Category Seminar

Denis Gratias et Marianne Quiquandon
CNRS-UMR 8247 IRCP, Chimie-ParisTech PSL, Paris

On se propose de discuter la symétrie résultant de la superposition de deux couches monoatomiques cristallines identiques désorientées l’une par rapport à l’autre d’une rotation-translation (α|τ).
Un réseau de coïncidence apparaît —défini par le groupe intersection des groupes de translation des réseaux des monocouches— pour un ensemble dense dénombrable de valeurs de la rotation α, qu’on discutera en toute généralité pour les quatre types de réseaux bidimensionnels, oblique, rectangle, carré et hexagonal. Ces valeurs singulières d’angle α associées aux normes σ des vecteurs unitaires du réseau de coïncidence se répartissent dans le plan (α, σ) selon des branches indexées par des suites de Farey et dont on discutera les propriétés.
Pour une rotation donnée, les symétries spatiales de ces bicouches se répartissent en un
petit nombre seulement de groupes selon la valeur de la translation τ. Ainsi les bicouches de
graphène à réseau de coïncidence ne peuvent présenter que 6 types de groupes d’espace quelles que soient la rotation a de coïncidence et la translation τ.
Dans le cas générique d’absence de réseau de coïncidence, la bicouche présente une
symétrie quasipériodique de rang 4 au plus qu’on peut décrire par une méthode de coupe à partir d’un espace de dimension 4. On montrera l’importance fondamentale du réseau-0 (0-lattice) pour décrire les symétries des figures de moiré de ces édifices.

Maxime Moreaud

IFPEN, Solaize

Since 2017, IFPEN has fully entered the race for accelerated design of new materials with models creating links between synthesis and effective properties. Its AI and materials teams propose new tools for the numerical generation and characterization of materials microstructures.  
This approach realistically considers the microstructure to capture morphological and topological details at scales of interest. Numerical models link to synthesis or processing parameters, and estimate textural and usage properties. In this talk, we will discuss the general ideas of this approach, examples of multi-scale microstructures, and some recent work on numerical textural characterizations such as tortuosity and deep learning accelerated physisorption simulation.

Sylvain Schwartz

Laboratoire QTECH (ONERA)

Rydberg atoms are by definition atoms which have been excited to a state with a large principal quantum number, resulting in exaggerated properties such as a large atomic size, a long lifetime compared to other excited states and large matrix elements for the dipole operator. In practice, dipole-dipole interactions between Rydberg atoms are at the heart of quantum simulations, where they are used to create entangled atomic states. But the large dipole of Rydberg atoms can also result in a strong coupling with external electromagnetic fields, making these atoms good candidates to be used as very sensitive probes of electromagnetic environment in the GHz to THz range. I this talk, I will give a brief overview of the state of the art of quantum simulation and quantum metrology with Rydberg atoms, and present the ongoing project that we have in the QTech lab at ONERA about quantum metrology with cold Rydberg atoms trapped in optical potentials. Possible applications include electromagnetic intelligence, THz imaging and scientific applications such as the calibration of black-body shifts in state-of-the-art optical clocks (in collaboration with SYRTE and laboratoire Aimé Cotton).

Baptiste Bienvenu, Chu Chun Fu and Emmanuel Clouet

Université Paris-Saclay, CEA, Service de Recherches de Métallurgie Physique, 91191 Gif-sur-Yvette

