Modeling of platinum-based nano-alloys: Co-Pt, emblematic system of the order, and Pt-Ag, hybrid system between order and demixtion.

Due to the strong correlation between chemical order and physical properties, nanoalloys with a tendency to order are particularly interesting in the field of catalysis, magnetism, or optics. By reducing the size of the system, i.e. from a solid alloy to a nanoalloy, many questions arise: Is the chemical order preserved? What is the morphology of nanoparticles? What is the composition and chemical order on the surface? What is the evolution of properties with size? This presentation is devoted to the study of two systems, both similar and different in their behavior: Co-Pt, a system emblematic of the chemical order, and Pt-Ag, a hybrid system presenting both a chemical order and a tendency to demix, as well as a strong tendency to segregation. In order to answer these various questions, we adopt a semi-empirical approach through an N-body potential, allowing atomic relaxations, in the approximation of the second moment of state density (SMA), coupled with Monte Carlo simulations in different ensembles. The SMA potential is adjusted, in order to reproduce the volume and surface properties, on calculations derived from the theory of density functional theory (DFT) or on experimental data. In a first step, the volume phase diagram of the two systems is determined by the model and compared to the experiment. Then the low index surfaces (111), (100) and (110) are studied in order to verify the segregation inversion observed for the Co-Pt system, where Pt segregates weakly on the dense surfaces (111) and (100) but where we observe a pure Co plane on the surface (110). On the contrary, the Pt-Ag system shows strong Ag segregation on surfaces (111) and (100). In a second step, aggregates of truncated octahedral morphology of different sizes (ranging from 1000 to 10000 atoms) will be analyzed in terms of chemical composition on the different unequal sites (top, edge, facets (100) and (111) and core) and then compared to the reference systems (surfaces, volume) over the whole concentration range. For the Co-Pt system, we observe ordered structures similar to those of the volume for the core and similar to those of the surfaces for the facets. The impact of the two-dimensional phase (√3 × √3)R30◦ specific to the surface, is all the more important on the chemical order at the core as the nanoparticle is small. For the Pt-Ag system, we observe an important segregation of Ag at the surface, as well as a Pt enrichment at the subsurface, and the stabilization of the L11 ordered phase at the core. This structure can appear in a single variant or by adopting all possible variants, leading to an onion peel structure.

 

Diffusion accélérée par l’irradiation et la déformation plastique: analyse microstructurale et élémentaire

L’exploitation prolongée des centrales nucléaires françaises implique la compréhension des mécanismes de vieillissement sous irradiation des réacteurs nucléaires à eau pressurisée (REP). Le séminaire abordera, en ce sens, deux éléments d’étude.

Le premier volet de la présentation s’applique aux cuves des REP qui subissent une fragilisation importante sous irradiation neutronique. Cette fragilisation est due à la formation et l’agglomération de défauts ponctuels (lacunes et interstitiels), constituant un obstacle au mouvement des dislocations. La contribution des amas de soluté au durcissement est également non négligeable [1,2]. Cette étude vise à identifier l’effet du Ni et du Mn sur la formation et l’évolution des défauts microstructuraux, en mettant en évidence les mécanismes de ségrégation de ces solutés sur les amas/puits de défauts ponctuels pouvant conduire à la précipitation de phases secondaires. Pour ce faire, deux alliages modèles sous-saturés Fe-3%at.Ni et Fe-3%at.Mn ont été caractérisés après irradiation aux ions et aux électrons, en couplant la Microscopie Electronique en Transmission (MET, conventionnelle et haute résolution) et la Sonde Atomique Tomographique (SAT). Les résultats obtenus (Fig. 1.a-c) montrent que l’irradiation à 400°C induit, dans le Fe3at.%Ni, la formation d’une phase γm non prédite par le diagramme de phases d’équilibre (Fig. 1.d). Ainsi, un modèle faisant le lien entre la chimie des phases, les déformations propres de la phase (eigenstrain) et la concentration d’excès des défauts ponctuels a été développé pour calculer les diagrammes de phases sous irradiation (Fig. 1.e).

Figure 1 : (a) Image MET-HR d’un précipité γ formé sous irradiation dans la matrice α-Fe3at.%Ni ; (b) cliché de diffraction associé ; (c) agrandissement des encarts bleu et rouge indiqués dans les variants 1 et 2 respectivement du précipité présenté en (a) ; (d) et (e) sont les diagrammes de phases d’équilibre et sous irradiation (calculés) respectivement.

