HEA

1. Heavy Ion Irradiation of FCC and BCC
High Entropy Alloys for Advanced Nuclear
Reactor Applications
Calvin Parkin1, Michael Moorehead1, Mohamed Elbakhshwan1,
Chuan Zhang2, Wei-Ying Chen3, Kumar Sridharan1, Adrien Couet1
1University of Wisconsin– Madison
2Computherm, LLC
3Argonne National Laboratory

2. Project Overview
Selected High
Entropy Alloys:
Irradiation
performance
for fast reactor
applications
Heavy ion,
proton and
dual beam
experiments
In-situ
heavy-ion
irradiation
under TEM
Advanced
microscopy
High-
temperature
liquid
sodium
compatibility
Unique
micro/nano-
mechanical
tests
2

3. Motivation
Yield stress of TerraPower HT9 compared to historical HT92
Sodium Fast Reactor materials (cladding/ducts):
• Adequate creep strength up to 650°C and fracture toughness down to 320°C
• Require up to 600 dpa, 600-700°C, Liquid Na coolant
• HT-9 being considered by TerraPower for TWR
Operating temperatures and radiation damage regimes for some advanced reactors1
600
3
1. Zinkle, S. J. and J. T. Busby (2009). "Structural materials for fission & fusion energy." Materials Today 12(11): 12-19.
2. Xu, C. H., M.J. (2017). "TerraPower HT9 Mechanical and Thermal Creep Properties." TMS2017.

4. Background: High Entropy Alloys
(a) Conventional Alloy, (b) High Entropy Alloy1
Irradiation of single-phase HEA needed to confirm benefits of compositional complexity.
Multiple primary elements with <35 at% in each
• High configurational entropy  high-temperature stability
• Modeling studies suggest radiation resistance of HEA matrix
• HEA are of interest as a replacement base matrix for nuclear alloy design
4
1. Miracle, D. and O. Senkov (2017). "A critical review of high entropy alloys and related concepts." Acta Materialia 122: 448 - 511.

5. HEA Radiation Resistance Theories
Variable atomic mass, size, and force constants
cause phonon broadening
• Lower mean free path of deposited heat1
• Longer time for quench more
recombination2
Complex energy landscape and distorted lattice
reduce defect mobility
• Increased average migration energy for
interstitials3
• Immobile interstitial loops slow vacancy
saturation4
5
1. Granberg, F., et al. (2015). "Mechanism of Radiation Damage Reduction in Equiatomic Multicomponent Single Phase Alloys." Physical Review Letters.
2. Béland, L. K., et al. (2015). "Lattice thermal conductivity of multi-component alloys." Journal of Alloys and Compounds 648: 408 -413.
3. Yeh, J. W. (2015). "Physical Metallurgy of High-Entropy Alloys." The Journal of The Minerals, Metals & Materials Society 67(10): 2254 - 2261.
4. Lu, C., et al. (2016). "Enhancing radiation tolerance by controlling defect mobility and migration pathways in multicomponent single-phase alloys." Nature
Communications 7: 13564.

6. Purpose of the Study
Investigate mechanisms of radiation resistance in single-phase HEA
• How does the microstructural evolution of HEA under irradiation
compare to less compositionally complex reference materials?
• Which proposed radiation damage mechanisms are influenced most
strongly by compositional complexity?
• How do these mechanisms extend to reactor conditions (high dpa, high
temperature)?
6

7. Alloy Selection
CrFeMnNi family
• Mechanical properties similar to SS316
• Cr18.1Fe27.3Mn27.3Ni27.3 shows no swelling
after 10 dpa Ni2+ ion irradiation at 700 °C1
Single phase predicted as low as ~775 °C
• Cr15Fe35Mn15Ni35 predicted by CALPHAD
to be single phase as low as 600 °C
Predicted to phase separate at SFR
operating temperatures.
Cr18.1Fe27.3Mn27.3Ni27.3
Cr63Fe15Mn19Ni3
Cr45Fe25Mn27Ni3
7
1.Kumar, N. A. P. K., et al. (2016). "Microstructural stability and mechanical behavior of FeNiMnCr high entropy alloy under ion irradiation." Acta Materialia 113: 230-244.

8. Alloy Selection
Cr15Fe35Mn15Ni35
8
CrFeMnNi family
• Mechanical properties similar to SS316
• Cr18.1Fe27.3Mn27.3Ni27.3 shows no swelling
after 10 dpa Ni2+ ion irradiation at 700 °C1
Single phase predicted as low as ~775 °C
• Cr15Fe35Mn15Ni35 predicted by CALPHAD
to be single phase as low as 600 °C
Single phase predicted at SFR operating
temperatures.
1.Kumar, N. A. P. K., et al. (2016). "Microstructural stability and mechanical behavior of FeNiMnCr high entropy alloy under ion irradiation." Acta Materialia 113: 230-244.

