Multiscale modelling of twinning in tin
Twinning is a displacive deformation mechanism characterized by a continuous deformation of the material. Although widely studied for other industrial materials such as titanium alloys, this inelastic mechanism remains poorly understood and incompletely modeled for complex crystallographic structures. However, due to the reduced number of symmetries in these structures, dislocation slip is insufficient to accommodate deformation in certain loading directions, requiring the activation of twinning. This is the case for tin, which has a tetragonal structure. In particular, twinning contributes significantly to the mechanical response of tin at high strain rates and low temperatures. At intermediate temperatures and strain rates, a competition between dislocation plasticity and twinning plasticity can occur, making it crucial to describe the coupling between these two phenomena. Proposing a better description of this coupling will shed new light on the experimental data available at CEA DAM. The objective of the thesis is to develop a multiscale approach, from molecular dynamics to continuum mechanics, validated by experiments, to converge on a model that describes the behavior of tin over a wide range of temperatures and strain rates.
Thermal Barrier Coatings with enhanced mechanical properties performed by plasma spraying
Increasing the performance of aircraft gas turbines requires improvements in the materials used in the combustion chamber and on the parts at the outlet of the chamber. Widely used in the aerospace industry, plasma spraying enables the application of low-conductivity ceramic coatings that provide a thermal barrier protection for metal parts. The mechanical stress observed require coatings that are increasingly resistant in mechanical terms. As a result, the thesis will focus on developing plasma-sprayed thermal barrier coatings with increased mechanical strength while maintaining good thermal insulation compared to the state of the art yttria stabilized zirconia thermal barrier coating currently used in gas turbine engines. For example, particular attention will be paid to toughness, which is the ability of a material to resist fracture in the presence of a crack. Factors that can influence toughness include composition, microstructure, and the addition of reinforcements. The use of original solutions, such as bio-inspired ones, is also a possibility.
Metallurgy under extreme conditions
The microstructure-properties relationship is a core concept of metallurgy, and of materials engineering in general. For instance, the hardness of quenched steels emerges from their martensitic microstructure, induced by a phase change in iron. Here we are concerned about metallurgy under extreme conditions in which metallic samples undergo pressurizations in the 100 GPa (=1 million atmospheres) range, making it possible to synthesise new crystalline phases with potentially interesting properties (hardness, magnetism, etc.).
Studied systems will include tin, then indium and cobalt. The three of them exhibit a rich polymorphism under high pressure and temperature. We will seek to elucidate the role of defects such as twinning and plasticity on the mechanism and kinetics of these transitions. This will be done by comparing experimental observations with microstructure predictions obtained through mesoscopic simulation. High pressure/ high temperature will be mainly generated by laser-heated diamond anvil cells, and characterisation tools will include in situ X-ray imaging by diffraction and tomography, as well as electron microscopy. The X-ray sources used will be synchrotron sources and the European free-electron X-ray laser.
Development of theoretical Raman spectra with application on minerals from the surface of Mars
As we push the boundaries of space exploration with new missions to nearby planets, improving our investigation tools is crucial. Mars rovers have revealed a surface mineralogy unlike anything on Earth, shaped by the planet’s former hydrosphere followed by an extended dry and cold environment. For example, this favors the formation of perchlorates, or mixed silicate–salts glassy phases — minerals that are difficult to synthesize and stabilize on Earth but remain surprisingly stable on Mars. Recent Raman spectrometry data confirms their presence, highlighting an opportunity for deeper research. Understanding these minerals could offer new insights into Martian chemistry and planetary evolution.
Here we want to calculate the theoretical Raman spectra of perchlorates and other Martian minerals using the density functional perturbation theory (DFPT) as implemented in the ABINIT package. We want to obtain not only the position and the intensity of the peaks, but also the peak widths. They are necessary to correctly identify between similar spectra and to assess, by integration, the actual intensity of the peaks, which are directly comparable to experimental values on the field. These allow us to choose the representative peaks that can be used in identification and to analyze the displacement patterns associated with the vibrations. The results of our simulations will be compared and interpreted in the light of measurements performed by the current rovers on the surface of Mars.
For this, we need to implement several third- and fourth-order derivatives of the energy. This will be done as a series of DFPT terms, where the perturbations can be atomic displacements or electric fields. We will use a combination of the 2n+1 theorem and finite differences. The implementation will be done within the "Projector Augmented-Wave" approach (PAW) to DFT. The entire development effort will be integrated into the ABINIT package and made available to the entire community. ABINIT (www.abinit.org) is a highly visible international collaborative project for ab initio simulations based on DFT and DFPT. The computed spectra will be made available to the community via the WURM database.
The successful candidate will be co-advised between the IPGP (Paris) and the CEA (Bruyères-le-Chatel, S of Paris) groups. IPGP is a world-renowned geosciences research institute founded in 1921, associated with the CNRS, a component of the Université Paris Cité and employing more than 500 people. The group of Razvan Caracas is highly active in computational mineralogy, study of matter at extreme conditions, and planetology. The Quantum simulation of Matter group at CEA Bruyères-le-Chatel led by Marc Torrent is a main developer of the ABINIT package, highly active in density functional theory, projector augmented-wave, and high-performance computing.
Synthetic methodologies towards functionalized azaheterocycles and application to energetic molecules
The objective of the PhD is to develop new synthesis and/or functionalization methods to obtain functionalized heterocyclic molecules. These molecules are based on 5- or 6-member azaheteroaromatic rings (diazines, triazines, triazoles, tetrazoles, etc.). The targeted structures make it possible to envisage high densities and enthalpies of formation, while maintaining low sensitivity (thermal, mechanical, etc.). They find applications in the energy field, notably propulsion, explosives and gas generators (airbags). In addition, these heterocyclic compounds as well as the intermediates are also structurally close to families of biologically active products and/or likely to exhibit fluorescence properties, as already shown in a previous PhD in the laboratory.
IO access scheduling on magnetic tapes using machine learning
Numerical simulations are used to obtain responses to physical phenomena that
cannot be reproduced, either because they are too dangerous or too expensive.
The models used for these simulations are increasingly complex, in terms of
size and precision, and require access to increasingly large computing and
data storage capacities. To this end, and in order to optimize costs, the use
of mass storage technologies such as magnetic tapes is critical. However, to
ensure good overall system performance, the development of algorithms and
mechanisms related to data placement and tape access scheduling is essential.
The objective of the thesis is to study the technology of magnetic tapes, as
well as existing mechanisms such as RAO (Recommended Access Order) or request
retention; and to implement new strategies for the optimization of magnetic
tape performance.