Defect in solid

Июль 29, 2022

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Defect in solid

Содержание

2. 2 Solids consist of crystal. Crystals are described by scientists using an idealized model, but such
3. 3 Dictionary Solid – твердое тело, твердое состояние Sufficiently – достаточно Impurities – примеси Invariably –
4. 4 Defects occur in a solid at any temperature. The number of defects increases with increasing
5. 5 Point defects Point defects include vacancies, atoms between the nodes of the crystal lattice, impurities.
6. 6 Defect of Schotky and Frenkel Frenkel defect: anion vacancy-interstitial cation pair When the temperature is
7. 7 Number of defect of Schotky The number of vacancies (defect of Shotky) is given by
8. 8 Number of defect of Frenkel The number of defect of pair Frenkel is given by
9. Line defect - dislocation The experimental data do not show that the observed values of the
10. 10 The two types of dislocations are: Edge dislocation Screw dislocation In Edge dislocations the Burger
11. 11 The other type of dislocation is the screw dislocation where the Burger vector is parallel
12. 12 Dislocations move in steps. When the half-plane reaches a free surface it produces a slip
13. 13 Dislocations appear as lines when observed under transmission electron microscope (TEM) Observing dislocations
14. 14 Grain boundaries is surface defect. Most crystalline solids are an aggregate of several crystals. Such
15. 15 Porosity Cracks Inclusions These defects form during manufacturing processes for various reasons and often harmful
16. 16 Volume defect Microphotograph of steel with inclusions of enriched Uranium
17. 17 Volume defect Another example is effect Hydrogen Embrittlement. It is generally agreed that hydrogen, in
18. 18 The best known methods for studying defects in solid: Measurement of electrical resistance Positron annihilation
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Solids consist of crystal. Crystals are described by scientists using an

idealized model, but such crystals are not found in nature. The deviation of the crystal structure from the periodic crystal structure is defects.
The real crystal are not perfect. A real crystal always has a large number of imperfections in the lattice crystal. One can reduce crystal defect considerably, but can never eliminate them entirely.

Perfect model of crystal

What is a defect in a solid?

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Dictionary
Solid – твердое тело, твердое состояние
Sufficiently – достаточно
Impurities – примеси
Invariably –

неизменно
Interstitial – междуузлье
Mechanical treatment – механическая обработка
Imperfections – несовершенство, дефект
Grain boundary – граница зерна
Porosity - пористость
Cracks - трещины
Inclusions – включение
Discontinuities – разрывы
Grinding – шлифование
Enriched uranium – обагащенный уран

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Defects occur in a solid at any temperature. The number of

defects increases with increasing temperature and under the influence of ionizing radiation and mechanical treatment

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Point defects
Point defects include vacancies, atoms between the nodes of the

crystal lattice, impurities.
Vacancy – regular position in a lattice which there is no atom. Vacancies are present in all materials.
An impurity is a foreign atom that does not form a chemical compound in the crystal lattice

Point defect-vacancy

Point defect-impurity

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Defect of Schotky and Frenkel
Frenkel defect: anion vacancy-interstitial cation pair
When the

temperature is sufficiently high, as the atoms vibrate around their regular positions, some acquire enough energy to leave the site. When the regular atom leaves, a vacancy is created.
A pair of one cation and one anion can be missed from an ionic crystal. Such a pair of vacant ion sites is called Schotky defect. In crystals, an ion displaced from a regular site to an interstitial site is called Frenkel deffect.

Schottky defect: anion -cation vacancy pair

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Number of defect of Schotky
The number of vacancies (defect of Shotky)

is given by formula:
N- number of atom;
n - number of vacancies;
kB- constant of Boltzman, 1,38*10-23 J /K;
T- temperature of solid;
Ev- energy of formation defect of Shotky;

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Number of defect of Frenkel
The number of defect of pair Frenkel

is given by formula:
N-number of atom;
N’- number of interstitial by can take up free atom;
n- number of defects pair of Frenkel;
kB- constant of Boltzman, 1,38*10-23 J /K;
T- temperature of solid;
Ef- energy of formation pair of Frenkel.

