Brief theoretical information. Laboratory work on the course "Materials Science"

A methodical development of a practical lesson for the academic discipline OP 08 "Materials Science" in the specialty of secondary vocational education 22.02.06 "Welding production" is presented.

In the course of this work, students study the types and characteristics of crystal lattices of metals, the influence of crystal lattices on the structure and properties of metals and their alloys.

At the end of the work, students are asked to answer control questions.

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Guidelines for practical work on the topic "Studying the types of crystal lattices and their influence on the structure and properties of metals and their alloys" for second-year students in the specialty of secondary vocational education 22.02.06 "Welding production" in the academic discipline OP 08 "Materials Science"

Nikiforuk Tatyana Alekseevna.

teacher of special disciplines,

TOGBPOU "Diversified College",

Morshansk, Tambov region

Morshansk, 2016

Basic theoretical provisions 3

1. Atomic-crystal structure of metals 3
2. Transformations in the solid state. Polymorphism 6
3. Procedure for performing practical work 8
4. Contents of the practice report 8

Objective: Familiarize yourself with the types and characteristics of crystal lattices of metals. To study the influence of crystal lattices on the structure and properties of metals.

Basic theoretical provisions

1. ATOMIC-CRYSTAL STRUCTURE OF METALS.

metal materials. 83 of the known 112 chemical elements of D. I.'s periodic table are metals. They have a number of characteristic properties:

- high thermal and electrical conductivity;

- a positive coefficient of electrical resistance (with increasing temperature, the electrical resistance increases);

- thermionic emission (emission of electrons during heating);

- good reflectivity (shine);

- ability to plastic deformation;

Polymorphism.

The presence of these properties is due to the metallic state of matter, the main of which is the presence of easily mobile collectivized conduction electrons.

The metallic state arises in a set of atoms, when, as they approach each other, the outer (valence) electrons lose their connection with individual atoms, become common and move freely between positively charged, periodically located ions. Attractive forces (coupling forces) in solids differ significantly in nature. Usually, four main types of bonds in solids are considered: van der Waals, covalent, metallic, and ionic.

The atomic-crystal structure is understood as the mutual arrangement of atoms in a crystal. A crystal consists of atoms (ions) arranged in a certain order, which is periodically repeated in three dimensions.

The smallest complex of atoms, which, when repeated many times in space, makes it possible to reproduce a spatial crystal lattice, is called an elementary cell.

To characterize the unit cell, the parameters of the crystal lattice are used:

Three ribs a, b, c , measured in angstroms (1Å = 1 * 10-8 cm) or in kiloix - kX (1kX = 1.00202 Å) and three anglesα , β , γ , ;

Compact structureη - the ratio of the volume occupied by atoms to the volume of the cell (for the bcc latticeη = 64%, for the fcc latticeη = 74%) ;

The coordination number K is the number of nearest neighbors of a given atom: for the bcc lattice, this number is 8, i.e. the atoms located at the top belong to eight unit cells (Fig. 1.a), for the fcc lattice this number is 12, i.e. atoms located at the top belong to twelve elementary cells (Fig. 1.b).

Figure 1. Scheme for determining the coordination number of the crystal lattice:

a – HCC;

b - BCC;

c - GPU

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 occupy places closest to each other leads to the formation of lattices of other types (Fig. 2.):

Body-centered cubic lattice (BCC) (Fig. 2.a) with the parameter

a \u003d 0.28 - 0.6 mm \u003d 2.8 - 6.0 Å

Face-centered cubic lattice (FCC) (Fig. 2.b) with the parameter

a = 0.25mm

Hexagonal densely packed lattice (HCP) (Fig. 2.c) with the parameter

c/a ≈ 1.633

Figure 2. Crystal lattices: a – face-centered cube (BCC); b – body-centered cube (fcc); c- hexagonal close-packed (hcp)

Knots (positions of atoms), directions in the plane and in space are designated using the so-called Miller indices (Fig. 3).

Node indices are written - (mnp),

Direction indices are written −[mnp] ,

The plane index is written - (hk1).

Figure 3. Symbols for some of the most important nodes, directions and planes in a cubic lattice.

Due to the unequal density of atoms in different planes and directions of the lattice, many properties of a single crystal (chemical, physical, mechanical) in a given direction differ from properties in another direction and, naturally, depend on how many atoms are found in this direction.

The difference in properties depending on the direction of the test is called anisotropy.

All crystals are anisotropic.

Anisotropy is a feature of any crystal, characteristic of the crystalline structure.

Technical metals are polycrystals, i.e. consist of a set of crystallites with different orientations. In this case, the properties in all directions are averaged.

1. TRANSFORMATIONS IN THE SOLID STATE. POLYMORPHISM.

The atoms of a given element can form, if we proceed only from geometric considerations, any crystal lattice. However, the stable and, therefore, actually existing type is the lattice with the lowest free energy reserve.

So, different metals form different types of crystal lattice:

Li, Na, K, Mo, W—bcc;

- Al, Ca, Cu, Au, Pt—fcc;

Mg, Zr, Hf - hcp.

However, in some cases, when the temperature or pressure changes, it may turn out that for the same metal a different lattice will be more stable than the one that exists at a given temperature or pressure. So, for example, there is iron with lattices of body-centered and face-centered cubes, cobalt with face-centered and hexagonal lattices has been found (Fig. 4).

The existence of the same metal (substance) in several crystalline forms is called polymorphism or allotropy.

Various crystalline forms of the same substance are called polymorphs or allotropic modifications (table 1).

Allotropic forms are denoted by Greek lettersα , β , γ and so on, which are added as subscripts to the symbol denoting the element. The allotropic form at the lowest temperature, denoted by the letterα , next - β etc.

The phenomenon of polymorphism is based on a unified law on the stability of the state with the least amount of energy. The stock of free energy depends on the temperature. Therefore, in one temperature range, one modification is more stable, and in another, another.

The temperature at which the transition from one modification to another is carried out is called the temperature of the polymorphic (allotropic) transformation.

The mechanism of growth of crystals of the new phase can be normal crystallization and martensitic.

The normal growth mechanism is the nucleation of a new phase at the boundaries of grains, blocks, fragments at low degrees of supercooling (Snα ↔ Sn β ).

The martensitic mechanism is realized at low temperatures and a high degree of supercooling, with a low diffusion mobility of atoms by means of their shift (displacement) along certain crystallographic planes and directions. The new phase is in the form of needles and grows very rapidly (Coα ↔ Co β ).

Allotropic transformation is accompanied by a change in properties, volume and the appearance of internal stresses.

Figure 4. Unit cells of crystal lattices:

I - cubic body-centered (α-iron),

II - cubic face-centered (copper),

III - hexagonal close-packed;

a and c are the lattice parameters.

Table 1. Allotropic modifications of metals.

Metal		Group		Modification		Crystal cell
Calcium		II-A		Ca α up to 450 Caαβ 450-851		Cubic FCC
Gallium		III-B		Gaα Gaβ		Rhombic tetragonal
Thallium		III-B		Tlα up to 262 Tlβ 262-304		Hexagonal close-packed
Titanium		IV-A		Tiα up to 882 Tiβ 882-1725		Hexagonal close-packed Cubic body centered
Zirconium		IV-A		Zrα up to 862 Zrβ 862-1830		Hexagonal close-packed Cubic body centered
Hafnium		IV-A		Hfα up to 1610 Hfβ 1610-1952		Hexagonal Cubic body centered
Tin		IV-B		Snα up to 18 Snβ 18-232		Diamond Tetragonal body-centered
Tungsten		VI-A		Wα up to 650 Wβ 650-3400		Cubic body centered Complicated (undeciphered)
Uranus		VI-A		Uα up to 660 Uβ 660-770

FEDERAL AGENCY FOR EDUCATION State educational institution of higher professional education

"South-Russian State University of Economics and Service" (GOU VPO "YURGUES")

MATERIALS SCIENCE

TECHNOLOGY OF STRUCTURAL MATERIALS

Laboratory workshop

for students of specialties 190601, 190603, 200503, 260704

full-time and part-time forms of education

MINES GOU VPO "YURGUES"

UDC 620.1(076.5) BBK 30.3ya73

Compiled by:

Candidate of Technical Sciences, Associate Professor of the Department of Applied Mechanics and Machine Design

Yu.E. damn

Ph.D., Art. Lecturer at the Department of Applied Mechanics and Machine Design

S.N. Baibara

Reviewers:

Ph.D., professor, head. Department of "Technical operation of vehicles"

SOUTH. Sapronov

Candidate of Technical Sciences, Professor of the Department of Technology of Leather Products, Standardization and Certification

M341 Materials Science: Technology of Structural Materials: Laboratory Workshop / compiled by Yu.E. Chertov, S.N. Baybara. - Mines: GOU VPO "YURGUES", 2010. - 71 p.

The use of a laboratory workshop will allow to consolidate the lecture material, to ensure independent study of individual didactic units of the discipline, the successful completion of tests and independent tasks.

Designed for students of specialties 190601, 190603, 200503, 260704 full-time and part-time forms of education.

UDC 620.1(076.5) BBK 30.3ya73

Access mode to the electronic analogue of the printed edition: http://www.libdb.sssu.ru

© GOU VPO "South Russian state University of Economics and Service", 20 10

FOREWORD .............................................................. .........................................
Laboratory work number 1. Study of the crystallization process
Laboratory work number 2. Study of macro- and microstructure
metals and alloys ............................................... ................................................
Laboratory work number 3. Exploring State Diagrams
double alloys ................................................................ ............................................
Laboratory work number 4. Study of phase transformations
according to the state diagram of iron-cementite .............................................. ......
Laboratory work number 5. Methods for measuring the hardness of metals......
Laboratory work number 6. Effect of heat treatment
on the mechanical properties of structural steel....................................................
Laboratory work number 7. Forming blanks by casting
into sand molds .............................................. .........................................
Laboratory work number 8. Learning the ways of electrical
metal welding ................................................................ ...............................................
Laboratory work number 9. Learning how to make
plastic products ............................................................... ................................................
BIBLIOGRAPHICAL LIST.................................................................. ..........

FOREWORD

The future specialist - a graduate of a higher educational institution will have to work in a rapidly changing production environment. Already, the cycle of technology renewal in some industries is shorter than the period of study at an institute or university. Therefore, the training of a new type of specialists who are able to quickly adapt to the new working conditions of enterprises is one of the main tasks of the university.

The laboratory workshop, as a form of training, maximally contributes to the activation of the mental activity of students and the development of their skills of creative application in practice of the acquired knowledge.

The proposed laboratory work will allow students to study the theoretical provisions of the course "Materials Science", gain practical skills in studying the structure and properties of metal engineering materials, assessing the impact on the structure and properties of metals of various types of heat treatment.

The performance of laboratory work in the conditions of a sharp reduction in the volume of lectures often does not coincide with the order of presentation of the lecture course. Therefore, each work contains general theoretical information that will facilitate the student's independent preparation for the work, contributing to its conscious implementation and understanding of the results obtained.

The laboratory workshop was prepared in accordance with the requirements of the State Educational Standard in the discipline “Materials Science. TKM" for students of engineering specialties of higher educational institutions.

Laboratory work No. 1 STUDYING THE PROCESS OF CRYSTALLIZATION OF METALS AND ALLOYS

The purpose of the work: to study the process of transition of metallic materials (metals and alloys) from a liquid to a solid state of aggregation, taking into account the influence of external factors, as well as to study the structure of a steel ingot.

1. Give a brief description of metals, alloys and the processes of their crystallization.

2. Familiarize yourself with the biological microscope.

3. To monitor the crystallization of salts from supersaturated aqueous solutions.

4. Draw, observing the crystallization of the drop, the most characteristic zones and give explanations. The size of the picture is a circle of 50 mm.

5. Sketch the longitudinal and transverse sections of the steel ingot. Give an explanation for the presence of three zones in the ingot.

6. Prepare a written report on the work.

General information from the theory

1. Brief description of metals and alloys

Metals and alloys are the most important structural materials widely used in engineering. Metals, in addition to brilliance and ductility, have high thermal and electrical conductivity.

Obtaining chemically pure metals is associated with significant difficulties, and the values ​​of their mechanical characteristics are not high. In this regard, metal alloys are widely used in technology.

Alloys are complex substances, which include several metals or metals and non-metals. Metal alloys have the properties of pure metals noted above.

Metallic materials in a solid state of aggregation have a crystalline structure, in which positively charged ions are arranged in a strictly defined order, periodically repeating in three dimensions of space. Since alloys are usually obtained by metallurgical technology, the liquid state precedes the solid state. The transition of a substance from a liquid to a solid state is called

crystallization.

2. Crystallization of metals and alloys

Crystallization proceeds under conditions when the system passes to a thermodynamically more stable state with a lower free energy. Free energy F is that part of the internal energy of the system that can be converted into work. With increasing temperature, the free energy of the liquid and solid states of the metal decreases (see Fig. 1.1).

Free energy F

condition

					condition
T cr
					T pl
Temperature,

Figure 1.1 - Change in the free energy of the liquid and solid states depending on temperature

When the equilibrium temperature T S is reached, the free energy of the liquid and solid states is equal, and therefore at this temperature neither the crystallization process nor the melting process can proceed to the end.

For the development of the crystallization process, it is necessary to create conditions under which the free energy of the solid phase will be less than the free energy of the liquid phase. As can be seen from the graph shown in Figure 1.1, this is possible only with some overcooling of the alloy.

The degree of hypothermia is the difference between the equilibrium (theoretical) and actual crystallization temperatures

T TS Tcr .

Some degree of overheating of the alloy is necessary for the development of the melting process.

T Tm TS .

The degree of supercooling is measured in degrees Celsius and depends on the cooling rate, the nature and purity of the melt. The faster the cooling rate, the greater the degree of subcooling. The purer the melt, the greater its stability and, consequently, the greater the degree of supercooling.

The presence of undissolved particles in the melt accelerates the crystallization process and refines the grain. Research D.K. Chernov, it was revealed that crystallization begins with the formation of crystalline nuclei (crystallization centers) and continues with an increase in their number and size.

The number of crystallization centers (C.C.) and their growth rate (S.R.) depend on the degree of supercooling. With an increase in the degree of supercooling, the number of crystallization centers increases and the rate of their growth increases; at a certain degree of hypothermia, a maximum occurs.

However, metals and alloys, which have a low tendency to supercooling in the liquid state, cannot be cooled to temperatures at which the number of crystallization centers and the crystal growth rate would reach a maximum. Therefore, for metals, the curves "Ch.Ts." and "S.R." break off already at low degrees of supercooling (solid curves in Figure 1.2).

S.R.

T ST

Degree of hypothermia T, C

Figure 1.2 - The influence of the degree of supercooling on the number of crystallization centers and the growth rate of crystals

For the degree of supercooling T, the rates of formation of crystallization centers and their growth are small, therefore, the crystallization process proceeds slowly, and the grains are large (since few crystallization centers are formed per unit volume of the liquid phase).

For the degree of supercooling T, both the rate of nucleation of crystallization centers and the rate of their growth have significantly increased, so the crystallization process will proceed much faster than with the degree of supercooling T, and since the number of crystallization centers per unit volume increases, the grains are small.

Thus, by changing the degree of supercooling, it is possible to obtain crystallites (grains) of various sizes. Many properties of the alloy depend on the grain size. In practice, grain refinement in alloys is also achieved by modification, i.e. the introduction of dispersed particles of modifier substances into the melt, which become additional centers of crystallization.

The process of crystallization of metals and alloys is similar to the process of crystallization of salts from aqueous solutions. At the same time, the formation of crystals becomes possible to observe with a biological microscope at room temperature as the water evaporates, which is convenient and safe.

3. Structure of a metal ingot

Crystals in the process of metal solidification can have a different shape depending on the cooling rate, the nature and amount of impurities. Most often, in the process of crystallization, branched or tree-like crystals are formed, called dendrites. Initially, long branches are formed, the so-called axes of the first order (the main axes of the dendrite). Simultaneously with the elongation of the axes of the first order, the same branches of the second order perpendicular to them arise and grow on their edges. In turn, third-order axes are born on the second-order axes, and so on.

		– zone of fine grains;
		– zone of columnar crystals;
		– zone of equiaxed crystals;
		- shrink shell;
		- gas bubbles, voids,
		shrinkage friability

Figure 1.3 - Scheme of the structure of a steel ingot of calm steel

Crystallization of liquid metal begins at the surface of a colder mold and occurs first in a thin layer of strongly supercooled liquid adjacent to the surface. This leads to the formation of a very narrow zone of small non-oriented grains on the surface of the ingot.

Behind zone 1, deep into the ingot, there is a second zone - the zone of columnar crystals. The growth of these crystals proceeds in the direction of heat removal, and since all crystals grow simultaneously, columnar (elongated) crystals are obtained, the growth of which continues as long as there is a directed heat removal. In the case of strong overheating and rapid cooling, the zone of columnar crystallites can fill the entire volume of the ingot.

This type of crystallization is called transcrystallization. In the inner part of the ingot, zone 3 is formed, consisting of equiaxed differently oriented dendritic crystals, larger due to the low cooling rate (due to a decrease in T). Since the liquid metal has a larger specific volume than the solid one, then in that part of the ingot that solidifies last, a void is formed - a shrinkage cavity. It is usually surrounded by the most contaminated metal containing micro- and macropores, gas bubbles and other defects. The crystallization of the zones of the ingot, as well as the axes of the dendrites, does not occur simultaneously, therefore the metal of the ingot has a heterogeneity in chemical composition - zonal and dendritic segregation.

4. Equipment and samples

Biological microscopes are used to observe the process of salt crystallization. The microscope stand is a stable base to which the remaining parts of the microscope are attached: tube, condenser holder, turret with objectives, eyepiece. As a rule, the microscope is equipped with several lenses of different magnifications, placed on a revolving nozzle, which allows you to set the lenses to the working position by moving them. Sample examination usually begins with the lowest magnification objective with the largest field of view. Details of interest are examined using lenses with high magnification.

The schematic diagram of a biological microscope is shown in Figure 1.4.

- mirror;

- subject table;

- glass slide;

- a drop of salt solution;

- lens;

- microscope tube;

- eyepiece;

- the eye of the observer.

Figure 1.4 - Schematic diagram of a biological microscope

Microscope adjustment is carried out as follows. By turning the glass 2 towards the light source, the brightest illumination is achieved in the eyepiece 8. Then a glass slide 4 with a drop 5 of the salt solution is placed on the table 3 so that the edge of the drop can be observed. The focal length is set by lowering/raising the object table 3 relative to the tube 7, achieving a clear image of the edge of the drop in the eyepiece 8.

5. Work order

Having studied the theoretical part and familiarized themselves with the task for work, students begin to observe the crystallization process. For this, a biological microscope and a glass slide with a drop of a supersaturated aqueous solution of sodium chloride are issued. After adjusting the microscope, place the glass on the microscope stage and observe the beginning of the crystallization process at the edge of the drop. As the water evaporates, crystals will also grow in the next drop zones. The process under study can be conditionally divided into three periods. The first is the crystallization of salt at the edge of the drop, where the amount of water is the smallest. During this period, small regular-shaped crystals form at the edge of the drop, since supercooling causes the formation of a large number of crystallization centers. During the second period, large columnar crystals are formed. The direction of their axes is normal to the edges of the drop. During this period, there is a high rate of crystal growth and a limited number of crystallization centers. During the third period, tree-like (dendritic) crystals are formed. In this case, the amount of water in the droplet is insignificant and its evaporation from the middle part proceeds rapidly.

Federal State Budgetary Educational Institution of Higher Education

"Volga State University of Water Transport"

PERM BRANCH

E.A . Sazonova

MATERIALS SCIENCE

COLLECTION OF PRACTICAL AND LABORATORY WORKS

26.02.06 "Operation of ship's electrical equipment and automation"

23.02.01 "Organization of transportation and transport management" (by type)

PERMIAN

2016

Introduction

Guidelines for the implementation of laboratory and practical work in the academic discipline "Materials Science" are intended for students of secondary vocational education in the specialty 26.02.06 "Operation of ship electrical equipment and automation"

This manual provides guidance on the implementation of practical and laboratory work on the topics of the discipline, indicates the topics and content of laboratory and practical work, forms of control on each topic and recommended literature.

