deformation & strengthening mechanisms of materials
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Chapter 8. Deformation & Strengthening Mechanisms of Materials. Deformation & Strengthening Mechanisms of Materials. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. +. ion cores. electron cloud. -. -. -. +. +. +. +. -. -. -. -. +. +. - PowerPoint PPT PresentationTRANSCRIPT
Deformation & Strengthening Mechanisms of
Materials
Chapter 8
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.Strengthening of materials… - restrict the dislocation motion & makes a material harder and stronger. -- 4 strategies/mechanisms to strengthen the materials.
Annealing of materials… - heat treatment for cold-worked metals. -- change mech. properties & microstructure.
Dislocation motion in selected materials…
1. Metals (Cu, Al): - dislocation motion easiest. -- non-directional bonding. -- close-packed directions for slip.
electron cloudion cores
+
+
+
+
+++++++
+ + + + + +
+++++++
2. Covalent Ceramics (Si, diamond): - dislocation motion difficult. -- directional (angular) bonding.
3. Ionic Ceramics (NaCl): - dislocation motion difficult. -- need to avoid nearest neighbors of like sign (- and +).
+ + + +
+++
+ + + +
- - -
----
- - -
slip
dislocation motion
me
cha
nis
ms
sta
ge
s
grain size reduction
solid solution strengthening
precipitation strengthening
strain hardening @ cold working
recovery
recrystallization
grain growth
deformation strengthening annealing
Deformation & Strengthening Mechanisms of Materials
Deformation mechanisms…
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.
- If dislocations can't move, plastic deformation doesn't occur!
example: Dislocation cell structure in lightlydeformed Aluminum
Wall of high dislocation density
Deformation of materials… - plastic deformation which is occur due to dislocation motion. -- consider metals only. -- show a significant plastic deformation before failure/rupture.
(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.
Slip system…
Screw dislocation
Edge dislocation
Deformation & Strengthening Mechanisms of Materials
Slip systems…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.
Close packedplane
Non-close-packedplane
- 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.
Adapted from Fig. 8.6, Callister & Rethwisch 3e.
Deformation & Strengthening Mechanisms of Materials
Strengthening mechanisms…Strengthening of materials
(1) grain size reduction
(2) solid solution strengthening
(3) precipitation strengthening
- grain boundaries are barriers to slip.- barrier "strength" increases with
increasing angle of misorientation.- smaller grain size: more barriers to slip.
- impurity atoms distort the lattice & generate
stress.- stress can produce a barrier to
dislocation motion.
Smaller substitutionalimpurity
Impurity generates local stress at A and B that
opposes dislocation motion to the right.
A
B
Larger substitutional impurity
Impurity generates local stress at C and D that opposes dislocation motion to the right.
C
D
- hard precipitates are difficult to shear. i.e: ceramics in metals (SiC in Iron @ Aluminum).
Side View
precipitate
Top View
Slipped part of slip plane
Unslipped part of slip plane
S
Large shear stress needed to move dislocation toward precipitate and shear it.
Dislocation “advances” but precipitates act as “pinning” sites with
S.spacing
- aim: -- restrict the dislocation motion. -- makes a material harder & stronger.
Deformation & Strengthening Mechanisms of Materials
Strengthening mechanisms…Strengthening of materials
(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.
-Forging
Ao Ad
force
die
blank
force
-Drawing
tensile force
Ao
Addie
die
-Extrusion
ram billet
container
containerforce
die holder
die
Ao
Adextrusion
-Rolling
roll
Ao
Adroll
100 x %o
do
A
AACW
- grain structure at different regions of brass rolled into a wedge. -- grains elongate in rolling direction. -- dislocations get rearranged.
Cold work (rolling process)
- aim: -- restrict the dislocation motion. -- makes a material harder & stronger.
cold-worked grains
- As cold work (%CW) is increased -- Yield strength (y) increases. -- Tensile strength (TS) increases. -- Ductility (%EL or %AR) decreases.
Impact of Cold Work
Deformation & Strengthening Mechanisms of
Materials
(4) strain hardening
Strain hardening @ Cold work analysis
What is the yield strength, tensile strength & ductility of copper rod after cold working?
Do=15.2mm
Cold Work
Dd=12.2mm
Copper
%6.35100 x %2
22
o
do
r
rrCW
Answer:
Brass
Copper
1040 Steel
200
100
300
400
500
600
700
Yie
ld s
tren
gth
(MP
a)
800
10 20 30 40 50 60 700
Yie
ld s
tren
gth
(ksi
)
120
100
80
60
40
20
Percent cold work
Example 1:
900
800
700
600
500
400
300
200 10 20 30 40 50 60 700
Percent cold work
120
100
80
60
40
140
Tens
ile
stre
ngth
(M
Pa)
Tens
ile
stre
ngth
(ks
i)
Brass
Copper
1040 Steel
Brass
Copper
1040 Steel
10 20 30 40 50 60 700
10
0
20
30
40
50
60
70
Percent cold work
Duc
tili
ty (
%E
L)
Yield strength = 310 MPa
Tensile strength = 340 MPa
Ductility = 7%
Deformation & Strengthening Mechanisms of
Materials
(4) strain hardening
Strain hardening @ Cold work analysis
Answer:
Brass
Copper
1040 Steel
200
100
300
400
500
600
700
Yie
ld s
tren
gth
(MP
a)
800
10 20 30 40 50 60 700
Yie
ld s
tren
gth
(ksi
)
120
100
80
60
40
20
Percent cold work
Example 2:
900
800
700
600
500
400
300
200 10 20 30 40 50 60 700
Percent cold work
120
100
80
60
40
140
Tens
ile
stre
ngth
(M
Pa)
Tens
ile
stre
ngth
(ks
i)
Brass
Copper
1040 Steel
Brass
Copper
1040 Steel
10 20 30 40 50 60 700
10
0
20
30
40
50
60
70
Percent cold work
Duc
tili
ty (
%E
L)
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).
1002
22
o
do
r
rrCW%
100
720
2
22
o
o
r
r
(a) %CW = 20(b) Yield strength = 250 MPa
Ductility (% EL) = 17 %
ro = mm
Deformation & Strengthening Mechanisms of
Materials
Stages of annealing process…Annealing of materials
(1) recovery
(2) recrystallization
(3) grain growth
- relieve stored internal strain energy.- enhance atomic diffusion at the elevated temperature.- reduce the number of dislocations.
- new grains are formed at TR that: -- have a small dislocation density. -- are small. -- replace cold-worked grains.
- 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).
- further recrystallization -- all cold-worked grains are replaced.
After 4 seconds After 8 seconds
0.6 mm0.6 mm
i.e: 33% cold-worked brass is reheat at 580oC
new crystalsnucleate
0.6 mm 0.6 mm
After 3 seconds
- at longer times, larger grains consume smaller ones. - why? Grain boundary area & energy is reduced.
After 8 s After 15 min
0.6 mm 0.6 mm
º
º
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.
• 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.
Summary
End of Chapter
8