Digital imaging to 1000x
Examination of the microstructure of a material provides information used to determine if the structural parameters are within certain specifications. The analysis results are used as a criterion for acceptance or rejection.
Microstructural examination is generally performed using optical or scanning electron microscopes to magnify features of the material under analysis. The amount or size of these features can be measured and quantified, and compared to acceptance criteria. These examinations are often used in failure analysis to help identify the type of material in question and determine if the material received the proper processing treatments. Metallurgical examinations may evaluate:
- Extent of decarburization and carburization, grain size, intergranular attack or corrosion
- Depth of alpha case in titanium alloys
- Percent spheroidization
- Inclusion ratings
- Volume fraction of various phases or second phase particles in metals
Sample Preparation
In order to identify and evaluate the microstructure of material, it is very important to prepare the test sample carefully and properly. The various steps in sample preparation for microstructural examination include:
- Selecting a representative sample of the materials
- Sectioning the sample to avoid altering or destroying the structure of interest
- Mounting the section without damage to the test sample
- Grinding to achieve a flat sample with a minimum amount of damage to the sample surface
- Polishing the mounted and ground sample
- Etching in the proper etchant to reveal the microstructural details
The Process
Selecting
Selecting a representative test sample to properly characterize the microstructure or the features of interest is a very important first step. For example, grain size measurements are performed on transverse sections, whereas general microstructure evaluations are performed on longitudinal sections. Therefore, it is important to provide the laboratory with information about the orientation or the rolling direction of the test specimen.
Sectioning
Test samples are carefully sectioned to avoid altering or destroying the structure of the materials. If an abrasive saw is used, it is important to keep the sample cool with coolant or lubricant so it doesn’t burn or overheat. However, no matter how carefully abrasive sawing or electric discharge machining is performed, a small amount of deformation occurs on the sample surface. This deformation must be removed during subsequent preparation steps.
Mounting
After the test sample is sectioned to a convenient size, it is mounted in a plastic or epoxy material to facilitate handling and the grinding and polishing steps. Mounting media must be compatible with the sample with respect to hardness and abrasion resistance. Typical mounting materials are thermosetting phenolics such as Bakelite, and thermoplastic materials such as methyl methacrylate (Lucite). Mounting involves putting the sample in a mold and surrounding it with the appropriate powder. When the mold is heated and pressurized at the correct levels, setting or curing of the media occurs. The mounted sample is removed from the mold. If the use of heat or pressure might alter the structure of the sample of interest, then castable cold mounting materials such as epoxies are employed.
Grinding
Grinding follows mounting to remove the surface damage that occurred during the sectioning step and to provide a flat surface. Grinding generally involves the use of water lubricated abrasive wheels and the use of a series of progressively finer abrasive grits. This procedure provides a flat surface that is nearly free of the disturbed or deformed metal that has been introduced by the previous sample preparation steps.
Polishing
The polishing step removes the last thin layer of the deformed metal. It leaves a properly prepared sample, ready for examination of the unetched characteristics such as inclusion content or any porosity that may exist.
Etching
The final step that might be used is etching to show the microstructure of the test sample. This step reveals features such as grain boundaries, twins and second phase particles not seen in the unetched sample.
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