Defects In Crys
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Defects In Crys as PDF for free.
More details
- Words: 1,141
- Pages: 45
Imperfection /Defects
10/15/08
1
Structure -insensitive Properties • • • • •
Elastic constants Melting point Density Specific heat Coefficient of thermal expansion.
10/15/08
2
Structure-sensitive Properties • • • • •
Electrical conductivity Semiconductor Properties Yield stress Fracture Strength Creep strength Practically all the mechanical properties are structure-sensitive properties.
10/15/08
3
Defects in Crystalline Materials • All real crystals contain imperfections which may be point, line , surface or volume defects. • Which disturb locally the regular arrangement of the atoms. • Their presence can significantly modify the properties of crystalline solids. 10/15/08
4
Defect, or imperfection • The term defect, or imperfection, is generally used to describe any deviation from an orderly array of lattice points. • When the deviation from the periodic arrangement of the lattice is localized to the vicinity of only a few atoms it is called a point defect, or point imperfection.
10/15/08
5
Lattice Imperfection • However if the defects extends through microscopic region of the crystal, it is called a lattice imperfection. • Lattice imperfections may be divided into Line defects and surface or Planer defects.
10/15/08
6
Types of defects
10/15/08
7
Point Defects
10/15/08
8
Point Defects
10/15/08
9
Line Defect • Line defects obtain their name because they propagate as lines or as a two dimensional net in the crystal. The edge and Screw dislocations are the common line defects encountered in materials. • Surface defects arise from the clustering of line defects into plane
10/15/08
10
10/15/08
11
Surface Defect • The stacking fault between two closed -packed regions of the crystal that have alternate stacking sequences are other example of surface defects. • Grain boundaries, a low angle boundaries and Twin boundaries are surface defects.
10/15/08
12
Point Defects • All the atoms in a perfect lattice are at specific atomic sites (ignoring thermal vibrations). • In pure metal two types of point defect are possible, I) Intrinsic defects ii) Extrinsic defects. • Intrinsic defects: i) A vacant atomic site or vacancy, ii) an interstitial atom. • Vacancy formed by the removal of an atom from an atomic site . • Interstitial by the introduction of an atom into a nonlattice site at 1/2, 1/2, 0 position. • 10/15/08
13
Point Defects
10/15/08
14
Vacancy & Interstitial • It is known that vacancies and interstitials can be produced in materials by plastic deformation and high- energy particle irradiation. • The latter process is particularly important in materials in nuclear reactor installations.
10/15/08
15
• The interstitial defect occurs in pure metals as a result of bombardment with high-energy nuclear particles ( radiation damage), • It does not occur frequently as a result of thermal activation. • Further more, intrinsic point defects are introduced into crystals simply by virtue of temperature, • For all temperature above 0K there is a thermodynamically stable concentration. 10/15/08
16
• The formation energy of interstitial is typically two to four times more than the formation energy of vacancy. • Therefore in metals in thermal equilibrium the concentration of intestinal may be neglected in comparison with that of vacancies
10/15/08
17
Extrinsic defects • Extrinsic defects . Impurity atoms in a crystal can be considered as a extrinsic point defect. Impurity atoms can take up two different types of sites. • Substitutional. An atom of the parent lattice lying in a lattice site is replaced by the impurity atom • Interstitial The impurity atom is at a nonlattice site 10/15/08
18
Point Defects
10/15/08
19
10/15/08
20
Dislocation • The most important two dimensional, or line, defect is the dislocation. • Dislocations are important for explaining the slip of crystals, • They are also intimately connected with nearly all other mechanical phenomena such as , • yield point, strain hardening /work hardening, creep, fatigue, and brittle fracture. 10/15/08
21
10/15/08
22
10/15/08
23
10/15/08
24
10/15/08
25
10/15/08
26
• One way of thinking of a dislocation is to consider that it is the region of localized lattice disturbance separating the slipped and un slipped region of a crystal.