 

Le second volet de ce séminaire porte sur le mécanisme de propagation de fissures de corrosion sous contrainte (CSC) dans les composants du circuit primaire des REP (alliages base Ni). A la température de fonctionnement (350°C), l’amplitude de la zone déchromée découlant de l’oxydation sélective de l’élément Cr aux joints de grains n’est pas expliquée [3]. En effet, le coefficient de diffusion du Cr dans ces alliages, extrapolé à partir des hautes températures, est trop faible [4]. Cette étude se propose ainsi d’évaluer l’hypothèse d’un effet accélérateur de la plasticité sur la diffusion. Pour cela, des expériences de diffusion sont réalisées à basses températures (jusqu’à 500°C) dans le Ni pur à l’état non déformé et déformé. Les coefficients de diffusion du Cr en volume et aux joints de grains sont déterminés à l’aide de deux techniques : la Spectrométrie de Masse des Ions Secondaires (SIMS) et la technique des radiotraceurs (Université de Münster, Allemagne).

[1] M.K. Miller, M.G. Burke, An atom probe field ion microscopy study of neutron irradiated pressure vessel steels, J. Nucl. Mater. 195 (1992) 68-82.

[2] M. Lambrecht et al., On the correlation between irradiation induced microstructural features and the hardening of reactor pressure vessel steels, J. Nucl. Mater. 406 (2010) 84-89.

[3] M. Sennour et al., Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment, J. Nucl. Mater. 393 (2009) 254.

[4] D. D Pruthi et al., Diffusion of Chromium in Inconel-600, J. Nucl. Mater. 64 (1977) 206-210.

Conférencière : Dr. Lisa Belkacemi-Rebrab  (Centre des Matériaux, MINES ParisTech) 

Date et Lieu : Vendredi 16 Octobre à 14h00 Salle de conférence du LEM (E2.01.20), Châtillon.

 

CPFEM simulations of grain size effect in FCC polycrystals: a new approach based on surface GND density

A multiscale modeling methodology involving discrete dislocation dynamics (DDD) and crystal plasticity finite element method (CPFEM) is used to study the physical origin and to simulate the grain size effect in FCC polycrystalline plasticity. This model is based on the dislocation density storage–recovery framework, expanded on the scale of slip systems. DDD simulations are used to establish a constitutive law incorporating the main dislocation mechanisms controlling strain hardening in monotonically deformed FCC polycrystals. This is achieved by calculating key quantities controlling the accumulation of the forest dislocation density within the grains and the polarized dislocation density at the grain boundaries during plastic deformation. The model is then integrated into the CPFEM at the polycrystalline aggregate scale to compute short- and long-range internal stresses within the grains. These simulations quantitatively reproduce the deformation curves of FCC polycrystals as a function of grain size. Because of its predictive ability to reproduce the Hall-Petch law, the proposed framework has a great potential for further applications.

Speaker: Maoyuan Jiang

Date and Location: Monday 09/03/20 14h00, LEM meeting room (E2.01.20), Châtillon.

Orientation imaging at the onset of plastic deformation


Diffraction Contrast Tomography (DCT) is a near-field X-ray diffraction technique for the inspection of ductile materials at the micron scale. It has traditionally been used for the study of undeformed polycrystalline materials with grain sizes of a few tenths of microns. It uses a box-sized monochromatic X-ray beam, which allows it to scan large regions of millimeter sized sample (with up to thousands of grains) in a relatively short time.
Recent work has introduced sub-grain orientation reconstruction (6D-DCT), which has made DCT a viable tool for the reconstruction of slightly deformed materials.
Topo-tomography (TT) is also a near-field X-ray diffraction technique, which, on the other hand, allows to focus on a single grain with a high-resolution detector and to obtain sub-micron level shape information.
In this talk, we will first present how the data is acquired and reconstructed in modern DCT and TT acquisitions. Then, we will present their 6D and 5D extensions (respectively) for the reconstruction of sub-grain level orientation information. Finally, we will discuss future applications, including the combined use of DCT and TT data in a single 6D reconstruction for the investigation of slip bands formation at the onset of deformation.

Speaker: Dr Nicola Viganò

Date and Location: Friday 21/02/20, 14h00 LEM meeting room (E2.01.20), Châtillon.

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.