Nanocrystalline metals exhibit a high strength/hardness but generally poor ductility during deformation regardless of their crystal structure which is often called the strength-ductility trade-off relationship and generally appears in most ultrafine-grained metals. The ultrafine-grained (UFG) high manganese austenitic twinning-induced plasticity (TWIP) steels have been found to overcome the strength-ductility trade-off but their underlying mechanism of discontinuous yielding behavior has not been well understood. In this study, our systematic TEM characterization suggests that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, show an obvious association with grain size and nucleation of deformation twin was promoted rather than suppressed in UFG. More specifically, the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 m. We also provide insights into the atomistic process of deformation twin nucleation at 3{111} twin boundaries, the dominant type of grain boundary in the UFG-TWIP steel of interest. In response to the external tensile stresses, the structure of coherent 3{111} twin boundary changes from atomistically smooth to partly defective by the grain boundary migration mechanism thus the "kink-like" defective step can act as a nucleation site for deformation twin, which deformation process is different from the one induced by dislocation pile-ups in coarse-grained counterparts and explain why UFG TWIP steel can retain the moderate ductility. In addition to the effect of grain size on deformation twin nucleation, grain boundary character was also taken into account. In coarse-grained TWIP steel, we experimentally reveal that deformation twin nucleation occurs at an annealing twin () boundary in a high-Mn austenitic steel when dislocation pile-up at boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as or  A periodic contrast reversal associated with a sequential stacking faults emission from boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism. The correlation between grain boundary character and deformation behavior was discussed both in low- and high-sigma value grain boundaries. On the other hand, localized strain concentration causes the nucleation of deformation twins at grain boundaries regardless of the grain boundary misorientation character in UFG TWIP steel. The invisibility of stacking fault (zero contrast) was also observed to be emitted at 3{111} boundaries in the coarse-grained TWIP steel, which deformation twin nucleation mechanism is found to be identical to UFG Fe-31Mn-3Si-3Al TWIP steel. / Doctor of Philosophy / High manganese (Mn) twin-induced plasticity (TWIP) steel is a new type of steels which exhibit pronounced strain hardening rate so that offering an extraordinary potential to adjust the strength-ductility relationship. This key advantage will help implement the current development of lightweighting components in automobile industry due to a considerable reduction of material use and an improved press formability. Such outstanding ductility can be contributed by the pronounced strain hardening rate during every such deformation processes, which is highly associated with several different controlling parameters, i.e., SFE, grain orientation, grain size, and grain boundary characters. In this study, we take particular attention to the effect of grain size and grain boundary characters on deformation twinning behavior besides well-known parameters such as SFE and grain orientation. The effect of grain size on deformation twinning behavior was found to be deeply associated with the yielding behavior in TWIP steel, i.e., a discontinuous yielding behavior with a unique yield drop was observed in ultrafine-grained TWIP while a continuous yielding behavior was observed in coarse-grained counterpart. Our TEM characterization indicates that the microstructural features of grains >10 m are different from the microstructural features in grains < 1 m. In over-10 m grains, normal dislocation slips and the formation of in-grain stacking faults are the main deformed microstructure. However, in the under-1 m grains, the in-grain dislocation slip is inhibited, but the deformation twinning is promoted at grain boundaries. This deformation transition from in-grain slip to twinning at grain boundary appears to be responsible for the discontinuous yielding behavior observed in stress-strain curve. The effect of grain boundary character on deformation twinning was examined in both coarse- and ultrafine-grained TWIP steels. In coarse-grained TWIP steel, we found that deformation twinning behavior varies as the function of boundary structure, i.e., different atomic configuration. Coherent twin boundary can act as a nucleation site for deformation twin as a localized strain concentration was introduced by dislocation pile-ups. On the other hand, incoherent boundaries can act as a deformation twin nucleation site by a boundary relaxation mechanism, i.e., grain-boundary dislocations can dissociate into partial dislocations to both side of boundary to accommodate the misfit between grains. In UFG TWIP steel, we found that the coherent twin boundary can act as a deformation twin nucleation site without presence of dislocation pile-ups. Alternatively, twin boundary becomes defective with a "kink-like" step by boundary migration. As a result, this defective step would progressively accumulate localized strain field thus stimulate the nucleation of deformation twin. Such study provides a novel insight into the UFG TWIP steel and a roadmap toward controlling TWIP effect.

High manganese TWIP (TWinning Induced Plasticity) steels are particularly attractive for automotive applications because of their exceptional properties of strength combined with an excellent ductility. However, as austenitic steels, they appear to be sensitive to liquid zinc embrittlement during welding, the liquid zinc arising from the melted coating due to the high temperatures reached during the welding process. In this framework, the cracking behaviour of a high manganese austenitic steel has been investigated in relation to the liquid metal embrittlement (LME) phenomenon by hot tensile tests carried out on electro-galvanized specimens using a Gleeble 3500 thermomechanical simulator. The influence of different parameters such as temperature and strain rate on cracking behaviour has been studied. Embrittlement appears within a limited range of temperature depending on experimental conditions. Conditions for which cracking occurs could be experienced during welding processes. The existence of a critical stress above which cracking appears has been evidenced and this critical stress can be used as a cracking criterion. Finally, the study of the influence of different parameters such as time of contact between steel and liquid zinc before stress application, coating and steel on LME occurrence provides understanding elements of LME mechanism and permits to suggest solutions for preventing cracking during spot welding of such steels.

