Transcript Chapter 7

CHAPTER 7:
DISLOCATIONS AND STRENGTHENING
• In Ch.6
– Plastic (permanent) & Elastic (reversible)
• Yield Strength and hardness are a measure of materials
resistance to deformation
– In microscopic scale what is going on ?
• Net movement of large number of atoms in response to
an applied stress
• Involves movement of dislocations, in Ch. 4
– How does strengthening happen ?
– Why do some materials are stronger than others ?
– How can one manipulate dislocations and their
motion ?
Chapter 7-
Dislocations & Materials Classes
• Metals: Disl. motion easier.
-non-directional bonding
-close-packed directions
for slip.
electron cloud
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• Covalent Ceramics
(Si, diamond): Motion hard.
-directional (angular) bonding
• Ionic Ceramics (NaCl):
Motion hard.
-need to avoid ++ and - neighbors.
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Chapter 7 -
Dislocation Motion
Dislocations & plastic deformation
• Cubic & hexagonal metals - plastic deformation by
plastic shear or slip where one plane of atoms slides
over adjacent plane by defect motion (dislocations).
• If dislocations don't move,
deformation doesn't occur!
Adapted from Fig. 7.1,
Callister 7e.
Chapter 7 -
DISLOCATION MOTION
• Produces plastic deformation !
• Bonds are incrementally broken and
re-formed. Much less force is needed ,
why ?
Plastically
stretched
zinc
single
crystal.
Adapted from Fig.
7.9, Callister 6e.
(Fig. 7.9 is from
C.F. Elam, The
Distortion of
Metal Crystals,
SLIP PLANE
Adapted from Fig. 7.1, Callister 6e. (Fig. 7.1 is adapted from A.G. Guy, Essentials of Materials
Science, McGraw-Hill Book Company,
New York, 1976. p. 153.)
SEE THE MOVIE AGAIN !
• If dislocations don't move,
plastic deformation doesn't happen!
How do we generate the dislocation motion ?
Oxford University
Press, London,
1935.)
Adapted from Fig.
7.8, Callister 6e.
Chapter 7- 3
DISLOCATION MOTION
deformed
Initial state
apply force
O1
O0
t3
t2
t1
t0
The motion of a single dislocation across the plane causes
the top half of the crystal to move (to slip) with respect to
the bottom half but we can not have to break all the bonds
across the middle plane simultaneously (which would
require a very large force).
The slip plane (O0-O1)– the crystallographic plane of dislocation
motion.
Chapter 7-
STRESS and Dislocation Motion
Dislocation line
NOT be parallel to the dislocation line !
Chapter 7-
Dislocation Motion
• Dislocation moves along slip plane in slip direction
perpendicular to dislocation line
• Slip direction same direction as Burgers vector
Edge dislocation
Adapted from Fig. 7.2,
Callister 7e.
Screw dislocation
Chapter 7 -
STRESS AND DISLOCATION MOTION
Chapter 7-
STRESS field around dislocations
Bonds are Compressed
Bonds are Stretched
WHY is there a stress field ?
Atoms try to relax by trying to achieve
their positions for the case if there was
not a dislocation in the vicinity !
Chapter 7-
Stress fields of dislocations interacting !
The strain fields around dislocations
interact with each other. Hence, they
exert force on each other.
Edge dislocations, when they are in
the same plane, they repel each
other if they have the same
sign (direction of the Burgers
vector). WHY ?
They can attract and annihilate if
they have opposite signs. PROVE !
Chapter 7-
Dislocation Density
• The number of dislocations in a material is expressed using the
term dislocation density - the total dislocation length per unit
volume or the # of dislocations intersecting a unit area. Units are
mm / mm3, or just / mm
• Dislocation densities can vary from 103 mm-2 in carefully
solidified metal crystals to 1010 mm-2 in heavily deformed
