Archive 23 June 2022

Data-driven models of atomic simulations in discrete and continuous state spaces

By in Seminar

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.

Optical properties of black phosphorus: from bulk crystal to atomic layers

By in Phd Defence

Abstract

Black phosphorus is a small gap semiconductor (about 0.3 eV) that has recently joined the family of two-dimensional materials. Due to its modulable band gap from mid-infrared to visible depending on the thickness, its strong anisotropy in the atomic plane as well as the high mobility of charge carriers it is promised to a high application potential in the field of optoelectronics. The objective of this thesis was to study the optical properties of the black phosphorus crystal and its atomic layers.


After a description of the different instrumental developments realized during this thesis, the methods of
fabrication of the samples are discussed. Two points have to be mastered: The elaboration of thin layers and their protection from environmental conditions to avoid their oxidation. In a first part, several methods known as “Top-Down”(mechanical exfoliation, gold assisted exfoliation, ion etching) are compared on the basis of the quality, the size, the thickness of the obtained samples as well as the ease of the operating mode execution. In a second part, two methods of thin film protection are presented: alumina passivation (by ALD or aluminum evaporation) and encapsulation of BP flakes into hBN flakes (hBN / BP / hBN heterostructures).


The strong anisotropy of black phosphorus makes the identification of the orientation of the crystallographic axes a key point in the study of the material. For this purpose, a procedure has been proposed using polarized Raman spectroscopy. It has been confronted and validated by different experimental means (TEM observations, EBSD) and theoretical means (modeling of the Raman intensity in thin films). The vibrational properties have also been studied as a function of the number of atomic layers. Several effects have been noticed at high (> 100 cm-1) and low (< 100 cm-1) frequencies and are attributed to dimensionality reduction and resonance phenomena. Thanks to the peculiar excitation conditions used in this study, a large number of modes related to inter-plane vibrations are for the first time identified and have been shown to be accurate indicators of crystallite thickness. The photoluminescence of the bulk crystal is for the first time studied at room and cryogenic temperatures.

Several band-edge emission components have been identified as excitonic, including a fine line due to the free exciton. The analysis of their behavior as a function of temperature as well as a calculation of the binding energy of the free exciton taking into account the anisotropy of the medium have made it possible to establish a new reference value for the black phosphorus gap at 0.287 eV at 2 K. The photoluminescence study of the exfoliated crystals revealed the disappearance of the fine line of luminescence in favor of a wide band. This change is attributed to the density of defects introduced by the mechanical exfoliation as evidenced by a broadening of Raman bands. The photoluminescence band was followed as a function of the thickness of the exfoliated layers down to 8 atomic layers. Below a threshold thickness evaluated at 25 nm, a shift of the band towards high energies is highlighted, and is very well described by a quantum confinement model. No significant difference is observed between the alumina passivated and hBN encapsulated samples, which indicates that the dielectric effects are not predominant in the thickness range studied.

Phd Candidate:
Etienne Carré

Jury:
Christophe TESTELIN – Directeur de recherche, CNRS, Sorbonne Université – Rapporteur
Laëticia MARTY – Chargée de recherche, CNRS,Université Grenoble Alpes – Rapporteur
Bruno MASENELLI – Professeur des universités, INSA Lyon – Examinateur
Aurélie PIERRET – Ingénieure de recherche, CNRS, École Normale Supérieure Paris – Examinatrice
Pierre SENEOR – Professeur des universités, CNRS, Université Paris Saclay – Examinateur
Annick LOISEAU – Directrice de recherche, ONERA, Sorbonne Université – Directrice de thèse
Julien BARJON – Professeur des universités, UVSQ – Directeur de thèse
Ingrid STENGER – Maîtresse de conférences, UVSQ – Encadrante

 

Thursday 23th June 2022, 14h30
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Modelling of the propagation of a short crack in ductile material coupling phase-field method and dislocation dynamics

By in Phd Defence

Abstract

The propagation of short cracks in FCC metals is strongly influenced by microstructures, in particular associated with the linear defects of the crystals, i.e. dislocations.

In this work, a new coupling between two methods at the mesoscale is proposed to investigate the interaction of moving cracks with three-dimensional dislocation microstructures. First, crack propagation is predicted by a phase field model. In this approach, cracks are described by some continuous damage field that evolves so as to minimize the total free energy, including stored elastic energy and surface energy associated with the crack. Second, dislocation microstructures are handled by a Dislocation Dynamics (DD) model that describes plastic deformation by the movement of dislocations under external loading.

To couple both models, the DCM (Discrete-Continuous Model) approach is used, where dislocations are described by continuous fields (eigenstrain or Nye tensor) in an elastic solver. Fast Fourier Transform (FFT) based solvers are used for their computational efficiency. Particular discretization schemes have been adopted to minimize the smoothing of dislocation cores, usually performed in MDC approaches. The different schemes are carefully analyzed with respect to the quality of the predicted fields. In addition, the resulting model is implemented using efficient parallelization solutions.

Thanks to this new coupling, we have been able to study the elastic shielding on crack propagation according to the nature of the slip systems and the dislocations density. We have also been able to investigate phenomena and ingredients rarely accounted for, such as dislocation cross slips close to the crack front or the influence of the number of sources. This mesoscale method constitutes a breakthrough for the thorough analysis of physical mechanisms controlling the early stages of fracture in metallic materials.

Keywords : Crack, Plasticity, Multiphysics modelling, Dislocation Dynamics, Phase Field

Phd Candidate:
Luis Eon

Jury:
Stéphane Berbenni – Directeur de Recherche CNRS, LEM3, Metz – Rapporteur
Samuel Forest –  Directeur de Recherche CNRS, CDM, Evry  – Rapporteur
Véronique Doquet – Directrice de Recherche CNRS, LMS, Palaiseau  – Examinatrice
Lionel Gélébart – Ingénieur-chercheur HdR,  CEA/DEN, Gif-sur-Yvette – Examinateur
Rénald Brenner – Directeur de Recherche CNRS, D’Alembert, Paris – Examinateur
Yoann Guilhem – Maître de conférences, LMPS, Gif-sur-Yvette – Examinateur
Riccardo Gatti – Chargé de Recherche CNRS, LEM, Châtillon – Encadrant de thèse
Benoît Appolaire – Professeur des Universités, IJL, Nancy – Directeur de thèse

 

Tuesday 14th June 2022, 10h00
Salle Contensou, ONERA, 29 Avenue de la Division Leclerc,92320, Chatillôn

Dislocation-free plasticity in small-grained metals

By in Seminar

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.

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