ISSUES TO ADDRESS... Why are dislocations observed primarily in metals and alloys? • How are strength and dislocation motion related? • How do we increase strength? 1 • How can heating change strength and other properties? CHAPTER 8: DEFORMATION AND STRENGTHENING MECHANISMS
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CHAPTER 8: DEFORMATION AND STRENGTHENING MECHANISMS
CHAPTER 8: DEFORMATION AND STRENGTHENING MECHANISMS. ISSUES TO ADDRESS. • Why are dislocations observed primarily in metals and alloys?. • How are strength and dislocation motion related?. • How do we increase strength?. • How can heating change strength and other properties?. 1. - PowerPoint PPT Presentation
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ISSUES TO ADDRESS...• Why are dislocations observed primarily in metals and alloys?
• How are strength and dislocation motion related?
• How do we increase strength?
1
• How can heating change strength and other properties?
CHAPTER 8: DEFORMATION AND
STRENGTHENING MECHANISMS
3
• Produces plastic deformation,• Depends on incrementally breaking bonds.
Plasticallystretchedzincsinglecrystal.
• If dislocations don't move, deformation doesn't happen!
• What is the tensile strength & ductility after cold working?
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Cold work ----->
Do=15.2mm Dd=12.2mm
Copper
%CW ro
2 rd2
ro2
x10035.6%
COLD WORK ANALYSIS
ductility (%EL)
7%
%EL=7%% Cold Work
20
40
60
20 40 6000
Cu
% Cold Worky=300MPa
100
300
500
700
Cu
200 40 60
yield strength (MPa)
300MPa
% Cold Work
tensile strength (MPa)
340MPa
TS=340MPa
200Cu
0
400
600
800
20 40 60
• Results for polycrystalline iron:
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• y and TS decrease with increasing test temperature.• %EL increases with increasing test temperature.• Why? Vacancies help dislocations past obstacles.
1. disl. trapped by obstacle
2. vacancies replace atoms on the disl. half plane
3. disl. glides past obstacle
obstacle
- BEHAVIOR VS TEMPERTURE
00 0.1 0.2 0.3 0.4 0.5
200
400
600
800
Str
ess
(M
Pa)
Strain
-200°C
-100°C
25°C
• 1 hour treatment at Tanneal... decreases TS and increases %EL.• Effects of cold work are reversed!
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• 3 Annealing stages to discuss...
EFFECT OF HEATING AFTER %CW
ten
sile
str
en
gth
(M
Pa)
du
ctilit
y (
%E
L)
Annealing Temperature (°C)
300
400
500
600 60
50
40
30
20Recovery
Recrystallization
Grain Growth
ductility
tensile strength300 700500100
Annihilation reduces dislocation density.
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• Scenario 1
• Scenario 2
atoms diffuse to regions of tension
extra half-plane of atoms
extra half-plane of atoms
Disl. annhilate and form a perfect atomic plane.
1. dislocation blocked; can’t move to the right
obstacle dislocation
2. grey atoms leave by vacancy diffusion allowing disl. to “climb”
4. opposite dislocations meet and annihilate
3. “Climbed” disl. can now move on new slip plane
R
RECOVERY
• New crystals are formed that: --have a small disl. density --are small --consume cold-worked crystals.
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33% coldworkedbrass
New crystalsnucleate after3 sec. at 580C.
0.6 mm 0.6 mm
RECRYSTALLIZATION
• All cold-worked crystals are consumed.
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After 4seconds
After 8seconds
0.6 mm0.6 mm
FURTHER RECRYSTALLIZATION
• At longer times, larger grains consume smaller ones. • Why? Grain boundary area (and therefore energy) is reduced.
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• Empirical Relation:
After 8 s,580C
After 15 min,580C
dn do
n Ktelapsed time
coefficient dependenton material and T.
grain diam.at time t.
exponent typ. ~ 2
0.6 mm 0.6 mm
GRAIN GROWTH
Example
Using the diagram, compute the time for the average grain size to increase form 0.01-mm to 0.1-mm at (a) 500 oC and (b) 600 oC
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0
unload/reload
0
brittle failure
plastic failure
20
40
60
2 4 6
(MPa)
x
x
semi- crystalline
case
amorphous regions elongate
crystalline regions align
crystalline regions
slide
8
onset of necking
aligned, cross- linked case
networked case
Initial
Near Failure
near failure
TENSILE RESPONSE: BRITTLE & PLASTIC
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• Drawing... --stretches the polymer prior to use --aligns chains to the stretching direction• Results of drawing: --increases the elastic modulus (E) in the stretching dir. --increases the tensile strength (TS) in the stretching dir. --decreases ductility (%EL)• Annealing after drawing... --decreases alignment --reverses effects of drawing.• Compare to cold working in metals!
Adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.)