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Twin Analytical has many instruments that are used to help characterize/prepare test samples in an investigation.
Unit with Vickers and Knoop measurement capability. Minor load range of 5gm to 100gm and major load range above 100gm to 1000gm.
The RMC MT 940 rotary microtome can prepare cross-sectioned samples that are free of deleterious artifacts produced by slower metallographic techniques. The microtome will cut sections ranging from 0.25 μm to 30 μm providing a practical, cost effective technique for specialized sample preparation.
This technique can produce thin uniform sections for TEM studies and thick cross-sections for SEM investigations, e.g. blisters under metallic coatings, paint pinholes and pencil pipe casting defects. Extensive work has gone into developing the technique to produce minimally deformed sections through a sample consisting of a substrate and coating of vastly differing hardness with intervening void pockets.
One of the most critical steps in any analysis is the proper preparation of the sample prior to analysis. The aim of metallographic sample preparation is to reveal the true structure of the sample, whether it is metal or any other solid material. Theoretically, a sample’s true structure shows a precise image of the structure to be analyzed. Ideally, true structure requires the following:
NO DEFORMATION — There are two types of deformation, elastic and plastic. Elastic deformation disappears when the applied load is removed. Plastic deformation (cold work) can result in subsurface defects after grinding, lapping or polishing. Remaining plastic deformation can first be seen after etching.
NO SCRATCHES — Scratches are grooves in the surface of a sample, produced by the points of abrasive particles.
NO PULL-OUTS — Pull-outs are the cavities left after grains of particles which are torn out of the sample during abrasion. They are found in hard and brittle materials, and in materials with inclusions. Hard or brittle materials can not be deformed plastically, so small parts of the surface material shatter and may fall out or be pulled out by the polishing cloth.
NO CONTAMINATION — Material from a source other than the sample itself that is deposited on the sample surface during mechanical grinding, polishing or careless handling is called contamination.
NO SMEARING — The plastic deformation of larger sample areas is called smearing.
NO EDGE ROUNDING — Using a polishing surface with high resilience will result in material removal from both the sample surface and around the sides. The effect of this is edge rounding. With mounted specimens this effect can be seen if the wear rate of the resin is higher than that of the sample material.
NO RELIEF — Material from different phases is removed at different rates due to varying hardness or wear rate of the individual phases.
NO CRACKS — Cracks are fractures in brittle materials and materials with different phases. The energy used to prepare the sample is greater than can be absorbed. The surplus energy results in cracks.
NO GAPS — Gaps are voids between mounting resin and sample material.
NO ADDITIONAL POROSITY — Some materials have natural porosity, e.g. cast metals, spray coatings or ceramics. It is important to get the correct values, and not wrong readings because of preparation faults.
NO COMET TAILS — Comet tails occur adjacent to inclusions or pores, when the motion between sample and polishing disc is unidirectional. A key factor in avoiding comet tails is the polishing dynamics.
NO EMBEDDED ABRASIVE — Loose abrasive particles pressed into the surface of the sample.
NO LAPPING TRACKS — These are indentations on the sample surface made by abrasive particles moving freely on a hard surface. There are no scratches, like from a cutting action. Instead, there are the distinct tracks of particles tumbling over the surface without removing material.
NO STAINING — Staining is often seen after cleaning or etching samples. Staining is a discoloration of the sample surface, typically because of contact with a foreign substance. Finger print staining may cause erroneous conclusions. Retaining a clean prepared surface is essential for proper analysis.
Using mechanical preparation, however, it is almost impossible to achieve all of the above-mentioned conditions. There will be minimal damage to the structure which, for the most part, cannot be revealed with an optical microscope. This damage does not influence the examination results. This nearly perfect condition, with only superficial damage remaining, is commonly called true structure.
Optical Microscopes
Optical microscopy is an efficient and inexpensive means for examining features of a material over a wide range of magnifications. The optical microscope reveals features to the trained observer that cannot be determined from a scanning electron microscope (SEM) examination; for example, color, relative opacity, birefringence and refractive index.
The following light optical metallograph is available for conducting microscopic analysis:
Olympus PMG3 Research Metallograph — Magnification range 50X - 1000X.
The following stereo microscope is available for low-magnification examination:
Olympus SZH Stereo Microscope — Magnification range 7.5X - 64X (zoom).
