Metal Science

Metal Science

 

the science of the relationships among the composition, structure, and properties of metals and alloys, as well as the laws governing their changes upon thermal, mechanical, and physicochemical actions. Metal science is the scientific basis for research on the composition and methods of production and processing of metallic materials with various mechanical, physical, and chemical properties. Techniques for the production of metal alloys (bronze and others), as well as the principle of increasing the hardness and strength of steel by quenching, have been known since ancient times.

Metal science was established as an independent discipline in the 19th century, initially under the name “metallography.” The term “metal science” was introduced in Germany during the 1920’s, and the retention of the term “metallography” for the study of the macrostructure and microstructure of metals and alloys was proposed. In many countries “metallography” continues to be used as before, and in some cases this area of science is called physical metallurgy.

The rise of metal science as a discipline resulted from the needs of technology. In 1831, P. P. Anosov, while working on methods for the production of damask steel, studied under a microscope the structure of a polished steel surface that had been etched with acid. In 1864, H. C. Sorby performed similar studies of the microstructure of iron meteorites and steel samples using photomicrography. In 1868, D. K. Chernov noted the existence of temperatures at which steel undergoes transformations upon heating and cooling (critical points). These temperatures were measured by F. Osmond (1888) using the thermoelectric thermometer invented by H. Le Chatelier. W. Roberts-Austen (Great Britain) used methods of thermal analysis to study the microstructure of several bimetallic systems, including iron-carbon alloys (1897). His results were critically reexamined in 1900 by H. W. Roozeboom on the basis of phase rules that had been theoretically derived by J. W. Gibbs (1873–76). Le Chatelier considerably advanced the techniques for the study of the microstructure. N. S. Kurnakov designed a recording pyrometer in 1903 and, on the basis of studies of a number of bimetallic systems performed with his co-workers (S. F. Zhemchuzhnyi, N. I. Stepanov, G. G. Urazov, and others), defined the relationships that became the foundation of the theory of singular points and physicochemical analysis. G. Tammann and his co-workers began their studies of phase diagrams in 1903. In Russia, A. A. Baikov studied phenomena of hardening in alloys (1902) and significantly improved the methods of metal science through the introduction of automatic recording of the differential curves of heating and cooling (1910) and the technique of etching of microsections at high temperatures (1909). Baikov founded the first educational laboratory of metal science in Russia at the St. Petersburg Polytechnic Institute; among the scientists who worked there were N. T. Gudtsov, G. A. Kashchenko, M. P. Slavinskii, and V. N. Svechnikov. Pioneers in the application of metal science to industrial practice were A. A. Rzheshotarskii, who started a physical metallurgy laboratory at the Obukhov Plant (1895), and N. I. Beliaev, who founded a similar laboratory at the Putilov Plant (1904). In 1908, A. M. Bochvar founded a metallographic laboratory at the Moscow Higher Technical School that was the first of its type in Moscow. Among the specialists in nonferrous metal science who worked there were I. I. Sidorin, A. A. Bochvar, and S. M. Voronov.

In 1918, A. Portevin and M. Garvin (France) established the relationship between the critical points of steel and the rates of cooling. Studies of the transitions in steel under isothermal conditions began in 1929–30 (E. Davenport and E. Bain, and R. Mehl in the USA; S. S. Steinberg and N. A. Minkevich in the USSR; and F. Wever in Germany). At the same time, the physical theory of the crystallization of metals (la. I. Frenkel’ and V. I. Danilov in the USSR, M. Volmer in Germany, and I. Stranski in Bulgaria) was developing; the experimental foundation for the theory had been laid in the early 20th century by Tammann.

Beginning in the 1920’s, an exceptionally important role in the development of metal science was played by X-ray structural analysis, which has made possible determination of the crystal structure of various phases and description of changes during phase transitions, heat treatment, and deformation (the structure of martensite and changes in the structure of solid solutions upon decomposition). Of great importance in this area was the work of G. V. Kurdiumov, S. T. Konobeevskii, and N. V. Ageev in the USSR and of A. Westgren (Sweden), W. Hume-Rothery (Great Britain), and U. Dehlinger and W. Koster (Germany). Kurdiumov, in particular, developed the theory of steel hardening and tempering and studied the main types of phase transitions in the solid state (“normal” and martensite). In the 1920’s, A. F. loflfe and N. N. Davidenkov originated the theory of the strength of crystals. The theory of phase transitions and the study of the atomic-crystalline and electron structure of metals and alloys, as well as of the nature of mechanical, thermal, electrical, and magnetic properties of metals, constituted new stages in the history of metal science as an interdisciplinary science between physical chemistry and solid-state physics.

The development of metal science in the second half of the 20th century has been characterized by a significant expansion of methodological resources. In addition to X-ray structural analysis, the atomic-crystalline structure of metals is studied by electron microscopy, which makes it possible to study the local changes in the structure of alloys, the mutual position of structural components, and defects in crystal structure. The methods of electron diffraction, neutronography, radioisotope tracers, internal friction, X-ray microanalysis, calorimetry, and magnetometry are also of considerable significance.

Metal science is arbitrarily divided into a theoretical branch, which studies the general principles of the structure of metals and alloys and the processes taking place in them as a result of various types of action, and the applied (engineering) branch, which studies the basis of industrial treatment processes (heat treatment, casting, and pressure treatment) and specific classes of metallic materials.

The main divisions of theoretical metal science are the theory of the metallic state and the physical properties of metals and alloys, crystallization, phase equilibrium in metals and alloys, diffusion in metals and alloys, phase transitions in the solid state, and the physical theory of processes of plastic deformation, hardening, degradation, and recrystallization. The subject matter of theoretical metal science is to a great extent related to metal physics.

