MATERIALS SCIENCE AND TESTING

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MICROSCOPY

Metal microscope

Angular resolution which is also called as spatial resolution, describes the ability of any device

that form an image such as the eye and telescopes, to distinguish small details of an object. The

eye which is an organ acts as an instrument in the human optical system. The resolution of the

human eye is 1 arcminute. It means that in the normal visual range (~250 mm) the eye is able

to detect a ~0.1 mm pixel (depending on the personal capabilities). So to see smaller objects

than that with the human eye, magnifiers or microscopes need to be used.

There are many types of microscopes. The most common (and the first to be invented) is the optical microscope, which uses light to image the sample. Other major types of microscopes are the electron microscope (e.g. transmission electron microscope: TEM, scanning electron microscope: SEM), the ultramicroscope, and the various types of scanning probe microscope (e.g. atomic force microscope: AFM, scanning tunnel microscope: STM). A compound light microscope is an optical instrument that uses visible light to produce

a magnified image of an object (or specimen) that is projected onto the retina of the eye or onto

an imaging device. The word ‘compound’ refers to the fact that two lenses, the objective lens

and the eyepiece (or ocular), work together to produce the final magnification Mfinal of the image

such that:

Mfinal = Mobjective × Mocular

Table 1. Different types of optical microscopy.

Microscopy Used for Contrast is due to Remarks

Reflected light

· observation of non-transparent specimen

· e.g. metals, ceramics, composites

· variations in surface topography and differences in reflectivity

· e.g. different phases, different grain orientations, boundary regions

· specimen preparation is necessary for some features

Transmitted light

· observation of transparent specimen

· specimen is in the form of a thin slice (~10-50 μm)

· e.g. minerals, rocks, glasses, ceramics, polymers

· differences in the absorption of light through different regions

· specimen preparation is necessary

Polarised light

· observation of grains, grain orientation and thickness

· differences in birefringence and thickness of the specimen

· specialised use of the transmission mode

Optical microscopes have a wide variety of applications (Table 1). Since

transparent preparations cannot be made from metals, microscopic examination cannot be performed by transmitted light, but reflected light. Most metallurgical microscopes use the Le Chatelier or inverted system (Figure 1).

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Figure 1. Schematic figure of the metal microscope.

The sample or specimen is above the vertical axis objective lens system. The light from the light source goes through several lenses, prisms and a plan-parallel plate to get through the objective lens to the surface of the sample. The resolution of the objective lens (d) is the smallest subject distance whose endpoints are mapped as a separate points by the lens. The nsin called the numerical aperture. Numerical aperture determines the resolving power of an objective, the higher the numerical aperture of the system, the better the resolution. The most commonly used immersion medium (after air) is cedar oil, in which n = 1.52.

sinn2d (nm),

where: - the wavelength of the illuminating light (450-650 nm); n - refractive index of the immersion fluid between the sample and the objective

(0.9-1.52); - half angle of the cone formed by the optical axis and the light entering the

objective (Figure 2).

Figure 2. Half angle of the cone formed by the optical axis and the light entering the objective,

in air (a) in oil (b).

The useful magnification is the quotient of the human eye lens and the objective lens. With further magnification the image will only be blurred, and a portion of the viewing area will be lost. Therefore, optical microscopes are designed to have maximum 2000 times magnification. The depth of field (the subject distance difference within the image sharpness does not deteriorate) in the case of optical microscopes decrease to 1 m with increasing the

Objective a)

Oil

Objective

Specimen Specimen

Air

b)

Sample

Ocular

Objective

Light

source

Plane-parallel

plate

Prism

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magnification. This is the basic reason that only flat samples can be examined. With higher magnification than the useful magnification the contrast of the image reduces. Preparation of specimens When preparing samples for microscopy, it is important to produce something that is representative of the whole specimen. It is not always possible to achieve this with a single sample. Indeed, it is always good practice to mount samples from a material under study in more than one orientation. The variation in material properties will affect how the preparation should be handled, for example very soft or ductile materials may be difficult to polish mechanically. Cutting a specimen It important to be alert to the fact that preparation of a specimen may change the microstructure of the material, for example through heating, chemical attack, or mechanical damage. The amount of damage depends on the method by which the specimen is cut and the material itself. Cutting with abrasives may cause a large amount of damage, whilst the use of a low-speed diamond saw can cause fewer problems. There are many different cutting methods, although some are used only for specific specimen types. Mounting Mounting of specimens is usually necessary to allow them to be handled easily. It also minimises the amount of damage likely to be caused to the specimen itself. Specimens can be hot mounted (at around 200°C) using a mounting press, either in a thermosetting plastic (e.g. phenolic resin), or a thermosoftening plastic (e.g. acrylic resin). If hot mounting will alter the structure of the specimen a cold-setting resin can be used, e.g. epoxy, acrylic or polyester resin. Porous materials must be impregnated by resin before mounting or polishing, to prevent grit, polishing media or etchant being trapped in the pores, and to preserve the open structure of the material. A mounted specimen usually has a thickness of about half its diameter, to prevent rocking during grinding and polishing. The edges of the mounted specimen should be rounded to minimise the damage to the grinding and polishing discs (Figure 3).