metals.
Where do Dislocations come from, what are their sources ?
• Most crystalline materials, especially metals, have dislocations
in their as-formed state, mainly as a result of stresses
(mechanical, thermal...) associated with the manufacturing
processes used.
• The number of dislocations increases dramatically during plastic
deformation.
• Dislocations spawn from existing dislocations, grain boundaries
and surfaces and other “defects” .
Chapter 7-
NICE SIMULATIONS => http://www.ims.uconn.edu/centers/simul/movie/
Deformation Mechanisms
Slip System
– Slip plane - plane allowing easiest slippage
• Wide interplanar spacings - highest planar densities
– Slip direction - direction of movement - Highest linear
densities
Adapted from Fig.
7.6, Callister 7e.
– FCC Slip occurs on {111} planes (close-packed) in <110>
directions (close-packed)
=> total of 12 slip systems in FCC
– in BCC & HCP other slip systems occur
Chapter 7 -
SLIP SYSTEMS !!!
• Dislocations move with ease on certain
crystallographic planes and along certain directions
on these planes !
– The plane is called a slip plane
– The direction is called a slip direction
– Combination of the plane of slip and direction is a
slip system
• The slip planes and directions are those of highest
packing density.
– The distance between atoms is shorter than the
average…High number of coordination along the
planes also important !
PLS SEE TABLE 7.1 in ur books , page 180 !!! Chapter 7-
Concept Check
Page 181, in 7th edition !
Chapter 7-
Stress and Dislocation Motion
• Crystals slip due to a resolved shear stress, tR.
• Applied tension can produce such a stress.
Applied tensile
stress: s = F/A
A
F
Resolved shear
stress: tR =Fs /A s
slip plane
tR = FS /AS
tR
normal, ns
AS
FS
F
Relation between
s and tR
Fcos l
F
tR
tR  s cos l cos f
l
FS
A/cos f
nS f
A
AS
l+f  90
Chapter 7 -
Critical Resolved Shear Stress
• Condition for dislocation motion:
tR  tCRSS
• Crystal orientation can make
it easy or hard to move dislocation
tR  s cos l cos f
s
tR = 0
l =90°
t
typically
10-4 GPa to 10-2 GPa
s
s
tR = s/2
l =45°
f =45°
maximum at l = f = 45º
tR = 0
f =90°
Chapter 7 -
Resolving the Applied Stress on a SLIP PLANE !
• Shear Stress has to be resolved on to the slip planes as;
– Shear Stress is needed for dislocations to move / slip
– Dislocations can only move on slip planes, and these
planes are rarely on axis with the applied force !
We should resolve the force applied in a tensile test,
F, on to the cross-sectional area A where the slip is
going to take place;
Chapter 7-
Critical Resolved Shear Stress => Slip in Single
X’tals
Macroscopically;
Q: when do materials plastically deform / yield ?
Stress > YS
Q: How does plastic deformation take place ?
Dislocation Motion ( Elastic deformation ??)
Q: On what planes does dislocations move ?
Slip Planes
Q: At stress = YS, what would be the minimum resolved shear
stress needed to act on dislocations to initiate dislocation
motion, onset of yield/plastic deformation ?
HIMMM ! Lets think !!!
Chapter 7-
The Critical Resolved Shear Stress
The minimum shear stress required to
initiate slip is termed:
the critical resolved shear stress
Chapter 7-
Single Crystal Slip
Plastically
stretched
zinc
single
crystal.
Adapted from Fig.
7.9, Callister 7e.
A large number of
dislocations are generated
on slip planes, as they leave
the system they form these
“shear bands”
Adapted from Fig. 7.8, Callister 7e.
Chapter 7 -
Ex: Deformation of single crystal
a) Will the single crystal yield?
b) If not, what stress is needed?
f=60°
l=35°
tcrss = 3000 psi
t  s cosl cosf
s  6500 psi
Adapted from
Fig. 7.7,
Callister 7e.
t  (6500 psi) (cos35 )(cos60 )

