Les aciers ferritiques à faible teneur en chrome tel que le T/P91 sont largement utilisés dans les centrales de productions d’électricité pour leurs bonnes propriétés mécaniques et leur faible coefficient d’expansion thermique. Cependant, la demande croissante en énergie alliée à la nécessité de réduire les émissions de gaz à effet de serre, conduisent à envisager l’augmentation des conditions d’utilisation (température et pression) de ces matériaux. Des études ont montré qu’en modifiant la température de fonctionnement et la pression de vapeur d’eau de 538°C/18.5 MPa à 650°C/30 MPa, le rendement des centrales thermiques progressait d’environ 8%. Se pose alors la question de la tenue à la corrosion à haute température des aciers à 9% de chrome. Au cours de ces travaux, le comportement d’un acier ferritique/ martensitique à 9% de chrome a été étudié à 650°C sous air sec et sous vapeur d’eau de matière isotherme et en conditions de cyclage thermique. La prise de masse des échantillons renseigne sur la cinétique de la réaction d’oxydation et l’adhérence des couches d’oxydes formées. Les produits de corrosion ont été caractérisés par plusieurs techniques d’analyses dans l’optique de clairement identifiés les oxydes en présences et leurs mécanismes de formation. Des oxydes mixtes de fer et de chrome (Cr,Fe)2O3 sont dans un premier temps formés et assurent s’avèrent être temporairement protecteur. Pour des longs temps d’oxydation ou des températures supérieures à 650°C, la magnétite Fe3O4 et l’hématite Fe2O3 sont les principaux oxydes formés, montrant ainsi l’inadéquation des nuances à faible teneur en chrome pour une utilisation dans des conditions aussi drastiques. Dans l’optique d’augmenter la résistance à la corrosion à haute température de cet alliage, diverses solutions ont été envisagées tel que l’aluminisation par cémentation en caisse, les revêtements d’oxydes de terre rare par MOCVD, ou encore l’ajout d’éléments d’addition. Ces solutions ont été également testées à 650°C sous air sec et sous vapeur d’eau. / The improvement of high temperature oxidation resistance of low chromium content steels, such as T/P91, is of great interest in regards with their application in thermal power generating plants. Indeed, they possess good creep properties, and low thermal expansion coefficient. Important needs in energy together with environmental issues place power generation plants under constraints which lead to develop high efficiency systems. A usual way to increase the efficiency consists in increasing temperature and pressure parameters of the power generating plant. Studies has shown that the total efficiency of a plant increases by nearly 8 % when changing the steam parameters from 538°C/18.5 MPa to 650°C/30 MPa. Then, the problem of corrosion resistance of 9% chromium steel in those conditions is asked. In this work, the behavior of a ferritic / martensitic 9% chromium steel has been studied at 650°C in dry air and in water vapor containing environment in both isothermal and thermal cyclic conditions. The weight gain of samples provides information on the kinetics of the oxidation reaction and the adhesion of formed oxide scale. Corrosion products were characterized by several analytical techniques in order to identify oxides with accuracy and to understand their formation mechanisms. Mixed iron and chromium oxides (Cr, Fe) 2O3 are initially formed and provide temporary protection to the substrate. For long time exposure or temperatures above 650°C, magnetite, Fe3O4 and hematite Fe2O3 are the main oxides formed, highlighting the fact that low chromium steel are inappropriate for applications in such drastic conditions. In order to increase the high temperature corrosion resistance of this alloy, various solutions have been proposed as aluminizing by pack cementation, reactive element oxides coatings of by MOCVD, or addition of alloying elements in the steel composition. These solutions were then tested at 650 ° C in dry air and in water vapor environments.

