Discrimination of dynamically and post-dynamically recrystallized grains based on EBSD data: application to Inconel 718.

The importance of angular resolution in EBSD analyses is discussed based on an Inconel 718 sample containing several populations of recrystallized grains, with subtle differences in dislocation contents. Classical EBSD analyses (with angular resolution in the range of 0.5-1°) do not allow for distinguishing recrystallized grains grown dynamically or post-dynamically. The angular resolution of EBSD orientation and misorientation data can be significantly improved (down to about 0.1-0.2°) either using more sophisticated Kikuchi pattern indexing methods and/or using the recently proposed LLASS denoising filter (Local Linear Automatic Smoothing Splines). Then the coexistence of both dynamically and post-dynamically recrystallized grains in the sample can be confirmed and quantified. ECCI images unambiguously confirm the conclusions drawn from the analysis of improved angular resolution EBSD data, and furthermore reveal the presence of thermal stress induced dislocations with typical patterns in water quenched Inconel 718 recrystallized grains. LAY DESCRIPTION: EBSD is widely used to study recrystallization phenomena. Conventional EBSD is nevertheless not able to distinguish dynamic recrystallized grains from post-dynamic recrystallized grains which differ by subtitle differences in dislocation contents. In this paper, we show that improving the orientation precision of EBSD data by means of different methods allows distinguishing these two recrystallized grains populations. Analyses and discussion are based on an Inconel 718, a famous Nickel-based superalloy in aeronautic.

Modeling dynamical electron scattering with Bethe potentials and the scattering matrix.

Bethe potentials were introduced by Bethe in 1928 as a first order perturbation approach to reducing the number of diffracted beams in dynamical electron scattering problems. The approach starts from the Bloch wave representation, and uses a threshold criterion to split the diffracted beams into two subsets, namely strong and weak beams. Since the use of Bloch wave based Bethe potentials for defect simulations is somewhat tedious, this paper applies the perturbation approach to the scattering matrix formalism, which is more readily adaptable for defect image simulations. The size of the dynamical matrix, and hence the computation time, can be reduced significantly. A threshold criterion for the separation of scattered beams into strong and weak sets is introduced. A general guideline in setting the threshold for strong or weak beam selection is discussed along with several parameters that may influence the threshold values, such as atomic number, accelerating voltage, structure complexity, incident beam tilt and temperature.

A phantom-based forward projection approach in support of model-based iterative reconstructions for HAADF-STEM tomography.

We introduce a forward model for the computation of high angle annular dark field (HAADF) images of nano-crystalline spherical particles and apply it to image simulations for assemblies of nano-spheres of Al, Cu, and Au with a range of sizes, as well as an artificial bi-sphere, consisting of solid hemispheres of Al and Cu or Al and Au. Comparison of computed intensity profiles with experimental observations on Al spheres at different microscope accelerating voltages provides confidence in the forward model. Simulated tomographic tilt series for both HAADF and bright field (BF) images are then used to illustrate that the model-based iterative reconstruction (MBIR) approach is capable of reconstructing sphere configurations of mixed atomic number, with the correct relative reconstructed intensity ratio proportional to the square of the atomic number ratio.

The Three-Dimensional Morphology of Growing Dendrites.

The processes controlling the morphology of dendrites have been of great interest to a wide range of communities, since they are examples of an out-of-equilibrium pattern forming system, there is a clear connection with battery failure processes, and their morphology sets the properties of many metallic alloys. We determine the three-dimensional morphology of free growing metallic dendrites using a novel X-ray tomographic technique that improves the temporal resolution by more than an order of magnitude compared to conventional techniques. These measurements show that the growth morphology of metallic dendrites is surprisingly different from that seen in model systems, the morphology is not self-similar with distance back from the tip, and that this morphology can have an unexpectedly strong influence on solute segregation in castings. These experiments also provide benchmark data that can be used to validate simulations of free dendritic growth.

Theory of dynamical electron channeling contrast images of near-surface crystal defects.

