Category Seminar

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

Dr.  Enrica Epifano

CIRIMAT, UMR 5085, toulouse France

In this talk, E. Epifano will present the results of her postdoctoral research conducted at the ONERA. The presentation will comprise two parts. In the first part, solubility and diffusion of light atoms (B, C, N, O) in the γ-TiAl intermetallic phase are investigated by first-principle calculations. The accommodation of the light atoms in the various interstitial positions is studied by density functional theory. Barrier energies for their diffusion among the different interstitial sites are computed using the Nudged Elastic Band (NEB) method and atomic jumping rates are obtained from the Transitional State Theory. Diffusion coefficients are obtained from the solution of the transport equation in the infinite time limit, using the analytical Multi-State Diffusion method.

In the second part, experimental studies of the oxidation resistance of MAX phases are shown. The MAX phases are a new class of materials that have an extraordinary combination of both metallic and ceramic features. Some of the MAXes are alumin-forming and they hence exhibit excellent oxidation resistance. The results herein shown concern a study on the quaternary (Ti_x,Ga_1-x)3AlC2 phase, realized in collaboration with the Drexel University of Philadelphia.

Friday 25 June 2021 14h00

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

Dr. Antoine GUITTON^1,2

¹Université de Lorraine – CNRS – Arts et Métiers – LEM3, Metz, France
²LabEx Damas – Université de Lorraine, Metz, France
antoine.guitton@univ-lorraine.fr
 www.antoine-guitton.fr

Although mechanics of materials is aged by more than one century, it still faces many conceptual challenges. One must relate two extreme scales: the sample scale (i.e. Macroscopic) and the scale of fundamental mech- anisms (i.e. Microscopic). In addition, statistics of observations are generally extremely low so that, on one hand the uncertainty is high and on the other hand the representativeness of mechanisms is very questionable.

The Transmission Electron Microscope (TEM) is one of the most well-known techniques for observing and characterizing dislocations in electron transparent thin foils (thickness of ≈ 100nm with a useful field of view of few μm) (1–4). Comprehensive dislocation studies at microscopic scale bring valuable information for extrapolating to the macroscopic mechanical response of materials and they can feed numerical advanced multiscale crystal plasticity models (4). However, fundamental questions on the representativeness of observed phenomenon must be raised, when extrapolating discussions to a millimeter-size specimen.

In this framework, we have successfully combined mechanical testing (nanoindentation and in-situ tensile tests) of bulk specimens with a dislocation-scale characterization technique: Accurate Electron Channeling Contrast Imaging (A-ECCI) (5). A-ECCI is a non-destructive procedure offering the ability to provide, inside a Scanning Electron Microscope (SEM), TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about 100 nm) on centimetric bulk specimen with still unsurpassed resolutions (6).

First, physics of defect contrasts and experimental procedures will be presented (5; 7; 8). Second, the full potentiality of A-ECCI for following the evolution of deformation microstructures will be detailed (9–11). Finally, statistical comparisons between crystal plasticity models and experiences will be highlighted in the framework of a first step towards feature engineering (12–14).

Thursday 27 May 2021 14h00

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

First synthesised in the 60’s, the metallic glasses are a very promising class of material thanks to their very high yield strength. Yet, these materials are also very brittle due to the formation of persistent shear bands which concentrate plastic deformation.

In this thesis, we perform atomistic simulations with a simple two-dimensional binary Lennard-Jones model glass. To link plasticity and the material structure, we use a novel structural indicator, the local yield stress. 

Through this measure, the material average local yield stress is shown to increase as the degree of relaxation increases. We also find the existence of a unique post-yield shear threshold distribution, independent on the initial state of the material.

By the mean of an elementary model, the origin of the Bauschinger effect in amorphous solids (a plasticity-induced  asymmetry of the mechanical behaviour) is found to arise from the inversion of the low yield barriers population anisotropy during the unloading.

Then, by considering systems of different sizes and degrees of relaxation the persistence of plasticity, and thus the formation of shear-bands, is shown to mostly depend on the degree of relaxation of the system.

Finally, in well relaxed glasses, a correlation between the location of the shear band and the initial soft regions is shown. As further loading is applied on the material, a diffusive broadening of the shear band is observed.

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.

Speaker :  Dr. Lisa Belkacemi-Rebrab (Centre des Matériaux, MINES ParisTech)
Date and Location:  Friday 16/10/20 14h00, LEM meeting room (E2.01.20), Châtillon.

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.

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.