Dislocation induced stress drop in cubic metals

Dislocation core has nonlinear properties which cannot be explained with linear elasticity theory. In respect that dislocation core is responsible for determining dislocation mobility, there have been many efforts to understand the core structures of dislocations. In this work, we firstly report that stress applied on free surfaces of nanoplate is dropped to certain value inside the plate when dislocation starts to move. We insist that phonon scattering induced by anharmonic strain field near the dislocation core reduces the applied stress during dislocation motion and derived relation between the applied stress and the amount of stress drop based on the model. We simulated edge dislocation in iron and aluminum by using molecular dynamics simulation and measured the amount of stress drop. Also, we observed that screw dislocation induces stress drop whose amount is much smaller than the edge dislocation. There was a good agreement between the model and simulation results when kink is not formed on dislocation line for both cases. Furthermore, our model predicts that the amount of stress drop decreases as temperature increases, which coincides with simulation result

A short course on DDLab and ParaDiS

DDLab and ParaDiS are dislocation dynamics simulation codes. They use the same algorithm for the calculation of node force, node velocity and topological changes, etc. The difference between them is that DDLab is a MATLAB code which is mainly used i

Dynamic drags acting on moving defects in discrete dispersive media: From dislocation to low-angle grain boundary

Although continuum theory has been widely used to describe the long-range elastic behavior of dislocations, it is limited in its ability to describe mechanical behaviors that occur near dislocation cores. This limit of the continuum theory mainly stems from the discrete nature of the core region, which induces a drag force on the dislocation core during glide. Depending on external conditions, different drag mechanisms are activated that govern the dynamics of dislocations in their own way. This is revealed by the resultant speed of the dislocation. In this work, we develop a theoretical framework that generally describes the dynamic drag on dislocations and, as a result, derive a phenomenological cubic constitutive equation. Furthermore, given that a lowangle grain boundary (LAGB) can be regarded as an array of dislocations, we extend the model to describe the mobility law of LAGBs as a function of misorientation angle. As a result, we prove that both dislocations and LAGBs follow the developed constitutive equation with the same mathematical form despite their different governing drag sources. The suggested model is also supported by molecular dynamics simulations. Therefore, this work has significance for a fundamental understanding of the dynamic drag acting on defects and facilitates a general description of various drag mechanisms

Relativistic effect inducing drag on fast-moving dislocation in discrete system

Phonon scattering, a dominant source of drag, is one of key issues to understand the dynamic behaviors of a dislocation. In this paper, it is found that a relativistic effect causes additional drag that is not ignorable when the dislocation's speed is comparable to the transverse shear wave speed. By considering the emission of lattice waves from the dislocation core, we theoretically derive an equation of dislocation motion wherein the relativistic effect is well considered in the frame of phonon scattering. Consequently, the relativistic drag force is characterized by two dimensionless constants that are newly defined in this study. Given that these constants depend on structural and oscillation properties of the dislocation core, a discrete nature of the core is well-reflected. Then, the solution of the equation, or the dislocation's speed, is compared with the result obtained by molecular dynamics simulation. Furthermore, the developed equation can explain a level-off behavior at high dislocation's speed by quantifying the relativistic drag force. Thus we can broaden our understanding of dislocation dynamics to fast-moving dislocations

Mixing behavior of Ti–Al interface during the ultrasonic welding process and its welding strength: Molecular dynamics study

In this study, we conducted molecular dynamics simulations to investigate the mechanical mixing and deformation behavior of hcp Ti/fcc Al bimetal formed by ultrasonic welding (UW). To analyze the effect of the interface shape, we considered sixteen sinusoidal interfaces of various heights and spatial periods along with the flat interface. Mechanical mixing between Ti and Al occurs mainly in the vibrational loading direction, while it is suppressed in the interface-normal direction, as the loading direction lies within the slip planes of both the hcp and fcc structures. The degree of mechanical mixing depended on the shape of the interface. According to the simulation results, mechanical mixing becomes active as the sinusoidal height increases, and the spatial period decreases because of the enlarged interface areas. During the bonding process, phase transformation is observed at the sinusoidal interface; hcp Ti is converted to fcc Ti as misfit dislocations formed at the interface glide as Shockley partials on the slip plane owing to the applied vibrational loading. A simple shear test was performed to analyze the welding strength. Although sinusoidal Ti/Al interfaces can have a welding strength that is higher than that of a flat interface, we found that the welding strength was not closely related to the degree of mechanical mixing. Rather, the welding strength was affected by the interaction between a wall of misfit dislocations, stacking fault tetrahedra, and lattice dislocations generated near the interface during the UW process