The main focus in the present work concerns calculations on steel/slag equilibria. Thermodynamic software and databases are now powerful and accurate enough to give reliable results when applied to complex metallurgical processes. One example is the decarburization process of high alloyed steels. It is shown that using advanced thermodynamic models, without a complicated kinetic description of the system, reasonable agreement with experimental data is obtained. The calculations are performed using the Thermo-Calc software. Within this work a Java interface for Thermo-Calc has been implemented. Java gives graphical possibilities and a graphical interface has been created that facilitates calculations that involve both metallic phases as well as oxides and make them feasible also for an industrial user. / QC 20101118

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Firstly, an existing thermodynamic model for the predicting of martensite start temperature of commercial steels has been improved to include more elements such as N, Si, V, Mo, Nb, W, Ti, Al, Cu, Co, B, P and S and their corresponding composition ranges for Martensitic transformation. The predicting ability of the existing model is improved considerably by critical assessment of different binary and ternary systems i.e. CALPHAD approach which is by wise selection of experimental data for optimization of the interaction parameters. Understanding the degree of variation in multi-component commercial alloys, various ternary systems such as Fe-Ni-X and Fe-Cr-X are optimized using both binary and ternary interaction parameters. The large variations between calculated and the experimental values are determined and reported for improvements in thermodynamics descriptions.Secondly, model for the prediction of Epsilon martensite start temperature of some commercial steels and shape memory alloys is newly introduced by optimizing Fe-Mn, Fe-Mn-Si and other Fe-Mn-X systems considering the commercial aspects in the recent development of light weight steels alloyed with Al and Si.Thirdly, the effect of prior Austenite grain size (pAGS) on martensite start temperature is introduced into the model in the form of non-chemical contribution which will greatly influence the Gibbs energy barrier for transformation. A serious attempt has been made to describe the dependency of transition between lenticular and thin-plate martensite morphologies on the refinement of prior Austenite grain size.Finally, the model is validated using a data-set of 1500 commercial and novel alloys. Including the newly modified thermodynamic descriptions for the Fe-based TCFE9 database by Thermo-Calc software AB, the model has the efficiency to predict the martensite start temperature of Multi-component alloys with an accuracy of (±) 35 K. The model predictability can be further improved by critical assessment of thermodynamic factors such as stacking faults and magnetism in Fe-Mn-Si-Ni-Cr systems.

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La mise au point de réacteurs nucléaires sous-critiques (Accelerator Driven System - ADS), intrinsèquement sûrs, pour incinérer les déchets nucléaires par transmutation d’actinides de durée de vie longue en radioéléments de plus courte durée de vie, réclament des sources intenses de neutrons. Ces sources sont produites par la technique de spallations « éjections », c’est-à-dire l’interaction d’un faisceau de protons de haute énergie et haute intensité avec une cible contenant des métaux lourds. Compte tenu de la puissance déposée par les protons dans la cible (de l'ordre de quelques MW) seuls des métaux sous forme liquide peuvent à la fois résister aux défauts induits et évacuer la chaleur. Dans ce contexte l'expérience MEGAPIE (MEGawatt PIlot Experiment) fonctionnant avec une fenêtre au niveau de la cible de spallation est pionnière et ouvre la voie vers des cibles de haute puissance. Pour la mise en œuvre de cette cible MEGAPIE, les choix de l’alliage de la cible de spallations, des matériaux de structures de la fenêtre et des parties chaudes sont portés respectivement sur l’eutectique plomb-bismuth, bon rendement neutronique, et l’acier T91 (1%Mo9%Cr en masse), bonne tenue à l’irradiation. Pour les parties froides l’acier 316L a été choisi. Toutefois, malgré sa bonne tenue à l’irradiation, le contact de ces deux alliages (eutectique plomb-bismuth et acier T91) présente quelques inconvénients qui nécessitent d’approfondir la connaissance de leurs propriétés physico-chimiques dans l’environnement MEGAPIE et motive cette présente étude. La contribution de ce travail au développement du réacteur de transmutation a consisté : - d’une part à l'élaboration de revêtements destinés à améliorer les performances de l'acier de structure T91 vis à vis de l'oxydation et de la fissuration par l'eutectique Pb-Bi liquide, - d'autre part à l'étude thermodynamique du système Pb-Bi-(Hg)-Fe-Cr-Al-O pour connaître l’ensemble des équilibres entre phases existant dans les conditions imposées par le fonctionnement du réacteur ADS. Le deuxième volet de ce travail trouve sa justification dans les résultats du premier qui ont mis en évidence le rôle primordial joué par les oxydes mixtes dans l'interaction corrosive entre l'acier et l'eutectique Pb-Bi faiblement oxygéné. / The development of sub-critical nuclear reactors (Accelerator Driven System - ADS), intrinsically safe to incinerate nuclear waste by transmutation of long-lived actinides to radionuclides with a shorter existence, requires intense sources of neutrons. These sources are produced by the "ejected" spallation technique, i.e. the interaction of a high energy, high intensity proton beam with a target containing heavy metals. Considering the power of the proton beam transferred to the target (of a few MW), only metals in the liquid form can be cooled in a closed cooling system and resist to nuclear reactor induced by the proton beam. In this context, the MEGAPIE project (MEGawatt PIlot Experiment), which works with a window at the spallation target, is pioneer and opens the way for high-power targets. The choice of the alloy for the MEGAPIE spallation target is a lead-bismuth eutectic, which is efficient for producing neutrons. For the window beam and hot parts, the best choice at present is the T91 steel (1%Mo9%Cr in mass), due to its good irradiation resistance. For cold parts, the 316L steel was chosen. However, the contact of the two alloys (eutectic lead-bismuth and T91 steel) presents some inconveniences that need to be investigated in relation to their physico-chemical properties in the MEGAPIE environment, which motivates this study. The contribution of this work to this subject consisted of : - In a part, the development of coatings designed to improve the chemical and mechanical performances of the T91 steel in contact of the lead-bismuth eutectic liquid alloy, - In another part, the investigation of the thermodynamic properties of the Pb-Bi-(Hg)-Fe-Cr-Al-O system for all phase equilibria within the conditions imposed by the operation of the ADS reactor. The second part of this work is justified by the results, which, for the first time, highlight the role played by the mixed oxide phases in the corrosive interaction between the T91 steel and Pb-Bi eutectic, in a poorly oxygenated environment.