11-08-2012, 12:11 PM
Microstructure Analysis
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Theory
Metallurgy is the study of microstructural features of materials. The structure studied by
metallography are indicative of the properties and hence the performance of material in service.
Typical application of metallography techniques in research centres and industry may include:
a. To monitor metal alloy heat treatment
b. To measure the thickness of coating
c. To evaluate/examine the weld or braze
d. To evaluate corrosion, etc.
e. For failure analysis
In this technique, planar surface is prepared by sectioning followed by mounting in a
thermosetting resin prior to grinding and polishing to obtain a reflective surface. In order to
delineate the microstructure chemical or other etching method is often employed prior to
microscopy investigation.
a) Sectioning and cutting
The areas of interest forming the metallography specimens need to be sectioned for ease of
handling. Depending on the type of material, the sectioning operation can be done by using
abrasive cutter (for metal and metallic composite), diamond wafer cutter (ceramics, electronics,
and minerals) or thin sectioning with a microtome (plastics).
In order not to damage the specimen, proper cutting requires the correct selection of abrasive
cutting wheel, proper cutting speed & cutting load and the use of coolant.
Mounting
The mounting operation accomplishes three important functions:
1. To protects the specimen edge and maintain the integrity of materials surface features.
2. Fill voids in porous materials.
3. Improves handling of irregular shaped samples.
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Samples for microstructure evaluation are typically encapsulated in a plastic mount for handling
during sample preparation. Large sample or samples for macrostructure evaluation can be
prepared without mounting.
The metallography specimen mounting is done by encapsulating the specimen into:
1. A compression/hot mounting compound (thermosets – e.g. phenolics, epoxies or
thermoplastics – e.g. acrylics)
2. A castable resin/cold mounting (e.g. acrylics resins, epoxy resins and polyester resins)
c) Grinding
Grinding is required to ensure the surface is flat & parallel and to reduce the damage created
during sectioning. Grinding is accomplished by decreasing the abrasive grit size sequentially to
obtain the required fine surface finish prior to polishing.
It is important to note that the final appearance of the prepared surface is dependent on the
machine parameters such as grinding/polishing pressure, relative velocity distribution and the
direction of grinding/polishing.
d) Polishing
For microstructure examination a mirror/reflective finish is needed whereas a finely ground finish
is adequate for macrostructure evaluation. Polishing can be divided into two main steps:
Rough polishing
The purpose is to remove the damage produced during grinding. Proper polishing will maintain
the specimen flatness and retain all inclusions or secondary phases by eliminating the previous
damage and maintaining the specimen integrity.
Fine polishing
The purpose is to remove only surface damage.
e) Etching
Etchans are specially formulated for the specific material and evaluation objectives. Etching
alters the microstructural features based on composition; stress or crystal structure and it will
develop the surface topology, which can be visible in the microscope.
Typically, chemical etching involve immersing the polished surface in the prepared chemical
solution for a specified time (usually seconds) followed by rinsing the etched specimen under
running tap water and drying.
Microscopic Analysis
For microscopic analysis, a reflective surface is required. The analysis can be done by using a
metallurgy microscope.