Evolution of Microstructure in Creep-Resistant Steels

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Evolution of Microstructure in Creep-Resistant
Steels
www.msm.cam.ac.uk/phase-trans
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Theme

• Methods used in the past
• Outcomes of past approach
• Inspiration for new theory
• Progress to date

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oxidation
thermodynamics
processing
kinetics
alloy design
cost models
microstructure
validation
bayesian neural networks
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40 mm
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Nickel base alloy FT750dc
wt
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Tancret Bhadeshia, 2002
1000
800
600
Yield stress / MPa
400
200
0
0
200
400
600
800
1000
1200
Temperature / C
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Tancret Bhadeshia, 2002
700
FT750dc
600
500
400
rupture stress / MPa
300
200
100
0
1
2
3
4
5
6
log (t /h)
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ferritic alloy C
wt
10
650 oC
Cole Bhadeshia, 2002
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oxidation
thermodynamics
processing
kinetics
alloy design
cost models
microstructure
validation
bayesian neural networks
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Problems

• During the development of these models, we have
  discovered some fundamental difficulties with
  theory
• In particular, theory dealing with small driving
  forces

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Rate of reaction
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Stored energy

• Fe-0.2C-1.5Mn wt
• stored energy drives changes

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fine grain ODS
ferrite (strain-free)
bainite, P-cementite
P-ferrite, P-cementite
ferrite, cementite
ferrite, cementite, graphite
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Coarsening
diffusion
r
r
2
1
flux
a
q
q
concentration
aq
c
r
aq
1
c
r
2
distance
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Strang, Vodarek Bhadeshia, 1999
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Trivedi, 1970
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diaz del Castillo Bhadeshia, 2001a
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diaz del Castillo Bhadeshia, 2001a
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Summary Binary Solutions

• Solutions now exist for small supersaturations,
  for needles, plates and spheres
• For maximum velocity hypothesis
• For local equilibrium at interface

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Summary Multicomponent Solutions

• Solutions now exist for needles and plates
• Unique solution exists for velocity (maximum
  velocity hypothesis fails)
• equilibrium impossible at small supersaturations
  when capillarity prominent

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Data from Abe, 1999 Fe-9Cr-W alloys
0W 1W 2W 4W
M_23C_6 size / µm
Time at 600 C / h
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M_23 C_6
Mole fraction of phase
Laves phase
Tungsten / wt
Bhadeshia, 2000
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Multicomponent coarsening
Venugopalan Kirkaldy, 1978
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Coarsening accelerated by tungsten!
-1
chromium alone
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Effective diffusivity / m s
multicomponent
Tungsten / wt
Bhadeshia, 2000
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Multiphase coarsening
Laves
concentration
in ferrite
q
q
distance
Bhadeshia, 2000
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Conclusions

• There has been progress in predicting alloy
  behaviour
• Much more progress needed in theory
• Oxidation models?