 (6500 psi) (0.41)
t  2662 psi  tcrss  3000 psi
s = 6500 psi
So the applied stress of 6500 psi will not cause the
crystal to yield.

Chapter 7 -
Ex: Deformation of single crystal
What stress is necessary (i.e., what is the
yield stress, sy)?
tcrss  3000 psi  sy cosl cosf  sy (0.41)
tcrss
3000 psi
 sy 

 7325 psi
cosl cosf
0.41
So for deformation to occur the applied stress must
be greater than or equal to the yield stress
s  sy  7325 psi
Chapter 7 -
Slip Motion in Polycrystals
• Stronger - grain boundaries
pin deformations
s
• Slip planes & directions
(l, f) change from one
crystal to another.
Adapted from Fig.
7.10, Callister 7e.
(Fig. 7.10 is
courtesy of C.
Brady, National
Bureau of
Standards [now the
National Institute of
Standards and
Technology,
Gaithersburg, MD].)
• tR will vary from one
crystal to another.
• The crystal with the
largest tR yields first.
• Other (less favorably
oriented) crystals
yield later.
300 mm
Chapter 7 -
Plastic Deformation in Polycrystalline Materials
Chapter 7-
Anisotropy in sy
• Can be induced by rolling a polycrystalline metal
- before rolling
- after rolling
Adapted from Fig. 7.11,
Callister 7e. (Fig. 7.11 is from
W.G. Moffatt, G.W. Pearsall,
and J. Wulff, The Structure
and Properties of Materials,
Vol. I, Structure, p. 140, John
Wiley and Sons, New York,
1964.)
rolling direction
235 mm
- isotropic
since grains are
approx. spherical
& randomly
oriented.
- anisotropic
since rolling affects grain
orientation and shape.
Chapter 7 -
Anisotropy in Deformation
2. Fire cylinder
at a target.
3. Deformed
cylinder
side view
rolling direction
1. Cylinder of
Tantalum
machined
from a
rolled plate:
end
view
• The noncircular end view shows
Photos courtesy
of G.T. Gray III,
Los Alamos
National Labs.
Used with
permission.
plate
thickness
direction
anisotropic deformation of rolled material.
Chapter 7 -
ANISOTROPY IN DEFORMATION
1. Cylinder of
Tantalum
machined
from a
rolled plate:
2. Fire cylinder
at a target.
3. Deformed
cylinder
side view
end
view
• The noncircular end view shows:
Photos courtesy
of G.T. Gray III,
Los Alamos
National Labs.
Used with
permission.
plate
thickness
direction
anisotropic deformation of rolled material.
Chapter 7- 10
STRENGTHENING MECHANISMS
The ability of a metal to deform depends on the ability of
dislocations to move with relative ease under loading
conditions
Restricting dislocation motion will inevitably make the material
stronger, need more force to induce same amount of
deformation !
• Mechanisms of strengthening in single-phase metals:
– grain-size reduction
– solid-solution alloying
– strain hardening
• Ordinarily, strengthening mechanisms reduces ductility,
why ???
Chapter 7-
4 STRATEGIES FOR STRENGTHENING:
1: REDUCE GRAIN SIZE
• Grain boundaries are
barriers to slip.
• Barrier "strength"
increases with
slip plane
mis-orientation.
Average grain size
1/ 2
Chapter 7- 7
y
s yield  so  k y d
ar
nd
Grain size can be changed
by processing !
Hall-Petch Equation :
u
bo
more barriers to slip.
Adapted from Fig. 7.12, Callister 6e.
(Fig. 7.12 is from A Textbook of Materials
Technology, by Van Vlack, Pearson
Education, Inc., Upper Saddle River, NJ.)
in
• Smaller grain size:
grain A
B
a
gr
High-angle boundaries
are better in blocking slip
!
a
r
g
in
GRAIN SIZE STRENGTHENING:
AN EXAMPLE
• 70wt%Cu-30wt%Zn brass alloy
s yield  so  k y d 1/ 2
• Data:
Adapted from Fig. 7.13,
Callister 6e.
(Fig. 7.13 is adapted
from H. Suzuki, "The
Relation Between the
Structure and
Mechanical Properties
of Metals", Vol. II,
National Physical
Laboratory Symposium
No. 15, 1963, p. 524.)
Chapter 7- 8
4 Strategies for Strengthening:
2: Solid Solutions
• Impurity atoms distort the lattice & generate stress.
• Stress can produce a barrier to dislocation motion.
• Smaller substitutional
impurity
• Larger substitutional
impurity
A
C
B
Impurity generates local stress at A
and B that opposes dislocation
motion to the right.
D
Impurity generates local stress at C
and D that opposes dislocation
motion to the right.
Chapter 7 -
Strategy #2: Solid Solutions
• Alloyed metals are usually stronger than
their pure base metals counter parts.
Why ? Interstitial or substitutional impurities in a
solution cause lattice strain, aka distortions in the
lattice
Then ?
•
•
Strain field around the impurities interact with dislocation strain
fields and impede dislocation motion.
Impurities tend to diffuse and segregate around the dislocation
core to find atomic sites more suited to their radii. This reduces
