Large quantities of cheap, nanocrystalline metals

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Large quantities of cheap, nanocrystalline metals

• Why do we need such materials?
• Production methods.
• Theory.
• Role of interfaces.

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an apple weighs 1 Newton
1 Pa
1 m
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Scifer, 5.5 GPa and ductile
Kobe Steel
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Hanson and Yang
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Hanson and Yang
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1 Denier weight in grams, of 9 km of fibre
50-10 Denier
Scifer is 9 Denier
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hexagonal close-packed
cubic close-packed
Christian, 1951
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Swallow and Bhadeshia, 1996
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Fe-2Si-3Mn-C wt
800
B
S
600
Temperature / K
400
M
S
200
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Carbon / wt
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Fe-2Si-3Mn-C wt
1.E08
1 year
1 month
Time / s
1.E04
1.E00
0
0.5
1
1.5
Carbon / wt
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Low transformation temperature Bainitic
hardenability Reasonable transformation
time Elimination of cementite Austenite grain
size control Avoidance of temper embrittlement
wt
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Isothermal
Austenitisation
Homogenisation
transformation
1200
C
o
2 days
1000
o
C
15 min
Temperature
125
o
C
-
325
o
C
Air
slow
hours
-
months
cooling
cooling
Quench
Time
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100
retained austenite
80
X-ray diffraction results
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Percentage of phase
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bainitic ferrite
20
0
200
250
300
325
o
Temperature/
C
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a
g
a
g
g
50 nm
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g
g
a
a
a
20 nm
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Peet, Babu, Miller, Bhadeshia, 2004
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Conclusions
Low temperature transformation 0.25 T/Tm Fine
microstructure 20-40 nm thick plates
Carbide-free Designed using theory alone Typical
mechanical properties
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Faster Transformation
Cobalt (1.5 wt) and aluminium (1 wt) increase
the stability of ferrite relative to austenite
Refine austenite grain size
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200oC
250oC
300oC
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Mechanically Alloyed Oxide Dispersion
Strengthened Metals
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Atom probe image of MA957
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MA956
Chou Bhadeshia, 1994
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MA956
Chou Bhadeshia, 1994
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Mechanical Mixture
free energy of
mechanical
o
m
mixture
B
G
o
m
Gibbs free energy per mole
A
x
1-x
A
B
Concentration x of B
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free energy of
mechanical
o
m
mixture
B
G
Gibbs free energy per mole
o
m
A
?G
M
Gx
free energy
of solution
A
B
x
Composition
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For a random mixture, number of configurations
given by
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Boltzmann
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Classical theory for entropy of mixing
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Solution-like behaviour when particles about 1000
atoms in size
Free energy of mixing due to configurational
entropy alone
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Enthalpy
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Surface per unit volume
Sv
Solution formation impossible!
Particle size
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coherent
coherent
incoherent
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Single barrier to solution formation when
components attract
Badmos and Bhadeshia
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Double barrier to solution formation when
components immiscible
Badmos and Bhadeshia
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o
m
B
o
m
A
free energy
of solution
A
B
Composition
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o
m
B
o
m
Paradox at concentration extremities vanishes in
the discrete model of concentration
A
Gibbs free energy per mole
A
B
Concentration x of B