A model to predict image formation in the three-dimensional field ion microscope

This article presents a numerical model dedicated to the simulation of field ion microscopy (FIM). FIM was the first technique to image individual atoms on the surface of a material. By a careful control of the field evaporation of the atoms from the surface, the bulk of the material exposed, and, through a digitally processing a sequence of micrographs, a three-dimensional reconstruction can be achieved. 3DFIM is particularly suited to the direct observation of crystalline defects such as vacancies, interstitials, vacancy clusters, dislocations, and any combinations of theses defects that underpin the physical properties of materials. This makes 3DFIM extremely valuable for many material science and engineering applications, and further developing this technique is becoming crucial. The proposed model enables the simulation of imaging artefacts that are induced by non-regular field evaporation and by the impact of the perturbation of the electric field distribution of the distorted distribution of atoms close to defects. The model combines the meshless algorithm for field evaporation proposed by Rolland et al. (Robin-Rolland Model, or RRM) with fundamental aspects of the field ionization process of the gas image involved in FIM

Reflections on the spatial performance of atom probe tomography in the analysis of atomic neighbourhoods

Atom probe tomography is often introduced as providing "atomic-scale" mapping of the composition of materials and as such is often exploited to analyse atomic neighbourhoods within a material. Yet quantifying the actual spatial performance of the technique in a general case remains challenging, as they depend on the material system being investigated as well as on the specimen's geometry. Here, by using comparisons with field-ion microscopy experiments and field-ion imaging and field evaporation simulations, we provide the basis for a critical reflection on the spatial performance of atom probe tomography in the analysis of pure metals, low alloyed systems and concentrated solid solutions (i.e. akin to high-entropy alloys). The spatial resolution imposes strong limitations on the possible interpretation of measured atomic neighbourhoods, and directional neighbourhood analyses restricted to the depth are expected to be more robust. We hope this work gets the community to reflect on its practices, in the same way, it got us to reflect on our work.Comment: Submitted to Microscopy & Microanalysis to be part of the special issue assocaited to the APT&M 2020 conferenc

Revealing atomic-scale vacancy-solute interaction in nickel

Imaging individual vacancies in solids and revealing their interactions with solute atoms remains one of the frontiers in microscopy and microanalysis. Here we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of tantalum with vacancies. Our results support solute-vacancy binding, which explains improvement in creep resistance of Ta-containing Ni-based superalloys and helps guide future material design strategies.Comment: Submitted to Physics Review Lette

Imaging individual solute atoms at crystalline imperfections in metals

Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials\u27 properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni–Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature

Étude de la fonction de transfert pointe-image de la sonde atomique tomographique

La sonde atomique Tomographique est un instrument de nanoanalyse quantitative. Cet instrument permet l'observation en trois dimensions, dans un volume voisin de 10x10x100 nm3, de la distribution chimique d'un matériau métallique à l'échelle subnanométrique. Ce travail de thèse est consacré à l'étude de la fonction de transfert entre l'échantillon analysé et l'image obtenue. Le principe fondamental est l'évaporation par effet de champ des atomes. Le champ électrique nécessaire est obtenu de l'application d'un potentiel intense sur l'échantillon préparé sous la forme d'une pointe très fine. L'image est produite par la projection des espèces chargées sur un détecteur tridimensionnel. L'évaporation et la projection des atomes de la pointe ont été étudiées au moyen d'un modèle de simulation prenant en compte les principes physiques fondamentaux de l'instrument. La confrontation des résultats simulés aux résultats expérimentaux a montré que le modèle décrit avec succès les processus d'évaporation pour les métaux purs, les alliages monophasés et les alliages multiphasés. Le champ électrique de surface est conforme aux observations de microscopie ionique. La rugosité de la pointe à l'échelle de l'atome a une influence notable sur les lignes de champ électrique et les trajectoires ioniques au voisinage de la pointe. L'influence des conditions initiales sur les aberrations de trajectoire a été évaluée. L'appui des simulations a permis la mise au point de méthodes de correction des images. Ces corrections réduisent notablement les grandissements locaux observés. La résolution spatiale de l'instrument a été calculée. Elle atteint, pour des cas favorables, 0,2 nm parallèlement à la surface d'analyse et 0,06 nm en profondeur. Cette excellente résolution permet la reconstruction tridimensionnelle du réseau cristallin du tungstène par des méthodes de transformée de Fourier développées au cours de cette thèse.ROUEN-BU Sciences (764512102) / SudocROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF

Enhanced dynamic reconstruction for atom probe tomography

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Enhanced dynamic reconstruction for atom probe tomography

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Dynamic evolution and fracture of multilayer field emitters in atom probe tomography: a new interpretation

International audienceSince Atom Probe Tomography reconstruction is based on ion back projection onto the emitter surface, understanding of the evolution dynamics of the tip shape is essential to get an accurate picture of the initial sample. In this article, an analytical approach is presented to dynamically describe the morphology evolution of complex multilayer structures during field evaporation. The model is mostly founded on the common continuity hypothesis, except for the classical hemispherical description of the tip apex, which is extended to a wider class of a constant mean curvature surface of revolution, the Delaunay surfaces. The results obtained from this approach are comparable with standard numerical simulations, but the analytical character of the model gives more insight into the principles driving the emitter morphology. In particular, a complete picture of curvature evolution during the transition from one layer to another is provided. Additionally, a field evaporation threshold for tip fracture in a bilayer sample is highlighted

A Meshless Algorithm to Model Field Evaporation in Atom Probe Tomography

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