Introduction

Metals -- simple substances, which under normal conditions have characteristic properties: high electrical and thermal conductivity, negative temperature coefficient of electrical conductivity, the ability to reflect electromagnetic waves well (brilliance and opacity), high strength and ductility.

The properties of metals can change significantly at very high pressures. Many metals, depending on temperature and pressure, can exist in the form of several crystalline modifications.

More than 80 chemical elements and many metal alloys have similar metallic properties. The number of metal alloys used in engineering is in the thousands and is constantly growing in accordance with the emerging new and diverse requirements of many industries.

The properties of metals are due to their crystal structure and the presence in their crystal lattice of numerous unrelated atomic nuclei mobile conduction electrons.

Metal alloys have much in common with metals in properties, so they are often referred to as metals.

Metals (alloys) in industry are divided into two main groups: ferrous and non-ferrous metals.

Ferrous metals are an alloy of iron and carbon, which may contain more or less other chemical elements. Cobalt, nickel, as well as manganese, close to them in properties, are often referred to as ferrous metals. Ferrous metals are the most widespread, due to the relatively high content of iron in the earth's crust, its low cost, high mechanical and technological properties.

Non-ferrous metals are divided into the following groups according to their properties:

light (Be, Mg, Al, Ti), with a relatively low density - up to 5000 kg / m 3;

refractory (Ti, Cr, Zr, Nb, Mo, W, V, etc.) with a melting point higher than that of iron (1539°C);

noble (Ph, Pd, Ag, Os, Pt, Au, etc.) with chemical inertness:

uranium (U, Th, Pa) - actinides used in nuclear technology;

rare earth metals (REM), lanthanides (Ce, Pr, Nd, Sm, etc.) and similar yttrium and scandium, used as additives to various alloys;

alkaline earth metals (Li, Na, K) used as coolants in nuclear reactors.

Classification of metal alloys by chemical composition, based on the indication of the main component of the alloy (iron, copper, aluminum, etc.), has a traditional character, and is most widely used.

Macro-, micro- and atomic structure of metals and alloys is studied by metallography.

The macrostructure is the structure of the metal, visible to the naked eye or with a magnifying glass in a fracture or on an etched section. The microstructure of the structure of a metal, observed under optical or electron microscopes, which makes it possible to enlarge the area under consideration from ten times to hundreds of thousands of times.

The atomic structure of metals is the spatial arrangement of atoms in a crystal lattice. This type of structure is investigated using X-ray structural analysis.

metal structure

All metals have a crystalline structure. Arranged in one way or another, atoms form an elementary cell of a spatial crystal lattice. The cell type depends on the chemical nature and state of the metal. The crystalline state, first of all, is characterized by a certain, regular arrangement of atoms in space. This leads to the fact that in a crystal each atom has the same number of nearest atoms - neighbors located at the same distance from it. In the process of crystallization, positively charged ions, arranged in series in the form of elementary crystal lattices, form crystals in the form of grains or dendrites. All metals and alloys have a crystalline structure. The resulting crystals grow, crystallize from the liquid melt at first freely, do not interfere with each other, then they collide and the growth of crystals continues only in those directions where there is free access to the liquid metal. As a result, the original geometrically correct shape of the crystals is violated. After hardening, the grains and dendrites have an irregular, geometrically distorted shape.

Figure 1. Scheme of crystallization: a - in the form of grains; b - in the form of dendrites.

The desire of metal atoms (ions) to settle down, perhaps closer to each other, denser, leads to the fact that the number of combinations of mutual arrangement of metal atoms in crystals is small.

There are a number of schemes and methods for describing variants of the mutual arrangement of atoms in a crystal. The mutual arrangement of atoms in one of the planes is shown in the atomic arrangement diagram (Figure 2).

Figure 2. Elementary crystal cell (simple cubic).

