J. Senger

Dislocation microstructure evolution in cyclically twisted microsamples: a discrete dislocation dynamics simulation

Miniaturization in technical devices has increased interest in the investigation of the deformation and fatigue behaviour of metals in the micrometre regime. Due to the small dimensions of these devices, mechanical properties depend on the motion of a marginal number of dislocations. In this paper, the evolution of dislocation microstructure in torsion loaded single crystalline aluminium wires is analysed by three-dimensional discrete dislocation dynamics simulations. It is shown that the size of pile-ups and the number of the active slip systems is significantly influenced by cross-slip events independent of the crystallographic orientation. Dislocations are driven by the stress gradient from the applied loading to move into the centre of the sample. These dislocations cannot escape through the surface because of the reversal of the sign of the stress in the centre of the sample. If the micrometre-sized specimens are untwisted, the remaining dislocation microstructure in these samples depends on the maximum torsion angle reached before unloading. The larger the torsion angle, the higher is the remaining dislocation density in the unloaded specimens. These results are discussed with respect to cyclic deformation mechanisms at small scale.

Evolution of mechanical response and dislocation microstructures in small-scale specimens under slightly different loading conditions

In small dimensions, the flow stress of metallic samples shows a size-dependence such that smaller is stronger, even in nominally strain gradient-free loading conditions. However, the role of the boundary conditions in miniaturised tension or compression tests on the mechanical response and dislocation structure has not been studied in detail. In simulations performed with a three-dimensional discrete dislocation dynamics tool, initial, well-defined dislocation microstructures are loaded in tension with different boundary conditions including superimposed torsion moments. The influence of the loading conditions on details of the evolving dislocation microstructure was investigated by using identical starting configuration. An additional torsion moment significantly influences the dislocation activity since forest-dislocations are generated, but size effect of the flow stress is found to be unchanged.

Discrete dislocation dynamics simulation and continuum modeling of plastic boundary layers in tricrystal micropillars

Since the mid 80s various gradient plasticity models have been developed for obtaining the plastic response of materials at the micron- and submicron- scales. In particular, gradient terms have been proven to be crucial for understanding size effects in constrained plastic flow, which are related to the emergence of plasticity boundary layers near passive (plastically not deformable) boundaries. In spite of the success of gradient theories in modeling boundary layer formation, there remain unresolved issues concerning the physical interpretation of the internal length scale involved in the theoretical formulation. Physically, boundary layer formation is related to the piling up of dislocations against the boundaries. This phenomenon is investigated by performing discrete dislocation dynamics (DDD) simulations on a tri-crystal with plastically non-deforming grain boundaries. Strain distributions are derived from the DDD simulations and matched with the results of gradient plasticity calculations, in order to identify the internal length scale governing the boundary layer width.

High Performance Computing and Discrete Dislocation Dynamics: Plasticity of Micrometer Sized Specimens

A parallel discrete dislocation dynamics tool is employed to study the size dependent plasticity of small metallic structures. The tool has been parallelised using OpenMP. An excellent overall scaling is observed for different loading scenarios.

Dislocation Microstructure Evolution in Cyclically Twisted Micrometer‐Sized Metallic Samples: A Discrete Dislocation Dynamics Analysis

Mechanical properties of metals at the micrometer scale are different compared to bulk behavior. At these dimensions, for plastic deformation increasing stresses with decreasing sample sizes are required. This result is explained by the marginal number of activated dislocations which control plasticity. However, for reliable technical devices at small scales, the behavior of dislocations has to be understood in detail. Three‐dimensional discrete dislocation dynamics simulations are an adequate tool to study the dislocation microstructure evolution during the complete deformation process.Dislocation motion in twisted single crystalline aluminum specimens is simulated. Under torsion loading, geometrically necessary dislocations have to be generated to accommodate the strain gradient. Screw dislocations pile‐up in the sample centre where shear stresses are low. Stress gradients hinder the dislocations to move through the sample and escape at the opposite surface. Dislocation density increases linearly as a f...