metallography


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metallography

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

Metallography

 

the science of the structure of metals and alloys; part of general metals science. Metallography studies the principles of formation of structure, investigating the macrostructure and microstructure of metals (through observation by the naked eye or using optical and electron microscopes), as well as changes in the mechanical, electrical, magnetic, thermal, and other physical characteristics of a metal as a function of changes in its structure. In addition, X-ray diffraction microscopy is also used to study the microstructure. Study of the structure is necessary to find the relationship between structure and characteristics, and establishment of the principles of formation of structure is necessary to predict the properties of new alloys on the basis of the relationship. For example, the strength of single-phase alloys is related to grain size; if second-phase inclusions are present, the distance between inclusions influences the strength and recrystallization temperature of the alloy. Also, the magnetic properties of ferromagnets depend on the size and number of second-phase inclusions.

The macrostructure is characterized by the shape and distribution of large crystallites (grains), the presence and distribution of various metal defects, and the distribution of impurities and nonmetallic inclusions. The microstructure of a metallic material is determined by the shape, dimensions, relative number, and mutual arrangement of crystals of individual phases or their homogeneous aggregates. The fine structure (substructure) is the structure of the individual grains, defined by the arrangement of dislocations and other lattice defects.

The formation and change of the internal structure of a metal result from phase transformations upon heating or cooling of the metal, and also from plastic deformation, irradiation, recovery, recrystallization, or sintering. The structure of cast metal, which is formed by the appearance and growth of centers of crystallization in the melt, depends on the rate of cooling of the melt, the content of impurities, and the direction of heat dissipation. An increase in the cooling rate may, for example, lead to a decrease in grain size. The grain size may be changed by subjecting the metal to plastic deformation and recrystallization. The microstructure changes sharply when phase transformations take place in a solid metal; the transformations may be caused by changes in temperature or pressure. In this case the structure also depends on the conditions under which the transformation takes place, above all on the temperature interval and cooling rate but also on the characteristics of the lattice structure of the phases participating in the transformation. For example, the dimensions of second-phase products and the distance between them decrease if the transformation takes place at low temperature or under accelerated cooling. The substructure of the metal changes during phase transformations, and also during plastic deformation and recrystallization. For example, after strong deformation the dislocations may form accumulations that separate the grains into distinct fragments.

In addition to the principles of formation of structure, metallography studies the conditions and causes of the emergence during crystallization, plastic deformation, and recrystallization of metal texture, which causes anisotropy of the properties of polycrystalline materials.

REFERENCES

Bochvar, A. A. Metallovedenie, 5th ed. Moscow, 1956.
Hume-Rothery, W., and G. V. Raynor. Struktura metallov i splavov. Moscow, 1959. (Translated from English.)
Laboratoriia metallografii, 2nd ed. Moscow, 1965.
Smallman, R., and K. Ashbee. Sovremennaia metallografiia. Moscow, 1970. (Translated from English.)
Livshits, B. G. Metallografiia, 2nd ed. Moscow, 1971.
V. IU. NOVIKOV

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.

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

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