Molecular-dynamics simulation of edge-dislocation dynamics in aluminum

Abstract

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].