light microscopy, of course, It is the "workhorse" of the aluminum industry. That's it basically makes it possible to rapidly identify the causes of deviations from the established quality criteria of the material of aluminum products. At the same time, light microscopy capabilities are limited both by increasing, and the depth of focus of some microscopic objects in the microstructure of aluminum alloys.
In such cases, light microscopy complements or replaces electron microscopy. Electron microscopy is capable of producing micro-chemical analysis of the various alloy components, it is very important to assess the completeness of the passage of various technological processes, such as homogenization ingots, solution annealing treatment, artificial aging. This analysis of the causes of damage to aluminum products – during manufacture or in use – results impossible without thorough SEM fracture surface.
Electron microscopy - scanning and transmission
Electron microscopy is a powerful tool, which allows you to analyze the microstructure, chemical composition and crystal structure of many materials, including, aluminum and aluminum alloy, on an area of less than 1 m. By research methods, electron microscopy is divided into:
- scanning (raster) electron microscopy and
- transmission (transmission) electron microscopy.
Figure - Diagram of the device of electron microscopes - scanning (b) and transmission (c) - compared with a light microscope (a)
(Light and Electron Microscopy / E. M. Slayter, H. S. Slayter / Cambridge University Press, 1997)
What is the difference between scanning and transmission electron microscopes?
The figure schematically shows the similarities and differences between the three types of microscopes – light, transmission and scanning.
A scanning electron microscope is based on a scan of the sample surface by a focused electron beam convergent-beam. This beam is very thin. Cross-sectional area is much smaller, than the area, which scans. Microscopic image based on the capture and measurement signals, that occur when the electron beam interaction with the sample.
Transmission electron microscope uses a parallel (coherent) stationary electron beam, directed onto the surface of the sample studied area. Microscopic image is due to the fact, that some of the electrons pass ("penetrate, shines through ") the sample through and through.
Therefore, images, which can be obtained with a scanning microscope, sometimes referred to as virtual images, since they are based on measuring the difference in the characteristics of the incident and reflected electron beam. At the same time the image of the transmission electron microscope is directly dependent on the intensity of the electrons penetrating through the sample and it can be called a true, so to speak, real image.
Scanning electron microscope
All methods, which are applied to scanning electron microscopes - and their number, based on the application of a focused (from 1 m to 0,15 nm) electron beam, which "hits" the sample. The dimensions of this electron probe - the beam cross-section - depend on the device of the electron gun (thermionic or field emission) and the magnitude of the current at the tip of the electron beam. This value of the current operator selects, which sits at a microscope. electron beam – electron probe – many times passes over the sample surface, and various signals from the sample surface are measured and form an "image" in the plane of the axes x-y.
Interaction between the sample and the electron beam produces several types of signals, which include:
- secondary electrons;
- reflected electrons;
- X-ray characteristic radiation.
resolution electron microscope for the reflected electrons and X-rays depends on the magnitude of its accelerating voltage, as well as the average atomic number and density of the sample. Image resolution scanning electron microscope electron beam depends on the size and contrast of the natural sample. Most of the materials do not have sufficient natural contrast, to reach the instrumental resolution electron microscope, which is equal to the size of the "tip" of the electron beam. As all techniques of scanning electron microscopy "work" near the sample surface, then the basic requirements for the sample are as follows:
- fit in size on the stage of the microscope;
- be compatible with high vacuum microscope;
- be electrically conducted.
X-ray spectrometers, electron microscopes
Modern electronic microanalysers and scanning electron microscopes are similar tools. They are equally based on the scanning of samples with electron beams. The difference between them lies in the type of detector, which is used for chemical analysis. Electronic microanalysers have three or more special energy-dispersive X-ray spectrometer, to provide accurate chemical analysis, whereas a conventional scanning electron microscope is only one, wavelength dispersive, X-ray spectrometer. This spectrometer provides a relatively "fast" to identify the majority of the chemical elements, quite easy to use and, what is important, relatively inexpensive.
An additional advantage energodispersionnnyh X-ray spectrometer - it is their small size compared with volnodispersnymi. This allows their use in close proximity to the samples in a transmission electron microscope, wherein the amount of generated X-rays is limited because of the small sample thickness.
Samples for scanning electron microscopy
Samples of aluminum and its alloys are electrically conductive and do not require additional coatings on its surface. Samples of materials, which are not electrically conductive, usually covered with thin (less 5 nm) electrically conductive layer, for example, by depositing on the surface of sample of amorphous carbon or vapor deposition of such metals, like gold, platinum, palladium or chromium.
Samples for transmission electron microscopy
Samples for transmission electron microscope must be thin enough, to electrons with energy 100 keV or more can pass through a sample. For aluminum and aluminum alloys, this thickness is about 50 m.
In just a few decades of work with a transmission electron microscope was developed many methods of thinning samples from almost all materials up to a thickness, to be suitable for penetrating electron microscopy. For preparation of the samples of aluminum and aluminum alloy, as for most other metals, electrolytic polishing is usually used.
Источник: Industrial Applications Of Electron Microscopy, ed. Zhigang Li/ Marsel Decker Inc., 2003