Types of Illumination
The optical microscope is an important tool of the metallurgist from both the scientific and technical standpoints. It is used to determine grain size and the size, shape, and distribution of various phases and inclusions which have a great effect on the mechanical properties of metals. The microstructure will also reveal the mechanical and thermal treatment of the metal. Optical investigations provide valuable microstructural information and can be used to select features requiring more detailed analysis. In many instances the optical investigation is sufficient to resolve the specific problem under investigation thus eliminating the need for more costly methods of analysis. Furthermore, microscopic studies depend upon the care taken in sample preparation. The most expensive microscope will not reveal the structure of a sample that has been poorly prepared.
Bright field illumination, dark field illumination, polarized light and Normarski interference contrast methods are utilized to highlight specific features or structures of a material.
Bright-Field Reflected Light Illumination — In bright-field illumination, sometimes referred to as vertical illumination, the surface of the sample which is perpendicular to the axis of the objective, appears bright. Furthermore, white light is used.
Dark-Field Reflected Light Illumination — Another method that often is used to distinguish features not in the plane of the polished and etched surface of a metallographic sample is dark-field illumination. This type of illumination gives contrast completely reversed from that obtained with bright-field illumination, the features that are light in bright field will be dark in dark field, and those that are dark in bright field will be light in dark field. This highlighting of angled surfaces (namely, those of pits or cracks) allows more positive identification of their nature than can be derived from a black image under bright-field illumination. Generally, the same resolution can be obtained by the two techniques, but often features that have poor contrast in bright field will have considerably increased contrast in dark field.
Polarized Reflected Light Illumination — Because many metals and metallic and non-metallic phases are optically anisotropic, polarized light is particularly useful in metallography. Polarized light is obtained by placing a polarizer in front of the condenser lens of the microscope and placing an analyzer behind the eyepiece. Using this arrangement, the specimen is illuminated by plane polarized light. When this light is reflected from an isotropic surface, it remains plane polarized and remains completely extinguished when the analyzer is rotated. On the other hand, the light that is reflected from an anisotropic surface has a component that is perpendicular to the plane of polarization of the incident light and therefore the image does not remain extinguished when the analyzer is rotated, but changes alternately from dark to light with every 90 of rotation. This occurs because the optical properties of anisotropic materials vary with crystallographic direction.
Polarized light is particularly useful in metallography for revealing grain structure and twinning in anisotropic metals and alloys and for identifying anisotropic phases and inclusions. Polarized light has also been used for direct observation of phase transformations in alloys in which one or more of the phases is anisotropic.
Normanski Interference Contrast Reflected Light Illumination — This occurs when in a bright-field system, with polarized light, a prism is inserted between the objective and the vertical illuminator. A ray of light emitted from the light source is linearly polarized after it passes through the polarizer. It enters the prism and is divided into two rays of linearly polarized light.
In order to equalize the intensities of the rays and therefore, to maximize contrast of the interference fringes, the prism must be placed with its principle axis 45o with respect to the direction of vibration of the linearly polarized light. Also to avoid glare, this prism is slightly inclined toward the optical axis of the microscope.
The two divided rays intersect at a point on the plane of fringe localization that is calculated to coincide with the rear focal plane of the objective. They then pass through the objective, become parallel to each other with a slight lateral separation, and impinge on the sample. They are reflected back and are focused on the plane of localization with the aid of the objective and are recombined by the prism. These recombined rays pass through the analyzer at a retardation equal to twice the difference in height where the two rays were reflected.
The net result of all this is to produce a so-called Nomarski-type image which appears three-dimensional with one side of individual features appearing less bright than the other. This pseudo-3-dimensionality results from differences in light intensity caused by the optical path differences described above.
The Nomarski system has the advantage of enabling you to use high numerical aperture with accompanying shallow depth of field to yield excellent resolution and less confusion in the image from features below or above the exact plane of focus. This is known as optical sectioning. The color resulting from the manipulation of the light is known as optical staining. The distance of the prism from the back focal plane of the objective is critical to ensure that a so-called interference fringe of color or gray fills the entire field of view. The greatest psuedo 3-dimensional effect is achieved when the prism or the polarizer/compensator is set to the position that gives a gray background.
