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Dynamic Recrystallisation (Dynamic Recrystallisation Mechanisms (Bulging,…
Dynamic Recrystallisation
Recovery
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
Result: misoriented lattice domains
Dynamic Recrystallisation Mechanisms
Bulging
Subgrain Rotation Recrystallisation
Grain Boundary Migration
Mechanism = f ( temp, strain rate etc )
New subgrains form via recovery & recrystallisation
Misorientation
up to 10 ° - subgrains
misoriented domains exist as subgrains
once > 10° -
new grain forms
via subgrain rotation recrystallisation
Recovery Process
1. Rigid backstop
neighbouring grains
cannot accept more dislocations
ice crystal stops moving dislocations to edge
2. Lattice concentrates dislocations
in one place/ plane
forms a deformation band
3. Subgrain boundary eventually forms
separating 2 new lattice domains
misoriented wrt eachother
with
comparatively lower
--> dislocation ρ & internal energy
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
during progressive deformation
if domains misoriented >10°
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
∴ minimises dislocations in neighbouring crystal
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
Recovery Premise
Crystal reduces internal energy by rearranging dislocations into subgrain walls