9. Alloy Selection
1. King, D. J. M., Parkin, C., Couet, A. (2019). "High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system." Acta Materialia 166: 435-446.
Nb25Ta25Ti25V25
Nb52Ta18Ti14V16
NbTaTiV(Zr) family
• BCC structure inherently resistant to void
swelling (0.2 %/dpa vs. 1%/dpa)
• Zr and Ta tend to segregate1
• NbTaTiV predicted by CALPHAD to be
single phase as low as ~500 °C
Single phase predicted at SFR operating
temperatures.
9

10. Alloy Fabrication
100 μm 100 μm
300 μm 300 μm
Cr18.1Fe27.3Mn27.3Ni27.3 Homogenized
1200 °C for 48 hours
NbTaTiV
Homogenized 1200 °C for 1 week
or 1500 °C for 3 days
Samples for irradiation made by arc melting or vacuum induction melting
• Typical dendritic microstructure forms due to solidification.
• Single phase HEA were successfully fabricated after homogenization.
10

11. Alloy Fabrication
Samples for irradiation made by arc melting or vacuum induction melting
• Typical dendritic microstructure forms due to solidification.
• Single phase HEA were successfully fabricated after homogenization.
40 60 80 100
{311}
{220}
{200}
{111}
{222}
{311}
{220}
{111}
Cr18.1Fe27.3Mn27.3Ni27.3 XRD
Intensity
[-]
2 [deg]
{200}
Homogenized 2 days 1200 C
Aged 700 C 1 day
40 60 80 100
Homogenized 7 days 1200 C
Aged 700 C 1 day
{310}
NbTaTiV XRD
Intensity
[-]
2 [deg]
{110}
{200} {211}
{220}
{220}
{211}
{200}
{110}
40 60 80 100
Homogenized 2 days 1200 C
Aged 700 C 1 day
Intensity
[-]
2 [deg]
Cr15Fe35Mn15Ni35 XRD
{111}
{200} {220}
{311}
{111}
{200} {220} {311}{222}
11
Cr18.1Fe27.3Mn27.3Ni27.3 and NbTaTiV phase separate after one day of
ageing at 700 °C, Cr15Fe35Mn15Ni35 remains single phase.

12. In situ IVEM investigation of defect evolution (1 MeV Kr2+ ions & TEM)
• Electropolished 3 mm discs prepared from bulk material
• Cryo temperature suppresses diffusion to test damage accumulation
• Elevated temperature then reveals effect on defect mobility
1.0 MeV Kr++ ions to
Material (Composition) Temp [K] Max dpa
Pure Ni 50
Model Alloy E-90
(Fe-Ni15.7-Cr15.6 wt%)
50, 300 0.3
FCC HEA1
(Cr18.1Fe27.3Mn27.3Ni27.3 at%)
50, 300, 773 0.5
FCC HEA2
(Cr15Fe35Mn15Ni35 at%)
50, 300, 773 2.0
Pure V 50, 300 2.0
BCC HEA
(NbTaTiV equimolar)
50, 300, 773 2.0
Two RTE at IVEM-Tandem awarded by NSUF
12

13. Results: Defect clusters appear with increasing dpa.
{200}
Cr15Fe35Mn15Ni35 near <110> zone axis at 50K.
0 dpa 0.1 dpa 0.5 dpa 1 dpa 2 dpa
Bright field
Weak beam
dark field:
g={200}
13

14. Faulted loops seen with relrod contrast.
300 K
50 K
1 dpa 2 dpa
0.5 dpa 1 dpa 2 dpa
A schematic for relrod
imaging condition1
Cr15Fe35Mn15Ni35 near <110> zone axis some mechanism for loop formation at low temperature.
14
1. Yang, Y. C., Y.; Huang, Y.; Allen, T.; Rao, A. (2012). "Irradiation Microstructure of Austenitic Steels and Cast Steels Irradiated in the BOR-60 Reactor at 320°C."
Proceedings of the 15th International Conference on Environmental Degradation of Materials in Nuclear Power Systems — Water Reactors.