As a result of mastering this academic discipline, the student should be able to:

˗ perform mechanical tests of material samples;

˗ use physical and chemical methods of metal research;

˗ use reference tables to determine the properties of materials;

˗ choose materials for the implementation of professional activities.

As a result of mastering this academic discipline, the student should know:

˗ basic properties and classification of materials used in professional activities;

˗ name, marking, properties of the processed material;

˗ rules for the use of lubricants and coolants;

˗ basic information about metals and alloys;

˗ basic information about non-metallic, gasket,

Sealing and electrical materials, steel, their classification.

Laboratory and practical work will allow to form practical work skills, professional competencies. They are included in the structure of the study of the academic discipline "Materials Science", after studying the topic: 1.1. "Basic information about metals and alloys", 1.2 "Iron-carbon alloys", 1.3 "Non-ferrous metals and alloys".

Laboratory and practical work is an element of the academic discipline and is evaluated according to the criteria presented below:

A grade of "5" is given to a student if:

˗ the subject of the work corresponds to the given one, the student shows systemic and complete knowledge and skills on this issue;

˗ the work is designed in accordance with the recommendations of the teacher;

˗ the amount of work corresponds to the given;

˗ the work was done exactly in the time specified by the teacher.

A grade of "4" is given to a student if:

˗ the topic of the work corresponds to the given one, the student makes small inaccuracies or some errors in this matter;

˗ the work is framed with inaccuracies in the design;

˗ the amount of work corresponds to the specified or slightly less;

˗ the work was submitted within the time specified by the teacher, or later, but no more than 1-2 days.

A grade of "3" is given to a student if:

˗ the topic of the work corresponds to the given one, but the work lacks significant elements in terms of the content of the work or the topic is presented illogically, the main content of the issue is not clearly presented;

˗ the work is framed with errors in the design;

˗ the amount of work is significantly less than the specified;

˗ the work was submitted with a delay of 5-6 days.

A grade of "2" is given to a student if:

˗ the main theme of the work is not disclosed;

˗ the work is not designed in accordance with the requirements of the teacher;

˗ the amount of work does not correspond to the given one;

˗ the work was submitted late in terms of more than 7 days.

Laboratory and practical work in their content have a certain structure, we propose to consider it: the progress of the work is given at the beginning of each practical and laboratory work; when performing practical work, students complete the task indicated at the end of the work (item "Assignment for students"); when performing laboratory work, a report is drawn up on its implementation, the content of the report is indicated at the end of the laboratory work (paragraph "Content of the report").

When performing laboratory and practical work, students follow certain rules, consider them below: laboratory and practical work is performed during training sessions; final registration of laboratory and practical work at home is allowed; it is allowed to use additional literature when performing laboratory and practical work; before performing laboratory and practical work, it is necessary to study the main theoretical provisions on the issue under consideration.

Practical work No. 1

"Physical properties of metals and methods for their study"

Objective : to study the physical properties of metals, methods for their determination.

Progress:

Theoretical part

Physical properties include: density, melting point (melting point), thermal conductivity, thermal expansion.

Density - the amount of substance contained in a unit volume. This is one of the most important characteristics of metals and alloys. By density, metals are divided into the following groups:lungs (density not more than 5 g/cm 3 ) - magnesium, aluminum, titanium, etc.;heavy - (density from 5 to 10 g/cm 3 ) - iron, nickel, copper, zinc, tin, etc. (this is the most extensive group);very heavy (density over 10 g/cm 3 ) - molybdenum, tungsten, gold, lead, etc. Table 1 shows the density values ​​of metals.

Table 1

Density of metals

The melting point is the temperature at which a metal changes from a crystalline (solid) state to a liquid state with the absorption of heat.

The melting points of metals range from −39°C (mercury) to 3410°C (tungsten). The melting point of most metals (with the exception of alkalis) is high, but some "normal" metals, such as tin and lead, can be melted on a conventional electric or gas stove.

Depending on the melting temperature, the metal is divided into the following groups:fusible (melting point does not exceed 600 o C) - zinc, tin, lead, bismuth, etc.;medium melting (from 600 o From to 1600 o C) - they include almost half of the metals, including magnesium, aluminum, iron, nickel, copper, gold;refractory (more than 1600 o C) - tungsten, molybdenum, titanium, chromium, etc. When additives are introduced into the metal, the melting point, as a rule, decreases.

table 2

Melting and boiling points of metals

Thermal conductivity is the ability of a metal to conduct heat at a given rate when heated.

Electrical conductivity - the ability of a metal to conduct an electric current.

Thermal expansion - the ability of a metal to increase its volume when heated.

The smooth surface of metals reflects a large percentage of light - this phenomenon is called metallic luster. However, in the powdered state, most metals lose their luster; aluminum and magnesium, however, retain their brilliance in powder. Aluminum, silver and palladium reflect light most well - mirrors are made from these metals. Rhodium is sometimes also used to make mirrors, despite its exceptionally high price: due to its much greater hardness and chemical resistance than silver or even palladium, the rhodium layer can be much thinner than silver.

Research methods in materials science

The main research methods in metal science and materials science are: fracture, macrostructure, microstructure, electron microscopy, X-ray research methods. Consider their features in more detail.

1. Fracture is the easiest and most affordable way to assess the internal structure of metals. The method of assessing fractures, despite its apparent roughness in assessing the quality of a material, is used quite widely in various industries and scientific research. Fracture evaluation in many cases can characterize the quality of the material.

The fracture may be crystalline or amorphous. Amorphous fracture is typical for materials that do not have a crystalline structure, such as glass, rosin, vitreous slags.

Metal alloys, including steel, cast iron, aluminium, magnesium alloys, zinc and its alloys give a granular, crystalline fracture.

Each face of a crystalline fracture is a plane of shearing of an individual grain. Therefore, the break shows us the size of the grain of the metal. Studying the fracture of steel, one can see that the grain size can vary over a very wide range: from a few centimeters in cast, slowly cooled steel to thousandths of a millimeter in correctly forged and hardened steel. Depending on the grain size, the fracture can be coarse-grained and fine-grained. Typically, a fine-grained fracture corresponds to a higher quality metal alloy.

If the destruction of the sample under study occurs with the previous plastic deformation, the grains in the fracture plane are deformed, and the fracture no longer reflects the internal crystalline structure of the metal; in this case, the fracture is called fibrous. Often in one sample, depending on the level of its plasticity, there may be fibrous and crystalline areas in the fracture. Often, the quality of the metal is evaluated by the ratio of the fracture area occupied by the crystalline areas under given test conditions.

A brittle crystalline fracture can result from fracture along grain boundaries or along slip planes crossing grains. In the first case, the fracture is called intergranular, in the second, transcrystalline. Sometimes, especially with very fine grains, it is difficult to determine the nature of the fracture. In this case, the fracture is studied using a magnifying glass or a binocular microscope.

Recently, the branch of metal science has been developing for the fractographic study of fractures on metallographic and electron microscopes. At the same time, they find new advantages of the old method of research in metal science - fracture studies, applying the concepts of fractal dimensions to such studies.

2. Macrostructure - is the next method for the study of metals. Macrostructural research consists in studying the sectional plane of a product or sample in the longitudinal, transverse or any other directions after etching, without the use of magnifying instruments or with a magnifying glass. The advantage of macrostructural research is the fact that with the help of this method it is possible to study the structure of a whole casting or ingot, forging, stamping, etc. directly. Using this research method, one can detect internal defects of the metal: bubbles, voids, cracks, slag inclusions, investigate the crystal structure of the casting, study the inhomogeneity of ingot crystallization and its chemical inhomogeneity (segregation).

With the help of sulfur imprints of macrosections on photographic paper according to Bauman, the uneven distribution of sulfur over the cross section of the ingots is determined. This research method is of great importance in the study of forged or stamped blanks to determine the correct direction of the fibers in the metal.

3. Microstructure - one of the main methods in metal science is the study of the metal microstructure on metallographic and electron microscopes.

This method makes it possible to study the microstructure of metal objects with high magnifications: from 50 to 2000 times on an optical metallographic microscope and from 2 to 200 thousand times on an electron microscope. The study of the microstructure is carried out on polished sections. On unetched sections, the presence of non-metallic inclusions, such as oxides, sulfides, small slag inclusions, and other inclusions that differ sharply from the nature of the base metal, is studied.

The microstructure of metals and alloys is studied on etched sections. Etching is usually done with weak acids, alkalis, or other solutions, depending on the nature of the cut metal. The effect of etching is that it dissolves various structural components in different ways, coloring them in different tones or colors. Grain boundaries that differ from the base solution usually have an etchability that differs from the base solution and is distinguished on a thin section in the form of dark or light lines.

The grain polyhedra visible under a microscope are sections of the grains by the surface of the thin section. Since this cross section is random and can pass at different distances from the center of each individual grain, the difference in the sizes of the polyhedra does not correspond to the actual differences in the sizes of the grains. The closest value to the actual grain size are the largest grains.

When etching a sample consisting of homogeneous crystalline grains, for example, a pure metal, a homogeneous solid solution, etc., etched surfaces of different grains are often observed differently.

This phenomenon is explained by the fact that grains with different crystallographic orientations emerge on the surface of the microsection, as a result of which the degree of action of the acid on these grains is different. Some grains look shiny, others are heavily etched and darken. This darkening is associated with the formation of various etching patterns that reflect light rays in different ways. In the case of alloys, individual structural components form a microrelief on the surface of the section, which has areas with different slopes of individual surfaces.

Normally located areas reflect the greatest amount of light and are the brightest. Other areas are darker. Often, the contrast in the image of a granular structure is associated not with the surface structure of the grains, but with the relief at the grain boundaries. In addition, different shades of structural constituents may be the result of the formation of films formed by the interaction of the etchant with structural constituents.

