Seminar: “Coupling Bragg Coherent Diffraction Imaging (BCDI) and Molecular Dynamics to investigate nanostructure “

Fig. 1 (top) Experimental reconstruction of the u111 displacement field on a 250 nm Pt NP (bottom)  u111 displacement field obtained by energy minimization of a simulated Pt NP (right) εxx, εyy and  εzz components of the strain tensor derived from the simulation

Physical properties at small length scale deviate strongly from the bulk counterpart, typically below the micrometer. For instance, mechanical strength increases with reducing size, large residual strain due to processing are present in nanostructures. Thus a better understanding of the physical properties in relationship with the microstructure is needed for nanoscale materials. Because of its good spatial resolution (~ 10 nm) and excellent sensitivity to atomic displacements and local strain [1,2], Bragg coherent diffraction imaging (BCDI) has emerged in the past two decades as a powerful tool to probe the structure and local displacement field inside nanoscale objects [3]. When combined with in situ mechanical loading, BCDI is particularly relevant for the study of defect nucleations in isolated nanoparticles [4] or to investigate intragranular deformation mechanisms in polycrystalline thin films [5].

Nowadays, the length scales that are accessible by BCDI and that can be simulated by Molecular Dynamics (MD) simulation are almost converging. The coupling between the two methods is therefore particularly relevant and allows to get a detailed picture of the deformation mechanisms in nanostructures at the atomic scale. This coupled approach has been used to study the surface relaxation of metallic nanoparticles (Au, Pt). An excellent quantitative agreement is obtained between the component of the displacement field measured experimentally and calculated by energy minimization (Molecular Statics) (Fig. 1). With this approach, the measurement of only one Bragg reflection is required to derive the 3D displacement field and the six independent components of the strain tensor from the simulation [6]. The two techniques can also be combined to  identify defect structures nucleated during in situ  mechanical loading [4,5] and to interpret the evolution of the strain field in nanoparticle catalysts during gas reaction [7,8]..

[1] Watari, M. et al. Nature Materials 10, 862–866 (2011).

[2] Labat, S. et al. ACS Nano 9, 9210–9216 (2015).

[3] Robinson, I. & Harder, R. Nat Mater 8, 291–298 (2009).

[4] Dupraz, M. et al. Nano Lett. 17(11) (2017).

[5] Cherukara, M. et al. Nat. Comm. (2018).

[6] Dupraz et al.  to be submitted (2019)

[7] Kim, D. et al. Nat. Comm. 9, 3422 (2018).

[8] Dupraz, M. et al. in preparation

Speaker: Dr. Maxime Dupraz

Date and Location: Monday 25/11/19, 14h00 LEM meeting room (E2.01.20), Châtillon.

Georges Saada passed away

Georges Saada was born on August 10, 1932 in Sfax, Tunisia. He left Tunisia at the age of seventeen, after high school and obtaining his baccalaureate. In Paris, he prepared the Grandes Ecoles at the Louis-le-Grand and Buffon high schools. He joined the École Polytechnique in 1952 and then prepared a thesis in metal physics. He also graduated at the École Nationale Supérieure des Télécommunications.

After military service, his career began in the army, which he left in 1960. Indeed his skills led him to scientific research. After five years at the Institut de Recherche de la Sidérurgie, as a research engineer, he chose to focus his career on university teaching. First a lecturer at the University of Lille, he participated in 1969 in the creation of the University of Paris XIII Villetaneuse where he became a professor in 1971. From 1973 to 1981, he was at the head of the Laboratoire des Propriétés Mécaniques et Thermodynamiques des Matériaux.

In 1981, he was appointed Head of Mission for Higher Education at the Ministry of National Education, Alain Savary.

He returned to the University of Paris XIII and joined the Laboratoire d’Etude des Microstructures in 1990.

Georges Saada played a pioneering role in the field of plasticity of materials. His work has had a major impact on the development of this discipline, with seminal contributions to the understanding of the physical mechanisms that cause deformation of metal alloys. His work has been recognized by the award of the Grande Médaille de la Société Française de Métallurgie et de Matériaux in 2008.

Phd Defence:” Investigation of grain size and shape effects on crystal plasticity by dislocation dynamics simulations”

Phd candidate: Maoyuan JIANG

Directeur de thèse : Benoit DEVINCRE

Co-directeur de thèse : Ghiath MONNET

Dislocation Dynamics (DD) simulations are used to investigate the Hall-Petch (HP) effect and back stresses induced by grain boundaries (GB) in polycrystalline materials.

The HP effect is successfully reproduced with DD simulations in simple periodic polycrystalline aggregates composed of 1 or 4 grains. In addition, the influence of grain shape was explored by simulating grains with different aspect ratios. A generalized HP law is proposed to quantify the influence of the grain morphology by defining an effective grain size.  The average value of the HP constant K calculated with different crystal orientations at low strain is close to the experimental values. 

The dislocations stored during deformation are mainly located at GB and can be dealt with as a surface distribution of Geometrically Necessary Dislocations (GNDs). We used DD simulations to compute the back stresses induced by finite dislocation walls of different height, width, density and character. In all cases, back stresses are found proportional to the surface density and their spatial variations can be captured using a set of simple empirical equations. The back stress calculation inside grains is achieved by adding the contributions of GNDs accumulated at each GB facet.

These back stresses are found to increase linearly with plastic strain and are independent of the grain size. The observed size effect in DD simulations is attributed to the threshold of plastic deformation, controlled by two competing mechanisms: the activation of dislocation sources and forest strengthening. Due to strain localization in coarse-grained materials, the pile-up model is used to predict the critical stress. By superposing such property to the analysis we made from DD simulations in the case of homogeneous deformation, the HP effect is justified for a wide range of grain sizes.

Tuesday 04/05/2019, 13h30
Amphi 3, Bâtiment Eiffel, CentraleSupélec, 8-10 rue Joliot-Curie, 91190 Gif-sur-Yvette