Considering high pressure torsion experiments as a motivation, plastic behavior of crystalline solids is treated as a highly viscous material flow through an adjustable crystal lattice. Instead of the traditional decomposition rule considering the deformation gradient as a product of the elastic and plastic parts, the proposed model is based on its rate form: the velocity gradient consists of the lattice velocity gradient and the sum of the velocity gradients corresponding to the slip rates of individual slip systems; the slip strains themselves are not defined in the model. The geometrical changes caused by material flow and the slip strains can be specified a posteriori. Crystal lattice distortions are measured with respect to a lattice reference configuration. In an adopted rigid plastic approximation the lattice distortions are reduced to rotations. Constitutive equations incorporate non-local hardening caused by close range interactions among dislocations.
This contribution deals with the numerical simulation of dislocation dynamics, their interaction, merging and changes in the dislocation topology. The glide dislocations are represented by parametrically described curves moving in slip planes. The simulation model is based on the numerical solution of the dislocation motion law belonging to the class of curvature driven curve dynamics. We focus on the simulation of the cross-slip of two dislocation curves where each curve evolves in a different slip plane. The dislocations evolve, under their mutual interaction and under some external force, towards each other and at a certain time their evolution continues outside slip planes. During this evolution the dislocations merge by the cross-slip occurs. As a result, there will be two dislocations evolving in three planes, two planes, and one plane where cross-slip occurred. The goal of our work is to simulate the motion of the dislocations and to determine the conditions under which the cross-slip occurs. The simulation of the dislocation evolution and merging is performed by improved parametric approach and numerical stability is enhanced by the tangential redistribution of the discretization points.
The interpretation of the experimentally determined critical distance of the screw dislocation annihilation in persistent slip bands is still an open question. We attempt to analyze this problem using the discrete dislocation dynamics simulations. Dislocations are represented by parametrically described curves. The model is based on the numerical solution of the dislocation motion law belonging to the class of curvature driven curve dynamics. We focus on the simulation of the cross-slip of one edge dislocation curve bowing out of the wall of a persistent slip band channel and one screw dislocation gliding through the channel. The dislocations move under their mutual interaction, the line tension and the applied stress. A cross-slip leads to annihilation of the dipolar parts. In the changed topology each dislocation evolves in two slip planes and the plane where cross-slip occurred. The goal of our work is to develop and test suitable mathematical and physical model of the situation. The results are subject to comparison with symmetric configuration of two screw dislocations studied in papers by Pauš et al. The simulation of the dislocation evolution and merging is performed by the improved parametric approach. Numerical stability is enhanced by the tangential redistribution of the discretization points.
For the purpose of estimation of possible inaccuracy in standard discrete dislocation dynamics simulations, we study the motion of interacting dislocations in two regimes: the standard stress control and the total strain control. For demonstration of the difference, we consider two dislocations of opposite signs, gliding in parallel slip planes in a channel of a persistent slip band. Exposed to the applied stress, the dislocations move, bow out, and form a dipole. We investigate the passing stress needed for the dislocations to escape each from other, considering the stress controlled regime and the total strain controlled regime. The motion is described by the mean curvature flow and treated by means of the direct (parametric) method. The results of numerical experiments indicate that the stress control and the total strain control provide upper and lower estimate of the passing stress, respectively, and that these two estimates differ by approximately 10%.
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