With the help of metallographic research, it is possible to carry out a qualitative identification of the structural constituents of alloys and a quantitative study of the microstructures of metals and alloys, firstly, by comparison with the known studied microcomponents of the structures and, secondly, by special methods of quantitative metallography.

The grain size is determined. The method of visual assessment, which consists in the fact that the considered microstructure is approximately estimated by the points of standard scales according to GOST 5639-68, GOST 5640-68. According to the relevant tables, for each point, the area of ​​​​one grain and the number of grains per 1 mm are determined. 2 and in 1 mm 3 .

The method of counting the number of grains per unit surface of the section according to the appropriate formulas. If S is the area on which the number of grains n is counted, and M is the magnification of the microscope, then the average grain size in the cross section of the microsection surface

Determination of the phase composition. The phase composition of the alloy is often assessed by eye or by comparing the structure with standard scales.

An approximate method for quantitative determination of the phase composition can be carried out by the secant method with the calculation of the length of segments occupied by different structural components. The ratio of these segments corresponds to the volumetric content of individual components.

Point method A.A. Glagolev. This method is carried out by estimating the number of points (points of intersection of the microscope eyepiece grid) falling on the surface of each structural component. In addition, the method of quantitative metallography produces: determination of the size of the interface between phases and grains; determination of the number of particles in the volume; determination of grain orientation in polycrystalline samples.

4. Electron microscopy. The electron microscope has recently found great importance in metallographic research. Undoubtedly, he has a great future. If the resolution of an optical microscope reaches 0.00015 mm = 1500 A, then the resolution of electron microscopes reaches 5-10 A, i.e. several hundred times more than the optical one.

An electron microscope is used to study thin films (replicas) taken from the surface of a thin section or a direct study of thin metal films obtained by thinning a massive sample.

The greatest need for the use of electron microscopy is the study of processes associated with the release of excess phases, for example, the decomposition of supersaturated solid solutions during thermal or deformation aging.

5. X-ray research methods. One of the most important methods in establishing the crystallographic structure of various metals and alloys is X-ray diffraction analysis. This research method makes it possible to determine the nature of the mutual arrangement of atoms in crystalline bodies, i.e. solve a problem that is not accessible to either a conventional or an electron microscope.

X-ray diffraction analysis is based on the interaction between X-rays and the atoms of the body under investigation, due to which the latter become, as it were, new sources of X-rays, being the centers of their scattering.

The scattering of rays by atoms can be likened to the reflection of these rays from the atomic planes of a crystal according to the laws of geometric optics.

X-rays are reflected not only from planes lying on the surface, but also from deep ones. Reflected from several equally oriented planes, the reflected beam is amplified. Each plane of the crystal lattice gives its own beam of reflected waves. Having received a certain alternation of reflected X-ray beams at certain angles, the interplanar distance, the crystallographic indices of the reflecting planes, and, ultimately, the shape and size of the crystal lattice are calculated.

Practical part

Report content.

1. In the report, you must indicate the name, purpose of the work.

2. List the main physical properties of metals (with definitions).

3. Record tables 1-2 in your notebook. Draw conclusions from the tables.

4. Fill in the table: "Basic research methods in materials science."

x-ray

research methods

Practical work No. 2

Topic: "Studying state diagrams"

Objective: familiarization of students with the main types of state diagrams, their main lines, points, their meaning.

Progress:

1. Study the theoretical part.

Theoretical part

The state diagram is a graphic representation of the state of any alloy of the system under study depending on concentration and temperature (see Fig. 1)

Fig.1 Status diagram

State diagrams show steady states, i.e. states that, under given conditions, have a minimum of free energy, and therefore it is also called an equilibrium diagram, since it shows which equilibrium phases exist under given conditions.

The construction of state diagrams is most often carried out using thermal analysis. The result is a series of cooling curves in which inflection points and temperature stops are observed at phase transformation temperatures.

Temperatures corresponding to phase transformations are called critical points. Some critical points have names, for example, the points corresponding to the beginning of crystallization are called liquidus points, and the end of crystallization are called solidus points.

According to the cooling curves, a diagram of the composition is built in coordinates: along the abscissa axis - the concentration of the components, along the ordinate axis - temperature. The concentration scale shows the content of component B. The main lines are the liquidus (1) and solidus (2) lines, as well as the lines corresponding to phase transformations in the solid state (3, 4).

From the state diagram, one can determine the phase transformation temperatures, the change in the phase composition, approximately, the properties of the alloy, the types of processing that can be applied to the alloy.

Below are the different types of state diagrams:

Fig.2. State diagram of alloys with unlimited solubility

components in the solid state (a); typical cooling curves

alloys (b)

Analysis of the resulting diagram (Fig. 2).

1. Number of components: K = 2 (components A and B).

2. Number of phases: f = 2 (liquid phase L, solid solution crystals)

3. Main lines of the chart:

acb is the liquidus line, above this line the alloys are in the liquid state;

adb is the solidus line, below this line the alloys are in the solid state.

Fig.3. State diagram of alloys with the absence of solubility of components in the solid state (a) and cooling curves of alloys (b)

Analysis of the state diagram (Fig. 3).

1. Number of components: K = 2(components A and B);

2. Number of phases: f = 3(component A crystals, component B crystals, liquid phase).

3. Main lines of the chart:

the solidus line ecf, parallel to the concentration axis, tends to the axes of the components, but does not reach them;

Rice. Fig. 4. State diagram of alloys with limited solubility of components in the solid state (a) and cooling curves of typical alloys (b)

State diagram analysis (Fig. 4).

1. Number of components: K = 2 (components A and B);

2. Number of phases: f = 3 (liquid phase and crystals of solid solutions (component B solution in component A) and (component A solution in component B));

3. Main lines of the chart:

the liquidus line acb, consists of two branches converging at one point;

line solidus adcfb, consists of three sections;

dm is the line of the limiting concentration of component B in component A;

fn - line of the maximum concentration of component A in component B.

Practical part

Task for students:

1. Write down the title of the work and its purpose.

2. Write down what a state diagram is.

Answer the questions:

1. How is a state diagram built?

2. What can be determined from the state diagram?

3. What are the names of the main points of the diagram?

4. What is indicated on the diagram along the x-axis? Axes of ordinates?

5. What are the main lines of the diagram called?

Assignment by options:

Students answer the same questions, different are the pictures that need to be answered. Option 1 gives answers according to figure 2, option 2 gives answers according to figure 3, option 3 gives answers according to figure 4. The figure must be fixed in a notebook.

1. What is the name of the diagram?

2. Name how many components are involved in the formation of an alloy?

3. What letters indicate the main lines of the diagram?

Practical work No. 3

Topic: "The study of cast irons"

Objective: familiarization of students with the marking and scope of cast iron; formation of the ability to decipher grades of cast iron.

Progress:

Theoretical part

Cast iron differs from steel: in composition - a higher content of carbon and impurities; in terms of technological properties - higher casting properties, low ability to plastic deformation, almost never used in welded structures.

Depending on the state of carbon in cast iron, they distinguish: white cast iron - carbon in a bound state in the form of cementite, in a fracture it has a white color and a metallic sheen; gray cast iron - all or most of the carbon is in the free state in the form of graphite, and no more than 0.8% of carbon is in the bound state. Due to the large amount of graphite, its fracture is gray; half - part of the carbon is in the free state in the form of graphite, but at least 2% of the carbon is in the form of cementite. Little used in technology.

Depending on the form of graphite and the conditions of its formation, the following groups of cast irons are distinguished: gray - with lamellar graphite; high-strength - with nodular graphite; malleable - with flaky graphite.

Graphite inclusions can be considered as a corresponding form of voids in the cast iron structure. Stresses are concentrated near such defects during loading, the value of which is the greater, the sharper the defect. It follows from this that lamellar graphite inclusions weaken the metal to the maximum extent. The flaky form is more favorable, and the spherical form of graphite is optimal. Plasticity depends on the shape in the same way. The presence of graphite most sharply reduces the resistance under hard loading methods: impact; gap. The compression resistance is reduced a little.

Gray cast irons

Gray cast iron is widely used in mechanical engineering, as it is easy to process and has good properties. Depending on the strength, gray cast iron is divided into 10 grades (GOST 1412).

Gray cast irons with low tensile strength have a fairly high compressive strength. The structure of the metal base depends on the amount of carbon and silicon.

Given the low resistance of gray iron castings to tensile and shock loads, this material should be used for parts that are subjected to compressive or bending loads. In the machine tool industry, these are basic, body parts, brackets, gears, guides; in the automotive industry - cylinder blocks, piston rings, camshafts, clutch discs. Gray iron castings are also used in electrical engineering, for the manufacture of consumer goods.

Marking of gray cast irons: indicated by the index SC (grey cast iron) and a number that shows the value of the tensile strength multiplied by 10 -1 .

For example: SCH 10 - gray cast iron, tensile strength 100 MPa.

malleable iron

Good casting properties are ensured if no graphitization occurs during the crystallization and cooling of the castings in the mold. To prevent graphitization, cast irons must have a reduced carbon and silicon content.

There are 7 grades of ductile iron: three with ferritic (KCh 30 - 6) and four with pearlite (KCh 65 - 3) base (GOST 1215).

In terms of mechanical and technological properties, malleable cast iron occupies an intermediate position between gray cast iron and steel. The disadvantage of ductile iron compared to ductile iron is the limitation of the wall thickness for casting and the need for annealing.

Ductile iron castings are used for parts operating under shock and vibration loads.

Ferritic cast irons are used to make gearbox housings, hubs, hooks, brackets, clamps, couplings, flanges.

Perlitic cast irons, characterized by high strength and sufficient ductility, are used to make forks of cardan shafts, links and rollers of conveyor chains, and brake shoes.

Marking malleable iron: indicated by the index CCH (ductile iron) and numbers. The first number corresponds to the tensile strength multiplied by 10 -1 , the second number is the relative elongation.

For example: KCh 30-6 - malleable cast iron, tensile strength 300 MPa, relative elongation 6%.

