Metallography is the science and art of preparing a metal surface for analysis by grinding, polishing, and etching to reveal microstructual constituents. After preparation, the sample can easily be analyzed using optical or electron microscopy. A skilled technician is able to identify alloys and predict material properties, as well as processing conditions by metallography alone.

Ceramic and polymeric materials may also be prepared using metallographic techniques.

Preparing Metallographic Samples

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Metallographic samples are typically "mounted" in a phenolic or epoxy resin. Mounting a sample provides a safe and ergonomic way by which to hold a sample during the grinding and polishing operations.

After mounting, the sample is wet sanded to reveal the surface of the metal. The sample is successively ground with finer and finer grades of sandpaper to remove scratches from the sample surface. After grinding the sample with 1000 grit or finer sand paper, polishing can begin. Typically, a sample is polished with a slurry of alumina, silica, or diamond on a low knap cloth to produce a scratch free mirror finish.

After polishing, the microstructural constituents of the sample are revealed by using a suitable chemical or electrolyic etchant. The ideal etchant is dependant on sample compostition, and the microstructural feature(s) of interest.

Analysis

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Prepared samples are most often inspected using an inverted metallographic microscope. This type of microscope is sufficient for magnifications less than 1000x. If a sample must be observed at higher magnification, it will be examined in a scanning electron microscope (SEM). When equipped with energy dispersive spectrometry, SEM analysis is able to determine the composition of the sample.

Quantitative Metallograpy

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A number of techniques exist to quantitatively analyze metallographic samples. These techniques are valuable in the research and production of metal matrix composites, precipitation hardened alloys, and duplex stainless steels. Quantitative metallography relies on systematic point counts and statistical analysis to approximate the volume fraction of phases present in a microstructure.

Grain size may also be estimated using quantitative metallurgy techniques.

References

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• www.metallography.com
• ASTM E562-02 - Procedure for determining volume fraction by systematic point count.
• ASTM E1382-97 - Standard Test Method for determining average grain size.

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