Imaginary lines drawn through the centers of atoms form a lattice, at the nodes of which atoms (positively charged ions) are located; this is the so-called crystallographic plane. Multiple repetition of parallel crystallographic planes reproduces a spatial crystal lattice, the nodes of which are the location of atoms (ions). The distances between the centers of neighboring atoms are measured in angstroms (1 A = 1 * 10 -8 cm) or in kiloisks - kX (1kX = 1.00202 A). The mutual arrangement of atoms in space and the value between the atomic distances are determined by X-ray diffraction analysis. The arrangement of atoms in a crystal is very conveniently depicted in the form of spatial schemes, in the form of the so-called elementary crystal cells. Under the elementary crystal cell is meant the smallest complex of atoms, which, when repeated in space, allows you to reproduce the spatial crystal lattice. The simplest type of crystal cell is the cubic lattice. In a simple cubic lattice, the atoms are not packed tightly enough. The desire of metal atoms to take places closest to each other leads to the formation of lattices of other types: cubic body-centered (Figure 3, a), face-centered cubic (Figure 3, b) and hexagonal close-packed (Figure 3, c).

Figure 3. Elementary crystal cells: a - cubic body-centered; b - cubic face-centered; c - hexagonal close-packed.

metal thermal conductivity electromagnetic temperature

Circles representing atoms are located in the center of the cube and along its vertices (body-centered cube), or in the centers of the faces and along the vertices of the cube (face-centered cube), or in the form of a hexagon, inside of which a hexagon is also half inserted, three atoms of the upper plane of which are inside hexagonal prism (hexagonal lattice).

The crystal lattice imaging method shown in Figure 3 is conditional (as in any other). Perhaps, the image of atoms in a crystal lattice in the form of contacting balls is more correct (left diagrams in Figure 3). However, such an image of the crystal lattice is not always more convenient than the accepted one (right diagrams in Figure 3).

The dimensions of the crystal lattice are characterized by parameters, or lattice periods. The cubic lattice is determined by one parameter - the length of the edge of the cube a (Figure 3, a, b). The parameters have values ​​on the order of atomic dimensions and are measured in angstroms.

For example, the lattice parameter of chromium, which has the structure of a body-centered cube, is 2.878 A, and the lattice parameter of aluminum, which has the structure of a face-centered cube, is 4.041 A.

The lattice parameter is an extremely important characteristic. Modern methods x-ray studies allow you to measure the parameter with an accuracy of the fourth or even fifth decimal place, i.e. one ten-thousandth - one hundred-thousandth of an angstrom.

From the consideration of the schemes of crystal lattices (Figure 3), if we assume that the atoms are, as it were, elastic balls touching each other, it follows that the lattice parameter a and the atomic diameter d are connected by simple geometric relationships.

For a body-centered cube

For a face-centered cube

Taking the shape of a sphere for an atom, we can calculate that in a cubic body-centered lattice, atoms occupy 68% of the volume, and in a cubic face-centered (as in a hexagonal close-packed) 74%, i.e. in the second case, the atoms are arranged more densely, more compactly.

For metals, a hexagonal lattice is common (Figure 3, c).

If the layers of atoms touch each other, i.e., three atoms depicted inside the lattice (Figure 3, c) touch the atoms located on the upper and lower planes, then we have the so-called hexagonal close-packed lattice.

The dimensions of the hexagonal close-packed lattice are characterized by a constant value c/a=1.633. For other values ​​of c/a, a loosely packed hexagonal lattice is obtained.

The face-centered cubic and hexagonal lattices represent the densest way to stack balls of the same diameter.

Some metals have a tetragonal lattice (Figure 4); it is characterized by the fact that the edge c is not equal to the edge a. The ratio of these parameters characterizes the so-called degree of tetragonality. For c/a = 1, a cubic lattice is obtained. Depending on the spatial arrangement atoms, a tetragonal lattice (as well as a cubic one) can be simple, body-centered, and face-centered.

Figure 4. Tetragonal lattice

The number of atoms in mutual contact is essential for the properties of a given metal or alloy. This is determined by the number of atoms equally spaced at the nearest distance from any atom.

The number of atoms that are at the closest and equal distance from a given atom is called the coordination number. So, for example, an atom in a simple cubic lattice has six nearest equidistant neighbors, i.e., the coordination number of this lattice is 6.

The central atom in a body-centered lattice has eight nearest equidistant neighbors, i.e., the coordination number of this lattice is 8. The coordination number for a face-centered lattice is 12. In the case of a hexagonal close-packed lattice, the coordination number is 12, and in the case of c/a? 1.633 each atom has six atoms at one distance and six at another (coordination number 6).

For a short designation of the crystal lattice, indicating in this designation the type of crystal lattice and the coordination number, one of the following systems was adopted (Table 1).

Table 1

Each metal has a specific crystal lattice.

An essential characteristic of the crystal structure is the number of atoms per unit cell.

In about. c. To the lattice, the atoms located at the top belong to eight elementary cells. Consequently, each atom contributes only one-eighth of its volume to a given unit cell. The central atom belongs entirely to the given elementary cell. Therefore, there are 1/8 * 8+1=2 atoms per elementary cell.

In a face-centered cube, there are four atoms per unit cell (1/8 × 8 atoms from the number located at the cube vertices + 1/2 × 6=3 atoms from the number of centering faces).

Typical metallic elements, located on the left side of D. I. Mendeleev’s table, crystallize in close packing, i.e., into simple crystalline cells with a large coordination number. Typical metal gratings are, as mentioned, gratings o. c. k., g. c. to. and g. p. at. Indeed, almost all metals, starting from zinc, cadmium and mercury and to the left, in most cases have simple lattices.

For non-metallic elements, a small value of the coordination number (K4 and less) is characteristic. Non-metals have a lower density and a lower specific gravity than metals.

Conclusion

Metals are simple substances that have free electrons not associated with certain atoms, which are able to move throughout the volume of the body. This feature of the state of a metallic substance determines the properties of metals.

Metal atoms easily donate their outer (valence) electrons, turning into positively charged nones. The free electrons given away by the atoms continuously chaotically, i.e., randomly, move throughout the entire volume of the metal. Such free electrons are often referred to as an electron gas. Positively charged ions, when colliding with free electrons, can turn into neutral atoms for some time.

Thus, metals consist of positively charged ions ordered in space, electrons moving among them, and a small number of neutral atoms. The metals are aluminium, iron, copper, nickel, chromium, etc.

Alloys are systems consisting of two or more metals or metals and non-metals. Alloys have all the characteristic properties of metals. For example, steel and cast iron are alloys of iron with carbon, silicon, manganese, phosphorus and sulfur; bronze - an alloy of copper with tin or other elements; brass - an alloy of copper with zinc and other elements.

In industry, alloys obtained by fusion of components with subsequent crystallization from a liquid state are widely used, much less - alloys obtained by sintering.

In the process of crystallization from the molten (liquid) state of a metal or alloy, positively charged ions and neutral atoms are grouped in a strictly defined sequence, forming crystal lattices - the correct ordered arrangement of atoms in an elementary cell. The crystal lattices of metals and alloys can be of various types: volume-concentrated cubic (b.c.c.), face-centered cubic (f.c.c.), hexagonal close-packed (h.c.c.). A volume-concentrated cubic lattice is formed by iron, copper, aluminum, lead, etc.; hexagonal close-packed - zinc, magnesium, cobalt, etc.

To characterize the crystal lattice, it is necessary to know the lattice periods - the distance a and c between the centers of atoms or ions located at the lattice nodes. The grating period is measured in angstroms (1A=10 -8 cm).

In the process of crystallization, positively charged ions, arranged in series in the form of elementary crystal lattices, form crystals in the form of grains or dendrites. All metals and alloys have a crystalline structure. The resulting crystals grow, crystallize from the liquid melt at first freely, do not interfere with each other, then they collide and the growth of crystals continues only in those directions where there is free access to the liquid metal. As a result, the original geometrically correct shape of the crystals is violated. After hardening, the grains and dendrites have an irregular, geometrically distorted shape.

When heated, the heat absorbed by metals is spent on the vibrational movements of atoms and, as a result, on the thermal expansion of the metal. When melting, the volume of metals increases by 3-4%. As the temperature rises, the vibrational motions of atoms or ions increase, the crystal grains disintegrate, and the alloy, passing through the solid-liquid state, turns into a melt.

The transition to the liquid state does not lead to the complete destruction of the crystal structure. In the melt of metals and alloys, there are always the smallest areas in which the original, hereditary structure of the metal, close to crystalline, is preserved. In addition, refractory particles (furnace lining residues, impurities of other elements) are always present, which can form additional crystallization centers and cause the onset of crystallization. On the artificial creation of crystallization centers in the melt with a simultaneous change in its cooling rate, the control of alloy crystallization is based in order to obtain a given structure of the alloy in the solid state.

Literature

1. Gulyaev A.P. Metal science. - 5th ed., revised. and additional - M.: Metallurgy Publishing House, 1977.

2. Materials science for plumbers, fitters, construction machine operators: Textbook for environments. prof.-tech. schools / Yu.G. Vinogradov, K.S. Orlov, L.A. Popova. - M.: Higher. school, 2nd ed., 1989.

3. Materials science. Lecture 5. Z.O.

4. Moizberg R.K. Materials Science, 1991.

5. Khanapetov M.V. Welding and cutting of metals. - 3rd ed., revised. and additional - M.: Stroyizdat, 1988.

In the manufacture of machines and working installations, metals and their alloys have become the most used.
Metals- these are substances that have high electrical and thermal conductivity, gloss, malleability and other properties that are easy and not very amenable to metalworking.

In industry, all metals and alloys are divided into two categories: colored And black. So called black metals- this is pure iron and alloys based on its material. TO colored- includes other types of metals. For the correct choice of metal for the manufacture of structures of mechanisms with further analysis of its use, mechanical and other properties that affect the reliability and performance of machines, you need to know the internal structure, mechanical, physico-chemical and technological properties, as well as what method to do metal processing and needs whether the material is in metal cutting (if the material needs to be processed by cutting, then it is better to do this using plasma metal cutting).

In the solid state, all metals and alloys have a crystalline structure. Metal molecules (atoms, ions) in space are arranged in a strictly defined order and form among themselves crystal lattice.
A crystal lattice is formed by metal processing, i.e. transition of its state from liquid to solid. This process is called - crystallization. For the first time these processes were studied by a scientist from Russia - D.K. Chernov.

crystallization process :
The process itself consists of two parts. In a metal that is in a liquid state, the atoms are constantly moving. If the temperature is lowered, then the speed of movement of atoms decreases, they approach and group into crystals (therefore, in order to change the shape and structure of the product, it is subjected to metalworking by heating) - this is the first part, during which crystallization centers are formed.
Then there is growth around the centers of crystallization - this is the second part of the process. At the very beginning, the growth of crystals proceeds freely, but then, the growth of some interferes with the growth of others, as a result, a irregular shape groups of crystals called grains. The size of the obtained grains significantly affects the further metalworking of products. A metal consisting of large grains has a low resistance to impact, if metal is cut, then it becomes difficult to obtain a low roughness on the surface of such a metal. The grain sizes depend on the crystallization conditions and the properties of the metal itself.

Ways to study the metal structure :
The study of the structure of metals and alloys is carried out through macro and micro analyzes, as well as in other ways. With the help of macro-analysis, the structure of the metal is studied, which can be seen with the naked eye or with a magnifying glass. This structure is determined by macrosections or breaks. macro section- This is a sample of metal, one of the sides of which is etched with acid and polished.
In micro-analysis, the sizes and shapes of grains, their structural components are studied, microdefects and the quality of metal heat treatment are revealed. This analysis is performed on microsections using a microscope. Microsection- this is a certain sample of metal that has a flat polished surface, etched with a weak solution of acid.

Metal properties :
Metallic properties are divided into physicochemical, technological and mechanical. Mechanical properties are the resistance of a metal to an external force. The mechanical properties are viscosity, strength, stamina and others.
Strength- these are the properties of the metal under certain conditions not to collapse, but to perceive the influence of external forces. This property is an important indicator when choosing a metal processing method.
Viscosity is the resistance of the material under impact loading.
Hardness- the properties of the material to the resistance of the penetration of another material into it.

The main technological properties include - ductility, weldability, melting properties, machinability and others.
Ductility- these are the properties of the material to be subjected to metalworking by forging and other methods of pressure treatment.
Weldability- material properties to create strong welded joints.
Melting properties- the properties of the material in molten form to fill molds and create dense castings with the desired configuration.
Machinability- the properties of the material to undergo metal cutting in order to give the part the desired shape, size and surface roughness. best method metal cutting is plasma cutting of metal. After this process, the metal practically does not need further metalworking.
In order to get a quality product with a good external and internal structure, you need to be well versed in the structure of metals, because this is the only way to get an excellent result.

Most alloys are obtained by fusing components in a liquid state. The components that make up the alloys in the solid state can interact with each other in different ways, forming mechanical mixtures, solid solutions and chemical compounds.

A mechanical mixture of two components is formed when they do not dissolve in each other in the solid state and do not enter into chemical interaction. Alloys - mechanical mixtures (for example, lead-antimony, tin-zinc) are heterogeneous in structure and represent a mixture of crystals of these components. In this case, the crystals of each component in the alloy completely retain their individual properties. That is why the properties of such alloys (for example, electrical resistance, hardness, etc.) are defined as the arithmetic mean of the magnitude of the properties of both components.

Alloys - solid solutions are characterized by the formation of a common spatial crystal lattice by the atoms of the base metal-solvent and the atoms of the soluble element. The structure of such alloys consists of homogeneous crystalline grains, like a pure metal. There are substitutional solid solutions (copper-nickel, iron-chromium, and other alloys) and interstitial solid solutions (for example, a solution of iron and carbon) (Fig. 5).

Alloys - solid solutions are the most common. Their properties differ from those of the constituent components. For example, the hardness and electrical resistance of solid solutions are much higher than those of pure components. Due to their high ductility, they lend themselves well to forging and other types of pressure treatment. The machinability of solid solutions is low.

Chemical compounds, like solid solutions, are homogeneous alloys. An important feature of them is that during solidification, a completely new crystal lattice is formed, which is different from the lattices of the components that make up the alloy. Therefore, the properties of a chemical compound are independent and do not depend on the properties of the components. Chemical compounds are formed at a strictly defined quantitative ratio of the alloyed components. The alloy composition of a chemical compound is expressed by chemical formula. These alloys usually have high electrical resistance, high hardness, low plasticity. So, the chemical compound of iron with carbon - cementite (Fe 3 C) is 10 times harder than pure iron.

Crystallization of alloys

Alloys have a more complex structure than simple metals. In this regard, the processes of crystallization of alloys are much more complicated than those of metals.

Alloys, unlike pure metals, during solidification or melting, have not one, but two critical points - temperatures, at which any transformations occur in metals or alloys (Fig. 6).

To facilitate the study of alloys, they are combined into systems.

Systems include all those alloys that consist of the same components and differ from each other only in the quantitative ratio of these components, i.e., concentration. For example, the lead-antimony alloy system includes all alloys consisting of lead and antimony and differing from each other only in the quantitative composition of these components.

