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48,80 €ISBN 978-3-8440-5762-1Paperback156 Seiten44 Abbildungen230 g21 x 14,8 cmEnglischDissertation
Februar 2018
Lutz Morsdorf
Fundamentals of ferrous low-carbon lath martensite: from the as-quenched, to tempered and deformed states
This thesis aims at extending the fundamental understanding of lath martensitic microstructures in low-carbon steels. Ferrous lath martensite is the crucial microstructure constituent to achieve high strength, not only in purely martensitic steels but also in multi-component materials such as dual-phase (DP), quenching & partitioning (QP) and transformation induced plasticity (TRIP) steels.
First, a novel hierarchical microstructure description based on the martensitic transformation sequence, i.e. martensite transforming step by step between martensite-start and martensite-finish temperatures, is formulated and added to the concept of crystallographic hierarchy in lath martensite. A combination of electron channeling contrast imaging, atom probe tomography and nanoindentation reveals that earliest transformed laths are rather coarse in size, least dislocated and most autotempered which is in significant contrast to the substructure in later transformed thinner laths.
Second and third, the relevance of those microstructure heterogeneities within the bounds of single prior austenite grains is demonstrated in martensite tempering and deformation behaviors. To investigate the nano-scale tempering reactions, the experimental techniques mentioned above are applied in a quasi-in-situ manner. This enables systematic monitoring of the tempering-induced microstructure evolution by always starting from the same autotempering condition in a single lath. Regarding the deformation response of such heterogeneous as-quenched microstructures, strain localization is studied by a multi-field strain mapping approach coupled to topographic trace analysis. Both are carried out in a scanning electron microscope on stepwise deformed samples.
First, a novel hierarchical microstructure description based on the martensitic transformation sequence, i.e. martensite transforming step by step between martensite-start and martensite-finish temperatures, is formulated and added to the concept of crystallographic hierarchy in lath martensite. A combination of electron channeling contrast imaging, atom probe tomography and nanoindentation reveals that earliest transformed laths are rather coarse in size, least dislocated and most autotempered which is in significant contrast to the substructure in later transformed thinner laths.
Second and third, the relevance of those microstructure heterogeneities within the bounds of single prior austenite grains is demonstrated in martensite tempering and deformation behaviors. To investigate the nano-scale tempering reactions, the experimental techniques mentioned above are applied in a quasi-in-situ manner. This enables systematic monitoring of the tempering-induced microstructure evolution by always starting from the same autotempering condition in a single lath. Regarding the deformation response of such heterogeneous as-quenched microstructures, strain localization is studied by a multi-field strain mapping approach coupled to topographic trace analysis. Both are carried out in a scanning electron microscope on stepwise deformed samples.
Schlagwörter: lath martensite; carbon; as-quenched; tempering; deformation; APT; ECCI; EBSD
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