L’objectif de ce travail est d’optimiser la résistance au revenu d’un acier martensitique à 0,6% de carbone, faiblement allié en Cr, V, et Si/Al. Une série d’alliage modèle ternaire de haute pureté (Fe-C-X, X=Cr, V, Si, Al) a été élaborée à l’EMSE par fusion en creuset froid.. L’étude du comportement des alliages a été réalisée au moyen d’une caractérisation fine de la précipitation (Diffraction RX, MEB-FEG, MET) complétée par des mesures dilatométriques. Le comportement des alliages modèles a été comparé à celui de l’alliage industriel. Le vanadium est responsable de deux effets : (i) les carbures de vanadium bloquent la croissance des grains au cours de l’austénitisation ; (ii) le vanadium dissous permet la précipitation d’une cémentite (V,Cr,Fe)3C à l’origine d’un pic de durcissement secondaire pour un revenu vers 550°C. Le silicium a un effet particulier : il stabilise la matrice martensitique et retarde la précipitation de la cémentite vers des températures de revenu plus élevées, augmentant ainsi la résistance au revenu. Cependant, le silicium se révèle nocif pour le procédé industriel de mise en forme utilisé. Deux nouveaux alliages ont alors été élaborés pour chercher un élément de substitution à l’aluminium : un alliage ou le silicium est remplacé par l’aluminium, et un alliage ou le silicium est absent. L’aluminium reste neutre dans l’alliage mais l’étude de ces deux nuances a permis de mettre en évidence la possibilité d’utiliser une nuance ne contenant pas de silicium mais présentant une résistance au revenu comparable a celle de la nuance industrielle, du fait de la présence conjointe du chrome et du vanadium. / The purpose of this work is to optimise the strength of a tempered martensitic medium C steel, containing low-level of Cr, V and Si/Al. A series of high purity "model" ternary alloys (Fe-C–X, X=Cr, V, Si, Al) was prepared (EMSE) by the cold crucible method. The initial structures of quenched alloys were optimised through appropriate austenitizing conditions. Analysis of alloys behaviour on tempering was performed through carbide precipitation characterisation (X-ray diffraction, SEM and TEM), completed by hardness and dilatometric measurements. The behaviour of "model" alloys was compared to that of the industrial material.Vanadium addition is responsible for two effects: (i) vanadium carbides lock the grain growth during the austenitizing; (ii) dissolved vanadium promotes the precipitation of (V,Cr)3C, that is at the origin of a secondary hardening peak near 550°C. Silicon addition has an interesting effect, as it stabilises the martensitic matrix and increases the temperature of cementite precipitation. Therefore, a high value of hardness is conserved up to high tempering temperatures.Unfortunately, because of some noxious properties, the industrial process in concern excludes the presence of Si. That’s why two new model alloys have been prepared and tested, in which the silicon addition (i) was absent, or (ii) replaced by Al.Aluminium has been shown to be "neuter" vs tempering behaviour. Moreover, the interesting effect of secondary hardening (550°C) has been observed in both Si-free steels, showing that alloying with only Cr and V can be enough to get the expected behaviour of the studied steel.

Avec la réduction de taille des composants microélectroniques, le monosiliciure de nickel (NiSi) a été largement utilisé dans les transistors CMOS (Complementary-Metal-Oxide-Semiconductor) en tant que contacts pour les sources, drains et grilles. Cependant, NiSi se dégrade lors du recuit à haute température. Il apparait essentiel d'étudier la séquence de formation de phases et la stabilité du monosilicide en présence d’éléments d'alliage. Les réactions à l'état solide entre des films de Ni allié en W et/ou Pt et un substrat de Si ont été étudiées principalement par diffraction des rayons X (DRX) in-situ et sonde atomique tomographique (SAT). L'analyse combinatoire de la réaction entre des films Ni avec différents composition gradients et le Si a permis de comprendre la séquence de formation. Les concentrations des éléments d'alliage (W et Pt) ont un impact significatif sur la séquence de formation des siliciures de Ni et sur la cinétique de formation des phases. Le mécanisme d'agglomération des films minces de NiSi a également été étudié au cours de cette thèse. Un film de Ni pur de 15 nm a été déposé sur un substrat de Si enrichi de multicouches de Si isotopique. Des analyses SAT ont été effectuées sur l'échantillon de Ni/Si (isotope) après un recuit à 600°C. En observant la distribution des isotopes de Si, un mécanisme d'agglomération de NiSi a été proposé selon que les isotopes de Si restent sous forme de multicouches ou qu'ils se mélangent. Cette étude rendue possible grâce à la capacité unique de la SAT d'observer les isotopes en 3D et à l'échelle atomique apporte une meilleure compréhension de l'agglomération de films minces poly-cristallin d'intermétallique / With the downscaling of microelectronic devices, NiSi has been widely used in complementary-metal-oxide-semiconductor (CMOS) transistors as contact on source, drain and gate. However, NiSi