This thesis aims to optimize the performance of future nuclear steels. Martensitic steels are particularly studied for the components of sodium-cooled fast reactor cores, as they exhibit lower swelling under irradiation compared to austenitic grades. To improve their creep properties, these steels are sometimes reinforced with a fine dispersion of stable nanometric oxides (Oxides Dispersed Strengthened). However, conventional martensitic ODS steels, with chromium content limited to 9-11 %Cr, often suffer from low toughness at room temperature.
Recent research indicates that the toughness of ODS steels could be significantly enhanced with very low carbon content. This thesis proposes an original approach that combines the exceptional toughness and corrosion resistance of Maraging steels with an ODS-type precipitation. Indeed, the Maraging stainless steels are rich in chromium (10-15 %) and nickel (4-9 %), with carbon content below 0.02 % by weight. After austenitization and quenching, these steels exhibit a martensitic structure, providing an outstanding balance between yield strength and toughness.
To evaluate the performance of these disruptive grades, compositions of interest will be selected, developed, and characterized at CEA with collaboration of academic partner teams.

JOB PROFIL: The applicant must be master-2 graduated with training in materials science and ideally metallurgy. The proposed subject is mainly experimental. A basic knowledge of electronic microscopy and/or XRay diffraction is required for this position. At the end of the PhD the applicant will be highly skilled in steel metallurgy and will have operated a large number of microstructural characterizations advanced tools (SEM, TEM, SAXS XRD, DSC). Naturally, he/she can pretend to a metallurgy researcher position in a large range of industries.

Molecular hydrogen H2 is an alternative energy carrier to traditional fossil fuels, gas or oil. It meet the current energy and environmental challenges, i.e. the need to store greenhouse gases free energy produced by intermittent means such as wind turbines or solar panel. Nevertheless, its safe storage and transportation is one of the keys to its use. The containers or pipes that carry the hydrogen must be leaktight and maintain their integrity over time, for both economical and safety reasons. Understanding and predicting the behavior of hydrogen in container/pipeline alloys and the associated mechanical degradation – such as embrittlement – is therefore crucial for the development of the hydrogen industry. These issues are also generic to all alloys exposed to a source of hydrogen, in corrosion or in the metallurgical industries where the hydrogen simply comes from contact with water, or in the oil&gas industry where hydrogen comes from hydrogen sulphides present in hydrocarbons.

If many experimental works have identified hydrogen embrittlement as the origin of the degradation of alloys exposed to hydrogen, large gray areas still remain on the mechanisms at work due to experimental difficulties and the great variability of the observed phenomena. In addition, the transport and trapping of hydrogen prior to mechanical degradation are poorly known and poorly documented at the nanoscale.

The objective of the thesis is to explore the mechanisms of hydrogen trapping / transport in austenitic materials, as well as its distribution in volume, prior to cracking in order to be able to report and explain the experimental observations.
To achieve this objective, the thesis work will be dedicated to the study of pure nickel, a model system for the austenite phase. The study will be carried out in three stages: (i) thermodesorption measurements and (ii) atomic scale simulations using molecular dynamics, both feeding chemical kinetics modeling coupled with Fick's law at the mesoscopic scale.

Metal alloys used in industrial applications can form oxide layers in the presence of a corrosive environment. These oxides may be uniformly distributed on the surface and/or localized at the grain boundaries. In the latter case, the oxidized grain boundaries may experience brittle fracture under mechanical loading, potentially leading to intergranular cracking of the material. This mechanism is, for example, a possible scenario for the failure of austenitic stainless steel bolts used in the internals structure of Pressurized Water Reactors (PWRs). Under the effect of mechanical loading,
neutron irradiation and the presence of a corrosive environment, these bolts fail through a phenomenon known as irradiation-assisted stress corrosion cracking. To model this phenomenon, we need to determine the fracture properties of intergranular oxides, and to take into account the coupling between cracking, oxidation and irradiation. In this thesis, experimental and numerical work will be combined. Firstly numerical simulations based on the variational approach to fracture approach will be assessed in order to design micro-beam micromechanics experiments aimed at reliably determining the fracture properties of oxides, and also to study the couplings between cracking, oxidation and irradiation. In particular, the cracking-oxidation coupling that prefigures the transition between initiation and propagation will be investigated in detail. These experiments will then be carried out on model and industry-relevant steels, and interpreted using numerical simulations. Finally, all the results obtained in this work will be incorporated into simulations of polycrystalline aggregates, in order to assess the possibility of quantitatively predicting intergranular cracking in the context of irradiation-assisted stress corrosion.