Дисертації з теми "Correlative EBSD"

Electron backscatter diffraction is a method that is well described and commonly used for orientation image mapping, including grain size estimation. The use of this method for measuring elastic deformation and rotations caused by plastic deformations is not so well decribed. This diploma thesis first describes the typical EBSD system. The information regarding the standard coordinate systems, grain orientation notation and system calibration is then used to create an open-source software for mapping elastic deformations and rotations inside a single grain or a monocrystal. This software uses data acquired during standard EBSD mapping on a commercial system.

One conventional method for studying dislocations uses the Transmission Electron Microscope (TEM), a complex and expensive piece of equipment which requires extensive specimen preparation in order to thin the specimens to electron transparent thickness. Newer High Resolution Electron Backscatter Diffraction (HREBSD) methods of determining geometrically necessary dislocation content via cross-correlation promise to be able to produce estimates of the dislocation density of the sample over a larger area with considerably less preparation time and using a much more accessible instrument. However, the accuracy of the new EBSD technique needs more experimental verification, including consideration of possible changes in the specimen dislocation density due to the different preparation methods. By comparing EBSD and TEM dislocation measurements of Electron Transparent platinum specimens prepared using the Focused Ion Beam (FIB), along with EBSD dislocations measurements of specimens prepared by both FIB and mechanical polishing techniques, this paper seeks to verify the accuracy of the new method and identify any changes in the specimens’ apparent dislocation density caused by the different preparation processes.

The development and example applications of a new EBSD-based texture analysis system are presented. This new system uses the cross-correlation function to compare two EBSD patterns at a number of corresponding regions in each pattern to calculate the deformation gradient tensor. Bragg's Law-based simulated EBSD patterns are used as reference patterns in the cross-correlation method to enable the measurements of absolute elastic strain and lattice orientation at discrete points in a crystalline sample. The resolution limits of this new method are explored using a variety of computational and physical experiments. The simulated pattern method is estimated to be able to measure lattice orientations to within +/-0.02° and elastic strains to within +/-3.6x10-4 for small strains and +/-1x10-3 for large strains. Two example applications are demonstrated. The first demonstration is estimating the dislocation density in a 5.5% compressed Mg-based AZ91 alloy. Nye's and Kröner's formulations are used to estimate the dislocation density. Comparisons are made with traditional OIM measurements and it is found that the simulated pattern method offers an order of magnitude improvement in dislocation density estimations over OIM. The second demonstration is tetragonality measurements of HSLA 65 steel along the weld line of a friction stir welded plate. Accurate tetragonality measurements in the bainite phase of the steel can be made using information from the diagonal components of the elastic strain tensor. The measured tetragonality can be related to the concentration of interstitial carbon atoms in the iron lattice to find the carbon distribution in the sample. From these experiments, it is demonstrated that the simulated pattern method presents a new and powerful methodology for texture analysis that exhibits both ease of use and access to high resolution orientation and elastic strain data.

Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocationdensity on the microscale for real samples. It is particularly difficult to resolve measureddislocation content onto individual dislocation systems at the length scales most commonly of interestin plasticity studies. Traditionally, dislocation content is analyzed at the continuum levelusing the Nye tensor and the fundamental relation of continuum dislocation theory to interpret informationmeasured by diffraction techniques, typically EBSD or High Resolution EBSD. In thiswork the established Nye-Kroner method for resolving measured geometrically necessary dislocationcontent onto individual slip systems is assessed and extended. Two new methods are alsopresented to relieve the ambiguity of the Nye-Kroner method. One of these methods uses modifiedclassical dislocation equations to bypass the Nye-Kroner relation, and the other estimates the bulkdislocation density via the entry-wise one-norm of the Nye tensor. These methods are validatedvia a novel simulation of distortion fields around continuum fields of dislocation density based onclassical lattice mechanics and then applied to actual HR-EBSD scans of a micro-indented singlecrystals of nickel and tantalum. Finally, a detailed analysis of the effect of the spacing betweenpoints in an EBSD scan (which is related to the step size of the numerical derivatives used in EBSDdislocation microscopy) on geometrically necessary dislocation measurements is conducted.

