Fracture, frictional grain-boundary sliding ->> Cataclastic flow: Favoured by low σc Crystal-plastic flow: Favoured by high σc T ε Plastic deformation results from the relative movement of structural elements: III-1-1 FRAGMENT OF ROCKS AND CRYSTALS Intracrystalline slip: Edge dislocation Dislocation line Dislocation line Intracrystalline slip: Screw dislocation Dislocation loop, combine pure edge and pure screw dislocation b Burgers vector Characterized by a sliding plane, and a slip direction. Sliding plane Dislocation glide + dislocation climb = dislocation flow CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION III-1 FLOW UNIT AND MECHANISM OF DEFORMATION III-1-2 INTRA-CRYSTAL DOMAINS 100 μm A dislocation has the ability to climb via the exchange atoms- vacancies by solid state diffusion in the crystal lattice.
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Fracture, frictional grain-boundary sliding ->> Cataclastic flow: Favoured by low σc
Crystal-plastic flow: Favoured by high σc
T
ε
Plastic deformation results from the relative movement of structural elements:
III-1-1 FRAGMENT OF ROCKS AND CRYSTALS
Intracrystalline slip: Edge dislocation Dislocation line
Dislocation line
Intracrystalline slip: Screw dislocation
Dislocation loop, combine pure edge and pure screw dislocation
b
Burgers vector
Characterized by a sliding plane, and a slip direction.
Sliding plane
Dislocation glide + dislocation climb = dislocation flow
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-1 FLOW UNIT AND MECHANISM OF DEFORMATION
III-1-2 INTRA-CRYSTAL DOMAINS
100 µm
A dislocation has the ability to climb via the exchange atoms-vacancies by solid state diffusion in the crystal lattice.
sub-grain
Dislocation wall
Diffusive mass-transfer: Favoured by fluids
In general, no single process operates alone
σ1
σ1
σ3 σ3
Material move away from areas of high normal stress towards regions of low normal stress through either solid-state diffusion (atoms-vacancies exchange through the lattice: Nabarro-Herring creep or along the grain boundaries: Cobble creep) or transport in solution (Solution creep).
Dislocations may aggregate within stable, low-energy arrays such as dislocation wall. Dislocation walls lead to subgrain boundaries (1 to 5º lattice misorientation) and grain boundaries when the misorientation is higher than 10º.
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-1 FLOW UNIT AND MECHANISM OF DEFORMATION
III-1-2 INTRA-CRYSTAL DOMAINS
III-1-3 MOLECULES, ATOMS, AND CRYSTAL DEFECTS
Dislocation distorts the crystal lattice and introduce elastic energy (red spots on the sketch on the left) that can be sufficient to drive their motion.
Twinning
Mechanical twinning <=> deformation twinning <=> glide twinning
twinning plane
FlexionTwinning
Fracture
III-2-2 AT MEDIUM TEMPERATURE (0.4 to 0.6 Tf)Diffusion become more active–>>RECOVERY PROCESSES and DYNAMIC RECRYSTALLIZATION
Recovery by climb of edge dislocations
Dislocation wall
Annihilation
AnnihilationOpposite dislocation edges
Hardening
Poligonization
The higher the density of dislocation , the more difficult it becomes for any dislocation to move: HARDENING
Climbing rate < gliding rate => Climb is the controling factor
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-2 DEFORMATION MECHANISMS
III-2-1 AT LOW TEMPERATURE (< 0.3 Tf)
γ
Dislocation gliding
τ
Flexion
Dislocation wallSubgrain rotationrecrystallization
Dislocation climb
Time t1 Time t2
Dynamic equilibrium between hardening and recovering processes
ε
σHardening
Creep: steady state flow
ε constant
h
T1
T2 Dynamic recrystallization
Creep: steady state flow T3 Dislocation climbs
-> Boundary migration
-> Subgrain rotation
Diffusive mass-transfer:
Nabarro-Herring creep: Diffusion through the lattice
Coble creep: Diffusion along grain boundaries
Solution creep: Dissolution-precipitation
Superplastic creep: Coherent grain boundary sliding without opening pores between adjacent crystal
1 11
2 22
333 4 4 4
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-2-3 AT HIGH TEMPERATURE (0.8 Tf)
III-2-2 AT MEDIUM TEMPERATURE (0.4 to 0.6 Tf)III-2 DEFORMATION MECHANISMS
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-2 DEFORMATION MECHANISMS
III-2-4 ABOVE SOLIDUS (T> Tf)
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-2 DEFORMATION MECHANISMS
III-2-4 ABOVE SOLIDUS (T> Tf)
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-3 THE EFFECT OF FLUIDS
σ1
σ1
σ3 σ3
Molecules move away from areas of high normal stress towards regions of low normal stress. They move along the grain boundaries by transport in solution. This mechanism of deformation is called solution creep.
σ1
Dissolution Transport Crystallization
σ1
σ1
σ3
σ3
Passive rotation of insoluble elements
λ1
c axis c axis c axis
Slip plane
Macroscopic pure shear Simple shear Rigid rotation
Preferred crystallographic orientation by dislocation glide
Ductile deformation by dislocation creep produces characteristic preferred orientations of mineral crystallographic axes. The pattern of CPO (Crystallographic Preferred Orientation) depends on:
the slip systems that are actived (depends on temperature and stress)
the geometry and the magnitude of the deformation
Coaxial deformation -> fabrics symetric to the principal axes of finite strain
Noncoaxial deformation -> asymmetric fabric
CHAPTER III : MECHANISM OF CONTINUOUS DEFORMATION
III-4 CRYSTALLOGRAPHIC FABRICS
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