Atomistic Study Research Articles

Ductile-brittle transition at edge cracks (001)[110] in BCC iron under different loading rates in mode I: a 3D atomistic study

3D atomistic simulations via molecular dynamics (MD) at temperature of 0 K and 295 K (22 °C) with a high quasi-static loading rate dP/dt of 2.92 kN s−1 show that cleavage fracture is supported by surface emission of oblique dislocations and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD. The loading rates are equivalent to the cross head speed of 5 mm min−1 and 1 mm min−1 used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations.

Adhesive Strength of Hexadecane on Different Iron Compounds: an MD Approach

The lubricity of alkane is a research target for numerous tribological applications in either industrial area or fundamental scientific studies. In the current work, a comparative investigation using a classical molecular dynamics (MD) method is carried out to investigate the effect of pure iron and its oxide surfaces on structural properties, adsorption ability of hexadecane (C16H34). A reliable force field (FF) of condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) is employed to describe the intra- and intermolecular interactions for hexadecane and its interaction with iron oxide surfaces, while the interaction between hexadecane and pure iron is derived from an ab initio result. Regarding the surfaces, the pure iron surfaces are considered using embedded-atom method/Finnis-Sinclair potential (EAM/FS), while the iron oxide surfaces are constructed using the traditional Buckingham force field. The results reveal that hexadecane shows preferential adsorption on iron oxide surfaces compared to pure iron.

An atomistic study of the influence of carbon on the core structure of screw dislocation in BCC Fe and its consequences on non-Schmid behavior

In the present work, we report on the effect of introducing carbon atom in the core of the screw dislocations in body centered iron (Fe) using classical interatomic potentials available in open literature. Reconstruction of dislocation core from easy to hard core configuration is observed irrespective of the initial position of the carbon atom, which eventually settles in the dislocation core occupying the prismatic sites formed by the rows of Fe atoms in hard core. Loading is carried out parallel and perpendicular to the Burgers vector to understand the effect of carbon on the non-Schmid characteristics of the slip. Twinning–antitwinning (T/AT) asymmetry of CRSS is observed when the dislocation core symmetry of the easy core configuration is retained after the core reconstruction occurs. The CRSS of dislocation in both easy and hard core configuration is sensitive to the loading conditions. The asymmetry in CRSS and choice of glide planes are correlated with the dislocation core spreading onto the {1 1 0} planes in the 〈1 1 1〉 zone under the influence of the applied loads.

Operando electrochemical TEM, ex-situ SEM and atomistic modeling studies of MnS dissolution and its role in triggering pitting corrosion in 304L stainless steel

Insights into the roles of geometry, size, and composition on MnS inclusion dissolution to initiate pits in 304L SS were obtained by combining operando and ex-situ electron microscopy, EDS, and atomistic modeling. The majority of thirty MnS on SS surface did not undergo dissolution even after over a week of exposure, and only a small number caused an attack of SS. Operando TEM during potentiodynamic polarization confirmed that MnS attack was followed by corrosion of the nearby SS substrate. Atomistic modeling revealed that elemental variations in the surface composition may play a role in differentiating pitting initiation by sulfur species.

Shock-induced spallation in a nanocrystalline high-entropy alloy: An atomistic study

High-entropy alloys are attracting an increasing interest due to their promising mechanical properties. However, their high-pressure properties are not fully understood. We study shock-induced spallation in a nanocrystalline high-entropy alloy using various grain sizes. Our results show that the spall strengths for the nanocrystals are significantly reduced in comparison to single crystals. In contrast to previous results on single crystals, we observe a large number of stacking faults, twins, and dislocations during the shock, which persist even during the release of the shock wave. This behavior is in good agreement with recent experiments of shock loading via high power lasers where pronounced nanotwinning has been observed in the recovered samples.

Ag nanoparticle interaction with complex lipidbilayer: an atomistic and coarse-grained study

Understanding nanoparticle (NP) translocation through complex cellular membranes is required for pre-evaluating their toxicity. Silver (Ag) is a common material, extensively applied in NP form for its apparent anti-bacterial properties. While so widely available, data about their interaction with lipid membranes, particularly the role of size and surface functionalization in membrane binding, is relatively scarce. As a first step, we performed all-atom biased and unbiased molecular dynamic simulations for bare Ag NPs for three sizes (1/3/5 nm diameter) with a complex membrane of DPPC:DOPC:POPC:Chol in 5:2:2:1 representing model lung membrane.

