5 defects notes
TRANSCRIPT
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Objectives
1. Describe the basic types of point defects
found in crystalline materials.
2. Explain the rules for alloy substitutional
solubility.
3. Describe the basic line and area defect
types in crystals.
4. Explain etching process and effects of
grain boundary and surface energy on
behavior of atoms
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Crystalline Defects
Defects occur any time the crystalline structure
is interrupted or distorted.
The scale of disruption determines the energy
level of neighboring atoms:
Atom Energy Level = F(degree of disruption)
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Crystalline Defects
1. Point Defects
A. Vacancy
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Number of Vacancies
(Arhennius Expression)
Nv = N exp – (Q
V/ kT)
QV, k – Tables
N – total number of atoms
T – Temperature (Kelvin)
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Crystalline Defects
1. Point Defects
A. Vacancy
B. Self-Interstitial
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Crystalline Defects
1. Point Defects
A. Vacancy
B. Self-Interstitial
C. Alloying Elements
i. substitutional solid
solution
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Requirements for
Elemental Solubility1. Atomic Size Factor +/- 15%
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Requirements for Elemental
Solubility
1. Atomic Size Factor +/- 15%
2. Matched _____________3. Same crystalline structures
4. Similar __________ (ionization
state)
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Substitutional Solubility
Means that one element can simply
________________ for atoms of another.
Poor solubility doesn’t mean you can’t mix,
only___________________________.
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Crystalline Defects
1. Point Defects
A. Vacancy
B. Self-Interstitial
C. Alloying Elements
i. substitutional
solid solution
ii. interstitial solid
solution
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Application of Substitutional Atoms
Dental Ceramics (ionic
crystals)
Tensile strength
improved by doping withlarger atoms.
Doped layer prestressed
Tension
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Crystalline Defects
2. Line Defects A. Edge Dislocations
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Crystalline Defects
2. Line DefectsA. Edge Dislocations
B. Screw Dislocations
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Crystalline Defects
3. Interfacial or Area Defects
A. Free Surfaces
“Surface Energy”
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Crystalline Defects
3. Interfacial or Area Defects
A. Free Surfaces
B. Grain Boundaries
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Grain Boundary Etching
Etching takes advantage of the elevated energy
of grain boundary atoms.
Grain boundary atoms will dissolve (react) more
rapidly than others on an exposed surface.
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Microscopy of Etched Grain
Surface
Light Beam
Boundary
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Crystalline Defects
3. Interfacial or Area DefectsA. Free Surfaces
B. Grain Boundaries
C. Twin Boundaries
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Twinning
Annealing Twins form during casting or heat
treating – typical in FCC alloys
Deformation Twins form during mechanical
deformation – usually in BCC or HCP alloys.