Internal structure of alloys. Crystal structure of metals

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Internal structure metals and alloys

1. Atomic structure of metals

2. Polymorphism. Anisotropy

3. Structure of real crystals and crystal lattice defects

1 . Atomic structure of metals

In a huge range of materials, since time immemorial known to man and widely used by him in his life and activities, metals have always occupied a special place.

Confirmation of this: both in the names of the eras (golden, silver, bronze, iron ages), into which the Greeks divided the history of mankind: and in archaeological finds metal products (forged copper jewelry, agricultural implements); and in the widespread use of metals and alloys in modern technology.

The reason for this is the special properties of metals, which distinguish them favorably from other materials and make them irreplaceable in many cases.

Metals are one of the classes of structural materials, characterized by a certain set of properties:

· “metallic luster” (good reflectivity);

· plasticity;

· high thermal conductivity;

· high electrical conductivity.

These properties are due to the structural features of metals. According to the theory of the metallic state, a metal is a substance consisting of positive nuclei around which electrons rotate in orbitals. At the last level, the number of electrons is small and they are weakly bound to the nucleus. These electrons have the ability to move throughout the entire volume of the metal, i.e. belong to a whole collection of atoms.

Thus, plasticity, thermal conductivity and electrical conductivity are ensured by the presence of “electron gas”.

All metals that solidify in normal conditions, are crystalline substances, that is, the arrangement of atoms in them is characterized by a certain order - periodicity, both in different directions and in different planes. This order is defined by the concept of a crystal lattice.

In other words, a crystal lattice is an imaginary spatial lattice, at the nodes of which particles are located that form a solid body.

A unit cell is a volume element of a minimum number of atoms, the repeated transfer of which in space can build the entire crystal.

The unit cell characterizes the structural features of the crystal. The main parameters of the crystal are:

Fig.1.1. Crystal lattice diagram

dimensions of the edges of the unit cell. a, b, c - lattice periods - distances between the centers of the nearest atoms. In one direction they are kept strictly defined.

angles between axes ().

coordination number (K) indicates the number of atoms located at the nearest equal distance from any atom in the lattice.

lattice basis is the number of atoms per unit cell of the lattice.

the packing density of atoms in a crystal lattice is the volume occupied by atoms, which are conventionally considered as rigid balls. It is defined as the ratio of the volume occupied by atoms to the volume of the cell (for a body-centered cubic lattice - 0.68, for a face-centered cubic lattice - 0.74)

The classification of possible types of crystal lattices was carried out by the French scientist O. Bravais, and accordingly they were called “Bravais lattices”. In total, there are fourteen types of lattices for crystalline solids, divided into four types;

Rice. 1.2. The main types of crystal lattices: a - body-centered cubic; b- face-centered cubic; c - hexagonal close-packed

primitive - lattice nodes coincide with the vertices of unit cells;

base-centered - atoms occupy the tops of cells and two places on opposite faces;

body-centered - atoms occupy the tops of the cells and its center;

face-centered - atoms occupy the vertices of the cell and the centers of all six faces

The main types of crystal lattices are:

1. Body-centered cubic (bcc) (see Fig. 1.2a), atoms are located at the vertices of the cube and at its center (V, W, Ti,)

2. Face-centered cubic (FCC) (see Fig. 1.2b), atoms are located at the vertices of the cube and at the center of each of the 6 faces (Ag, Au,)

3. Hexagonal, based on a hexagon:

o simple - atoms are located at the vertices of the cell and in the center of 2 bases (carbon in the form of graphite);

o close-packed (hcp) - there are 3 additional atoms in the middle plane (zinc).

2 . Polymorphism.Anisotropy

metal crystalline atomic polymorphism

The properties of a body depend on the nature of the atoms of which it is composed and on the strength of interaction between these atoms. The forces of interaction between atoms are largely determined by the distances between them. In amorphous bodies with a chaotic arrangement of atoms in space, the distances between atoms in different directions are equal, therefore, the properties will be the same, that is, the amorphous bodies are isotropic.

In crystalline bodies, atoms are correctly located in space, and in different directions the distances between atoms are not the same, which determines significant differences in the forces of interaction between them and, as a final result, different properties. The dependence of properties on direction is called anisotropy

To understand the phenomenon of anisotropy, it is necessary to identify crystallographic planes and crystallographic directions in the crystal.

