Slide 1
Solid state physics. Part 2.
Real crystals (just like “real boys”) are ideal crystals that grow in the wrong places.
Slide 2
Crystal growth You know, of course, that water (at normal pressure) freezes at 0°. If the temperature drops, then exactly at 0° the water will begin to freeze and turn into ice crystals. Until all the water freezes, its temperature will not drop further. If, on the contrary, you heat an ice crystal to 0°, it will remain unchanged. As soon as the temperature reaches 0°, the crystal will immediately begin to melt. No matter how much we heat further, the temperature of the ice will not increase until all the ice has melted. Only when the entire crystal, having melted, turns into water (in other words, until the structure of all particles disintegrates), the temperature of the water can begin to rise. Any crystalline substance melts and crystallizes at a strictly defined melting point: iron - at 1530°, tin - at 232°, quartz - at 1713°, mercury - at minus 38°. Non-crystalline solids do not have a constant melting point (and therefore no crystallization temperature); when heated, they gradually soften.
Slide 3
Methods for growing crystals One of them is cooling a saturated hot solution. At each temperature, no more than a certain amount of substance can dissolve in a given amount of solvent (for example, water). If the solution is cooled slowly, few nuclei are formed, and, gradually growing on all sides, they turn into beautiful crystals of regular shape. With rapid cooling, many nuclei are formed, and particles from the solution will “fall” onto the surface of the growing crystals, like peas from a torn bag; Of course, this will not produce the right crystals, because the particles in the solution may simply not have time to “settle” on the surface of the crystal in their proper place. Another method for obtaining crystals is to gradually remove water from a saturated solution. The “excess” substance crystallizes. And in this case, the slower the water evaporates, the better the crystals are obtained.
Slide 4
The third method is to grow crystals from molten substances by slowly cooling the liquid. When using all methods, the best results are obtained if a seed is used - a small crystal of the correct shape, which is placed in a solution or melt. In this way, for example, ruby crystals are obtained. Growing gemstone crystals is done very slowly, sometimes over years. If you accelerate crystallization, then instead of one crystal you will get a mass of small ones. This method can only be carried out in special devices. Currently, more than half of the technically important crystals are grown from melt. One of the most widely used industrial methods for producing semiconductor and other single crystals is the Czochralski method. Developed in 1918. The starting material (charge) is loaded into a refractory crucible and heated to a molten state. Then the seed crystal in the form of a thin rod with a diameter of several mm is installed in a cooled crystal holder and immersed in the melt
Slide 5
Jan Czochralski (1885 - 1953) - Polish chemist, inventor of the now widely known method of growing single crystals from a melt by pulling them upward from a free surface, which was later named after him. According to some accounts, Czochralski discovered his famous method in 1916, when he accidentally dropped his pen into a crucible of molten tin. Pulling the pen out of the crucible, he discovered that a thin thread of frozen tin was trailing behind the metal pen. By replacing the pen nib with a microscopic piece of metal, Czochralski was convinced that the metal thread thus formed had a single-crystal structure. In experiments conducted by Czochralski, single crystals measuring about one millimeter in diameter and up to 150 cm in length were obtained
Slide 6
Crystal defects In describing the structure of crystals, we have so far used their ideal models. The difference between real crystals and ideal ones is that real crystals do not have a regular crystal lattice. They always contain violations of strict periodicity in the arrangement of atoms. These irregularities are called crystal defects. Defects are formed during the growth of crystals under the influence of thermal movement of molecules, mechanical influences, irradiation by particle flows, due to the presence of impurities, etc. Crystal defects are any violation of the translational symmetry of the crystal - the ideal periodicity of the crystal lattice. There are several types of defects based on size. Namely, there are zero-dimensional (point), one-dimensional (linear), two-dimensional (flat) and three-dimensional (volumetric) defects.
Slide 7
Zero-dimensional (or point) defects in a crystal include all defects that are associated with the displacement or replacement of a small group of atoms (intrinsic point defects), as well as with impurities. They arise during heating, doping, during crystal growth and as a result of radiation exposure. They can also be introduced as a result of implantation. The properties of such defects and the mechanisms of their formation have been best studied, including motion, interaction, annihilation, and evaporation. Defects, called point defects, arise when one of the atoms of the crystal lattice is replaced by an impurity atom (a), the introduction of an atom between lattice sites (b), or as a result of the formation of vacancies - the absence of an atom in one of the lattice sites (c).
Slide 8
Substitutional impurities, replacing particles of the main substance at lattice sites, are introduced into the lattice more easily, the closer the atomic (ionic) radii of the impurity and main substance are. Interstitial impurities occupy interstices and, moreover, the more easily, the greater the volume of space between atoms. The introduced atoms or ions that differ from the main atoms in size or valency can be either intrinsic or impurity atoms or ions. If a foreign atom is in a node, then this is a substitution defect; if it is in an interstice, then it is an interstitial atom. The equilibrium positions occupied by interstitial atoms depend on the material and lattice type. Neighboring atoms at the sites of the crystal lattice are slightly displaced, causing slight deformation. Vacancies are the most important type of point defects; they accelerate all processes associated with the movement of atoms: diffusion, sintering of powders, etc. In technically pure metals, point defects increase electrical resistance, but have almost no effect on mechanical properties. Only at high concentrations of defects in irradiated metals does ductility decrease and other properties noticeably change.
Slide 9
How can pinpoint defects appear? According to the basic principles of statistical physics, even in the case when the average kinetic energy of atoms is very small, there will always be a certain number of atoms with higher energy, sufficient for the atom to leave the crystal lattice site. Moving around the crystal and giving part of its energy to other atoms, such an atom can be located in interstices. The combination of an atom in an interstitial site and a vacancy is called a Frenkel defect (or Frenkel pair). The vacancy and the interstitial atom are connected by significant elastic forces.
Frenkel defects easily arise in crystals containing significant interatomic voids. Examples of such crystals are substances with the structure of diamond or rock salt.
Slide 10
Schottky point defects are mainly found in close-packed crystals, where the formation of interstitial atoms is difficult or energetically unfavorable. Some atoms from the near-surface layer, as a result of thermal movement, can leave the crystal to the surface (Fig.). The vacancy at the vacated site can then migrate into the bulk of the crystal. The formation of Schottky defects reduces the density of the crystal, since its volume increases at a constant mass, whereas with the formation of Frenkel defects, the density remains unchanged, since the volume of the entire body does not change.
