Physics-based Simulation Models for EBSD: Advances and Challenges

EBSD has evolved into an effective tool for microstructure investigations in the scanning electron microscope. The purpose of this contribution is to give an overview of various simulation approaches for EBSD Kikuchi patterns and to discuss some of the underlying physical mechanisms

The external capsule and cognition: A high angular resolution diffusion MRI study of cholinergic fibers

Introduction : Les changements de la matière blanche dans les faisceaux cholinergiques contribuent à la dysfonction exécutive en contexte de vieillissement cognitif. Les objectifs de cette étude étaient de : 1) caractériser les faisceaux de la matière blanche de la capsule externe, plus précisément les faisceaux cholinergiques latéraux (LCT), ainsi que le faisceau longitudinal supérieur (SLF), en utilisant la tractographie HARDI; 2) évaluer l’association entre les mesures de diffusion obtenues dans ces faisceaux et la performance cognitive. Méthodologie : Une analyse HARDI (High angular resolution diffusion imaging) de 34 participants âgés avec des résultats normaux au MMSE (Mini-Mental State Examination) a été faite, suivie de reconstruction par tractographie probabiliste contrainte anatomiquement robuste aux croisements de fibres et à l’effet de volume partiel. La capsule externe a été manuellement segmentée sur une image T1 moyenne avant d’être fusionnée à un atlas, permettant l’extraction des LCT (et SLF) présumés. Des mesures de diffusion et HARDI ont été obtenues. Tous les participants ont effectué une batterie neuropsychologique incluant le Trail-Making-test-B, le Coding and Symbol search, l’empan mnésique visuospatial et le Stroop. Résultats : Les LCT présumés ont été isolés avec succès en utilisant la tractographie probabiliste HARDI. Des corrélations entre les mesures de diffusion dans les LCT et le temps de complétion du Stroop (diffusivités radiales 0,371 p=0,03 et médiales 0,348 p=0,04 du LCT gauche), le score au Symbol Search (densité de fibres apparente du LCT droit -0,362 p=0,04) et la partie motrice du Trail Making Test partie B (diffusivités axiale 0,432 p=0,01, radiale 0,412 p=0,02 et médiale0,450 p=0,01 du LCT gauche) ont été observées. Des corrélations ont aussi été retrouvées entre les mesures de diffusion dans le SLF et les mêmes 3 tests. Ces résultats se sont négativés aux corrections de Bonferroni. Discussion : HARDI, utilisé dans l’isolation de faisceaux clés en fonctions cognitives, et permettant une meilleure analyse du croisement de fibres, représente un outil novateur pour la compréhension du vieillissement cérébral. Malgré que les résultats de l’étude aient été négativés après les corrections faites pour le nombre d’analyses, ceux-ci demeurent intéressants afin d’orienter de futures études de plus grande amplitude.Abstract: Introduction : White matter changes in the cholinergic tracts contribute to executive dysfunction in the context of cognitive aging. The objectives of this study were to: 1) Characterize white matter bundles from the external capsule, comprising presumed lateral cholinergic tracts (LCT), among others, and the superior longitudinal fasciculus (SLF), using HARDI tractography. 2) Evaluate the association between the obtained diffusion measures within these bundles and cognitive performance. Methods : High angular resolution diffusion imaging (HARDI) of 34 elderly participants with normal MMSE (Mini-Mental State Examination) scores was done, followed by anatomically constrained probabilistic tractography reconstruction robust to crossing fibers and partial volume fraction. The external capsule was manually segmented on a mean T1 image then merged with an atlas, allowing extraction of the presumed lateral cholinergic bundles and SLF. Diffusion tensor and HARDI-based measures were obtained. All subjects underwent a detailed neuropsychological battery including the MMSE, MoCA or Montreal Cognitive Assessment, Trail-Making-test-B, Coding and Symbol search, visuospatial memory span from WMS-III and Stroop test. Results : The presumed lateral cholinergic tracts were successfully isolated using probabilistic HARDI tractography. Correlations between diffusion measures in the LCT and the time of completion of Stroop (left LCT radial 0,371 p=0,03 and medial diffusivity 0,348 p=0,04), the Symbol Search score (right LCT apparent fiber density -0,362 p=0,04) and the motor part of TMT-B (left LCT axial 0,432 p=0,01, radial 0,412 p=0,02 and medial diffusivity 0,450 p=0,01) were observed. Correlations were also found between diffusion measures in the SLF and the same three tests. However, these results did not survive Bonferroni corrections. Discussion : HARDI, with isolation of strategic white matter tracts for cognitive functions, represents a new tool to better understand the complex process of brain aging. Although the results of the study were negated by the corrections done for the number of analyses, the correlations found remain an interesting starting point for future studies

