1. Deformation & Strengthening Mechanisms of Materials
Chapter 8
Deformation & Strengthening Mechanisms of Materials

2. Deformation & Strengthening Mechanisms of Materials
Deformation of materials…
is refer to plastic deformation which is occur
due to motion of dislocations.
-- consider metals only.
-- show a significant plastic deformation
before failure/rupture.
Dislocation motion in selected materials…
1. Metals (Cu, Al):
- dislocation motion easiest.
-- non-directional bonding.
-- close-packed directions for slip.
Strengthening of materials…
restrict the dislocation motion & makes a
material harder and stronger.
-- 4 strategies/mechanisms to strengthen the
materials.
electron cloud
ion cores +
Annealing of materials…
heat treatment for cold-worked metals.
-- change mech. properties & microstructure.
2. Covalent Ceramics (Si, diamond):
- dislocation motion difficult.
-- directional (angular) bonding.
deformation
strengthening
annealing
dislocation motion
grain size reduction
slip
solid solution strengthening
mechanisms
precipitation strengthening
3. Ionic Ceramics (NaCl):
- dislocation motion difficult.
-- need to avoid nearest neighbors of
like sign (- and +).
strain cold working
recovery + -
stages
recrystallization
grain growth

3. Deformation & Strengthening Mechanisms of Materials
Deformation of materials…
plastic deformation which is occur due to
dislocation motion.
-- consider metals only.
-- show a significant plastic deformation
before failure/rupture.
Deformation mechanisms…
(1) slip
- atom slide over each other (slip) due to
movement of dislocations.
-- exist external shear stress.
- Occurs on specific crystallographic planes & within these planes only in certain direction.
Wall of high dislocation density
example: Dislocation cell structure in lightly
deformed Aluminum
Edge dislocation
Slip system…
Represent a slip plane & slip direction combination.
Slip plane - plane on which easiest slippage occurs
-- Highest planar densities (and large interplanar spacings).
Slip directions - directions of movement
-- Highest linear densities.
Screw dislocation
If dislocations can't move, plastic
deformation doesn't occur!

4. Deformation & Strengthening Mechanisms of Materials
Slip in crystals…
slip occurs in densely or close packed
planes.
lower shear stress is required for slip to
occur in densely packed planes.
if slip is restricted in close planes, then less
dense planes become operative.
Less energy is required to move atoms
along denser planes.
Slip systems…
Slip systems are combination of slip planes
& slip direction.
Each crystal has a number of characteristic
slip systems.
i.e: In FCC crystal, slip takes place in {111}
octahedral planes and <110> directions.
-- 4 (111) type planes and 3 [110] type
directions.
4 x 3 = 12 slip systems.
Close packed
plane
Non-close-packed
plane
Adapted from Fig. 8.6, Callister & Rethwisch 3e.

5. Deformation & Strengthening Mechanisms of Materials
Strengthening of materials
Strengthening mechanisms…
- aim:
-- restrict the dislocation motion.
-- makes a material harder & stronger.
(1) grain size reduction
- grain boundaries are barriers to slip.
barrier "strength" increases with increasing
angle of misorientation.
- smaller grain size: more barriers to slip.
(3) precipitation strengthening
hard precipitates are difficult to shear.
i.e: ceramics in metals
(SiC in Aluminum).
(2) solid solution strengthening
impurity atoms distort the lattice & generate
stress.
stress can produce a barrier to dislocation
motion.
precipitate
Large shear stress needed
Smaller substitutional
impurity
Larger substitutional impurity
Side View
to move dislocation toward A B C D
precipitate and shear it.
Unslipped part of slip plane
Dislocation
Top View
“advances” but
precipitates act as S
“pinning” sites with
spacing S.
Slipped part of slip plane
Impurity generates local stress at A and B that opposes dislocation motion to the right.
Impurity generates local stress at C and D that opposes dislocation motion to the right.

