metallography

Metallography is the microscopic examination of metals and alloys to reveal their internal structure, grain boundaries, phases, and defects. A metallographer prepares a polished cross-section of material, etches it with acid or other reagent to make structural features visible, then observes it under an optical or electron microscope at magnifications from 50x to 100,000x or higher. The technique answers the question: what is actually inside this piece of metal, and how is it arranged?

The preparation step is critical and time-consuming. A sample is cut from the material, mounted in resin or bakelite, then ground progressively with finer abrasives (typically silicon carbide papers from 120 grit to 2000 grit or finer) until flat and smooth. Polishing follows, often with alumina or diamond paste on soft cloth wheels. The polished surface is then etched, usually with dilute acids like nitric acid or ferric chloride, which attack grain boundaries and different phases at different rates, creating contrast visible under magnification. Without etching, most steel or aluminum looks blank under the microscope.

What metallography reveals

Optical metallography exposes grain size and shape, which directly govern hardness, toughness, and ductility. Coarse grains generally make material weaker and more brittle. It shows phases present in an alloy: ferrite, austenite, martensite, carbides, and so on. It reveals defects: porosity, segregation, cracks, inclusions, and improper heat treatment. Scanning electron microscopy (SEM) with elemental analysis (EDS, energy dispersive spectroscopy) identifies which elements are present in specific regions. Transmission electron microscopy (TEM) resolves features at the nanometer scale. Fractography, a related discipline, examines broken surfaces to determine whether failure was brittle, ductile, fatigue, or stress-corrosion cracking.

The name derives from the Greek metallum (metal) and graphia (description or writing). Metallography is both art and science. Success depends on skill in preparation, choice of etchant, and interpretation of what appears. A poorly polished sample or wrong etchant yields misleading images. Experience teaches a metallographer to recognize telltale patterns: the Widmanstätten structure in titanium alloys, the cored dendrites in as-cast steel, the tempered martensite in heat-treated tool steel.

Metallography is the primary tool for quality control in foundries, forges, and heat treat shops. It validates that material meets specification before delivery, diagnoses failures in service, and supports development of new alloys and processes. Every metallurgical laboratory contains at least one optical microscope and specimen preparation equipment. For high-stakes applications, such as aerospace forgings or bearing steel, metallographic examination is mandatory.