Doctorat de Physique

used to trace significant geodynamic processes, such as orogenesis, rifting or subduction. In metallic materials, the development of multiple phases is critical for optimizing performance and is an important ingredient in the design of high performance parts in aeronautics, energy, transport, and plenty of other domains. The theory behind the aforementioned PTs relies on the assumption that the stress has the same effect on phase equilibrium as equivalent hydrostatic pressure, meaning the effects of deviatoric stress are neglected. Such hypothesis, however, have recently triggered discussions in both Earth science and Metallurgy. In fact, there are now several experimental results that show non-trivial relation between mechanical pressure and mineral reactions [1], including the quartz/coesite transformation [2], a PT emblematic of Earth’s Science. Similarly, the α/ω transformation of titanium has been shown to be influenced by the applied stress [3].

The PhD thesis will take place at PPRIME Institute (Poitiers, France) in the context of the international ANR project MULTI-STEP: Multiscale Stress-driven Thermodynamic Equilibrium Predictions, in collaboration with the Centre de Mise en Forme des Matériaux (Mines-ParisTech, Sophia Antipolis, France), the Heidelberg University (Germany) and the Goethe University (Frankfurt, Germany). This project aims at understanding the effect of deviatoric stress on the phase fraction equilibrium of titanium and silica. It proposes to overcome the classical hypotheses for phase equilibrium calculations using a multiscale numerical approach, from atomistic to large-scale thermomechanical modeling, coupled with micromechanical experiments.

The PhD thesis will mostly focus on the WP1 of the MULTI-STEP project lying on atomic scale simulations. The main objective of this WP is to provide deviatoric stress-dependant phase diagram for both titanium and silica. In this aim, atomistic simulations will be used in combination with state of the art approaches for computing Gibbs free energies, such as thermodynamics integration [4] or umbrella sampling [5]. The atomic scale approach will also allow to obtain two main ingredients needed for the homogenization performed in WP5. First, the atomistic simulation will allow to derive equations of state for all phases of interest, which will be used to compute the volumetric part of eigenstrains. Second, the bicrystallography will permits from the atomic structure of the crystals to infer the deviatoric part of the eigenstrains related to the different phase transformations.

Depending on the progress of the subject and the candidate's interests, participation in other WPs of the project may be considered.

References:

1. S. Cionoiu and et al. “Impact of interseismic deformation on phase transformations and rock properties in subduction zones”. In: Sci. Rep. 9.1 (2019), p. 19561.
2. B. Richter and et al. “Stresses and pressures at the quartz-to-coesite phase transformation in shear deformation experiments”. In: J. Geophys. Res.: Solid Earth 121.11 (2016), pp. 8015–8033.
3. K. Dang and et al. “The role of deviatoric stress and dislocations on the α to ω phase transformation in Ti”. In: Acta Mater. 244 (2023), p. 118510.
4. P. Y. Chew and A. Reinhardt. “Phase diagrams—Why they matter and how to predict them”. In: J. Chem. Phys. 158.3 (2023).
5. Kästner, J. “Umbrella sampling”. Comp. Mol. Sc., (2011), 1(6), p. 932-942.

Techniques/methods in use: Molecular dynamics, Ab-Initio Molecular Dynamics, Thermodynamics Integration, Umbrella Sampling

Funding category: Financement public/privé

ANR project

PHD title: Doctorat de Physique

PHD Country: France

Requirements

Specific Requirements

Knowledges in material physics, thermodynamics, interest on numerical simulations