Transmission electron microscopy study of extended defect evolution and amorphization in silicon carbide under silicon ion irradiation

International audienceThe damage induced in 3C-SiC epilayers on a silicon wafer by 2.3-MeV Si ion irradiation for fluences of 10$^{14}$, 10$^{15}$, and 10$^{16}$ cm$^{-2}$, was studied by conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The evolution of extended defects and lattice disorder is followed in both the 3C-SiC film and Si substrate as a function of ion fluence, with reference to previous FTIR spectroscopy data. The likelihood of athermal unfaulting of native stacking faults by point defect migration to the native stacking faults is discussed in relation to damage recovery. Threshold energy densities and irradiation doses for dislocation loop formation and amorphous phase transformation are deduced from the damage depth profile by nuclear collisions. The role of electronic excitations on the damage recovery at high fluence is also addressed for both SiC and Si

Phase field modelling of irradiated materials

International audienceAlmost forty years after Turings seminal paper on patterning, progress on modeling instabilitiesleading to pattern formation has been achieved. The initial concept of dissipative structure isnow clearly understood within the Phase-Field framework. So far, such an approach obtainedpromising results in various aspects of materials research from pattern formation during solidificationto defect dynamics. In this work, we will try discussing experimental results observed during agingof solids under irradiation within this framework. The approach followed in this presentation iscomprehensive and not specialized in specific aspects of the Phase-Field modelling (mechanics,mathematics, or numerical methods) at the expense of a holistic picture

X-Ray diffraction study of oxygen deficient Y2Ti2O7-δ pyrochlore powders synthesized by high-energy ball milling (HEBM)

International audienceY2Ti2O7 pyrochlore are widely used for various applications, from oxygen transport to materials strengthening. Ultra-pure oxygen deficient Y2Ti2O7-δ pyrochlore powders synthesized by high-energy ball milling (HEBM) are studied by X-ray diffraction (XRD). Rietveld analysis of diffraction patterns reveals that a large oxygen sub-stoichiometry can be accommodated in this material with only minor structural changes. This accommodation results from the ability of Ti ions to change their valence state, which is confirmed by complementary EELS analysis. It may also explain why Y2Ti2O7 is both radiation and mechanical milling resistant. This work highlights the fact that selection of cations allows rigid ionic structures like pyrochlore to accommodate a large amount of point defects

Comportement en corrosion de l'acier inoxydable 316 L produit par les procédés WAAM et WLAM en milieu aqueux à haute température

International audienceL'acier inoxydable austénitique 316L est couramment utilisé dans le circuit primaire des réacteurs nucléaires à eau pressurisée (REP) pour sa bonne résistance à la corrosion à haute température. La fabrication additive par dépôt fil permet de réparer et d'ajouter des caractéristiques aux composants existants à faible coût. Cette étude a donc pour objectif d'évaluer la durabilité du 316L élaboré par fabrication additive par arc-fil (WAAM) et laser-fil(WLAM) en milieu primaire de REP.Des échantillons de 316L déposé par WAAM et WLAM ont été soumis au préalable à des traitements de détensionnement et d'hypertrempe. Ils ont ensuite été exposés à de l'eau hydrogénée à 340 °C et 160 bar jusqu'à 3 000 h. Pour évaluer la sensibilité à la CSC dans ce milieu, des essais à déformation imposée ont été réalisés sur des éprouvettes U-bend. Des essais de traction lente ont également été réalisés sur des éprouvettes prédéformées à 11% dans le domaine plastique.La microstructure a été finement caractérisée par microscopie optique et électronique (MEB et MET) ainsi que par diffraction d'électrons rétrodiffusés (EBSD). Les échantillons détensionnés possèdent tous une microstructure dendritique composée de grains austénitiques colonnaires et de ferrite δ squelettique. De la phase intermétallique σ ne se serait formé uniquement que dans les échantillons WAAM. A l'état brut et détensionné, les aciers WAAM et WLAM présentent une texture morphologique et cristallographique qui consiste en des grains colonnaires le long de la direction de solidification [001]. Après hypertrempe, à la recristallisation est de 100% pour le WAAM et de 40% pour le WLAM qui présente une microstructure duale.Après exposition, une couche duplex d'oxyde est formée en surface. Elle est composée d'une fine couche continue d'oxyde cohérente avec une structure spinelle (Ni,Fe)(Fe,Cr)2O4 et recouverte de cristallites d'oxyde de magnétite Fe3O4, similairement au 316L conventionnel [1].Les résultats des essais de CSC seront également présentés et comparés aux résultats obtenus sur le 316L conventionnel [1]. Une attention particulière sera portée sur le rôle des phases δ et σ sur la fissuration

Corrosion and stress corrosion cracking behavior of 316 L stainless steel produced by additive manufacturing wire processes in water environments

International audienceAustenitic 316L stainless (SS) is widely used in the primary circuit of pressurized water reactors (PWR) for its good corrosion resistance in high temperature water. Wire additive manufacturing allows to repair and add features on existing components at low cost. This study aims to assess the durability of materials obtained by Wire Laser Additive Manufacturing (WLAM) in PWR primary water.Stress-relief and solution annealing heat treatments were performed on WLAM samples. To evaluate the effect of thermal post-treatment on stress corrosion cracking (SCC) susceptibility, U-bends specimens were exposed to hydrogenated water at 340°C and 160 bar for 3000h at 340°C, as well as pre-deformed polished flat specimens to conduct slow strain rate tests (SSRT) up to 6% plastic strain. Material characterizations were carried out by using optical microscopy, scanning electron microscopy, EBSD and transmission electron microscopy. Microstructure of WLAM stress-relief samples is composed of dendrites and mainly cellular austenitic grains with lathy-δ and skeletal-δ. A complete recrystallization is observed after solution annealing, altering mechanical properties. After the exposition, a duplex oxide layer is formed which is composed of a thin continuous (Ni,Fe)(Fe,Cr)2O4 oxide layer, covered by oxide crystallites of magnetite Fe3O4, similar to conventional 316L. Surface characterization of U-bends and SSRT samples clearly shows the influence of microstructure and heat post-treatment on the corrosion behavior of WLAM parts

Intricate disorder in defect fluorite/pyrochlore: a concord of chemistry and crystallography

International audienceIntuitively scientists accept that order can emerge from disorder and a significant amount of effort has been devoted over many years to demonstrate this. In metallic alloys and oxides, disorder at the atomic scale is the result of occupation at equivalent atomic positions by different atoms which leads to the material exhibiting a fully random or modulated scattering pattern. This arrangement has a substantial influence on the material's properties, for example ionic conductivity. However it is generally accepted that oxides, such as defect fluorite as used for nuclear waste immobilization matrices and fuel cells, are the result of disorder at the atomic scale. To investigate how order at the atomic scale induces disorder at a larger scale length, we have applied different techniques to study the atomic composition of a homogeneous La$^2$ Zr$^2$ O$^7$ pyrochlore, a textbook example of such a structure. Here we demonstrate that a pyrochlore, which is considered to be defect fluorite, is the result of intricate disorder due to a random distribution of fully ordered nano-domains. Our investigation provides new insight into the order disorder transformations in complex materials with regards to domain formation, resulting in a concord of chemistry with crystallography illustrating that order can induce disorder