Spinodal decomposition is a phase separation mechanism within the miscibility gap. Its importance in case of Fe-Cr system, the basis of the whole stainless steel family, stems from a phenomenon known as the “475oC embrittlement” which results in a ruin of mechanical properties of ferritic, martensitic and duplex stainless steels. This work is aimed at a better understanding of the phase separation process in   the Fe-Cr system. Alloys of 10 to 55 wt.% Cr , each five percent, were homogenized to achieve fully ferritic microstructure and then isothermally aged at 400, 500 and 600oC for different periods of time ranging from 30min to 1500 hours. Hardness of both homogenized and aged samples were measured by the Vickers micro-hardness method and then selected samples were studied by means of Transmission Electron Microscopy (TEM).  It was observed that hardness of homogenized samples increased monotonically with increasing Cr content up to 55 wt.% which can be attributed to solution hardening as well as higher hardness of pure chromium compared to pure iron.  At 400oC no significant change in hardness was detected for aging up to 1500h, therefore we believe that phase separation effects at 400oC are very small up to this time. Sluggish kinetics is imputed to lower diffusion rate at lower temperatures. At 500oC even after 10h a noticeable change in hardness, for alloys containing 25 wt.% Cr and higher, was observed which indicates occurrence of phase separation. The alloy with 10 wt.% Cr did not show change in hardness up to 200h which suggests that this composition falls outside the miscibility gap at 500oC. For compositions of 15 and 20 wt.% Cr only a small increase in hardness was detected even after 200h of aging at 500oC, which could be due to the small amounts of α´ formed. However, it means that alloys of 15 wt.% Cr and higher are suffering phase separation. For compositions inside the miscibility gap, hardening effect is a result of phase separation either by nucleation and growth or spinodal decomposition. To distinguish between these two mechanisms, TEM studies were performed and we found evidence that at 500oC the Fe-25 wt.% Cr sample decomposes by nucleation  and growth  while that of 35 wt.% Cr  shows characteristics of the spinodal mechanism. For compositions inside the miscibility gap, with increasing Cr content up to 40% the change in hardness generally increased and for 45% and higher it always decreased. This suggests that the composition range corresponding to the spinodal region at 500oC is biased towards the Fe-rich side of the phase diagram. At 600oC only samples of 25, 30 and 35 wt.% Cr were studied because according to the previous studies, the spinodal boundary is most probably located in this composition range. However, no change in hardness was observed even up to 24h. We believe that this means the miscibility line lies below 600oC for alloys containing 35 wt.% Cr and lower. Further investigations are needed to confirm and explain this result.

Dans la section UO2-PuO2-Pu2O3, les équilibres de phases décrivent un domaine monophasé (U1-y,Puy)O2-x stable pour y<0,20 à 25°C et jusqu'à l'équilibre solide-liquide. Aux teneurs Pu supérieures, ils sont plus complexes avec l'apparition d'une démixtion et la précipitation de phase(s) additionnelle(s). L'objectif de la thèse a consisté à améliorer la représentation du système pour 0,15≤y≤0,65 et 25≤T(°C)≤1500.A 25°C, une lacune de miscibilité composée de deux phases (U1-y,Puy)O2-X a été observée pour y<0,45, dont l'une est de rapport Oxygène/Métal proche de la stœchiométrie et une autre très réduite. Pour la première fois, un domaine triphasé a été caractérisé à teneurs Pu supérieures avec deux phases (U1-y,Puy)O2-X de teneurs proches de y=0,45, et une phase (U1-y,Puy)2O3 comprenant une faible proportion d'uranium solubilisée.L'étude en fonction de la température a démontré que la température de démixtion augmente avec la teneur Pu. Plusieurs représentations ont été établies. A 200°C, les limites d'existence du domaine multiphasé évoluent peu par rapport à 25°C. A 400°C, la démixtion survient à une teneur Pu proche de 0,35, largement inférieure à celle suggérée par la littérature. A 600°C, les résultats précisent les équilibres de phases jusqu'alors très méconnus avec une démixtion apparaissant à partir de y=0,60.L'analyse microstructurale des échantillons a clairement démontré l'impact significatif de la démixtion sur le matériau se traduisant par des fissures au sein des échantillons, d'autant plus nombreuses que la teneur en Pu est élevée. / In the UO2-PuO2-Pu2O3 section, a monophasic (U1-y,Puy)O2-x domain is stable for y<0,20 at 25°C and up to solid-liquid equilibrium. At higher Pu content, phase equilibria are more unclear with a phase separation process. The main objective of this work consisted in upgrading the representation of this system for 0,15≤y≤0,65 and 25≤T(°C)≤1500.At 25°C, a miscibility gap composed by two different (U1-y,Puy)O2-X phases has been observed for y<0,45, with one very closed to stoichiometric state (Oxygen/Metal=2) and one other very reduced. For the first time, a triphasic domain has been characterized at higher Pu contents, with two (U1-y,Puy)O2-X phases near y=0,45 and one (U1-y,Puy)2O3 phase with a low U content inside. Concerning the study in function of temperature, we have demonstrated that phase separation temperature increase when Pu content grows. Several representations have been established. At 200°C, the representation is closed to that at 25°C. At 400°C, the phase separation have been specified at a lower Pu content than that of literature : y=0,35. At 600°C, our results have clarified the section, until then very unclear, with a phase separation appearing at y=0,60.The microstructural analysis has clearly demonstrated the significant impact of the phase separation on the material. Indeed many cracks have been observed in our samples, and quantity of these defects increases when Pu content grows.