Crystallization of amorphous Ni-Zr alloys during heating with molecular dynamics simulations

The heating processes of amorphous NixZr100-x(x=10, 16.7, 33.3) alloys were investigated with molecular dynamics simu- lations. The simulation results show that the crystallization of amorphous alloys during heating is controlled by the heating rate and the alloy’s composition. The slower heating rate depresses the crystallizing temperatures and the melting temperatures of the amor- phous alloys. Crystallization can be eliminated at rapid heating rates, the critical value of which decreases with increasing Ni content of the studied amorphous Ni-Zr alloys. Different crystalline structures formed during crystallizing depend on the heating rate, and the transition between crystalline structures was observed in the heating processes.

Determination of ion quantity by using low-temperature ion density theory and molecular dynamics simulation

In this paper, we report a method by which the ion quantity is estimated rapidly with an accuracy of 4%. This finding is based on the low-temperature ion density theory and combined with the ion crystal size obtained from experiment with the precision of a micrometer. The method is objective, straightforward, and independent of the molecular dynamics （MD） simulation. The result can be used as the reference for the MD simulation, and the method can improve the reliability and precision of MD simulation. This method is very helpful for intensively studying ion crystal, such as phase transition, spatial configuration, temporal evolution, dynamic character, cooling efficiency, and the temperature limit of the ions.

Crystallization of polymer chains induced by graphene: Molecular dynamics study

The present work is devoted to a study of the molecular mechanisms of the crystallization of polymer chains induced by graphene by using molecular dynamics （MD） simulations. From the atomic configuration translation, the number distri- bution of the atoms, and the order parameter S, the crystallization process can be summarized in two steps, the adsorption and the orientation. By analyzing the diffusion properties of the polymer chains, we find that a graphene substrate has a great adsorption for the polymer molecules and the polymer molecules need more time to adjust their configurations. Therefore, the adsorption step and the orientation step are highly cooperative.

Molecular Dynamics Simulation of Zero-Fluence and Low-Bombarding-Energy ^(63)Cu-^(65)Cu Sputtering

A molecuIar dynamics simulation has been used to study zero-fluence and low- bombarding63Cu-65Cu sputtering. Calculations show that the isotopic ratios at both θ≤35° and energy 63Cu-65Cu sputtering. Calculations show that the isotopic ratios at both θ≤35° and θ >35°, and the total isotopic ratio increase when the bombarding energy decreases. This result might impty the existence of the bombarding-energy-dependent momentum asymmetry.

Study of structural and magnetic properties of Fe(80)P-9B(11) amorphous alloy by ab initio molecular dynamic simulation

The structural and magnetic properties of Fe80P9B11 amorphous alloy are investigated through ab initio molecular dynamic simulation. The structure evolution of Fe（80）P9B（11） amorphous alloy can be described in the framework of topological fluctuation theory, and the fluctuation of atomic hydrostatic stress gradually decreases upon cooling. The left sub peak of the second peak of Fe–B partial pair distribution functions（PDFs） becomes pronounced below the glass transition temperature, which may be the major reason why B promotes the glass formation ability significantly. The magnetization mainly originates from Fe 3d states, while small contribution results from metalloid elements P and B. This work may be helpful for developing Fe-based metallic glasses with both high saturation flux density and glass formation ability.

Etching Mechanisms of CF_3 Etching Fluorinated Si:Molecular Dynamics Simulation

Molecular dynamics simulations are performed to investigate CF3 continuously bom- barding the amorphous silicon surface with energies of 10 eV, 50 eV, 100 eV and 150 eV at normal incidence and room temperature. The improved Tersoff-Brenner potentials were used. The simulation results show that the steady-state etching rates are about 0.019, 0.085 and 0.1701 for 50 eV, 100 eV and 150 eV, respectively. With increasing incident energy, a transition from C-rich surface to F-rich surface is observed. In the region modified by CF3, SiF and CF species are dominant.

Molecular Dynamics Simulation of Dealloyed Layer-induced Tensile Stress in Cu_3Au

During stress corrosion cracking of Cu3Au alloy, there is a dealloyed layer on its surface because of preferential dissolution of Cu, and there is a linear distribution of Cu vacancy concentration in the dealloyed layer. Molecular dynamics simulation has been done on the three-dimensional crystal (about 148 000 atoms) by employing the embedded-atom method (EAM) potential. Simulation shows that Cu3Au crystal in which there is a dealloyed layer on one surface and one end is fixed will be deflected after relaxing for a long time because of a tensile stress generated at or near the dealloyed layer interface. The deflection and then the tensile stress increase with increasing the depth of dealloyed layer and the vacancy concentration in the dealloyed layer.