Ductile iron

These cast irons are obtained from gray ones, as a result of modification with magnesium or cerium. Compared to gray cast irons, the mechanical properties are improved, this is due to the absence of uneven stress distribution due to the nodular shape of the graphite.

These cast irons have high fluidity, linear shrinkage is about 1%. Foundry stresses in castings are somewhat higher than for gray cast iron. Due to the high modulus of elasticity, the machinability is quite high. They have satisfactory weldability.

From high-strength cast iron, thin-walled castings (piston rings), forging hammers, beds and frames of presses and rolling mills, molds, tool holders, and faceplates are made.

Castings of crankshafts weighing up to 2..3 tons, instead of forged steel shafts, have a higher cyclic viscosity, are insensitive to external stress concentrators, have better antifriction properties and are much cheaper.

Ductile iron marking: indicated by the HF index (ductile iron) and a number that shows the tensile strength value multiplied by 10 -1 .

For example: VCh 50 - high-strength cast iron with a tensile strength of 500 MPa.

Practical part

Task for students:

1. Write down the name of the work, its purpose.

2. Describe the production of pig iron.

3.Fill in the table:

3.High strength

cast irons

Practical work No. 4

Topic: "Study of carbon and alloy structural steels"

Objective:

Progress:

1. Familiarize yourself with the theoretical part.

2. Complete the tasks of the practical part.

Theoretical part

Steel is an alloy of iron and carbon, in which carbon is contained in an amount of 0 -2.14%. Steels are the most common materials. They have good technological properties. Products are obtained as a result of processing by pressure and cutting.

Quality depending on the content of harmful impurities: sulfur and phosphorus steel is divided into steel:

˗ Ordinary quality, up to 0.06% sulfur and up to 0.07% phosphorus.

˗ High-quality - up to 0.035% sulfur and phosphorus each separately.

˗ High quality - up to 0.025% sulfur and phosphorus.

˗ Extra high quality, up to 0.025% phosphorus and up to 0.015% sulfur.

Deoxidation is the process of removing oxygen from steel, i.e., according to the degree of its deoxidation, there are: calm steels, i.e., completely deoxidized; such steels are designated by the letters "sp" at the end of the brand (sometimes the letters are omitted); boiling steel - slightly deoxidized; are marked with the letters "kp"; semi-quiet steels, occupying an intermediate position between the two previous ones; denoted by the letters "ps".

Ordinary quality steel is also subdivided according to deliveries into 3 groups: steel of group A is supplied to consumers by mechanical properties (such steel may have a high content of sulfur or phosphorus); steel group B - by chemical composition; group B steel - with guaranteed mechanical properties and chemical composition.

Structural steels are intended for the manufacture of structures, machine parts and devices.

So in Russia and in the CIS countries (Ukraine, Kazakhstan, Belarus, etc.), an alphanumeric system for designating grades of steels and alloys, developed earlier in the USSR, was adopted, where, according to GOST, the names of the elements and methods of steel smelting are conditionally denoted by letters, and the content is indicated by numbers. elements. Until now, international organizations for standardization have not developed a unified steel marking system.

Structural carbon steel marking

ordinary quality

˗ Designated according to GOST 380-94 with the letters "St" and the conditional brand number (from 0 to 6) depending on the chemical composition and mechanical properties.

˗ The higher the carbon content and strength properties of steel, the higher its number.

˗ The letter "G" after the grade number indicates an increased content of manganese in steel.

˗ The steel group is indicated before the grade, and the group "A" in the designation of the steel grade is not put.

˗ To indicate the category of steel, a number is added to the grade designation at the end of the corresponding category, the first category is usually not indicated.

For instance:

˗ St1kp2 - carbon steel of ordinary quality, boiling, grade No. 1, second category, supplied to consumers in terms of mechanical properties (group A);

˗ VSt5G - carbon steel of ordinary quality with a high content of manganese, calm, grade No. 5, first category with guaranteed mechanical properties and chemical composition (group B);

˗ VSt0 - carbon steel of ordinary quality, grade number 0, group B, first category (steels of grades St0 and Bst0 are not divided according to the degree of deoxidation).

Marking of structural carbon quality steels

˗ In accordance with GOST 1050-88, these steels are marked with two-digit numbers showing the average carbon content in hundredths of a percent: 05; 08; 10 ; 25; 40, 45 etc.

˗ For calm steels, letters are not added at the end of their names.

For example, 08kp, 10ps, 15, 18kp, 20, etc.

˗ The letter G in the steel grade indicates a high content of manganese.

For example: 14G, 18G, etc.

˗ The most common group for the manufacture of machine parts (shafts, axles, bushings, gears, etc.)

For instance:

˗ 10 - structural carbon quality steel, with a carbon content of about 0.1%, calm

˗ 45 - structural carbon quality steel, with a carbon content of about 0.45%, calm

˗ 18 kp - structural carbon quality steel with a carbon content of about 0.18%, boiling

˗ 14G - high-quality structural carbon steel with a carbon content of about 0.14%, calm, with a high manganese content.

Marking of alloy structural steels

˗ In accordance with GOST 4543-71, the names of such steels consist of numbers and letters.

˗ The first digits of the grade indicate the average carbon content in steel in hundredths of a percent.

˗ The letters indicate the main alloying elements included in the steel.

˗ The numbers after each letter indicate the approximate percentage of the corresponding element, rounded up to a whole number, with an alloying element content of up to 1.5%, the number after the corresponding letter is not indicated.

˗ The letter A at the end of the grade indicates that the steel is high quality (with a reduced content of sulfur and phosphorus)

˗ N - nickel, X - chromium, K - cobalt, M - molybdenum, V - tungsten, T - titanium, D - copper, G - manganese, S - silicon.

For instance:

˗ 12X2H4A - structural alloy steel, high quality, with a carbon content of about 0.12%, chromium about 2%, nickel about 4%

˗ 40KhN - structural alloy steel, with a carbon content of about 0.4%, chromium and nickel up to 1.5%

Marking of other groups of structural steels

Spring-spring steels.

˗ The main distinguishing feature of these steels is that the carbon content in them should be about 0.8% (in this case, elastic properties appear in the steels)

˗ Springs and springs are made of carbon (65,70,75,80) and alloyed (65S2, 50KhGS, 60S2KhFA, 55KhGR) structural steels

˗ These steels are alloyed with elements that increase the elastic limit - silicon, manganese, chromium, tungsten, vanadium, boron

For example: 60С2 - structural carbon spring steel with a carbon content of about 0.65%, silicon about 2%.

Ball bearing steels

˗ GOST 801-78 is marked with the letters "ШХ", after which the chromium content is indicated in tenths of a percent.

˗ For steels subjected to electroslag remelting, the letter Ш is also added at the end of their names through a dash.

For example: ШХ15, ШХ20СГ, ШХ4-Ш.

˗ They are used to make parts for bearings, they are also used to make parts that work under high loads.

For example: ШХ15 - structural ball bearing steel with a carbon content of 1%, chromium 1.5%

Automatic steels

˗ GOST 1414-75 begin with the letter A (automatic).

˗ If the steel is alloyed with lead, then its name begins with the letters AC.

˗ To reflect the content of other elements in steels, the same rules are used as for alloyed structural steels. For example: A20, A40G, AS14, AS38HGM

For example: AC40 - automatic structural steel, with a carbon content of 0.4%, lead 0.15-0.3% (not indicated in the brand)

Practical part

Task for students:

2. Write down the main features of the marking of all groups of structural steels (ordinary quality, quality steels, alloyed structural steels, spring steels, ball bearing steels, free cutting steels), with examples.

Assignment by options:

Decipher the steel grades and write down the scope of a particular grade (i.e. what it is intended for).

Practical work No. 5

Topic: "Study of carbon and alloy tool steels"

Objective: familiarization of students with the marking and scope of structural steels; formation of the ability to decipher the marking of structural steels.

Progress:

1. Familiarize yourself with the theoretical part.

2. Complete the task of the practical part.

Theoretical part

Steel is an alloy of iron and carbon, in which carbon is contained in an amount of 0-2.14%.

Steels are the most common materials. They have good technological properties. Products are obtained as a result of processing by pressure and cutting.

The advantage is the ability to obtain the desired set of properties by changing the composition and type of processing.

Depending on the purpose, steels are divided into 3 groups: structural, tool and special-purpose steels.

Quality depending on the content of harmful impurities: sulfur and phosphorus steel is divided into: steel of ordinary quality, content up to 0.06% sulfur and up to 0.07% phosphorus; quality - up to 0.035% sulfur and phosphorus each separately; high quality - up to 0.025% sulfur and phosphorus; especially high-quality, up to 0.025% phosphorus and up to 0.015% sulfur.

Tool steels are intended for the manufacture of various tools, both for manual processing and for mechanical processing.

The presence of a wide range of manufactured steels and alloys manufactured in different countries has necessitated their identification, but so far there is no single system for marking steels and alloys, which creates certain difficulties for the metal trade.

Marking of carbon tool steels

˗ These steels in accordance with GOST 1435-90 are divided into high-quality and high-quality.

˗ High-quality steels are designated by the letter U (carbon) and a number indicating the average carbon content in the steel, in tenths of a percent.

For example: U7, U8, U9, U10. U7 - carbon tool steel with a carbon content of about 0.7%

˗ The letter A is added to the designations of high-quality steels (U8A, U12A, etc.). In addition, in the designations of both high-quality and high-quality carbon tool steels, the letter G may be present, indicating an increased content of manganese in the steel.

For example: U8G, U8GA. U8A - carbon tool steel with a carbon content of about 0.8%, high quality.

˗ They make tools for manual work (chisel, center punch, scriber, etc.), mechanical work at low speeds (drills).

Marking of alloyed tool steels

˗ The rules for designating alloyed tool steels according to GOST 5950-73 are basically the same as for structural alloyed steels.

The difference lies only in the numbers indicating the mass fraction of carbon in steel.

˗ The percentage of carbon is also indicated at the beginning of the name of the steel, in tenths of a percent, and not in hundredths, as for structural alloy steels.

˗ If the carbon content in tool alloy steel is about 1.0%, then the corresponding figure at the beginning of its name is usually not indicated.