Synchrotron Sub-μ X-ray Tomography of Kirkendall Porosity in a Diffusion Couple of Nickel-Base Superalloy and Nickel after Annealing at 1250 \ub0C

Kirkendall porosity that forms during interdiffusion in a diffusion couple of nickel-base superalloy CMSX-10 with pure nickel is investigated. The diffusion experiments are conducted at a temperature of 1250 \ub0C, where the strengthening γ′-phase is partially dissolved. The porosity is studied by X-ray sub-μ tomography with a spatial resolution of about 0.353 μm3 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. It is found that depending on the distance from the interface the Kirkendall pores take different shapes: octahedral, rounded pyramidal, drop shaped, dendritic, pear shaped, and joint shapes. Such a variety of pore morphologies indicates a complex multistage process of porosity nucleation and growth under vacancy supersaturation of different degrees. The experimental findings are interpreted on the basis of the results of diffusion modeling. It is shown that the kinetics of porosity growth is essentially influenced by the dissolution of the γ′-phase

Elucidation of orientation relations between Fe-Cr alloys and corrosion products after high temperature SO2 corrosion

The early stages of corrosion of Fe-Cr-model alloys (2 and 9 % Cr) were investigated after exposure at 650 degrees C in 0.5 % SO2 containing gas by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The impact of the grain orientation of the base alloy on the orientation relations of the corrosion products is presented. After 2 min - 5 min exposure the formation of a multi-layered corrosion zone was discovered. A clear orientation relationship between ferrite and the (Fe,Cr)(3)O-4 spinel could be demonstrated. The obtained results show the importance of the grain orientation on oxidation resistance

EBSD orientation analysis based on experimental Kikuchi reference patterns

Orientation determination does not necessarily require complete knowledge of the local atomic arrangement in a crystalline phase. We present a method for microstructural phase discrimination and orientation analysis of phases for which there is only limited crystallographic information available. In this method, experimental Kikuchi diffraction patterns are utilized to generate a self-consistent master reference for use in the technique of Electron Backscatter Diffraction (EBSD). The experimentally derived master data serves as an application-specific reference in EBSD pattern matching approaches. As application examples, we map the locally varying orientations in samples of icosahedral quasicrystals observed in a Ti40Zr40Ni20 alloy, and we analyse AlNiCo decagonal quasicrystals

Zircon to monazite phase transition in CeVO4

X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to 12 and 16 GPa, respectively. Experiments reveal that at 5.3 GPa the onset of a pressure-induced irreversible phase transition from the zircon to the monazite structure. Beyond this pressure, diffraction peaks and Raman-active modes of the monazite phase are measured. The zircon to monazite transition in CeVO4 is distinctive among the other rare-earth orthovanadates. We also observed softening of external translational Eg and internal B2g bending modes. We attributed it to mechanical instabilities of zircon phase against the pressure-induced distortion. We additionally report lattice-dynamical and total-energy calculations which are in agreement with the experimental results. Finally, the effect of non-hydrostatic stresses on the structural sequence is studied and the equations of state of different phases are reported.Comment: 45 pages, 8 figures, 8 table