Metallography and Microstructural Evaluation
By definition, metallography is the science dealing with the identification
of the constitution and structure of metals and alloys by use of low-magnification,
optical microscopy and scanning electron microscopy. The following discussion
will concentrate on optical microscopy, as well as the sample preparation
required to evaluate microstructures.
Here at Laboratory Testing, the routine optical microscopy testing is
performed to evaluate the extent of decarburization and carburization,
grain size, intergranular attack and/or corrosion, the depth of alpha
case in titanium alloys, the percent spheroidization, inclusion ratings,
the volume fraction of various phases or second phase particles in metals,
the evaluation of welds, as well as the evaluation and identification
of typical microstructures of metals to confirm structure - property
relationships such as tempered martensite in steels.
In order to identify and evaluate the microstructure of metals, it is very important to prepare the samples properly and with great care. The various steps in sample preparation 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 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
Selecting a representative 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 for the laboratory personnel to be given information about
the orientation or to be shown the rolling direction of the test specimen
that the customer provides. (Photo: Cross section of a copper tube sample
etched to show grain boundaries and twinning. This sample was used to obtain
the average grain size of the copper tubing.)Sectioning of the test sample is performed carefully to avoid altering or destroying the structure of the materials. Thus, if an abrasive saw is used, it is important that sufficient amounts of coolant or lubricant are provided to keep the sample cool. This will keep the sample from burning or overheating. 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.
After the 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.
After the sample is mounted, grinding is the next preparation step. Grinding generally involves the use of water lubricated abrasive wheels. This step is required to remove the surface damage that occurred during the sectioning step and to provide a flat surface. The grinding procedure includes 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.
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.
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.