Characterisation of shear bands and plasticity in model glasses at the atomic scale

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.

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.

 

Confinement of dyes inside boron nitride nanotubes: photostable and shifted fluorescence down to the near infrared

Scientists of LEM, LP2N (France), Polytechnique Montréal, Université de Montréal (Canada) have succeeded in encapsulating organic dye molecules inside a boron nitride nanotube. This encapsulation protects efficiently organic dye molecules against degradations inherent to their surrounding conditions and improves the fluorescence over a time scale longer by 104 with respect to that of free dyes.

More details on the of CNRS INP website

René Caudron passed away

Originally from Belgium near Mons, René Caudron did most of his studies there, before joining Onera in 1964 where he was part of the small group that, around Paul Costa, set up the ONERA Solid State Physics Laboratory created at the initiative of Raimond Castaing. He spent his entire career at ONERA in the Materials Department and then at the LEM. He was one of the essential members of the laboratory, an extraordinary engineer-physicist, originally by many aspects of its national and international reputation.
At a time when almost all experiments were set up “at home”, René participated in all the research “manips” of LEM during the first twenty-five years of its existence, which were devoted to the study of the electronic structure of transition compounds: carbides, nitrides, hydrides, borides. These were low-temperature experiments, the highlight of which was undoubtedly its specific low-temperature heat measuring device, one of the most efficient at the time. He thus contributed to validating the models developed at Orsay and Strasbourg on diluted alloys. This was his thesis work.
He took part in all the experimental studies of the laboratory, specialising for a long period in the study of spin glasses, before moving on to the study of chemical effects in alloys. On this occasion, he built his famous G4.4 diffuse scattering spectrometer, installed on the CEA’s Orphée nuclear reactor in Saclay, which he was in charge of until his retirement in 2003, and which was also among the most efficient in the world.
An outstanding physicist and experimentalist, René Caudron made a deep impression on his colleagues, interns and doctoral students, all of whom testify to having met in him an extraordinary researcher and, above all, a man of conviction of unspeakable kindness and modesty.

 

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.

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