Mechanical properties of Fe-based amorphous–crystalline composite: a molecular dynamics simulation and experimental study

A new Fe-based amorphous–crystalline composite without non-metallic elements, Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5), was prepared by melt-spinning. The formation ability and structure information were investigated by X-ray diffractometer(XRD), energy-dispersive spectrometer(EDS) and scanning electron microscope(SEM). The mechanical properties of the amorphous–crystalline composite were investigated by nanoindentation. A molecular dynamics simulation study was performed to simulate the formation of Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5) amorphous alloy. The mechanical properties were obtained by compression simulations simultaneously. The results indicate that the Fe_(55)Cr_(15)Mo_(15)Ni_(10)W_(5) ribbon is an amorphous–crystalline composite structure with good ductility, and the hardness of the amorphous–crystalline composite is about 75%higher than that of master ingot. The simulation mechanical properties are in good agreement with the results of nanoindentation at the nanoscale.

Rotational viscosity of a liquid crystal mixture: a fully atomistic molecular dynamics study

Fully atomistic molecular dynamics （MD） simulations at 293, 303 and 313 K have been performed for the four- component liquid crystal mixture, E7, using the software package Material Studio. Order parameters and orientational time correlation functions （TCFs） were calculated from MD trajectories. The rotational viscosity coefficients （RVCs） of the mixture were calculated using the Nemtsov-Zakharov and Fialkowski methods based on statistical-mechanical approaches. Temperature dependences of RVC and density were discussed in detail. Reasonable agreement between the simulated and experimental values was found.

Investigating the Calculation of Rotational Viscosity of the Mixture Comprising Different Kinds of Liquid Crystals： Molecular Dynamics Computer Simulation Approach

Molecular dynamics （MD） computer simulation techniques, as a powerful tool commonly utilized by the liquid crystal display （LCD） community, usually are employed for computing the equilibrium and transport properties of a classical many body system, since they are very similar to real experiments in many respects. In this paper we pre- sent molecular dynamics computer simulation results taken for a mixture of the two different kinds of nematic liq- uid crystals （LCs）. We calculated rotational viscosity from Brownian behavior with friction of the mean director of the mixture comprising pentylcyanobiphenol （5CB） and decylcyanobiphenol （10CB） by using molecular dynamics computer simulation, where intermolecular potential parameter is Generalized AMBER force field （GAFF）. Our computed results show a good agreement with the experimental results.

Dissipative particle dynamics thermostat: a novel thermostat for molecular dynamics simulation of liquid crystals with Gay–Berne potential

The Gay-Berne （GB） model has been proved to be highly successful in the simulation of liquid crystal phases via both molec- ular dynamics （MD） and nonequilibrium molecular dynamics （NEMD）. However, the conventional thermostats used in the simulations of GB systems, such as Nose-Hoover and Langevin thermostats, have serious shortcomings especially in NEMD simulations. Recently, dissipative particle dynamics （DPD） has established itself as a useful thermostat for soft matter simulations, whereas the application of DPD thermostat in （NE）MD simulations is limited to the spherically isotropic potential models, such as the Lennard-Jones model. Considering the virtues of the DPD thermostat, that is, local, momentum conserved, and Galilean invariant, we extend the DPD thermostat to the non-spherical GB model. It is interesting to find that the translational DPD and rotational DPD thermostats can be used in the GB system independently and both can achieve the thermostatting effects. Also, we compared the performance of the DPD thermostat with other commonly used thermostats in NEMD simulations by investigating the streaming velocity profiles and the dynamics of phase separation in a typical but simple binary GB mixture under shear field. It is revealed that the known virtues of DPD thermostats, such as Galilean invariant, shear velocity profile-unbiased, and unscreened hydrodynamic interactions, are still intact when applying to GB systems. Finally, the appro- priate parameters for the DPD thermostat in the GB system are identified for future investigations.

Research on the nanometric machining of a single crystal nickel via molecular dynamics simulation

Molecular dynamics simulations are employed to study the nanometric machining process of single crystal nickel. Atoms from different machining zones had different atomic crystal structures owing to the differences in the actions of the cutting tool. The stacking fault tetrahedral was formed by a series of dislocation reactions, and it maintained the stable structure after the dislocation reactions. In addition, evidence of crystal transition and recovery was found by analyzing the number variations in different types of atoms in the primary shear zone, amorphous region, and crystalline region. The effects of machining speed on the cutting force, chip and subsurface defects, and temperature of the contact zone between the tool and workpiece were investigated. The results suggest that higher the machining speed, larger is the cutting force. The degree of amorphousness of chip atoms and the depth and extent of subsurface defects increase with the machining speed. The average friction coefficient first decreases and then increases with the machining speed because of the temperature difference between the chip and machining surface.

