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Application of polarized light in the metallographic annalysis of metals and alloys is considered, its application for the analysis of ninmetallic inclusions is shown. Examples of application of differential and interferential contrast for the annalysis of structure of metals in reflected light are shown.
A. G. ANISOVICH, State Scientific Institution "Physico-Technical Institute of the National Academy of Sciences of Belarus"
UDC 620.186.1 + 535-4
APPLICATION OF POLARIZED LIGHT IN THE ANALYSIS OF METALS AND ALLOYS
The observation method in polarized light (polarizing microscopy) serves both for microscopic studies of minerals, biological objects, and for studying the structure of metals and non-metallic materials. The optical properties of anisotropic micro-objects are different in different directions and manifest themselves differently depending on the orientation of these objects relative to the axis of the lens and the plane of polarization of the light incident on them. The light emitted from the illuminator passes through the polarizer; the polarization imparted to it in this case changes with subsequent reflection from the sample, and these changes are studied using an analyzer and various optical compensators. Polychromatic polarized light is effective in metallography for detecting and studying
of transparent objects, therefore a limited number of problems are solved using white polarized light. Traditionally, non-metallic inclusions are studied in metallography using polarized light. Since a certain part of nonmetallic inclusions is optically transparent, the study is based on the difference in the optical properties of the inclusion in different directions, i.e., their optical anisotropy. Optical anisotropy appears when light passes inside an inclusion when light is reflected from its surface. A flat surface and a transparent inclusion interact differently with the light flux. Plane polarized light reflected from a flat surface is blocked by the analyzer and the surface appears dark. Part of the light is refracted
Figure: 1. Spherical transparent slag inclusions in light (a) and dark 10 msh | (b) fields and polarized light (c)
on the outer surface of the inclusion, passes inward and, being reflected on the surface of the inclusion-metal, goes outward, again experiencing refraction on the inner surface. As a result, the light ceases to be polarized. Therefore, with the crossed position of the analyzer and the polarizer, a light image of the inclusion on a dark background is visible. The inclusion color can change due to interference, which is associated with anisotropic effects when polarized light is reflected.
Using polarized light, one can draw conclusions about the shape of transparent inclusions. If the inclusion has a regular round shape, then concentric rings appear on the image of the structure in both the light and dark fields (Fig. 1, a, b), associated with the interference of rays reflected from the inner surface of the inclusion. In some cases, you can observe the interference color of the rings, the formation of which depends on the angle of inclination of the rays. In polarized light with crossed nicols, the effect of a dark cross is observed (Fig. 1, c). The contrast of concentric rings and a dark cross depends on the perfection of the shape of the inclusion. The "dark cross" phenomenon is associated with optical phenomena in converging polarized light. The branches of the dark cross expand towards the ends
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and are parallel to the main sections of the nicols. Since the optical axis of the inclusion coincides with the optical axis of the microscope system, the center of the inclusion is not illuminated. In accordance with the optical cross give in polarized light, in particular, globular transparent inclusions of silicates.
If the inclusion is opaque (Fig. 2), then concentric rings are not formed in the light- and dark-field images. The circular contrast around the inclusion in the bright field (Fig. 2, a) does not belong to the inclusion itself and can be associated with stresses in the alloy. In a dark field (Fig. 2b), the edges of the inclusion glow due to the reflection of light from non-planar areas. In polarized light (Fig. 2, c, d), the dark cross effect is absent.
A transparent inclusion of irregular shape “glows” in a dark field (Fig. 3, a, b) and polarized light (Fig. 3, c) without specific optical effects.
The images shown in Fig. 1-3 have good contrast. However, when using bright field lighting, it is not always possible to obtain a high contrast image. In fig. 4 shows photographs of a transparent particle of aluminum oxide. In a bright field (Fig. 4, a) the image has low contrast and clarity; focusing is carried out
Figure: 2. Round opaque inclusion of slag in silumin: a - bright field; b - dark field; c, d - polarized light
(c - nicolas are parallel; d - nicolas are crossed)
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Figure: 3. Vitrified inclusion in alloyed silumin: a - bright field; b - dark field; c - polarized light
poured onto the surface of the particle. The surface relief is visible in the dark field (Fig. 4, b). Special methods can be used to increase the contrast of the image. It is possible to change the phase of the reflected rays. The human eye does not perceive phase differences, but is able to distinguish between changes in intensity and wavelength (color). Therefore, a change in phase is translated into a change in intensity (or color) using the phase contrast method, which makes the features of the structure visible. Get color-
the structure can be imaged using polarized light and special devices. It should be remembered that the resulting colors are conditional and not related to physical properties phases. Such methods include the method of differential interference contrast. In fig. 4c shows an image of the inclusion obtained using differential interference contrast. Its application increased the clarity of the image and the depth of field. Focusing on the surface
Fig. 4. Particles of aluminum oxide in the AK21M2.5N2.5 alloy in a bright field (a), dark field (b), using differential interference contrast (c)
Figure: 5. Wollaston prism (a) and light beam splitting scheme (b)
the inclusion also allows one to see excess and eutectic silicon.