the overall strain energy and “anchor” the dislocation.
Motion of the dislocation core away from the impurities moves
it to a region of lattice where the atomic strains are greater,
where lattice strains due to dislocation is no longer
compensated by the impurity atoms.
Chapter 7-
Interactions of the Stress Fields
COMPRESSIVE
TENSILE
TENSILE
Chapter 7COMPRESSIVE
Interactions of the Stress Fields
Chapter 7-
Stress Concentration at Dislocations
Adapted from Fig. 7.4,
Callister 7e.
Chapter 7 -
Impurity Segregation
Impurities tend to segregate at
energetically favorable areas
around the dislocation core and
partially decrease the overall stress
field generated around the
dislocation core.
However, when stress is applied
more load is needed to move
dislocations with impurity atoms
segregated to its core !
Chapter 7-
Strengthening by Alloying
• small impurities tend to concentrate at dislocations
• reduce mobility of dislocation  increase strength
Adapted from Fig.
7.17, Callister 7e.
Chapter 7 -
Strengthening by alloying
• large impurities concentrate at dislocations on low
density side
Adapted from Fig.
7.18, Callister 7e.
Chapter 7 -
Ex: Solid Solution
Strengthening in Copper
Yield strength (MPa)
Tensile strength (MPa)
• Tensile strength & yield strength increase with wt% Ni.
400
300
200
0 10 20 30 40 50
180
120
60
wt.% Ni, (Concentration C)
• Empirical relation:
Adapted from Fig.
7.16 (a) and (b),
Callister 7e.
0 10 20 30 40 50
wt.%Ni, (Concentration C)
sy ~ C1/ 2
• Alloying increases sy and TS.
Chapter 7 -
4 Strategies for Strengthening:
3: Precipitation Strengthening
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
precipitate
Large shear stress needed
to move dislocation toward
precipitate and shear it.
Side View
Top View
Unslipped part of slip plane
S
Dislocation
“advances” but
precipitates act as
“pinning” sites with
spacing S.
Slipped part of slip plane
• Result:
1
sy~
S
Chapter 7 -
Application:
Precipitation Strengthening
• Internal wing structure on Boeing 767
Adapted from chapteropening photograph,
Chapter 11, Callister 5e.
(courtesy of G.H.
Narayanan and A.G.
Miller, Boeing Commercial
Airplane Company.)
• Aluminum is strengthened with precipitates formed
by alloying.
Adapted from Fig.
11.26, Callister 7e.
(Fig. 11.26 is courtesy
of G.H. Narayanan
and A.G. Miller,
Boeing Commercial
Airplane Company.)
1.5mm
Chapter 7 -
4 Strategies for Strengthening:
4: Cold Work (%CW)
• Room temperature deformation.
• Common forming operations change the cross
sectional area:
-Forging
force
die
A o blank
-Drawing
die
Ao
-Rolling
Ad
Ao
Adapted from Fig.
11.8, Callister 7e.
Ad
roll
force
Ad
roll
-Extrusion
Ao
tensile
force
force
die
container
ram
billet
container
Ao  Ad
%CW 
x 100
Ao
die holder
extrusion
die
Chapter 7 -
Ad
Dislocations During Cold Work
• Ti alloy after cold working:
• Dislocations entangle
with one another
during cold work.
• Dislocation motion
becomes more difficult.
0.9 mm
Adapted from Fig.
4.6, Callister 7e.
(Fig. 4.6 is courtesy
of M.R. Plichta,
Michigan
Technological
University.)
Chapter 7 -
Result of Cold Work
Dislocation density =
total dislocation length
unit volume
– Carefully grown single crystal
 ca. 103 mm-2
– Deforming sample increases density
 109-1010 mm-2
– Heat treatment reduces density
 105-106 mm-2
s
• Yield stress increases
sy1
as rd increases:
s
y0
large hardening
small hardening
e
Chapter 7 -
RESULT OF COLD WORK
• Dislocation density (rd) goes up:
Carefully prepared sample: rd ~ 103 mm/mm3
Heavily deformed sample: rd ~ 1010 mm/mm3
• Ways of measuring dislocation density:
40mm
OR
• Yield stress increases
as rd increases:
r N
d
A
Area, A dislocation
pit
N dislocation
pits (revealed
by etching)
Micrograph
adapted from
Fig. 7.0, Callister
6e. (Fig. 7.0 is
courtesy of W.G.
Johnson,
General Electric
Co.)
Chapter 7- 18
STRENGTHENING STRATEGY 4: COLD
WORK (%CW)
• An increase in sy due to plastic deformation.
BUT actually # of dislocations are increasing !!!
• Curve fit to the stress-strain response:
Chapter 7- 22
Stress fields of dislocations interacting !
The strain fields around dislocations
interact with each other. Hence, they
exert force on each other.
Edge dislocations, when they are in
the same plane, they repel each
other if they have the same
sign (direction of the Burgers
vector). WHY ?
They can attract and annihilate if
they have opposite signs. PROVE !
Chapter 7-
Effects of Stress at Dislocations
Adapted from Fig.
7.5, Callister 7e.
