Cobalt-based Superalloys Development in CHiMaD - Tony Chung

1. Cobalt-based Superalloys
Development in CHiMaD
1
Tony Chung, Fernando Reyes,
David Seidman and David Duand
CHiMaD Phase Field Workshop, Nov 06, 2019

2. 2International Civil Aviation Organization
Global air traffic growth by continents 2017-2018

3. Nickel superalloys have dominated the turbine
industry for the past 70 years, but we are now
approaching its thermodynamic limit
3
• Minimum degradation over extended operation
• Ability to tolerate severe environment at elevated temperature
• Superior high temperature mechanical properties
γ′ (L12) precipitate
γ (FCC) matr

4. Cobalt superalloys offer potential
for higher jet engine efficiency
4
• Potentially higher operation
temperature than Ni based
superalloys since its solidus and
liquidus temperature are 50-150˚C
higher
• Challenges:
• lower γ′ solvus temperature
• higher density
• lower oxidation resistance
• inferior high T mechanical
properties
Sato, J. et al. Cobalt-base high-temperature alloys. Science 312, 90–91 (2006).
Lee, C. S. Precipitation-hardening characteristics of ternary cobalt-aluminum-X alloys. (1971).

5. CHiMaD effort on cobalt
superalloys development
5
Database
Development
Experiments
Computational
Alloy Design

6. Cobalt superalloys offer potential
for higher jet engine efficiency
6
• Potentially higher operation
temperature than Ni based
superalloys since its solidus and
liquidus temperature are 50-150˚C
higher
• Challenges:
• inferior high T mechanical
properties
• higher density
• lower γ′ solvus temperature
• lower oxidation resistance
Sato, J. et al. Cobalt-base high-temperature alloys. Science 312, 90–91 (2006).
Lee, C. S. Precipitation-hardening characteristics of ternary cobalt-aluminum-X alloys. (1971).

7. 7
Effect of directional coarsening on creep
resistance in cobalt superalloys
Microstructural evolution in low-density
tungsten-free cobalt superalloys

8. Lattice parameter misfit in
superalloys
8
Ni superalloys (-): aγ′ < aγ
Co superalloys (+): aγ′ > aγ
aγ′
aγ
matrix
precipitate

9. Stress-induced directional
coarsening (rafting) in superalloys
9
Jokisaari, A. M., et al., Predicting the morphologies of γʹ precipitates
in cobalt-based superalloys. Acta Materialia 141, 273–284 (2017).
Ni-based
Co-based

10. What is the influence of rafting directionality
on creep resistance?
Can we engineer microstructure to enhance
creep properties?
10
Jokisaari, A. M., et al., Predicting the morphologies of γʹ precipitates
in cobalt-based superalloys. Acta Materialia 141, 273–284 (2017).

11. Pre-rafting to create three
different γ′ morphologies
11
150MPa Compression
150MPa Tension

12. Tensile creep response at
305MPa
12

13. Tensile creep response at
250MPa
13

14. Tensile creep response at
205MPa
14

15. Creep behaviors vary drastically
at different stress level
15
Intermediate stressHigh stress Low stress

16. 20
Effect of directional coarsening on creep
resistance in cobalt superalloys
Microstructural evolution in low-density
tungsten-free cobalt superalloys
• Co superalloys exhibits opposite rafting behavior to Ni superalloys
• Rafting can be reversed by applying stress in opposite direction
• Interfacial dislocations likely play an essential role in reversal of rafting
• Rafting is associated with significant amount of interface diffusion
which also drives creep deformation

17. V Fe Co
Ru
Ta
Partition to γ': increase γ' volume fraction
Partition to γ: slow γ' coarsening
Omori et al., Intermetallics (2013)
Co-9Al-10W
γ’-formerSolid solution
In Co-Al-W
Computationally predicted ternary systems
2017: Nysadham et al. predicts γ’-precipitates in the Co-Ta-V and Co-Nb-V
systems
21
Nysadham et al. Acta Mat. (2017), Omori et al. Intermetallics (2013)
Nb
Kγ’/γ = Partitioning
coefficient
Kγ’/γ > 1 →
γ'
Kγ’/γ < 1 →
γ

18. Computationally predicted ternary systems
22
No γ’-phase was seen in these systems
Selected Nominal Composition
Co-6Ta-6V(-10Ni) Selected Nominal Composition
Co-6Nb-6V
900 ⁰C 900 ⁰C
Ruan et al. J. Alloys & Comp. (2016), Wang et al. (), Reyes Tirado et al. Acta Mat.
(2018)
Ruan et al. J. Alloys & Comp. (2016) Wang et al. J. Pha. Eq. & Diff. (2015)
Co-Ta-V Co-Nb-V