Walter Hermann Schottky (1886 - 1976) - famous German physicist, invented the electron tube with a screening grid in 1915 and the tetrode in 1919. In 1938, Schottky formulated a theory predicting the Schottky effect, now used in Schottky diodes.
Slide 11
Thus, while representing a less than perfect, ordered, and somewhat monotonic sequence of alternating positive and negative ions, real crystals contain a wide range of interesting point defects, which, as we will see, can greatly influence many of their properties. These, as we have already said, are intrinsic defects, the concentration of which depends on temperature, and in addition, non-intrinsic, impurity defects that are either present by chance or added purposefully during crystal growth. All these defects can be considered quasiparticles. Like real particles in a vacuum, they can move and interact with each other over long distances to form more complex structures.
Slide 12
Transport Processes in Crystals It is often mistakenly believed that such well-known alkali halide compounds as sodium chloride and potassium chloride are insulators, but in fact they are relatively good conductors, this is especially true at elevated temperatures. The fact that conductivity exists, as well as the fact that both self-diffusion and diffusion of impurity ions occur quite easily in ionic solids, serve as irrefutable evidence of the presence of point defects in them. Many of these materials do not have electronic conductivity - measurements show that the conductivity is due to the migration of ions. However, without the existence of vacancies or interstitial atoms, the movement of ions in such a classical ionic conductor is impossible: this would require too much energy. Thanks to defects and their movements (Fig.), the process of ion movement turns into an exchange of places between the ion and the defect; in this case, the amount of energy required decreases.
Slide 13
Diffusion (Latin diffusio - spreading, spreading, scattering, interaction) is the process of mutual penetration of molecules of one substance between the molecules of another, leading to spontaneous equalization of their concentrations throughout the occupied volume. In some situations, one of the substances already has an equalized concentration and they talk about diffusion of one substance in another. In this case, the substance is transferred from an area of high concentration to an area of low concentration (along a concentration gradient). In crystals, both the lattice’s own atoms can diffuse (self-diffusion or homodiffusion), and atoms of other chemical elements dissolved in the substance (impurity or heterodiffusion), as well as point defects in the crystal structure - interstitial atoms and vacancies.
Slide 14
Diffusion is a process at the molecular level and is determined by the random nature of the movement of individual molecules. The rate of diffusion is therefore proportional to the average speed of the molecules. If in a mixture of gases the mass of one molecule is four times greater than another, then such a molecule moves twice as slow as its movement in a pure gas. Accordingly, its diffusion rate is also lower. This difference in the rate of diffusion of light and heavy molecules is used to separate substances with different molecular weights. An example is the separation of isotopes. If a gas containing two isotopes is passed through a porous membrane, the lighter isotopes pass through the membrane faster than the heavier ones. For better separation, the process is carried out in several stages. This process has been widely used to separate uranium isotopes (separation of 235U from the bulk 238U). (Currently, to separate uranium isotopes, the centrifugation method is used, in which gas containing uranium is rotated very quickly and, due to the difference in the mass of the molecules, the isotopes are separated, which are then converted back into the metal.)
Slide 15
Diffusion phenomenologically obeys Fick's laws. Fick's 1st law establishes the proportionality of the diffusion flux of particles to their concentration gradient; Fick's 2nd law describes the change in concentration due to diffusion. The phenomenon of diffusion was first studied by the Würzburg scientist A. Fick using the example of salt solutions. Fick, through careful research, showed that the free diffusion of salt solutions occurs according to laws completely analogous to the laws of heat propagation in solids
Slide 16
Diffusion in Crystals Some general crystallographic features of the diffusion process are quite obvious if we take into account the geometry of the crystal. First of all, diffusion almost always occurs gradually, with the length of the elementary “steps” being of the order of one atomic diameter, i.e., several angstroms. Atoms move by jumping from one position in the lattice to another. In total, these elementary jumps ensure the movement of atoms over long distances. Let's find out what the mechanism of individual atomic jumps is. There are several possible schemes: the movement of vacancies, the movement of interstitial atoms, or some method of mutual exchange of places between atoms (Fig.).
Atomic movements that lead to diffusion: a – movement of vacancies; b – movement of interstitial atoms; c – exchange of places of two atoms; d – ring exchange of places of four atoms
Slide 17
Based on the idea of point defects in crystals, Frenkel proposed two main mechanisms of diffusion in solids: vacancy (Fig. a: an atom moves, exchanging places with a vacancy) and interstitial (Fig. b: an atom moves along interstices). The second method moves small (in size) impurity atoms, and the first method moves all the rest: this is the most common diffusion mechanism.
Yakov Ilyich Frenkel (1894 - 1952) - Soviet scientist, theoretical physicist, one of the founders of solid state physics. From 1921 until the end of his life, Frenkel worked at the Leningrad Institute of Physics and Technology. Since 1922, Frenkel published a new book literally every year. He became the author of the first course on theoretical physics in the USSR.
Slide 18
Dislocations A dislocation is a linear defect in the crystal lattice of a solid, which represents the presence of an “extra” atomic half-plane. The simplest visual model of an edge dislocation is a book in which a part has been torn off from one of the inner pages. Then, if the pages of a book are likened to atomic planes, then the edge of the torn part of the page models a dislocation line. There are screw and edge dislocations.
Slide 19
In order for a dislocation to form in an ideal crystal, it is necessary to produce a shift in some part of the slip plane
The dislocation density varies over a wide range and depends on the state of the material. After careful annealing, the dislocation density is low; in crystals with a strongly deformed crystal lattice, the dislocation density reaches very high values.
Slide 20
The dislocation density largely determines the plasticity and strength of the material. If the density is less than a certain value, then the resistance to deformation increases sharply, and the strength approaches the theoretical one. Thus, an increase in strength is achieved by creating a metal with a defect-free structure, and also, on the other hand, by increasing the density of dislocations, which impedes their movement.
Slide 21
During plastic deformation, one part of the crystal moves relative to another under the influence of tangential stresses. When the loads are removed, the shear remains, i.e. plastic deformation occurs. The application of a shear stress leads to the movement of an edge dislocation, and the displacement of its axis by one translation means a change in the half-plane that currently forms the dislocation. The movement of an edge dislocation through the entire crystal will lead to a shift of part of the crystal by one interatomic distance. The result of this is plastic deformation of the crystal (Fig.), i.e., parts of the crystal are displaced relative to each other by one translation.
A metal in a stressed state always experiences normal and tangential stresses under any type of loading. An increase in normal and shear stresses leads to different consequences. An increase in normal stresses leads to brittle fracture. Plastic deformation is caused by tangential stresses.
