Transcript Document 7814924

NEEP 541 – Microstructure
Fall 2002
Jake Blanchard
Outline
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Microstructure
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Definitions
Defects
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Point Defects
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Dislocations
Strings of pt defects
Planar Defects
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Vacancies
Interstitials
Impurities
Clusters
Line Defects
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Grain boundaries
Interphase boundaries
Twin boundaries
Domain boundaries
Stacking defaults
Volume Defects
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Cavities
Precipitates
cracks
Pre-Irradiation
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Point defects, dislocations, grain
boundaries are key
These act as sinks and traps for moving
defects
Typically in thermodynamic equilibrium
Defect
concentration
  EF
c  AF exp 
 kT



Formation
energy
Pre-Irradiation
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Vacancy formation energy is lower than
interstitial formation energy
Equilibrium vacancy concentration
greater than interstitial
Interstitials are more mobile
Dislocations not in equilibrium
Dislocation density ~1012 /m2
Damage Structure
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Displacements create v’s and i’s
These move by diffusion
Get recombination
…and clustering
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Di-vacancies, di-interstitials
Voids
Stacking fault tetrahedra
FCC interstitial configuration
BCC interstitial configuration
Octahedral
http://www.techfak.uni-kiel.de/matwis/amat/def_en/makeindex.html
Tetrahedral
Lattice Effects
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Interstitials deform surrounding lattice
They “share” spot with lattice atom
Called a dumb-bell configuration
Di-Interstitials in FCC and BCC
Dumbell - FCC
Dumbell - BCC
Vacancies
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Produce weaker lattice deformation
Form planar and 3-D clusters
Lattice collapses around planar clusters
(dislocation loops)
3-D clusters are voids or stacking fault
tetrahedra (SFT)
Impurities (inert gases) stabilize voids
Dislocation loops
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Lattice collapses around platelet of
vacancies or interstitials
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Frank loops form
Loops grow
Loops convert to perfect loop
Loops rotate
Interstitial Loop
Vacancy loop
Loop Growth
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Loops grow by collecting point defects
as they diffuse
Typical growth equation is:
2
2





b

b
 drL  1 
v
i
  Ds exp 


   DvCv  Z i Di Ci  Ds exp 
 dt v b 
 rL kT 
 rL kT 
 b 2 
 b 2 
 drL  1 
i
v
  Ds exp 


   Z i Di Ci  DvCv  Ds exp 
 dt i b 
 rL kT 
 rL kT 
Point defect
flux to loop
Thermal
emission
Loop Growth
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Z represents preferential attractive
interaction between dislocation loops
and moving interstitials over that with
vacancies
That is, dislocation loops are more likely
to absorb interstitials than vacancies
This tends to leave an excess of
vacancies in the lattice
Hence, interstitial loops tend to grow
and vacancy loops tend to shrink
Voids
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Excess vacancies can form 3-D clusters
Usually some inert gas is needed to stabilize
these voids
At low temperature (<0.25Tm), diffusion is
slow so voids don’t form
At high temperature (>0.5Tm), thermal
emission eliminates voids
In between, void formation is likely
This causes swelling (we’ll come back to this)
Microstructure Development
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3 changes occur as damage takes place
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Changes in dislocation structure
Void formation and growth
Changes in chemical state (segregation,
precipitation)
Destination of Au Interstitials
Dislocation Changes
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Dislocations change shape and size
Length increases by loop growth and
interactions with other dislocations
Length decreases due to annihilation
In annealed materials, length increases
In cold-worked materials, length
decreases
Dislocation Density Evolution
Microchemical Changes
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Cascades can destroy clusters and
dissolve precipitates (effective diffusion)
Increased point defect densities
enhance diffusion
Diffusion Coeff. for Ni alloy