DFT and HAADF-STEM Investigations of the Zn Effects on β″ Phase Structure in a Zn Added Al-Mg-Si-Cu Alloy

2021 ◽  
Vol 1026 ◽  
pp. 93-101
Author(s):  
Rui Yu ◽  
Yong An Zhang
Keyword(s):  
First Principles ◽  
Dark Field ◽  
First Principles Calculations ◽  
Β Phase ◽  
Cu Alloy ◽  
Formation Enthalpies ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Peak Aging ◽  
Stem Image

First-principles calculations were conducted to investigate the effects of Zn on the structure of β″ phase. The effects of Cu, which was often added in the alloy, were also taken into consideration. Firstly, single Zn or Cu atom was doped on different sites of the β″ phase. Then the formation enthalpies and lattice constants of doped β″ phases were calculated. The results showed that it was more energetically favorable for single Zn or Cu atom to occupy Si3/Al sites than other sites. Furthermore, different quantities of Zn or Cu atoms were doped on Si3/Al sites. With the amounts of doping atoms increasing, the formation enthalpies of β″ phases doped by Zn were lower than which doped by Cu, indicating that it was more preferential for Zn to enter the β″ phase when Zn content was higher than Cu. Additionally, the doping of Zn could reduce the formation enthalpies of the β″ phase, which promoted the formation of the β″ phases. As a result, the aging hardening response of the alloy was improved. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was also conducted on a peak-aging Zn added Al-Mg-Si-Cu alloy. The HAADF-STEM image of β″ phase showed that the occupancies of Zn atoms were just on the Si3/Al sites and substituted all the Al atoms, which was consistent with the results of first-principles calculations.

Single atom visibility on (111) silicon in simulated ADF STEM images

1988 ◽  
Vol 46 ◽  
pp. 820-821
Author(s):  
R. F. Loane
Keyword(s):  
Silicon Crystal ◽  
Gold Atom ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Single Atom ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Crystal Substrate ◽  
Annular Dark Field ◽  
Stem Image

The multislice method has been adapted to simulate annular dark field (ADF) scanning transmission electron microscope (STEM) images. In the STEM image simulation, a highly focused electron probe is scanned across a specimen and the scattered intensity, accumulated over an annular detector, is recorded as a function of probe position. Each pixel in the STEM image is determined by an entire multislice calculation for a particular position of the incident probe. This N4 process is very computationally expensive and currently requires the use of a supercomputer to achieve runtimes of less than a day.The simulated specimen consisted of a (111) silicon crystal substrate, which was a multiple of 94 Å (30 slices) thick, followed by an additional slice containing a single gold atom. Slice potentials were 38.4 Å x 39.9 Å (256 x 256 pixels) in size, which set the maximum included scattering angle to 79 mrad.

Structure Models of Massively Transformed High Niobium Containing TiAl Alloys

2006 ◽  
Vol 980 ◽  
Author(s):  
Christina Scheu ◽  
Limei Cha ◽  
Saso Sturm ◽  
Harald F. Chladil ◽  
Paul H. Mayrhofer ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Ab Initio ◽  
Dark Field ◽  
Tial Alloys ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Stem Image ◽  
Antisite Defects

AbstractAb-initio calculations using the Vienna ab-initio simulation package (VASP) were performed for a high Nb bearing γ TiAl based alloy with a composition of Ti-46at.%Al-9at.%Nb in order to evaluate the effect of Nb on the crystal structure. The calculations revealed that upon doping with Nb the resulting structure can have Ti and Nb atoms on Al-sites, which leads to a reduction of the c/a ratio of the tetragonal γ TiAl cell to ~1.In contrast, the c/a ratio is increased, compared to the binary phase, if the Nb atoms occupy solely Ti sites and if Ti antisite defects (i.e. Ti on the Al sublattice) are formed. The relaxed structure models were used to perform high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image simulations. The results showed that the positions of the Nb atoms should be detectable by these high spatial resolution methods, although it might be easier by HAADF-STEM investigations due to the stronger dependence of the signal on the atomic number Z.

Stem Observations of the Microstructures Present in Polymer Blend Thin Films.

