Dynamic Recrystallisation
Recovery
Dynamic Recrystallisation Mechanisms
- Bulging
- Subgrain Rotation Recrystallisation
- Grain Boundary Migration
Mechanism = f ( temp, strain rate etc )
Crystal rearranges dislocations into subgrain walls to minimise internal energy
Crystal tidies up / straightens out portions of lattice
∴ Reduces internal energy
Result - tidier domains with lower dislocation ρ
Previously - lattice bent due to dislocation presence
New subgrains form via recovery & recrystallisation
Misorientation
up to 10 ° - subgrains
once > 10° -
new grain forms
Recovery Process
1. Rigid backstop
2. Lattice concentrates dislocations in one place/ plane
3. Subgrain boundary eventually forms
neighbouring grains cannot accept more dislocations
ice crystal stops moving dislocations to edge
forms a deformation band
separating 2 new lattice domains
misoriented wrt eachother
with comparatively lower
--> dislocation ρ & internal energy
via subgrain rotation recrystallisation
misoriented domains exist as subgrains
1. Bulging
2 neighbouring grains have differing dislocation density
∴ Tidy grain grows (accepts molecules)
at expense of untidy grain
2. Subgrain Rotation Recrystallisation
Progressive misorientation of domains wrt eachother
Dislocations concentrated into subgrain walls
if domains misoriented >10°
during progressive deformation
High angle boundaries ∴ new grains form
3. Grain Boundary Migration
'Cannibalism' - Grain rapidly eats into neighbour
VERY HIGH TEMP PROCESS
Grain boundaries become very mobile
∴ may sweep material in any direction
moving disclocations & subgrain boundaries
Strain accomodation
Limited shortening possible by basal slip
Limited void space in ice
Once aggregate cannot shorten further
Dislocations accumulate in lattice
Raising its internal free energy
∴ minimises dislocations in neighbouring crystal
Recovery Premise
Crystal reduces internal energy by rearranging dislocations into subgrain walls
Result: misoriented lattice domains
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