10/15/08
27
• The two basic types of dislocations : • Edge dislocation , Burger vector is normal to the line of the dislocation • Two types Positive edge dislocation and negative edge dislocation. • Screw dislocation, burger vector is parallel to the line of dislocation. • Two types , Right hand screw and left hand screw dislocation. 10/15/08
28
• Two important rules. • I) The burger vector of edge dislocation is normal to the line of the dislocation . • II) The burger vector of screw dislocation is parallel to the line of the dislocation . • All crystals, apart from some whiskers, contain dislocations and in well annealed crystals the dislocation are arranged in a rather ill- defined net work, the frank net. 10/15/08
29
Dislocation Density • The dislocation density is defined as the total length of dislocation line per unit volume of crystal, normally quoted in units of mm-2. • Thus for a volume V containing line length l, Density = l/V. • An alternative definition, the number of dislocations intersecting a unit area, again measured in units of mm-2 . • If all the dislocations are parallel, the two density values are the same, but for completely random arrangement the volume density is twice the surface density. 10/15/08
30
Stacking Faults • A stacking fault is a planer defects , • it is a local region in the crystal where the regular sequence has been interrupted. • The atomic arrangement on the plane of an fcc structure and the plane of an hcp structure could be obtained by the stacking of closed- packed planes of spheres. • For the fcc structure, the stacking sequence of the planes of atom is given by ABCABCABC. • For the hcp structure, the sequence is given by ABABAB and there is no alternate site for an A layer resting on B layer. 10/15/08
31
• For the hcp structure, the sequence is given by ABABAB and there is no alternate site for an A layer resting on B layer. • In case of ABCABCABC stacking, A layer can rest equally well on either B or C position and geometrically there is no reason for the selection of a particular position. • Therefore in fcc lattice two types of stacking fault are possible. Either by removal or introduction of stacking sequence. 10/15/08
32
• i) Intrinsic stacking fault part of the layer has been removed which results in a break of the stacking sequence. • ii) Extrinsic stacking fault. An extra layer has been introduced between B and C layer. There are two breaks in the stacking sequence. 10/15/08
33
10/15/08
34
10/15/08
35
10/15/08
36
10/15/08
37
10/15/08
38
10/15/08
39
10/15/08
40
10/15/08
41
10/15/08
42
10/15/08
43
10/15/08
44
10/15/08
45
10/15/08
1
Structure -insensitive Properties • • • • •
Elastic constants Melting point Density Specific heat Coefficient of thermal expansion.
10/15/08
2
Structure-sensitive Properties • • • • •
Electrical conductivity Semiconductor Properties Yield stress Fracture Strength Creep strength Practically all the mechanical properties are structure-sensitive properties.
10/15/08
3
Defects in Crystalline Materials • All real crystals contain imperfections which may be point, line , surface or volume defects. • Which disturb locally the regular arrangement of the atoms. • Their presence can significantly modify the properties of crystalline solids. 10/15/08
4
Defect, or imperfection • The term defect, or imperfection, is generally used to describe any deviation from an orderly array of lattice points. • When the deviation from the periodic arrangement of the lattice is localized to the vicinity of only a few atoms it is called a point defect, or point imperfection.
10/15/08
5
Lattice Imperfection • However if the defects extends through microscopic region of the crystal, it is called a lattice imperfection. • Lattice imperfections may be divided into Line defects and surface or Planer defects.
10/15/08
6
Types of defects
10/15/08
7
Point Defects
10/15/08
8
Point Defects
10/15/08
9
Line Defect • Line defects obtain their name because they propagate as lines or as a two dimensional net in the crystal. The edge and Screw dislocations are the common line defects encountered in materials. • Surface defects arise from the clustering of line defects into plane
10/15/08
10
10/15/08
11
Surface Defect • The stacking fault between two closed -packed regions of the crystal that have alternate stacking sequences are other example of surface defects. • Grain boundaries, a low angle boundaries and Twin boundaries are surface defects.
10/15/08
12
Point Defects • All the atoms in a perfect lattice are at specific atomic sites (ignoring thermal vibrations). • In pure metal two types of point defect are possible, I) Intrinsic defects ii) Extrinsic defects. • Intrinsic defects: i) A vacant atomic site or vacancy, ii) an interstitial atom. • Vacancy formed by the removal of an atom from an atomic site . • Interstitial by the introduction of an atom into a nonlattice site at 1/2, 1/2, 0 position. • 10/15/08
13
Point Defects
10/15/08
14
Vacancy & Interstitial • It is known that vacancies and interstitials can be produced in materials by plastic deformation and high- energy particle irradiation. • The latter process is particularly important in materials in nuclear reactor installations.
