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Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen 1 Online-Ressource : Illustrationen (2022). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, The cyclic plasticity behavior of metallic materials is of great interest for design optimization and service life-time calculation of industrial components in complex loading conditions. A central role is assigned to the Bauschinger effect, which describes a loss of strength during load reversal. The Bauschinger effect is attributed to various microstructural origins and influenced by diverse material-specific factors as well as experimental boundary conditions. Numerous influencing factors have already been quantified and explained phenomenologically, usually focusing on a specific material and selected influencing factors. However, the relationships between the Bauschinger effect in single crystals, single-phase polycrystals, and multiphase materials have not been systematically investigated. In addition, it is not known whether and how the effect of individual influencing parameters, such as precipitates, differs in these microstructures. Against this background, the Bauschinger effect was analyzed in i) single crystals of Ni-base superalloy 718, ii) chemically identical Alloy 718 polycrystals, and iii) the two-phase duplex stainless steel (DSS) 1.4462 within the scope of this study. A particular focus was on the influence of nanoscale precipitates, which was investigated from γ'' precipitates in Alloy 718 and α' precipitates in DSS. The Bauschinger effect was quantified by various experimental methods on different size scales. Fully reversed cyclic micro-bending tests and cyclic micro-indentation tests on Alloy 718 single crystals with different crystal orientations allow the determination of slip system related material properties and the investigation of the Bauschinger effect depending on activated slip systems. The relationship between single crystal and polycrystal plasticity was analyzed using macroscopic tension-compression tests. Furthermore, the evaluation of dislocation slip-traces as well as quasi in-situ electron backscatter diffraction analyses give insights into local plasticity behavior at different loading conditions during tension-compression tests. The experimental analyses have shown that the Bauschinger effect is only weakly pronounced in precipitation-free single crystals and results mainly from local dislocation activities. Plasticity behavior is characterized by predominantly planar dislocation slip, dislocation annihilation, and the formation and dissolution of dislocation pile-ups in front of obstacles. Nanoscale γ'' precipitates increase dislocation interaction and lead to the activation of additional slip systems. Planar dislocation slip is obstructed and dislocation sub-structures with local gradients of dislocation density are developed. These result in long-range back stresses, which lead to a more pronounced Bauschinger effect with increasing precipitation content. The Bauschinger effect in solution-annealed Alloy 718 polycrystals is more pronounced than in the single crystal, but a decrease in Bauschinger effect with increasing γ'' precipitation content is observed. Of particular importance are back stresses at grain boundaries, which result from the deformation incompatibility of adjacent grains with different crystal orientation. The experimental results indicate that γ'' precipitates reduce the orientation dependence of elastic-plastic properties in fcc crystal lattice, leading to a decrease in back stresses at grain boundaries and thus a less pronounced Bauschinger effect. DSS 1.4462 exhibits a highly pronounced Bauschinger effect during load reversal. Main reason is the strong deformation incompatibility at phase boundaries due to different elastic-plastic properties of ferrite and austenite. Nanoscale α' precipitates due to spinodal decomposition of the ferrite (475°C embrittlement) increase the Bauschinger effect by amplifying this phase difference, as demonstrated by cyclic micro-indentation tests in single austenitic and ferritic grains. In summary, the experimental investigations have shown that various factors influencing the Bauschinger effect are interdependent in a complex relationship. Therefore, the effect of specific factors, such as nanoscale precipitates, cannot be individually quantified. Rather, they always have to be analyzed in the context of the existing microstructure and experimental boundary conditions. The quantification and phenomenological explanation of influencing factors on the Bauschinger effect enhances the understanding of these microstructure-property correlations, and thereby contributes to the targeted design and simulative description of industrial components in cyclic loading conditions., Published by Aachen

Eine physikalisch begründete dreidimensionale Kontinuumsformulierung der plastischen Verformung von Metallen benötigt neben einer kinematischen Beschreibung der Bewegung gekrümmter Versetzungslinien eine adäquate Homogenisierung der Versetzungswechselwirkungen. Insbesondere die Beschreibung von Versetzungsnetzwerken erfordert eine Verbindung der gleitsystemgebundenen Versetzungsbewegung mit der gleitsystemübergreifenden Ausprägung der Interaktion und Reaktion von Versetzungen. Basierend auf Beobachtungen in diskreten Versetzungsdynamiksimulationen wird ein Modell der Mobilität von Versetzungsnetzwerken entwickelt, dass neben der Versetzungsbewegung und Versetzungsinteraktion auch explizite Versetzungsreaktionen, sowie Quergleiten berücksichtigt. Neben der Versetzungsmultiplikation haben die berücksichtigten Mechanismen auch eine Stabilisierung von Versetzungen zur Folge. Vereinfachte Beispiele zeigen, dass die Homogenisierung zugrundeliegender physikalischer Mechanismen zu einem Anstieg der Versetzungsdichte auf unbelasteten Gleitsystemen, sowie zu einer gleitsystemübergreifenden Limitierung der Versetzungsbewegung führt. Anhand von Vergleichen mit Ergebnissen diskreter Versetzungsdynamiksimulationen wird deutlich, dass das Modell im Gegensatz zu existierenden Formulierungen in der Lage ist die Mobilität von Versetzungsnetzwerken adäquat zu reproduzieren. Damit wird die Verfestigung als eine Folge der Evolution von Versetzungsnetzwerken erreicht, welche wiederum aus einem Zusammenspiel aus plastischer Scherung, Versetzungsmultiplikation und Limitierung von Versetzungsbewegung entsteht.