Les aciers TWIP (TWinning Induced Plasticity) à haute teneur en manganèse sont particulièrement prometteurs pour les applications automobiles de par leur excellent compromis entre résistance mécanique et ductilité. Cependant, la microstructure austénitique leur confère une sensibilité à la fragilisation par le zinc liquide durant les procédés de soudage ; le zinc liquide provenant de la fusion du revêtement résultant de l’élévation de température à la surface de l’acier. Dans cette étude, la fissuration d’un acier austénitique à haute teneur en manganèse a été étudiée en rapport avec le phénomène de fragilisation par les métaux liquides par des essais de traction à chaud réalisés sur des éprouvettes électrozinguées au moyen d’un simulateur thermomécanique Gleeble 3500. L’influence de nombreux paramètres tels que la température et la vitesse de déformation sur la fissuration a été étudiée. La fragilisation apparaît dans un domaine de température limité qui dépend des conditions expérimentales. Les conditions pour lesquelles la fissuration apparaît peuvent être rencontrées durant les procédés de soudage. L’existence d’une contrainte critique pour laquelle la fissuration apparait a été mise en évidence et celle-ci peut être utilisée comme critère de fissuration. Enfin, l’étude de l’influence de différents paramètres tels que le temps de contact entre l’acier et le zinc liquide avant l’application des contraintes, le revêtement et l’acier sur l’apparition de la fragilisation apporte des éléments de compréhension du mécanisme de fissuration et permet de proposer des solutions pour éviter la fissuration durant le soudage par point de l’acier étudié. / High manganese TWIP (TWinning Induced Plasticity) steels are particularly attractive for automotive applications because of their exceptional properties of strength combined with an excellent ductility. However, as austenitic steels, they appear to be sensitive to liquid zinc embrittlement during welding, the liquid zinc arising from the melted coating due to the high temperatures reached during the welding process. In this framework, the cracking behaviour of a high manganese austenitic steel has been investigated in relation to the liquid metal embrittlement (LME) phenomenon by hot tensile tests carried out on electro-galvanized specimens using a Gleeble 3500 thermomechanical simulator. The influence of different parameters such as temperature and strain rate on cracking behaviour has been studied. Embrittlement appears within a limited range of temperature depending on experimental conditions. Conditions for which cracking occurs could be experienced during welding processes. The existence of a critical stress above which cracking appears has been evidenced and this critical stress can be used as a cracking criterion. Finally, the study of the influence of different parameters such as time of contact between steel and liquid zinc before stress application, coating and steel on LME occurrence provides understanding elements of LME mechanism and permits to suggest solutions for preventing cracking during spot welding of such steels.

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Das Elektronenstrahlschweißen im Vakuum hat sich als zuverlässiges Verfahren für die Herstellung schmaler und hochpräziser Schweißnähte beim Schweißen von TRIP-Stählen bewährt. Das Verständnis für die dabei auftretenden Mechanismen und wirkenden Kräfte stellt einen wichtigen Baustein für die Weiterentwicklung des Verfahrens dar. Um zur Erweiterung dieses Verständnisses beizutragen, wird auf Basis vorhandener Berechnungsmethoden in OpenFOAM ein numerisches Modell für das Elektronenstrahlschweißen entwickelt. Es ist in der Lage, die dafür relevanten Einflussfaktoren zu berücksichtigen. So werden die Wärmeübertragung im Feststoff und der Schmelze, alle Aggregatzustandsänderungen und die auf die Dynamik der Schmelze wirkenden Kräfte einbezogen. Das entwickelte Simulationsmodell ist in der Lage zu zeigen, dass außer der natürlichen Konvektion vor allem der beim Verdampfen der Schmelze entstehende Überdruck und die thermokapillare Konvektion an der Schmelzeoberfläche für hohe Strömungsgeschwindigkeiten verantwortlich sind. Darüber hinaus haben neben der Schmelzbaddynamik die thermophysikalischen Eigenschaften des Stahls einen starken Einfluss auf die Ausprägung der Schweißnaht. Vor allem die Wärmeleitfähigkeit verändert diese erheblich, was die Simulationen unter Berücksichtigung der Temperaturabhängigkeit verdeutlichen. Die in dieser Arbeit erreichten Erkenntnisse helfen, die beim Elektronenstrahlschweißen entstehenden Nahtgeometrien und die Gründe für hohe Strömungsgeschwindigkeiten im Schmelzbad besser einordnen und verstehen zu können. Darüber hinaus dient das entwickelte numerische Modell mit der Berücksichtigung aller relevanten Mechanismen als Grundlage für Weiterentwicklungen hinsichtlich vielerlei Anwendungen, beispielsweise für das Schweißen anderer Werkstoffe, zusätzliche Effekte wie dem Spiking oder anderen Elektronenstrahltechnologien wie dem Elektronenstrahlschmelzen im Bereich der additiven Fertigung.