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TY  - THES
AU  - Dunlap, Anthony
TI  - Elektronenmikroskopische Untersuchung der schädigungsrelevanten physikalischen Mechanismen in umgeformten Einsatzstahl 16MnCrS5
PB  - Rheinisch-Westfälische Technische Hochschule Aachen
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2023-10841
SP  - 1 Online-Ressource : Illustrationen
PY  - 2023
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2024
N1  - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023
AB  - Metal forming is a complex process, influenced by a multitude of microstructural parameters. Damage is such an influence and it is defined as cavity or void, which negatively affects the mechanical properties of a material. Damage caused by metal forming is classified as ductile damage. It represents an early stage of material failure. However, not every damage site necessarily results in failure. A complete understanding of damage allows prolonging the lifetime of a component and offers a potential for lightweight construction of formed work pieces. Electron microscopy is a versatile technique, to investigate damage mechanisms on a microscopic scale. The aim of this work is analyzing the physical mechanisms, that influence the evolution of damage during metal forming, using electron microscopy. An additional major objective is the reliable quantification of damage, which is necessary to assess the damage state of a formed work piece. A ferritic-pearlitic 16MnCrS5 case hardening steel is used to study damage in this thesis. In the investigated state, the steel matrix features a very ductile microstructure with a good formability. Inclusions, specifically manganese sulfides, are the focus of this investigation. Hot-caliber rolled and forward rod extruded work pieces are studied in this work. Additionally, the evolution of damage is observed using in-situ bending experiments in a large chamber scanning electron microscope (LC-SEM). Electron backscatter diffraction (EBSD) is applied to investigate the grain structure and the orientation of the microstructure. Electron channeling contrast imaging (ECCI) and scanning transmission electron microscopy (STEM) to investigate the interaction of dislocations and damage. The quantitative analysis of damage is performed by an automated particle analysis system in SEM using energy-dispersive X-ray spectroscopy. Delamination and fracturing of manganese sulfide is identified as the primary damage mechanism in the case hardening steel. EBSD measurements reveal that the MnS inclusions have a polycrystalline structure. Fracture of inclusions occurs within grains (transcrystalline) and along grain boundaries (intercrystalline). In-situ bending tests allow an observation of damage evolution with increasing strain. Damage is observed at MnS inclusions in formed samples. Due to the elemental contrast present at the interfaces between voids, matrix and inclusions, the EDX particle analysis is shown to be a suitable method for damage quantification. Large areas (≈ 1 mm²) are analyzed automatically and the method delivers reproducible results. Investigating caliber rolled samples revealed a low amount of damage during this forming process. Damage occurred solely at MnS along rolling direction if inclusions contain calcium. Damage quantification of forward rod extruded samples show an increase of void area at forming parameter with stress states, which have a positive triaxiality. This describes a state with a hydrostatic tensile stress component. An extrusion process with a negative triaxiality results in the reduction of void area and damage, even at a high extrusion strain. This shows that the formation of damage does not depend linearly on the strain, although EBSD measurements detect a high degree of deformation in form of texture and increase of small angle grain boundaries. Therefore, deformation does not necessarily result in an increase of damage. Investigation of the dislocation structure along bending samples also shows, that the formation of dislocation not inevitably result in the fracture of MnS inclusions. In Areas of the bending sample under compressive stresses, no voids are observed, although accumulations of dislocations are present in the vicinity of the inclusions. Areas under tensile stresses however, show both, fracture in direct correlation with dislocations as well as fracture that is formed independently of dislocations. Altogether, the applied methods of this work show, that electron microscopy is a suitable method for the investigation of damage mechanisms. EDX particle analysis presents an accessible, large-scale method, which efficiently quantifies damage in 16MnCrS5 case hardening steel. STEM and ECCI are two methods with distinct benefits and drawbacks, which allow the investigation of the dislocation structure. The resolution in STEM is distinctively better. However, the preparation of an individual sample is very complex and time consuming. Application of ECCI in contrast is less complex and it is possible to investigate bulk samples, with varying stress states.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2023-10841
UR  - https://publications.rwth-aachen.de/record/973501
ER  -