Slide 22
An increase in strength is achieved by creating a metal with a defect-free structure, as well as by increasing the density of dislocations, which impedes their movement. Currently, defect-free crystals have been created - whiskers up to 2 mm long, 0.5...20 microns thick - “whiskers” with strength close to theoretical. Dislocations affect not only strength and ductility, but also other properties of crystals. As the density of dislocations increases, their optical properties change and the electrical resistance of the metal increases. Dislocations increase the average rate of diffusion in the crystal, accelerate aging and other processes, reduce chemical resistance, therefore, as a result of treating the surface of the crystal with special substances, pits are formed at the points where dislocations emerge.
Slide 23
Epitaxy is the natural growth of one crystalline material on another (from the Greek επι - on and ταξισ - ordering), i.e. the oriented growth of one crystal on the surface of another (substrate). The minimum energy is consumed if the crystal grows along a screw dislocation.
Slide 24
Thank you for your attention!
- Size: 2.2 Megabytes
- Number of slides: 37
Description of the presentation Presentation Defects in crystals on slides
Energy changes that occur during the formation of defects in a perfect crystal. The gain in entropy associated with the presence of a choice of positions is called configuration entropy and is determined by the Boltzmann formula S = k ln. W, where W is the probability of the formation of a single vacancy, proportional to the number of regular atoms forming the lattice (10 23 per 1 mole of substance).
Various types of defects in crystals: a) vacancy; b) interstitial atom; c) a small replacement defect; d) large replacement defect; e) Frenkel defect; e) Schottky defect (a pair of vacancies in the cation and anion sublattices)
The energy of displacement of an atom from its position in the lattice. Energy barrier. To move an atom from its position, activation energy is required. ΔE – defect formation energy; E * - activation energy. 1 / 1 1 E k. T sn C N e , 2/ 2 2 E k. T mn C N e Equilibrium will be established if n 1 = n 2: under equilibrium conditions, there are vacancies and interstitial atoms in the metal lattice! //Ek. T m s. N N Ce
Dislocations. Mechanical properties and reactivity of solids. 1) - metals usually turn out to be much more ductile than can be expected based on calculations. The calculated value of shear stress in metals is 10 5 - 10 6 N/cm 2, while experimentally found values for many metals do not exceed 10 - 100 N/cm 2. This indicates that there are some “weak links” in the structure of metals , thanks to which metals are deformed so easily; 2) - on the surfaces of many well-cut crystals, under a microscope or even with the naked eye, spirals along which the crystal grew are visible. Such spirals cannot form in perfect crystals; 3) - without ideas about the existence of dislocations, it would be difficult to explain such properties of metals as plasticity and fluidity. Plates of magnesium metal, for example, can be stretched, almost like rubber, to several times their original length; 4) - hardening in metals could not be explained without invoking ideas about dislocations.
Arrangement of atoms around an edge dislocation An edge dislocation is an “extra” atomic half-plane that does not pass through the entire crystal, but only through part of it. Edge dislocation projection.
Movement of an edge dislocation under the action of shear stress. If you connect points A and B, then this will be a projection of the slip plane along which dislocations move. Dislocations are characterized by the Burgers vector b. To find the magnitude and direction of b, it is necessary to describe a contour around the dislocation, mentally drawing it from atom to atom (Fig. e). In a defect-free region of the crystal, such a contour ABCD, constructed from translations to one interatomic distance in each direction, is closed: its beginning and end coincide at point A. On the contrary, contour 12345 surrounding the dislocation is not closed, since points 1 and 5 do not coincide. The magnitude of the Burgers vector is equal to the distance 1 - 5, and the direction is identical to the direction 1 - 5 (or 5 - 1). The Burgers vector of an edge dislocation is perpendicular to the dislocation line and parallel to the direction of motion of the dislocation line (or direction of shear) under the action of an applied stress.
Screw dislocation With continued shear stress, indicated by the arrows, the SS ' line and slip marks reach the back face of the crystal. To find the Burgers vector of a screw dislocation, let us again imagine contour 12345 (Fig. a) “circling” around it. Vector b is determined by the magnitude and direction of the segment 1 - 5. For a screw dislocation, it is parallel to the dislocation line SS ' (in the case of an edge dislocation, it is perpendicular) and perpendicular to the direction of movement of the dislocation, coinciding, as in the case of an edge dislocation, with the direction of shear or slip.
A dislocation line that changes the nature of the dislocation from screw to edge. Origin and movement of a dislocation loop The nature of dislocations is such that they cannot end inside the crystal: if in some place on the crystal surface a dislocation enters the crystal, this means that somewhere on another part of the surface it leaves the crystal.
Scheme of the appearance of a dislocation loop (ring) Scheme of the appearance of vacancies (b) by the annihilation of two dislocations of the opposite sign (a). In reality, direct application of an external deforming force is not necessary for the formation of dislocations. This force can be thermal stresses arising during crystallization, or, for example, similar stresses in the area of foreign inclusions in a solidifying metal ingot during cooling of the melt, etc. In real crystals, excess extraplanes can arise simultaneously in different parts of the crystal. The extraplane, and therefore the dislocations, are mobile in the crystal. This is their first important feature. The second feature of dislocations is their interaction with the formation of new dislocations, dislocation loops similar to those shown in the figures below, and even the formation of vacancies due to the annihilation of two dislocations of opposite sign.
Mechanical strength of metals. Frenkel's model. The destructive force is usually called stress and denoted by σ. According to this model, the resistance σ first increases as the shift along the x axis increases and then drops to zero as soon as the atomic planes shift by one interatomic distance a. When x>a the value of σ increases again and again falls to zero at x = 2a, etc., i.e. σ(x) is a periodic function that can be represented as σ = A sin (2 π x/a ) , for the region of small x A = G /(2π), where G is Young’s modulus. A more rigorous theory subsequently gave a refined expression σ m ax = G /30. Diagram of the shift of atomic planes (a) and the dependence of voltage on distance in the crystal (b).
Experimental and theoretical values of the shear strength of some metals. Roller model of shift of atomic planes of a crystal | F 1 + F 2 |=| F 4 + F 5 | the entire roller system is in balance. One has only to slightly change the balance of forces with a weak external influence, and the top row of rollers will move. Therefore, the movement of a dislocation, i.e., a collection of defective atoms, occurs at low loads. The theory gives σ m ax, which shifts a dislocation, in the form σ m ax = exp ( - 2 π a / [ d (1- ν) ]), where ν is Poisson's ratio (transverse elasticity), d is the distance between slip planes, and - period of the crystal lattice. Assuming a = d, ν = 0.3, we obtain the values of σ m ax in the last column of the table, from which it can be seen that they are much closer to the experimental ones.
