Microscope is an important tool for a mechanical / metallurgical engineer. Metallurgical Microscope is very helpful in determining the size, shape and distribution of various phases, including inclusions in metals. Grain size determination is also made with the help of metallurgical microscope. All these studies have a great bearing on the mechanical properties of metals. No structure-property correlation can be made without the help of metallography.
For study of metallurgical microstructures, microscope is used. Various microscopes used are as follows:
Scanning electron microscope,
Transmission electron microscope etc,
It consists of
|(a) main frame||(b) eye piece lens|
|(c) objective lens||(d) revolving nose piece for objectives|
|(e) knobs for coarse and fine adjustment||(f) light source|
|(g) light intensity knob||(h) specimen stage with x and y direction movement|
Fig: CCTV and Optical Metallurgical Microscope setup
All the above parts are fitted on the main frame. The lens from which the microstructure is observed is called an eyepiece. Objective lens is the lens near the specimen. Revolving nosepiece consist of objective lens of different magnification. The coarse and fine adjustment knobs are used to focus the image sharply. The light source provides illumination. For adjustment of brightness, the light intensity knob is used. The specimen may be scanned without touching it, by using x and y direction movement knobs. Adding or removing some of the filters may obtain better contrast.
A horizontal beam of light from a suitable source is diverted down ward by a plane glass reflector through the objective lens on surface of specimen. The light gets reflected back and passed through the objective and then eyepiece. By observing through the eyepiece, an enlarged image of illuminated area may be studied. An aperture diaphragm opening should be optimum to observe the structure fully.
Photographic attachment may be used to make the permanent record of microstructure. A close circuit television attachment is also possible so that more than one person may observe the structure at a time. A special quality metallograph with a projecting screen is used with video camera and suitable software system called as image analyzer. This helps in calculating volume fraction of various phases, nodule count, number of nodules etc. in nodular cast iron.
The resolving power of an objective lens is its ability, which makes if possible to see distinctly two separate lines in a structure. Therefore, magnification by microscope is increased to see distinctly the details of an object.
The resolution d is related to wavelength l of illumination, refractive index m of medium between the lens and specimen and the numerical aperture by the equation,
d = 0.5l/ Numerical aperture = 0.5l / msina
Where a is half the angle subtended by the maximum cone of light rays coming to the objective lens. The numerical aperture is the quantitative measurement of light gathering power of an objective lens.
Magnification depends on the focal length. The shorter the focal length of the lens, the greater will be the magnification of structure. The total magnification of a microscope is obtained by multiplication of the magnifying powers of the eyepiece and objective lenses used at that instant. The maximum magnification that can be obtained with the optical microscope is about 2000X. The principal limitation is the wavelength of the visible light, which limits the resolution of the fine detail in the metallographic specimen.
Bright field illumination gives un-etched (or cathodic) area of the specimen as bright and etched (or anodic) area of specimen as dark. This means, in this type of illumination, single-phase metal and alloy shows bright grains and dark grain boundaries. This occurs because the lenses regather the reflected light while scattered light does not get recollected. Bright field illumination is normally used to study the metallographic structure.
Dark field illumination gives exactly reverse feature to bright field illumination. In this type of illumination, only scattered light is recollected, while the reflected rays are blocked. This produces very strong image contrast. Under such illumination, it is possible to see features, which are not visible using bright field illumination. However, the photo-microscopy becomes difficult in this illumination.
Several special instruments, as mentioned below, can be used with optical microscope to obtain additional information:
1. Micro-hardness testing,
2. Hot stage microscopy, and
3. Comparison microscope
Metallographic is used to study
1. Size, shape and distribution of various phases,
2. Grain size determination and its distribution,
3. Prediction of probable mechanical properties,
4. Presence of secondary phases and its distribution,
5. Non-metallic inclusions,
6. Segregation of elements,
7. Heterogeneous conditions,
8. Abnormal structure,
9. Directionality in metal working process, and
10. Effect of heat treatments.
Metallography is widely used in industries and research centers for:
1. Routine testing during production,
2. Failure analysis, and
3. Research and development of new alloys and processes.