Dislocations and Strengthening Mechanisms
Dislocations and Strengthening Mechanisms
Dislocations and Strengthening Mechanisms
2
Dislocations & Materials Classes
• Metals: Disl. motion easier.
+ + + + + + + +
-non-directional bonding + + + + + + + +
-close-packed directions + + + + + + + +
for slip. electron cloud ion cores
• Covalent Ceramics
(Si, diamond): Motion hard.
-directional (angular) bonding
A C
B D
Lo-Carbon Steel!
Adapted from Fig. 7.20,
Callister 7e.
Cold Work Analysis
• What is the tensile strength &
ductility after cold working?
Copper
Cold
Work
D o =15.2mm D d =12.2mm
2 2
ro rd
%CW x 100 35.6%
2
ro
Cold Work Analysis
• What is the tensile strength &
ductility after cold working to 35.6%?
40
500 600
Cu
300 Cu 400 340MPa 20
Cu 7%
100 200
0 20 40 60 00
0 20 40 60 20 40 60
% Cold Work % Cold Work % Cold Work
YS = 300 MPa TS = 340MPa %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.)
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Effect of Heating After %CW
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are reversed!
annealing temperature (ºC)
100 200 300 400 500 600 700
tensile strength (MPa)
600 60
tensile strength
ductility (%EL)
50
500
• 3 Annealing
40
stages to
400 30 discuss...
ductility 20
Adapted from Fig. 7.22, Callister 7e. (Fig.
7.22 is adapted from G. Sachs and K.R. van
300 Horn, Practical Metallurgy, Applied
Metallurgy, and the Industrial Processing of
Re Re Gr Ferrous and Nonferrous Metals and Alloys,
co c ry a in
ve sta Gr American Society for Metals, 1940, p. 139.)
ry lliz ow
ati th
on
Recovery
Annihilation reduces dislocation density.
• Scenario 1 extra half-plane
of atoms Dislocations
Results from annihilate
diffusion atoms
and form
diffuse
a perfect
to regions
atomic
of tension
extra half-plane plane.
of atoms
• Scenario 2
3. “Climbed” disl. can now R
move on new slip plane
2. grey atoms leave by
4. opposite dislocations
vacancy diffusion
meet and annihilate
allowing disl. to “climb”
1. dislocation blocked; Obstacle dislocation
can’t move to the right
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Recrystallization
• New grains are formed that:
-- have a low dislocation density
-- are small
-- 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.)
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 After 8
seconds seconds
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.)
TR = recrystallization
temperature
TR
º
Recrystallization Temperature, TR
27
Example 7.3 Design of a
Stamping Process
One method for producing fans for cooling automotive and truck
engines is to stamp the blades from cold-rolled steel sheet, then
attach the blades to a “spider’’ that holds the blades in the proper
position. A number of fan blades, all produced at the same time,
have failed by the initiation and propagation of a fatigue crack
transverse to the axis of the blade (Figure 7.11). All other fan
blades perform satisfactorily. Provide an explanation for the failure
of the blades and redesign the manufacturing process to prevent
these failures.
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Example 7.3 (continued)
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