Abstract Measurements have been made of the energy losses in fast electrons transmitted through thin films of Al-Zn alloys containing up to 50 wt. % Zn. The measurements show that the determination of concentrations in this alloy system by energy loss techniques will not be very accurate. The variation of the plasma energy with concentration cannot be described simply by accounting for changes in the electron density. A simple model is developed to account for the measurements, and the information deduced from this model is used to identify prominent detail in the energy loss spectrum of zinc.
Abstract An analysis of electron energy loss spectra of some 12 transition metal dichalcogenides has given useful information towards the understanding of the band structures of these layer-type solids. Using the experience gained from studying graphite in which the valence band consists of two groups, namely π and σ bands, a similar band scheme has also been established for the transition metal dichalcogenides. The plasma resonance energies of π electrons are discussed in terms of the screening by σ electrons and the finite oscillator coupling between the two valence bands. Single electron interband excitations have been observed, which are expected to form the basis for further detailed study into the band structures of these materials. Many atomic-like excitations associated with the core d and core f electrons have also been observed.
Observations of the effect of thermal diffuse scattering, single electron excitations and plasmon excitation on the quality of electron microscope images of crystals are presented and discussed. Preservation of image contrast after thermal diffuse scattering seems to be poor in practice but only a small fraction of the electrons accepted by the aperture are affected in this way. To a first approximation, contrast appears to be preserved after single electron excitation or plasmon excitation especially at small scattering angles where no evidence of interband transitions of the fast electron has been found. At larger scattering angles some loss of contrast occurs so that the size of the aperture can be important. The significance of the results is discussed with reference to electron microscopy of thick crystals, direct resolution of atomic positions and out-of-focus images.
Direct observations of the energy loss spectra from θ phase precipitates in Al + 4 wt. % Cu alloy have been made by means of combined electron microscopy and energy analysis in order to examine the difficulties involved in using this technique for qualitative microanalysis of precipitated phases. It is concluded that a microanalysis will be completely reliable if the peaks in the characteristic energy loss spectra (0 to 50 ev) from matrix and precipitate are reasonably well defined and well separated. If this is not so, then the precipitate must extend from the top to the bottom surface of the electron microscope specimen before any reliable information can be obtained. As far as the extension of the technique to the study of segregation effects is concerned, it is concluded that the boundaries at which such effects are expected must be aligned parallel to the incident electron beam.
A new high resolution technique has been developed for the study of segregation and associated phenomena; a microanalysis is, in effect, performed on a highly magnified image of a specimen when examined by transmission electron microscopy. The method depends on the facts that electrons lose energy on transmission through the specimen, that the energy losses are characteristic of the material of the specimen, and that these energy loss electrons contribute to a normal image in the electron microscope. The spatial resolution of the technique is estimated to be about 10 nm. In the special case presented here, the measurements are semi-quantitative and concentration changes of about 1 wt. % are detectable. The technique is applied to the study of precipitation and associated segregation of magnesium at high angle grain boundaries in an aluminium 7 wt. % magnesium alloy after it has been quenched from different solution treatment temperatures. The results of the investigation are consistent with precipitate nucleation at the boundaries during the quench at a quench rate of 3 x 10 -4 °C s -1 . Non-equilibrium segregation of magnesium to the boundaries is also indicated, and a model based on the converse of the Kirkendall effect is proposed to explain this.
It is shown that the difference between the mean absorption coefficient measured from the intensity oscillation of a zero-loss electron microscope dark-field image and that measured from the ordinary image is the reciprocal of the mean free path for inelastic scattering.
Experiments are reported which show that contrast effects in electron microscope images are preserved by electrons which have been scattered elastically or quasi-elastically (i.e. energy loss ≤1·5 ev) near Bragg beams. It is uncertain whether phonon scattering is entirely responsible for the contrast observed. Scattering from amorphous oxide or contamination layers on the surfaces of the specimen can produce contrast effects similar to those observed. Calculations supporting this point of view are reported.
The conventional electron microscopy of an internally oxidized 0·05 wt. % Si+Ni alloy is reported. The method of oxidation and the morphology and identity of the precipitates are described. Possible effects of this type of precipitation on the magnetic properties of this and similar alloy systems are briefly considered. Confirmatory evidence for the nature of the precipitates has been obtained by a new technique of combined electron microscopy and energy analysis of the transmitted electrons.
An instrument is described which combines the facilities of transmission electron microscopy of thin foils with energy analysis of the transmitted electrons in the low-lying loss region. The instrument consists of a standard Siemens electron microscope and a Möllenstedt electron energy analyser placed below the final viewing screen. Energy spectra may be obtained from selected regions of the specimen at high magnification and for operating voltages up to 100 kv. Two examples of the use of such an instrument are given, namely, the study of diffraction contrast effects for inelastically scattered electrons and its use in qualitative microanalysis of specimens containing precipitate phases.
