structural imperfections - 2013
TRANSCRIPT
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STRUCTURALIMPERFECTIONS
(DEFECTS)IN CRYSTALLINE SOLIDS
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Real Crystalline solids are almost neverperfect. These imperfections can be classified
according to their dimensionality:
1. Point defects (0-Dimension)
2. Line defects (1-D)
3. Interfacial defects (2-D)
4. Bulk defects (3-D)
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Relative Size Ranges of Defects
10-610-8 10210110010-210-410-12
Electronicpointdefect
Atomicpointdefect
Line defect Bulk defect
Interfacial defect
cm
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Solidification- result of casting of molten material 2 steps
Nuclei form
Nuclei grow to form crystalsgrain structure
Start with a molten material
all liquid
Imperfections in Solids
Adapted from Fig.4.14 (b), Callister 7e.
Crystals grow until they meet each other
nuclei crystals growing grain structureliquid
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Imperfections in Solids
There is no such thing as a perfect crystal.
What are these imperfections?
Why are they important?
Many of the important properties of materials
are due to the presence of imperfections.
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1. POINT DEFECTS
These are defects of atomic dimensions thatusually result from:
1. The presence of an impurity atom Substitutional larger atoms
Interstitial smaller atoms
2. The absence of a matrix atom (vacancy)3. The presence of a matrix atom in a wrong
place (self-interstitial)
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Vacancies:-vacant atomic sites in a structure.
Self-Interstitials:-"extra" atoms positioned between atomic sites.
Point Defects
Vacancy
distortion
of planes
self-interstitial
distortionof planes
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Presence of an impurity atom:-"extra" atoms positioned between atomic sites.
Substitutional
Interstitial
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The point defects discussed so far occur in metallicstructures. Those in ionic structures differ because ofthe charge neutrally requirement.
An anion
and a cation
is missing
An anion or acation is at an
insterstital site
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are line defects, slip between crystal planes result when dislocations move,
produce permanent (plastic) deformation.
Dislocations:
Schematic of Zinc:
before deformation after tensile elongation
slip steps
2. Line Defects (Dislocations)
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Linear Defects (Dislocations)
Are one-dimensional defects around which atoms
are misaligned Edge dislocation:
extra half-plane of atoms inserted in a crystalstructure
Screw dislocation:
spiral planar ramp resulting from shear deformation
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Edge Dislocations
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Screw Dislocations
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Dislocations are simply slide or slip of oneportion of crystal system over another.
When dislocations pass through the wholesystem, the system permanently deforms.
Dislocations are on boundary between theregions where slip has occurred and where ithas not.
On either side of the dislocation crystallinesystem is essentially perfect.
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3.INTERFACIAL DEFECTS
(BOUNDARIES)
1. Free surfaces: Interfaces between liquids andgases.
2. Grain boundaries: Interfaces between crystalsystems having different orientation.
In each crystal system the atoms arearranged orderly. However, at the boundarythere is a transition zone which is not alingedwith either of the crystal systems.
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Grain Boundaries
Tilt boundary: Result of a
set of edge dislocations.
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O TA C O CT O S
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IMPORTANCE OF IMPERFECTIONS
Most of the properties of materials are affected
by imperfections: Small amount ofimpurity atoms may increase the electrical
conductivity of semi-conductors.
Dislocations are responsible for ductility. Strength of
materials can be increased to a large extent by themechanism strain-hardening which produces line defectsthat act as a barrier to control the growth of otherimperfections.
Presence ofbulk defects such as cracks, notches, holescauses brittle materials, which break at very low stresseswithout showing large deformations.
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Point, Line, and Area defects exist in solids.
The number and type of defects can be varied
and controlled (e.g.,T
controls vacancy concentration)
Defects affect material properties (e.g., grain
boundaries control crystal slip).
Defects may be desirable or undesirable(e.g., dislocations may be good or bad, depending
on whether plastic deformation is desirable or not.)
Summary