Papers by Keyword: Molecular Dynamics Simulation

Abstract: Aluminum alloys development always exit in the manufacturing process. Al/Mg alloys have been attracted significant attention because of their excellent mechanical properties. The microstructural evolution and deformation mechanisms are still challenging issues, and it is hard to observe directly by experimental methods. Accordingly, in this paper atomic simulations are performed to investigate the uniaxial compressive behavior of Al/Mg phases; with different ratio of Mg ranging from 31% to 56%. The compression is at the same strain rate (3.10¹⁰ s⁻¹), at the same temperature (300K) and pressure, using embedded atom method (EAM) potential to model the interactions and the deformation behavior between Al and Mg.From these simulations, we get the radial distribution function; the stress–strain responses to describe the elastic and plastic behaviors of β-Al₃Mg₂, ε-Al₃₀Mg₂₃, Al₁Mg₁ and γ-Al₁₂Mg₁₇ phases with 31, 41, 50 and 56% of Mg added to pure aluminum, respectively. The mechanical properties, such as Young’s modulus, elasticity limit and rupture pressure, are determined and presented. The engineering equation was used to plot the stress-strain curve for each phase.From the results obtained, the chemical composition has a significant effect on the properties of these phases. The stress-strain behavior comprised elastic, yield, strain softening and strain hardening regions that were qualitatively in agreement with previous simulations and experimental results. These stress-strain diagrams obtained show a rapid increase in stress up to a maximum followed by a gradual drop when the specimen fails by ductile fracture. Under compression, the deformation behavior of β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases is slightly similar. From the results, it was found that ε-Al₃₀Mg₂₃ phase are brittle under uniaxial compressive loading and γ-Al₁₂Mg₁₇ phase is very ductile under the same compressive loading.The engineering stress-strain relationship suggests that β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ phases have high elasticity limit, ability to resist deformation and also have the advantage of being highly malleable. From this simulation, we also find that the mechanical properties under compressive load of ε-Al₃₀Mg₂₃ phase are evidently less than other phases, which makes it the weakest phase. The obtained results were compared with the previous experimental studies, and generally, there is a good correlation.The Al-Mg system was built and simulated using molecular dynamics (MD) software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).

Abstract: Molecular dynamics simulation was used to investigate two models of aqueous solution ofcalcium carbonate system between 283K and 373K. The diffusion coefficients of water moleculesdemonstrated that both the electropositive surface (110) on Model-I and neutral surface (104) onModel-II showed interaction with the water molecules, and the surface (110) exhibited strongerelectrostatic interaction with water molecules than the latter, besides obvious anomaly appeared near343K on Model-I. On the other hand, surface (110) exhibited anomalous influences on Ca2+ andCO32- ions near 313K and 343K on Model-I, and only a broad anomaly on CO32- ions near 343K onModel-II. The binding energies between surface (110) / (104) and Ca2+ /CO32- ions demonstrated thatthe surface (104)were more favorable for the growth of the new crystal but weak for the diffusion.

Abstract: The effects of temperature and graphite-like structure additive on the graphitization process of amorphous carbon were investigated through molecular dynamics simulation. The molecular models of amorphous carbon and graphite-like structure-amorphous carbon were constructed with the initial density of 1.62 g/cm³ and carbon atoms number of 4096 by rapid quenching method. After annealing treatment at 3200 K, 3600 K and 4000 K respectively, the evolution rules of sp² C atoms and the instantaneous conformations of the graphite-like structure-amorphous carbon system were analyzed to investigate the effects of temperature and graphite-like structure on the graphitization process. It could be found that increasing graphitization temperature properly could improve graphitization degree of amorphous carbon. Addition of graphite-like structure could promote recrystallization of the irregular carbon atoms in amorphous carbon materials, thus accelerating graphitization process and promoting graphitization of the system.

Abstract: In-service welding has been gaining considerable attentions due to its significant economic benefits. At high temperature, several technical difficulties exist during repair process and burn-through has been one of the critical issues. To reveal the physical nature of burn-through, finite element simulation and molecular dynamics simulation are combined to investigate the micro dynamic properties of different micro regions in welded joint and the effect of crack on the microdynamic behavior in the process of material failure. The results indicate that burn-through is a failure process under the effect of tensile stress and high temperature. The performance near fusion line is the worst and a burn-through tends to occur at the regions behind maximum melting depth. The failure process of welded joints experience the initiation and development of micro defects. Continuous expansion of micro holes and micro cracks causes the structural fracture. Furthermore, the micro crack would decrease the structural strength and the failure mode differs for cracks in different direction. Failure process of crack structure has experienced the crack tip passivation and dislocation emission. And the formation of stacking fault is carried out in failure process. This paper reveals the microscopic mechanism of burn-through at the atomic level and provides a scientific basis for the continuous and safe operation of gas pipelines.