Atomistic study of Al partitioning and its influence on nanoscale precipitation of Cu-rich nanocluster-strengthened steels

The intrinsic partitioning behavior of Al and its influence on the nanoscale precipitation of Cu-rich nanocluster-strengthened steels were investigated by using atom probe tomography (APT) and first-principles calculations. The APT results reveal that Al partitions to Cu-rich nanoclusters, which results in a slight decrease in the volume fraction of the nanoclusters. The first-principles calculations indicate that the Al substitution in body-centered cubic (bcc) Cu is more energetically favorable as compared with that in bcc Fe. In addition, the Al partitioning has no significant influence on the chemical driving force and interfacial energy but slightly increases the strain energy for nucleation, thereby increasing the critical energy for the formation of Cu-rich nanoclusters. As a result, the nanoscale Cu precipitation is slightly inhibited in the Al-containing ferritic steels. In addition, the effects of Al on the precipitation strengthening response were quantitatively evaluated, and the results indicate that the degree of precipitation strengthening depends majorly on the combined effect of cluster size and inter-cluster spacing.

Effects of elasticity and dislocation core structure on the interaction of dislocations with embedded CNTs in aluminium: An atomistic simulation study

The interaction between edge dislocations and carbon nanotubes (CNTs) embedded into an Al matrix is investigated. Both pristine and Ni-coated CNTs are considered. It is demonstrated that the embedded CNTs lead to a yield stress contribution equivalent to that of an array of non-shearable inclusions that are by-passed via the Orowan mechanism. The strain hardening mechanism associated with multiple dislocation passes shows, however, strong effects of atomistic core structure such as crystallographic and non-crystallographic cross slip of near-screw segments on the CNT-metal interface, dislocation lock formation and annihilation, jog formation and shedding of prismatic dislocation loops.

Atomistic Molecular Dynamics Simulation Study on the Interaction between Atomically Precise Thiolate-Protected Gold Nanoclusters and Phospholipid Membranes

The interaction of atomically precise monolayer thiolate (SR) protected gold nanoclusters (Au NCs) with the phospholipid membranes has been studied by the all-atom molecular dynamics (AAMD) simulations. The effect of cluster size, type, and the surface charge density of protection ligand was studied. The simulation results show gold nanoclusters with different size and surface modifications have much different transmembrane behaviors. The Au25(SR)18 cluster was found to possess the best affinity to the phospholipid membranes among six atomically accurate clusters Au25(SR)18, Au36(SR)24, Au44(SR)28, Au68(SR)32, Au144(SR)60, and Au314(SR)96. Using the Au25 NC as a model, this work also found that the aggregation mode of the surface ligands and the surface charge density are the important factors affecting the interaction between the gold nanoclusters and the phospholipid membranes. Moreover, the balance of hydrophilic and hydrophobic ligands on the surface of Au NCs is beneficial to the high permeability.

The impact of misorientation on the grain boundary energy in bi-crystal copper: an atomistic simulation study.

Atomistic simulations were performed to investigate the relationships among the misorientation, dislocation density, and grain boundary energy of twist and tilt bi-crystal grain boundaries. In this work, the grain boundary energies were calculated based on the embedded-atom method interatomic potential for Cu. The results show that the dislocation density of the grain boundary changes with the rotation angle, thereby affecting the grain boundary energy. Furthermore, the grain boundary energy of a grain boundary with no dislocations is greater than that of a grain boundary with dislocations, which results from the distribution of the atomic potential energy on the grain boundaries. Additionally, the grain boundary energy increases with the dislocation density of the grain boundary in the case of dislocations on the grain boundary. On this basis, a new relationship is proposed for the misorientation angle and grain boundary energy. We assume that when the driving force of dislocation nucleation breaks through the grain boundary energy barrier, the grain boundary energy declines.

Predicting the Mechanical Response of Polyhydroxyalkanoate Biopolymers Using Molecular Dynamics Simulations.

