ecture dislocation slip
TRANSCRIPT
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Chapter 7-
Quiz is: (1) Closed book
(2) Covers material from beginning of lecture 1 to
the end of lecture 13 (previous lecture!)
(3) Worth 30% of total assessment
(4) A similar format to example quiz (will be available
after pacing break).
Remember to bring a calculator and ruler but not
cell phone. These will not be provided!
Midterm Exam Wednesday 23rd October
11.10
12.00Please be seated by 11.00am sharp!
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Chapter 7-
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?
Dislocation Slip
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Chapter 7-
Design uncertainties mean we do not push the limit.
Factor of safety, N
working y
N
Often N isbetween
1.2 and 4
Example: Using a safety factor of 5, calculate the diameter,
d, needed to ensure that yield does not occur in the 1045
carbon steel rod shown below.
working y
N
Design or Safety Factors
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Chapter 7- 3
1. Initial 2. Small load 3. Unload
Plastic means permanent!
F
linearelastic
linearelastic
plastic
Plastic Deformation (Metals)
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Chapter 7-
Analogy of Dislocation Motion
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Chapter 7-
Produces plastic deformation
-
Depends on
Simulation of Dislocation Motion
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Chapter 7-
Line Defects : Edge Dislocations
Edge
dislocation
line
Burgers vector
Dislocation: some of atoms are misaligned
Half plane
Squeezed
Pulled
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Chapter 7-Screw Dislocations Mixed Dislocations
Other Types of Dislocations
Edge
Mixed
Screw
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Chapter 7-9
Dislocation Motion
A dislocation moves
perpendicular to the dislocation line
The slip direction is the same as the
Edge dislocation
Screw dislocation
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Chapter 7- 2
Metals: Disl. motion
electron cloud ion cores
Covalent Ceramics(Si, diamond): Motion
Ionic Ceramics (e.g. NaCl):
Motion
Dislocations versus Materials Class
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Chapter 7- 3
Plastically
deformedzinc
single
crystal.
Plastic Shear in Metals
The slip plane has
The slip direction has
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Chapter 7-
Plastic Shear in Metals
Compression: pure Ni (fcc)
Uchic et al., Science 305 (2004), 986
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Chapter 7-
Slip Plane - {111} Slip Direction -
FCC Slip occurs on planes (close-packed) in
directions (close-packed)
=> total of
Slip SystemSlip plane -
Highest planar densities (and large interplanar spacings)
Slip directions-Highest linear densities
Slip System in FCC Metals
For BCC & HCP there are other slip systems.
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Chapter 7-
BCC
FCC
HCP
Common Slip Systems in Metals
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Chapter 7- 6
Structure: close-packed
planes & directionsare preferred.
Comparison among crystal structures:FCC/BCC: many close-packed planes/directions;
HCP: only one plane, 3 directions;
Mg (HCP)
Al (FCC)
tensile direction
Results of tensiletesting.
view onto two
close-packedplanes.
Dislocations & Crystal Structure
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Chapter 7- 4
Crystals slip due to
Applied tensile stress can
R cos cos
ns
AAs
Stress and Dislocation Motion
slip plane
normal, ns
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Chapter 7- 5
Condition for dislocation motion:
Slip plane orientation relative to tensile
axis affects ease of dislocation motion
R cos cos
Critical Resolved Shear Stress
10-4G to 10-2G
typically
R >CRSS
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Chapter 7-
Slip planes & directions
(, )
Rwill
The crystal with the
R yields first.
Other (less favorably
oriented) crystals
yield later.300 mm
Dislocation Motion in Polycrystals