Abstract: As a type of nanostructured material with nanosized porosity and ultrahigh specific surface area, nanoporous metals attract much attention in both industrial and theoretical fields. Through molecular dynamics simulations, the strain energy of nanoporous copper is investigated with special consideration on the effect of temperature and strain rate. First, with the variation of temperature and strain rate, the change of both stress and strain energy is plotted. Dislocation movement and structural response of nanoporous copper are explored in different stages of strain. Secondly, yield points under different conditions are analyzed to demonstrate the super plasticity of nanoporous copper. It is interesting that critical points appears. Based on above mentioned investigation, it is expected to provide a simple description on mechanical property and performance of nanoporous metals.

Abstract: The morphological and structural transitions in CdSe hollow nanoparticles (hNPs) with zinc blende structure have studied by molecular dynamics (MD) simulation method under heating. The seven samples of CdSe-hNPs are constructed with different thicknesses from the solid NPs at 10nm and 15nm sizes. Morphological changes in CdSe-hNPs have presented by describing the first stage melting in hollow semiconductor NPs. The thermal effect on the atomic arrangement has also examined by the cubic zinc blende-to-wurtzite transformation occurred during the melting in hNPs. MD results show that the inner shells of those with thin walls have begun to melt at lower temperatures due to the thickness of the NPs. The first stage melting, which resulted in the filling of the void within the particle, takes place almost at the same temperature for hNPs with the thick wall thickness. Then, the melting of the particles is completed at higher temperatures. The cubic diamond structure disappears with the collapse of the inner cavity, and the hcp structure begins to appear at later temperatures.

Abstract: The effects of CNR diameter and CNR number on tensile properties of the CNR-graphene hybrid structure (CGHS) were studied by molecular dynamics simulation in this paper. Results show that interactions between adjacent graphene sheets are significantly strengthened by the cross-linked CNRs. For CGHSs, the maximum strength is ~64.0 GPa and the maximum Young’s modulus strength is ~763 GPa. When the diameter of CNRs is large or the CNR linkers are dense, the tensile strength of CGHSs reached the maximum and the fracture mechanism of CGHSs changed from CNR-graphene junction fracture to graphene sheet fracture. Present work should serve as guide to experiments concerning physical properties of this novel material.

Abstract: The He separation performance of the N-modified graphdiyne monolayer (N-GDY) was studied by using both the first-principles density functional theory (DFT) and molecular dynamics (MD) simulations. The high cohesive energy of 7.24 eV/atom confirmed the strong stability of N-GDY for a gas separation membrane. Based on the calculations, the N-GDY membrane was found to exhibit extremely high He permeance (4.8 ×10^-3 mol/m²·s·Pa at 100 K) and selectivities of He/H₂O, He/Ar, He/N₂, He/CO, He/CO₂, and He/CH₄ (10²~10¹² at 300 K). Therefore, N-GDY should be a good candidate for He separation from natural gas.

Abstract: Physical phenomena in a nanometric machining process were studied by molecular dynamics simulations. A cylindrical tool was indented and then moved laterally on an initially flat workpiece. The focus of the study is on the effect of lubrication on the nanoscale. Therefore, the indentation and the scratching were studied both in vacuum and submersed in a lubricant. All materials were modeled by Lennard-Jones truncated and shifted potential sites. It is observed, that in the lubricated case, a substantial part of the cutting edge of the tool is in dry contact with the workpiece. Nevertheless, compared to the dry scenario, the lubrication lowers the coefficient of friction. However, the work which is needed for the indentation and the scratching is not reduced. The processed surface is found to be smoother in the lubricated case. As expected, the lubrication has an important influence on the temperature field observed in the simulation.

Abstract: This work was try to study the number and types of hydrogen bonds (H-bonds) formed in hindered phenol AO-70/nitrile butadiene rubber (NBR) composites and their contributions to the damping properties by molecular dynamic (MD) simulation and experimental methods. MD simulation results showed that there were four types of H-bonds, namely, type A (AO-70) –OH...NC– (NBR) H-bonds in AO-70/NBR composites, type B (AO-70) –OH...O=C– (AO-70) H-bonds, type C (AO-70) –OH...OH–(AO-70) and D (AO-70) –OH...O–C– (AO-70) H-bonds, what's more, type A and type B H-Bonds formed more easily than others. Fourier transform infrared spectroscopy (FTIR) confirmed the existence of H-bonds. Meanwhile, the AO-70/NBR composites with AO-70 content of 109 phr had the largest number of H-bonds, smallest fractional free volume (FFV) and resulting in the optimistic damping performance of the composites.