CRYSTAL IMPERFECTIONS
Definition of crystal
Whenever atoms arrange themselves in an orderly repetitive three dimensional pattern a crystal is formed.
It is a solid which consists of atoms arranged in a pattern in 3-D.
A perfect crystal is constructed by the infinite regular repetition in space of identical structural units or building blocks.
The symmetry is an important characteristic of most of the crystals. e.g cube and octahedrons are simple form of the crystal.
All metals are crystalline, where atoms are arranged in a definite periodic order.
Classification of Crystals
On the basis of periodic arrangement of atoms crystals are grouped into seven systems.
The systems are :
Cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic.
In the present context, only cubic and hexagonal crystal structures are considered as most of the metals and alloys belong to these two systems.
In crystal structure, the smallest unit is one unit cell which characterizes the specific arrangement and location of atoms.
There are three types of unit cells with cubic crystal structure such as SC, BCC, FCC.
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| 1. SC,2. BCC, 3. FCC |
Hexagenal Crystal Structure
Example - Metals like Be, Tc, Mg and Zn
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| Hexagenal Crystal Structure |
Ideal Crystals
In ideal crystals, the angles between the faces required to determine the crystal form are same.
Crystal Imperfections
Crystals are not perfect. An important characteristic which determines some important properties of crystalline materials is the presence of imperfections. Except some ideal crystals most of the crystals have some type of defects or imperfections. All crystals are not composed of identical atoms on identical sites throughout a regularly repeating 3D lattice. These imperfection or defects are used to describe any deviation from an orderly periodic array of atoms and influence the characteristics like mechanical strength, electrical properties and chemical reactions.
Classification of imperfections
Defects one classified into point, line or place and volume imperfections.
Types and causes of point defects
Point Defects
In crystal lattice, point defect is completely local in its effect. When point defect gets introduced in crystal lattice, internal energy of the crystal increases.
Types of Point Defects
Vacancies, interstitialcies and impurities are example of point defctes.
Causes of Point Defects
A vacant lattice site is a point defect.
Vacancies
The number of vacancies at equilibrium present in a crystal at a given temperature can be determined by the equation.
n0 = N* ex- E/KT
Where n0 = number of vacancies per mole
N = total number of atomic sites per mole.
E = activation energy for formation of vacancy.
K = Boltzman’s constant.
T = Temperature in absolute scale.
Vacancies are atomic sites from which the atoms are missing and exist in metal at all temperatures above absolute zero. It play a great role in diffusion of atoms in the crystal lattice. It arises from thermal vibrations and introduced during solidification.
Interstitialcies
When an atom is displaced from a regular site and occupies an interstitial site, an interstitialcy is formed. It also gives rise to lattice distortion because interstitial atom tends to push the surrounding atoms apart. The smaller the size of interstitial atoms smaller the defect.
Impurities
Impurities are foreign atoms which are present in the crystal lattice. Impurity atoms may occupy either interstitial or substitutional position. It is a small atom occupies an interstitial void space between atoms at lattice points of the crystal.
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| defect. |
Types and causes of line defects, Edge dislocation and screw dislocation
Line Deffects
Line defects are also known as dislocations. Dislocation is the region of localized lattice disturbance between slipped and unslipped regions of a crystal. Due to lattice disturbances, elastic strain fields and stresses are associated with dislocations.
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| (1) Edge dislocation and (2) Screw dislocation |
Types of Line Deffects
Dislocations are of two types
(1) Edge dislocation
(2) Screw dislocation
Edge dislocation
In the figure of edge dislocation in which a burger’s vector lies perpendicular to the dislocation line. A burger circuit is drawn around the dislocation line and the vector required to close the circuit RS is known as the burger vector of the dislocation. An edge dislocation moves in the direction of the burger vector. It has an extra row of atoms either above or below the slip plane in crystal. When the extra row of atoms is above the slip plane it is called positive. When the extra row of atoms is below the slip plane, it is called negative edge dislocation. Here the atoms above the edges are in compression and those below are in tension.
Screw dislocation
Here the burger vector is parallel to the dislocation line and distortion is of shear type. It follows a helical path and it may follow right hand or left hand screw rule. Positive and negative dislocations are shown by clockwise and anticlockwise signs, respectively. It shows cross slip, where it moves from one slip plane to another. Either edge or screw of opposite signs if present in the same line, attract each other and can annihilate each other.
Effect of imperfections on material properties
It affects or influence the characteristics like mechanical strength, electrical properties and chemical reactions. The role of imperfections in heat treatment is very important. Imperfections account for crystal growth , diffusion mechanism, annealing and precipitation, besides this, other metallurgical phenomena, such as oxidation, corrosion, yield strength, creep, fatigue and fractures’ are governed by imperfections. Imperfections are not always harmful to metals. Sometimes they are generated to obtain the desired properties. For example, carbon is added to steel as interstitial impurity to improve the mechanical properties and this properties are further improved by heat treatment.
Deformation by slip and twinning
Slip
Metals deform plastically by slip. Slipping is facilitated in the presence of dislocation.
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| Mechanism of slip |
Slip is defined as the process or mechanism by which a large displacement of one part of the crystal relative to another along particular crystallographic planes takes place.
There may be one or more slip planes and one or more slip directions in each crystal. Slip begins when the shearing stress acting along the slip planes in the direction of slip exceeds a certain value known as critical slip planes are planes of high atomic densities while the direction of slip along these planes is always the direction of highest atomic density.
Twins and Twinning
Other than slip, twinning also gives rise to plastic deformation in crystals. It may be called as a special case of slip movement. In twinning, instead of whole blocks of atoms moving different distances along the slipping planes, each plane of atoms concerned moves a definite distance and the total movement at any point relative to the twinning plane is proportional to the distance from this plane. In bcc and hcp it occurs frequently.
Effect of deformation on material properties
The mechanical properties are greatly affected by deformation i.e plastic deformation. The deformation process like rolling, forging, extrusion, drawing. Strain hardening takes place, so hardness changes. Elasticity changes, cracking takes place, grain growth takes place. Residual stress are produce in cold working.
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