Imperfections, Defects and Diffusion
Lattice Defects
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1
Types of Defects
o Point defects (composition)
• Vacancies
(missing atoms)
• Interstitials
(extra atoms)
• Impurities (unwelcome segregation)
o Extensive chemical changes
- Solid solutions
- Not a defect in intentional alloying or doping
o Line defects (1-dimensional) (Deformation)
• dislocations - in metals
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1
Types of Defects, cont.
o Interfacial defects (2-dimensional) (Properties)
- surfaces - both interior (pore walls)
and exterior (surface of material)
- interfaces -(grain boundaries)
o Bulk-Volume defects (3-dimensional)
- cracks, foreign inclusions, other phases
(including pores).
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Point Defects in Metals
Self Interstitial
Interstitial
Impurity
Vacancy
Substitutional
Impurity
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2
Point defects
o Vacancy
An empty atomic site
o Interstitial
An atom somewhere other than an atomic site
- Self-interstitial
- Impurity interstitial
o Substitutional impurity
Some “foreign” species on an atomic site
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Line Defects: Dislocation in Metals
Responsible for large mechanical deformation in crystalline solids
-Linear (one dimensional) defect around which
some of the atoms are misaligned
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3
Types of Dislocations
- Edge Dislocation:
A portion of an extra plane of atoms
- Screw Dislocation:
Helical atomic displacement around a
line
extending through the crystal
- Mixed Dislocation:
Some edge, some screw nature
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Edge Dislocation
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Burgers vector
b
Perfect crystal
dislocated crystal
b-represents the magnitude of
the structural defect
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Screw Dislocation
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Mixed Dislocation
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Why are Dislocations Important?
The motion of dislocations is the principle
mechanism whereby metals deform
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6
Why? Energy, Energy, Energy!
o Lower
energy
than
breaking
all bonds
in a
plane at
once
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13
Shear occurs by dislocation movement producing
permanent (plastic) deformation by “slip”
Textbook ch6, pp 212-218
Direction of
Slip plane
dislocation movement
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7
Edge Dislocation Motion - 3D
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Slip occurs along densely packed
directions on densely packed planes
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8
(Plane)[Direction] pairs designate “slip
systems” (e.g., in FCC and HCP)
pp 215
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Dislocation movement and ductility
•
A large number of independent slip systems are required
for good ductility in polycrystalline materials so grains
can deform to accommodate their neighboring grains
Common in many metal structures (bcc and fcc)
•
Dislocations are very complex in ceramic structures.
This and complications of like charged ions encountering
each other during slip make dislocation movement
almost impossible in ceramics
Therefore ceramics are not ductile, they are brittle
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9
Impediments to easy dislocation movement
• Impurity atoms (“solute hardening”)
• Intersection with other dislocations
(entanglement) (“work hardening”)
• Grain boundaries (dislocations “pile up”)
• Small dispersed inclusions (“precipitation
hardening”)
• All of these affect ductility and yield strength of a
metals
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19
Grain boundaries and other dislocations impede the
movement of dislocations causing “hardening”
Dislocations
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10
Material Science Investigative Techniques “Microscopy”
• Optical microscope
surface microstructure (~ 1µm)
• Scanning electron microscope (SEM)
Surface microstructure, analytical chemistry (~50-100 nm)
• Transmission electron microscope
resolve the atomic structure from a very thin foil (30 Ao),
(~1 Ao)
• Atomic force microscope
3D surface topography, electrical, magnetic scanning (~1 nm)
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21
Reading Assignment
Shackelford 2001(5th Ed)
– Read Chapter 4, pp 115-136, 145-150
Read ahead Chapter 5, pp 158-181
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