Alexandra M. Goryaeva

Modélisation à l’échelle atomique des dislocations et de la plasticité dans la post-perovskite MgSiO3: La couche D" située à la frontière manteau-noyau est une région complexe caractérisée par une forte anisotropie à différentes échelles. Inaccessible de par sa profondeur et caractérisée par des conditions P-T extrêmes, l’étude de cette région de la Terre représente un défi majeur qui ne peut être abordé qu’au travers d’observables géophysiques et d’expériences de hautes pressions. Les causes de l’anisotropie sismique de D" sont toujours l’objet de débats. La contribution de l’orientation préférentielle des cristaux reste cependant une piste privilégiée compte-tenu de la structure très anisotrope de la post-perovskite (ppv). De plus, D" est une couche limite thermique à l’interface entre le noyau constitué d’un alliage de fer liquide et le manteau inférieur constitué de silicates visqueux. Les propriétés physiques de D" sont donc particulièrement importantes pour comprendre les transferts thermiques en provenance du noyau et leur contribution à la convection mantellique. Ce phénomène implique l’écoulement plastique de roches contrôlé par le déplacement de défauts cristallins. Cependant, pour la ppv, les informations concernant les systèmes de glissement majeurs ou les défauts sont extrêmement parcellaires. Pour les phases de hautes pressions, la modélisation numérique représente une approche de choix pour obtenir des informations sur les mécanismes de déformations élémentaires difficiles à obtenir par voie expérimentale. Le but de ce travail est d ‘étudier à l’échelle atomique les défauts majeurs de la ppv MgSiO3 ainsi que leurs mobilités afin d’évaluer la capacité de cette phase à se déformer plastiquement par glissement de dislocations dans les conditions de D".

The activation entropy of dislocation glide, a key process controlling the strength of many metals, is often assumed to be constant or linked to enthalpy through the empirical Meyer-Neldel law-both of which are simplified approximations. In this study, we take a more direct approach by calculating the activation Gibbs energy for kink-pair nucleation on screw dislocations of two body-centered cubic metals, iron and tungsten. To ensure reliability, we develop machine learning interatomic potentials for both metals, carefully trained on dislocation data from density functional theory. Our findings reveal that dislocations undergo harmonic transitions between Peierls valleys, with an activation entropy that remains largely constant, regardless of temperature or applied stress. We use these results to parameterize a thermally-activated model of yield stress, which consistently matches experimental data in both iron and tungsten. Our work challenges recent studies using classical potentials, which report highly varying activation entropies, and suggests that simulations relying on classical potentials-widely used in materials modeling-could be significantly influenced by overestimated entropic effects.

The evolution of the microstructure of materials is primarily governed by defect-mediated diffusion. Here, we examine the intriguing role played by vacancies in tungsten, a strategic metal for high-temperature fusion-energy systems. We address the existing apparent contradictions between experimental observations indicating the presence of voids and theoretical predictions of vacancy self-repulsion by employing both experimental and theoretical methods. We have designed a transmission-electron-microscopy experiment and developed a theoretical data-driven approach using the Bayesian adaptive biasing force method to investigate the finite-temperature properties of mono- and di-vacancies and characterize their relative stability via their formation free energies. Our investigation reveals that di-vacancies are energetically unfavorable at low temperatures but stabilize entropically as the temperature increases. This result is entirely consistent with experimental observations of the clustering of quenched-in vacancies in tungsten during reannealing treatments at intermediate temperatures. This work provides critical insights into the temperature-dependent stability of defects, bridging the gap between theoretical predictions and experimental observations, with significant implications for the design and optimization of high-temperature materials for fusion.

The asymmetric distribution of geometrically equivalent defects is a long-standing problem in materials science. In this study, we investigate the preferential nucleation of interstitial dislocation loops in specific planes in stressed aluminum, commonly observed experimentally, and seek to clarify the underlying mechanism. For this purpose, we consider a structural change in the geometry of defects, specifically the transformation of 3D compact A15 clusters into 2D Frank loops. Using object kinetic Monte Carlo and ab initio calculations, we show that a symmetry breaking in the transformation of A15 clusters significantly impacts the dislocation loop distributions, resulting in the emergence of a preferential orientation when the material is under stress. This discovery not only calls for a critical revision of established theories but also has tangible applications for materials under extreme conditions.

It is generally considered that the elementary building blocks of defects in face-centred cubic (fcc) metals, e.g., interstitial dumbbells, coalesce directly into ever larger 2D dislocation loops, implying a continuous coarsening process. Here, we reveal that, prior to the formation of dislocation loops, interstitial atoms in fcc metals cluster into compact 3D inclusions of A15 Frank-Kasper phase. After reaching the critical size, A15 nano-phase inclusions act as a source of prismatic or faulted dislocation loops, dependent on the energy landscape of the host material. Using cutting-edge atomistic simulations we demonstrate this scenario in Al, Cu, and Ni. Our results explain the enigmatic 3D cluster structures observed in experiments combining diffuse X-ray scattering and resistivity recovery. Formation of compact nano-phase inclusions in fcc structure, along with previous observations in bcc structure, suggests that the fundamental mechanisms of interstitial defect formation are more complex than historically assumed and require a general revision. Interstitial-mediated formation of compact 3D precipitates can be a generic phenomenon, which should be further explored in systems with different crystallographic lattices.

The elastic properties of tungsten, a ubiquitous material in future energy systems, are investigated up to its melting temperature by means of a data-driven approach. The proposed workflow combines machine learning of the force field and enhanced sampling of the crystalline structure. While the machine learning force field achieves the accuracy of ab initio calculations, its implementation in sampling methods is often limited due to its high computational cost, which is commonly a few orders of magnitude larger than that of traditional potentials. To overcome this limitation, we propose a fast and robust Bayesian sampling scheme aiming at estimating the fully anharmonic free energy of crystalline solids with the help of an improved adaptive biasing force method. This method performs a thermodynamic integration from a harmonic reference system, wherein zero frequencies associated with the periodic boundaries are screened off. The proposed sampling method drastically improves the speed of convergence and overall accuracy. We demonstrate the efficiency of the improved method by calculating the second order derivatives of the free energy, such as the elastic constants, which are performed almost 100 times faster than with the standard methods. The proposed method enables the prediction of the elastic properties of tungsten in the range of temperatures that cannot be investigated experimentally, from 2100 K to the melting temperature. The accuracy and numerical efficiency of the proposed strategy open up many avenues for the reliable prediction of finite-temperature properties of materials, such as the relative stability of structural defects and elastic constants.