Polyhydroxyalkanoates (PHAs) have emerged as a promising class of biosynthesizable, biocompatible, and biodegradable polymers to replace petroleum-based plastics for addressing the global plastic pollution problem. Although PHAs offer a wide range of chemical diversity, the structure–property relationships in this class of polymers remain poorly established. In particular, the available experimental data on the mechanical properties is scarce. In this contribution, we have used molecular dynamics simulations employing a recently developed forcefield to predict chemical trends in mechanical properties of PHAs. Specifically, we make predictions for Young’s modulus, and yield stress for a wide range of PHAs that exhibit varying lengths of backbone and side chains as well as different side chain functional groups. Deformation simulations were performed at six different strain rates and six different temperatures to elucidate their influence on the mechanical properties. Our results indicate that Young’s modulus and yield stress decrease systematically with increase in the number of carbon atoms in the side chain as well as in the polymer backbone. In addition, we find that the mechanical properties were strongly correlated with the chemical nature of the functional group. The functional groups that enhance the interchain interactions lead to an enhancement in both the Young’s modulus and yield stress. Finally, we applied the developed methodology to study composition-dependence of the mechanical properties for a selected set of binary and ternary copolymers. Overall, our work not only provides insights into rational design rules for tailoring mechanical properties in PHAs, but also opens up avenues for future high throughput atomistic simulation studies geared towards identifying functional PHA polymer candidates for targeted applications.

Introducing the potency of new boron-based heterocyclic anion receptor additives to regulate the solvation and transport properties of Li-ions in ethylene carbonate electrolyte of Li-Ion battery: An atomistic molecular dynamics study

Development of new solvent/additive electrolyte is emerging field of research towards the improvement of ion transport and solid-electrolyte-interface formation in Li-ion battery (LIB). We perform extensive atomistic molecular dynamics simulations with LiPF6 in ethylene carbonate (EC) in presence of a new class of boron (B) based heterocyclic anion receptor additives; C2HBNO(NO2)2, and C2HBNS(NO2)2 and their trimers B[C2HBNO(NO2)2]3, and B[C2HBNS(NO2)2]3 to investigate the efficacy of this class of additives in regulating the solvation and transport properties of Li+ ions as compared to the reference LiPF6/EC solution that contained TPFPB, a commonly used B-based additive. Our designed additives in several aspects show their efficacy. Among all, C2HBNS(NO2)2 is found to be the most efficient PF6- trapper that reduces ion pair formation. It promotes diffusion of Li+ and EC, improves the transference number, and reduces viscosity of solution, as compared to TPFPB. Further, it improves the ionic conductivity and drift velocity under a wide range of temperatures and external electric fields, respectively. Some of the computed properties are in agreement with the available experimental results. Our study would facilitate the design and synthesis of this class of additives to open up new possibilities of accelerating the LIB power sources for portable appliances.

Atomistic studies of the responses of composites with thermal residual stresses and defects under uniaxial loading

The thermal residual stresses and plastic deformation caused by the coefficient of thermal expansion (CTE) mismatch in the cooling process play an important role in the physical and mechanical properties of metal matrix composites (MMCs). In this study, the Cu/SiCp composites with and without cooling process are subjected to uniaxial tensile and compressive loadings through molecular dynamics (MD) simulation. We found that the cooling process leads to a decrease in strength and earlier yielding. The mechanical response depends on the temperature drop, and samples cooled from a higher temperature have a lower yield strength. The introduction of reinforcements and thermally induced defects reduce the tension/compression (C/T) asymmetry as compared with that of single crystal and defective Cu. These pre-existing defects can be the source for the initiation of plasticity in early stage of external loading, and the defect evolution shows a C/T asymmetry. For the density of two predominant dislocation types, the Shockley partial increases at the stress buildup stage, while the stair-rod dislocation decreases, regardless of tension and compression. The mechanisms for the dissociation of stair-rod dislocation and incomplete stacking fault tetrahedrons (ISFTs) at the stress buildup stage have been revealed. Our results provide an atomic-scale perspective to understand the influence of thermally residual stresses and defects on mechanical behavior of MMCs.