Scheme of caterpillar movement Schemes of dislocation-type movement: a - tensile dislocation, b - compressive dislocation, c - carpet movement. “First, let’s try to drag the caterpillar along the ground. It turns out that this is not easy to do; it requires significant effort. They are due to the fact that we are trying to simultaneously lift all pairs of caterpillar legs off the ground. The caterpillar itself moves in a different mode: it tears off only one pair of legs from the surface, carries them through the air, lowers them to the ground, then repeats the same with the next pair of legs, etc., etc. After doing this all pairs of legs will be transported through the air, the entire caterpillar as a whole will move the distance by which each pair of legs alternately shifted. The caterpillar does not drag any pair of legs along the ground. That’s why it crawls easily.”
Ways to control dislocation defects. Fixation by impurities. An impurity atom interacts with a dislocation and the movement of such a dislocation, burdened with impurity atoms, turns out to be difficult. Therefore, the efficiency of dislocation pinning by impurity atoms will be determined by the interaction energy E, which in turn consists of two components: E 1 and E 2. The first component (E 1) is the energy of elastic interaction, and the second (E 2) is the energy of electrical interaction. Fixation by foreign particles. Foreign particles are microscopic inclusions of a substance different from the base metal. These particles are introduced into the metal melt and remain in the metal after it solidifies when the melt cools. In some cases, these particles enter into a chemical interaction with the base metal, and then these particles already represent an alloy. The mechanism of dislocation pinning by such particles is based on different speeds of movement of dislocations in the metal matrix and in the material of foreign particles. Fixation with inclusions of the second phase. The second phase is understood as the release (precipitates) of an excess concentration of an impurity from a metal-impurity solution compared to the equilibrium one. The separation process is called solid solution decomposition. Intertwining of dislocations. When the density of dislocations in a metal is high, they become intertwined. This is due to the fact that some dislocations begin to move along intersecting slip planes, preventing the movement of others.
Qualitative view of the solubility curve. If the crystal contained a concentration of C m at a temperature T m and was quickly cooled, then it will have a concentration of C m at low temperatures, for example, at T 1, although the equilibrium concentration should be C 1. The excess concentration ΔC = C m – C 1 should be at sufficiently long heating will drop out of the solution, because only then will the solution assume a stable equilibrium state corresponding to the minimum energy of the system A 1- x B x.
Methods for detecting dislocations a) Micrograph (obtained in a transmission electron microscope, TEM) of a Sr crystal. Ti. O 3 containing two edge dislocations (100) (marked in the figure). b) Schematic representation of an edge dislocation. c) Micrograph of the surface of a Ga crystal. As (obtained in a scanning tunneling microscope). At point C there is a screw dislocation. d) Scheme of a screw dislocation.
Visualization of dislocations using a transmission electron microscope. a) Dark lines on a bright background are dislocation lines in aluminum after 1% stretching. b) The reason for the contrast of the dislocation region - and the curvature of crystallographic planes leads to electron diffraction, which weakens the transmitted electron beam
a) Etching pits on the surface (111) of bent copper; b) on the surface (100) c) (110) recrystallized Al -0.5% Mn. Dislocations can also be made visible in a conventional optical microscope. Since the areas around the point where dislocations reach the surface are more susceptible to chemical etching, so-called etch pits are formed on the surface, which are clearly visible in an optical microscope. Their shape depends on the Miller indices of the surface.
To obtain a metal material with increased strength, it is necessary to create a large number of dislocation pinning centers, and such centers must be evenly distributed. These requirements led to the creation of superalloys. New metal functional materials. "Designing" the structure of alloys A superalloy is at least a two-phase system in which both phases differ primarily in the degree of order in the atomic structure. The superalloy exists in the Ni - Al system. In this system, an ordinary mixture can be formed, i.e., an alloy with a chaotic distribution of Ni and Al atoms. This alloy has a cubic structure, but the nodes of the cube are replaced by Ni or Al atoms randomly. This disordered alloy is called the γ phase.
Along with the γ phase in the Ni - А l system, an intermetallic compound Ni 3 А l can also be formed, also with a cubic structure, but ordered. Cuboids Ni 3 А l are called γ ‘ -phase. In the γ '-phase, Ni and A l atoms occupy the sites of the cubic lattice according to a strict law: for one aluminum atom there are three nickel atoms. Scheme of dislocation movement in an ordered crystal
C diagram of dislocation pinning by inclusions of another phase. DD – moving dislocation. To create a superalloy, nickel is melted and mixed with aluminum. When the molten mixture is cooled, the disordered γ phase first solidifies (its crystallization temperature is high), and then small-sized cuboids of the γ '-phase are formed inside it as the temperature decreases. By varying the cooling rate, it is possible to regulate the kinetics of formation, and hence the size of inclusions of the γ ‘-phase Ni 3 А l.
The next step in the development of high-strength metallic materials was the production of pure Ni 3 Al without the γ phase. A type of fine-grained mosaic structure of metal. This material is very fragile: chipping occurs along the grain boundaries of the mosaic structure. Here other types of defects are revealed, in particular the surface. Indeed, on the surface of the crystal there is a break in chemical bonds, i.e. a violation is a break in the crystal field, and this is the main reason for the formation of a defect. Dangling chemical bonds are unsaturated, and in contact they are already deformed and therefore weakened. Scheme of breaking chemical bonds on the crystal surface.
To eliminate these defects it is necessary: - either to produce a monocrystalline material that does not contain individual grains-crystallites; - or find a “buffer” in the form of impurities that would not penetrate in noticeable quantities into the volume of Ni 3 Al, but would be well adsorbed on the surface and fill vacancies. Isovalent impurities have the greatest affinity for vacancies, i.e. impurities whose atoms are in the same group of the Periodic Table as the atom removed from the crystal lattice and forming the vacancy. Superalloys Ni 3 Al and Ni 3 Al are widely used today as heat-resistant materials at temperatures up to 1000°C. Similar cobalt-based superalloys have slightly lower strength, but retain it up to a temperature of 1100°C. Further prospects are associated with the production of intermetallic compounds of Ti. Al and T i 3 A l in their pure form. Parts made from them are 40% lighter than the same parts made from nickel superalloy.
