tungsten ...grain boundaries (GBs). This entailed the utilization of a quasi-static mode I displacement-controlled setup with cohesive zone volume elements (CZVEs) to study failure mechanisms and evaluate the fracture toughness of the GB cracks. The fracture toughness was estimated using three approaches: computing (i) the individual and (ii) the average energy release rate of CZVEs along the fractured surfaces and using them as inputs for the Griffith model, and (iii) relating the fracture toughness to crack propagation initiation. The cracks in all the pristine GBs evolved in a brittle fashion, occasionally forming facetted cleavage planes. Upon introduction of impurities, other mechanisms such as void formation and crack-tip transformation were also observed. Depending on the GB proximity of the occupied segregation sites, local strengthening was seen occasionally for individual CZVEs and at the crack-tip, which was triggered by local impurity-induced crack deflection onto planes with higher cohesion. But when the fracture toughness from the averaged energy release rate was considered, an overall reduction with increasing impurity segregation was found, although to a varying degree for different GBs. This indicates an overall increased degree of embrittlement with increasing P-segregation at the GBs, which concurs with most experimental results reported in the literature.
Display omitted
•The impact of phosphorus (P) on the brittle fracture toughness of tungsten grain boundaries (GBs) is investigated.•Three different approaches to assess the GB fracture toughness are evaluated.•Introduction of P impurities weakens the GBs and alters the crack paths.•Increasing P coverage gradually reduces the energy release rate and fracture toughness for GBs.•The results observed are in line with experimental observations of P-induced GB embrittlement.
We present a new classical interatomic potential designed for simulation of the W-Mo-Nb system. The angular-dependent format of the potential allows for reproduction of many important properties of ...pure metals and complex concentrated alloys with good accuracy. Special attention during the development and validation of the potential was paid to the description of vacancies, screw dislocations and planar defects, as well as thermo-mechanical properties. Here, the applicability of the developed model is demonstrated by studying the temperature dependence of the elastic moduli and average atomic displacement in pure metals and concentrated alloys up to the melting point.
In this work we have studied transgranular cleavage and the fracture toughness of titanium hydrides by means of quantum mechanical calculations based on density functional theory. The calculations ...show that the surface energy decreases and the unstable stacking fault energy increases with increasing hydrogen content. This is consistent with experimental findings of brittle behaviour of titanium hydrides at low temperatures. Based on Griffith-Irwin theory we estimate the fracture toughness of the hydrides to be of the order of 1 MPa⋅m1/2, which concurs well with experimental data. To investigate the cleavage energetics, we analyse the decohesion at various crystallographic planes and determine the traction-separation laws based on the Rose's extended universal binding energy relation. The calculations predict that the peak stresses do not depend on the hydrogen content of the phases, but it is rather dependent on the crystallographic cleavage direction. However, it is found that the work of fracture decreases with increasing hydrogen content, which is an indication of hydrogen induced bond weakening in the material.
Display omitted
In the present work, we have generated a new second-nearest neighbour modified embedded atom method potential (2NN-MEAM) for the W–P system to investigate the impact of P impurity segregation on the ...strength of symmetric 〈110〉 tilt coincident site lattice grain boundaries (GBs) in tungsten. By incorporating the impurity-induced reduction of the work of separation in the fitting strategy, we have produced a potential that predicts decohesion behaviour as found by ab initio density functional theory (DFT) modelling. Analysis of the GB work of separation and generalized stacking fault energy data derived from DFT and the 2NN-MEAM potential show that P-impurities reduce the resistance to both cleavage and slip. Mode I tensile simulations reveal that the most dominant mode of GB failure is cleavage and that pristine GBs, which are initially ductile, on most accounts change to brittle upon introduction of impurities. Such tendencies are in line with experimentally observed correlations between P-impurity content and reduced ductility.
Display omitted
•We fit a 2NN-MEAM potential to model P-induced embrittlement of W grain boundaries.•The potential accurately predicts the impact of P on brittle and ductile mechanisms.•Increased impurity coverage is found to promote reduced grain boundary strength.•P-impurities contribute to reduced plasticity and preference for cleavage.
PDF