First principle insight into co-doped MoS2 for sensing NH3 and CH4

In this work we present the atomistic computational study of the adsorption properties of Co doped MoS2 adsorbed ammonia (NH3) and methane (CH4). The adsorption distance, adsorption energy (Ead), charge transfer (Qt), bandgap, Density of States (DOS), Projected Density of States (PDOS), transport properties, sensitivity and recovery time have been reported. The diffusion property of the system was calculated using Nudge Elastic Band (NEB) method. The calculated results depict that after suitable doping of Co on MoS2 monolayer decreases the resistivity of the system and makes it more suitable for application as a sensor. After adsorbing NH3 and CH4, Co doped MoS2 bandgap, DOS and PDOS become more enhanced. The adsorption energy calculated for NH3 and CH4 adsorbed Co doped MoS2 are -0.9 eV and -1.4 eV. The reaction is exothermic and spontaneous. The I-V curve for Co doped MoS2 for CH4 and NH3 adsorption shows a linear increase in current up to 1.4 V and 2 V, respectively, then a rapid decline in current after increasing a few volts. The Co doped MoS2 based sensor has a better relative resistance state, indicating that it can be employed as a sensor. The sensitivity for CH4 and NH3 were 124 % and 360.5 %, respectively, at 2 V. With a recovery time of 0.01s, the NH3 system is the fastest. In a high-temperature condition/environment, the Co doped MoS2 monolayer has the potential to adsorb NH3 and CH4 gas molecules. According to NEB, CH4 gas molecules on Co doped MoS2 has the lowest energy barrier as compared to NH3 gas molecules. Our results indicate that adsorbing NH3 and CH4 molecules in the interlayer is an effective method for producing Co doped MoS2 monolayers for use as spintronics sensor materials.

Mechanical compression in cofacial porphyrin cyclophane pincers.

Intra- and intermolecular interactions are dominating chemical processes, and their concerted interplay enables complex nonequilibrium states like life. While the responsible basic forces are typically investigated spectroscopically, a conductance measurement to probe and control these interactions in a single molecule far out of equilibrium is reported here. Specifically, by separating macroscopic metal electrodes, two π-conjugated, bridge-connected porphyrin decks are peeled off on one side, but compressed on the other side due to the covalent mechanical fixation. We observe that the conductance response shows an exceptional exponential rise by two orders of magnitude in individual breaking events during the stretching. Theoretical studies atomistically explain the measured conductance behavior by a mechanically activated increase in through-bond transport and a simultaneous strengthening of through-space coupling. Our results not only reveal the various interacting intramolecular transport channels in a molecular set of levers, but also the molecules' potential to serve as molecular electro-mechanical sensors and switches.

Effect of Concentration of Mo on the Mechanical behavior of γ UMo: an Atomistic Study

We performed molecular dynamics simulation on nanoindentation ofγ phase Uranium Molybdenum alloys using spherical indenter. A ternary potential developed for UMoXe was utilized. We calculated the updated values for hardness and reduced elastic modulus at different concentrations of Mo. The whole process of deformation and dislocation analysis was visualized using OVITO. We found an increase in deformation withincreasein stress while dislocations are not found anyhow induced defects have been distributed throughout the simulation cell randomly. The increase in concentration affected the hardness and reduced elastic modulus significantly. This study provides insights into the structure and mechanical characteristics of γ UMo under deformation.

Deformation behaviors and inverse Hall-Petch effect in nanoindentation of silicon: An atomistic simulation study with experimental validation

In this research, the unique perspectives on deformation behaviors of monocrystalline and polycrystalline silicon materials in nanoindentation are studied by the atomistic simulation approach. First, the validity of molecular dynamics (MD) simulation model for nanoindentation is verified by comparing the indentation morphology and force of simulation with those of nanoindentation experiment on monocrystalline silicon using atomic force microscopy (AFM). Further simulation on nanoindentation of polycrystalline silicon reveals the intriguing phenomena such as phase transformation and the effect of grain size. The indentation force with respect to indentation depth is analyzed for the monocrystalline silicon and polycrystalline silicon with grain size ranging from 20.48 nm to 6.45 nm. Inverse Hall-Petch effect, which indicates that stress decreases with the decrease of grain size, is observed in the nanoindentation process of silicon. The grain boundary-induced stress during indentation process hardly affects the formation of bct-5 silicon significantly, but it is responsible for the reduced formation of β‑silicon. The amount of bct-5 silicon atoms at the maximum indentation depth is not significantly different among the polycrystalline cases. On the other hand, the amount of bct-5 atoms of monocrystalline silicon is 14–18% less than that of polycrystalline silicon. In addition, it is discovered that residual indentation morphology and residual phase transformation are not significantly affected by the grain size. As such, this research reflects the rare efforts that bring direct validation to atomistic simulation of nanomanufacturing and provide analysis on deformation and phase transformation under various grain sizes for silicon nanoindentation.