Ti-6Al-4V is the most widely used titanium alloy and is typically used in stages of gas turbine engines, due to its high strength-to-weight ratio, corrosion resistance and high strength at moderate temperatures. However, the alloy is susceptible to the development of strong textures during thermomechanical processing that leads to a preferred crystallographic orientation. These are referred to as macrozones and are thought to develop during the β to α phase transformation, as a result of the retention of large prior β grains during processing and variant selection. Macrozones are clusters of neighbouring grains with a common crystallographic orientation that may act as one single grain during loading and have been shown to cause scatter in the fatigue life. The focus of the current work was based on the analysing the strain behaviour of soft, hard and no macrozones within the microstructure, during various loading conditions. The local strain behaviour was studied at a micro and nanoscale, using the digital image correlation (DIC) technique, which utilises microstructural images recorded during mechanical loading. On a microscale, the no-macrozone and strong-macrozone condition loaded at 0% exhibited homogeneous strain behaviour. The strong-macrozone condition loaded at 45% and 90% to the extrusion direction, respectively, developed pronounced high strain bands correlating to regions that were favourably oriented for prismatic and basal slip, respectively. Characterisation of the slip bands provided a detailed understanding of the deformation behaviour at the nanoscale and the slip system was subsequently determined for each grain using slip trace analysis. Prismatic slip was the dominant slip system in all conditions, particularly in the soft-oriented macrozone regions of the strong-macrozone condition loaded at 45 degrees. Shear strains of 10 times the appliedstrain were observed. Further investigations on the strong-macrozone condition loaded at 45 degrees to ED during standard and dwell fatigue demonstrated early failure in the dwell sample, with higher strain accumulation for dwell.

It has been found that commercially pure (CP) Titanium (Ti) undergoes substantial amount of deformation twinning during plastic forming in a wide range of temperatures and strain rates giving CP-Ti good ductility and allowing up to 90% thickness reduction by cold rolling. Aluminium (Al) rich ! Ti-alloys lack this superior ductility but exhibit therefore up to five times higher yield strength, which was connected experimentally to reduced activity of deformation twinning with addition of Al to Ti. Ultimately this is also valid in the ! phase of two-phase alloys such as Ti6Al4V and thought to be key to the reduced ductility in Al rich alloys. It is to date unclear if ordering of Al in the Ti matrix, a change in the stacking fault energy (SFE) with alloying or a transition of the cellular dislocation structures in CP-Ti to planar slip patterns at high Al contents reduces twin activity. The focus of this dissertation project is therefore the transition of microstructural details and the deformation structures in the ! phase with increasing Al concentration. For simplified investigations binary Ti-Al alloys containing 3.5, 7, 10 and 13at.% Al have been created with comparable grain morphology and texture within this study. For a better understanding of the role of Al also binary Ti-Sn (Tin) alloys (1 & 3.4at.% Sn) and Ti-Zr (Zirconium) alloys (3.6 & 10at.%) as well as an Oxygen (O) rich Ti-10at.%Al and the industrial compositions of Ti6Al4V were produced on the same route and investigated by the same methods. This alloy range allows evaluation of the effects of the c/a ratio, ordering phenomena and the SFE on the twin activity. The knowledge was finally transferred to industrially forged CP-Ti and Ti5Al2.5Sn. TEM and neutron diffraction confirmed the onset of Ti3Al formation from Al concentrations above 7at.% (4wt%), but no ordering of Zr or Sn atoms was found after solution treatments. The evolution of lattice strain and lattice reorientation due to twinning with increasing compressive strain was captured by in-situ experiments under neutron diffraction at Engin-X, ISIS. Post-mortem EBSD micro and macro texture mappings revealed that the twin fraction in Al reduces above a critical concentration of 7at.% (4wt%), which was enhanced with increasing ordering towards Ti3Al. Sn and Zr addition showed no significant effect on the overall twin fraction, but increased twin numbers with facilitated nucleation and impeded twin growth, which may be related to the SFE. Increasing slip planarity and a transition from prismatic slip towards basal slip with addition of Al was found with means of Digital image correlation (DIC). DIC also revealed intense prismatic slip in grains undergoing !"!! tension twinning and virtually barely any strain accumulation within a twin below 9% plastic strain, rationalised by much increased nanohardness in the twin in comparison to the parent. Nanoindentation also revealed that alloying with Al reduces the crystal anisotropy. Finally it is believed that ordering and the closely related transition of slip patterns lead to the reduction in twin activity, while c/a ratio, crystal anisotropy and SFE seem less important.