Alloys with easy deformability under load. The method for creating such metallic materials is to produce a structure with very small crystallite grains. Grains with dimensions less than 5 microns slide over each other under load without destruction. A sample consisting of such grains can withstand a relative tension Δ l / l 0 = 10 without destruction, i.e., the length of the sample increases by 1000% of the original length. This is the effect of superplasticity. It is explained by the deformation of bonds in grain contacts, i.e., a large number of surface defects. Superplastic metal can be processed almost like plasticine, giving it the desired shape, and then a part made of such material is heat treated to enlarge the grains and quickly cooled, after which the effect of superplasticity disappears, and the part is used for its intended purpose. The main difficulty in producing superplastic metals is achieving a fine grain structure.
It is convenient to obtain nickel powder by the leaching method, in which the Al - Ni alloy is crushed using Na alkali. OH leach aluminum to produce a powder with a particle diameter of about 50 nm, but these particles are so chemically active that they are used as a catalyst. The activity of the powder is explained by a large number of surface defects - broken chemical bonds that can attach electrons from adsorbed atoms and molecules. Scheme of rapid crystallization of a metal melt sprayed in a centrifuge: 1 - cooling gas; 2 - melt; 3 - melt jet; 4 - small particles; 5 - rotating disk Scheme of dynamic pressing of metal powders: 1 - projectile, 2 - powder, 3 - mold, 4 - gun barrel
Laser glazing method. The term is borrowed from porcelain (ceramic) production. Using laser radiation, a thin layer on the metal surface is melted and rapid cooling is applied at rates of the order of 10 7 K/s. The limiting case of ultra-fast hardening is the production of amorphous metals and alloys - metallic glasses.
Superconducting metals and alloys Material Al V In Nb Sn Pb Nb 3 Sn Nb 3 Ge Т с, К 1, 19 5, 4 3, 4 9, 46 3, 72 7, 18 18 21. . . 23In 1911 in Holland, Kamerlingh Onnes discovered a decrease in the resistivity of mercury at the boiling point of liquid helium (4.2 K) to zero! The transition to the superconducting state (ρ = 0) occurred abruptly at a certain critical temperature Tc. Until 1957, the phenomenon of superconductivity had no physical explanation, although the world was busy searching for more and more new superconductors. Thus, by 1987, about 500 metals and alloys with different Tc values were known. Niobium compounds had the highest Tc.
Continuous current. If an electric current is excited in a metal ring, then at normal, for example, room temperature, it quickly dies out, since the flow of current is accompanied by heat losses. At T ≈ 0 in a superconductor, the current becomes undamped. In one of the experiments, the current circulated for 2.5 years until it was stopped. Since the current flows without resistance, and the amount of heat generated by the current is Q = 0.24 I 2 Rt, then in the case of R = 0 there are simply no heat losses. There is no radiation in the superconducting ring due to quantization. But in an atom the momentum and energy of one electron are quantized (take on discrete values), and in a ring the current, i.e. the entire set of electrons, is quantized. Thus, we have an example of a cooperative phenomenon - the movement of all electrons in a solid is strictly coordinated!
Meissner effect Discovered in 1933. Its essence lies in the fact that an external magnetic field at T< Т с не проникает в толщу сверхпроводника. Экспериментально это наблюдается при Т=Т с в виде выталкивания сверхпроводника из магнитного поля, как и полагается диамагнетику. Этот эффект объясняется тем, что в поверхностном слое толщиной 0, 1 мкм внешнее магнитное поле индуцирует постоянный ток, но тепловых и излучательных потерь нет и в результате вокруг этого тока возникает постоянное незатухающее магнитное поле. Оно противоположно по направлению внешнему полю (принцип Ле-Шателье) и экранирует толщу сверхпроводника от внешнего магнитного поля. При увеличении Н до некоторого значения Н с сверхпроводимость разрушается. Значения Н с лежат в интервале 10 -2 . . . 10 -1 Т для различных сверхпроводников. http: //www. youtube. com/watch? v=bo 5XTURGMTM
If there were no Meissner effect, the conductor without resistance would behave differently. When transitioning to a state without resistance in a magnetic field, it would maintain a magnetic field and would retain it even when the external magnetic field is removed. It would be possible to demagnetize such a magnet only by increasing the temperature. This behavior, however, has not been observed experimentally.
In addition to the superconductors considered, which were called superconductors of the first kind, superconductors of the second kind were discovered (A, V. Shubnikov, 1937; A. Abrikosov, 1957). In them, an external magnetic field, upon reaching a certain H c1, penetrates into the sample, and electrons, whose velocities are directed perpendicular to H, begin to move in a circle under the influence of the Lorentz force. Vortex filaments appear. The “trunk” of the thread turns out to be a non-superconducting metal, and superconducting electrons move around it. As a result, a mixed superconductor is formed, consisting of two phases - superconducting and normal. Only when another, higher value of Hc is reached, the 2 filaments, expanding, come closer together, and the superconducting state is completely destroyed. The values of Нс2 reach 20. . . 50 T for such superconductors as Nb 3 Sn and Pb. Mo 6 O 8 respectively.
Josephson structure diagram: 1-dielectric layer; 2-superconductors The structure consists of two superconductors separated by a thin dielectric layer. This structure is located at a certain potential difference specified by the external voltage V. From the theory developed by Feynman, the expression for the current I flowing through the structure follows: I= I 0 sin [(2e. V/h)t+ φ 0 ], where I 0 = 2Kρ/ h (K is the interaction constant of both superconductors in the Josephson structure; ρ is the density of particles carrying the superconducting current). The quantity φ 0 = φ 2 - φ 1 is considered as the phase difference between the wave functions of electrons in contacting superconductors. It can be seen that even in the absence of external voltage (V = 0), a direct current flows through the contact. This is the stationary Josephson effect. If we place the Josephson structure in a magnetic field, then the magnetic flux Ф causes a change in Δ φ, and as a result we get: I= I 0 sinφ 0 cos (Ф / Ф 0), where Ф 0 is the magnetic flux quantum. The value of Ф 0 = h с/е is equal to 2.07·10 -11 T cm 2. Such a small value of Ф 0 allows the production of ultra-sensitive magnetic field meters (magnetometers) that detect weak magnetic fields from the biocurrents of the brain and heart.