19. Co-xNi-5.4Ta-6.6V (x: 0 and 10) aged at 900 °C
23
Reyes Tirado et al. Acta Mat. (2018)
2 h 16 h 64 h
0 Ni
10 Ni
No changes in γ’ morphology with the addition of Ni
Coarsening and coalescence
Time

20. Other phases show signs of metastability after
long-term aging at 900 °C
24
Reyes Tirado et al. Acta Mat. (2018)
2 h 64 h
0 Ni
10 Ni
Solidification
Precipitates
Discontinuous precipitation of C36 consumes the
microstructure as aging progresses
2 µm

21. Co-6Nb-6V aged at 900 °C
25
As aging progresses, all cuboidal precipitates disappear and the more stable D019
phase with a Co3X composition and Widmanstätten morphology is seen.
2 h0 h
Low-magnificationHigh-magnification
Reyes Tirado et al. Acta Mat. (2018)
16 h

22. Compositions
Co-Ta-V-based: Co-10Ni-5Al-3Ta-3V-2Ti-0.04B-xCr (x = 0 and 4Cr)
Co-Nb-V-based: Co-10Ni-5Al-3Nb-3V-0.04B-6Ti-xCr (x = 0 and 4Cr)
: Co-10Ni-5Al-3Nb-3V-0.04B-2Ti-xCr (x = 4 and 8Cr)
Ti V Co
Ta
Al
[1] Klein et al. Corrosion Sci. (2011), [2] Omori et al. Intermetallics (2013)
γ’-phase can be stabilized by alloying with strong
γ’- formers like Al, Ti and Ni
26
NiCr
B
Nb
Partition to γ': increase γ' volume fraction
Partition to γ: slow γ' coarsening
Inhibit GB sliding during creep

23. Only γ’-precipitates are seen after 1000 h of aging at 850 °C
Time
0 h 168 h 500 h 1000 h
0 Cr
4 Cr
Microstructural evolution
Co-10Ni-5Al-3Ta-3V-2Ti-0.04B-xCr (850 °C)
27
Reyes Tirado et al. Acta Mat. (2019)

24. 0 h 168 h 500 h 1000 h
0 Cr
4 Cr
Time
0 Cr alloy:
• The microstructure does not change over time
4 Cr alloy
• Cuboidal precipitates at short aging times,
indicating a change in lattice misfit
Microstructural evolution
Co-10Ni-5Al-3Ta-3V-2Ti-0.04B-xCr (850 °C)
28
Reyes Tirado et al. Acta Mat. (2019)

25. Microstructural evolution
Co-10Ni-5Al-3Nb-3V-0.04B-6Ti-xCr (850 °C)
29
Time
24 h
0 Cr
4 Cr
72 h 168 h
Only γ’-precipitates are seen up to 1000 h. Cr does not seem to
affect the microstructural evolution
500 h 1000 h

26. Microstructural evolution
Co-10Ni-5Al-3Nb-3V-0.04B-2Ti-xCr (850 °C)
30
Time
24 h 168 h
4 Cr
8 Cr
72 h

27. Plate-like Structure

28. Co-10Ni-6.25Al-3.75V-3.3(Ta or Nb)-2.5Ti-0.04B-4Cr
Aged at 850 °C for 168 h – 25% Increase of γ;-formers:
32
Ta Nb
High Magnification
Low Magnification
γ+γ
’
γ+γ
’
γ+γ’
γ+γ’
Eutectic – like
region at GB
Co3Ta?
(C36 Laves)
γ+γ
’

29. Other W-freeAlloysAged at 850 °C for 168 h:
33
Co-10Ni-8Cr-7Al-4Cr-4.5V-3NB-2.5Ti-0.04BCo-10Ni-6.25Al-4Cr-3.75V-3.3Nb-1Ta-2.5Ti-0.0
4B
Other phases
Eutectic-like regions at GB and inside
grains in a dendritic arrangement
γ+γ
’
γ+γ
’
γ+γ’
γ’
Laves phase
γ+γ’
Eutectic – like
region at GB
Hybrid

30. 34
Effect of directional coarsening on creep
resistance in cobalt superalloys
Microstructural evolution in low-density
tungsten-free cobalt superalloys
• Co superalloys exhibits opposite rafting behavior to Ni superalloys
• Rafting can be reversed by applying stress in opposite direction
• Interfacial dislocations likely play an essential role in reversal of rafting
• Rafting is associated with significant amount of interface diffusion
which also drives creep deformation
• Metastable γ’-precipitates are present in the Co-Ta-V and Co-Nb-V
ternary systems with the C36 and D019 phases consuming the γ+γ’
microstructure, respectively
• The γ’-phase in both systems is stabilized by Al, Ti, Ni, and Cr
additions producing elongated precipitates arranges in a plate-like
structure.

31. Acknowledgement
35
Dunand Research Group
Seidman Research Group