With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys. / <p>QC 20130503</p>

Cette étude se propose de fournir une description précise des microstructures lamellaires de deux alliages, le Zircaloy-4 et le TA6V, et de caractériser leur déformation à haute température. A cette fin, des techniques expérimentales nouvelles pour ces matériaux ont été développées : essai Jominy instrumenté, compression plane encastrée à parois mobiles, microtomographie X. Les principaux résultats soulignent le rôle de la phase alpha_GB formée aux anciens joints de grains bêta sur la sélection de variants dans le Zircaloy-4 et le TA6V. L'agencement tridimensionnel des colonies dans le TA6V est également mis en lumière pour la première fois et discuté en relation avec la genèse de la microstructure. Dans le Zircaloy-4 comprimé à haute température, plusieurs mécanismes de localisation de la déformation sont observés. L'activité du maclage à 750 °C, en particulier, est clairement mise en évidence. Une classification de ces hétérogénéités est proposée et leur influence sur la recristallisation est discutée.