PLASTIC DEFORMATION
Dislocations and their role in
plastic deformation
1
What are dislocations?
Dislocations are line defects that exist in
metals
There are two types of dislocations:
edge and screw
The symbol for a dislocation is
The dislocation density in annealed
metals is normally ρ = 106/cm2
2
Types of dislocations
Screw
Edge
3
Dislocation motion⇔plastic deformation
Note: Dislocations normally move under a shear stress
4
How does a dislocation move?
5
Stress field of a dislocation
6
Analog to an electric charge
7
Modes of deformation
Slip
Twinning
Shear band formation
8
Slip
Dislocations move on a certain
crystallographic plane: slip plane
Dislocations move in a certain
crystallographic direction: slip direction
The combination of slip direction and
slip plane is called a slip system
9
Slip…..
Slip planes are normally close-packed planes
Slip directions are normally close-packed
directions
Recall for fcc close-packed planes are {111}
Close-packed directions are <110>
10
Slip systems
Crystal
system
Slip plane Slip
direction
<110>
Total
number of
slip
systems
12
fcc
{111}
hcp
bcc
Active
slip
systems
5
{0001}
<2110>
3
2/3
{110}
{100}
<111>
48
2
11
Dislocation interaction
Positive
⊥
Positive
⊥
Positive
⊥
Repulsion
Negative
Attraction
&
Annihilation
Note: More positive-positive interactions in reality
12
Positive-positive dislocation interaction
Results in more stress to move
dislocations (or cause plastic
deformation):called work hardening
This type of interaction also leads to
dislocation multiplication which leads to
more interactions and more work
hardening
13
Twinning
Common in hcp and bcc structures
Limited deformation but help in plastic
deformation in hcp and bcc crystals
Occurs on specific twinning planes and
twinning directions
14
Compare slip and twinning
SLIP
TWINNING
HOMOGENEOUS
LOCALIZED
COMMON IN FCC
COMMON IN HCP &
BCC
OCCURS UNDER
STATIC LOADING
OCCURS UNDER
SHOCK LOADING
15
Shear band formation
Limited non-homogeneous deformation
Very large localized strain ε~1 or 100%
Occurs especially under high strain
rates
Mechanism of deformation still unclear
16
Plastic deformation ⇔movement of dislocations
Strengthening methods
17
Cold working
Deformation at temperatures below 0.4
Tm
Dislocation density increases from
106/cm2 to 1010-12/cm2
High dislocation density results in a
large number of dislocation interactions
which results in high strength and
hardness
18
Solid solution strengthening
Interaction between stress fields of alloy
atoms and dislocations
This is the purpose of alloying
19
Grain size refinement
Small grains result in higher strength
Small grains is equivalent to a large
number of grain boundaries in the same
volume
Grain boundaries act as barriers to
dislocation motion
20
Mechanism
Strength is inversely proportional to grain size
σ = σ0 + kyd-1/2
Hall-Petch equation
Smaller grains have more boundary area and hence more
barriers to dislocation motion
21
Precipitation hardening
Precipitates are second-phase particles
Hard precipitates act as barriers to
dislocation motion
Applicable only to some alloy systems
22