The equation I= I 0 sin [(2e. V/h)t+ φ 0 ] shows that in the case of V ≠ 0 the current will oscillate with a frequency f = 2 e. V/h. Numerically, f falls into the microwave range. Thus, the Josephson contact allows you to create alternating current using a constant potential difference. This is the non-stationary Josephson effect. An alternating Josephson current, just like an ordinary current in an oscillating circuit, will emit electromagnetic waves, and this radiation is actually observed experimentally. For high-quality Josephson S - I - S contacts, the thickness of the dielectric layer I must be extremely small - no more than a few nanometers. Otherwise, the coupling constant K, which determines the current I0, is greatly reduced. But the thin insulating layer degrades over time due to the diffusion of atoms from superconducting materials. In addition, the thin layer and the significant dielectric constant of its material lead to a large electrical capacitance of the structure, which limits its practical use.
Basic qualitative ideas about the physics of the phenomenon of superconductivity. Mechanism of formation of Cooper pairs Let us consider a pair of electrons e 1 and e 2, which are repelled by the Coulomb interaction. But there is also another interaction: for example, electron e 1 attracts one of the ions I and displaces it from the equilibrium position. The I ion creates an electric field that acts on the electrons. Therefore, its displacement will affect other electrons, for example, e 2. Thus, the interaction of electrons e 1 and e 2 occurs through the crystal lattice. An electron attracts an ion, but since Z 1 > Z 2, the electron, together with the ion “coat,” has a positive charge and attracts a second electron. At T > T c, thermal motion blurs the ion “coat”. The displacement of an ion is the excitation of lattice atoms, i.e., nothing more than the birth of a phonon. During the reverse transition, a phonon is emitted and is absorbed by another electron. This means that the interaction of electrons is the exchange of phonons. As a result, the entire collective of electrons in the solid body turns out to be bound. At any given moment, an electron is more strongly connected to one of the electrons in this collective, i.e., the entire electronic collective seems to consist of electron pairs. Within a pair, electrons are bound by a certain energy. Therefore, only those influences that overcome the binding energy can affect this pair. It turns out that ordinary collisions change the energy by a very small amount, and it does not affect the electron pair. Therefore, electron pairs move in the crystal without collisions, without scattering, i.e., the current resistance is zero.
Practical application of low-temperature superconductors. Superconducting magnets, made of Nb 3 Sn superconducting alloy wire. At present, superconducting solenoids with a field of 20 T have already been built. Materials corresponding to the formula M x Mo 6 O 8, where the metal atoms M are Pb, Sn, Cu, Ag, etc., are considered promising. The highest magnetic field (approximately 4 0 T) obtained in Pb solenoid. Mo 6 O 8. The colossal sensitivity of Josephson junctions to a magnetic field served as the basis for their use in instrument making, medical equipment and electronics. SQUID is a superconducting quantum interference sensor used for magnetoencephalography. Using the Meissner effect, a number of research centers in different countries are conducting work on magnetic levitation - “floating” above the surface to create high-speed magnetic levitation trains. Induction energy storage devices in the form of a circuit with undamped current and electric power transmission lines (EPL) without losses through superconducting wires. Magnetohydrodynamic (MHD) generators with superconducting windings. They have an efficiency of converting thermal energy into electrical energy of 50%, while for all other power plants it does not exceed 35%.
Defects in the crystal structureReal metals that are used as structural
materials consist of a large number of irregularly shaped crystals. These
crystals
called
grains
or
crystals,
A
structure
polycrystalline or granular. Existing production technologies
metals do not allow obtaining them of ideal chemical purity, therefore
real metals contain impurity atoms. Impurity atoms are
one of the main sources of defects in the crystal structure. IN
Depending on their chemical purity, metals are divided into three groups:
chemically pure - content 99.9%;
high purity - content 99.99%;
ultrapure - content 99.999%.
Atoms of any impurities are sharply different in size and structure
differ from the atoms of the main component, so the force field around
such atoms are distorted. An elastic zone appears around any defects.
distortion of the crystal lattice, which is balanced by volume
crystal adjacent to a defect in the crystal structure.
inherent in all metals. These violations of the ideal structure of solids
have a significant impact on their physical, chemical,
technological and operational properties. Without use
ideas about defects in real crystals, it is impossible to study the phenomena
plastic deformation, hardening and destruction of alloys, etc. Defects
crystal structure can be conveniently classified according to their geometric
shape and size:
surface (two-dimensional) are small in only one direction and have
flat shape - these are the boundaries of grains, blocks and twins, the boundaries of domains;
point (zero-dimensional) are small in all three dimensions, their sizes are not
more than several atomic diameters are vacancies, interstitial atoms,
impurity atoms;
linear (one-dimensional) are small in two directions, and in the third
direction they are commensurate with the length of the crystal - these are dislocations, chains
vacancies and interstitial atoms;
volumetric (three-dimensional) have in all three dimensions relatively
large sizes mean large inhomogeneities, pores, cracks, etc.; Surface defects are interfaces
between individual grains or subgrains in a polycrystalline metal, to
This also includes “packing” defects in crystals.
A grain boundary is a surface on either side of which
crystal lattices differ in spatial orientation. This
the surface is a two-dimensional defect having significant dimensions in
two dimensions, and in the third - its size is comparable to the atomic. Grain boundaries
- these are areas of high dislocation density and inconsistency
structure of adjacent crystals. Atoms at grain boundaries have increased
energy compared to the atoms inside the grains and, as a consequence, more
tend to engage in various interactions and reactions. At grain boundaries
there is no ordered arrangement of atoms. At the grain boundaries during metal crystallization, they accumulate
various impurities, defects, non-metallic inclusions are formed,
oxide films. As a result, the metallic bond between the grains is broken
and the strength of the metal decreases. As a result of the broken border structure
weaken or strengthen the metal, which leads, respectively, to
intercrystalline (intergranular) or transgranular (along the grain body)
destruction. Under the influence of high temperatures, the metal tends to reduce
surface energy of grain boundaries due to grain growth and contraction
the length of their borders. When chemically exposed to grain boundaries
turn out to be more active and, as a result, corrosion destruction
begins at grain boundaries (this feature underlies microanalysis
metals in the manufacture of polished sections).
There is another source of surface distortion of the crystalline
metal structure. The metal grains are mutually misoriented into several
degrees, the fragments are misoriented by minutes, and the blocks that make up
fragment, mutually misoriented for only a few seconds. If
examine the grain at high magnification, it turns out that inside it
There are areas misoriented relative to each other at an angle of 15"...30".