At low temperature, plasticity of body-centered cubic (BCC) transition metals is governed by the glide in compact {110} planes of screw dislocations with a ½<111> Burgers vector, experiencing a high friction with the crystal lattice. The aim of this work is to build laws to predict the plastic flow stress based on atomic scale modeling of the core properties and mobility of these dislocations (using ab initio calculations and molecular dynamics), allowing to link them to macroscopic mechanical properties (yield stress, slip system activity).
In this context, a special care is given to the case of chromium (Cr), the only BCC transition metal having a structure close to antiferromagnetism, a spin-density wave, below ambient temperature. To qualify the impact of magnetism on the plasticity of Cr, ab initio calculations at zero temperature were coupled to Monte Carlo simulations at finite temperature. This allowed to conclude that magnetism has only a marginal influence, except at very low temperature where the ½<111> Burgers vector of these dislocations generates magnetic faults given that it does not respect the magnetic order of Cr.
In the following, a systematic study across all seven BCC transition metals (vanadium, niobium, tantalum, chromium, molybdenum, tungsten and iron) helped develop a yield criterion reproducing the experimental features of the so-called “non-Schmid” effects, characteristic of these metals at low temperature. However, some effects cannot be captured by this criterion, accounting for the motion of isolated dislocations only. This is for instance the case of anomalous slip, observed in all BCC transition metals except iron, and characterized by slip activity of ½<111> dislocations in low-stressed {110} planes. Through in situ observations in a transmission electron microscope performed by Daniel Caillard (CEMES-CNRS, Toulouse), coupled with atomistic simulations, a new mechanism explaining this phenomenon in all BCC metals has been evidenced, based on the high mobility of multi-junctions. Finally, the mobility of dislocations with a <100> Burgers vector, most often observed as junctions between ½<111> dislocations but rarely considered as possible slip systems, is studied using atomistic simulations. It was evidenced that, even if the mobility of <100> screw dislocations is competitive with the conventional ½<111> in {110} planes, <100> dislocations are locked at low temperature along a mixed orientation requiring a very high stress to start moving, thus explaining their low slip activity.

Thomas Swinburne
CINaM, Marseille

Building models for the plasticity, thermodynamics and kinetics of metals is challenging as subtle aspects of atomic cohesion must be faithfully reproduced, and predictions often require averaging over large, complex configuration ensembles. I will discuss how the energy landscapes of atomic systems can be rapidly explored at scale and “coarse-grained” when the dynamics are thermally activated thus thus scale separated[1,2] and how data-driven techniques, typically used to regress energies for modern cohesive models, can be used to capture a much wider range of properties such as defect entropics[3] or dislocation properties. When the dynamics do not have a clear timescale separation, coarse graining is much more challenging. I will discuss how a data-driven approach can provide a solution, producing efficient surrogate models which can predict the evolution of nanoparticle ensembles and the yielding of complex microstructures, offering new perspectives for multiscale modelling approaches[4].

[1]  TD Swinburne and D Perez, NPJ Comp. Mat 2020, MSMSE 2022
[2]  TD Swinburne and DJ Wales JCTC 2020, 2022
[3]  C Lapointe et al. PRMat 2020
[4]  TD Swinburne, In Prep.

Marc Legros, Romain Gauthier, Armin Rajabzadeh, Frédéric Mompiou et Nicolas Combe

CEMES-CNRS, Toulouse

Most crystalline materials around us (metals, alloys, ceramics) are polycrystalline, made of “grains”, separated by “grain boundaries”. These boundaries between domains of different orientation determine certain physical properties and especially their mechanical behavior. For example, we can make a ceramic malleable or on the contrary harden a metal by reducing the size of its crystallites through the famous Hall-Petch law [1,2], established in a phenomenological way for steels 70 years ago. Physically, this relationship can be explained by the obstacle effect that grain boundaries have on dislocations, which are the main vectors of plastic deformation. When grains become nanometric, the plasticity threshold saturates or decreases, which is generally attributed to plastic processes carried by the grain boundaries themselves, such as rotation, intergranular slip and/or migration/shear coupling. These mechanisms are mostly observed in small-grained metals, but rarely quantified experimentally, except in experiments on bicrystals [3]. The Cahn & Mishin (C&M) model [4,5], which popularized the migration-shear coupling, predicts that the coupling factor increases with the disorientation of the joint. In other words, when a joint migrates, it produces more shear the higher its disorientation. The rare measurements made on polycrystals, experimentally more complex to realize, do not seem to attest this trend. And metallic nanocrystals are not known for their deformability.

To be sure, we have been studying the deformation mechanisms related to grain boundary migration for the last ten years, both by in situ transmission electron microscopy (TEM), using atomic simulations by molecular dynamics and more recently by atomic force microscopy (AFM), all coupled with crystal orientation mapping techniques. It is thus possible to follow the motion of the identified boundaries and even to statistically quantify the shear produced in ultra-fine-grained aluminum. In the absence of dislocation, this migration-shear coupling is the main driver of plastic deformation [6]. However, this coupling is much weaker than that predicted by the C&M model, which explains the low yield of grain boundary plasticity mechanisms, and thus the low ductility of metallic nanocrystals.

