This document summarizes the development of microstructural models to predict the lifetime of nickel-based superalloys and MCrAlY coatings used in jet engine components. It describes characterizing the microstructure of an IN738 alloy and two MCrAlY coatings after various heat treatments. Models were developed relating the carbide ratios in IN738 and thickness of diffusion zones in coatings to temperature and time. The models aim to act as virtual time-temperature recorders to predict component lifetime based on microstructural evolution.

1. Microstructural Methods of
Life Prediction for Nickel
Base Superalloys
M R Shaheedullah

2. Inlet compressor burner nozzle
Introduction
blades used in jet engines and gas turbines
nickel base alloy used because heat and
creep resistant
coatings for better thermal stability
full details of operating conditions not
always available in gas turbines
use microstructure to predict temp.

3. Contents
Introduction
Aims
Heat treatment programme
Characterisation of IN738
Characterisation of MCrAlY coatings
IN738 alloy model
Coating/substrate interaction model
Conclusions and further work

4. Aims
To develop a model that will act as a time
temperature recorder IN738 industrial gas
turbines
To characterise two MCrAlY coatings on
IN738
To develop a model that will act as a time
temperature recorder for MCrAlY coatings
on an IN738 substrate

5. Heat Treatment Programme
IN738 samples with both coatings to apply
to substrate model and coating model
Creep test schedule from sponsoring
company
Heat treatment schedule at IPTME
Temperatures 850°C - 1000°C
Times 1000 - 10000 hours

6. Short term Long term
Phases
γ Ni
γ′ Ni3Al
M23C6
MC
M23C6 M23C6
MC
MCM23C6
Characterisation of IN738
MC
G.b

7. Carbides in IN738
• Matrix dissolved to leave
MC and M23C6 carbides
• processed on XRD
• ratio of two carbides
changes with temp and time
850 ºC / 300 hours 950 º C / 1000 hours

8. Characterisation of
MCrAlY Coatings
Two coatings studied
NiCoCrAlYRe coating A
CoNiCrAlY coating D
Applied to IN738 by plasma spray
Study effect of long term thermal
exposure
Study interaction with substrate

9. Microstructure of Coatings
4800 hours 9600 hours
A
D
Phases
β NiAl
σ TCP
α Cr

10. Microstructure of
Interdiffusion Layer
4800 hours 9600 hours
A
D

11. Substrate Model
Data on carbide ratios from XRD
Adapted Starink and Thomson model for
MARMOO2 to give predicted ratio
Use spreadsheet to fit empirical parameter

12. Substrate model
[ ] ⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎥
⎦
⎤
⎢
⎣
⎡
=−=
n
CM
Cr
CMCM
eq
CMCM
l
tD
CfTtyTyTty 2
623
623623623623 ),0()(),(
Amount of C in M23C6 at
time and temperature
Equilibrium
(final) amount
(MTDATA)
Initial
amount
(exptl.)
),(),(),( 623623 TtyTtyTty eq
CMCMTiC ≅+
Diffusion parameters

13. Substrate model
Averaged Model Curves for M23C6 ratio vs time
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0.00 2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00 16000.00
time/ hours
M23C6/MC
850
885
900
915
930
950
1000

14. Coating Model
Width of interdiffusion zone increases
with time and temperature
Beta depletion zones increase in width
time and temperature
It is possible to base models on both
these effects

15. 1
1.1 Schematic of Surface Layers (2), (41), (35)
γ + β
}
Al2O3
γ + β → γ′
β depleted
zone
Co, Ni solid
soln. Matrix
substrate
Al diffusion
Al
β
γ + β (Al) dif. couple
γ + γ′
} β depleted
zone
Oxide
Coating
Interdiffusion
layer
γ + γ′
Schematic of Surface Layers

16. Predicted Equilibrium Data for Coatings
400 500 600 700 800 900 1000 1100 1200 1300
0
10
20
30
40
50
60
70
80
Temperature (C)
Weight%Phase
al 8
BETA
GAMMA
SIGMA
GAMMA'
400 500 600 700 800 900 1000 1100 1200 1300
0
10
20
30
40
50
60
70
80
90
100
Temperature (C)
Weight%Phase
al 6
GAMMA
BETA
SIGMA
GAMMA'
400 500 600 700 800 900 1000 1100 1200 1300
0
10
20
30
40
50
60
70
80
90
100
Temperature (C)
Weight%Phase
al 5
GAMMA
BETA
GAMMA'
SIGMA
400 500 600 700 800 900 1000 1100 1200 1300
0
10
20
30
40
50
60
70
80
90
100
Temperature (C)
Weight%Phase
al 3
GAMMA
SIGMA
BETA
Example : Sensitivity of coating D to Al content
(shows beta depletion, yellow)
8% Al
5% Al
6% Al
3% Al
Atom% Atom%

17. Conclusions and Further Work
IN738
Thermal exposure results in
increase in amount of M23C6 at expense of
MC carbide
increase in ‘width’ of grain boundary
carbide ratio model gives close agreement
to experimental results

18. Conclusions and Further Work
Coating
Thermal exposure results in
depletion of beta phase in coatings
model to be developed
coating inter layer substrate
• Treat as a series of shells of different compositions
• Allow diffusion between shells
• Link kinetic code to equilibrium phase conditions