A. Yu. Kuksin

Molecular-dynamics simulation of edge-dislocation dynamics in aluminum

The ability of a crystal to exhibit plastic deformation is related to the presence of dislocations in the crystal lattice. The motion of dislocations provides for the formation of a real atomic structure in crystalline solids and the kinetics of deformation of crystals under load; it underlies the control of many important physical properties of solids [1‐3]. It is known that the dependence of the yield stress on deformation rate in many metals sharply increases when the deformation rate exceeds ~10 3 –10 4 s ‐1 . This phenomenon can be interpreted as the consequence of a change in the mechanism of dislocation motion. Moving at low velocities, dislocations overcome obstacles as a result of the joint action of the applied stress and thermal fluctuations. Due to this, an increase in the temperature is accompanied by a decrease in the yield stress of the material. For a high-rate deformation, it is necessary to apply higher stresses. At a deformation rate exceeding a certain threshold ( ~10 4 s ‐1 for pure metals), the acting stresses prove to be sufficient for the dynamic overcoming of obstacles without an additional contribution from thermal fluctuations. In this case, the pumping of the dislocation energy to the crystal lattice vibrations or, depending on temperature, to the electron subsystem becomes the dominating mechanism of the retardation of dislocations. In contrast to the region of thermofluctuational mobility, the dislocation velocity in the dynamic region decreases with temperature in accordance with an increase in the density of the gas of elementary excitations. For this reason, an anomalous increase in the yield stress with increasing temperature is observed for some materials at very high rates of deformation [1]. In this study, we have compared the results of simulation with the values of the dynamic yield stress for single-crystalline aluminum in the shock-wave experiments, which offer a powerful method of studying the properties of materials dynamically loaded under well controllable conditions. The behavior of materials under high-rate deformation in shock-wave experiments is very diverse, which is manifested both in the temperature dependence of the yield stress and in the character of deformation in the samples upon storage [1, 4].

Plastic deformation under high-rate loading: The multiscale approach

A two-level approach has been proposed for describing the plastic deformation under high-rate loading of metals. The characteristics of the motion of dislocations under shear stresses have been investigated at the atomistic level by using the molecular dynamics simulation. The macroscopic motion of a material has been described at the continuum level with the use of the model of continuum mechanics with dislocations, which uses information obtained at the atomistic level on the dislocation dynamics. The proposed approach has been used to study the evolution of the dislocation subsystem under shock-wave loading of an aluminum target. The behavior of the dynamic yield stress with an increase in the temperature has been analyzed. The results of the calculations are in good agreement with experimental data.

Atomistic simulation of the motion of dislocations in metals under phonon drag conditions

The mobility of dislocations in the over-barrier motion in different metals (Al, Cu, Fe, Mo) has been investigated using the molecular dynamics method. The phonon drag coefficients have been calculated as a function of the pressure and temperature. The results obtained are in good agreement with the experimental data and theoretical estimates. For face-centered cubic metals, the main mechanism of dislocation drag is the phonon scattering. For body-centered cubic metals, the contribution of the radiation friction becomes significant at room temperature. It has been found that there is a correlation between the temperature dependences of the phonon drag coefficient and the lattice constant. The dependences of the phonon drag coefficient on the pressure have been calculated. In contrast to the other metals, iron is characterized by a sharp increase in the phonon drag coefficient with an increase in the pressure at low temperatures due to the α-∈ phase transition.

Atomistic simulation of plasticity and fracture of nanocrystalline copper under high-rate tension

A molecular dynamics simulation of the plastic deformation and the onset of fracture of nanocrystalline metals is performed using the example of copper. Successive stages of the response of the microstructure of a metal to deformation are considered, namely, grain boundary sliding, the nucleation and gliding of dislocations, and the formation and growth of microdamage nuclei. The influence of the grain size of a nanocrystal on its plasticity and strength is studied.

The Phase Diagram and Spinodal Decomposition of Metastable States of Lennard-Jones System

The method of molecular dynamics is used to treat the metastable states of Lennard-Jones crystal and liquid at different temperatures and pressures, including the negative-pressure region. Analysis is made of the pattern of relative position of surfaces of the equation of state of the crystal and liquid phases in the metastable region. The limits of stability of metastable states of liquid and crystal on the phase diagram are determined. The mechanism of crystal decomposition in the vicinity of the stability limit is treated. The stochastic properties of a many-particle system are investigated in the region in the vicinity of the spinodal, and the dynamic memory time and K-entropy are calculated.

Atomic positions and diffusion paths of h and he in the α-Ti lattice

The solution energy of H and He in various interstitial and substitution positions in the hcp lattice of α-Ti has been calculated based on the method of electron density functional. The lowest solution energy of He corresponds to the basal octahedral position and that of H corresponds to the octahedral position (next in energy is the tetrahedral position). The calculated vibration frequencies of H in various positions are used for identification of lines in the vibration spectrum obtained by the method of neutron inelastic scattering. Taking into account these spectra, it can be concluded that hydrogen atoms occupy in the hcp lattice of Ti both the octahedral and tetrahedral positions even at 600 K. The available experimental data do not contradict the conclusion that the octahedral position is more preferable in α-Ti. The energy barriers are estimated for various diffusion paths of H and He.

Influence of plastic deformation on fracture of an aluminum single crystal under shock-wave loading

The plastic deformation and the onset of fracture of single-crystal metals under shock-wave loading have been studied using aluminum as an example by the molecular dynamics method. The mechanisms of plastic deformation under compression in a shock wave and under tension in rarefaction waves have been investigated. The influence of the defect structure formed in the compression wave on the spall strength and the fracture mechanism has been analyzed. The dependence of the spall strength on the strain rate has been obtained.

Calculation of diffusion coefficients of defects and ions in UO2

This paper has presented molecular dynamics calculations of the diffusion coefficients of interstitials, vacancies, and vacancy complexes of oxygen and uranium in UO2, as well as the coefficients of ion diffusion provided by these defects. The interatomic potentials have been chosen by comparing the defect formation energies with data of the DFT + U calculations. The results of the calculations have been compared with experimental data on the annealing of defects and the measurements of self-diffusion coefficients of ions. The limitations of the model of point defects for the description of the self-diffusion in nominally stoichiometric UO2 have been discussed.

Atomistic modeling of the self-diffusion in γ-U and γ-U-Mo

Results of investigations of the self-diffusion in gamma-uranium and metallic U-Mo alloys are presented. Calculations are performed using the method of atomistic modeling with the help of interatomic potentials based on the embedded-atom model and its modifications. Proposed potentials are verified by calculating thermodynamic and mechanical properties of uranium and U-Mo alloys. The formation energies of point defects and atomic diffusivities due to the diffusion of defects are calculated for gamma-uranium and alloy containing 9 wt % molybdenum. Self-diffusion coefficients of uranium and molybdenum are evaluated. Based on the data obtained, it has been concluded that the experimentally observed features of the self-diffusion in gamma-uranium can be explained by the prevalence of the interstitial mechanism.

A kinetic model of fracture of simple liquids

The molecular-dynamic (MD) simulation is performed of the processes of generation and growth of cavities in stretched Lennard-Jones liquid. The process of homogeneous generation of cavities in a constant-volume cell is considered. The averaging of the lifetime of the homogeneous phase over the ensemble of MD trajectories is used to determine the nucleation rate as a function of pressure and temperature. The resultant correlation is compared with the classical theory of homogeneous nucleation. The initial stage of growth of spherical cavity is simulated, and the dependence of the rate of growth on pressure is determined along two isotherms. A kinetic model is suggested of fracture of liquid upon stretching at a constant rate. This model relates the volume of pores at an arbitrary instant of time to the kinetic characteristics of their generation and growth determined in MD models for single isolated cavities. The spallation strength of liquid, calculated using this kinetic model and MD data, only slightly depends on the rate of stretching. The calculation results agree well with the experimentally obtained dependence of spallation strength of hexane on the rate of stretching.