metallography (redirected from metallographic)

metallography

Study of the structure of metals and alloys, particularly using microscopic and X-ray diffraction techniques. Visual and optical microscopic observation of metal surfaces and fractures can reveal valuable information about the crystalline, chemical, and mechanical makeup of the material. In electron microscopes a beam of electrons instead of a beam of light is directed onto the specimen. The development of transmission electron microscopes has made it possible to examine internal details of very thin metal foils. X-ray diffraction techniques are used to study phenomena related to the grouping of the atoms themselves. See also materials science, Henry C. Sorby.

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metallography

the branch of metallurgy concerned with the composition and structure of metals and alloys

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metallography [‚med·əl′äg·rə·fē]

(metallurgy)

The study of the structure of metals and alloys by various methods, especially by the optical and the electron microscope, and by x-ray diffraction.

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Metallography

The study of the structure of metals and alloys by various methods, especially light and electron microscopy. Light microscopy of metals is conducted with reflected light on surfaces suitably prepared to reveal structural features. The method is often called optical microscopy or light optical microscopy. A resolution of about 200 nanometers and a linear magnification of at most 2000× can be obtained. Electron microscopy is generally carried out by the scanning electron microscope (SEM) on specimen surfaces or by the transmission electron microscope (TEM) on electron-transparent thin foils prepared from bulk materials. Magnifications can range from 10× to greater than 1,000,000×, sufficient to resolve individual atoms or planes of atoms.

Metallography serves both research and industrial practice. Light microscopy has long been a standard method for observing the morphology of phases resulting from industrial processes that involve phase transformations, such as solidification and heat treatment, and plastic deformation and annealing. Microscopy, both light and electron, is also indispensable for the analysis of the causes of service failures of components and products.

In light microscopy, microstructural features observed in photomicrographs include the size and shape of the grains (crystals) in single-phase materials (see illustration), the structure of alloys containing more than one phase such as steel, the effects of deformation, microcracking, and the effects of heat treatment. Other structural features investigated by light microscopy include the morphology and size of precipitates, compositional inhomogeneities (microsegregation), microporosity, corrosion, thickness and structure of surface coatings, and microstructure and defects in welds.

The electron microscope offers improved depth of field and higher resolution than the light microscope, as well as the possibility of in-place spectroscopy techniques. The scanning electron microscope images the surface of a material, while the transmission electron microscope reveals internal microstructure. Images produced by the scanning electron microscope are generally easier to interpret; in addition, the instrument operates at lower voltages, offers lower magnification, and requires less specimen preparation than is necessary for the transmission electron microscope. Consequently it is important to view a specimen with light microscopy and often with the scanning electron microscope before embarking on transmission electron microscopy.

However there are some disadvantages. Electron microscope specimens are viewed under vacuum, the instruments cost significantly more than light microscopes, electron beam damage is always a danger, and representative sampling becomes more difficult as the magnification increases.

The ionizing nature of electron irradiation means that x-ray spectrometry and electron spectrometry, both powerful tools in their own right, can be performed in both scanning electron microscopy and transmission electron microscopy. The various signals detected spectroscopically can also be used to form images of the specimen, which reveal elemental distribution among other information. In particular, the characteristic x-ray signal can be detected and processed to map the elemental distribution quantitatively on a micrometer scale in the scanning electron microscope and a nanometer scale in the transmission electron microscope. Electron spectroscopic signals permit not only elemental images to be formed but also images that reveal local changes in bonding, dielectric constant, thickness, band gap, and valence state. See Metallurgy