“Angular resolution expected from iCHORD orientation maps through a revisited ion channeling model”

International audienceCrystalline orientation maps are obtained in a Focused Ion Beam (FIB) microscope using the ion CHanneling ORientation Determination (iCHORD) method, which relies on the channeling phenomenon observed in ion-induced secondary electron images. The current paper focuses on the angular resolution that can be expected from such orientation maps, obtained using a revisited ion channeling model. A specific procedure was developed to evaluate the angular resolution, based on the distribution of orientation errors when evaluating controlled sample disorientation. The main advantage is that no external reference is required. An angular resolution of 1° is obtained on a nickel based sample using standard acquisition conditions. This value fulfills most of the needs in terms of microstructural characterization usually carried out by Electron Back Scattered Diffraction

Electron CHanneling ORientation Determination (eCHORD): An original approach to crystalline orientation mapping

International audienceWe present a proof-of-concept attesting the feasability to obtain orientation maps of polycrystalline materials within a conventional Scanning Electron Microscope (SEM) using a standard goniometer and Back Scattered Electron (BSE) detector. The described method is based on the analysis of the contrast variation of grains due to the channeling of incident electrons on a rotating sample. On each pixel of the map, experimental intensity profiles as a function of the rotation angle are obtained and compared with simulated ones to retrieve the orientation. From first results on aluminum polycrystals, the angular resolution is estimated to be better than one degree

The peculiarity of the metal-ceramic interface

Important properties of materials are strongly influenced or controlled by the presence of solid interfaces, i.e. from the atomic arrangement in a region which is a few atomic spacing wide. Using the quantitative analysis of atom column positions enabled by C(S)-corrected transmission electron microscopy and theoretical calculations, atom behaviors at and adjacent to the interface was carefully explored. A regular variation of Cu interplanar spacing at a representative metal-ceramic interface was experimentally revealed, i.e. Cu-MgO (001). We also found the periodic fluctuations of the Cu and Mg atomic positions triggered by the interfacial geometrical misfit dislocations, which are partially verified by theoretical calculations using empirical potential approach. Direct measurements of the bond length of Cu-O at the coherent regions of the interface showed close correspondence with theoretical results. By successively imaging of geometrical misfit dislocations at different crystallographic directions, the strain fields around the interfacial geometrical misfit dislocation are quantitatively demonstrated at a nearly three-dimensional view. A quantitative evaluation between the measured and calculated strain fields using simplified model around the geometrical misfit dislocation is shown

About the automatic measurement of the dislocation density obtained by R-ECCI

International audienceA proof of concept of a new method for automatic characterization of the dislocation density from scanning electron microscopy images is presented. A series of backscattered electron images are acquired while the sample is rotated. For each pixel of the region of interest, the variation of the grey-level intensity as a function of the rotation angle, called the intensity profile, is calculated. This profile can be used to determine the nature of each pixel (dislocation, matrix or noise), such that an automatic dislocation density can be determined within the region of interest. The method is well adapted for dislocation densities ranging from 1012 to 1014 m−2. The simulation of a volume containing dislocations enabled the determination of the maximum and minimum densities attainable as well as the theoretical and experimental measurement errors related to the projection of this volume on a two-dimensional image. The theoretical measurement error due to the projection of dislocation on a surface, is 3% for low dislocation densities (1012m−2) and 20% for higher dislocation densities (1014 m−2). Experimentally, measurement errors are limited by image analysis conditions, which leads to total measurement errors of 15% for 1012m−2 and 34% error for 1014 m−2. These uncertainties were obtained considering a given analyzed depth value, that could not be experimentally verified. This uncertainty on the depth value leads to large errors bars in the final measurement, which can reach an order of magnitude

The Influence of Vanadium on Ferrite and Bainite Formation in a Medium Carbon Steel

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Rotational-Electron Channeling Contrast Imaging analysis of dislocation structure in fatigued copper single crystal

International audienceThe dislocation structure of copper single crystal during cyclic fatigue has been characterized by the Rotational-Electron Channeling Contrast Imaging (R-ECCI) method. This technique is based on the acquisition of series of BackScattered Electron (BSE) images during the sample rotation. It facilitates the determination of orientation conditions in the Scanning Electron Microscope (SEM) for fast and accurate dislocation characterization, regardless of the initial orientation of the sample. The technique was applied to copper, observed in its as-received state as well as after several cyclic fatigue loadings. The evolution of the dislocation structure is described as a function of the applied stress

Can micro-compression testing provide stress–strain data for thin films? A comparative study using Cu, VN, TiN and W coatings

Micro-compression testing using an instrumented micro- or nanoindenter equipped with a flat punch is a promising approach to measure the stress–strain response of miniaturized materials and to complement hardness and modulus measurements gained by nanoindendation. Focussed ion beam milling is employed to fabricate micron-sized compression pillars from 1 μm thick single crystal Cu(001), TiN(001), and VN(001) films grown on MgO(001), and from a 6.7 μm thick polycrystalline W coating deposited on Si(001). In situ micro-compression tests in a scanning electron microscope reveal reproducible stress–strain curves for W, a considerable statistical scatter in the flow stress for Cu and VN, and failure of TiN pillars by cleavage. A linear work-hardening rate of 2.7±1.2 GPa is determined for the polycrystalline W coating. The results are critically discussed taking into account material defects and the stiffness of the film-substrate system

Toward an automated tool for dislocation density characterization in a scanning electron microscope

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Advanced nanomechanics in the TEM: effects of thermal annealing on FIB prepared Cu samples

The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surfacemediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength