查看更多>>摘要：We investigated the effects of size and interstitial elements of the L1(2)-type Al6Y8 cluster on the formation of the long-period stacking ordered (LPSO) structure in the 10H-type Mg-Al-Y alloy, Mg92Al12Y16-i(2) , i=interstitial elements, Mg, Al, Y or vacancy (no interstitial atom), by using first-principles calculations. We found that there exist two local minima of total energy (two stable states) in a function of the Al-Al distance of the vacancy-centered L1(2) cluster. The Al-type stacking is the most stable at the 1st energy minimum with the smaller Al-Al distance whereas the A3-type stacking is the most stable for the 2nd minimum with the greater Al-Al distance. Meanwhile, the Al-Y distances are identical at both minima. We also found that the sizes of the solute atom-centered L1 2 clusters in Mg92Al12Y16-i(2) (i=Mg, Al, Y) are similar to that for the 2nd minimum with the larger cluster size. The Al-Y distances are again identical for all the solute atom-centered clusters. We show that these behaviors in the Al-Al and Al-Y distances can be explained by the softness of the Al-Al bonds and the strong Al-Y bonds predicted from the distance dependence of the pair interaction energies between the solute atoms in the Mg-matrix. These results suggest that the Al-Y bond is a key factor to determine the structure in the solute-enriched layer in the Mg-Al-Y LPSO alloy, especially in the L1(2) cluster.
Jam, Amir NamazianJam, Negin NamazianIzadifar, MohammadrezaRabczuk, Timon...
12页
查看更多>>摘要：In this paper, we carried out molecular dynamic (MD) simulations to study the crack growth and its effects on the mechanical properties of carbon doped polycrystalline boron-nitride nanosheets. We also quantified the influence of the grain and crack size on the mechanical responses. Therefore, models of polycrystalline carbon-doped h-BN with primary center crack are tested. In the models with bigger cracks, we observe a linear hardening until fracture occurs completely. Furthermore, the crack propagation speed during the fracture process is measured.
查看更多>>摘要：Smart composite materials have been an active field of research in the last few decades. Magnetorheological elastomers (MREs) are examples of such smart composites and they show a coupling between magnetism and elasticity. MREs are heterogeneous materials and they consist of magnetic particles and a silicone-based elastomer. To describe and predict the behaviour of the macroscopic continuum accurately, microstructural information needs to be taken into account in the analysis of heterogeneous materials. Therefore, a combined approach called multi-scale analysis is used to consider various scales of observation simultaneously. The concept of Representative Volume Element (RVE) is typically employed by multi-scale approaches to describe the micro scale, and thus the size of RVE becomes a model parameter in such techniques. This has motivated the determination of the RVE size and the derivation of magnetoelastic constitutive relations in terms of the RVE sizes in our paper. The finite element method and a statistical analysis based on the coefficient of variation have been used to determine the RVE size of MREs. The results show that it is possible to determine a lower bound of the RVE size for an MRE. Furthermore, a parametric study has been conducted to examine the sensitivity of the RVE size on the different material properties of the constituents. It was found that the RVE size is primarily set by the contrast of the different material properties, i.e. the stiffness, permeability and magnetoelastic coupling coefficients.
查看更多>>摘要：In contrast to their more often detrimental role, large-angle (60 degrees) grain boundaries (GBs) provide opportunities to tailor the properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs), bringing new functionalities and exciting opportunities to form intrinsic electronic heterostructures within monolayer TMDCs. In domains facing the GBs, polarization vectors along armchair direction are inversed and thus polar discontinuity arises at the GBs, as a consequence of nonzero bulk polarization of TMDCs. In order to screen the polarization charge, which gives rise to an electric field, depending on the bonding nature at GBs free electrons or holes are driven to accumulate on the GBs leading to the dispersive boundary states of metallicity. In particularly, 60 degrees GBs in 2D TMDCs can behave as one-dimensional (1D) metallic quantum wires, serving as 1D channels for free carriers or excitons.
查看更多>>摘要：In this paper we are going to present a computational study on the growth behaviour of Guinier-Preston zones (GPZ) in binary Al-Cu alloys. For this purpose we performed atomistic Monte-Carlo (MC) simulations and calculations of static configurational energies by applying a Cluster Expansion Hamiltonian for the binary Al-Cu system derived from density functional theory, which we already developed in a former publication. The MC simulations were designed to represent Cu diffusion and growth events of GPZ and were utilised to display atomistically resolved concentration fields and diffusion currents of mobile Cu in the vicinity of GPZ. For the static calculations we constructed sequences of configurations mimicking growth paths of Cu atoms and recorded their formation energy profiles. From both parts an energetically as well as kinetically stabilised mechanism for GPZ growth is proposed and the origin for the transition from GP(I) to GP(II) is elucidated.
查看更多>>摘要：Water is a driving factor in the degradation process of biodegradable polymers. However, water diffusion is typically not incorporated in the kinetic models of biodegradation such as kinetic Monte Carlo (kMC), because water diffusion events occur at a much shorter time scale than the hydrolysis reactions. As such, there is a need to improve the computational efficiency of water diffusion in the kMC models of diffusion processes in biodegra-dation. In this work, a new dimensionality reduction scheme for kMC diffusion models is developed to signifi-cantly reduce the computation time, where a two-dimensional kMC diffusion model for porous microstructures is reduced to one-dimensional ones. The dimensionality reduction is accomplished by calibrating the model pa-rameters with multi-objective Bayesian optimization. The reduced-order diffusion model shows a 675-fold faster computation compared to the original two-dimensional model.