Chapter 7 -
IMPACT OF COLD WORK
Stress
• Yield strength (sy ) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
Adapted from Fig. 7.18,
Callister 6e. (Fig. 7.18 is
from Metals Handbook:
Properties and Selection:
Iron and Steels, Vol. 1, 9th
ed., B. Bardes (Ed.),
American Society for
Metals, 1978, p. 221.)
Chapter 7- 21
Impact of Cold Work
As cold work is increased
• Yield strength (sy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
Adapted from Fig. 7.20,
Callister 7e.
Chapter 7 -
Cold Work Analysis
• What is the tensile strength &
ductility after cold working?
ro2  rd2
%CW 
x 100  35.6%
2
ro
yield strength (MPa)
700
500
600
300MPa
100
0
Cu
Do =15.2mm
tensile strength (MPa)
800
300
Copper
Cold
Work
40
% Cold Work
sy = 300MPa
60
40
20
400 340MPa
200
0
ductility (%EL)
60
Cu
20
Dd =12.2mm
20
40
60
% Cold Work
TS = 340MPa
Cu
7%
00
20
40
60
% Cold Work
%EL = 7%
Adapted from Fig. 7.19, Callister 7e. (Fig. 7.19 is adapted from Metals Handbook: Properties and Selection:
Iron and Steels, Vol. 1, 9th ed., B. Bardes (Ed.), American Society for Metals, 1978, p. 226; and Metals
Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker
(Managing Ed.), American Society for Metals, 1979, p. 276 and 327.)
Chapter 7 -
s-e Behavior vs. Temperature
800
Stress (MPa)
• Results for
polycrystalline iron:
Adapted from Fig. 6.14,
Callister 7e.
-200C
600
-100C
400
25C
200
0
0
0.1
0.2
0.3
0.4
Strain
• sy and TS decrease with increasing test temperature.
• %EL increases with increasing test temperature.
3. disl. glides past obstacle
• Why? Vacancies
2. vacancies
help dislocations
replace
move past obstacles. atoms on the
obstacle
disl. half
plane
1. disl. trapped
by obstacle
Chapter 7 -
0.5
Effect of Heating After %CW
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are reversed!
100 200 300 400 500 600 700
600
60
tensile strength
50
500
40
400
30
ductility
300
20
ductility (%EL)
tensile strength (MPa)
annealing temperature (ºC)
• 3 Annealing
stages to
discuss...
Adapted from Fig. 7.22, Callister 7e. (Fig.
7.22 is adapted from G. Sachs and K.R. van
Horn, Practical Metallurgy, Applied Metallurgy,
and the Industrial Processing of Ferrous and
Nonferrous Metals and Alloys, American
Society for Metals, 1940, p. 139.)
Chapter 7 -
Recovery
Annihilation reduces dislocation density.
• Scenario 1
Results from
diffusion
extra half-plane
of atoms
atoms
diffuse
to regions
of tension
extra half-plane
of atoms
Dislocations
annihilate
and form
a perfect
atomic
plane.
• Scenario 2
3. “Climbed” disl. can now
move on new slip plane
2. grey atoms leave by
vacancy diffusion
allowing disl. to “climb”
1. dislocation blocked;
can’t move to the right
tR
4. opposite dislocations
meet and annihilate
Obstacle dislocation
Chapter 7 -
Recrystallization
• New grains are formed that:
-- have a small dislocation density
-- are smalller then initial cold worked ones
-- consume cold-worked grains.
0.6 mm
0.6 mm
Adapted from
Fig. 7.21 (a),(b),
Callister 7e.
(Fig. 7.21 (a),(b)
are courtesy of
J.E. Burke,
General Electric
Company.)
33% cold
worked
brass
New crystals
nucleate after
3 sec. at 580C.
Chapter 7 -
Further Recrystallization
• All cold-worked grains are consumed.
0.6 mm
0.6 mm
Adapted from
Fig. 7.21 (c),(d),
Callister 7e.
(Fig. 7.21 (c),(d)
are courtesy of
J.E. Burke,
General Electric
Company.)
After 4
seconds
After 8
seconds
Chapter 7 -
Grain Growth
• At longer times, larger grains consume smaller ones.
• Why? Grain boundary area (and therefore energy)
is reduced.
0.6 mm
0.6 mm
Adapted from
Fig. 7.21 (d),(e),
Callister 7e.
(Fig. 7.21 (d),(e)
are courtesy of
J.E. Burke,
General Electric
Company.)
After 8 s,
580ºC
After 15 min,
580ºC
• Empirical Relation:
exponent typ. ~ 2
grain diam.
n
d
at time t.
 don  Kt
coefficient dependent
on material and T.
elapsed time
Ostwald Ripening
Chapter 7 -
º
TR = recrystallization
temperature
TR
Adapted from Fig.
7.22, Callister 7e.
º
Chapter 7 -
Coldwork Calculations
A cylindrical rod of brass originally 0.40 in (10.2 mm)
in diameter is to be cold worked by drawing. The
circular cross section will be maintained during
deformation. A cold-worked tensile strength in excess
of 55,000 psi (380 MPa) and a ductility of at least 15
%EL are desired. Further more, the final diameter
must be 0.30 in (7.6 mm). Explain how this may be
accomplished.
Chapter 7 -
Coldwork Calculations Solution
If we directly draw to the final diameter what
happens?
Brass
Cold
Work
Do = 0.40 in
Df = 0.30 in
 Ao  Af
%CW  
 Ao


