JIC MME 323 Materials Science Teaching Slide Week 7-8

1. MME 323: MATERIALS SCIENCE
WEEK 7-8 :
Dislocations and Strengthening Mechanisms*
Adhi Primartomo, PhD
Email: primartomo_a@jic.edu.sa
Office: Room 191 – JIC Academic Building
* Source: Materials Science and Engineering; 9th Edition; W.D.Callister;
Wiley; 2011
https://sites.google.com/site/primartomo/file-cabinet

2. ~ LECTURE OUTLINE ~
Chapter 9: Dislocations and Strengthening
Mechanisms (p. 253 - 279)*
• Why Study Dislocations and Strengthening
Mechanisms?
• Introduction,
• Dislocations and Plastic Deformation,
• Characteristics of Dislocations,
• Slip Systems,
• Mechanism of Strengthening in Metals,
• Strengthening by Grain Size Reduction,
• Solid Solution Strengthening,
• Strain Hardening.
2

3. WHY STUDY DISLOCATIONS AND STRENGTHENING
MECHANISMS?
(page 254)
3
With knowledge of the nature of dislocations and the
role they play in the plastic deformation process, we
are able to understand the underlying mechanism of
the techniques that are used to strengthen and harden
metals and their alloys.

4. INTRODUCTION
(page 254)
4
• On a microscopic scale, plastic deformation
corresponds to the movement of a large number of
atoms in response to an applied stress.
• During this process, interatomic bond must be broken
and then re-formed.
• In crystalline solids, plastic deformation involves the
motion of dislocations (linear crystal defects – Ch.6)
• Characteristics of dislocations and their involvement
in plastic deformation will be discussed.

5. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
5
Basic Concepts:
Edge Dislocation
Screw Dislocation

6. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
6
Basic Concepts:
• Plastic deformation corresponds to the motion of large
numbers of dislocations.
• An edge dislocation moves in response to a shear stress
applied in a direction perpendicular to its line.

7. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
7
Basic Concepts:

8. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
8
Basic Concepts:
• The process by which
plastic deformation is
produced by dislocation
motion  Slip
• The crystallographic plane
along which the dislocation
line moves  Slip Plane
• Macroscopic plastic
deformation simply
corresponds to permanent
deformation that results
from the movement of
dislocations or slip in
respond to applied shear
stress.

9. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
9
Basic Concepts:
Edge Dislocation
Screw Dislocation

10. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
10
Basic Concepts:
• All metals and alloys contain some dislocations that were
introduced during: solidification, during plastic
deformation and as consequence of thermal stress from
rapid cooling.
• The number of dislocations  dislocation density: total
dislocation length per unit volume or per square
millimeter, i.e.: 109 – 1010 mm-2  heavily deformed
metals.

11. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
11
Characteristics of Dislocations:
Strain energy:
• When metals plastically deformed, some fraction of the
deformation energy (5%) retained internally.
Lattice strain:
• Some atomic lattice
distortion exist around
dislocation line because of
the presence of the extra
half plane of atoms.

12. DISLOCATIONS AND PLASTIC DEFORMATION
(page 254-259)
12
Slip Systems:
• Dislocations don’t move with the same degree of ease on all
planes.
• There is a preferred plane  Slip Plane
• In the plane, there is preferred direction  Slip Direction
• Slip plane usually has the densest atomic packing  greatest
planar density

13. MECHANISMS OF STRENGTHENING
(page 266-271)
13
Strengthening by Grain Size Reduction:
• The size of the grains influences mechanical properties.
• During plastic deformation, dislocation motion must take
place across this boundary.
• The grain boundary acts as a barrier to dislocation motion
for two reasons:
1. The two grains are of different orientations, a
dislocation passing into next grain must change its
direction of motion  becomes more difficult as the
misorientation increases.
2. The atomic disorder within a grain boundary region
results in a discontinuity of slip planes from one
grain into the other.

14. MECHANISMS OF STRENGTHENING
(page 266-271)
14
Strengthening by Grain Size Reduction:

15. MECHANISMS OF STRENGTHENING
(page 266-271)
15
Strengthening by Grain Size Reduction:
• Hall-Petch Equation – dependence of yield strength on
grain size.
“A fine-grained material
(small grain) is harder and
stronger than coarse-grained
(big grain) because fine-
grained has a greater total
grain boundary to restrict
dislocation motion”

16. MECHANISMS OF STRENGTHENING
(page 266-271)
16
Solid-Solution Strengthening:
• High-purity metals are almost softer and weaker than
alloys composed of the same base metal.
• Increasing concentration of the impurity results in
increase in yield strength.
• Alloys stronger than pure metals because impurity atoms
that go into solid solution impose lattice strains on the
surrounding host atoms.
• Lattice strain field interaction between dislocation and
these impurities atoms result: dislocation movement is
restricted.

17. MECHANISMS OF STRENGTHENING
(page 266-271)
17
Solid-Solution Strengthening:

18. MECHANISMS OF STRENGTHENING
(page 266-271)
18
Strain Hardening / Work Hardening:
• A ductile material becomes harder and stronger as it is
plastically deformed.
• The dislocation density increases with cold work because of
dislocation multiplication of formation of new dislocation.
• Consequently the average distance of separation between
dislocation decreases – dislocation positioned closer together.
• This result that the motion of a dislocation is restricted by the
presence of other dislocation.

19. MECHANISMS OF STRENGTHENING
(page 266-271)
19
Strain Hardening / Work Hardening:

20. MECHANISMS OF STRENGTHENING
(page 272)
20
Example Problem 9.2: