2. Strain Hardening in Metals
When a piece of metal
is deformed, the
dislocations run into
each other
This traffic jam
increases the material’s
strength
Deforming a piece of
metal also actually
increases the number of
dislocations
This increases the
strength too!!
3. Tensile Test
You can understand this better by
relating it to the results of the tensile
test.
4. John D Russ – Materials Science – A Multimedia
Approach
9. n is the slope of the
PLASTIC portion of the
curve, when graphed on a
logarithmic scale
10. Log True Stress, σ
Log plot of the plastic portion of a
tensile test
0.001
Slope=n
K
Log(σ) = Log(K) + n Log(ε)
0.010
0.100
Log True Strain, ε
1.0
11. Effect of Crystal Structure
HCP metals are already brittle
Little strain hardening is possible
Strain hardening coefficient around 0.05
BCC metals are less brittle than HCP
Some strain hardening is possible
N around 0.15
FCC metals are ductile
Strain hardening is easy
N around 0.5
12. Frank-Read Source
Strain hardening actually increases the
strength of a material PAST its original
tensile strength
Why?
Additional dislocations are formed as
dislocations run into point defects
13. Strain Hardening – The effect of
dislocation generation
Yield Point
Tensile
Strength
17. Frank-Read Source
Before deformation
a typical dislocation
density is about 106
cm of dislocation per
cm3 of metal
After strain
hardening it may
increase to as much
as 1012 cm per cm3 of
metal
18. Strain Hardening in Polymers
When you pull on a polymer, the chains
line up
Van der Waal bonds form between the
chains
The polymer becomes stronger
Try it with a 6-pack ring!!
The mechanism for strain hardening in plastics is different
from the mechanism in metals
21. Strain Hardening in
Ceramics?
Ceramics are already brittle – so strain
hardening is not usually possible
Ceramics break because of flaws – the
mechanism of deformation is different
Annealing ceramics causes grain
growth
May or may not be bad
22. Back to Metals
Cold Work
There is only a certain amount you can
deform a material before it breaks
Cold work is strain hardening measured
in % - The percent change in cross
sectional area of the material
Different materials have varying %
allowable cold work
24. Rod
Deformation of a rod (or a piece of wire)
Initial cross sectional area minus
Final cross sectional area
Over the initial cross sectional area
A0 − A
*100
A0
25. 2
2
Π * r02 − Π * r 2 r0 − r d 02 − d 2
=
=
%CW =
r2 d 2
Π * r02
0
0
All times 100 of course
26. Copper is often drawn into wire
Copper rod feeding
into drawing
machine
28. Cold Rolling
Metal is often rolled into sheets from
thicker stock
The width of the sheet is usually kept
the same, and only the thickness varies
A0 − A W ( h0 − h ) h0 − h
=
%CW =
=
A
Wh0
ho
0
30. Cold Work
What if you want to deform the sample
more than is “possible?
For example, what if you want to draw a
piece of wire, from a rod of copper?
You can anneal the material, and
“undo” the strain hardening
Annealing is a heat treatment
31. Problem
Propose a series of steps to reduce a
rod of copper-zinc alloy from 1 “
diameter to .1”diameter.
The maximum cold work allowable for
this copper zinc alloy is 85%.
You will have to draw the copper, then
anneal it several times.
32. One solution
Draw the 1” rod to 0.5”
Anneal
Draw the 0.5” rod to 0.25”
12 − 0.52
CW =
= 0.75
2
1
The maximum cold
work is not
0.52 − 0.252
exceeded
CW =
0.5
2
= 0.75
Anneal
Draw the 0.25” rod to
0.125”
Anneal
0.252 − 0.1252
CW =
= 0.75
2
0.25
33. Final Step
Draw the 0.125” rod
to 0.1”
The final cold work is 36%
0.1252 − 0.12
CW =
= 0.36
2
0.125
34. Problem
What if you want a certain tensile
strength in your final product?
Look at one of the graphs of properties
vs. cold work from the book.
Make sure that your final cold work step
is the right size to give you the
properties you want.
36. Cold Work is Anisotropic
When you deform a piece of metal you
elongate the grain.
Slip occurs only in the favored
directions
You strengthen the material in the
direction it is deformed, but properties in
the other directions do not change as
much.
40. Cold Working Wire
When you draw wire, you strengthen in
the longitudinal direction
It is not strengthened axially
This makes it easy to cut, but hard to
break by pulling on it!!
41. Annealing
You can’t just haphazardly heat up a
piece of metal to “undo” the strain
hardening
It’s a temperature dependent process
43. Recovery or Stress Relief
If you only add a small amount of
thermal energy (heat it up a little) the
dislocations rearrange themselves
into networks to relieve residual
stresses
Polyganized subgrain structure
Ductility is improved
Strength does not change
45. Three EBSD maps
of the stored energy
in an Al-Mg-Mn alloy
after exposure to
increasing
recrystallization
temperature. The
volume fraction of
recrystallized grains
(light) increases with
temperature for a
given time.
46. Sometimes Residual Stresses
are good
Shot Peening
Tempered Glass
Side and Rear Windows in Cars
http://abrasivefinishingcom
pany.com/images/shot_pe
ening_1.jpg
47. Recrystallization
Add more heat, and new grains start to
grow at the grain boundaries.
The new grains have not been strain
hardened
The recrystallized metal is ductile and
has low strength
51. Check out the CD animations
Try the quiz on the CD!!
On the next page explore how
properties change during the annealing
process
The whole process depends not only on
the temperature, but on how long you
keep the metal hot.
53. Sometimes annealing
happens by itself!!
Is cold working a good way to
strengthen a metal used at high
temperatures?
What about a tungsten filament in a
light bulb?
54.
55. SEM of a tungsten filament
http://ion.asu.edu/descript_depth.htm
56.
57. How hot is hot?
Most metals have a recrystallization
temperature equal to about 40% of the
melting point measured in Kelvin
Tr = 0.4Tm
58. For Example
If a metal melts at 1000K, it’s
recrystallization temperature is
approximately 400K
If the metal is exposed to temperatures
above the recrystallization temperature
while in service, the strengthening
achieved with cold work will be
eliminated
59. Factors Contributing to
Recrystallization Temperature
Melting Point
Original Grain Size
Amount of Cold Work
Pure metals recrystallize at lower
temperatures than alloys
Time at temperature
61. What should you do if cold
working isn’t applicable?
Try solid solution strengthening
Try hot working
http://www.bbc.co.uk/shropshire/content/image_galleries/friendshi
p_through_iron_gallery.shtml
62. Hot Working
Shape the metal while it is hot.
Above the recrystallization temperature
Blacksmiths use a combination of hot work
and cold work.
Can not fine tune the final properties this way
Dimensional control is hard
Surface finishes may be hard to produce
66. Welding affects the surrounding
material
An incomplete weld
of a bike frame
which failed.
Apparent in the image
is the bright weld
material in the center,
the surrounding lighter
heat affected zone
(HAZ), and dark outer
unaffected base metal.
Field of view is
approximately 15 mm.
Used by Permission of Ruth Kramer
http://www.mse.mtu.edu/slides/slide_2.html