Af 
 x 100  1 
 x 100

 Ao 
  0.30 2 
 Df2 4 
 x 100  43.8%
 x 100  1  
 1 

2
  0.40  
 Do 4 


Chapter 7 -
Coldwork Calc Solution: Cont.
420
540
6
Adapted from Fig.
7.19, Callister 7e.
• For %CW = 43.8%
– sy = 420 MPa
– TS = 540 MPa > 380 MPa
– %EL = 6
< 15
• This doesn’t satisfy criteria…… what can we do?
Chapter 7 -
Coldwork Calc Solution: Cont.
15
380
27
12
For TS > 380 MPa
> 12 %CW
For %EL < 15
< 27 %CW
Adapted from Fig.
7.19, Callister 7e.
 our working range is limited to %CW = 12-27
Chapter 7 -
Coldwork Calc Soln: Recrystallization
Cold draw-anneal-cold draw again
• For objective we need a cold work of %CW  12-27
– We’ll use %CW = 20
• Diameter after first cold draw (before 2nd cold draw)?
– must be calculated as follows:
2
 Df 2 2 
Df 2
%CW

%CW  1 
x
100

1


2 
2
100
D
D
02 
02

Df 2  %CW 
 1 

D02 
100 
0.5

D02 
Df 2
 %CW 
1 

100 

20 


Intermediate diameter = Df 1  D02  0.30 1 
 100 
0.5
0.5
 0.335 m
Chapter 7 -
Coldwork Calculations Solution
Summary:
1. Cold work
D01= 0.40 in  Df1 = 0.335 m

2


0.335 
%CW1  1 
x 100  30
  0.4  


2. Anneal above D02 = Df1
3. Cold work D02= 0.335 in  Df 2 =0.30 m
Fig 7.19
  0.3 2 
 x 100  20

%CW2  1  

  0.335  


Therefore, meets all requirements
s y  340 MPa
TS  400 MPa
%EL  24
Chapter 7 -
Rate of Recrystallization
E
log R  log t  log R0 
kT
B
log t  C 
T
note : R  1 / t
50%
start
1
TR
• Hot work  above TR
• Cold work  below TR
• Smaller grains
– stronger at low temperature
– weaker at high temperature
finish
log t
No Strain hardening occurs
TR ~ 0.3 Tm-0.7 Tm
TR depends on %CW
Chapter 7 •Decrease with increasing % CW
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.
Chapter 7 -
ANNOUNCEMENTS
Reading: Chapter 7
Core Problems: 7.6, 7.7, 7.13 (see page 1834), 7.14, 7.23, 7.31, 7.41 (figure 7.25)
Bonus Problems: 7.40, 7.36, 7.39
Due date is April 3th
Chapter 7- 0