1986 ◽  
Vol 79 ◽  
Author(s):  
K. E. Sickafus ◽  
S. D. Berger ◽  
A. M. Donald
Keyword(s):  
Polymer Blend ◽  
Scanning Transmission Electron Microscopy ◽  
Dark Field ◽  
Two Phase ◽  
Local Composition ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Stem Image ◽  
Multi Phase

AbstractThis paper examines the effectiveness of scanning transmission electron microscopy (STEM) as an analytical tool for determining composition in multi-phase polymer blend microstructures. The polymer blend polystyrene (PS) - polyether sulphone (PES) thin film used in this study exhibited a two-phase microstructure consisting of PES-rich inclusions, ranging from 0.2μm to 1.2μm in diameter, in a PS-rich matrix. Emphasis in this presentation is placed on the use of annular dark-field (ADF) STEM image contrast to infer information concerning the local composition of adjacent microstructural features.

High-angle annular dark-field stem imaging: more than just z-contrast!

1992 ◽  
Vol 50 (2) ◽  
pp. 1336-1337
Author(s):  
D.D. Perovic
Keyword(s):  
Experimental Studies ◽  
Dark Field ◽  
High Angle ◽  
Contrast Imaging ◽  
Atomic Displacements ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Stem Image ◽  
Incoherent Imaging ◽  
Z Contrast

Following the development of dedicated scanning-transmission electron microscopy (STEM), significant advances have been made in atomic number (Z)-contrast imaging using a high-angle annular detector (HAAD). With the exclusion of coherent (ie. Bragg) scattering, the HAAD allows for truly incoherent imaging with high compositional sensitivity approaching the simple Z2-dependence of unscreened Rutherford scattering. However, recent experimental studies have indicated that HAADF-STEM imaging is not always straightforward. For example, Fig. 1 shows a digitally acquired HAADF-STEM image of a (B,As)-doped Si multilayer. The B-doped (˜ 0.7 at.%B) layers appear significantly brighter than the adjacent Si matrix in contradiction with a simple Z-contrast argument. It was found that an increase in incoherent scattering from the B-doped regions results due to the presence of atomic displacements of the surrounding Si atoms which effectively behave as “frozen-in” static phonons. Accordingly, the B-doped layers quasi-elastically scatter electrons to relatively high angles giving rise to enhanced contrast in HAADF.

scholarly journals Quantification and optimization of ADF-STEM image contrast for beam-sensitive materials

2018 ◽  
Vol 5 (5) ◽  
pp. 171838 ◽  
Author(s):  
Karthikeyan Gnanasekaran ◽  
Gijsbertus de With ◽  
Heiner Friedrich
Keyword(s):  
Functional Materials ◽  
Polymer Nanocomposite ◽  
Dark Field ◽  
Low Contrast ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Stem Image ◽  
Liquid Cell ◽  
Imaging Conditions

Many functional materials are difficult to analyse by scanning transmission electron microscopy (STEM) on account of their beam sensitivity and low contrast between different phases. The problem becomes even more severe when thick specimens need to be investigated, a situation that is common for materials that are ordered from the nanometre to micrometre length scales or when performing dynamic experiments in a TEM liquid cell. Here we report a method to optimize annular dark-field (ADF) STEM imaging conditions and detector geometries for a thick and beam-sensitive low-contrast specimen using the example of a carbon nanotube/polymer nanocomposite. We carried out Monte Carlo simulations as well as quantitative ADF-STEM imaging experiments to predict and verify optimum contrast conditions. The presented method is general, can be easily adapted to other beam-sensitive and/or low-contrast materials, as shown for a polymer vesicle within a TEM liquid cell, and can act as an expert guide on whether an experiment is feasible and to determine the best imaging conditions.

Design of a new objective lens for an HB-501A STEM

1991 ◽  
Vol 49 ◽  
pp. 416-417
Author(s):  
Earl J. Kirkland ◽  
Robert J. Keyse
Keyword(s):  
High Resolution ◽  
Spherical Aberration ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Objective Lens ◽  
Specimen Holder ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
High Resolution Microscopy

An ultra-high resolution pole piece with a coefficient of spherical aberration Cs=0.7mm. was previously designed for a Vacuum Generators HB-501A Scanning Transmission Electron Microscope (STEM). This lens was used to produce bright field (BF) and annular dark field (ADF) images of (111) silicon with a lattice spacing of 1.92 Å. In this microscope the specimen must be loaded into the lens through the top bore (or exit bore, electrons traveling from the bottom to the top). Thus the top bore must be rather large to accommodate the specimen holder. Unfortunately, a large bore is not ideal for producing low aberrations. The old lens was thus highly asymmetrical, with an upper bore of 8.0mm. Even with this large upper bore it has not been possible to produce a tilting stage, which hampers high resolution microscopy.