An emerging terpolymeric nanoparticle pore former as an internal recrystallization inhibitor of celecoxib in controlled release amorphous solid dispersion beads:Experimental studies and molecular dynamics analysis

Solid oral controlled release formulations feature numerous clinical advantages for drug candidates with adequate solubility and dissolution rate.However,most new chemical entities exhibit poor water solubility,and hence are exempt from such benefits.Although combining drug amorphization with controlled release formulation is promising to elevate drug solubility,like other supersaturating systems,the problem of drug recrystallization has yet to be resolved,particularly within the dosage form.Here,we explored the potential of an emerging,non-leachable terpolymer nanoparticle(TPN)pore former as an internal recrystallization inhibitor within controlled release amorphous solid dispersion(CRASD)beads comprising a poorly soluble drug(celecoxib)reservoir and insoluble polymer(ethylcellulose)membrane.Compared to conventional pore former,polyvinylpyrrolidone(PVP),TPN-containing membranes exhibited superior structural integrity,less crystal formation at the CRASD bead surface,and greater extent of celecoxib release.All-atom molecular dynamics analyses revealed that in the presence of TPN,intra-molecular bonding,crystal formation tendency,diffusion coefficient,and molecular flexibility of celecoxib were reduced,while intermolecular H-bonding was increased as compared to PVP.This work suggests that selection of a pore former that promotes prolonged molecular separation within a nanoporous controlled release membrane structure may serve as an effective strategy to enhance amorphicity preservation inside CRASD.

The reaction mechanism and interfacial crystallization of Al nanoparticle-embedded Ni under shock loading

The shock-induced reaction mechanism and characteristics of Ni/Al system,considering an Al nanoparticle-embedded Ni single crystal,are investigated through molecular dynamics simulation.For the shock melting of Al nanoparticle,interfacial crystallization and dissolution are the main characteristics.The reaction degree of Al particle first increases linearly and then logarithmically with time driven by rapid mechanical mixing and following dissolution.The reaction rate increases with the decrease of particle diameter,however,the reaction is seriously hindered by interfacial crystallization when the diameter is lower than 9 nm in our simulations.Meanwhile,we found a negative exponential growth in the fraction of crystallized Al atoms,and the crystallinity of B2-NiAl(up to 20%)is positively correlated with the specific surface area of Al particle.This can be attributed to the formation mechanism of B2-NiAl by structural evolution of finite mixing layer near the collapsed interface.For shock melting of both Al particle and Ni matrix,the liquid-liquid phase inter-diffusion is the main reaction mechanism that can be enhanced by the formation of internal jet.In addition,the enhanced diffusion is manifested in the logarithmic growth law of mean square displacement,which results in an almost constant reaction rate similar to the mechanical mixing process.

Unveiling the early stage evolution of local atomic structures in the crystallization process of a metallic glass

The early stage evolution of local atomic structures in a multicomponent metallic glass during its crystallization process has been investigated via molecular dynamics simulation.It is found that the initial thermal stability and earliest stage evolution of the local atomic clusters show no strong correlation with their initial short-range orders,and this leads to an observation of a novel symmetry convergence phenomenon,which can be understood as an atomic structure manifestation of the ergodicity.Furthermore,in our system we have quantitatively proved that the crucial factor for the thermal stability against crystallization exhibited by the metallic glass is not the total amount of icosahedral clusters,but the degree of global connectivity among them.

Molecular dynamics simulation of the tribological performance of amorphous/amorphous nano-laminates

Because the amorphous/amorphous nano-laminates could enhance signifcantly the mechanical properties of the amorphous materials,they have been widely studied as a new group of structural materials.In this study,the nano-scratch performance of the Cu_(80)Zr_(20)/Cu_(20)Zr_(80)(A/B-type)and the Cu_(20)Zr_(80)/Cu_(80)Zr_(20)(B/A-type)amorphous/amorphous nano-laminates was evaluated by molecular dynamics simulation.Their dependences on the type of indenter,layer thickness,stacking mode and scratch depth were systematically analyzed.There is a signifcant size effect for the tribological properties of amorphous/amorphous nanolaminates that the friction force of the A/B-type increases with the increase of layer thickness,but the friction force of the B/A-type decreases as the layer thickness increase.The interface has an obvious obstruction effect on the shear deformation and reduces the plastic affected region below the scratched groove.Particularly,the contact environment of the indenter bottom has an important influence on the normal force,so it’s not that the deeper the depth,the greater the normal force.Hence it should not be ignored when evaluating the tribological properties of the amorphous/amorphous nano-laminates.This work can deepen the understanding of hetero-interface on the deformation mechanism during nano-scratch,and help to design amorphous nano-laminates with tailored tribological performance for practical applications.