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Line defect - dislocation
The experimental data do not show that the

observed values of the yield strength are much smaller than the theoretical values. To explain this effect, a new type of defect was introduced, which the scientists called a dislocation.
Dislocation is the region of localized lattice distortion which separates the slipped and unslipped portion of the crystal. The upper region of the crystal over the slip plane has slipped relative to the bottom portion.

The line (AD) between the slipped and unslipped portions is the dislocation. The magnitude and direction of slip produced by dislocation (pink shaded) is the Burger vector of the dislocation.

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The two types of dislocations are:
Edge dislocation
Screw dislocation
In Edge

dislocations the Burger vector is
perpendicular to the dislocation line and the distortion produces an extra half-plane above the slip plane

Slip vector

Edge dislocation

The Burgers vector is a quantitative characteristic describing the distortions of the crystal lattice around a dislocation.

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The other type of dislocation is the screw dislocation where the

Burger vector is parallel to the dislocation line (AD). The trace of the atomic planes around the screw dislocation makes a spiral or helical path (pink shade) like a screw and hence, the name.

Screw dislocation

Atomic positions along a screw dislocation

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Dislocations move in steps. When the half-plane reaches a free surface

it produces a slip step.
Edge dislocations can move only on the slip plane while screw dislocations do not have a fixed glide plane.
Since plastic deformation takes place by movement of dislocations, any hindrance to their motion will increase the strength of metals. Many types of increase in ductility of metals are based on an increase in the number of dislocations or an increase in the barriers on the path of dislocation.

Dislocation movement

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Dislocations appear as lines when observed under
transmission electron microscope (TEM)
Observing dislocations

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Grain boundaries is surface defect.
Most crystalline solids are an aggregate of

several crystals. Such materials are called polycrystalline. Each crystal is known as a grain. The boundary between the grains is the grain boundary.

Optical micrograph of
a polycrystalline material

Schematic of orientation change across the grain boundary

Grain boundaries

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Porosity
Cracks
Inclusions
These defects form during manufacturing processes for various reasons and often

harmful to the material.
A lot of inclusions fall into the material during welding, thermal, mechanical or other processing. For example during grinding.
Example porosity reduces thermal conductivity of material, but sometimes porosity specifically increase for increase thermal isolate.

Volume defect

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Volume defect
Microphotograph of steel with inclusions of enriched Uranium

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Volume defect
Another example is effect Hydrogen Embrittlement. It is generally agreed

that hydrogen, in atomic form, will enter and diffuse through a metal surface whether at elevated temperatures or ambient temperature. Once absorbed, dissolved hydrogen may be present either as atomic or molecular hydrogen or in combined molecular form (e.g., methane). Since these molecules are too large to diffuse through the metal, pressure builds at crystallographic defects (dislocations and vacancies) or discontinuities (voids, inclusion/matrix interfaces) causing minute cracks to form.

Hydrogen Induced Cracks (HIC)

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The best known methods for studying defects in solid:
Measurement of electrical

resistance
Positron annihilation
Studying of diffusion properties

Methods for studying defects in solid

Defect in solid

Содержание

2 Solids consist of crystal. Crystals are described by scientists using an

3DictionarySolid – твердое тело, твердое состояниеSufficiently – достаточноImpurities – примесиInvariably –

4Defects occur in a solid at any temperature. The number of

5Point defects Point defects include vacancies, atoms between the nodes of the

6Defect of Schotky and FrenkelFrenkel defect: anion vacancy-interstitial cation pairWhen the

7Number of defect of SchotkyThe number of vacancies (defect of Shotky)

8Number of defect of FrenkelThe number of defect of pair Frenkel

Line defect - dislocation The experimental data do not show that the

10The two types of dislocations are: Edge dislocation Screw dislocationIn Edge

11 The other type of dislocation is the screw dislocation where the

12Dislocations move in steps. When the half-plane reaches a free surface

13Dislocations appear as lines when observed undertransmission electron microscope (TEM)Observing dislocations

14Grain boundaries is surface defect.Most crystalline solids are an aggregate of

15PorosityCracksInclusionsThese defects form during manufacturing processes for various reasons and often

16Volume defectMicrophotograph of steel with inclusions of enriched Uranium

17Volume defectAnother example is effect Hydrogen Embrittlement. It is generally agreed

18The best known methods for studying defects in solid:Measurement of electrical

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