This structure is called block or mosaic, and areas are called blocks
mosaics. The properties of metals will depend both on the sizes of blocks and grains, and
and on their mutual orientation. Oriented blocks are combined into larger fragments in
whose general orientation remains arbitrary, thus all grains
misoriented relative to each other. As the temperature rises
misorientation of grains increases. Thermal process causing grain division
into fragments is called polygonization.
The difference in properties depending on the direction in metals is
the name is anisotropy. Anisotropy is characteristic of all substances with
crystalline structure. The grains are located randomly in the volume, therefore
There are approximately the same number of atoms in different directions and
properties remain the same, this phenomenon is called quasi-anisotropy
(false – anisotropy). Point defects are small in three dimensions and sizes
approaching the point. One of the common defects is
vacancies, i.e. a place not occupied by an atom (Schottky defect). To replace a vacant position
node, a new atom can move, and a vacant place—a “hole”—is formed along
neighborhood. With increasing temperature, the concentration of vacancies increases. So
like atoms. located near the surface. may come to the surface
crystal. and atoms will take their place. located further from the surface.
The presence of vacancies in the lattice imparts mobility to the atoms. those. allows them
move through the process of self-diffusion and diffusion. and thus provides
influence on processes such as aging, release of secondary phases, etc.
Other point defects are dislocated atoms
(Frenkel defect), i.e. atoms of own metal leaving the node
lattice and took place somewhere in the internodes. At the same time in place
moving atom, a vacancy is formed. The concentration of such defects
small. because their formation requires a significant expenditure of energy. Any metal contains foreign impurity atoms. IN
Depending on the nature of the impurities and the conditions under which they enter the metal, they can
be dissolved in the metal or exist in the form of separate inclusions. On
properties of the metal are most influenced by foreign dissolved
impurities whose atoms can be located in the voids between atoms
base metal - interstitial atoms or at crystal lattice sites
base metal - substitution atoms. If the impurity atoms are significantly
fewer base metal atoms, then they form interstitial solutions, and if
more - then they form substitution solutions. In both cases the lattice becomes
defective and its distortions affect the properties of the metal. Linear defects are small in two dimensions, but in the third they can
reach the length of the crystal (grain). Linear defects include chains
vacancies. interstitial atoms and dislocations. Dislocations are special
type of imperfections in the crystal lattice. From the perspective of dislocation theory
strength, phase and structural transformations are considered. Dislocation
called a linear imperfection that forms a zone inside the crystal
shift Dislocation theory was first applied in the mid-thirties
20th century physicists Orowan, Polyany and Taylor to describe the process
plastic deformation of crystalline bodies. Its use allowed
explain the nature of strength and ductility of metals. Dislocation theory gave
the ability to explain the huge difference between theoretical and practical
strength of metals.
The main types of dislocations include edge and screw. Regional
a dislocation is formed if an extra
half-plane of atoms, which is called an extraplane. Her edge is 1-1
creates a linear lattice defect called an edge dislocation.
It is conventionally accepted that a dislocation is positive if it is in the upper
part of the crystal and is indicated by the sign “ ” if the dislocation is located at the bottom
parts - negative “T“. Dislocations of the same sign repel each other, and
the opposite - they attract. Under the influence of edge tension
a dislocation can move across the crystal (along the shear plane) until
will reach the grain (block) boundary. This creates a step the size of
one interatomic distance. Plastic shear is a consequence
gradual movement of dislocations in the plane
shift Propagation of slip along a plane
sliding occurs sequentially. Every
the elementary act of moving a dislocation from
one position to another is accomplished by
rupture of only one vertical atomic
plane. To move dislocations it is required
significantly less force than for hard
displacement of one part of the crystal relative to another in the shear plane. At
movement of a dislocation along the shear direction through the entire crystal
there is a displacement of its upper and lower parts by only one interatomic
distance. As a result of the movement, the dislocation comes to the surface
crystal and disappears. A sliding step remains on the surface. Screw dislocation. Formed by incomplete displacement of the crystal along
density Q. Unlike an edge dislocation, a screw dislocation
parallel to the shift vector.
Dislocations are formed during the crystallization of metals during
“collapse” of a group of vacancies, as well as in the process of plastic deformation
and phase transformations. An important characteristic of the dislocation structure
are the dislocation density. The dislocation density is understood as
total dislocation length l (cm) per unit volume V
crystal (cm3). Thus. dimension of dislocation density, cm-2. U
annealed metals - 106...108 cm-2. When cold plastic
deformation, the dislocation density increases to 1011...1012 cm-2. More
high dislocation density leads to the appearance of microcracks and
metal destruction.
Near the dislocation line, the atoms are displaced from
their places and the crystal lattice is distorted, which
causes the formation of a stress field (above the line
dislocations, the lattice is compressed, and below it is stretched).
The value of a unit displacement of planes
characterized by the Burger vector b, which
reflects both the absolute value of the shift and its
direction. Mixed dislocation. Dislocation cannot end inside
crystal without connecting to another dislocation. This follows from the fact that
a dislocation is the boundary of a shear zone, and there is always a shear zone
a closed line, and part of this line can pass along the outer
crystal surface. Therefore, the dislocation line must close
inside the crystal or end on its surface.
When the shear zone boundary (dislocation line abcdf) is formed
straight sections parallel and perpendicular to the shear vector, and
a more general case of a curved dislocation line gh. In sections av, cd and
ef is an edge dislocation, and in the sections all and de there is a screw dislocation. Separate
sections of a curved dislocation line have an edge or screw
orientation, but part of this curve is neither perpendicular nor parallel
shear vector, and in these areas there is a mixed dislocation
orientation. Plastic deformation of crystalline bodies is related to the amount
dislocations, their width, mobility, degree of interaction with defects
lattices, etc. The nature of the bond between atoms affects plasticity
crystals. Thus, in nonmetals with their rigid directional bonds
dislocations are very narrow, they require high stresses to start - in 103
times greater than for metals. Resulting in brittle fracture in non-metals
occurs earlier than the shift.
The main reason for the low strength of real metals is
the presence of dislocations and other imperfections in the structure of the material
crystalline structure. Obtaining dislocation-free crystals
leads to a sharp increase in the strength of materials.