Here are examples: steel 4Kh2V5MF, KhVG, KhVCh.

˗ 9Kh5VF - alloy tool steel, with a carbon content of about 0.9%, chromium about 5%, vanadium and tungsten up to 1%

Marking of high-alloyed (high-speed)

tool steels

˗ Designated with the letter "P", the number following it indicates the percentage of tungsten in it: Unlike alloy steels, the percentage of chromium is not indicated in the names of high-speed steels, because it is about 4% in all steels, and carbon (it is proportional to the content of vanadium).

˗ The letter F, indicating the presence of vanadium, is indicated only if the content of vanadium is more than 2.5%.

For example: R6M5, R18, R6 M5F3.

˗ Usually these steels are used to make high-performance tools: drills, milling cutters, etc. (to reduce the cost of only the working part)

For example: R6M5K2 - high-speed steel, with a carbon content of about 1%, about 6% tungsten, about 4% chromium, up to 2.5% vanadium, about 5% molybdenum, about 2% cobalt.

Practical part

Task for students:

1. Write down the name of the work, its purpose.

2. Write down the basic principles for marking all groups of tool steels (carbon, alloy, high alloy)

Assignment by options:

1. Decipher the steel grades and write down the scope of a particular grade (that is, what it is intended for).

Practical work No. 6

Topic: "The study of alloys based on copper: brass, bronze"

Objective: familiarization of students with the marking and scope of non-ferrous metals - copper and alloys based on it: brass and bronze; formation of the ability to decipher the markings of brass and bronze.

Recommendations for students:

Progress:

1. Familiarize yourself with the theoretical part.

2. Complete the task of the practical part.

Theoretical part

Brass

Brass can contain up to 45% zinc. Increasing the zinc content to 45% leads to an increase in tensile strength up to 450 MPa. Maximum ductility occurs at a zinc content of about 37%.

According to the method of manufacturing products, wrought brass and casting brass are distinguished.

Wrought brass are marked with the letter L, followed by a number showing the percentage of copper, for example, brass L62 contains 62% copper and 38% zinc. If, in addition to copper and zinc, there are other elements, then their initial letters are put (O - tin, C - lead, F - iron, F - phosphorus, Mts - manganese, A - aluminum, C - zinc).

The number of these elements is indicated by the corresponding numbers after the number showing the copper content, for example, the LAZH60-1-1 alloy contains 60% copper, 1% aluminum, 1% iron and 38% zinc.

Brass has good corrosion resistance, which can be further improved by adding tin. Brass LO70 -1 is resistant to corrosion in sea water and is called “marine brass“. The addition of nickel and iron increases the mechanical strength up to 550 MPa.

Cast brass is also marked with the letter L. After the letter designation of the main alloying element (zinc) and each subsequent one, a number is placed indicating its average content in the alloy. For example, brass LTS23A6Zh3Mts2 contains 23% zinc, 6% aluminum, 3% iron, 2% manganese. The best fluidity has brass brand LTS16K4. Foundry brasses include brasses of the type LS, LK, LA, LAZh, LAZhMts. Foundry brasses are not prone to segregation, have concentrated shrinkage, castings are obtained with a high density.

Brass is a good material for structures operating at low temperatures.

Bronzes

Alloys of copper with elements other than zinc are called bronzes. Bronzes are divided into deformable and cast.

When marking wrought bronzes, the letters Br are put in the first place, then the letters indicating which elements, except for copper, are part of the alloy. The letters are followed by numbers indicating the content of the components in the alloy. For example, the brand BrOF10-1 means that bronze contains 10% tin, 1% phosphorus, and the rest is copper.

The marking of foundry bronzes also begins with the letters Br, then the letters of the alloying elements are indicated and a number is put indicating its average content in the alloy. For example, bronze BrO3Ts12S5 contains 3% tin, 12% zinc, 5% lead, the rest is copper.

Tin Bronzes When copper is fused with tin, solid solutions are formed. These alloys are very prone to segregation due to the large temperature range of crystallization. Due to segregation, alloys with a tin content of more than 5% are favorable for parts such as plain bearings: the soft phase provides good run-in, hard particles create wear resistance. Therefore, tin bronzes are good anti-friction materials.

Tin bronzes have low volumetric shrinkage (about 0.8%), therefore they are used in artistic casting. The presence of phosphorus provides good fluidity. Tin bronzes are divided into wrought and cast bronzes.

In deformable bronzes, the tin content should not exceed 6%, to ensure the necessary plasticity, BrOF6.5-0.15. Depending on the composition, wrought bronzes are characterized by high mechanical, anti-corrosion, anti-friction and elastic properties, and are used in various industries. Rods, pipes, tape, wire are made from these alloys.

Practical part

Task for students:

1. Write down the title and purpose of the work.

2.Fill in the table:

Name

alloy, his

definition

Main

properties

alloy

Example

markings

Decryption

stamps

Region

applications

Practical work number 7

Topic: "The study of aluminum alloys"

Objective: familiarization of students with the marking and scope of non-ferrous metals - aluminum and alloys based on it; study of the features of the use of aluminum alloys depending on their composition.

Recommendations for students: Before proceeding with the practical part of the task, carefully read the theoretical provisions, as well as the lectures in your workbook on this topic.

Progress:

1. Familiarize yourself with the theoretical part.

2. Complete the task of the practical part.

Theoretical part

The principle of marking aluminum alloys. At the beginning, the type of alloy is indicated: D - alloys of the duralumin type; A - technical aluminum; AK - malleable aluminum alloys; B - high-strength alloys; AL - casting alloys.

Next, the conditional number of the alloy is indicated. The conditional number is followed by a designation characterizing the state of the alloy: M - soft (annealed); T - heat treated (hardening plus aging); H - cold-worked; P - semi-hardened.

According to technological properties, alloys are divided into three groups: wrought alloys, not hardened by heat treatment; wrought alloys hardened by heat treatment; casting alloys. Sintered aluminum alloys (SAS) and sintered aluminum powder alloys (SAP) are produced by powder metallurgy methods.

Wrought cast alloys not hardened by heat treatment.

The strength of aluminum can be increased by alloying. In alloys that are not hardened by heat treatment, manganese or magnesium is introduced. The atoms of these elements significantly increase its strength, reducing plasticity. Alloys are designated: with manganese - AMts, with magnesium - AMg; after the designation of the element, its content (AMg3) is indicated.

Magnesium acts only as a hardener, manganese strengthens and increases corrosion resistance. The strength of alloys increases only as a result of deformation in the cold state. The greater the degree of deformation, the greater the increase in strength and the decrease in ductility. Depending on the degree of hardening, hard-worked and semi-work-hardened alloys (AMg3P) are distinguished.

These alloys are used for the manufacture of various welded tanks for fuel, nitric and other acids, low- and medium-loaded structures. Deformable alloys hardened by heat treatment.

Such alloys include duralumins (complex alloys of aluminum - copper - magnesium or aluminum - copper - magnesium - zinc systems). They have reduced corrosion resistance, to increase which manganese is introduced. Duralumins are usually hardened with a temperature of 500 O C and natural aging, which is preceded by a two- to three-hour incubation period. Maximum strength is reached after 4.5 days. Duralumins are widely used in the aircraft industry, automotive industry, and construction.

High-strength aging alloys are alloys that contain zinc in addition to copper and magnesium. Alloys V95, V96 have a tensile strength of about 650 MPa. The main consumer is the aircraft industry (skin, stringers, spars).

Forging aluminum alloys AK, AK8 are used for the manufacture of forgings. Forgings are produced at a temperature of 380-450 O C, subjected to hardening from a temperature of 500-560 O C and aging at 150-165 O C within 6 hours.

Nickel, iron, titanium are additionally introduced into the composition of aluminum alloys, which increase the recrystallization temperature and heat resistance up to 300 O WITH.

Pistons, blades and disks of axial compressors, turbojet engines are manufactured.

Cast alloys

Cast alloys include alloys of the aluminum-silicon system (silumins) containing 10-13% silicon. The additive to magnesium and copper silumins contributes to the effect of hardening of cast alloys during aging. Titanium and zirconium grind the grain. Manganese enhances anti-corrosion properties. Nickel and iron increase heat resistance.

Cast alloys are marked from AL2 to AL20. Silumins are widely used for the manufacture of cast parts for devices and other medium- and light-loaded parts, including thin-walled castings of complex shape.

Practical part

Task for students:

1. Write down the title and purpose of the work.

2. Fill in the table:

Name

alloy, his

definition

Main

properties

alloy

Example

markings

Decryption

stamps

Region

applications

Lab #1

Topic: "Mechanical properties of metals and methods for their study (hardness)"

Objective:

Progress:

1. Familiarize yourself with the theoretical provisions.

2. Complete the task of the teacher.

3. Make a report in accordance with the task.

Theoretical part

Hardness is the ability of a material to resist the penetration of another body into it. When testing for hardness, the body introduced into the material and called the indenter must be harder, have a certain size and shape, and must not receive permanent deformation. Hardness tests can be static or dynamic. The first type includes tests by the indentation method, the second - by the impact indentation method. In addition, there is a method for determining hardness by scratching - sclerometry.

By the value of the hardness of the metal, you can get an idea of ​​the level of its properties. For example, the higher the hardness determined by the pressure of the tip, the lower the ductility of the metal, and vice versa.

Hardness testing by the indentation method consists in the fact that an indenter (diamond, hardened steel, hard alloy) in the form of a ball, cone or pyramid is pressed into the sample under the action of a load. After removing the load, an imprint remains on the sample, by measuring the value of which (diameter, depth or diagonal) and comparing it with the dimensions of the indenter and the load, one can judge the hardness of the metal.

Hardness is determined on special devices - hardness testers. Most often, hardness is determined by the methods of Brinell (GOST 9012-59) and Rockwell (GOST 9013-59).

There are general requirements for sample preparation and testing by these methods:

1. The surface of the sample must be clean, without defects.

2. Samples must be of a certain thickness. After receiving the imprint, there should be no signs of deformation on the reverse side of the sample.

