Engineering 45
Dislocations &
Strengthening (2)
Bruce Mayer, PE
Registered Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering-45: Materials of Engineering
1
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Learning Goals
 Understand Why DISLOCATIONS
observed primarily in METALS and
ALLOYS
 Determine How are Strength and
Dislocation Motion Related
 Techniques to Increase Strength
 Understand How can HEATING
and/or Cooling change strength and
other properties
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
2-Phase Metal Strengthening
 Last Time Studied SINGLE-Phase
(ONE Xtal Structure) Strengthening
1. Grain Size Reduction
2. Solid Solution Alloying
3. Strain Hardening
 This Time Examine TWO Phase (2)
Strengthening by the Formation of
Solid Precipitates
• a.k.a., Precipitation Hardening
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Strengthen-4 → Precipitates
 Adjustment of Alloy
Composition and/or
Processing can
Produce a (more or
less) Uniform
Distribution of Small
2nd Phase Particles
(Precipitates) within
the Base-Alloy
matrix
Engineering-45: Materials of Engineering
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CuAl2 Precipitates
Within an Al Matrix
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
PreCip Strength Mechanism
 Hard Precipitates are Difficult to shear
• Ex: Ceramics in Metals (SiC in Iron or Aluminum)
precipitate
Large shear stress needed
to move dislocation toward
precipitate and shear it.
Side View
Top View
Uns
lipped part of slip plane
S
Dislocation
“advances” but
precipitates act as
“pinning” sites with
spacing S.
lipped
S
part of slip plane
 Empirical Relation: σy ~ 1/S
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
App  PreCip Strengthening
 Internal wing structure on Boeing 767
 Aluminum is
strengthened with
precipitates formed
by alloying.
1.5mm
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Simulation: Precip Hardening
 View onto slip
plane of Nimonic
PE16
• 45Ni-31Fe-16.5Cr3.5Mo-1.2Ti-1.2Al1Co
 Precipitate volume
fraction: 10%
 Average precipitate
size: 64 b (b = 1
atomic slip distance)
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
σ-ε Behavior vs. Temperature
 Tensile Tests on
PolyXtal Iron
Stress (MPa)
800
 σy and TS DEcrease
with INcreasing test
temperature.
-200C
600
-100C
400
25C
200
0
0
0.1
0.2
0.3
0.4
0.5
 %EL INcreases with
INcreasing test
temperature.
Strain
 Note Trends
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
σ-ε Behavior vs. Temperature
• Increased
Vacancy
Concentration
Stress (MPa)
 Why this
Behavior?
800
-200C
600
Engineering-45: Materials of Engineering
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25C
200
0
0
0.1
0.2
0.3
0.4
0.5
Strain
 Mechanism:
• Vacancies help
dislocations
move past
obstacles
-100C
400
3. disl. glides past obstacle
2. vacancies
replace
atoms on the
disl. half
plane
obstacle
1. disl. trapped
by obstacle
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Post-ColdWork Heat Treatment
 1 hour treatment at Tanneal...
• Decreases σu, and Increases %El

Heat Treating
REVERSES the
Effects of CW

Three Stages
1. Recovery
2. ReXtalization
3. Grain Growth
1
2
Engineering-45: Materials of Engineering
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3
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Recovery
 Annihilation reduces dislocation density (ρd ↓)

Scenario1
•

Hi-Temp
Increases
Diffusion
Scenario2
3
2
4
1
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
ReCrystallization
 New crystals are formed that
• have a much smaller dislocation density
• Are Small in Physical Size Relative to Existing Xtals
• Are More Energetically Favorable than CW Xtals
0.6 mm
33% cold
worked
brass
Engineering-45: Materials of Engineering
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0.6 mm
New crystals
nucleate after
3 sec. at 580C.
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
ReCrystallization cont
 All cold-worked crystals are consumed
by the New Crystals
0.6 mm
After 4
seconds
Engineering-45: Materials of Engineering
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0.6 mm
After 8
seconds
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Grain Growth
 At Longer Times Some of the NEW Xtals
consume OTHER NEW Xtals
 Grain Boundaries are Hi-Energy Regions, Thus
Their Reduction is Thermodynamically Favored
0.6 mm
After 8 s,
580C
Engineering-45: Materials of Engineering
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0.6 mm
After 15 min,
580C
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Grain Growth Quantified
 Many Metals Follow
This Grain Growth
Reln d n  Kt  d n
0
• Where
– d  Grain Size (m)
– t  Time (s)
– d0  BaseLine Grain
Size at t = 0 (m)
– K  Slope Constant
(mn/s)
– n  Power Constant
(unitless)
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Summary
 Dislocations are Observed Primarily in
Metals And Alloys
 Metal/Alloy Strength Is Increased By
Making Dislocation Motion Difficult
 Techniques to increase strength
•
•
•
•
Decrease Grain Size
Solid Solution Lattice Straining
Cold Work to Increase Dislocation Density
Precipitates to Impede Dislocation Motion
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Summary cont.


Post-CW Heating (annealing) can
reduce Dislocation density and
increase grain size
Anneal Process Time-Phases
1. Recovery
2. ReCrystallization
3. Grain Growth
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
WhiteBoard Work
 Problem 7.27
Parallel
(Elastic Recovery)
~0.16
σ-ε for C46400 (Naval) Brass
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt
σ-ε Curve for Steel
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-18_DisLoc-Strength-2.ppt