15. Cluster density in HEA lower than less compositionally
complex reference materials at 50 K
FCC HEA have fewer clusters than Ni or
E90 at 50 K
Dependence of loop density of annealed Fe-15Cr-16Ni
on temperature, dpa, and dpa rate.1
0.0 0.5 1.0 1.5 2.0
1E20
1E21
1E22
1E23
1E24 FCC Materials- 50 K
Cluster
Density
[m
-3
]
dpa
HEA1: Cr18.1Fe27.3Mn27.3Ni27.3
Ref: Fe-15.6Cr-15.7Ni (E90)
HEA2: Cr15Fe35Mn15Ni35
Ref: Ni
15
1E+22
1E+23
1E+24
0 5 10 15 20 25 30
Loop
Density
(m
-3
)
Irradiation Dose (dpa)
E90 irradiated at 300 °C
1E-4 dpa/sec
1E-3 dpa/sec
4E-4 dpa/sec
1. Okita, T., et al. (2001). Investigation of The Synergistic Influence of Irradiation Temperature and Atomic Displacement Rate on the Microstructural Evolution of Ion-
Irradiated Model Austenitic Alloy Fe-15Cr-16Ni. 10th International Conference on Environmental Degradation of Materials in Nuclear Power Systems. Lake Tahoe NV.

16. 0.0 0.5 1.0 1.5 2.0
1E21
1E22
1E23
1E24 BCC Materials- 50 K
Cluster
Density
[m
-3
]
dpa
V
NbTaTiV
NbTaTiV has fewer clusters at 50 K.
16
Cluster density in HEA lower than less compositionally
complex reference materials at 50 K

17. Loops grow more quickly in Cr18.1Fe27.3Mn27.3Ni27.3
2 dpa: Bright field at 773 K. Small compositional changes have a strong effect on loop growth.
HEA 2: Cr15Fe35Mn15Ni35
HEA 1: Cr18.1Fe27.3Mn27.3Ni27.3
17

18. Loops grow more quickly in Cr18.1Fe27.3Mn27.3Ni27.3
Average loop sizes vs. dpa.
Loop size distributions.
0 10 20 30 40 50 60
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Relative
Frequency
Loop Size [nm]
HEA1: Cr18.1Fe27.3Mn27.3Ni27.3— 773 K
0.1 dpa
0.3 dpa
0.5 dpa
1.0 dpa
2.0 dpa
0 10 20 30 40 50 60
0.0
0.1
0.2
0.3
0.4
0.5
HEA2: Cr15Fe35Mn15Ni35— 773 K
0.1 dpa
0.3 dpa
0.5 dpa
1.0 dpa
2.0 dpa
Relative
Frequency
Loop Size [nm]
0.0 0.5 1.0 1.5 2.0
0
5
10
15
20
25
30
35
40
Average
Loop
Size
[nm]
dpa
HEA1: Cr18.1Fe27.3Mn27.3Ni27.3
HEA2: Cr15Fe35Mn15Ni35
773 K FCC HEA Loop Size Distribution
18

19. Conclusions
• Reduced cascade defect accumulation in HEA at 50 K compared
to less compositionally complex reference materials
- Both FCC HEA have lower cluster density than pure Ni or E90 at 50 K.
- NbTaTiV has lower cluster density than pure V at 50 K.
- Faulted loops in relrod contrast at low temperature imply some mobility
of point defects.
• Effect on defect mobility is not conclusive
- Need high temperature data for reference materials.
- Loops grow more quickly in Cr18.1Fe27.3Mn27.3Ni27.3 vs. Cr15Fe35Mn15Ni35.
- Small compositional changes can have a significant effect on kinetics of
loop growth.
19

20. High-throughout mapping of radiation effects needed
• Combinatorial thin film materials libraries
• Laser metal-deposition 3D printed arrays
• XY stage for high-throughput irradiations at UW IBL
20
0 5 10 15 20
0
2
4
6
8
10
12
14
16
18
20
Ni in at%
Y-Pos.
X-Pos.
13
15
17
19
21
23
25
27
29
31
33
34
36
38
40
42
0 5 10 15 20
0
2
4
6
8
10
12
14
16
18
20
Mn in at%
Y-Pos.
X-Pos.
12
15
17
20
23
26
28
31
34
37
39
42
45
48
50
53
0 5 10 15 20
0
2
4
6
8
10
12
14
16
18
20
Fe in at%
Y-Pos.
X-Pos.
12
14
17
19
22
24
27
29
32
34
37
39
42
44
47
49 0 5 10 15 20
0
2
4
6
8
10
12
14
16
18
20
Cr in at%
Y-Pos.
X-Pos.
10
12
13
15
17
18
20
21
23
25
26
28
29
31
33
34

