Microdiffraction and convergent-beam diffraction from surfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100154603 ◽

1989 ◽

Vol 47 ◽

pp. 526-527

Author(s):

J. A. Eades

Keyword(s):

Surface Characterization ◽

Normal Incidence ◽

Grazing Incidence ◽

High Energy ◽

Large Area ◽

High Energy Electrons ◽

Convergent Beam ◽

Beam Diffraction ◽

Convergent Beam Diffraction

Microdiffraction from surfaces at near grazing incidence is an important method of surface characterization. It is very much akin to RHEED (reflection high-energy electron diffraction) except that in RHEED a large area of sample (∼ 1 mm2) contributes to the diffraction. In this respect the relationship between RHEED and surface microdiffraction is analogous to that between selected-area diffraction and microdiffraction in transmission. In addition RHEED systems usually have no post-specimen lenses and therefore operate at a fixed camera length.Surface microdiffraction can contribute important information for the characterization of surfaces but there are some important factors that make it more complex than in the case of convergent-beam diffraction in transmission.At grazing incidence, even with high-energy electrons, refraction at the surface is important -whereas in transmission (at near-normal incidence) it may be neglected.

Download Full-text

convergent-beam diffraction from surfaces

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100125208 ◽

1987 ◽

Vol 45 ◽

pp. 30-33

Author(s):

J. A. Eades ◽

A. E. Smith ◽

D. F. Lynch

Keyword(s):

Reciprocal Lattice ◽

Three Dimensional ◽

Grazing Incidence ◽

Three Dimensions ◽

Plane Figure ◽

Transmission Electron ◽

Convergent Beam ◽

Beam Patterns ◽

Beam Diffraction

It is quite simple (in the transmission electron microscope) to obtain convergent-beam patterns from the surface of a bulk crystal. The beam is focussed onto the surface at near grazing incidence (figure 1) and if the surface is flat the appropriate pattern is obtained in the diffraction plane (figure 2). Such patterns are potentially valuable for the characterization of surfaces just as normal convergent-beam patterns are valuable for the characterization of crystals.There are, however, several important ways in which reflection diffraction from surfaces differs from the more familiar electron diffraction in transmission.GeometryIn reflection diffraction, because of the surface, it is not possible to describe the specimen as periodic in three dimensions, nor is it possible to associate diffraction with a conventional three-dimensional reciprocal lattice.

Download Full-text

Convergent-Beam Diffraction in the Characterization of Crystalline Phases

MRS Proceedings ◽

10.1557/proc-62-143 ◽

1985 ◽

Vol 62 ◽

Author(s):

J. A. Eades ◽

M. J. Kaufman ◽

H. L. Fraser

Keyword(s):

Electron Microscope ◽

Transmission Electron Microscope ◽

Zone Axis ◽

Crystalline Materials ◽

Powerful Technique ◽

Transmission Electron ◽

Convergent Beam ◽

Beam Diffraction ◽

Convergent Beam Diffraction

ABSTRACTConvergent-beam diffraction in the transmission electron microscope is a powerful technique for the characterization of crystalline materials. Examples are presented to show the way in which convergent-beam zone-axis patterns can be used to determine: the unit cell; the symmetry; the strain of a crystal. The patterns are also recognizable and so can be used, like fingerprints, to identify phases.

Download Full-text

The Microstructure of Electron-Beam Welds in Inconel 718

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s1431927600001409 ◽

1980 ◽

Vol 38 ◽

pp. 332-333

Author(s):

R. Vincent ◽

J.W. Steeds

Keyword(s):

Electron Beam ◽

Cross Sections ◽

Inconel 718 ◽

Large Area ◽

Edx Analysis ◽

Preliminary Identification ◽

The Matrix ◽

Convergent Beam ◽

Beam Diffraction ◽

Convergent Beam Diffraction

Electron-beam welds in the Ni-base superalloy, Inconel 718 are freauently found to contain an extensive system of micro-cracks radiating from the weld boundary into the heat-affected zone (HAZ) of the matrix. To study precipitation in cross-sections of weld samples* over a large area adjacent to the weld, extraction replicas were prepared from the electro-polished and etched surfaces. Some preliminary identification of the extracted particles has been made by EDX analysis and convergent-beam diffraction, combined with scanning microscopy and EDX analysis of the electro-polished or ion-eroded surfaces. The welds are 1 cm deep, and near the top surface, where most of the long U+2018cracks’ occur (Fig.1a), the weld is 1.5 mm wide. The FAZ, as defined by a suitable etchant (10% HC1-methanol), extends for a further 1mm on each side. The grain size (50-100μm) is unchanged across the matrix-FAZ boundary. EDX spectra from the large precipitates (2-10μm) in the matrix and HAZ (Fig.la) show a strong Mb peak coupled with a weak Ti peak; the other alloy elements are entirely absent. Additional evidence from convergent-beam diffraction patterns (Fig.3) and measurement of the lattice constant2 (ao = 4.42 + 0.015 Å), proves that the majority of the particles are carbides, (Nb,Ti)C, with a F-centred cubic structure. These precipitates are absent from the weld, and the backbone of the dendritic weld precipitation is formed by thin, branched platelets (Fig.4). These are also carbides, (Nb,Ti)C, which recrystal 1ize from the melt. However, the FDX snectra now show weak additional peaks from most of the alloy elements (Ni, Cr, Fe, Mo).

Download Full-text

One of my Failures: Diffraction in the TEM

Microscopy Today ◽

10.1017/s1551929510001252 ◽

2011 ◽

Vol 19 (1) ◽

pp. 72-72 ◽

Cited By ~ 1

Author(s):

Alwyn Eades

Keyword(s):

Electron Microscope ◽

Transmission Electron Microscope ◽

Standard Method ◽

Transmission Electron ◽

Diffraction Patterns ◽

Convergent Beam ◽

Beam Diffraction ◽

Convergent Beam Diffraction

There are two principal techniques for obtaining diffraction patterns in the transmission electron microscope (TEM). They are selected-area diffraction (SAD) and convergent-beam diffraction (CBED). CBED is quicker and easier to use, and it provides a much richer characterization of the sample. Thus, it is clear that CBED should be used in the vast majority of cases. It should be the diffraction technique that students learn first, and students should be taught to consider it the standard method of doing diffraction in the TEM.

Download Full-text

Using the TEM Condenser Lens to Switch between Image and Diffraction Modes

Microscopy Today ◽

10.1017/s1551929513000072 ◽

2013 ◽

Vol 21 (2) ◽

pp. 40-40

Author(s):

Lydia Rivaud

Keyword(s):

Electron Microscope ◽

Diffraction Pattern ◽

Transmission Electron Microscope ◽

Condenser Lens ◽

Selected Area Diffraction Pattern ◽

Transmission Electron ◽

Set Up ◽

Convergent Beam ◽

Beam Diffraction ◽

Convergent Beam Diffraction

Central to the operation of the transmission electron microscope (TEM) (when used with crystalline samples) is the ability to go back and forth between an image and a diffraction pattern. Although it is quite simple to go from the image to a convergent-beam diffraction pattern or from an image to a selected-area diffraction pattern (and back), I have found it useful to be able to go between image and diffraction pattern even more quickly. In the method described, once the microscope is set up, it is possible to go from image to diffraction pattern and back by turning just one knob. This makes many operations on the microscope much more convenient. It should be made clear that, in this method, neither the image nor the diffraction pattern is “ideal” (details below), but both are good enough for many necessary procedures.

Download Full-text