A bottom-up continuum approach of crystal plasticity for the analysis of fcc microwires under torsion

The microstructural evolution of face‐centered cubic microwires is studied using a physical motivated, homogenized continuum model of crystal plasticity. The dislocation configuration in the three‐dimensional space is thereby described via a Continuum Dislocation Dynamics (CDD) theory including a dislocation source term. The resulting spatial distribution of dislocation densities and strain components are shown for a relaxation problem with torsional loading

Analyse von Gleitsysteminteraktionen und deren Auswirkungen auf die Versetzungskonfigurationen in Mikrosystemen

Im Zuge der Miniaturisierung von Bauteilen stößt die Anwendung von mechanischen Komponenten in den Bereich der Mikroskala vor. In diesem Bereich unterscheidet sich das plastische Verformungsverhalten metallischer Mikrosysteme aufgrund von sogenannten Größeneffekten von dem bekannten Verhalten makroskopischer Bauteilkomponenten. Zur Gewährleistung der Zuverlässigkeit der Mikrosysteme ist ein tiefgreifendes Verständnis für die Mikrostrukturevolution von elementarer Bedeutung, wobei die genauen Auswirkungen der auftretenden Gleitsysteminteraktionen bisher größtenteils unbekannt sind. Im Rahmen dieser Arbeit wird eine mechanismusbasierte Kontinuumsformulierung der Kristallplastizität weiterentwickelt sowie datengetriebene Analysemethoden und theoretische Systemanalysen eingesetzt, um direkte Einblicke in die Interaktionen von Gleitsystemaktivitäten und deren Auswirkungen auf die Versetzungskonfigurationen in einkristallinen, kubisch-flächenzentrierten Metallen unter verschiedenen Belastungen zu erhalten. Hierbei lässt sich eine Klassifikation der Gleitsysteme vornehmen, wobei sich die einzelnen Gleitsystemgruppen bezüglich ihrer Aktivitäten, Spannungsrelaxationsmechanismen sowie Versetzungskonfigurationen unterscheiden. Die beobachtete Akkumulation von sogenannter geometrisch notwendiger Versetzungsdichte lässt sich in homogenen Spannungsfeldern auf den inaktiven Gleitsystemen und in inhomogenen Spannungsfeldern auf den aktiven Gleitsystemen lokalisieren, wobei sich die Stabilisierung der Versetzungsdichten in den Systemen auf die jeweiligen Spannungskonfigurationen und die Bildung von Versetzungsnetzwerken zurückführen lässt. Der interne Versetzungsaufstau induziert dabei einen Größeneffekt, sofern der Einfluss interner Spannungen verglichen zum externen Spannungsfeld ausreichend groß ist. Folglich vertieft diese Arbeit signifikant das Verständnis für die Gleitsysteminteraktionen bzw. für das Materialverhalten der Mikrosysteme im Allgemeinen

Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

With decreasing system sizes, the mechanical properties and dominant deformation mechanisms of metals change. For larger scales, bulk behavior is observed that is characterized by a preservation and significant increase of dislocation content during deformation whereas at the submicron scale very localized dislocation activity as well as dislocation starvation is observed. In the transition regime it is not clear how the dislocation content is built up. This dislocation storage regime and its underlying physical mechanisms are still an open field of research. In this paper, the microstructure evolution of single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed under compression loading. Experimental in situ HR-EBSD measurements as well as 3d continuum dislocation dynamics simulations are presented. The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities. Results are presented that show, how the plastic deformation of the material takes place and how the different slip systems are involved. A central finding is, that an increasing amount of GND density is stored in the system during loading that is located dominantly on the slip systems that are not mainly responsible for the production of plastic slip. This might be a characteristic feature of the considered size regime that has direct impact on further dislocation network formation and the corresponding contribution to plastic hardening.Comment: submitted, not yet accepte

Characterization of Lomer junctions based on the Lomer arm length distribution in dislocation networks

During the plastic deformation of crystalline materials, 3d dislocation networks form based on dislocation junctions. Particularly, immobile Lomer junctions are essential for the stability of dislocation networks. However, the formed Lomer junctions can unzip and dissolve again, if the linked mobile dislocations of the Lomer junction - the Lomer arms - experience sufficiently high resolved shear stresses. To generate a better understanding of the dislocation network stability and to pave the way to a general stability criterion of dislocation networks, we investigate the Lomer arm length distribution in dislocation networks by analyzing discrete dislocation dynamics simulation data of tensile-tested aluminum single crystals. We show that an exponential distribution fits best to the Lomer arm length distribution in the systems considered, which is independent of the crystal orientation. The influence of the slip system activity on the Lomer arm length distribution is discussed

An Empirical Evaluation of Constrained Feature Selection

While feature selection helps to get smaller and more understandable prediction models, most existing feature-selection techniques do not consider domain knowledge. One way to use domain knowledge is via constraints on sets of selected features. However, the impact of constraints, e.g., on the predictive quality of selected features, is currently unclear. This article is an empirical study that evaluates the impact of propositional and arithmetic constraints on filter feature selection. First, we systematically generate constraints from various types, using datasets from different domains. As expected, constraints tend to decrease the predictive quality of feature sets, but this effect is non-linear. So we observe feature sets both adhering to constraints and with high predictive quality. Second, we study a concrete setting in materials science. This part of our study sheds light on how one can analyze scientific hypotheses with the help of constraints

Irreversible evolution of dislocation pile-ups during cyclic microcantilever bending

In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability

Irreversible evolution of dislocation pile-ups during cyclic microcantilever bending

In metals geometrically necessary dislocations (GNDs) are generated primarily to accommodate strain gradients and they play a key role in the Bauschinger effect, strain hardening, micron-scale size effects and fatigue. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pileups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems to provide an in-depth explanation of the Bauschinger-effect and strain hardening at this scale. We conclude that the most dominant role in this behaviour is played by short-range dislocation interactions