Les travaux de cette thèse ont été dédiés à l’étude de pièces minces en acier inoxydable 316L fabriquées ou réparées par Directed Energy Deposition (DED). La nouveauté principale est l’observation de la déformation à l’échelle de la microstructure. Pour ce faire, une expérience associant un essai de traction in situ dans un microscope électronique à balayage, de la corrélation d’images à haute résolution et une carte EBSD de la microstructure ont été réalisés. Les résultats obtenus permettent alors de mieux appréhender les différentes propriétés en traction ainsi que les comportements d’autoéchauffement lors d’essais cycliques. Le premier objectif a été de qualifier et quantifier les mécanismes de déformation à l’échelle de la microstructure pour expliquer le comportent anisotrope révélé lors d’essais de traction. Par conséquent, deux orientations de sollicitation permettant d’effectuer un chargement perpendiculaire ou parallèle à la direction d’impression ont été définies. Dans le premier cas, la localisation de déformation s’effectue au niveau de certaines intercouches. Pour le second, une localisation dans certaines régions correspondant à de gros grains particuliers a été observée. Le deuxième objectif a été d’évaluer la capacité du DED à réparer. Pour ce faire, des éprouvettes moitié tôle / moitié imprimées ont été fabriquées afin de solliciter l’interface en traction. Une grande différence de microstructure a été observée entre les deux matériaux. Néanmoins, l’interface n’a jamais été la zone de rupture lors d’essais de traction. De plus, une déformation homogène a été observée dans la partie tôle alors qu’une déformation hétérogène avec des pics de concentration au niveau de certaines intercouches a été remarquée dans la partie imprimée lors d’essais in situ. Enfin, une zone de faible déformation a été constatée de part et d’autre de l’interface, zone dans laquelle une plus grande dureté a été mesurée. Le dernier objectif a été d’évaluer les propriétés en fatigue par des essais d’autoéchauffement. Il a été montré que les difficultés liées à la faible épaisseur de nos éprouvettes pouvaient être surmontées en maitrisant le protocole expérimental. Un comportement anisotrope a été observé durant ces essais cycliques avec des éprouvettes perpendiculaires au sens d’impression montrant une plus grande limite d’endurance par rapport aux éprouvettes sollicitées parallèlement. Pour ces dernières, des analyses post-mortem ont montré un scénario de fatigue classique avec une fissure dominante et donc une source de dissipation de chaleur localisée. A l’inverse, une multitude de fissures aux intercouches créant de nombreux sites de dissipation a été constatée pour les éprouvettes testées perpendiculairement
This thesis was dedicated to the study of 316L stainless steel additively manufactured or repaired specimens by Directed Energy Deposition (DED). Different configurations were manufactured under optimal process parameters. The novelty of this work is the observation of the microstructural strain localization. This experiment combined an in situ tensile test inside a scanning electron microscope with high resolution digital image correlation and an electron backscatter diffraction map. These results allowed for a fresh interpretation of monotonic tensile tests as well as of self-heating experiments under cyclic loading and the failure patterns observed at the surface of specimens. The first objective was to understand the deformation mechanisms at the grain scale which could explain the observed macroscopic anisotropy of the tensile properties as reported in literature. Two loading directions, along and perpendicular, were considered with respect to the printing direction for fully printed specimens. We observed that for a tensile load perpendicular to the printing direction, the strain localization is mainly situated at some interlayers. For a tensile load along the printing direction, the strain localization was observed in some particular regions of large grains. The second objective was the assessment of DED as a repair technology. Dog bone shaped repaired specimens (half hot rolled sheet and half printed) were designed and they exhibited an important hierarchical microstructural gradient. We noticed that the interface is not a weak area during a monotonic tensile test. Moreover, while homogeneous strain was observed in the substrate half, the printed half showed a strain heterogeneity, with the highest localization found at some interlayers. An unstrained zone was observed at both sides of the interface and was associated with higher hardness. The last objective was to evaluate the fatigue properties by self-heating tests. The experiment has proven that the difficulties due to the small dimensions of the single-track thickness specimens can be overcome by careful construction of the experimental set-up. The results revealed a certain correlation between the pattern of the microstructure, the deformation pattern at this scale and the self-heating results. Anisotropy was highlighted during these cyclic tests where specimens tested perpendicularly to the printing direction showed higher fatigue limits in comparison to the ones tested along the printing direction. Post mortem analysis revealed a multitude of cracks at interlayers for the specimens tested perpendicularly to the printing direction creating several sites of heat diffusion. For the specimens tested along the printing direction, a more classical fatigue scenario was observed with one dominating crack and thus a localized heat dissipation