3. The specimen must lie firmly and firmly on the stage.

4. The load must act perpendicular to the sample surface.

Determination of Brinell hardness

The hardness of the metal according to Brinell is determined by indenting a hardened steel ball (Fig. 1) with a diameter of 10 into the sample; 5 or 2.5 mm and is expressed by the hardness number HB obtained by dividing the applied load P in N or kgf (1N = 0.1 kgf) by the surface area of ​​the imprint F formed on the sample in mm

Brinell hardness number HB expressed as the ratio of the applied loadFto the squareSspherical surface of the imprint (hole) on the measured surface.

HB = , (MPa),

where

S– area of ​​the spherical surface of the imprint, mm 2 (expressed throughDandd);

D– ball diameter, mm;

d– imprint diameter, mm;

load valueF, ball diameterDand the duration of exposure under load τ, are selected according to table 1.

Figure 1. Scheme of Brinell hardness measurement.

a) Scheme of pressing the ball into the test metal

FDis the diameter of the ball,d otp - the diameter of the imprint;

b) Measurement of the imprint diameter with a magnifying glass (in the figured=4.2 mm).

Table 1.

Choice of ball diameter, load and dwell time under load depending on

on the hardness and thickness of the sample

over 6

6…3

less than 3

29430 (3000)

7355 (750)

1840 (187,5)

Less than 1400

over 6

6…3

less than 3

9800 (1000)

2450 (750)

613 (62,5)

Non-ferrous metals and alloys (copper, brass, bronze, magnesium alloys, etc.)

350-1300

over 6

6…3

less than 3

9800 (1000)

2450 (750)

613 (62,5)

30

Non-ferrous metals (aluminum, bearing alloys, etc.)

80-350

over 6

6…3

less than 3

10

5

2,5

2450 (250)

613 (62,5)

153,2 (15,6)

60

Figure 2 shows a diagram of a lever device. The sample is mounted on the object table 4. Rotating the flywheel 3, the screw 2 lifts the sample until it comes into contact with the ball 5 and then until the spring 7, put on the spindle 6, is fully compressed. The spring creates a preload on the ball equal to 1 kN (100 kgf), which provides a stable position of the sample during loading. After that, the electric motor 13 is turned on and through the worm gear of the gearbox 12, the connecting rod 11 and the system of levers 8.9 located in the body 1 of the hardness tester with weights 10 creates a given full load on the ball. A spherical imprint is obtained on the test specimen. After unloading the device, the sample is removed and the diameter of the imprint is determined with a special magnifying glass. The arithmetic mean value of measurements in two mutually perpendicular directions is taken as the calculated diameter of the indent.

Figure 2. Diagram of the Brinell device

Using the above formula, using the measured indentation diameter, the hardness number HB is calculated. The hardness number depending on the diameter of the resulting imprint can also be found from the tables (see the table of hardness numbers).

When measuring hardness with a ball with a diameter D = 10.0 mm under a load F = 29430 N (3000 kgf), with a holding time τ = 10 s, the hardness number is written as follows:HB2335 MPa or according to the old designation HB 238 (in kgf / mm 2 )

When measuring Brinell hardness, remember the following:

It is possible to test materials with a hardness of not more than HB 4500 MPa, since with a higher sample hardness, an unacceptable deformation of the ball itself occurs;

To avoid punching, the minimum thickness of the sample must be at least ten times the depth of the indentation;

The distance between the centers of two adjacent prints must be at least four print diameters;

The distance from the center of the imprint to the side surface of the sample must be at least 2.5d.

Rockwell hardness determination

According to the Rockwell method, the hardness of metals is determined by indenting a hardened steel ball with a diameter of 1.588 mm or a diamond cone with an apex angle of 120 into the test sample. O under the action of two sequentially applied loads: preliminary Р0 = 10 kgf and total Р, equal to the sum of preliminary Р0 and main Р1 loads (Fig. 3).

Rockwell hardness numberHRis measured in arbitrary dimensionless units and is determined by the formulas:

HR c = - when the diamond cone is indented

HR v = - when a steel ball is pressed in,

where 100– the number of divisions of the black scale C, 130 is the number of divisions of the red scale B of the dial of the indicator measuring the depth of indentation;

h 0 - the depth of indentation of the diamond cone or ball under the action of the preload. Mm

h– depth of indentation of the diamond cone or ball under the action of the total load, mm

0.002 - price of division of the indicator dial scale (moving the diamond cone when measuring hardness by 0.002 mm corresponds to moving the indicator pointer by one division), mm

The type of tip and the load value is selected according to table 2, depending on the hardness and thickness of the test sample. .

Rockwell hardness number (HR) is a measure of the depth of indentation and is expressed in arbitrary units. A unit of hardness is a dimensionless value corresponding to an axial displacement of 0.002 mm. The Rockwell hardness number is indicated directly by an arrow on the C or B scale of the indicator after automatic removal of the main load. The hardness of the same metal, determined by different methods, is expressed in different units of hardness.

For instance,HB 2070, HR c 18 orHR v 95.

Figure 3. Rockwell hardness measurement scheme

table 2

V

HR V

steel ball

981 (100)

0,7

25…100

on a B scale

2000 to 7000 (hardened steels)

WITH

HR WITH

diamond cone

1471 (150)

0,7

20…67

on a scale C

From 4000 to 9000 (parts carburized or nitrided, hard alloys, etc.)

A

HR A

diamond cone

588 (60)

0,4

70…85

on a B scale

The Rockwell method is characterized by simplicity and high productivity, ensures the preservation of a high-quality surface after testing, and allows testing metals and alloys, both low and high hardness. This method is not recommended for alloys with an inhomogeneous structure (gray cast iron, malleable and high-strength cast iron, antifriction bearing alloys, etc.).

Practical part

Report content.

Indicate the title of the work, its purpose.

Answer the questions:

1. What is called hardness?

2. What is the essence of the definition of hardness?

3. What 2 hardness testing methods do you know? What is their difference?

4. How should the sample be prepared for testing?

5. How to explain the lack of a universal method for determining hardness?

6. Why is hardness most often determined of the many mechanical characteristics of materials?

7. Record in your notebook the scheme for determining Brinell and Rockwell hardness.

Lab #2

Topic: "Mechanical properties of metals and methods for their study (strength, elasticity)"

Objective: to study the mechanical properties of metals, methods for their study.

Progress:

1. Familiarize yourself with the theoretical provisions.

2. Complete the task of the teacher.

3. Make a report in accordance with the task.

Theoretical part

The main mechanical properties are strength, elasticity, viscosity, hardness. Knowing the mechanical properties, the designer reasonably chooses the appropriate material that ensures the reliability and durability of structures with their minimum weight.

Mechanical properties determine the behavior of the material during deformation and destruction from the action of external loads. Depending on the loading conditions, the mechanical properties can be determined at:

1. Static loading - the load on the sample increases slowly and smoothly.

2. Dynamic loading - the load increases at a high speed, has a shock character.

3. Repeated-variable or cyclic loading - the load during the test repeatedly changes in magnitude or in magnitude and direction.

To obtain comparable results, the samples and the methodology for conducting mechanical tests are regulated by GOSTs. In a static tensile test: GOST 1497, strength and ductility characteristics are obtained.

Strength - the ability of a material to resist deformation and destruction.

Plasticity is the ability of a material to change its size and shape under the influence of external forces; measure of plasticity - the value of residual deformation.

The device that determines the strength and ductility is a tensile testing machine that records the tension diagram (see Fig. 4), expressing the relationship between the elongation of the sample and the acting load.

Rice. Fig. 4. Stretch diagram: a – absolute, b – relative.

Section oa on the diagram corresponds to the elastic deformation of the material when Hooke's law is observed. The stress corresponding to the elastic ultimate strain at point a is called the proportional limit.

The proportional limit is the maximum voltage up to which Hooke's law is valid.

At stresses above the proportionality limit, uniform plastic deformation occurs (lengthening or narrowing of the cross section).

Point b - elastic limit - the highest stress, until which no residual deformation occurs in the sample.

Area cd is the yield point, it corresponds to the yield strength - this is the stress at which an increase in deformation occurs in the sample without increasing the load (the material "flows").

Many grades of steel, non-ferrous metals do not have a pronounced yield point, therefore, a conditional yield strength is set for them. The conditional yield strength is the stress that corresponds to a residual deformation equal to 0.2% of the initial length of the sample (alloyed steel, bronze, duralumin, and other materials).

Point B corresponds to the strength limit (local thinning appears on the sample - a neck, the formation of thinning is typical for plastic materials).

Tensile strength is the maximum stress that the sample can withstand before resolution (tensile strength).

Beyond point B, the load drops (due to elongation of the neck) and failure occurs at point K.

Practical part.

Report content.

1. Indicate the name of the work, its purpose.

2. What mechanical properties do you know? What methods determine the mechanical properties of materials?

3. Write down the definition of the concepts of strength and ductility. What methods are used to determine them? What is the name of the device that determines these properties? How are properties defined?

4. Record the absolute tension curve of the plastic material.

5. After the diagram, specify the names of all points and sections of the diagram.

6. What limit is the main characteristic when choosing a material for the manufacture of any product? Justify the answer.

7. What materials are more reliable in work, brittle or ductile? Justify the answer.

Bibliography

Main:

Adaskin A.M., Zuev V.M. Materials science (metalworking). - M .: JIC "Academy", 2009 - 240 p.

Adaskin A.M., Zuev V.M. Materials science and technology of materials. - M.: FORUM, 2010 - 336 p.

Chumachenko Yu.T. Materials Science and Plumbing (NPO and SPO). - Rostov n / D .: Phoenix, 2013 - 395 p.

Additional:

Zhukovets I.I. Mechanical testing of metals. - M.: Vyssh.shk., 1986. - 199 p.

Lakhtin Yu.M. Fundamentals of materials science. – M.: Metallurgy, 1988.

Lakhtin Yu.M., Leontieva V.P. Materials Science. - M .: Mashinostroenie, 1990.

Electronic resources:

1. Journal "Materials Science". (Electronic resource) – access form http://www.nait.ru/journals/index.php?p_journal_id=2.

