Description du sujet de thèse

Embrittlement of titanium in liquid lithium
Compact high intensity neutron sources (CANS) are a new alternative to critical nuclear reactors for neutron diffraction or neutronography analysis of matter. The applications of these new devices range from materials science, nanoscience, life sciences to fundamental sciences (quantum or topological materials, cosmology, quantum gravitation, fundamental interactions). Neutron production is based on the low energy nuclear reaction (p/d,n) with light nuclei targets such as beryllium or lithium.
The use of a lithium target in liquid state in a loop configuration with a heat exchanger has several major advantages: lithium allows both a high neutron yield, but also an efficient extraction of the thermal power deposited by the primary beam because of its use in circulation in the liquid state (the melting point of lithium is 180°C). The reference structural materials for this type of device are steels but the current project wishes to develop the innovative use of titanium to reduce the radiological activation of the structures. Lithium is a medium that can be very corrosive, it is especially known to promote the propagation of cracks in steels, in the presence of residual stresses for example, in case of wetting by the liquid metal (liquid metal embrittlement or LME), or in connection with intergranular corrosion related to impurities of lithium in solution (dissolved nitrogen). From this point of view, the behavior of titanium is not known for this type of application under the conditions envisaged. One of the potentially critical issues is the propagation of cracks in brittle mode induced by the adsorption of liquid metal. Fractures can be sudden with a fast propagation (of the order of one centimeter per second) in the worst case, and intervene in the elastic stress domain of the material or lead to reduce the ductility of the material if the damage occurs in the plastic deformation domain. It is therefore necessary to study the conditions that could allow the propagation of cracks in this environment in order to eventually propose strategies to improve the resistance to MLF.
The proposed thesis topic is therefore focused on the question of the interaction of liquid lithium with titanium. The subject is completely new and there is no experimental data published in the literature. It is planned to look at this topic according to the following axes:
- Corrosion and wettability of titanium by liquid lithium.
- Influence of lithium on the mechanical properties of titanium (MLF).
o Sensitivity to MLF under conditions as close as possible to nominal operating conditions (dissolved impurity level)
o Quantification of the degree of MLF if it is present (CCT).
- Measurement of residual stresses in welded parts and effect of liquid metal on weld failure. An equivalent program will be conducted on parts produced by additive manufacturing.
- Advanced characterization by electron microscopy of the failure modes of titanium in contact with lithium and understanding of the various mechanisms at work: thermal activation and intergranular penetration/diffusion, effect of metallurgy, interaction with dynamic aging and creep.
The thesis subject will be carried out between the laboratories of the two partners using the test and characterization means of the two laboratories or of the outside (electronic microscopy, FIB, Nano-SIMS, etc.). The project includes a simulation part with which the candidate will interact. The start can be envisaged at the beginning of next year.
Required profile: The candidate should have an initial education (engineering degree and/or Master 2) in materials science (physical/chemistry, metallurgy, structural metallic materials, etc.). A strong taste for experimentation and characterization in materials science would be a plus.

Contexte de travail

The thesis will be carried out in 2 laboratories, the PIMM at ENSAM Paris and the CEA/DEN/DANS/DPC/SCCME laboratory at CEA-Saclay

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The applicant will be required to pass the security screening for access to the restricted areas for both sites