Mechanics and Dislocation Structures at the Micro-Scale: Insights on Dislocation Multiplication Mechanisms from Discrete
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Mechanics and Dislocation Structures at the Micro-Scale: Insights on Dislocation Multiplication Mechanisms from Discrete Dislocation Dynamics Simulations D. Weygand1 1 Institute for Applied Materials, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany ABSTRACT The plasticity of micro-pillar deformation has widely been studied by discrete dislocation dynamics simulations to explain the so-called size effect. In this study the role of glissile junctions forming during plastic deformation under various loading scenarios is in the center of interest. The activity of these naturally forming dislocation sources is followed in detail. Surprisingly these junctions are rather active sources and not just another obstacle as often assumed. Their relative contribution to the overall dislocation density for the simulated specimens reaches often values of 20% or even more. The formation of such a glissile junction is often correlated to stress drops or the end of a stress drop. It is therefore suggested – at least for the sample sizes considered – that this dislocation multiplication mechanism should be take into account in continuum models such as crystal plasticity of higher order dislocation continuum theories. INTRODUCTION Since the intriguing observation of a size effect in the plastic flow of micro-specimens under nominally uniaxial loading [1], many groups investigated such samples by use of three dimensional discrete dislocation dynamics tools [2,3,4]. The recent overviews [5,6] provide comprehensive insight to the main deformation mechanisms at these small length scales, including the size effects observed in thin metallic films too. In the present study we will not elaborate further on the size effect, but only on the evolution of dislocation densities and multiplication of dislocations within such small specimens, which is still lacking in the literature. The systems studied are micro-specimens whose smallest dimension is in the range of 0.5µm to 1.5µm and with aspect ratio of at least 2 to 3 in order to reduce effects of boundary constraints from the top and bottom surface, where displacements are prescribed. The systems considered here are essentially similar to those of the size-effect study for pillar geometries [2,4,7-9] and the study on the torsion of wires [10] for different torsion axis. THEORY Plasticity is governed by the glide of dislocations. In a discrete dislocation dynamics code, the dislocations are represented as discretized lines and their motion is calculated based on the elastic interactions between dislocation and the externally applied load. The discrete dislocation dynamics tool, described in [11-13] is used here for studying the evolution of a given dislocation population within a finite volume. The calculations assume isotropic elasticity and the elastic constants mimic isotropic Al. For this study, the history of the dislocation is tracked, such that each dislocation can be followed back to its initial appearance in the system. A dislocation may first enter the simulati
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