Antoine Guitton

1. Various surface-sensitive techniques have been developed to analyse dislocation structures, each offering unique advantages and limitations.In Transmission electron microscopy (TEM), individual ...

The Scanning Transmission Electron Microscopy in a Scanning Electron Microscope (STEM‐in‐SEM) approach proposed in this work addresses the possibilities and limitations of a prototype developed for microstructural characterization in thin foils. The developed detector comprises a digital micromirror device (DMD) that can generate user‐defined apertures for electron microscopy. As such, TEM‐like characterizations are possible while minimizing the presence of artifacts that sometimes hinder analyses. In addition, the developed detector benefits from an On‐Axis Transmission Kikuchi Diffraction (On‐Axis TKD) geometry, allowing relatively fast diffraction pattern acquisition with a high signal‐to‐noise ratio. The experimental results demonstrate both the ability to obtain diffraction patterns and to image microstructural defects—including diffraction contrast to characterize dislocations—under adjustable bright‐field and dark‐field conditions. The proposed Pixelated STEM‐in‐SEM technique is part of a broader framework aimed at transferring TEM techniques to the Scanning Electron Microscope (SEM).

Kink bands (KBs) are localized regions that rotate relative to adjacent, undeformed areas. They typically form in crystalline materials characterized by strong structural anisotropy and limited slip activity, such as hexagonal close-packed (HCP) structures and particularly in high- c / a nano-layered HCP crystals such as MAX phases. KBs induces lattice rotation and reorients the structure to activate additional slip systems, aiding in meeting the von Mises criterion. In this work, we propose a new model for KB formation based on the concept of zonal dislocations (ZDs). Continuum-scale thermodynamic analyses demonstrate that this model is both physically plausible and structurally stable. These findings are corroborated by atomic-scale simulations, which confirm the feasibility of ZD formation and reveal its internal structure. This model provides a coherent framework for interpreting plasticity in MAX phases and offers a compelling explanation for the hysteresis often observed during deformation, an open question in their mechanical behavior. Our results show that the formation of a single wall of basal dislocations, followed by their dissociation into ZDs, is significantly more energetically favorable than existing models that rely on the formation of two opposing walls of dislocation dipoles, currently considered the dominant pathway for KB initiation.

Il est couramment admis que la déformation plastique des phases MAX est dueau glissement de dislocations dans les plans de base s'organisant en empilements et murs. Cesderniers peuvent former des zones de désorientation locale appelées kink bands. Cependant, lesmécanismes élémentaires et le rôle exact des défauts microstructuraux sont encore mal connus. Cemanuscrit présente une étude expérimentale multi-échelle des mécanismes de déformation de laphase MAX Ti2AlN. A l'échelle macroscopique, deux types d'expériences ont été menés. Des essaisde compression in-situ à température et pression ambiantes couplés à la diffraction neutroniqueont permis de mieux comprendre le comportement des différentes familles de grains dans le Ti2AlNpolycristallin. Des essais de compression sous pression de confinement ont également été réalisés dela température ambiante jusqu'à 900 °C. À l'échelle mésoscopique, les microstructures des surfacesdéformées ont été observées par MEB et AFM. Ces observations complétées par des essais denanoindentation ont montré que la forme des grains et leur orientation par rapport à la directionde sollicitation gouvernent l'apparition de déformations intra- et inter-granulaires ainsi que lalocalisation de la plasticité. Finalement à l'échelle microscopique, une étude détaillée par METdes échantillons déformés sous pression de confinement a révélé la présence de configurations dedislocations inédites dans les phases MAX, telles que des réactions entre dislocations, des dipôleset des dislocations hors plan de base. À la vue de ces résultats nouveaux, les propriétés mécaniquesdes phases MAX sont rediscutées.

Defect–grain boundary interactions have been proven essential for the development of tailored microstructures for high-performance materials. We aim to unravel the mechanical response of grain boundaries by investigating dislocation activity in the vicinity of a tensile twin boundary (TB) in Mg–Y. A combined approach of quasi in-situ nanoindentation, electron channeling contrast imaging (ECCI), and HR-EBSD was employed to induce localized deformation and analyze the resulting TB response. Indentation in the parent grain near the TB produced distinct pop-ins, indicating deformation events close to the boundary. Observed slip included non-basal activity in the twin and prismatic slip in the parent grain. Dislocation pile-ups caused TB bulging rather than transmission, consistent with low slip transmission factors. However, limited slip transmission is observed. Peach–Koehler force calculations based on HR-EBSD measurements confirm stress-induced TB motion.

In this paper is presented an application of the hardfacing process using the cored wire Hardface HC-O (or T Fe15 as per EN 14700), highly alloyed with carburizing elements (≈4 wt.% C, ≈5 wt.% Si, ≈25 wt.% Cr). A multi layer-layer surface deposit is applied to a S355MC steel substrate, and its final structure analyzed at both macroscopic and microscopic scales. Samples were taken from different points of the structure and were first digitally sectioned using X-ray tomography for defect segmentation. Chemical composition, microstructure, and crystallographic orientations of the present phases were analyzed for each layer and then correlated with tomographic observations and hardness properties. The results confirmed the advantage of using an over-alloyed cored wire, as the hardness increased by more than 500% compared to the substrate. The formation of primary M(Fe,Cr)₇C₃ chromium carbides within a carbon-enriched austenitic matrix enabled surface hardness values close to 1000 HV in the deposited layer. Microstructural formation mechanisms were identified and are, in fact, directly governed by the chemical composition of the wire, combined with the multilayer geometry of the deposit and the cladding kinetics. These results are promising as they could extend the use of hardfacing to applications requiring higher energy absorption, such as blast scenarios or even kinetic threats impacts.