Differential interference contrast (DIC) is an advanced polarization contrast technique that can be used to visualize minimal differences in height or unevenness in surfaces. In this case, a birefringent prism of Nomarskii or Wollaston is used (Fig. 5, a), which splits the polarized light beam on its way to the sample into two partial beams (Fig. 5, b).
This prism consists of two rectangular prisms glued together, made of birefringent crystals (Icelandic spar, natural quartz). The prisms are glued so that their optical axes are mutually perpendicular. A ray of light falling on the side face of the first prism is divided into two plane polarized rays - ordinary and extraordinary, which propagate in such a crystal at different speeds. Getting into the second prism at a different angle to the direction of the optical axis, they are refracted at the interface of two glued prisms at different angles (in this case, an ordinary ray becomes extraordinary and vice versa). Coming out of the second prism, each of the two beams is refracted again, almost symmetrically deviating from one another in different directions from the direction of the beam entering the first prism. Visually, this principle is expressed in the fact that the surfaces of the sample are illuminated with polarized monochromatic light, that is, having a certain wavelength (\u003d color blue or red, or green, etc.). If the sample surface is perfectly flat, then it is colored the same way. When moving the prism horizontally, the color of the flat surface will change in accordance with the diagram shown in Fig. 6 (the color scale is shown here for clarity and does not correspond
scale of interference colors). When the prism is moved horizontally, the surface first has, for example, yellow, then green, etc.
However, if there is a small step (height difference) on the surface of the sample, then one of these two partial rays must travel 25 k (k is the height of the drop, 5 is the difference in the path of the rays) longer and acquire a path difference. Therefore, the areas of the sample lying above or below the main plane of its surface will have their own color. This is illustrated in Fig. 7. Under bright-field illumination, the silicon carbide particles located on the inclusion of excess silicon have the form of dark spots (Fig. 7, a). When using differential interference contrast (Fig. 7, b), SiC particles have their own color due to the fact that they are located above the plane of the microsection.
If the surface is curved, then several colors or the entire spectrum can be seen simultaneously. For illustration, a flat surface was photographed, in this case a micrometer object (Fig. 8, a). After that, without changing the settings of the microscope optical system, the surface of the steel ball was photographed (Fig. 8, b). The top point of the spherical surface corresponds to the white spot; color, approximately corresponding
Figure: 6. Scheme of staining the sample surface
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Figure: 7. Particles of silicon carbide in excess silicon crystals of hypereutectic silumin in a bright field (a);
DIK - contrast (b)
Figure: 8. Fragment of the scale of the object-micrometer (a) and the image of a curved surface in the DIC (b)
the color of the plane in Fig. 8, a, indicated by an arrow. The color of the stripes changes according to the curvature of the spherical surface. The sequence of colors corresponds to the scale of interference colors with interference on a wedge-shaped plate. In practice, this method is "ob-
mate "that is used in crystallography to determine the thickness of transparent crystals.
When studying objects in reflected light using differential-interference devices, an increase in the con-
trust of individual sections of the object, with similar values \u200b\u200bof the reflection coefficients, which provides additional information about the structure of the object. In this case, the object appears to be embossed. The method allows you to analyze a sample with an accuracy of measuring the height of the unevenness (thickness) in the nanometer range. An example of how can
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the color of the sample changes when the prism is moved, shown in Fig. 9. This shows the joining of dissimilar materials by welding. Different halves of the specimen have different properties and are not polished evenly. The material on different sides of the seam has a slight difference in height and, accordingly, is painted in different colors.
Literature
1. Chervyakov A. N., Kiseleva S. A., Ryl'nikova A. G. Metallographic determination of inclusions in steel. M .: State. scientific and technical publishing house of literature on ferrous and non-ferrous metallurgy, 1962.
2. Panchenko EV, Skakov Yu. A., Krimer BI et al. Laboratory of metallography / Ed. B. G. Livshits. Moscow: Metallurgy, 1965.
3. Tatarsky VB Crystal optics and emersion method. Moscow: Nedra, 1965.
4. Levin EE Microscopic research of metals. M .; L .: State. scientific and technical publishing house of engineering literature, 1951.
