An affordable creep-resistant nickel-base alloy for power plant Franck Tancret Harry Bhadeshia.
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Transcript of An affordable creep-resistant nickel-base alloy for power plant Franck Tancret Harry Bhadeshia.
The problem
Future power plant: 750°C
But:
Commercial superalloys are too expensive(Nb, Ta, Co, Mo…)
New steels: 650°C
=> Use of Ni-base alloys
=> Design an affordable creep-resistant Ni-base alloy
Industrial requirements
- Affordable
- 100 000 h creep lifetime under 100 MPa at 750°C
- Stable at service temperature
- Forgeable
- Weldable
- Corrosion resistance
- Toughness
Design procedure
- Empirical nonlinear multiparametric modelling of mechanical properties as a function of composition and processing conditions (Gaussian processes)
Based on a huge database on the properties of many existing alloys => captures trends and interactions
- Phase diagram and segregation simulation (Thermo-Calc)- Processability- General metallurgy principles
Materials Science & Technology, 19 (2003)
Ni – 20Cr – 3.5 W – 2.3 Al – 2.1 Ti – 5 Fe – 0.4 Si – 0.07 C – 0.005 B
Experimental results
1 2 3 4 50
100
200
300
400
500
600
700
Experimentalresults
Target
Prediction anderror bounds
617
625
Creep rupture stressat 750°C (MPa)
log (lifetime) (h)
Next issue:PROCESSING
Melting and solidification (primary chemical segregation) Forging (’-free temperature window) Heat treatment (precipiation hardening…) Welding
How modelling can be used to address these issues?
Is the designed alloy easy to process?
Phase diagram simulation (Thermo-Calc)
600 800 1000 1200 14000.00
0.02
0.04
MB
2
'
liquid
. %mol
(° )Temperature C
0
2
α-CrM23C6
M7C3
. %mol
0
20
40
60
80
100
. %mol forgingwindow
melting
No undesirable phasesat service temperature
Primary segregation simulation(Thermo-Calc)
Scheil’s approximation: homogeneous liquid diffusion-free solids LIQUID
T = T – 1 K
SOLID(S) LIQUID
V andcomposition
Primary segregation simulation(Thermo-Calc)
Simple dendrite geometrical models=> composition profiles
2r
Vr
∝
sphere
rV
r
∝
cylinder
Vr ∝
plate
Primary segregation simulation(Thermo-Calc)
0.0 0.2 0.4 0.6 0.8 1.00
20
40
60
80
100
0
1
2
3
4
Ti K
α ( )counts
Relative dendrite thickness
sphere
cylinder
plate ( %)Ti concentration mol
Precipitation hardening kinetics
Solutionising1175°C, WQ
Isothermal heat treatmentsbelow ’ solvus, WQ
Vickers hardness
0.1 1 10 100 1000 10000
2.5
3.0
3.5
4.0
Avrami model+ Friedel-type hardening:
H = H0 + A V
f
1/2
850°C
800°C
750°C
700°CV
icke
rs h
ard
nes
s (G
Pa)
Ageing time (h)
Precipitation hardening kineticsDiffusion-controlled growth model:
3 Ni + diffusing (Al, Ti) => Ni3(Al,Ti)
Before ageing:solutionised
During ageing: ’
Cav
Precipitation hardening kinetics
Diffusion-controlled growth model (Al + Ti)
dN(i)dN(i-1)dNp
distance fromprecipitate
C(1)=
Ceq
C(2) C(i-1) C(i) C(i+1)… …
/ ’interface
surface S dtdx
)1i(C)i(CDS)i(dN
Sdx
)i(dN
Sdx
)1i(dN)i(C)i(C tdtt
adjustableparameter
Precipitation hardening kinetics
Diffusion-controlled growth model (Al + Ti)
C
d
Cav
Ceq
t = 0
C
d
Cav
Ceq
t
C
d
Cav
Ceq
t =
Precipitation hardening kinetics
Diffusion-controlled growth model (Al + Ti)
0.1 1 10 100 1000 10000
2.5
3.0
3.5
4.0
4.5
Friedel-type hardening:
H = H0 + A N
p
1/2
850°C
800°C
750°C
700°C
Vic
kers
har
dn
ess
(GP
a)
Ageing time (h)
Precipitation hardening kinetics
Diffusion-controlled growth model (Al + Ti)
8.5x10-4 9.0x10-4 9.5x10-4 1.0x10-3 1.0x10-3-6
-5
-4
-3
-2
-1
0
1
Q = 348 kJ/mol
D = D0 exp(-Q/RT)
ln D
(n
m2 /s
)
1/T (K-1)
CONCLUSIONS
Design of an affordable Ni-base alloy for power-plant 100 000 h creep lifetime under 100 MPa at 750°C Corrosion-resistant, stable, forgeable, weldable…
Extensive use of modelling:• Mechanical properties (Gaussian processes)• Phase diagram simulation
- Stability- Forgeability- Weldability- Age-hardening- Solidification segregation (Scheil’s model)
• Precipitation-hardening kinetics (diffusion model)