This paper describes the dynamical simulation of electron channeling contrast images (ECCIs) of dislocations. The approach utilizes both the Bloch wave formalism and the scattering matrix formalism to generate electron channeling patterns (ECPs). The latter formalism is then adapted to include the effect of lattice defects on the back-scattered electron yield, resulting in a computational algorithm for the simulation of ECCIs. Dislocations of known line direction and Burgers vector are imaged experimentally by ECCI and match well with simulated ECCIs for various channeling conditions. Experiment/simulation comparisons for ECPs and ECCIs are demonstrated for metals (Al), semiconductors (Si), and ceramics (SrTiO3).

Separation of electrostatic and magnetic phase shifts using a modified transport-of-intensity equation.

We introduce a new approach for the separation of the electrostatic and magnetic components of the electron wave phase shift, based on the transport-of-intensity equation (TIE) formalism. We derive two separate TIE-like equations, one for each of the phase shift components. We use experimental results on FeCoB and Permalloy patterned islands to illustrate how the magnetic and electrostatic longitudinal derivatives can be computed. The main advantage of this new approach is the fact that the differences in the power spectra of the two phase components (electrostatic phase shifts often have significant power in the higher frequencies) can be accommodated by the selection of two different Tikhonov regularization parameters for the two phase reconstructions. The extra computational demands of the method are more than compensated by the improved phase reconstruction results.

On the computation of the magnetic phase shift for magnetic nano-particles of arbitrary shape using a spherical projection model.

The magnetic phase shift of an electron wave traveling through a magnetized object is computed by considering the object to be made up of a collection of uniformly magnetized spheres arranged on the nodes of a cubic grid. In the limit of vanishing grid size, this approach becomes equivalent to other numerical approaches. Update equations are derived for the change of the magnetic phase shift when the magnetization of a single object voxel is modified. Example phase shift calculations are presented for a uniformly magnetized sphere, circular disks with an infinitely sharp vortex core and a smooth core, and an oval disk with a pair of vortices and an antivortex.

Diffraction contrast STEM of dislocations: imaging and simulations.

The application of scanning transmission electron microscopy (STEM) to crystalline defect analysis has been extended to dislocations. The present contribution highlights the use of STEM on two oppositely signed sets of near-screw dislocations in hcp α-Ti with 6wt% Al in solid solution. In addition to common systematic row diffraction conditions, other configurations such as zone axis and 3g imaging are explored, and appear to be very useful not only for defect analysis, but for general defect observation. It is demonstrated that conventional TEM rules for diffraction contrast such as g·b and g·R are applicable in STEM. Experimental and computational micrographs of dislocations imaged in the aforementioned modes are presented.

Microstructural tomography of a Ni(70)Cr(20)Al(10) superalloy using focused ion beam microscopy.

A focused ion beam (FIB) microscope has been used to simultaneously depth profile and image the gamma-gamma' microstructure of a nickel base superalloy using normal incidence milling in order to characterize the precipitate microstructure in three dimensions (3D). The normal incidence milling rates of the gamma and gamma' phases in this alloy are closely matched when the orientation of the depth-profiled surface is near 001, which allows for uniform material removal to depths up to a couple of microns. Depth-profiling experiments consisted of automated ion milling and collection of ion-generated secondary-electron images at specified intervals, and was demonstrated for a voxel resolution of roughly 15 x 15 x 16 nm(3). Image-processing software was used for automated processing of the 2D image sequence to render the gamma precipitate structure in 3D.

Determination of magnetic vortex polarity from a single Lorentz Fresnel image.

Nanoscale confinement of the magnetization in a magnetic element often results in the creation of a vortex structure. The vortex equilibrium state is characterized by the curling of the in-plane magnetization (chirality) and an out-of-plane core magnetization. The polarity of the vortex core can point up or down, independent of the chirality, and, thus, magnetic elements with a vortex core are interesting as four-state logic elements. We present an easy-to-use, quantitative method for the determination of both chirality and polarity from a single Fresnel image. This method offers direct evidence of the three-dimensional structure of a magnetic vortex and has significant advantages over the more complex methods currently in use.