5. Anisovich A. G., Rumyantseva I. N. The art of metallography: the possibility of using dark-field image for analysis of the structure of metals: Sat. materials of the 4th International. scientific and technical conf. "Modern methods and technologies for creating and processing materials." Minsk, October 19-21, 2009 Book. 1.S. 7-12.
6. Anisovich AG, Rumyantseva IN Application of the method of differential interference contrast in metal science: Sat. materials of the 3rd Intern. scientific and technical conf. "Modern methods and technologies for creating and processing materials." Minsk, October 15-17, 2008 T. 1.P. 130-135.
7. Klark ER, Eberhardt KN Microscopic methods of material research. Moscow: Technosphere, 2007.
8. Egorov and OV Technical microscopy. With a microscope on "you". Moscow: Technosphere, 2007.
9. Wollaston prisms // Optics Provider LLC [Electronic resource]. 2012-Access mode: http://opticsprovider.ru.
10. Wollaston's prism // Elan LLC [Electronic resource]. 2012-Access mode: http: // www.elan-optics.com.
11. Chetverikov SD Methodology of crystal-optical studies of thin sections. M .: State. publishing house geologist. literature, 1949.
The applications of light polarization for practical purposes are quite diverse. For example, some application examples were developed many years ago, but are still in use today. Other examples of applications are just being implemented
Figure 1. Application of light polarization. Author24 - online exchange of student papers
In a methodological sense, they all have one common property - either they contribute to the solution specific tasks in physics, are either completely inaccessible in relation to other methods or allow solving them in non-standard, but at the same time more operational and effective way.
The phenomenon of light polarization
With the aim of a more detailed acquaintance with the use of light polarization, one should understand the essence of the phenomenon of polarization itself.
Definition 1
The phenomenon of light polarization is an optical phenomenon that has found its application in a technical sense, but is not found in everyday life. Polarized light literally surrounds us, but for human eye polarization itself remains virtually inaccessible. We thus suffer from "polarization blindness."
Created by the sun (or some other common source, such as a lamp), natural light is a collection of waves that are emitted by a huge number of atoms.
A polarized wave is a transverse wave, where the oscillations of all particles are performed within the same plane. At the same time, it can be obtained, thanks to a rubber cord, in the event that a special barrier with a thin gap is placed in its path. The slot, in turn, will pass exclusively the vibrations occurring along it. A plane-polarized wave is emitted by a single atom.
Examples of polarization of light and Umov's law
In nature, there are many different examples of light polarization. In this case, you can consider the most common of them:
- The simplest and most widely known example of polarization is clear sky, which is believed to be its source.
- Glare on glass display cases and water surfaces are other common occurrences. If necessary, they are eliminated by the appropriate polaroid filters, which are often used by photographers. These filters become indispensable if you need to capture any glass-protected paintings or exhibits from a museum on photographs.
The principle of operation of the above filters is based on the fact that absolutely any reflected light (depending on the angle of incidence) has a certain degree of polarization. When looking at the flare, thus, you can easily choose the optimal angle of the filter, at which it is suppressed, up to its complete disappearance.
A similar principle is used by manufacturers of high-quality sunglasses with a sun filter. By using polaroid filters in their glass, those glare that interfere with it are removed. They, in turn, emanate from the surfaces of a wet highway or sea.
Remark 1
The effective application of the phenomenon of polarization is demonstrated by Umov's law: any scattered light from the sky is the sun's rays that have previously undergone multiple reflections from air molecules, and repeatedly refracted in water droplets or ice crystals. Along with that, the polarization process will be characteristic not only of directional reflection (from water, for example), but also of diffuse one.
In 1905, physicists presented proof of the version that the darker the surface of the reflection of a light wave, the higher the degree of polarization turns out to be, and it was this dependence that was proved in Umov's law. If we consider this dependence on a specific example with an asphalt highway, it turns out that in a wet state it becomes more polarized in comparison with a dry one.
Application of light polarization in history and in everyday life
The polarization of light, therefore, turns out to be a difficult phenomenon to study, but important in terms of wide practical application in physics. In practice, the following examples are encountered in everyday life:
- A striking example, familiar to everyone, is 3D cinema.
- Another common example is polarized goggles, which hide glare from water and headlights on the track.
- The so-called polarizing filters are used in photographic technology, and wave polarization is used to transmit signals between the antennas of different spacecraft.
- One of the main daily tasks of lighting engineering is considered to be a gradual change and regulation of the intensity of light fluxes. Solving this problem using a pair of polarizers (polaroids) has certain advantages over other control methods. Polaroids can be manufactured in large format, which implies the use of such pairs not only in laboratory installations, but also in steamer windows, railway carriage windows, etc.