[1]   EO Hall. The deformation and ageing of mild steel: III Discussion of results. Proceedings of the Physical Society Section B 1951;64:747–53.
[2]   NJ Petch. The cleavage strength of polycrystals: Journal of the Iron and Steel Institute, v. 174. 1953
[3]   T Gorkaya, DA Molodov, G Gottstein. Stress-driven migration of symmetrical 〈100〉 tilt grain boundaries in Al bicrystals. Acta Materialia 2009;57:5396–405.
[4]   JW Cahn, JE Taylor. A unified approach to motion of grain boundaries, relative tangential translation along grain boundaries, and grain rotation. Acta Materialia 2004;52:4887–98.
[5]   JW Cahn, Y Mishin, A Suzuki. Coupling grain boundary motion to shear deformation. Acta Materialia 2006;54:4953–75.
[6]   R Gautier, A Rajabzadeh, M Larranaga, N Combe, F Mompiou, M Legros. Shear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals. Comptes Rendus Physique 2021;22:1–16.

Clément Ribart, Henry Proudhon

Centre des Matériaux, MINES Paristech, CNRS UMR 7633

Diffraction Contrast Tomography remains the fastest 3D grain mapping characterization method and allows further characterization of specific crystallographic locations. Coupled to in situ mechanical testing, something we have developed for a few years at CDM, it provides a unique way to probe deformation and fracture mechanisms of structural materials. Correlative experiments at higher resolution can now be performed to achieve “zoom-in” observation in selected grains. We will review several technique that can be used at the European synchrotron, such as X-ray topo-tomography, scanning 3DXRD or Dark Field X-ray Microscopy. Example to characterize plastic strain localization in metals will be presented. As the method becomes more and more automated, it allows both quantitative and statistical measurements in the bulk of the microstructure. Finally, coupling experiments with crystal plasticity finite element computations at the grain scale will be discussed as a key to unlock microstructure-deformation mechanisms.

Contact to attend the conference : Mathieu.Fevre@onera.fr

Thursday 31st Mars 2022 at 14h00

Contensou conference room, ONERA Châtillon

Frederico Orlacchio

LEM, UMR ONERA-CNRS – SIAM, ONERA

Additive Manufacturing is arousing great interest in both academic and industrial communities, in particular with a view to reducing raw material waste and optimising manufactured components. During the internship carried out between the SIAM unit of DMAS/ONERA and the LEM (CNRS/ONERA), particular attention was paid to the impact of thermal hardening on the precipitation of intermetallic phases in the aluminum alloy Al-4Fe, manufactured by Laser Powder Bed Fusion (L-PBF). First, an experimental study at different scales was carried out in order to characterise the microstructure of the Al-4Fe material in its metallurgical state after fabrication. Various manufacturing conditions were studied. The grain microstructure as well as the precipitation of iron-rich phases were studied by different microscopy techniques (optical, SEM, TEM). Secondly, the evolution of the hardness and the modifications of the microstructure induced by various post-manufacturing heat treatments were characterised. The small disorientations inside the grains were measured by EBSD in order to obtain information on the strain hardening of the material during these treatments. Finally, the various contributions necessary for the development of a model of the hardness of these alloys were discussed.

Tuesday 28th September 2021 at 9h30

videoconference at the followink link: https://rdv.onera.fr/seminaireLEM

Djahid Oucheriah

LEM, UMR ONERA-CNRS

At the nanoscale, nanomaterials possess unique properties that differ strongly from those of the bulk material. In the case of AgPt nanoparticles, we aimed to study the solid-liquid transition of nanoparticles of different sizes and compositions. For this purpose, we performed atomic scale simulations using a semi-empirical potential integrated in a Monte Carlo code to relax the structures. In doing so, we observed that the melting temperature decreases with the size of the nanoparticles (pure systems and alloys). However, our detailed analysis shows that the melting of the nanoparticle systematically passes through an intermediate stage with a crystalline core (pure Pt or AgPt depending on the composition) and a layer of liquid Ag.

Thursday 29th July 2021 at 11h00

videoconference at the following link: https://cnrs.zoom.us/j/98877547159