2. Materials science: educational resource, access form http://www.supermetalloved/narod.ru.

3. Steel grader. (Electronic resource) – access form www.splav.kharkov.com.

4. Federal Center for Information and Educational Resources. (Electronic resource) – www.fcior.ru access form.

Questions for the exam for the 2nd year of the faculty of MI
Questions for the exam for undergraduates of the 1st year of MI

Laboratory works

Laboratory journals for the course "Materials Science"

(Students are required to carry a printed version of the lab journals for lab work)

Laboratory work on the course "Materials Science"

Laboratory work on the course "Materials Science"

The main educational and educational-methodical literature on the disciplines read at the department

Cycle Materials Science

1. Bogodukhov S.I., Kozik E.S. Materials Science. Textbook for high schools. – M.: Mashinostroenie, 2015. – 504 p.
2. Solntsev Yu.P., Pryakhin E.I. Materials Science. Textbook for high schools. - St. Petersburg: KHIMIZDAT, 2007. - 784 p.
3. Arzamasov V.B., Cherepakhin A.A. Materials Science. Textbook. - M.: Exam, 2009. - 352 p.: ill.
4. Oskin V.A., Baikalova V.N., Karpenkov V.F. Workshop on Materials Science and Technology of Structural Materials: Textbook for High Schools (ed. Oskin V.A., Baikalova V.N.) . - M.: KolosS, 2007. - 318 p.: ill.
5. Material science and technology of metals: a textbook for universities / G.P. Fetisov and others - 6th ed., add. - M.: Higher School, 2008. - 878 p.
6. Materials science and technology of metals: a textbook for universities in engineering specialties / G.P. Fetisov, M.G. Karpman and others - M .: Higher School, 2009. - 637 p.
7. Medvedeva M.L., Prygaev A.K. Notebook on materials science. Methodological guide - M .: Publishing Center of the Russian State University of Oil and Gas. THEM. Gubkina, 2010, 90 p.
8. Efimenko L.A., Elagina O.Yu., Prygaev A.K., Vyshemirsky E.M., Kapustin O.E., Muradov A.V. Promising and traditional pipe steels for the construction of gas and oil pipelines. Monograph. – M.: Logos, 2011, 336 p.
9. Prygaev A.K., Kurakin I.B., Vasiliev A.A., Krivosheev Yu.V. Substantiation of the choice of structural materials and development of their heat treatment modes for the manufacture of machine parts and equipment for the oil and gas industry. Methodical manual for course work in the discipline "Materials Science" - M .: Russian State University of Oil and Gas named after I.M. Gubkina, 2015
10. Fektistov G.P., Karpman M.G., Miatyukhin V.M. etc. Material science and technology of materials. - M .: Higher School, 2000
11. Gulyaev A.P. Materials Science. - M .: Metallurgy, 1986
12. Efimenko L.A., Prygaev A.K., Elagina O.Yu. Metal science and heat treatment of welded joints. Tutorial. - M.: Logos, 2007. - 455 p.: ill.
13. Guidelines for laboratory work on the course "Materials Science" part 1 and part 2, - M .: RGU of Oil and Gas, 2000
14. Trofimova G.A. Guidelines for laboratory work "Construction and analysis of a thermomechanical curve for amorphous polymers" and "Determination of the mechanical properties of plastics and rubbers." - M .: Russian State University of Oil and Gas named after I.M. Gubkina, 1999

Cycle Corrosion and protection of equipment NGP

1. Semenova I.V., Florianovich G.M., Khoroshilov A.V. Corrosion and corrosion protection. - M: Fizmatlit, 2010. - 416 p.
2. Medvedeva M.L. Corrosion and protection of equipment in oil and gas processing. Tutorial. M.: Federal State Unitary Enterprise Publishing House "Oil and Gas" Russian State University of Oil and Gas. I.M. Gubkina, 2005. - 312 p.: ill.
3. Medvedeva M.L., Muradov A.V., Prygaev A.K. Corrosion and Protection of Main Pipelines and Tanks: Textbook for Oil and Gas Universities. - M .: Publishing Center of the Russian State University of Oil and Gas named after I.M. Gubkina, 2013. - 250 p.
4. Sorokin G.M., Efremov A.P., Saakiyan L.S. Corrosion-mechanical wear of steels and alloys. -M.: Oil and gas, 2002

Cycle Tribology

1. Sorokin G.M., Malyshev V.N., Kurakin I.B. Tribology of steels and alloys: Textbook for universities. - M.: Russian State University of Oil and Gas named after I.M. Gubkina, 2013. - 383 p.: ill.
2. Sorokin G.M., Kurakin I.B. System analysis and complex criteria for the strength of steels. - M .: Nedra Publishing House LLC, 2011. - 101 p.
3. Sorokin G.M. Tribology of steels and alloys. Moscow: Nedra, 2000
4. V. N. Vinogradov and G. M. Sorokin, Acoust. Mechanical wear of steels and alloys: Textbook for universities. - M.: Nedra, 1996. - 364 p.: ill.
5. V. N. Vinogradov and G. M. Sorokin, Acoust. Wear resistance of steels and alloys: Textbook for universities. - M.: Oil and gas, 1994. - 417 p.: ill. 246.

Topic:Study of the process of crystallization of metals

Objective: to study the mechanism of crystallization of metals, the energy conditions for the course of the crystallization process.

Work order

1. Study theoretical information.

2. In a notebook for practical work, answer control questions in writing.

Theoretical information

A common property of metals and alloys is their crystalline structure, which is characterized by a certain arrangement of atoms in space. To describe the atomic-crystal structure, the concept of a crystal cell is used - the smallest volume, the translation of which in all dimensions can completely reproduce the structure of the crystal. In a real crystal, atoms or ions are brought close to each other to the state of direct contact, but for simplicity they are replaced by diagrams where the centers of attraction of atoms or ions are shown as dots; the cells most characteristic of metals are shown in fig. 1.1.

Fig.1.1. Types of crystal lattices and the arrangement of atoms in them:

a) face-centered (fcc), b) body-centered (bcc), c) hexagonal close-packed (HS)

Any substance can be in three states of aggregation: solid, liquid and gaseous, and the transition from one state to another occurs at a certain temperature and pressure. Most technological processes occur at atmospheric pressure, then phase transitions are characterized by the temperature of crystallization (melting), sublimation and boiling (evaporation).

With an increase in the temperature of a solid body, the mobility of atoms in the nodes of the crystal cell increases, and their oscillation amplitude increases. When the melting temperature is reached, the energy of the atoms becomes sufficient to leave the cell - it collapses with the formation of a liquid phase. Melting temperature is an important physical constant of materials. Among metals, mercury has the lowest melting point (-38.9 ° C), and tungsten has the highest (3410 ° C).

The opposite picture takes place when the liquid is cooled with its further solidification. Near the melting point, groups of atoms are formed, packed into cells, as in a solid. These groups are the centers (germs) of crystallization, and then a layer of crystals grows on them. When the same melting temperature is reached, the material passes into a liquid state with the formation of a crystal lattice.

Crystallization is the transition of a metal from a liquid to a solid state at a certain temperature. According to the law of thermodynamics, any system tends to move into a state with a minimum value of free energy - a composite internal energy that can be converted into work isothermally. Therefore, the metal solidifies when the solid state has less free energy and melts when the free energy in the liquid state is less.

The crystallization process consists of two elementary processes: the nucleation of crystallization centers and the growth of crystals from these centers. As noted above, at a temperature close to crystallization, the formation of a new structure, the center of crystallization, begins. With an increase in the degree of supercooling, the number of such centers around which crystals begin to grow increases. At the same time, new centers of crystallization are formed in the liquid phase, so the increase in the solid phase simultaneously occurs both due to the appearance of new centers and the growth of existing ones. The total crystallization rate depends on the course of both processes, and the rates of nucleation of centers and growth of crystals depend on the degree of supercooling ΔТ. On fig. 1.2 schematically shows the mechanism of crystallization.

Rice. 1.2. Crystallization mechanism

Real crystals are called crystallites, they have an irregular shape, which is explained by their simultaneous growth. Crystallization nuclei can be fluctuations of the base metal, impurities and various solid particles.

The grain sizes depend on the degree of supercooling: at low values ​​of ΔT, the crystal growth rate is high, so an insignificant amount of large crystallites is formed. An increase in ΔT leads to an increase in the rate of formation of nuclei, the number of crystallites increases significantly, and their sizes decrease. However, the main role in the formation of the metal structure is played by impurities (non-metallic inclusions, oxides, deoxidation products) - the more of them, the smaller the grain size. Sometimes the modification of the metal is carried out on purpose - the deliberate introduction of impurities in order to reduce the grain size.

In the formation of a crystal structure, the direction of heat removal plays an important role, because the crystal grows faster in this direction. The dependence of the growth rate on the direction leads to the formation of branched tree-like crystals - dendrites (Fig. 1.3).

Rice. 1.3 Dendritic crystal

During the transition from a liquid to a solid state, selective crystallization always takes place - the purer metal solidifies first. Therefore, the grain boundaries are more enriched with impurities, and the heterogeneity of the chemical composition within the dendrites is called dendritic segregation.

On fig. 1.4. the structure of a steel ingot is shown, in which 3 characteristic zones can be distinguished: fine-grained 1, a zone of columnar crystals 2, and a zone of equilibrium crystals 3. Zone 1 consists of a large number of crystals not oriented in space, formed under the influence of a significant temperature difference between the liquid metal and cold walls.

Rice. 1.4. The structure of the steel ingot

After the formation of the outer zone, the conditions for heat removal deteriorate, supercooling decreases, and fewer crystallization centers appear. Crystals begin to grow from them in the direction of heat removal (perpendicular to the walls of the mold), forming zone 2. In zone 3, there is no clear direction of heat removal, and the crystallization nuclei in it are foreign particles displaced during the crystallization of previous zones.

Control questions

1. In what states of aggregation can a material exist?

2. What is called a phase transformation of the first kind?

3. What process is called crystallization, what type of phase transformation does it belong to?

4. Describe the mechanism of metal crystallization and the conditions necessary to start it.

5. What caused the dendritic shape of crystals?

6. Describe the structure of a metal ingot