suffers from degradation upon annealing at high temperatures. Adding alloying elements is an effective method to increase the stability of nickel monosilicide but the formation sequence of Ni silicides is substantially modified. Therefore, the studies of the phase formation sequence and the stability of monosilicide are of great importance.The solid-state reactions between Ni films alloyed with W and/or Pt and Si substrates were studied mainly by in-situ X-ray diffraction (XRD) and atom probe tomography (APT) using combinatorial analysis of co-deposited gradient films. The phase sequence was monitored by in-situ XRD and APT was used to examine the silicides and reveal the redistribution of alloying elements. The content of alloying elements (W and Pt) has a large impact on the phase sequence of Ni silicides and the kinetics of phase formation. The basic agglomeration mechanism of NiSi thin films was studied. A 15nm pure Ni film was deposited on a Si substrate enriched with isotope multilayers. APT analyses were performed on the sample of Ni/Si (isotope) after an annealing at 600°C. By observing the distribution of Si isotopes (30Si, 29Si and 28Si), whether they maintain a multilayer structure or are mixed together, a mechanism of the agglomeration of NiSi was proposed. This was possible because of the unique capability of APT to observe isotopes in 3D at the atomic scale and it allows a better understanding and to control of the agglomeration of poly-crystalline compound thin films

Among Al alloys, 2219 Al alloy possesses highest strength at elevated temperatures. However, the application of this alloy is also restricted to a maximum temperature of 150°C, above which, the strengthening precipitates coarsen rapidly resulting in a steep loss in strength. In the present investigation, an attempt has been made to improve the elevated as well as the room temperature properties of commercial 2219 alloy by the addition of small amounts of Sc & Mg, Sc & Zr, and Nb & Zr, and these are designated as 2219ScMg, 2219ScZr and 2219NbZr alloys, respectively. All the three alloys were cast in the form of strips in a water cooled copper mould using suction casting technique with a cooling rate of 102 to 103 K/s. The as-cast strips of 2219ScMg alloys were naturally aged and cold rolled by following three different routes (a) cold rolling, (b) homogenization and cold rolling and (c) hot rolling and cold rolling. A significant improvement in strength has been achieved by all the three wrought processing routes with greater than 140 MPa increase in 0.2% proof stress at room temperature and greater than 110 MPa increase in 0.2% proof stress at 200°C as compared to 2219-T851 alloy having 0.2% proof stress of 345 MPa at room temperature and 205 MPa at 200°C. Hardness values, measured at room temperature after exposure at 200°C, remain stable up to 1000 h. Microstructural analysis of 2219ScMg alloy reveals that Al3Sc or Al3(Sc,Zr) dispersoids form during casting and GP zones form on {100} and {111} plane during natural ageing. Subsequently, rolling introduces higher dislocation densities in the matrix. All these microstructural features contribute to the improvement of the room temperature strength of the alloy. On exposure at 200°C, GP zones transform to mainly θ′ and a few Ω precipitates. A finer, homogeneous distribution of θ′ and Ωprecipitates yields higher strength. Sc and Mg atoms are segregated at the θ′/matrix interface, which gives rise to slower growth kinetics of θ′ precipitates. As a result, the alloy exhibits better thermal stability at 200°C. For 2219ScZr and 2219NbZr alloys, the processing of the cast strip involves a two stage ageing procedure. This includes first stage ageing at 375°C for 2219ScZr alloy and at 400°C for 2219NbZr alloy. This is followed by solution treatment at 535°C for 30 minutes and second stage ageing at 200°C for both the alloys. For 2219ScZr alloy, tensile tests performed at room temperature, 200°C and 250°C show 0.2% proof stress of 456 ± 22 MPa, 295 ± 20 MPa and 227 ± 2 MPa respectively. The alloy is found to be thermally stable at 200°C. It is found that the addition of Sc and Zr results in the formation of Al3(Sc,Zr) precipitates during ageing at 375°C. These precipitates are fully coherent with the matrix and have a significant precipitation hardening effect. They also stimulate the nucleation of θ′′ and θ′precipitates during ageing at 200°C making them finer, homogeneously distributed and thermally stable. Therefore, the strength of the alloy at ambient and elevated temperature is improved. For 2219NbZr alloy, the tensile tests show that 0.2% proof stress is 409 ± 10 MPa at room temperature and 252 ± 22 MPa at 200°C. Microstructural observations reveal that the increase in strength is mainly due to the high volume fraction of Al3Zr precipitates, which form during ageing at 400°C, and due to the formation of θ′′ and θ′precipitates during ageing at 200°C. It is observed that Al3Zr precipitates facilitate the nucleation of θ′′ and θ′ precipitates making them finer, homogeneously distributed and thermally stable, as in the case of 2219ScZr alloy.