The plane passing through the nodes of the crystal lattice is called the crystallographic plane.

The straight line passing through the nodes of the crystal lattice is called the crystallographic direction.

Miller indices are used to designate crystallographic planes and directions. To establish the Miller indices, the unit cell is inscribed in the spatial coordinate system ( X,Y axes, Z - crystallographic axes). The unit of measurement is the grating period.

Fig.1.3. Examples of designation of crystallographic planes (a) and crystallographic directions (b)

To determine the indices of the crystallographic crystallographic plane it is necessary:

set the coordinates of the points of intersection of the plane with the coordinate axes in units of the lattice period;

take the reciprocal values ​​of these quantities;

reduce them to the smallest integer multiple of each of the resulting numbers.

The resulting values ​​of prime integers that do not have a common factor are Miller indices for the plane and are indicated in parentheses. Examples of designation of crystallographic planes in Fig. 1.3 a.

In other words, the axis index shows how many parts the plane divides the axis unit along a given axis. Planes parallel to the axis have an index of 0 (110)

The orientation of a straight line is determined by the coordinates of two points. To determine the crystallographic direction indices it is necessary:

align one direction point with the origin of coordinates;

set the coordinates of any other point lying on a straight line, in units of lattice period

reduce the ratio of these coordinates to the ratio of the three smallest integers.

Indices of crystallographic directions are indicated in square brackets

In a cubic lattice, the direction indices are perpendicular to the plane(hkl) have the same indices.

The ability of some metals to exist in different crystalline forms depending on external conditions (pressure, temperature) is called allotropy or polymorphism.

Each type of lattice represents an allotropic modification or modification.

An example of allotropic modification depending on temperature is iron (Fe).

Fe: - bcc - ;

BCC - ; (high temperature)

The transformation of one modification into another occurs at a constant temperature and is accompanied by a thermal effect. Modifications of an element are indicated by letters of the Greek alphabet in the form of an index next to the main designation of the metal.

An example of an allotropic modification caused by changes in pressure is carbon: at low pressures graphite is formed, and at high pressures diamond is formed.

Using the phenomenon of polymorphism, it is possible to strengthen and soften alloys using heat treatment.

3 . The structure of real crystals and crystal lattice defects

A single crystal can be grown from a liquid melt. They are usually used in laboratories to study the properties of a particular substance.

Metals and alloys obtained under normal conditions consist of a large number of crystals, that is, they have a polycrystalline structure. These crystals are called grains. They have an irregular shape and are differently oriented in space. Each grain has its own orientation of the crystal lattice, different from the orientation of neighboring grains, as a result of which the properties of real metals are averaged, and the phenomenon of anisotropy is not observed. In the crystal lattice of real metals there are various defects (imperfections) that disrupt the bonds between atoms and affect the properties of metals. The following structural imperfections are distinguished:

· point - small in all three dimensions;

· linear - small in two dimensions and arbitrarily extended in the third;

· superficial - small in one dimension.

Chiseled defects

One of the common imperfections in the crystal structure is the presence of point defects: vacancies, dislocated atoms and impurities. (Fig. 2.1.)

Fig.2.1. Point defects

A vacancy is the absence of atoms at the sites of a crystal lattice, “holes” that were formed as a result of various reasons. Formed during the transition of atoms from the surface to environment or from lattice nodes to the surface (grain boundaries, voids, cracks, etc.), as a result of plastic deformation, when the body is bombarded with atoms or high-energy particles (irradiation in a cyclotron or neutron irradiation in nuclear reactor). The concentration of vacancies is largely determined by body temperature. Moving through the crystal, single vacancies can occur. And unite in divacancies. The accumulation of many vacancies can lead to the formation of pores and voids.

A dislocated atom is an atom that has left a lattice site and taken a place in an interstitial site. The concentration of dislocated atoms is much lower than that of vacancies, since their formation requires significant energy expenditure. In this case, a vacancy is formed in place of the displaced atom.

Impurity atoms are always present in a metal, since it is almost impossible to smelt a chemically pure metal. They can be larger or smaller than the size of the main atoms and are located at lattice sites or interstices.

Point defects cause minor lattice distortions, which can lead to changes in the properties of the body (electrical conductivity, magnetic properties); their presence promotes diffusion processes and the occurrence of phase transformations in the solid state. As the defects move through the material, they can interact.

Linear defects:

The main linear defects are dislocations. The a priori concept of dislocations was first used in 1934 by Orowan and Theiler in their study of plastic deformation of crystalline materials, to explain the large difference between the practical and theoretical strength of a metal.

Dislocations are defects in the crystal structure, which are lines along and near which the correct arrangement of atomic planes characteristic of a crystal is disrupted.

The simplest types of dislocations are edge and screw.

An edge dislocation is a line along which the edge of an “extra” half-plane breaks off inside the crystal (Fig. 2.2)

Rice. 2.2. Edge dislocation (a) and the mechanism of its formation (b)

An incomplete plane is called an extraplane.

Most dislocations are formed by a shear mechanism. Its formation can be described using the following operation. Cut the crystal along the ABCD plane, shift the lower part relative to the upper one by one lattice period in the direction perpendicular to AB, and then bring the atoms together again at the edges of the cut below.

The greatest distortions in the arrangement of atoms in the crystal occur near the lower edge of the extraplane. To the right and left of the edge of the extraplane, these distortions are small (several lattice periods), and along the edge of the extraplane, the distortions extend across the entire crystal and can be very large (thousands of lattice periods) (Fig. 2.3).

If the extraplane is located in the upper part of the crystal, then the edge dislocation is positive (), if in the lower part, then it is negative (). Dislocations of the same sign repel, and those of the opposite sign attract.

Rice. 2.3. Distortions in the crystal lattice in the presence of an edge dislocation

Another type of dislocation was described by Burgers and was called a screw dislocation.

A screw dislocation is obtained using a partial shift along the Q plane around the line EF (Fig. 2.4). A step is formed on the surface of the crystal, passing from point E to the edge of the crystal. Such a partial shift disrupts the parallelism of the atomic layers, the crystal turns into one atomic plane, twisted along a screw in the form of a hollow helicoid around the line EF, which represents the boundary separating the part of the slip plane where the shift has already occurred from the part where the shift has not begun. Along the EF line, the macroscopic character of the imperfection region is observed; in other directions, its dimensions are several periods.

If the transition from the upper to the lower horizons is carried out by turning clockwise, then the dislocation is right-handed, and if turning counter-clockwise, then the dislocation is left-handed.

Rice. 2.4. Mechanism of screw dislocation formation

A screw dislocation is not associated with any slip plane; it can move along any plane passing through the dislocation line. Vacancies and dislocated atoms do not flow to the screw dislocation.

During crystallization, atoms of a substance precipitated from vapor or solution easily attach to the step, which leads to a spiral crystal growth mechanism.

Dislocation lines cannot break inside the crystal; they must either be closed, forming a loop, or branch into several dislocations, or go to the surface of the crystal.

The dislocation structure of a material is characterized by the density of dislocations.

The dislocation density in a crystal is defined as the average number of dislocation lines crossing an area of ​​1 m 2 inside the body, or as the total length of dislocation lines in a volume of 1 m 3

(cm -2; m -2)

The dislocation density varies over a wide range and depends on the state of the material. After thorough annealing, the dislocation density is 10 5 ... 10 7 m -2; in crystals with a highly deformed crystal lattice, the dislocation density reaches 10 15 ... 10 16 m -2.

Dislocation density largely determines the plasticity and strength of the material (Fig. 2.5)

Rice. 2.5. Effect of dislocation density on strength

The minimum strength is determined by the critical dislocation density

If the density is less than the value a, then the resistance to deformation increases sharply, and the strength approaches the theoretical one. An increase in strength is achieved by creating a metal with a defect-free structure, as well as by increasing the density of dislocations, which impedes their movement. Currently, defect-free crystals have been created - whiskers up to 2 mm long, 0.5...20 microns thick - “whiskers” with strength close to theoretical: for iron = 13000 MPa, for copper = 30000 MPa. When strengthening metals by increasing the dislocation density, it should not exceed values ​​of 10 15 ... 10 16 m -2. Otherwise, cracks will form.

Dislocations affect not only strength and ductility, but also other properties of crystals. As the dislocation density increases, the internal density increases, the optical properties change, and the electrical resistance of the metal increases. Dislocations increase average speed diffusion in the crystal, accelerate aging and other processes, reduce chemical resistance, therefore, as a result of treating the surface of the crystal with special substances, pits are formed at the points where dislocations emerge.

Dislocations are formed during the formation of crystals from a melt or gaseous phase, during the accretion of blocks with small misorientation angles. When vacancies move inside the crystal, they concentrate, forming cavities in the form of disks. If such disks are large, then it is energetically favorable to “slam” them with the formation of an edge dislocation along the edge of the disk. Dislocations are formed during deformation, during crystallization, and during heat treatment.

Surface defects are the boundaries of grains, fragments and blocks (Fig. 2.6).

Rice. 2.6. Misorientation of grains and blocks in metal

The grain sizes are up to 1000 microns. The misorientation angles are up to several tens of degrees ().

The boundary between grains is a thin surface zone of 5 - 10 atomic diameters with a maximum violation of the order in the arrangement of atoms.

The structure of the transition layer promotes the accumulation of dislocations in it. At the grain boundaries there is an increased concentration of impurities, which reduce the surface energy. However, even inside the grain, the ideal structure of the crystal lattice is never observed. There are areas misoriented one relative to the other by several degrees (). These areas are called fragments. The process of dividing grains into fragments is called fragmentation or polygonization.

In turn, each fragment consists of blocks less than 10 microns in size, misoriented at an angle of less than one degree (). This structure is called block or mosaic.

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Internal structure of metals called the structure and relative arrangement of their atoms, as well as the larger structure visible through a microscope or with the naked eye.

Metals in their internal structure are a collection of neutral atoms, positively or negatively charged ions and free electrons, forming the so-called “electron gas”. The presence of “electron gas” determines the high electrical and thermal conductivity of metals, and the interaction of free electrons with each other and with ions creates a strong bond, called metallic. The specificity of the metallic bond makes metals ductile (malleable).

In addition to the nature of the atoms, the properties of metals are influenced by the nature of the bonds between the atoms, the distance between them and the order of their arrangement.

All metals in the solid state have a crystalline structure, i.e. their atoms (ions) are arranged in a strict, periodically repeating order, forming an atomic-crystalline lattice in space (as opposed to amorphous solids, the atoms of which are randomly located in space).

The order of arrangement of atoms in different metals is not the same. It is usually determined by simple crystal lattices characteristic of most metals (Fig. 6) or complex crystal lattices. Lines in Fig. 6 conditional Atoms actually vibrate near equilibrium positions, i.e., at the nodes of the crystal lattice. The distance between atoms in a crystal lattice is measured in angstroms (1 Å=10 -9 nm). For most metals, the distance between atoms is in the range of 0.28-0.8 nm.

Figure 6. The order of atoms in simple latticesA- volumetricIcentered cubic (9 atoms), b - face-centered cubic (14 atoms), c - hexagonal close-packed (17 atoms)

The smallest volume of a crystal, which gives an idea of ​​the atomic structure of the metal in its entire volume, is called unit crystal cell.

Metals obtained in the usual way are polycrystalline bodies consisting of many elementary cells oriented relative to each other in a variety of ways. The cells have an irregular shape and are called crystallites, or grains. If the combination of elementary cells is correct, repeating the elementary cell in the arrangement of atoms, then the resulting body is called a single crystal.

Metal alloys, like metals, have a crystalline structure. Moreover, depending on the interaction of the components, they are divided into solid solutions, chemical compounds and mechanical mixtures.

Solid solutions are formed when, during fusion, atoms of one element in different quantities enter the crystal lattice of another element without significantly changing its shape. An element that has retained the shape of its lattice is called a solvent, and an element whose atoms are included in this lattice is called a dissolved element. Based on the placement of the atoms of the dissolved element in the solvent lattice, they are distinguished substitutional solid solutions(the atoms of the dissolved element are located in the lattice sites of the solvent) and interstitial solid solutions(the atoms of the dissolved element are located between the atoms of the solvent and its lattice sites).

If the components included in the substitutional solid solution have a similar structure of lattices and atoms, then such elements can form a continuous series of solid solutions, i.e., the number of substituted atoms can vary from 0 to 100%.

In this case, it is considered that the solvent is the element whose content in the alloy is more than 50%.

Interstitial solutions are formed by elements with very different lattice structures and atomic sizes.

Solid solutions are homogeneous (homogeneous) alloys, since their structure consists of grains that are identical in composition and properties. The properties of solid solutions can differ significantly from the properties of its constituent components. All metals, to one degree or another, can dissolve in one another, forming solid solutions.

Chemical compounds are formed during the chemical interaction of atoms of the alloy components, accompanied by a significant thermal effect. In this case, the crystal lattice of a chemical compound and all its properties can differ sharply from the lattice and properties of the components. Unlike solid solutions, chemical compounds usually form between components that have a large difference in electronic structure atoms. Typical examples chemical compounds are compounds of magnesium with tin, lead, antimony, bismuth, sulfur, selenium, tellurium, etc. They are homogeneous in their structure.

Chemical compounds of metals are called intermetallic(intermetallic compounds), and compounds of metals with non-metals (nitrides, hydrides, borides, carbides) having a metallic bond - metal connections.

Mechanical mixtures are formed when, during solidification of the melt, the atoms of its components do not mix, but crystallize into a lattice characteristic of each. The structure of such alloys is heterogeneous (inhomogeneous) and is a mixture of crystals of alloy components that have retained their structure.

Rice. 7. Cooling curves of amorphous ( A), crystalline body (b) and metals (V), Where t to t n - crystallization and supercooling temperature, °C; (T 1 -T 2) - crystallization time, s.

The structure of a crystalline body determines the following special properties compared to amorphous ones:

§ difference in the properties of single crystals in different directions, i.e. anisotropy, or vectoriality, of properties;

§ the presence of sliding planes, the application of external forces leads to sliding (shear) of one plane relative to another;

§ the existence of a critical temperature during solidification or melting, at which a transition from the liquid (molten) state to the solid or vice versa occurs.

The transition of a metal from a liquid to a solid state is called crystallization, and from solid to liquid - melting. If the formation of crystals occurs from a liquid when it is cooled, then this process is called primary crystallization, if the formation of crystals occurs in the solid state of the body, it is called secondary crystallization.

Crystallization processes are graphically represented by curves plotted in temperature - time coordinates (Fig. 7).

The phenomenon of overcooling in a crystallizing metal is explained by the fact that during the period of solidification there is a sharp decrease in the mobility of atoms, as a result of which its internal energy. This is accompanied by the release of heat, which warms up the liquid bath and for some time (T 1-T 2) keeps its temperature constant until the liquid completely crystallizes.

The greater the cooling rate, the greater the degree of supercooling.

The Russian metallurgist D.K. Chernov established in 1878 that the crystallization process consists of several stages. The first stage is the formation of nuclei (centers) of crystallization. At subsequent stages, dendrites (tree-like formations) are formed from these centers, which, merging, form grains (crystallites). However, they do not have the correct geometric shape, since the growth of the faces stops at the points of contact of the growing crystals.

Metal grain size - most important characteristic, which determines all its basic properties. Fine-grained metal has higher characteristics of hardness, strength, and impact strength, but it has reduced electrical conductivity and worse magnetic properties.

The grain size depends on the number of crystallization centers and the rate of crystal growth (cooling rate). The more crystallization centers and the lower their growth rate, the smaller the grain will be.

The formation of crystallization centers can occur spontaneously or on impurity particles present in the liquid metal, which is used in modification- introduction of impurities (modifiers) into liquid metal.

The formation of crystallization centers, and therefore the grain size, is affected by the degree of supercooling t to -t p. The greater the degree of supercooling, the more crystallization centers and the smaller the grains formed.

In the manufacture of machines and working installations, metals and their alloys are the most used.
Metals- these are substances that have high electrical and thermal conductivity, shine, 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. The so-called ferrous metals– this is pure iron and alloys based on its material. TO in color– include other types of metals. For the right choice metal for the manufacture of mechanical structures 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, physical-chemical and technological properties, as well as what method to perform metal processing and whether the material needs 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 located in a strictly defined order and form with each other 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 you lower the temperature, the speed of movement of atoms decreases, they come closer and group into crystals (therefore, in order to change the shape and structure of the product, it is subjected to metal processing using 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, an irregularly shaped group of crystals is formed, which are called grains. The size of the resulting grains significantly affects the further metalworking of products. A metal consisting of large grains has low resistance to impact; if metal is cut, it becomes difficult to obtain low roughness on the surface of such metal. The grain sizes depend on the crystallization conditions and the properties of the metal itself.

Methods for studying metal structure :
The study of the structure of metals and alloys is carried out through macro and micro analyzes, as well as other methods. Using 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 fractures. Macro sanding- This is a sample of metal, one of the sides of which is etched with acid and polished.
Microanalysis studies the sizes and shapes of grains, their structural components, and identifies microdefects and the quality of heat treatment of the metal. This analysis is carried out on microsections using a microscope. Microsection- this is a certain sample of metal that has a flat polished surface, etched with a weak acid solution.

Properties of metals :
Metallic properties are divided into physicochemical, technological and mechanical. Mechanical properties mean the resistance of a metal to the influence of external forces on it. Mechanical properties include viscosity, strength, durability and others.
Strength- these are the properties of a metal under certain conditions not to be destroyed, but to perceive the influence of external forces. This property is important indicator when choosing a metal processing method.
Viscosity is the resistance of a material under impact load.
Hardness– properties of a material to resist the introduction of another material into it.

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

Engineering materials include metals and their alloys, wood, plastics, rubber, cardboard, paper, glass, etc. Metals and their alloys are most widely used in the manufacture of machines.

Metals are substances that have high thermal and electrical conductivity; ductility, shine and other characteristic properties.

In technology, all metals and alloys are usually divided into ferrous and non-ferrous. Ferrous metals include iron and its alloys. Non-ferrous - all other metals and alloys. In order to choose the right material for the manufacture of machine parts, taking into account their operating conditions, mechanical loads and other factors affecting the performance and reliability of machines, it is necessary to know the internal structure, physico-chemical, mechanical and technological properties of metals.

Metals and their alloys in the solid state have a crystalline structure. Their atoms (ions, molecules) are located in space in a strictly defined order and form a spatial crystal lattice.

The smallest complex of atoms, which, when repeated many times in space, reproduces a lattice, is called a crystalline unit cell.

The shape of the elementary crystalline cell determines the set of properties of metals: luster, fusibility, thermal conductivity, electrical conductivity, workability and anisotropy (difference in properties in different planes of the crystal lattice).

Spatial crystal lattices are formed during the transition of a metal from a liquid to a solid state. This process is called crystallization. Crystallization processes were first studied by the Russian scientist D. K. Chernov.

Crystallization consists of two stages. In the liquid state of a metal, its atoms are in continuous motion. As the temperature decreases, the movement of atoms slows down, they come closer and group into crystals. So-called crystallization centers are formed (first stage). Then comes the digging of crystals around these centers (second stage). At first, the crystals grow freely. With further growth, the crystals are repelled, the growth of some crystals interferes with the growth of neighboring ones, as a result of which irregularly shaped groups of crystals are formed, which are called grains.

The grain size significantly affects the operational and technological properties of metals. Coarse-grained metal has low impact resistance; when it is processed by cutting, it becomes difficult to obtain a low surface roughness of parts. The grain sizes depend on the nature of the metal itself and the crystallization conditions.

Methods for studying the structure of metal. The study of the structures of metals and alloys is carried out using macro- and microanalysis, as well as other methods.

The macroanalysis method is used to study the macrostructure, i.e., the structure of the metal, visible to the naked eye or with a magnifying glass. The macrostructure is determined by metal fractures or macrosections.

A macrosection is a sample of a metal or alloy, one of the sides of which has been ground and etched with acid or another reagent. This method reveals large defects: cracks, shrinkage cavities, gas bubbles, uneven distribution of impurities in the metal, etc.

Microanalysis allows you to determine the size and shape of grains, structural components, the quality of heat treatment, and identify microdefects.

Microanalysis is carried out on microsections using a microscope (modern metallographic microscopes provide magnification up to 2000, and electronic microscopes - up to 25,000).

Mi microscopic is a sample of metal that has a flat polished surface, etched with a weak solution of acid or alkali to reveal the microstructure. Properties of metals. The properties of metals are usually divided into physicochemical, mechanical and technological. Physicochemical and mechanical properties solids, including metals, are familiar to you from physics and chemistry courses. Let us dwell on the consideration of some mechanical and technological properties that are important from the point of view of metal processing.

Mechanical properties, as is known, mean the ability of a metal or alloy to resist external forces. Mechanical properties include strength, toughness, hardness, etc.

Strength characterizes the ability of a metal or alloy, under certain conditions and limits, to withstand certain influences of external forces without collapsing.

An important property of metal is impact strength - the material’s resistance to destruction under impact load.

Hardness is understood as the property of a material to resist the penetration of another, harder body into it.

The mechanical properties of materials are expressed through a number of indicators (for example, tensile strength, relative elongation and contraction, etc.)

Tensile strength, or temporary tensile strength, is the conditional stress corresponding to the maximum load that the sample can withstand during testing before failure.

The hardness of metals and alloys is determined mainly using three methods, named after their inventors: the Brinell method, the Rockwell method and the Vickers method. I Hardness measurement using the Brinell method consists of pressing a hardened steel ball with a diameter of 2.5, 5 or 10 mm into the surface of the test metal using a TS hardness tester under the influence of a static load P. The ratio of the load to the surface area of ​​the indentation (hole) gives the hardness value , denoted NV.

Rockwell hardness is measured using a TK device by pressing a ball with a diameter of 1.59 mm (1/16 inch) or a diamond cone with an apex angle of 120° (for especially hard steels and alloys) into the test metal. Hardness readings are determined using the device indicator.

Vickers hardness is measured using a TP device by pressing a diamond tetrahedral pyramid into the metal with an apex angle of a = 136°. Based on the length of the diagonal of the resulting print, the hardness number HV is found using a table.

The use of a particular method depends on the hardness of the test sample, its thickness or the thickness of the test layer. For example, the Vickers method is used to measure the hardness of hardened steels, part materials up to 0.3 mm thick, and thin external cemented, nitrided and other surfaces of parts.

To the main technological properties of metals and alloys

include the following:

malleability - the property of a metal to be subjected to forging and other types of pressure treatment;

fluidity - the property of molten metal to fill the casting mold in all its parts and produce dense castings of precise configuration;

weldability - the property of a metal to produce strong welded joints;

Cutting machinability is the property of metals to be processed by cutting tools to give parts a certain shape, size and surface roughness.

Option 1.

In metals, type of bond:

covalent polar; 2) ionic; 3) metal; 4) covalent nonpolar.

The internal structure of metals contains:

1) only cations; 2) only anions; 3) cations and anions; 4) cations and neutral atoms.

Liquid metal at room temperature is:

1) iron; 2) mercury; 3) gold; 4) lithium.

Alchemists considered gold a symbol:

Bad judgment, that all metals:

1) have malleability; 2) have a metallic luster; 3) have electrical conductivity; 4) volatile substances.

The hardest metal:

1) sodium; 2) chrome; 3) lead; 4) lithium.

Metal with the highest density:

1) iron; 2) copper; 3) gold; 4) titanium.

Reflects light better:

1) lead; 2) silver; 3) zinc; 4) iron.

Among the listed substances, indicate those that are metals:

silicon; 2) beryllium; 3) boron; 4) aluminum; 5) potassium; 6) argon; 7) sulfur; 8) tin.

Give your answer as a sequence of numbers in ascending order.

Test No. 4 Topic “Simple substances - metals”

Option 2.

Metals to complete the layer:

1) donate electrons; 2) accept electrons; 3) give or receive electrons; 4) their layer is complete.

2. Bonding in metals between cations is carried out by:

1) free electrons; 2) anions; 3) protons; 4) neutrons.

3. The most ductile of precious metals:

1) silver; 2) platinum; 3) gold; 4) mercury.

Alchemists considered copper a symbol:

1) Venus; 2) Mars; 3) the Sun; 4) Saturn.

5. The softest metal:

1) chrome; 2) titanium; 3) molybdenum; 4) lead.

6. The most refractory metal:

1) tungsten; 2) mercury; 3) gold; 4) titanium.

7. Metal with the lowest density:

1) sodium; 2) tin; 3) lead; 4) iron.

8. Has the highest electrical conductivity:

1) iron; 2) gold; 3) aluminum; 4) silver.

9. Arrange the following metals in order of increasing density:

1) copper; 2) iron; 3) lead; 4) aluminum; 5) gold.

Give your answer as a sequence of numbers.

Answers. Topic “Simple substances - metals”

Option 1.

Option 2.