6. Deformation & Strengthening Mechanisms of Materials
Strengthening of materials
Strengthening mechanisms…
- aim:
-- restrict the dislocation motion.
-- makes a material harder & stronger.
(4) strain hardening
also known as cold work (%CW).
-- low temperature deformation.
common forming operations change the
cross sectional area.
i.e: forging, extrusion, rolling & drawing
process.
- As cold work (%CW) is increased
-- Yield strength (y) increases.
-- Tensile strength (TS) increases.
-- Ductility (%EL or %AR) decreases.
Impact of Cold Work
-Rolling
roll A o d
Cold work (rolling process)
-Extrusion
ram
billet
container
force
die holder
die A o d
extrusion
cold-worked grains
- grain structure at different regions of brass
rolled into a wedge.
-- grains elongate in rolling direction.
-- dislocations get rearranged.
-Forging A o d
force
die
blank
-Drawing
tensile
force A o d
die

7. Deformation & Strengthening Mechanisms of Materials
(4) strain hardening
Strain Cold work analysis
Example 1: D o
=15.2mm
Cold
Work d
=12.2mm
Copper
What is the yield strength, tensile strength & ductility of copper rod after cold working?
Answer:
Yield strength = 310 MPa
Tensile strength = 340 MPa
Ductility = 7%
900
800
700
600
500
400
300
200 10 20 30 40 50 60 70
Percent cold work
120
100 80
140
Tensile strength (MPa)
Tensile strength (ksi)
Brass
Copper
1040 Steel
Brass
Copper
1040 Steel
200
100
300
400
500
600
700
Yield strength (MPa)
800 10 20 30 40 50 60 70
Yield strength (ksi)
120 80
Percent cold work
Brass
Copper
1040 Steel 10 20 30 40 50 60 70
Percent cold work
Ductility (%EL)

8. Deformation & Strengthening Mechanisms of Materials
(4) strain hardening
Strain Cold work analysis
Example 2:
A cylindrical specimen of cold-worked copper has a tensile strength of 300 MPa and the cold-worked radius is 7.0 mm.
(a) calculate its radius before deformation.
(b) estimate its yield strength & ductility (% EL).
Answer:
(a) %CW = 20
(b) Yield strength = 250 MPa
Ductility (% EL) = 17 %
ro = mm
900
800
700
600
500
400
300
200 10 20 30 40 50 60 70
Percent cold work
120
100 80
140
Tensile strength (MPa)
Tensile strength (ksi)
Brass
Copper
1040 Steel
Brass
Copper
1040 Steel 10 20 30 40 50 60 70
Percent cold work
Ductility (%EL)
Brass
Copper
1040 Steel
200
100
300
400
500
600
700
Yield strength (MPa)
800 10 20 30 40 50 60 70
Yield strength (ksi)
120 80
Percent cold work

9. Deformation & Strengthening Mechanisms of Materials
Annealing of materials
Stages of annealing process…
- cold worked metals become brittle.
reheating, which increases ductility results in
recovery, recrystallization & grain growth.
this is called annealing & changes material
properties.
-- reduce tensile strength.
-- reduce hardness.
-- increase ductility (% EL).
(1) recovery
- relieve stored internal strain energy.
enhance atomic diffusion at the
elevated temperature.
- reduce the number of dislocations.
(2) recrystallization
- new grains are formed at TR that:
-- have a small dislocation density.
-- are small.
-- replace cold-worked grains.
i.e: 33% cold-worked brass is reheat at 580oC
(3) grain growth
at longer times, larger grains
consume smaller ones.
why? Grain boundary area &
energy is reduced.
new crystals
nucleate
0.6 mm
After 3 seconds
0.6 mm
0.6 mm
- further recrystallization
-- all cold-worked grains are replaced.
After 8 s
After 15 min
After 4 seconds
After 8 seconds
0.6 mm
If metal is held at recrystallization temperature, TR long enough, cold worked structure is completely replaced with recrystallized grains.
TR = recrystallization temperature
- point of highest rate of property change.

10. Summary • Dislocations are observed primarily in metals and alloys.
• Strength is increased by making dislocation
motion difficult.
• Particular ways to increase strength are to:
-- decrease grain size
-- solid solution strengthening
-- precipitate strengthening
-- cold work
• Heating (annealing) can reduce dislocation density
and increase grain size. This decreases the strength. 10

11. End of Chapter 8 11