The left branch of the curve corresponds to the creation
perfect
dislocation-free
filamentous
crystals (so-called “whiskers”), strength
which is close to theoretical. With limited
dislocation density and other distortions
crystalline
gratings
process
shift
occurs more easily the more dislocations there are
located in the bulk of the metal. One of the characteristics of a dislocation is the displacement vector - vector
Burgers. The Burgers vector is an additional vector that needs
insert into the contour described around the dislocation to close
the corresponding circuit in the lattice of an ideal crystal, open
due to the presence of dislocation. A contour drawn along a grid around the area, in
which has a dislocation will turn out to be open (Burgers contour). Gap
contour characterizes the sum of all elastic displacements of the lattice accumulated in
the area around the dislocation is the Burgers vector.
For an edge dislocation the Burgers vector is perpendicular, and for a screw dislocation
dislocation – parallel to the dislocation line. The Burgers vector is a measure
distortion of the crystal lattice due to the presence in it
dislocations. If a dislocation is introduced into the crystal by pure shear, then the vector
shift and is the Burgers vector. Burgers outline may be displaced
along the dislocation line, stretched or compressed in a direction perpendicular to
dislocation lines, while the magnitude and direction of the Burgers vector
remain constant. As stress increases, the number of dislocation sources in the
metal and their density increases. In addition to parallel dislocations
dislocations arise in different planes and directions. Dislocations
influence each other, prevent each other from mixing, their
annihilation (mutual destruction), etc. (which allowed J. Gordon to figuratively
call their interaction in the process of plastic deformation “intimate”
life of dislocations"). As the density of dislocations increases, their movement
becomes increasingly difficult, which requires an increase in the applied
load to continue deformation. As a result, the metal is strengthened, which
corresponds to the right branch of the curve.
Dislocations, along with other defects, participate in phase transitions.
transformations, recrystallization, serve as ready-made centers during precipitation
the second phase from solid solution. Along dislocations, the diffusion rate is
several orders of magnitude higher than through a crystal lattice without defects.
Dislocations serve as a place for concentration of impurity atoms, especially
interstitial impurities, as this reduces lattice distortion. If, under the influence of external forces, dislocations arise in the metal,
then the elastic properties of the metal change and the influence begins to affect
sign of initial deformation. If the metal is subjected to weak
plastic deformation by a load of the same sign, then when the sign changes
load, a decrease in resistance to initial plastic
deformations (Bauschinger effect).
Dislocations arising during primary deformation cause
the appearance of residual stresses in the metal, which, when combined with
operating voltages when the sign of the load changes, cause a decrease
yield strength. With increasing initial plastic deformations
the amount of reduction in mechanical characteristics increases.
Effect
Bauschinger
obviously
manifests itself
at
insignificant
initial
cold hardening
Short
vacation
riveted
materials
eliminates all manifestations
Bauschinger effect. Effect
is significantly weakened by
multiple
cyclical
loads
material
With
presence of small plastic
deformations of different signs. All of the above defects in the crystal structure lead to
the appearance of internal stresses. By volume, where they are
are balanced, stresses of the 1st, 2nd and 3rd kind are distinguished.
Internal stresses of the first kind are zonal stresses,
occurring between individual section zones or between individual
parts parts. These include thermal stresses that appear
with accelerated heating and cooling during welding and heat treatment.
Internal stresses of the second kind - occur inside the grain or between
neighboring grains are due to the dislocation structure of the metal.
Internal stresses of the third kind - arise inside a volume of the order
several elementary cells; the main source is point
defects.
Internal residual stresses are dangerous because
add up to the current operating voltages and can lead to
premature destruction of the structure.
Defects in crystals are divided into:
Zero-dimensional
One-dimensional
Two-dimensional
Point defects (zero-dimensional) - violation of periodicity at lattice points isolated from each other; in all three dimensions they do not exceed one or more interatomic distances (lattice parameters). Point defects are vacancies, atoms in interstices, atoms in sites of a “foreign” sublattice, impurity atoms in sites or interstices.
Vacancies– absence of an atom or ion in a crystal lattice site; Implemented or interstitial atoms or ions can be both intrinsic and impurity atoms or ions that differ from the main atoms in size or valency. Substitutional impurities replace particles of the main substance at lattice nodes.
Linear(one-dimensional) defects – The main linear defects are dislocations. The a priori concept of dislocations was first used in 1934 by Orowan and Theiler in their study of plastic deformation of crystalline materials, to explain the large difference between the practical and theoretical strength of a metal. Dislocation– these are defects in the crystal structure, which are lines along and near which the correct arrangement of atomic planes characteristic of the crystal is disrupted.
Surface defects of the crystal lattice. Surface lattice defects include stacking faults and grain boundaries.
Conclusion: All types of defects, regardless of the cause of their occurrence, lead to a violation of the equilibrium state of the lattice and increase its internal energy.
Slide 1
Ideal crystals, in which all atoms would be in positions with minimal energy, practically do not exist. Deviations from the ideal lattice can be temporary or permanent. Temporary deviations arise when the crystal is exposed to mechanical, thermal and electromagnetic vibrations, when a stream of fast particles passes through the crystal, etc. Permanent imperfections include:
Slide 2
point defects (interstitial atoms, vacancies, impurities). Point defects are small in all three dimensions, their sizes in all directions are no more than several atomic diameters;
Slide 3
linear defects (dislocations, chains of vacancies and interstitial atoms). Linear defects have atomic sizes in two dimensions, and in the third they are significantly larger in size, which can be commensurate with the length of the crystal;
Slide 4
flat, or surface, defects (grain boundaries, boundaries of the crystal itself). Surface defects are small in only one dimension;
Slide 5
volumetric defects, or macroscopic disturbances (closed and open pores, cracks, inclusions of foreign matter). Volume defects have relatively large sizes, incommensurate with the atomic diameter, in all three dimensions.
Slide 6
Both interstitial atoms and vacancies are thermodynamic equilibrium defects: at each temperature there is a very certain number of defects in the crystalline body. There are always impurities in lattices, since modern methods of crystal purification do not yet allow obtaining crystals with a content of impurity atoms of less than 10 cm-3. If an impurity atom replaces an atom of the main substance at a lattice site, it is called a substitutional impurity. If an impurity atom is introduced into an interstitial site, it is called an interstitial impurity.
Slide 7
A vacancy is the absence of atoms at the sites of a crystal lattice, “holes” that were formed as a result of various reasons. It is formed during the transition of atoms from the surface to the environment or from lattice nodes to the surface (grain boundaries, voids, cracks, etc.), as a result of plastic deformation, when the body is bombarded with atoms or high-energy particles. The concentration of vacancies is largely determined by body temperature. Single vacancies can meet and combine into divacancies. The accumulation of many vacancies can lead to the formation of pores and voids.