Atomistic Study on the Energetic and Mechanical Behaviors of Helium Bubble Nucleation and Growth in RAFM Steel

Helium produced by neutron irradiation is a crucial inducement to bring about the property of deterioration of structural materials served in a fusion reactor. To investigate the nucleation and growth behavior of helium bubbles in reduced activation ferritic/martensitic steels, which comprise one of the most promising candidate structural materials, the Molecular Statics method and the Metropolis Monte Carlo algorithm are combined to investigate the energetic and mechanical behaviors of HenVm clusters in α-Fe. The simulation results show that the vacancy and helium atom binding energy are inclined to reach a saturation state, i.e., 4.0 eV for the vacancy and 2.4 eV for the helium atom; however, the binding energy of self-interstitial atoms decreases to minus values at high helium-to-vacancy (He/V) ratios. The crossover of the binding energy curve of the helium and vacancy indicates that the equilibrium He/V ratio is 1.68 during the nucleation of helium bubbles. Meanwhile, the dissociation energy analysis indicates that the stable He/V ratio of the clusters is 1.3 at high temperatures. Moreover, the pressure analysis of the HenVm clusters indicates that the He/V ratio corresponding to their mechanical equilibrium state varies from 0.50 to 0.65 at 0 K. Furthermore, the analysis combined with the relevant experimental data of helium density in helium bubbles indicates that the actual He/V ratio of helium bubbles in the served materials is closely relevant to the irradiation condition, such as helium production rate, temperature, etc. The investigation results in this paper contribute to elucidate the microscopic process of helium bubble nucleation and growth and provides the energetic and mechanical parameters of small-sized helium bubbles with different sizes for large-scale simulation studies.

Atomistic Studies on Water-Induced Lithium Corrosion.

It is well known that lithium reacts violently with water under the release of molecular hydrogen and the formation of lithium hydroxide. In this work, the initial mechanisms for the surface reactions of metallic lithium with water from the gas phase were investigated by means of periodic density functional theory calculations. For this purpose, adsorption/absorption structures and diffusion and dissociation processes of hydrogen, OH, and H2O on low‐index metallic lithium surfaces were investigated. Through thermodynamic and kinetic considerations, negatively charged centers on the surface were identified as the origin of hydrogen formation. The strikingly low reaction barriers for the reaction at these centers implied a self‐supporting effect of hydrogen evolution and the associated lithium degradation.

Atomistic study of the effect of crystallographic orientation on the twinning and detwinning behavior of NiTi shape memory alloys

Understanding the effect of crystallographic orientation on the twinnin/detwinning mechanisms in NiTi shape memory alloys at an atomistic scale can help to control and tune the mechanical properties and failure behavior of such materials. In this work, we employed classical molecular dynamics (MD) and density functional theory (DFT) computational methods to better understand how twinning and detwinning occurs through a combination of slip, twin, and shuffle on 〈010〉, 〈110〉, and 〈111〉 crystallographic orientations under uniaxial tensile test. Elastic constants including Young's Modulus (E), Bulk modulus (B), Poisson’s ratio (ν), and Shear Modulus (G) are obtained and computed for resultant stress-induced martensite variants as a function of crystallographic orientation using DFT calculations. In addition, computational nanoindentation tests are carried out using MD simulations to evaluate the effect of crystallographic orientation on the twinning and detwinning characteristics in martensite in NiTi alloys under sphere indenter, both qualitatively and quantitatively. Based on a careful polyhedral template matching (PTM) and dislocation analysis (DXA) by taking into account the textures, it is determined that the microscopic stress-strain and load-displacement responses strongly depend on the crystallographic orientation. Our findings reveal that the size of twinned and detwinned zones in martensite increases in the order of 〈111〉 < 〈010〉 < 〈110〉. Based on DFT results, against 〈111〉 direction, abrupt changes in the free energy-strain curves occurs at 4% strain in 〈001〉, and 8% strain in 〈110〉 directions. The twinning and detwinning mechanisms are controlled by monoclinic martensite (B19′) → orthorhombic martensite (B19) phase transformation in 〈110〉 orientation and by body-centered orthorhombic martensite (BCO) → an intermediate structure (B19″) → monoclinic martensite (B19′) phase transformation in 〈001〉 orientation. Finally, the predicted orientation-dependent critical energy release rate is analyzed to examine the effect of the twinning and detwinning process on the fracture toughness of the material. Our results show that reducing the density of twins results in increasing the critical energy release rate. Therefore, the fracture stress intensity increases in the order of 〈001〉 < 〈110〉 < 〈111〉.