The number of alloys of the same system, but of different concentrations, is so large that it is practically impossible, and even irrational, to study all the transformations occurring in each of them from the cooling or heating curves. To study the state of the alloys of the selected system, depending on the temperature and concentration, a state diagram is built.

Substances in the solid state have a crystalline or amorphous structure. In a crystalline substance, atoms are arranged according to a geometrically correct pattern and at a certain distance from each other, while in an amorphous substance (glass, rosin) the atoms are arranged randomly.

All metals and their alloys have a crystalline structure. On fig.12 shows the structure of pure iron. Crystal grains indefinite form do not look like typical crystals - polyhedra, that's why they are called crystallites, grains or granules. However, the structure of crystallites is just as regular as that of developed crystals.

Fig.12. Microstructure of pure iron (x - 150)

Types of crystal lattices . When solidified, metal atoms form geometrically regular systems called crystal lattices. The arrangement of atoms in the lattice can be different. Many of the most important metals form lattices, the simplest (elementary) cells of which represent the shape of a centered cube (  - And  - iron, chromium, molybdenum, tungsten, vanadium, manganese), cube with centered faces (  - iron, aluminum, copper, nickel, lead) or a hexagonal, like a hexagonal prism, cell (magnesium, zinc,  - titanium,  - cobalt).

elementary cell repeats continuously in three dimensions, forming a crystal lattice, so the position of atoms in the unit cell determines the structure of the entire crystal.

Unit cell of a centered cube ( fig.13) consists of nine atoms, of which eight are located at the vertices of the cube, and the ninth is in its center.

Fig.13. elementary cell Fig.14. Part of the spatial lattice

centered cube and centered cube

To characterize the crystal lattice (the atomic structure of the crystal), spatial lattice, which is a geometric scheme of the crystal lattice and consists of points (nodes) regularly located in spaces.

Fig.15. Cube unit cell Fig.16. Part of the spatial re-

with centered cube mesh faces with centered

On rice.14 a part of the spatial lattice of a centered cube is shown. Here eight adjacent elementary cells are taken; nodes located at the vertices and in the center of each cell are marked with circles. Unit cell of a cube with centered faces ( fig.15) consists of 14 atoms, of which 8 atoms are located along the vertices - the cube and 6 atoms - along the faces.

On fig.16 a part of the spatial lattice of a cube with centered faces (face-centered cube) is shown. There are eight elementary cells in the diagram; the nodes are located at the vertices and at the centers of the faces of each cell. Hexagonal cell ( fig.17) consists of 17 atoms, of which 12 atoms are located at the vertices of the hexagonal prism, 2 atoms - in the center of the bases and 3 atoms - inside the prism. To measure the distance between atoms of crystal lattices, a special unit is used, called angstromcm.

Fig.17. Hexagonal cell

The lattice parameter (side or hexagon) for copper is 3.6 A, and for aluminum 4.05 A, for zinc 2.67 A, etc.

Each atom consists of a positively charged nucleus and several layers (shells) of negatively charged electrons moving around the nucleus. Electrons in the outer shells of metal atoms, called valence, are easily split off, move quickly between nuclei and are called free. Due to the presence of free electrons, metal atoms are positively charged ions.

Thus, at the lattice sites indicated by circles fig.14 And 16 are positively charged ions. The ions, however, are not at rest, but continuously fluctuate in their equilibrium positions. As the temperature rises, the amplitude of oscillations increases, which causes the expansion of crystals, and at the melting temperature, oscillations of particles increase so much that the crystal lattice is destroyed.

In all crystals, small deviations from the ideal lattice are observed - unoccupied sites and various kinds of atomic displacements.

Anisotropy and cleavage of crystals . In individual crystals, the properties are different in different directions. If you take a large crystal (there are laboratory and even production methods for growing large crystals), cut out several samples of the same size, but differently oriented, and test their properties, then sometimes there is a very significant difference in properties between individual samples. For example, when testing samples cut from a copper crystal, the relative elongation varied from 10 to 50%, and the tensile strength from 14 to 35 kg/mm ​​2 for various samples. This property of crystals is called anisotropy. The anisotropy of crystals is explained by the peculiarities of the arrangement of atoms in space.