Further understanding of mesoscale slip mechanics is crucial to future development of polycrystalline metals with improved performance. The research contained within this thesis aims to characterize localized mesoscale slip on slip bands further through two studies. First, a comprehensive comparison of slip system identification techniques was carried out to further validate each method as well as compare advantages and disadvantages of each. Second, slip bands in the presence of grain boundaries were studied to better characterize the dislocation content and behavior. In the first study, the use of SEM-DIC, AFM, ECCI, and HR-EBSD to characterize slip-system activity was assessed on the same material volume of Ti-7Al. This study presents a robust comparison of the various methods for the first time, including an assessment of their advantages and disadvantages, and how they can be used effectively in a complementary manner. The analysis of the different approaches was carried out in a blind manner independently at three different universities. A Ti-7Al specimen was deformed in uniaxial tension to approximately 3% axial strain, and the active slip systems were independently identified using (i) trace analysis; (ii) in-SEM digital image correlation, (iii) observations of residual dislocations from ECCI, and (iv) long-range rotation gradients through HR-EBSD, with consistent trace identification in all cases. Displacement data from AFM was used to augment the SEM-DIC displacement data by providing complementary out of plane displacement information. Furthermore, short-range dislocation gradients (measured by DIC) provided insight into the residual geometrically necessary dislocation (GND) content, and was consistent with the GND content extracted from EBSD data and ECCI images, confirming the presence of residual GNDs on the dominant slip systems resulting in visible slip bands. These approaches can be used in tandem to provide multi-modal information on slip band identification, strain and orientation gradients, out-of-plane displacements, and the presence of GNDs and SSDs, all of which can be used to inform and validate the development of dislocation-based crystal plasticity and strain gradient models. In the second study, shear strain profiles along slip bands in a modified Rolls-Royce nickel superalloy (RR1000) were analyzed for a tensile sample deformed by 2%. The strain increased with distance away from a grain boundary (GB), with maximum shear strain towards the center of the grain, indicating that dislocation nucleation generally occurred in the grain interior. The strain gradients in the neighborhood of the GBs were quantified and generally correlated with rotation about the active slip system line direction. This leads to an ability to determine the active slip system in these regions. The dislocation spacing and pileup stresses were inferred. The dislocation spacing closely follows an Eshelby analytical solution for a single ended pileup of dislocations under an applied stress. The distribution of pileup stress values for GBs of a given misorientation angle follows a log-normal distribution, with no correlation between the pileup stress and the GB misorientation angle. Furthermore, there is no observed correlation between various transmissivity factors and slip band pileup stress. Hence it appears that the obstacle strength of any of the observed GBs is adequate to facilitate the dislocation pileups present in the slip bands. However, slip band transmission does correlate with transmissivity factors, with the current study focusing on the Luster and Morris m'-factor. Observation of strain profiles of transmitted bands indicate dislocation nucleation locations.

