Pour répondre à la demande croissante d’allègement des véhicules automobiles, les aciéristes développent une nouvelle gamme d’aciers à Très Haute Résistance (THR), dite de troisième génération. Cette thèse, inscrite dans le projet ANR MATETPRO « MeMnAl Steels », s’intéresse plus particulièrement à deux nouvelles familles d’aciers THR Fe-C-Mn-Al, produites par ArcelorMittal et potentielles candidates pour la caisse en blanc des futurs véhicules. Elle vise à mieux cerner les paramètres microstructuraux permettant de contrôler et optimiser le comportement à rupture de ces aciers.Pour représenter les deux familles d’aciers, deux matériaux modèles ont été élaborés par laminage puis recuit intercritique, conduisant à une microstructure duplex : austénite retenue (γr, pouvant se transformer en martensite par effet TRIP) et ferrite. La microstructure du premier acier, dite UFG, est ultrafine (grains de taille inférieure au micromètre) tandis que celle du second est bimodale, mêlant gros grains de ferrite δ et régions à grains fins de ferrite α et d’austénite retenue γr.Les propriétés mécaniques de la microstructure UFG dépendent fortement de la température de recuit, en raison des variations de stabilité de l’austénite retenue. A l’inverse, la microstructure bimodale est très robuste vis-à-vis de la température de recuit mais très sensible à la température d’essai. L’endommagement en traction et en résilience est très peu développé pour ces deux familles. Il est localisé aux interfaces ferrite-martensite (formée pendant l’essai). Le lien entre les modes de rupture et la microstructure bimodale, étudié à l’aide d’essais Charpy, a montré l’existence de deux transitions distinctes de mode de rupture : une transition entre rupture ductile à grandes cupules et clivage pour les gros grains de ferrite δ et une transition entre rupture interfaciale et rupture ductile à fines cupules pour les zones à grains fins {α + γr}. La rupture de la microstructure UFG est ductile à température ambiante et interfaciale à plus basse température. Cette microstructure UFG peut être vue comme un matériau modèle représentant les régions à grains fins {α + γr} de la microstructure bimodale.Pour les deux familles d’aciers, le comportement élastoplastique comme le comportement à rupture semblent dominés par la stabilité de l’austénite retenue. / To fulfil the increasing demand on lightweighting automotive vehicles, steelmakers are developing a third generation of Advanced High Strength Steels (AHSS). This work, part of the ANR project MATETPRO “MeMnAl Steels”, addressed two new families of third generation AHSS produced by ArcelorMittal which may be used for the body in white of upcoming cars. It aimed at improving our current understanding of the microstructural features allowing controlling and optimizing the fracture behavior of this steel family.Two model materials were manufactured by hot and cold rolling followed by intercritical annealing. The resulting, so-called duplex microstructure is a mixture of ferrite and retained austenite (γr, which can transform into martensite by TRIP effect). The microstructure of the first steel was made of ultra-fine grains (UFG) of ferrite and retained austenite (grain size below one micrometer), while the second steel possessed a bimodal microstructure made of coarse δ-ferrite grains and fine-grained regions of α-ferrite and retained austenite γr.The mechanical properties of the UFG microstructure were strongly sensitive to the annealing temperature, due to variations in the stability of retained austenite. On the contrary, the bimodal microstructure was very robust regarding the annealing temperature but very sensitive to the test temperature. For these two families, damage development is scarce and mainly located at ferrite-martensite interfaces. Charpy impact tests on steels with the bimodal microstructure showed that each microstructural region presents its own fracture mechanisms and a specific ductile-to-brittle transition. A transition from brittle cleavage to large-dimpled, ductile fracture was observed for coarse δ-ferrite grains, while fined-grained regions presented a transition from interfacial fracture to fine-dimpled, ductile fracture. Fracture of the UFG microstructure was ductile at room temperature and interfacial at lower temperatures. This UFG microstructure can be interpreted as a model material embodying the behavior of the fine-grained {α + γr} regions in the bimodal microstructure.For both two steels, the constitutive and fracture behavior seem to be dominated by the stability of retained austenite.

γ-TiAl based alloys have attracted considerable research interest in the past few decades and have gained niche high temperature applications in aero-engines and automobiles. As high temperature structural materials, these alloys require stable microstructures. This thesis aims at addressing knowledge gaps in the understanding of microstructural stability in two technologically important γ-TiAl based alloys in different microstructures, viz. fully lamellar (FL) and duplex. Creep and exposure tests were complemented with a variety of microstructural characterization tools (SEM, EBSD, TEM, XRD). Density functional theory based calculations were also performed to further the understanding of stability of phases. In the first part of the thesis, microstructural stability of a FL microstructure was studied under creep and high temperature exposure conditions. An aim of these studies was to probe the effect of stress orientation with respect to lamellar plates on microstructural changes during primary creep. It was observed that retention of excess α2 resulted in an unstable microstructure and so under stress and temperature, excess α2 was lost. However, depending on stress orientation, the sequence of precipitates formed was different. In particular, for certain stress orientations, the formation of the non-equilibrium C14 phase was observed. The stress dependence of microstructural evolution was found to be stem from internal stresses due to lattice misfit and elastic moduli mismatch between α2 and γ. In the second part of this thesis, microstructural stability of a duplex alloy was probed, with an emphasis on understanding mechanisms that lead to tertiary creep. The as-extruded microstructure consisted of bands of equiaxed grains and lamellar grains. During creep, loss of lamellar grains was observed and this was attended by kinking of laths and formation of dynamically recrystallized equiaxed grains. Significant dislocation activity was seen in both lamellar and equiaxed grains at all stages of creep. Initially, dislocation activity leads to strengthening and primary creep behavior, but at later stages, it triggers dynamic recrystallization. Dynamic recrystallization was found to be the rate controlling creep mechanism. Accelerating creep behavior was due to strain localization during the constant load tensile test resulting from microstructural instabilities such as kinking.