Simulation of high-resolution annular dark field STEM images

1995 ◽  
Vol 53 ◽  
pp. 40-41
Author(s):  
E. J. Kirkland
Keyword(s):  
Electron Beam ◽  
Atomic Layer ◽  
Fresnel Diffraction ◽  
Dark Field ◽  
Objective Lens ◽  
Image Simulation ◽  
Small Probe ◽  
Annular Dark Field ◽  
Stem Image ◽  
Uniform Plane

In a STEM an electron beam is focused into a small probe on the specimen. This probe is raster scanned across the specimen to form an image from the electrons transmitted through the specimen. The objective lens is positioned before the specimen instead of after the specimen as in a CTEM. Because the probe is focused and scanned before the specimen, accurate annular dark field (ADF) STEM image simulation is more difficult than CTEM simulation. Instead of an incident uniform plane wave, ADF-STEM simulation starts with a probe wavefunction focused at a specified position on the specimen. The wavefunction is then propagated through the specimen one atomic layer (or slice) at a time with Fresnel diffraction between slices using the multislice method. After passing through the specimen the wavefunction is diffracted onto the detector. The ADF signal for one position of the probe is formed by integrating all electrons scattered outside of an inner angle large compared with the objective aperture.

Atomic resolution Z-contrast imaging of bulk crystal surfaces in Scanning Reflection Electron Microscopy

1990 ◽  
Vol 48 (4) ◽  
pp. 414-415
Author(s):  
Z. L. Wang ◽  
J. Bentley
Keyword(s):  
Electron Microscopy ◽  
Elastic Scattering ◽  
Dark Field ◽  
Contrast Imaging ◽  
Crystal Lattices ◽  
Small Probe ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Image Position ◽  
Z Contrast

The success of obtaining atomic-number-sensitive (Z-contrast) images in scanning transmission electron microscopy (STEM) has shown the feasibility of imaging composition changes at the atomic level. This type of image is formed by collecting the electrons scattered through large angles when a small probe scans across the specimen. The image contrast is determined by two scattering processes. One is the high angle elastic scattering from the nuclear sites,where ϕNe is the electron probe function centered at bp = (Xp, yp) after penetrating through the crystal; F denotes a Fourier transform operation; D is the detection function of the annular-dark-field (ADF) detector in reciprocal space u. The other process is thermal diffuse scattering (TDS), which is more important than the elastic contribution for specimens thicker than about 10 nm, and thus dominates the Z-contrast image. The TDS is an average “elastic” scattering of the electrons from crystal lattices of different thermal vibrational configurations,

STEM measurement of subcellular water distributions

1993 ◽  
Vol 51 ◽  
pp. 568-569
Author(s):  
R.D. Leapman ◽  
S.Q. Sun ◽  
S-L. Shi ◽  
R.A. Buchanan ◽  
S.B. Andrews
Keyword(s):  
Water Content ◽  
Internal Standard ◽  
Dry Weight ◽  
Dry Mass ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Bulk Water ◽  
Inelastic Electron ◽  
Scanning Transmission ◽  
Annular Dark Field

Recent advances in rapid-freezing and cryosectioning techniques coupled with use of the quantitative signals available in the scanning transmission electron microscope (STEM) can provide us with new methods for determining the water distributions of subcellular compartments. The water content is an important physiological quantity that reflects how fluid and electrolytes are regulated in the cell; it is also required to convert dry weight concentrations of ions obtained from x-ray microanalysis into the more relevant molar ionic concentrations. Here we compare the information about water concentrations from both elastic (annular dark-field) and inelastic (electron energy loss) scattering measurements.In order to utilize the elastic signal it is first necessary to increase contrast by removing the water from the cryosection. After dehydration the tissue can be digitally imaged under low-dose conditions, in the same way that STEM mass mapping of macromolecules is performed. The resulting pixel intensities are then converted into dry mass fractions by using an internal standard, e.g., the mean intensity of the whole image may be taken as representative of the bulk water content of the tissue.

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