Cross-correlation based analysis of electron backscatter diffraction (EBSD) patterns and the use of simulated reference patterns has opened up entirely new avenues of insight into local lattice properties within EBSD scans. The benefits of accessing new levels of orientation resolution and multiple types of previously inaccessible data measures are accompanied with new challenges in characterizing microscope geometry and other error previously ignored in EBSD systems. The foremost of these challenges, when using simulated patterns in high resolution EBSD (HR-EBSD), is the determination of pattern center (the location on the sample from which the EBSD pattern originated) with sufficient accuracy to avoid the introduction of phantom lattice rotations and elastic strain into these highly sensitive measures. This dissertation demonstrates how to greatly improve pattern center determination. It also presents a method for the extraction of grain boundary plane information from single two-dimensional surface scans. These are accomplished through the use of previously un-accessed detail within EBSD images, coupled with physical models of the backscattering phenomena. A software algorithm is detailed and applied for the determination of pattern center with an accuracy of ~0.03% of the phosphor screen width, or ~10µm. This resolution makes it possible to apply a simulated pattern method (developed at BYU) in HR-EBSD, with several important benefits over the original HR-EBSD approach developed by Angus Wilkinson. Experimental work is done on epitaxially-grown silicon and germanium in order to gauge the precision of HR-EBSD with simulated reference patterns using the new pattern center calibration approach. It is found that strain resolution with a calibrated pattern center and simulated reference patterns can be as low as 7x10-4. Finally, Monte Carlo-based models of the electron interaction volume are used in conjunction with pattern-mixing-strength curves of line scans crossing grain boundaries in order to recover 3D grain boundary plane information. Validation of the approach is done using 3D serial scan data and coherent twin boundaries in tantalum and copper. The proposed method for recovery of grain boundary plane orientation exhibits an average error of 3 degrees.

Formable Advanced High-Strength Steels (AHSS) have a unique combination of strength and ductility, making them ideal in the effort to lightweight vehicles. The AHSS in this study, Quenched and Partitioned 1180, rely on the Transformation Induced Plasticity (TRIP) effect, in which retained austenite (RA) grains transform to martensite during plastic deformation, providing extra ductility via the transformation event. Understanding the factors involved in RA transformation, such as local strain and grain attributes, is therefore key to optimizing the microstructure of these steels. This research seeks to increase understanding of those attributes and the correlations between microstructure and RA transformation in TRIP steels. To measure local strain, the viability of using forescatter detector (FSD) images as the basis for DIC study is investigated. Standard FSD techniques, along with an integrated EBSD / FSD approach (Pattern Region of Interest Analysis System), are both analyzed. Simultaneous strain and microstructure maps are obtained for tensile deformation up to around 6% strain. The method does not give sub-grain resolution, and surface feature evolution prevents DIC analysis across large strain steps; however, the data is easy to obtain and provides a natural set of complementary information for the EBSD analysis. In-situ tensile tests combined with EBSD allow RA grain and neighboring attributes to be characterized and corresponding transformation data to be obtained. However, pseudo-symmetry of the ferrite (BCC) and martensite (BCT) phases prevents EBSD from accurately identifying all phases. Measuring the relative distortion of the crystal lattice, tetragonality, is one approach to identifying the phases. Unfortunately, small errors in the pattern center can cause significant errors in tetragonality measurement. Therefore, this research utilizes a new approach for accurate pattern center determination using a strain minimization routine and applies it to tetragonality maps for phase identification. Tetragonality maps based on dynamically simulated patterns result in the most accurate maps and can also be used to predict approximate local carbon content. Machine learning is then used on the collected data to isolate key attributes of RA grains and provide a decision tree model to predict transformation based on those attributes. Among the most relevant attributes found, RA grain area, RA grain shape aspect ratio, a “hardness” factor, and major axis orientation are included. Possible correlations between these factors and transformation improve understanding of relevant attributes and show the advantage that machine learning can have in unravelling complex material behavior.