- Another example is the polarization blocking used in the lighting equipment in the workplace of operators who must see at the same time, for example, the oscilloscope screen and certain tables, maps or graphs.
- Polaroids can be useful for those whose work is related to water (sailors, fishermen), in order to extinguish reflections specularly reflected from the water, partially polarized.
Figure 2. Application of polarizing devices. Author24 - online exchange of student papers
Remark 2
Extinguishing the reflected light in conditions of normal or close to normal incidence can be carried out by means of circular polarizers. Previously, science has proven that in this case, the right circular light is converted to the left circular (and vice versa). The same polarizer, thus creating circular polarization of the incident light, will cause the reflected light to be extinguished.
In astrophysics, spectroscopy, lighting engineering, the so-called polarizing filters are widely used, which make it possible to isolate narrow bands from the investigated spectrum and provoke changes in saturation or color shades.
The action of such filters is based on the properties of the main parameters of the phase plates (dichroism of polaroids) and polarizers, which are directly dependent on the wavelength. For this reason, various combinations of such devices can be used to change the spectral energy distribution in light fluxes.
Example 1
So, for example, a pair of chromatic polaroids, which are characterized by dichroism exclusively within the visible sphere, in the crossed position will begin to transmit red light, and in the parallel position only white. Such a simple device will be effective in practical application when illuminating photographic laboratories.
Thus, the scope of application of light polarization is quite diverse. For this reason, the study of the phenomenon of polarization is of particular relevance.
Lighting control and glare suppression. One of the common uses for polarized light is to adjust the light intensity. A pair of polarizers allows you to smoothly change the light intensity within a huge range - up to 100,000 times.Polarized light often used to extinguish specularly reflected light from smooth dielectric surfaces. For example, polaroid sunglasses are based on this principle. When natural unpolarized light strikes the surface of a pond, some of it is mirrored and polarized. This reflected light makes it difficult to see objects underwater. If you look at the water through an appropriately oriented polarizer, then most of specularly reflected light will be absorbed and the visibility of underwater objects will be greatly improved. When observing through such glasses, "noise" - the light reflected from the surface - decreases by 5-20 times, and the "signal" - light from underwater objects - decreases by only 2-4 times. Thus, the signal-to-noise ratio increases significantly.
Polarizing microscopy. Polarization microscopy is widely used in a number of studies. A polarizing microscope is equipped with two polarizing prisms or two polaroids. One of them - the polarizer - is located in front of the condenser, and the second, the analyzer, is behind the objective. In recent years, special polarization compensators have been introduced into polarizing microscopes, which significantly increase sensitivity and contrast. With the help of microscopes with compensators, such small and non-contrast objects as intracellular birefringent structures and details of the structure of cell nuclei, which could not be detected by another method, were detected and photographed.
Enhanced contrast. Polarizing filters are often used to enhance the contrast of transparent and low-contrast elements. For example, they are used when photographing a cloudy sky in order to enhance the contrast between clouds and a clear sky. The light scattered by the clouds is almost completely unpolarized, the light of the clear blue sky polarized significantly. The use of polarizing filters is the most effective remedy enhance contrast.
Crystallographic studies and photoelastic analysis. In crystallography, polarization studies are carried out especially often. Many crystals and oriented polymeric materials exhibit significant birefringence and dichroism. By studying these characteristics and determining the direction of the corresponding axes, it is possible to identify materials, as well as obtain data on the chemical structure of new substances.
Of particular importance in technology is photoelastic analysis... This is a method that allows you to judge mechanical stresses by phase displacement. For photoelastic analysis, the part under study is made of a transparent material with a high coefficient of photoelasticity. The main part of the installation for photoanalysis is a polariscope, which consists of an illumination system, a polarizer, an analyzer and an eyepiece. If a flat glass strip is subjected to stretching, then the glass will be somewhat deformed, and mechanical stresses will arise in it. As a result, it will become birefringent and will shift the phase of the light wave. By measuring the phase shift, you can determine the magnitude of the voltage.
Photoelastic analysis method can be used in ophthalmology, since photoelastic phenomena are found in the membranes of the eye.
Accordingly, ordinary light is used in metallography for research isotropic objects, or in those cases (and there are most of them) in which the anisotropy data is not important or is not the goal. The optical properties of anisotropic micro-objects are different in different directions and manifest themselves in different ways depending on the orientation of these objects relative to the direction of observation and the plane of polarization of the light incident on them, therefore, when studying them, polarized light, possessing the property anisotropy.