查看更多>>摘要：Micro-spall is a large-area of liquid cavitation debris cloud formed near the free surface when metal melts during shocking or releasing process. We performed a molecular dynamics simulation to investigate the evolution of micro-spall and the related physical mechanism during the re-compaction process of a single crystal Pb by the second shock wave loading. The temperature and pressure obviously increase during re-compaction process. The ultra-high temperature-rise and local high pressure in the damage area is due to the collision between atoms and the collapse of voids. There is also one more pressure-rise and temperature-rise in the damage region, which is originated from the formation of high-speed internal micro jet and its striking on the back wall of large voids. Due to the uneven distribution of pressure and temperature after the 2nd shock wave passing through the micro spall damage region, the microstructures are also unevenly distributed. Besides, the damage region will swell due to the high temperature-rise during re-compaction process. Therefore, the micro-spall damaged region closed to the free surface is difficult to be compacted to dense state. Because that the voids in the damage region near the free surface are larger for the longer time interval, the temperature-rise originated from the collapse of voids is more severe. The wavefront plane is gradually broadened in the process of re-compaction.
查看更多>>摘要：The atomistic simulations of isolated poly (ethylene oxide) (PEO) in ionic liquid Ethylammonium nitrate (EAN) at 300 K, for degrees of polymerization, n = 9, 18, 27 and, 40 were carried out to understand the polymer dynamics using advanced sampling methods. We used various simulation techniques for this study: free energy simulations using a coupling parameter, replica-exchange molecular dynamics, and umbrella sampling simulations. The contributions from entropy and energy to the solvation free energy were also calculated from the finite-difference temperature derivative of the free energy at each reaction coordinate interval. We found that the polymer shows self-avoiding walk behavior evident from the scaling exponent is 0.55, and the end-to-end distance distribution has a single peak. EAN acts as a poor solvent compared to water in correlation with a previous experimental study. The solvation is entropically unfavorable and energetically favorable in IL, whereas it is entropically favorable and energetically unfavorable in water. However, PEO is more expanded in water for PEO with n = 9, whereas it is more expanded in EAN for other degrees of polymerization values. The mean force of the potential is found to be profoundly repulsive between two PEO chains with n = 9 in EAN compared to that in water, and the energetic solvent-polymer interactions attribute to repulsion. The cation alkyl chains of the IL interact with the polymer's surface, and ammonium groups of the cations stay away from the polymer surface due to the strong hydrogen bonding between the IL's negative and positive ions.
查看更多>>摘要：Graphene has demonstrated tremendous potentials as solid lubricants to reduce the friction force at the sliding interfaces. Its lubrication performance was found to be dependent on the mechanical properties of substrate. Here, we investigate the effect of substrate stiffness on the load-oriented thickness-dependent friction behavior of graphene sliding against a diamond tip using molecular dynamics simulations. It is found that the friction force increases about 6.4 times immediately when the load reaches a critical value to induce the formation of covalent bonds at the sliding interface. Below the critical load, single-layer graphene exhibits the smaller friction force than double-layer graphene when supported by high substrate stiffness. The friction reduction can be achieved by increasing the substrate stiffness or decreasing the sliding velocity. However, the critical load of rigid substrate-supported graphene (out-of-plane stiffness: 2167.5 N/m) is 37% lower than that of suspended graphene (out-of-plane stiffness: 115.1 N/m), which promotes the occurrence of tribo-chemical reactions during the sliding process. Such promotion is attributed to the enrichment of atomic charge at the sliding interface confirmed by density function theory calculations. This work offers a useful guidance to fabricate the high-performance nanodevices lubricated by nanomaterials with lamellar atomic structures for the tunable friction characteristics.
Eberhardt, Michael G.Hodge, Andrea M.Branicio, Paulo S.
9页
查看更多>>摘要：Molecular Dynamics simulations of Cu physical vapor deposition are employed to investigate the effects of substrate topology and line of sight on the synthesis of thin films on Cu substrates. The deposition and surface relaxation processes of sputtering Cu ions are simulated considering various angles of incidence (0-60 degrees) and sputtering energies (1, 10, 25, and 50 eV). Cu substrates are constructed with three different surface geometries: i) a flat Cu (1 1 1) surface; ii) a sinusoidal shaped substrate with a period of 127.6 angstrom and amplitude 20 angstrom ; and iii) a combined substrate composed of a flat Cu (1 11) surface and a cylinder of radius 63.8 angstrom, floating 50 angstrom above the flat surface. Results from simulations with low impact energy (1 eV) show a strong effect of the angle of incidence on the roughness of the thin film produced on the flat surface. For non-flat substrates, areas with no line of sight have diminished film coverage, which is offset by increasing the deposition energy. Higher energy atoms have greater adatom mobility enabling the coverage of regions with no line of sight and the reduction of surface roughness. The results show a lower sticking probability for highly energetic deposition at high angles of deposition, effectively promoting film coverage in areas with no line of sight through physical redeposition processes, which is augmented by particle-induced sputtering. These atomistic insights are relevant for the understanding of physical vapor deposition processes on convoluted topology substrates, such as magnetron sputtering on metamaterial lattices.
NSTL
Elsevier