L’amélioration des performances spécifiques des alliages métalliques de l’aéronautique est une démarche constante. Les alliages de titane sont des matériaux privilégiés par les constructeurs aéronautiques car ils allient hautes propriétés mécaniques et faible densité.Parmi ces matériaux, les alliages β-métastables qui ont pour particularité de retenir jusqu’à 40% de phase β connaissent un fort regain d’intérêt pour les motoristes (Ti-17) comme pour des applications de structure type trains d’atterrissage (Ti-5553 et Ti-10-2-3). Ce travail a pour but d’analyser le comportement mécanique et la durabilité de ces alliages soumis à des sollicitations monotones et cycliques en lien avec les microstructures.Des essais mécaniques ont pour cela été développés à partir de différentes microstructures métallurgiques qu’elles soient issues d’un traitement industriel ou spécifique visant à simplifier ces dernières. Les mécanismes de déformation (systèmes de glissement) et d’endommagement (amorçage de fissures) ont été identifiés et analysés à différentes échelles par microcopie optique et électronique à balayage en intégrant les notions d’orientation cristallographique (EBSD). Le recours à des essais réalisés in situ sous microscope (optique et MEB) et à une métrologie adaptée aux échelles pertinentes a permis d’identifier les éléments micro-structuraux clés et les cinétiques de développement de ces processus. Un des faits marquants est le rôle majeur de l’anisotropie de propriétés mécaniques de la phase β qui a également fait l’objet de simulations numériques
The improvement of specific performances of metallic materials used for aerospaceapplications needs continuous researches and developments. Titanium alloys are materials ofchoice for aerospace companies thanks to their high mechanical properties and low density.Among them, the β-metastable alloys that retain up to 40% of β phase are more and moreintroduced in aircraft engines (Ti-17) and for structural parts (e.g. landing gears in Ti-5553and Ti-10-2-3).This work aims to analyse the mechanical behaviour and durability of these alloyssubmitted to monotonic or cyclic loadings. Mechanical tests have been developed on differentindustrial microstructures as on academic simplified ones produced by specific thermaltreatments. Deformation mechanisms (slip systems) and damage processes (cracks initiation)were identified and analyzed at different scales using microscopes (optical and SEM) andcrystallographic features were studied by EBSD. Specific in situ tests performed undermicroscopes (optical and SEM) and digital images correlation techniques at scales of interesthave permitted to identify and to quantify the key microstructural parameters and the kineticsof these processes. One major result concerns the influence of the anisotropy of mechanicalproperties associated to the β phase

