metallography

Metallography is the science of evaluating the microstructure of metals and their alloys by preparing specimens and examining them with light or electron microscopes. The microstructure includes grains and grain boundaries, phases, precipitates, inclusions, and defects that influence mechanical properties such as strength, hardness, ductility, and toughness.

Specimen preparation is central to metallography. It involves sectioning a specimen, mounting it in a resin, grinding with progressively finer abrasives, and polishing to a mirror finish. Most features are revealed after etching with a chemical reagent that selectively attacks constituents at different rates. Etchant choice depends on the material; common examples include Nital for steels and Keller or similar reagents for aluminum alloys. Etching exposes grain boundaries, phase distribution, and defect structures.

Imaging methods include optical metallography (light microscopy, often with polarized light) for general microstructure, as well as scanning or transmission electron microscopy for higher resolution. Electron backscatter diffraction (EBSD) and other orientation techniques provide crystallographic information. Quantitative analysis may involve grain-size measurement, phase fraction estimation, and porosity or inclusion counting, typically aided by image analysis software. Standards from organizations such as ASTM and ISO govern specimen preparation, imaging, and reporting for metallographic analysis.

Applications of metallography span quality control in manufacturing, failure analysis, heat-treatment evaluation, materials development, and academic research. While powerful, metallography requires careful handling and interpretation, as preparation steps can introduce artifacts.