Dislocations & Materials Classes , and strenthning mechanism

In brittle materials, failure in the film occurs when the stress exceeds a critical stress defined by the intrinsic atomic strength of the material and the nature of any critical defects. If the strength of the material can be increased or the size (or sharpness) of the defects decreased, then the film will be able to withstand higher levels of stress. It is important to emphasize that this approach will not reduce the stress in the system; so, bending and other detrimental effects will still occur.

The strength of the material can be increased by adding second-phase reinforcements that can be either permanent (e.g. fibers) or temporary (long-chain polymers). The size of critical defects can be modified through appropriate processing (either selection of route or control of processing) to ensure that the samples are free of critical defects. Large pores, introduced due to contamination, and poor powder packing or large grains are common strength-limiting defects in powder-based thick films – the use of fine grains and ensuring well-homogenized powders with no contaminants are therefore critical, as is high-quality deposition processing (Chapter 3).

Such strengthening mechanisms can play an important role, as there is a significant change in the mechanical properties of thick films during processing due to the rapidly evolving microstructure and chemistry of the system. Often, stresses in the system will increase before the strength of the material increases, leading to situations where the film is at a higher risk of failing mid-way through processing.

Overcoming Challenges of Integration
Reduce temperature
Reducing the temperature used for processing is by far the most effective way to overcome the challenges. It alleviates all the thermally induced issues, reduces (or even eliminates) chemical reactions, and reduces differential strains caused by reactions and temperature.

Separate reactants
Two reactive materials can be separated either by removing one material completely or by placing a barrier between the two materials. Protective atmospheres and barrier layers are frequently used.

Reduce differential strains
Select materials with comparable thermal expansions, those that do not undergo volume changes due to reactions or phase changes, or reduce the need to consolidate materials during processing.

Reduce film thickness
Building up multiple thin layers can allow much thicker films to be created, as each single layer is better able to withstand relative shrinkage during processing.

Strengthening
Modifying the materials to increase strength or interface strength of system can be used to prevent mechanical failure.

Read more
Creep of Intermetallics
M.-T. Perez-Prado, M.E. Kassner, in Fundamentals of Creep in Metals and Alloys (Third Edition), 2015

4.2.3 Strengthening Mechanisms
Several strengthening mechanisms have been utilized in order to improve the creep strength of NiAl alloys. Solid solution of Fe, Nb, Ta, Ti, and Zr produced only

2. 2
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
™
is a trademark used herein under license.
Figure 6.28 The
Charpy V-notch
properties for a
BCC carbon steel
and a FCC
stainless steel.
The FCC crystal
structure typically
leads top higher
absorbed energies
and no transition
temperature

3. Dislocations & Materials Classes
• Covalent Ceramics
(Si, diamond): Motion hard.
-directional (angular) bonding
• Ionic Ceramics (NaCl):
Motion hard.
-need to avoid ++ and --
neighbors.
+ + + +
+++
+ + + +
- - -
----
- - -
• Metals: Disl. motion easier.
-non-directional bonding
-close-packed directions
for slip.
electron cloud ion cores
+
+
+
+
+++++++
++++++
+++++++

4. Dislocation Motion
Dislocations & plastic deformation
•Cubic & hexagonal metals -plastic deformation is by plastic
shear or slipwhere 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.

5. • Stronger since grain boundaries
pin deformations
• Slip planes & directions
(l, f) change from one
crystal to another.
• tRwill vary from one
crystal to another.
• The crystal with the
largest tRyields first.
• Other (less favorably
oriented) crystals
yield (slip) later.
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].)
Slip Motion in Polycrystals
s
300 mm

6. After seeing the effect of poly crystalline materials
we can say (as related to strength):
•Ordinarily ductility is sacrificed when an alloy
is strengthened.
•The relationship between dislocation motion
and mechanical behavior of metals is
significance to the understanding of
strengthening mechanisms.
•The ability of a metal to plastically deform
depends on the ability of dislocations to
move.
•Virtually all strengthening techniques rely on
this simple principle: Restrictingor Hindering
dislocation motion renders a material harder
and stronger.
•We will consider strengthening single phase
metalsby: grain size reduction, solid-solution
alloying, and strain hardening

7. Strategies for Strengthening:
1: Reduce Grain Size
• Grain boundaries are
barriers to slip.
• Barrier "strength"
increases with
Increasing angle of
misorientation.
• Smaller grain size:
more barriers to slip.
• Hall-Petch Equation:21

/
yoyield
dk

ss
Adapted from Fig. 7.14, Callister 7e.
(Fig. 7.14 is from A Textbook of Materials
Technology, by Van Vlack, Pearson Education,
Inc., Upper Saddle River, NJ.)

8. Impurity atoms distort the lattice & generate stress.
Stress can produce a barrier to dislocation motion.
Strategies for Strengthening:
2: Solid Solutions
• Smaller substitutional
impurity
Impurity generates local stress at A
and Bthat opposes dislocation
motion to the right.
A
B
• Larger substitutional
impurity
Impurity generates local stress at C
and Dthat opposes dislocation
motion to the right.
C
D

9. Stress Concentration at
Dislocations
Adapted from Fig. 7.4,
Callister 7e.

10. Strengthening by Alloying
•small impurities tend to concentrate at dislocations on the
“Compressive stress side”
•reduce mobility of dislocation increase strength
Adapted from Fig.
7.17, Callister 7e.