Les superalliages base Nickel employés dans les disques de turbine sont soumis à des sollicitations sévères allant du fluage à la fatigue à des températures pouvant atteindre 700°C. Dans ces conditions, la littérature montre que les joints de grains sont l’élément structural le plus faible. En fonction de ces conditions, les joints de grains peuvent glisser, transmettre ou accumuler de la déformation et s’oxyder. Ainsi,l’endommagement intergranulaire est étroitement lié à ces processus de déformation.L’objectif de ces travaux est de contribuer à la compréhension des mécanismes de déformation et d’endommagement qui ont lieu aux joints de grains. Pour étudier de manière quantitative les premiers stades de plasticité et d’endommagements, une nouvelle approche de mesure sans contact prenant en comptes les discontinuités de déplacement est présentée : la Heaviside-DIC. Celle-ci a fait l’objet d’une validation avant d’être utilisée dans l’étude du glissement inter et intragranulaire à haute température. Une première étude qualitative a permis de mettre en évidence des endommagements intergranulaires plus importants en fluage à 700°C/700 MPa comparé à la traction à différentes températures. La mise en œuvre de la H-DIC à 2échelles couplée à l’EBSD, a permis d’identifier une configuration microstructurale néfaste dans les superalliages polycristallins pour disques de turbine : les joints de macles cohérents. La comparaison à un superalliage colonnaire pour aubes ne possédant aucun joint de macle confirme ces observations.A la lumière de ces résultats et en s’appuyant sur des travaux de la littérature, un scénario permettant d’expliquer la transition entre déformation et endommagement intergranulaire est proposé
Nickel-based superalloys used in turbine disks are subjected to severe stresses ranging from creep to fatigue at temperatures up to 700°C. Under these conditions, the literature shows that grain boundaries are the weakest structural element. Depending on these conditions, the grain boundaries can slide, transmit or accumulate deformation and are favorable locations for oxidation. Thus, intergranular damage is closely related to these deformation processes.The purpose of this work is to contribute to the understanding of the deformation and damage mechanisms that occur at grain boundaries. To study quantitatively the early stages of plasticity and damage, a new non-contact measurement approach taking into account displacement discontinuities is presented: Heaviside-DIC. It was validated before being used in the study of high temperature inter and intragranular sliding. Afirst qualitative study has shown greater intergranular damage in creep at 700°C / 700 MPa compared to tension at different temperatures. The implementation of H-DIC at 2 scales coupled with EBSD, allowed to identify a detrimental microstructural configuration in turbine disks polycrystalline superalloys: coherent twin boundaries. Comparison with a turbine blade columnar superalloy with no twins confirms these observations.In this light, and based on literature, a scenario explaining the transition between deformation and intergranular damage is proposed