A simulation study of microstructure evolution during solidification process of liquid metal Ni

A molecular dynamics simulation study has been performed for the microstructure evolution in a liquid metal Ni system during crystallization process at two cooling rates by adopting the embedded atom method （EAM） model potential. The bond-type index method of Honeycutt-Andersen （HA） and a new cluster-type index method （CTIM-2） have been used to detect and analyse the microstructures in this system. It is demonstrated that the cooling rate plays a critical role in the microstructure evolution： below the crystallization temperature Tc, the effects of cooling rate are very remarkable and can be fully displayed. At different cooling rates of 2.0 × 10^13 K·s^-1 and 1.0 × 10^12 K·s^-1, two different kinds of crystal structures are obtained in the system. The first one is the coexistence of the hcp （expressed by （12 0 0 0 6 6） in CTIM-2） and the fcc （12 0 0 0 12 0） basic clusters consisting of 1421 and 1422 bond-types, and the hcp basic cluster becomes the dominant one with decreasing temperature, the second one is mainly the fcc （12 0 0 0 12 0） basic clusters consisting of 1421 bond-type, and their crystallization temperatures Tc would be 1073 and 1173 K, respectively.

Stress Relaxation Behaviors of Monocrystalline Silicon Coated with Amorphous SiO_(2) Film:A Molecular Dynamics Study

Long-lasting constant loading commonly exists in silicon-based microelectronic contact,as well as the chemical mechanical polishing area.In this work,the stress relaxation analysis of single crystal silicon coated with an amorphous SiO_(2) film is performed by varying the maximum indentation depth using molecular dynamics simulation.It is found that during holding,the applied indentation force declines sharply at the beginning and then steadily towards the end of the holding period.The stress relaxation amount of bilayer composites increases as the maximum indentation depth increases.It is also found that the deformation features of SiO_(2) film and silicon substrate during holding are inherited from the loading process.The SiO_(2) film during holding is further densified when the maximum indentation depth is equal to or less than a certain value(5.5 nm for the 0.8-nm film).The amount of generated phases and phase distributions of silicon substrate during holding are affected by the plastic deformation of silicon during loading.

Molecular Dynamics Simulation of the Evolution of Interfacial Dislocation Network and Stress Distribution of a Ni-Based SingleCrystal Superalloy

The evolution of misfit dislocation network at γ/γ＇ phase interfaces and the stress distribution characteristics of Ni-based single-crystal superalloys under different temperatures of 0, 100 and 300 K are studied by molecular dynamics （MD） simulation. It was found that a closed three-dimensional misfit dislocation network appears on the γ/γ＇ phase interfaces, and the shape of the dislocation network is independent of the lattice mismatch. Under the influence of the temperature, the dislocation network gradually becomes irregular, a/2 [110] dislocations in the γ matrix phase emit and partly cut into the γ＇ phase with the increase in temperature. The dislocation evolution is related to the local stress field, a peak stress occurs at γ/γ＇ phase interface, and with the increase in temperature and relaxation times, the stress in the γ phase gradually increases, the number of dislocations in the γ phase increases and cuts into γ＇ phase from the interfaces where dislocation network is damaged. The results provide important information for understanding the temperature dependence of the dislocation evolution and mechanical properties of Ni-based single-crystal superalloys.

Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

By molecular dynamics simulations employing an embedded atom method potential, we have investigated structural transformations in single crystal A1 caused by uniaxial strain loading along the [001], [011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction, and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings, and the appearance of the hcp phase is due to the partial dislocation moving forward on {lll}fcc family. For [011] loading, the hcp phase grows to form laminar morphology in four planes, which belong to the {111}fcc family; while for [111] loading, the hcp phase grows into a laminar structure in three planes, which belong to the {111}fcc family except for the （111） plane. In addition, the phase transition is evaluated by using the radial distribution functions.