In polarized light, vibrations take place only in one specific direction in a plane perpendicular to the direction of propagation of light (Fig. 1, b). It is impossible to visually distinguish between ordinary and polarized light. Obtaining and analyzing polarized light is based solely on its interaction with matter. An indispensable condition for this is the anisotropy of the substance itself. Microscopy uses two Nicolas prisms to capture and analyze polarized light (the common term is simply Nicolas). Nicolas are made from transparent crystals of Icelandic spar, which has the property of birefringence. Therefore, Nicole lets through vibrations in only one direction. The scheme for obtaining polarized light is shown in Fig. 2. Since ordinary light contains vibrations of different directions, the first nicole will always miss some part of them, in accordance with the direction of its optical axis. If the orientations of the optical axes of Nicolas 2 and Nicolas 1 coincide (Nicolas are parallel, Fig. 2, a), then Nicolas 2 will let light through. If the orientations of the optical axes of the nicols are mutually perpendicular (the nicols are crossed, Fig. 2, b), then the sample surface will be perceived as dark; Nicole 2 only transmits elliptically polarized light. This issue is discussed in detail in.
Figure 2. Scheme of the path of rays parallel and crossed nicholas [1].
Nicole 1 is called a polarizer, nicole 2 - analyzer.
The observation method in polarized light (polarizing microscopy) is used both for microscopic studies of minerals, biological objects, and for the analysis of the structure of metals and non-metallic materials.
Traditionally, in metallography, polarized light is used to study non-metallic inclusions. Since a certain part of nonmetallic inclusions is optically transparent, the study is based on the difference in the optical properties of the inclusion in different directions, i.e. them optical anisotropy ... Optical anisotropy is manifested when light passes through the inclusion and when light is reflected from its surface. A flat surface and a transparent inclusion interact differently with the luminous flux. Plane polarized light reflected from a flat surface is blocked by the analyzer and the surface appears dark. Part of the light is refracted on the outer surface of the inclusion, passes inward, is reflected on the inclusion-metal surface, and goes outward, again experiencing refraction on the inner surface. As a result, the light ceases to be polarized. Therefore, with the crossed position of the analyzer and the polarizer, a light image of the inclusion on a dark background is visible. The inclusion color can change as a result of interference, which is associated with anisotropic effects when polarized light is reflected.
Using polarized light, you can draw conclusions about the shape of transparent inclusions. If the inclusion has a regular circular shape, then concentric rings appear on its bright-field (Fig. 3, a) and dark-field images, associated with the interference of rays reflected from the inner surface of the inclusion. In polarized light with crossed nicols, dark cross effect (Fig. 3, b). The contrast of concentric rings and a dark cross depends on the perfection of the shape of the inclusion.
Figure 3. Spherical vitrified inclusions metallurgical slag in a bright field (a) and polarized light (b).
Figure 4. Circular inclusion of slag in silumin: a - bright field, b - dark field, c, d - polarized light (c - nichols are parallel, d - nicolas are crossed)
If the inclusion is not transparent, then concentric rings do not appear in the bright-field and dark-field images. In polarized light (Fig. 4, c-d), the dark cross effect is absent.
The specific effects arising in polarized light are also considered in the article "Optical effects". These are, first of all, etching pits and light figures on surface defects.
Here we will dwell on what can be obtained in polarized light for objects that are fairly common in metal science. Figure 5 shows a comparison of photographs of the gray cast iron structure obtained by different contrasting methods. For this material, the most informative is the bright field, the maximum amount of image details is visible. All non-planar details of the structure - cementite and iron phosphide - “glow” in the dark field. The planes - ferrite and the matrix of the phosphide eutectic - are dark. The inclusion of graphite is gray, its borders are slightly visible. We can say that in a dark field, this image is mostly black and white. In polarized light, the picture changes. Perlite cementite "glows". Moreover, each colony has its own color shade, depending on the orientation. Cementite in the composition of the phosphide eutectic should also "glow", but this is not visible at the given image scale. The Fe3P compound is illuminated. Since ferrite has a cubic body-centered crystal lattice, it does not change the plane of polarization; therefore, ferrite is dark in polarized light.
Figure 5. The structure of gray cast iron: a - bright field, b - dark field, c - polarized light.
Figure 6 shows the structure of niobium alloyed cast iron. The phase composition is carbides and austenite. In polarized light, the carbide phase is colored in shades of blue. The dark component is austenite in the eutectic composition.
Figure 6. The structure of cast iron: a - bright field, b - polarized light
1. A. N. Chervyakov, S. A. Kiseleva, A.G. Rilnikov. Metallographic determination of inclusions in steel. Moscow: Metallurgy, 1962.
2. EV Panchenko and other Laboratory of metallography. Moscow: Metallurgy, 1965.