A consequence of the anisotropy of crystals is cleavage, which is revealed upon destruction. In places where crystals are fractured, regular planes can be observed, indicating the displacement of particles under the influence of external forces, not randomly, but in regular rows, in a certain direction, corresponding to the arrangement of particles in the crystal. These planes are called cleavage planes.

Amorphous bodies are isotropic, that is, all their properties are the same in all directions. A fracture of an amorphous body always has an irregular curved, so-called conchoidal surface.

Metals solidified under ordinary conditions do not consist of a single crystal, but of many individual crystallites, differently oriented towards each other, so the properties of the cast metal are approximately the same in all directions; this phenomenon is called quasi-isotropy(seemingly isotropic).

Allotropy of metals (or polymorphism) - their property to rearrange the lattice at certain temperatures during heating or cooling. Allotropy is found by all elements that change valence with a change in temperature: for example, iron, manganese, nickel, tin, etc. Each allotropic transformation occurs at a certain temperature. For example, one of the transformations of iron occurs at a temperature of 910°C, below which the atoms form the lattice of a centered cube (see Fig. fig.14), and above - the lattice of a face-centered cube (see Fig. fig.16).

This or that structure is called an allotropic form or modification. Various modifications are denoted by Greek letters  ,  ,  etc., with the letter  designate a modification that exists at temperatures below the first allotropic transformation. Allotropic transformations are accompanied by a return (decrease) or absorption (increase) of energy.

Crystallization of metals . Crystallization is the formation of crystals in metals (and alloys) during the transition from a liquid to a solid state ( primary crystallization). Recrystallization from one modification to another during cooling of the solidified metal is called ( secondary crystallization). The process of metal crystallization is easiest to follow with a time counter and a thermoelectric pyrometer, which is a millivoltmeter connected to a thermocouple. A thermocouple (two dissimilar wires soldered at the ends) is immersed in molten metal. The thermal current arising in this case is proportional to the temperature of the metal and the millivoltmeter needle deviates, indicating this temperature on a graduated scale.

The pyrometer readings are automatically recorded in time and, according to the data obtained, cooling curves are built in the coordinates "temperature - time" (such curves are drawn by a recorder).

The temperature corresponding to any transformation in the metal is called critical point.

On fig.18, a the metal heating curve is shown. Here is the point A- start of melting, point b- end of melting.

Fig.18. Heating curves ( A) and cooling ( b- no loop

V- with loop) metal

Plot Ab indicates the invariance of temperature over time with continued heating. This shows that thermal energy is spent on internal transformation in the metal, in this case. on the transformation of a solid metal into a liquid one (latent heat of fusion).

The transition from a liquid to a solid state upon cooling is accompanied by the formation of a crystal lattice, i.e., crystallization. To cause crystallization, liquid metal needs supercool slightly below the melting point. Therefore, the area on the cooling curve ( fig.19.6) is somewhat lower t pl at subcooling temperature t etc .

Some metals have hypothermia ( t pl - t etc) can be very significant (for example, for antimony up to 40 ° C) and at a supercooling temperature t etc (rice.18 , V), crystallization immediately begins violently, as a result of which the temperature abruptly rises almost to t pl. In this case, a thermal hysteresis loop is drawn on the graph.

During solidification and during allotropic transformation in the metal, crystal nuclei (crystallization centers) first appear, around which atoms are grouped, forming the corresponding crystal lattice.

Thus, the crystallization process consists of two stages: the formation of crystallization centers and the growth of crystals.

In each of the emerging crystals, the crystallographic planes are randomly oriented, in addition, during primary crystallization, the crystals can rotate, since they are surrounded by liquid. Adjacent crystals grow towards each other and their points of contact define the boundaries of crystallites (grains).