L’objectif principal de ce travail est d’optimise la tomographie par coupe sériée dans un microscope ‘FIB’, en utilisant soit l’imagerie électronique du microscope à balayage (tomographie FIB-MEB), soit la diffraction des électrons rétrodiffusés (tomographie dite EBSD 3D). Dans les 2 cas, des couches successives de l’objet d’étude sont abrasées à l’aide du faisceau ionique, et les images MEB ou EBSD ainsi acquises séquentiellement sont utilisées pour reconstruire le volume du matériau. A cause de différentes sources de perturbation incontrôlées, des dérives sont généralement présentes durant l'acquisition en tomographie FIB-MEB. Nous avons ainsi développé une procédure in situ de correction des dérives afin de garder automatiquement la zone d'intérêt (ROI) dans le champ de vue. Afin de reconstruction le volume exploré, un alignement post-mortem aussi précis que possible est requis. Les méthodes actuelles utilisant la corrélation-croisée, pour robuste que soit cette technique numérique, présente de sévères limitations car il est difficile, sinon parfois impossible de se fier à une référence absolue. Ceci a été démontré par des expériences spécifiques ; nous proposons ainsi 2 méthodes alternatives qui permettent un bon alignement. Concernant la tomographie EBSD 3D, les difficultés techniques liées au pilotage de la sonde ionique pour l'abrasion précise et au repositionnement géométrique correct de l’échantillon entre les positions d'abrasion et d’EBSD conduisent à une limitation importante de la résolution spatiale avec les systèmes commerciaux (environ 50 nm)3. L’EBSD 3D souffre par ailleurs de limites théoriques (grand volume d'interaction électrons-solide et effets d'abrasion. Une nouvelle approche, qui couple l'imagerie MEB de bonne résolution en basse tension, et la cartographie d'orientation cristalline en EBSD avec des tensions élevées de MEB est proposée. Elle a nécessité le développement de scripts informatiques permettant de piloter à la fois les opérations d’abrasion par FIB et l’acquisition des images MEB et des cartes EBSD. L’intérêt et la faisabilité de notre approche est démontrée sur un cas concret (superalliage de nickel). En dernier lieu, s’agissant de cartographie d’orientation cristalline, une méthode alternative à l’EBSD a été testée, qui repose sur l’influence des effets de canalisation (ions ou électrons) sur les contrastes en imagerie d’électrons secondaires. Cette méthode corrèle à des simulations la variation d’intensité de chaque grain dans une série d’images expérimentales obtenues en inclinant et/ou tournant l’échantillon sous le faisceau primaire. Là encore, la méthode est testée sur un cas réel (polycritsal de TiN) et montre, par comparaison avec une cartographie EBSD, une désorientation maximale d'environ 4° pour les angles d’Euler. Les perspectives d’application de cette approche, potentiellement beaucoup plus rapide que l’EBSD, sont évoquées
The aim of current work is to optimize the serial-sectioning based tomography in a dual-beam focused ion beam (FIB) microscope, either by imaging in scanning electron microscopy (so-called FIB-SEM tomography), or by electron backscatter diffraction (so-called 3D-EBSD tomography). In both two cases, successive layers of studying object are eroded with the help of ion beam, and sequentially acquired SEM or EBSD images are utilized to reconstruct material volume. Because of different uncontrolled disruptions, drifts are generally presented during the acquisition of FIB-SEM tomography. We have developed thus a live drift correction procedure to keep automatically the region of interest (ROI) in the field of view. For the reconstruction of investigated volume, a highly precise post-mortem alignment is desired. Current methods using the cross-correlation, expected to be robust as this digital technique, show severe limitations as it is difficult, even impossible sometimes to trust an absolute reference. This has been demonstrated by specially-prepared experiments; we suggest therefore two alternative methods, which allow good-quality alignment and lie respectively on obtaining the surface topography by a stereoscopic approach, independent of the acquisition of FIB-SEM tomography, and realisation of a crossed ‘hole’ thanks to the ion beam. As for 3D-EBSD tomography, technical problems, linked to the driving the ion beam for accurate machining and correct geometrical repositioning of the sample between milling and EBSD position, lead to an important limitation of spatial resolution in commercial softwares (~ 50 nm)3. Moreover, 3D EBSD suffers from theoretical limits (large electron-solid interaction volume for EBSD and FIB milling effects), and seems so fastidious because of very long time to implement. A new approach, coupling SEM imaging of good resolution (a few nanometres for X and Y directions) at low SEM voltage and crystal orientation mapping with EBSD at high SEM voltage, is proposed. This method requested the development of computer scripts, which allow to drive the milling of FIB, the acquisition of SEM images and EBSD maps. The interest and feasibility of our approaches are demonstrated by a concrete case (nickel super-alloy). Finally, as regards crystal orientation mapping, an alternative way to EBSD has been tested; which works on the influence of channelling effects (ions or electrons) on the imaging contrast of secondary electrons. This new method correlates the simulations with the intensity variation of each grain within an experimental image series obtained by tilting and/or rotating the sample under the primary beam. This routine is applied again on a real case (polycrystal TiN), and shows a max misorientation of about 4° for Euler angles, compared to an EBSD map. The application perspectives of this approach, potentially faster than EBSD, are also evoked

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.

The design techniques of the components on the macro level are established in the scientific community, however are far behind from the real material performance limits. To obtain those limits, the deeper understanding of the material structure is required. The methods of a new comonents production through standard alloying are the basis of the modern material science manufacturing. The design of the materials with expected required performance limits is the next conceptual step for the materials scientist. As results, to make this step, the problem of a precise material structure analyses on the microstructural level is one os the major importance for the next generation materials design. The complexity of the material structure across the scales(macro-micro) requires a new non deterministic methods for better understanding of the connectivity betwen a materials performance and material microstructure features. This work presents a various new research methodologies and techniques of the material microstructure characterization and numerical design with future applications to the anlyses of the material behavior. The focus of the particular research was to analyse a new cross correlation function of the material structure on the micro length scale and develop a novel framework which allows a better understanding of various important material phenomenas such as failure initiation and recrystallization.