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Large Dataset Electron Diffraction Patterns for the Structural Analysis of Metallic Nanostructures
Published in Alina Bruma, Scanning Transmission Electron Microscopy, 2020
Arturo Ponce, José Luis Reyes-Rodríguez, Eduardo Ortega, Prakash Parajuli, M. Mozammel Hoque, Azdiar A. Gazder
Commercial instrumentation, used to perform precession electron diffraction (PED) or crystal orientation mapping, can be used to control the different sets of coils of the microscope (Moeck and Rouvimov 2010). In the PED method, the electron diffraction is registered while the electron beam is precessing on a cone surface; in this way, only a few reflections are simultaneously excited at the same time, therefore, dynamical effects are strongly reduced. For this type of setting, the scan and de-scan coils need to be calibrated to maintain the beam movement on range so that electron DPs remain stationary during precession and scanning routines. Because these systems use external cameras collecting data from the inclined focus screen of the microscope, camera length and distortions need to be corrected during data treatment. The collected DP areas are then cross-correlated from database templates of the crystal structures present in the sample (Santiago et al. 2016). The commercial use of software like ASTAR has found a range of applications going from metallurgy to in situ heating of experiments inside the TEM (Rauch and Veron 2005; Kobler et al. 2013; Deng et al. 2015).
Aberration Correction in Electron Microscopy
Published in Orloff Jon, Handbook of Charged Particle Optics, 2017
Ondrej L. Krivanek, Niklas Dellby, Matthew F. Murfitt
The electron source of the microscope is a CFEG, which provides excellent brightness (B > 109 A/(cm2 sr)) and good energy spread (ΔE ~ 0.3 eV). The four round condenser lenses of the column (one condenser is mounted in the electron gun and is not shown in the schematic) allow the source demagnification to be adjusted as needed, allow the beam’s angular convergence to be changed, and allow the height of the crossover that the beam entering the corrector appears to emanate from to be set to the value needed by the corrector. This section of the column also includes an electrostatic beam blanker, which can turn the beam on the sample off (and on) in a few microseconds, which is useful for preventing radiation damage when no data is being acquired. It also includes a beam-defining aperture and a precorrector set of scan coils. Placing the scan coils before the corrector means that the beam is scanned in the entire corrector-OL assembly, which makes sure that the scanning is not affected by uncorrected aberrations of the OL. This is particularly important for beam rocking, as needed for precession electron diffraction (Own et al., 2007).
Many-beam dynamical scattering simulations for scanning and transmission electron microscopy modalities for 2D and 3D quasicrystals
Published in Philosophical Magazine, 2019
Saransh Singh, William C. Lenthe, Marc De Graef
Automated indexing of electron diffraction patterns has received considerable attention in the past three decades, starting with the commercialisation of an indexing package for EBSD patterns in the early 1990s [35], and the introduction of the AStar indexing package for precession electron diffraction patterns in 2005 [36], to name just a few. The EBSD indexing algorithms are based on the extraction of the locations and orientations of the Kikuchi bands in the pattern by means of a Hough transform; the interplanar/interzonal angles are then compared to a look-up table based on the crystallography of the material, and this leads, in principle, to the orientation of the crystal lattice with respect to the sample reference frame. Recently, an alternative indexing approach was introduced [37,38] in which one pre-computes a dictionary of diffraction patterns for a uniform sampling of orientation space and a particular set of detector parameters; the dictionary patterns are then compared to the experimental patterns using an appropriate similarity metric and this leads to the direct indexing of the patterns without any feature extraction.
Structure and orientation of an intermetallic phase in a W-Ni-Co alloy
Published in Philosophical Magazine, 2019
Rajdeep Sarkar, Vajinder Singh, Suraj Kumar, Y. Venkateswara Rao, P. Ghosal, T. K. Nandy
An intermetallic phase, Co2W, forms in a W-Ni-Co alloy during heat-treatment at 800°C.Complete structure solution using PED (precession electron diffraction) and ADT (automated diffraction tomography) in TEM shows that the phase has an orthorhombic crystal structure with Pnam (62) space group.Co2W phase forms in the matrix with equiaxed and lath morphologies. While equiaxed phase has a crystallographic orientation relationship with W grains only, lath shaped precipitate shows OR with both W and matrix phase.The Ni-rich solid solution grains which form in between the Co2W laths in matrix have different orientation than the Ni-rich matrix.
Revealing texture architecture in a surface gradient nanostructured Al-Cu-Mg alloy
Published in Philosophical Magazine Letters, 2018
Zongqiang Feng, Yanxia Chen, Wei Zhang, Guilin Wu
To date, the scanning electron microscope (SEM)-based electron backscatter diffraction (EBSD) technique has been extensively used for orientation mapping of deformation microstructures. However, for spatial resolution limitation, EBSD is not ideal for mapping nanograins smaller than 30 ∼ 50 nm [13–16]. As to the SSPD-induced SGN layers, the heavily refined grains near the topmost surface often contain a high defect density and residual strains, making it a challenge to acquire high quality orientation maps of nanograins. To break through the dilemma in nanoscale orientation mapping, several novel characterisation techniques have been developed, such as SEM-based transmission Kikuchi diffraction [17,18], transmission electron microscope (TEM)-based automatic crystal orientation mapping (ACOM-TEM) [15,16,19] and three-dimensional orientation mapping in the TEM (3D-OMiTEM) [20]. Specifically, with the help of precession electron diffraction (PED), typical dynamic effects in ACOM-TEM can be avoided, remarkably improving orientation sensitivity and indexing rate of diffraction patterns [15,16,19]. The PED-assisted orientation mapping technique has been widely used for phase and orientation mapping of nanocrystalline materials [16,19]. In this work, we concentrated on grain structures and crystallographic textures in the SGN layer of a surface-sliding-friction treated (SSFTed) Al-Cu-Mg alloy. By means of EBSD- and PED- assisted orientation mapping, we have analyzed the through-thickness texture components in detail and here discuss the mechanism for textural development in response to increasing strain, strain rate and temperature in a single sample during SSFT processing. The present study provides new crystallographic insights into textural evolution in surface gradient nanostructured materials.