Advanced characterization techniques for high-angular and high-spatial resolutions in the scanning electron microscope

High-angular resolution electron diffraction-based techniques aim at measuring relative lattice rotations and elastic strains with an accuracy about 1.10-4 (<0.01°) in the scanning electron microscope (SEM). These metrics are essential for the fine characterization of deformation structures in terms of grain internal disorientations and geometrically necessary dislocation densities. To this purpose, relative deformations between electron diffraction patterns are retrieved with subpixel accuracy using digital image correlation (DIC) techniques. Here, a novel DIC approach is proposed. It relies on a linear homography [1], i.e., a geometric transformation often met in photogrammetry to model projections. The method is implemented in ATEX-software [2], developed at the University of Lorraine. Its performances are illustrated from both a semi-conductor and a metal. First, lattice rotation and elastic strain fields are investigated in the vicinity of a giant screw dislocation in GaN single crystal using the electron backscattered diffraction technique (Fig. 1). Second, the proposed method is coupled with the on-axis Transmission Kikuchi Diffraction (TKD) configuration to characterize a nanocrystalline aluminium obtained by severe plastic deformation. On-axis TKD consists in observing a thin foil in transmission in the SEM, using a scintillator is placed beneath the specimen, perpendicularly to the electron beam. Thanks to this coupling, high-spatial (3-6 nm) and high-angular (~0.01°) resolutions are simultaneously achieved in SEM. [3]

Contribution du glissement non octaédrique à l'évolution de texture des polycristaux CFC sous cisaillement simple

La contribution du glissement non octaédrique (NOC) à l'évolution de la texture sous cisaillement simple pour les métaux à structure cubique à faces centrées a été étudiée. Les orientations idéales produites par la famille octaédrique {111} ne changent pas après l’ajout de glissement NOC. Toutefois, la stabilité des orientations idéales, le champ de rotation et la vitesse d'évolution de la fonction de densité des orientations ont été affectés. L’impact du glissement NOC sur la forme des équipotentiels, l'état de contrainte ainsi que la distribution de glissement ont été examinés

Crystallography of growth blocks in spheroidal graphite

A better understanding of spheroidal graphite growth is expected in a near future thanks to widespread use of transmission electron microscopy. However, common transmission electron microscopy is quite time consuming and new indexing techniques are being developed, among them is transmission Kikuchi diffraction in a scanning electron microscope, a recent technique derived from electron backscatter diffraction. In the present work, on-axis transmission Kikuchi diffraction in scanning electron microscope, completed by transmission electron microscopy, was used with the objective of producing new observations on the microstructure of spheroidal graphite. This study shows that disorientations between blocks and sectors in spheroidal graphite are quite large in the early growth stage, which may be indicative of a competition process selecting the best orientations for achieving radial growth along the c direction of graphite

Characterization of the near-surface nanocrystalline microstructure of ultrasonically treated Ti-6Al-4V using ASTAR™/precession electron diffraction technique

The surface of Ti-6Al-4V was treated mechanically by applying ultrasonic nanocrystal surface modification. The effect of this treatment on the hardness, compressive residual stresses and fatigue performance were investigated. It is shown that in terms of the measured nanoindentation hardness values and the presence of compressive residual stresses, the treated sample only differed from the as-received sample in the first 200–300 µm area far from the surface. Also, the microstructure very close to the treated surface (\u3c5 µm) was characterized using a relatively new transmission orientation microscopy technique named ASTAR™/precession electron diffraction. Based on different types of results (e.g., index map and virtual bright field image) acquired by this technique, it is concluded that titanium grains smaller than 10 nm exist within the distance of less than 1 µm from the treated surface. Difficulties associated with ASTAR™/precession electron diffraction technique to characterize this challenging near-surface area are discussed

Mechanical properties, microstructure and crystallographic texture of magnesium AZ91-D alloy welded by Friction Stir Welding (FSW)

The objective of the study was to characterize the properties of a magnesium alloy welded by friction stir welding (FSW). The results led to a better understanding of the relationship between this process and the microstructure and anisotropic properties of alloy materials. Welding principally leads to a large reduction in grain size in welded zones due to the phenomenon of dynamic recrystallization. The most remarkable observation was that crystallographic textures appeared from a base metal without texture in two zones: the thermo-mechanically affected and stir welded zones. The latter zone has the peculiarity of possessing a marked texture with two components on the basal plane and the pyramidal plane. These characteristics disappeared in the TMAZ, which had only one component following the basal plane. These modifications have been explained by the nature of the plastic deformation in these zones, which occurs at a moderate temperature in the TMAZ and high temperature in the SWZ

Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Plasticité cristalline des matériaux hexagonaux sous cisaillement : application au magnésium

The properties of materials with hexagonal crystalline structure are currently of interest for technical applications and for academic research. This work is articulated around the case of magnesium used in particular in aeronautics for its lightness. However, because of the restricted number of symmetries of the hexagonal crystal structure, these materials can present certain ?difficulties? of forming. Forming usually imposes large deformations on the material; this is why it is of primary importance to know its behavior. Large plastic deformations imply the development of a plastic anisotropy which can be particularly strong in hexagonal polycrystals. This work initially shows the bases of comprehension and a review of literature on the plasticity of hexagonal materials. Then the role of strain?rate sensitivity in the crystal plasticity of materials with hexagonal structures is discussed. The ideal orientations of texture and their characteristics of persistence of hexagonal closed?packed crystals in simple shearing are identified. Then an analysis of texture evolution in magnesium during equal angular extrusion is carried out. The texture and mechanical behavior of magnesium during free end torsion are also analyzed. Finally, a modeling of the deformation during equal channel angular extrusion by a general flow function is proposedLes propriétés mécaniques des matériaux à structure cristalline hexagonale présentent actuellement un intérêt pour des applications techniques ainsi que pour la recherche académique. Ce travail s?articule autour du cas de magnésium utilisé notamment en aéronautique pour sa légèreté. Cependant, du fait du nombre restreint de symétries de leur structure cristalline, ces matériaux peuvent présenter certaines « difficultés » de mise en forme. La mise en forme impose la plupart de temps de grandes déformations au matériau, c?est pourquoi il est primordial d?y connaître son comportement. De grandes déformations plastiques impliquent le développement d?une anisotropie plastique qui peut être particulièrement forte dans les polycristaux hexagonaux. Ce travail propose en premier lieu les bases de compréhension et une revue de littérature sur la plasticité des matériaux hexagonaux. Le rôle de la sensibilité à la vitesse de déformation sur la plasticité des matériaux à structures hexagonales est ensuite discuté. Les orientations idéales de texture et leurs caractéristiques de persistance dans les cristaux hexagonaux en cisaillement simple sont déterminées. Une analyse de l?évolution de texture dans le magnésium durant une extrusion angulaire à section constante est ensuite effectuée. Puis l?évolution de texture et le comportement mécanique du magnésium en torsion est analysé. Finalement, une modélisation de la déformation en extrusion angulaire à section constante par une ligne de courant générale est proposé