Iron crystallization. Consider, as an example, the crystallization and critical points of iron.

Fig.19. Iron cooling and heating curves

On fig.19 curves of cooling and heating of pure iron are given, which melts at a temperature of 1539 0 C. The presence of critical points at lower temperatures indicates allotropic transformations in solid iron.

Critical points are indicated by the letter A, when heated, denote Ac and on cooling Ar indices 2, 3, 4 serve to distinguish allotropic transformations (index 1 denotes a transformation in the state diagram Fe - Fe 3 C.

At temperatures below 768 0 С, iron is magnetic and has a crystal lattice of a centered cube. This modification is called  -iron; when heated, it is at the point ace 2 goes into non-magnetic modification  -iron. The crystal structure does not change.

At the point ace 3 at a temperature of 910 0 C  -iron goes into  -iron with a crystal lattice of a face-centered cube.

At the point ace 4 at a temperature of 1401 0 C  -iron goes into  -iron, and the crystal lattice is again rearranged from a face-centered cube to a centered cube.

During cooling, the same transitions occur, only in reverse order.

Of these transformations, the largest practical value have transformations A 3 as when heating ace 3 ) and on cooling ( Ar 3 ).

Transformation at a point A 3 accompanied by a change in volume, since the density of the crystal lattice  -gland more lattice density  -gland, at the point ace 3 the volume decreases, at the point Ar 3 - increases.

§ 2. The structure of metals and alloys and methods for studying it

Crystal structure of metals. The study of the internal structure and properties of metals and alloys is the science called metallurgy.

All metals and alloys are built from atoms, in which the outer electrons are weakly bound to the nucleus. The electrons are negatively charged and if you create a slight potential difference, then the electrons will go to the positive pole, forming electricity. This explains the electrical conductivity of metallic substances.

All metals and alloys in the solid state have a crystalline structure. Unlike non-crystalline (amorphous) bodies, in metals, atoms (ions) are arranged in a strictly geometric order, forming a spatial crystal lattice. The mutual arrangement of atoms in space and the distances between them are established by X-ray diffraction analysis. The distance between nodes in the crystal lattice is called the lattice parameter and is measured in angstroms Å (10 -8 cm). The lattice parameters of various metals range from 2.8 to 6 Å (Fig. 23).

a - cubic body-centered; b - cubic face-centered; c - hexagonal

For a visual representation of the arrangement of atoms in a crystal, spatial schemes are used in the form of elementary crystal cells. The most common types of crystal lattices are body-centered cubic, face-centered cubic and hexagonal.

There are nine atoms in a body-centered cubic lattice. Chromium, tungsten, molybdenum, vanadium and iron have such a lattice at temperatures up to 910 ° C.

There are 14 atoms in a face-centered cubic lattice. Such a lattice have: copper, lead, aluminum, gold, nickel and iron at a temperature of 910-1400 ° C.

There are 17 atoms in a hexagonal close-packed lattice. Such a lattice have: magnesium, zinc, cadmium and other metals.

The mutual arrangement of atoms in space, the number of atoms in the lattice and interatomic spaces characterize the properties of the metal (electrical conductivity, thermal conductivity, fusibility, plasticity, etc.).

The distance between atoms in a crystal lattice can be different in different directions. Therefore, the properties of the crystal in different directions are not the same. This phenomenon is called anisotropy. All metals are crystalline bodies, therefore they are anisotropic bodies. Bodies that have the same properties in all directions are called isotropic.

A piece of metal, consisting of many crystals, has on average properties that are the same in all directions, so it is called quasi-isotropic (imaginary isotropy).

Anisotropy is of great practical importance. For example, by forging, stamping, rolling in parts, the correct orientation of the crystals is obtained, as a result of which different mechanical properties are achieved along and across the part. By means of cold rolling, high magnetic and electrical properties are achieved in a certain direction of the part.