The theory of the metallic state regards metals as an assembly of electrons moving in the periodic field of positive ions. The theoretical strength of metals, which was estimated on the basis of calculations of interatomic forces, is 100–1,000 times greater than the actual strength. The electrical resistance of metals is regarded as a consequence of disruptions of the ideal arrangement of atoms in the crystal lattice caused by vibrations and by the presence of static defects and impurities. Various phases, such as ordered solid solutions, electron compounds, interstitial phases, and sigma phases, are formed, depending on the special features of atomic interaction. The development of the electron theory of metals and alloys has played an important role in the production of alloys with special physical properties (such as superconductivity or magnetic properties).

The crystallization of metals is characterized by high rates of nucleation and crystal growth over small intervals of supercooling in which solidification takes place. The structure of a real metal ingot is determined by the laws governing crystallization, by the conditions of heat transfer, and by the effect of impurities. The mechanism of eutectic crystallization of alloys was studied by A. A. Bochvar (1935).

One of the most important subdivisions of theoretical metal science is the study of the phase equilibriums in alloys. Phase diagrams have been constructed for many binary, ternary, and more complex systems, and the temperatures of phase transitions have been determined. Under certain conditions (for example, rapid cooling), metastable states may arise with a relative minimum of free energy (for the given thermodynamic conditions). The most important examples of such states are marten-site in steel and supersaturated solid solutions of metals, such as Al-Cu. The kinetics of phase transitions and the conditions under which metastable states arise are determined by the extent of deviation of the system from equilibrium, by the mobility of atoms (diffusion characteristics), and by the structural and chemical correspondence of the resultant and initial phases.

Transformations in the solid state (phase transitions) under conditions of strong atomic interactions in crystal phases are accompanied by the generation of stress fields. Under certain conditions and in the presence of polymorphic modifications, an ordered rearrangement of the crystal lattice is observed at the phase boundary (martensite transformation). In the range of temperatures in which relaxation processes proceed rapidly, crystals of a new phase may be formed by disordered diffusional transitions of individual atoms (“normal” transformation). The kinetic diagrams of austenite transformations are of importance in the physical metallurgy of ferrous alloys. Processes of disproportionation of supersaturated solid solutions take place frequently in metal alloys. In many cases, the most profound changes in properties occur before the generation of the second phase. X-ray research has shown that these changes are related to the processes of redistribution of atoms in the lattice of the matrix and to the formation of enriched zones within the matrix. The equilibriums and kinetics of the phase transitions may vary to a considerable degree as a result of the action of high pressures. Because of the forces of chemical interaction between atoms of various elements.in unsaturated solid solutions, the redistribution of the atoms of elements may also occur. An ordered arrangement of atoms in particular nodes of the crystal lattice arises in substitutional solid solutions (for example, CuAl) and interstitial solid solutions (martensite, Ta-O, and other materials). Intraphase inhomogeneities (segregations) appear in some cases.

The physical theory of plastic deformation and defects of crystal structure is of great significance for the development of metal science. The discrepancies between the calculated and experimental values of the strength led in 1933–34 to a hypothesis on the presence of special defects (dislocations) in crystals. Displacement of the dislocations under the action of relatively small forces leads to plastic deformation. The existence of dislocations was confirmed by research conducted using various methods, particularly electron diffraction microscopy of thin foil. Methods of internal friction made possible determination of the role of point defects (vacancies). The presence of vacancies affects the physical properties of crystals and plays an important role in diffusion processes during heat treatment, relaxation of metals, recrystallization of metals, and sintering. The study of the properties of defect-free filamentary crystals has proved the correctness of theoretical estimates of strength. In cases of practical importance, an increase in strength is achieved by increasing the density of dislocations (for example, by plastic deformation, martensite transformation upon quenching, or a combination of the two). The impurities may accumulate near the dislocations and block them. One of the strongest manifestations of the effect of the real structure on processes in metals and alloys is the differences in the rates of diffusion and redistribution of the elements along the boundaries and within the volume of polycrystals. In some cases, very small quantities of impurities change the rate of boundary diffusion. Since numerous processes of decay of solid solutions begin mainly in the region of the boundary layers, small quantities of impurities may substantially affect the kinetics of the processes and the final structure. The interaction between the dislocations and interstitial impurities (such as carbon and nitrogen in iron) is one of the main causes of cold shortness of metals with body-centered cubic lattices. The motion and interaction of the dislocations leads to strengthening or weakening of metals and to creep, polygonization, and recrystallization. The most effective means of changing the structure and properties of metallic materials are alloying, heat treatment, surface hardening, thermochemical treatment, and thermomechanical treatment.

Applied (engineering) metal science is the study of the composition, structure, processes of treatment, and properties of various specific classes of metallic materials (for example, iron-carbon alloys, structural steel, stainless steel, heat-resistant alloys, aluminum alloys, magnesium alloys, and cermets). In connection with the development of new areas of technology, problems have arisen concerning the study of the behavior of metals and alloys subjected to radiation, very low temperatures, and high pressures.

REFERENCES

Bunin, K. P. Zhelezouglerodistye splavy. Kiev-Moscow, 1949.
Fizicheskie osnovy metallovedeniia. Moscow, 1955.
Bochvar, A. A. Metallovedenie, 5th ed. Moscow, 1956.
Kurdiumov, G. V. lavleniia zakalki i otpuska stali. Moscow, 1960.
Livshits, B. G. Metallografiia. Moscow, 1963.
Fizicheskoe metallovedenie, fascs. 1–3. Moscow, 1967–68. (Translated from English.)

R. I. ENTIN

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