Figure 3. Specimen mounting (before and after mounting into resin)

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Grinding Surface layers damaged by cutting must be removed by grinding. Mounted specimens are ground with rotating discs of abrasive paper flushed with a suitable cooling agent to remove debris and heat, for example wet silicon carbide paper. The coarseness of the paper is indicated by a number: the number of grains of silicon carbide per square inch. So, for example, 180 grit paper is coarser than the 1200 grit paper. The grinding procedure involves several stages, using a finer paper (higher number) for each successive stage. Each grinding stage should remove the scratches originate from the previous coarser paper. This is more easily achieved by orienting the specimen perpendicular to the previous scratches, and watching for these previously oriented scratches to be obliterated. Between each grade the specimen is washed thoroughly with soapy water to prevent contamination from coarser grit present on the specimen surface. Typically, the finest grade of paper used is the 1200, and once the only scratches left on the specimen are from this grade, the specimen is thoroughly washed with water, followed by alcohol and then allowed to dry. Cleaning specimens in an ultrasonic bath can also be helpful, but is not essential. Polishing Polishing discs are covered with soft cloth impregnated with abrasive diamond particles and an oily lubricant. Particles of two different grades are used : a coarser polish - typically with diamond particles 6 microns in grain size which should remove the scratches produced from the finest grinding stage, and a finer polish – typically with diamond particles 1 micron in grain size, to produce a smooth (mirror like) surface. Before using a finer polishing wheel the specimen should be washed thoroughly with warm soapy water followed by alcohol to prevent contamination of the disc. Mechanical polishing will always leave a layer of disturbed (deformed) material on the surface of the specimen, if the specimen is particularly susceptible to mechanical damage (or excessive force is used in the grinding and polishing stages) debris can become embedded in the surface and plastic deformation may exist below the surface. Electropolishing or chemical polishing can be used to remove this, leaving an undisturbed surface. Etching Etching is used to reveal the microstructure of the metal through selective chemical attack. It also removes the thin, highly deformed layer introduced during grinding and polishing. In alloys with more than one phase, etching creates contrast between different regions through differences in topography or reflectivity (Figure 4). The rate of etching is affected by crystallographic orientation, the phase present and the stability of the region. This means contrast may arise through different mechanisms – therefore revealing different features of the sample. In all samples, etchants will preferentially attack high energy sites, such as boundaries and defects.

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Figure 4. Examples of how contrast arise from an etched surface.

The specimen is etched using a reagent. For example, for etching stainless steel or copper and its alloys, a saturated aqueous solution of ferric chloride, containing a few drops of hydrochloric acid is used. This is applied using a cotton bud wiped over the surface a few times. The specimen should then immediately be washed in alcohol and dried. An example is shown in Figure 5.

Figure 5. Result of etching in the case of ledeburit.

Following the etching process there may be numerous small pits present on the surface. These are etch pits caused by localised chemical attack and, in most cases, they do not represent features of the microstructure. They may occur preferentially in regions of high local disorder, for example where there is a high concentration of dislocations. If the specimen is over etched, ie. etched for too long, these pits tend to grow, and obscure the main features to be observed. If this occurs it may be better to grind away the poorly etched surface and re-polish and re-etch, although it is important to remember what features you are trying to observe – repeated grinding a very thin sample may leave nothing to see. Metallographic examinations during the laboratory There are prepared specimens for the measurements, which are carried out on an Epityp 2 metallurgical microscope (Figure 6). As usually the deduction have to made by evaluating a very small surface, so it is important to search for a typical area. The entire surface of the specimen had to be scanned before choosing the representative area. If there are several different representative areas all of them have to be investigated.

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Figure 6. Epityp 2 metal microscope.

Calibration of the ocular scale with a glass calibration scale.

Measuring average grain size on an Al-Mg2Si alloy specimen (Figure 7 and 8).

a) b) Figure 7. Typical microscope pictures of the Al-Mg2Si alloy specimen.

Figure 8. Overall dimensions of a grain (a and b)

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Measuring average grain size on a steel specimen (Figure 9)

Figure 9. Typical microscope picture of the steel specimen.

References

Douglas B. Murphy: Fundamentals of light microscopy and electronic imaging.

Wiley-Liss, Inc, 2001.

(online: www.biology.uoc.gr/courses/BIOL493/documents/book.pdf)

Optical Microscopy and Specimen Preparation. 2009

(online: www.doitpoms.ac.uk/tlplib/optical-microscopy/index.php)