10/15/08
15
• The interstitial defect occurs in pure metals as a result of bombardment with high-energy nuclear particles ( radiation damage), • It does not occur frequently as a result of thermal activation. • Further more, intrinsic point defects are introduced into crystals simply by virtue of temperature, • For all temperature above 0K there is a thermodynamically stable concentration. 10/15/08
16
• The formation energy of interstitial is typically two to four times more than the formation energy of vacancy. • Therefore in metals in thermal equilibrium the concentration of intestinal may be neglected in comparison with that of vacancies
10/15/08
17
Extrinsic defects • Extrinsic defects . Impurity atoms in a crystal can be considered as a extrinsic point defect. Impurity atoms can take up two different types of sites. • Substitutional. An atom of the parent lattice lying in a lattice site is replaced by the impurity atom • Interstitial The impurity atom is at a nonlattice site 10/15/08
18
Point Defects
10/15/08
19
10/15/08
20
Dislocation • The most important two dimensional, or line, defect is the dislocation. • Dislocations are important for explaining the slip of crystals, • They are also intimately connected with nearly all other mechanical phenomena such as , • yield point, strain hardening /work hardening, creep, fatigue, and brittle fracture. 10/15/08
21
10/15/08
22
10/15/08
23
10/15/08
24
10/15/08
25
10/15/08
26
• One way of thinking of a dislocation is to consider that it is the region of localized lattice disturbance separating the slipped and un slipped region of a crystal.
10/15/08
27
• The two basic types of dislocations : • Edge dislocation , Burger vector is normal to the line of the dislocation • Two types Positive edge dislocation and negative edge dislocation. • Screw dislocation, burger vector is parallel to the line of dislocation. • Two types , Right hand screw and left hand screw dislocation. 10/15/08
28
• Two important rules. • I) The burger vector of edge dislocation is normal to the line of the dislocation . • II) The burger vector of screw dislocation is parallel to the line of the dislocation . • All crystals, apart from some whiskers, contain dislocations and in well annealed crystals the dislocation are arranged in a rather ill- defined net work, the frank net. 10/15/08
29
Dislocation Density • The dislocation density is defined as the total length of dislocation line per unit volume of crystal, normally quoted in units of mm-2. • Thus for a volume V containing line length l, Density = l/V. • An alternative definition, the number of dislocations intersecting a unit area, again measured in units of mm-2 . • If all the dislocations are parallel, the two density values are the same, but for completely random arrangement the volume density is twice the surface density. 10/15/08
30
Stacking Faults • A stacking fault is a planer defects , • it is a local region in the crystal where the regular sequence has been interrupted. • The atomic arrangement on the plane of an fcc structure and the plane of an hcp structure could be obtained by the stacking of closed- packed planes of spheres. • For the fcc structure, the stacking sequence of the planes of atom is given by ABCABCABC. • For the hcp structure, the sequence is given by ABABAB and there is no alternate site for an A layer resting on B layer. 10/15/08
31
• For the hcp structure, the sequence is given by ABABAB and there is no alternate site for an A layer resting on B layer. • In case of ABCABCABC stacking, A layer can rest equally well on either B or C position and geometrically there is no reason for the selection of a particular position. • Therefore in fcc lattice two types of stacking fault are possible. Either by removal or introduction of stacking sequence. 10/15/08
32
• i) Intrinsic stacking fault part of the layer has been removed which results in a break of the stacking sequence. • ii) Extrinsic stacking fault. An extra layer has been introduced between B and C layer. There are two breaks in the stacking sequence. 10/15/08
33
10/15/08
34
10/15/08
35
10/15/08
36
10/15/08
37
10/15/08
38
10/15/08
39
10/15/08
40
10/15/08
41
10/15/08
42
10/15/08
43
10/15/08
44
10/15/08
45
Related Documents
Defects In Crys
November 2019 14
Defects In Airdeccan
November 2019 9
Defects In Paint
May 2020 26
Defects In Construction.pptx
November 2019 17
Heart Defects
October 2019 38