Defects in Solids

1
Defects in Solids

• 0-D or point defects
• vacancies, interstitials, etc.
• control mass diffusion
• 1-D or linear defects
• dislocations
• control deformation processes
• 2-D or planar defects
• grain boundaries, surfaces, interfaces
• 3-D or volume defects
• voids, secondary components (phases)

concentrations ?
2
Diffusional Processes
diffusion coefficient
concentration gradient
Pd
flux
H2
response ? driving force
CO
CO2
Ficks first law (similar to Ohms
law) phenomenological
H
Applies under steady state conditions c(x) ? f(t)
x
hydrogen separation membrane
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Diffusional Processes
Steady-state c(x) ? f(t)
Non steady-state c(x) f(t)
Continuity requirements
Ficks second law
if D ? f(x)
4
Defects in Solids

• 0-D or point defects
• vacancies, interstitials, etc.
• control mass diffusion
• 1-D or linear defects
• dislocations
• control deformation processes
• 2-D or planar defects
• grain boundaries, surfaces, interfaces
• 3-D or volume defects
• voids, secondary components (phases)

concentrations ?
5
Linear (line) defects
Dislocations intimately tied to mechanical
properties

• small stress
• ? elastic deformation
• recoverable
• large stress
• ? plastic deformation
• permanent

s F/A stress
huge energy penalty, yet happens easily in metals
answer slip, dislocation glide
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Dislocation Motion Deformation
dislocation
slip plane
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Dislocations
vector between finish and start
Burgers vector
Edge
extra half-plane
b ? dislocation line
8
line of disruption dislocation line
no disruption of planes here
travel around dislocation line steps only in the
x-y plane
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Dislocations
Screw
b dislocation line
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Dislocations Mixed
Burgers vector is constant
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Dislocation Loops
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Dislocation Loops
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Dislocation Loops
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Dislocation Loops
in projection
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Dislocation Loops
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Dislocation Loops
Dislocation lines cannot simply terminate. They
either form complete loops or terminate at
crystal edge