11. Strengthening by alloying
•Large impurities concentrate at dislocations on
“Tensile Stress” side –pinning dislocation
Adapted from Fig.
7.18, Callister 7e.

12. Strategies for Strengthening: 4. Cold Work (%CW)
• Room temperature deformation.
• Common forming operations change the cross
sectional area:
Adapted from Fig.
11.8, Callister 7e.
-Forging
Ao Ad
force
die
blank
force
-Drawing
tensile
force
Ao
Ad
die
die
-Extrusion
rambillet
container
container
force
die holder
die
Ao
Ad
extrusion100 x %
o
do
A
AA
CW


-Rolling
roll
Ao
Ad
roll

13. • Ti alloy after cold working:
• Dislocations entangle and
multiply
• Thus, Dislocation motion
becomes more difficult.
Adapted from Fig.
4.6, Callister 7e.
(Fig. 4.6 is courtesy
of M.R. Plichta,
Michigan
Technological
University.)
During Cold Work
0.9 mm

14. Result of Cold Work
Dislocation density =
–Carefully grown single crystal
ca. 10
3
mm
-2
–Deforming sample increases density
10
9
-10
10
mm
-2
–Heat treatment reduces density
10
5
-10
6
mm
-2
• Yield stress increases
as rdincreases:
total dislocation length
unit volume
large hardening
small hardening
s
e
s
y0
s
y1

15. Impact of Cold Work
Lo-Carbon Steel!
Adapted from Fig. 7.20,
Callister 7e.
• Yield strength (s
y) increases.
• Tensile strength (TS) increases.
• Ductility (%ELor %AR) decreases.
As cold workis increased

16. • What is the tensile strength &
ductility after cold working?%6.35100 x %
2
22




o
do
r
rr
CW
Cold Work Analysis
D
o
=15.2mm
Cold
Work
D
d
=12.2mm
Copper

17. • What is the tensile strength &
ductility after cold working to 35.6%?
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.)
Cold Work Analysis
% Cold Work
100
300
500
700
Cu
200 4060
yield strength (MPa)
% Cold Work
tensile strength (MPa)
200
Cu
0
400
600
800
204060
340MPa
TS = 340MPa
ductility (%EL)
% Cold Work
20
40
60
2040600
0
Cu
7%
%EL= 7%YS = 300 MPa

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™
is a trademark used herein under license.

19. • 1 hour treatment at Tanneal...
decreases TSand increases %EL.
• Effects of cold work are reversed!
• 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.)
Effect of Heating After %CW
tensile strength (MPa)
ductility (%EL)
tensile strength
ductility
600
300
400
500
60
50
40
30
20
annealing temperature (ºC)
200100 300400500600700

20. Annihilation reduces dislocation density.
Recovery
• Scenario 1
Results from
diffusion
• Scenario 2
4. opposite dislocations
meet and annihilate
Dislocations
annihilate
and form
a perfect
atomic
plane.
extra half-plane
of atoms
extra half-plane
of atoms
atoms
diffuse
to regions
of tension
2. grey atoms leave by
vacancy diffusion
allowing disl. to “climb”
t
R
1. dislocation blocked;
can’t move to the right
Obstacle dislocation
3. “Climbed” disl. can now
move on new slip plane

21. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
™is a trademark used herein under license.

22. • New grains are formed that:
--have a low dislocation density
--are small
--consume cold-worked grains.
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.
0.6 mm 0.6 mm
Recrystallization

23. • All cold-worked grains are consumed.
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
0.6 mm0.6 mm
Further Recrystallization

24. • At longer times, larger grains consume smaller ones.
• Why? Grain boundary area (and therefore energy)
is reduced.
After 8 s,
580ºC
After 15 min,
580ºC
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.)
Grain Growth
• Empirical Relation:Ktdd
n
o
n

coefficient dependent on
material & Temp.
grain dia. At time t.
elapsed time
exponent typ. ~ 2
This is: Ostwald Ripening

25. T
R
Adapted from Fig.
7.22, Callister 7e.
º
º
T
R= recrystallization
temperature

26. Recrystallization Temperature, T
R
T
R= recrystallization temperature= point
of highest rate of property change
1.T
R0.3-0.6 T
m(K)
2.Due to diffusion annealing timeT
R= f(t)
shorter annealing time => higher T
R
3.Higher %CW=> lower T
R–strain hardening
4.Pure metals lower T
Rdue to dislocation
movements
•Easier to move in pure metals => lower T
R

27. 27
Figure 7.9 The fibrous
grain structure of a low
carbon steel produced by
cold working: (a) 10% cold
work, (b) 30% cold work,
(c) 60% cold work, and (d)
90% cold work (250).
(Source: From ASM
Handbook Vol. 9,
Metallography and
Microstructure, (1985) ASM
International, Materials
Park, OH 44073. Used with
permission.)

28. 28
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.

29. 29
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
™is a trademark used herein under license.
Figure 7.11 Orientations of samples (for Example 7.3)
Example 7.3 (continued)