Vector field electron tomography of magnetic materials: theoretical development.

The theory of vector field electron tomography, the reconstruction of the three-dimensional magnetic induction around a magnetized object, is derived within the framework of Lorentz transmission electron microscopy. The tomographic reconstruction method uses as input two orthogonal tilt series of magnetic phase maps and is based on the vector slice theorem. An analytical reconstruction of the magnetic induction of a single magnetic dipole is presented as a proof-of-concept. The method is compared to two previously reported approaches: a reconstruction starting from the gradient of the magnetic phase maps, and a direct reconstruction of the magnetic vector potential. Numerical examples as well as estimates of the reconstruction errors for a range of magnetic particle shapes are reported.

Origin of magnetic and magnetoelastic tweedlike precursor modulations in ferroic materials.

Based on experimental observations of modulated magnetic patterns in a Co0.5Ni0.205Ga0.295 alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.

[Acquired uterine arteriovenous fistula: report of one case]. / Fistule artério-veineuse utérine acquise: à propos d'une observation.

A case of acquired uterine arteriovenous fistula in a 28 Year old patient is reported that probably is secondary to a "difficult" delivery. Sono-graphic, MRI and angiographic findings are reported, before and after endovascular management.

High signals in the uterine cervix on T2-weighted MRI sequences.

The aim of this pictorial review was to illustrate the normal cervix appearance on T2-weighted images, and give a review of common or less common disorders of the uterine cervix that appear as high signal intensity lesions on T2-weighted sequences. Numerous aetiologies dominated by cervical cancer are reviewed and discussed. This gamut is obviously incomplete; however, radiologists who perform MR women's imaging should perform T2-weighted sequences in the sagittal plane regardless of the indication for pelvic MR. Those sequences will diagnose some previously unknown cervical cancers as well as many other unknown cervical or uterine lesions.

A new symmetrized solution for phase retrieval using the transport of intensity equation.

We propose a novel symmetrization method for solving the transport of intensity equation (TIE) using fast Fourier transforms for situations where the input images may or may not exhibit spatial periodicity. The method is derived from the analysis of intensity conservation law and the internal symmetry of the TIE, and is illustrated for both a computational and an experimental data set.

Interventional radiology in female infertility: technique and role.

The purpose of this paper is to describe the modifications in the radiologic approach to female infertility. The role of hysterosalpingography (HSG) has evolved from being the only source of information about the uterus to a more minor role, after ultrasound, that essentially deals with the morphology of the fallopian tubes. But if its diagnostic yield in the uterus is challenged by ultrasound and hysterosonography, it retains a major impact in the work-up of female infertility. Hysterosalpingography brings decisive diagnostic information concerning the state of the tubes and peritoneum. The interventional procedures of selective salpingography and tubal recanalization have a definite therapeutic effect and allow numerous pregnancies that would otherwise have required in vitro fertilization or tubal microsurgery.

[Imaging of gynecologic malformations]. / Imagerie des malformations gynécologiques.

Female genital tract anomalies are common (1 to 2% of the female population), and may lead to multiple clinical manifestations: amenorrhea, infertility, spontaneous repeated miscarriage, pelvic pain, endometriosis. They are caused by intra-uterine insults between weeks 6 and 18 of gestation. They are classified according to their embryologic origin. Imaging relies essentially on ultrasound and MRI, and indications for hysterosalpingography are less common. Imaging must classify the malformation and detect complications in order to assess the fertility prognosis and treat complications.

[Imaging of the cervix and the vagina]. / Imagerie du col et du vagin.

US and MRI currently are the best imaging modalities to evaluate pathology of the uterine cervix and vagina. Carcinoma of the cervix is the most frequent indication for imaging. MRI allows preoperative staging of cervical carcinoma based on FIGO classification, and post treatment follow-up. Other uterine cervix diseases are less frequently imaged and include a wide range of entities that most frequently cause increased T2W signal at MR imaging. Pathology of the pelvic floor, vagina, vulva, and perineum also includes a wide range of entities that have seldom been described in the imaging literature.