21. Thank you!
Questions?
21

22. HEA Radiation Resistance: Heat Transport
• Vibrational frequencies and electronic states are smeared by lattice distortion,
slowing propagation of energy via phonon scattering (Granberg, 2016)
• Shorter phonon mean free path  increased lifetime of thermal spike, more point
defect recombination (M. Caro, L. K. Beland, 2015)
CHEMICAL COMPLEXITY
V-W Nb-V-W Nb-Ta-V-W
Ni Fe-Ni Cr-Co-Fe-Ni
Phonons
Electrons
Simulated vibrational frequencies and
electronic band diagrams with
increasing compositional complexity.
(F. Körmann, 2017) (Y. Zhang, 2016)
Nb-Ta-Ti-V-W
22

23. HEA Radiation Resistance: Mass Transport
Complex energy landscape leads to an increase in migration energy.
• Sluggish diffusion- point defect clustering may be slower overall (Yeh, 2016)
• Immobile interstitial loops slow vacancy saturation (Lu, 2016)
Q/T
m
(J/mol∙K)
Normalized Ni Migration Energies (K.Y. Tsai, 2013)
Surface
Ions Ions
(a) Glissile loops move to sinks
(b) Sessile interstitial dislocation loops stay put
(a) (b)
23

24. Results- Loop densities of FCC materials.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
1E21
1E22
1E23
Cluster
Density
[m
-3
]
dpa
50 K
773 K
50 K relrod
300 K
300 K relrod
HEA2: Cr15Fe35Mn15Ni35
24

25. Results- Loop densities of FCC materials.
25
Mechanical properties of equimolar NbTiVZr vs. Inconel 718 (in compression)
and 18.1Cr-27.3Fe-27.3Mn-27.3Ni vs. SS316 (in tension)12
1. Miracle, D. B. and O. N. Senkov (2017). "A critical review of high entropy alloys and related concepts." Acta Materialia 122: 448-511.
2. Kumar, N. A. P. K., et al. (2016). "Microstructural stability and mechanical behavior of FeNiMnCr high entropy alloy under ion irradiation." Acta Materialia 113: 230-244.

26. Results- Fringes in old data (Campaign 1) at high dpa.
5 dpa: 2 beam dark field g={220} at 50 K.
Fringes are more pronounced with this dpa/contrast.
HEA at 5 dpa
E-90 at 5 dpa
26

27. Results- Edge-on loops in 15Cr-35Fe-15Mn-35Ni (50 K).
1 dpa: Bright field at 50 K.
27

28. Text Slide Sample
Ballistic stage can be split into
supersonic and sonic stages.
“subcascades in Ni and NiFe are
created with nearly identical energies
and distributed similarly in space.
This suggests that the differences in
production of extended defect
structures is not related to processes
taking place in the ballistic phase of
the collision cascade.”
Note NiFe is slightly longer and
remaining FP are slightly lower, but
effect is small.
28

29. Literature
• Jin et al. 2015- 3 MeV Ni irr, void swelling reduced with more elements.
• Jin et al. 2018- # of alloying elements less important than transport properties, thermal conductivity effect not dominant.
• Zhang et al. 2015- 3 MeV Au irr, Rutherford backscatter yield decreases with more elements.
• Körmann et al. 2017- Mass and force constant fluctuations both have an effect on phonon scattering and broadening.
• Zhao et al. 2019- Spontaneous recombination volume is increased from Ni to Ni-Fe to Ni-Fe-Cr. Fe- and Cr-containing
dumbbells are less stable than Ni-Ni
• Beland et al. 2015- Mass, size, and force constant defects reduce thermal conductivity. Strong coupling between mass
and size. A change in thermal conductivity by a factor α implies a change in quenching time by a factor 1/ α.
• Egami et al. 2015- Atoms in HEA experience strong atomic level stresses, makes them more susceptible to
amorphization by irradiation. Poorly supported connection to irradiation tolerance
• Granberg et al. 2016- Substantial reduction of damage accumulation under prolonged irradiation in single-phase NiFe
and NiCoCr alloys compared to elemental Ni. Attributed to reduced dislocation mobility. Fraction of damage in large
clusters is smaller for NiFe and NiCoCr than for Ni
• Lu et al. 2016- TEM study: 1D motion of small interstitial clusters is promoted in pure Ni but highly suppressed in NiFe,
NiCoFe, NiCoFeCr, and NiCoFeCrMn because compositional complexity defocuses 1D motion preventing long range
migration
• Lu et al. 2017- Higher fraction of loops are faulted loops in HEA, meaning the incubation period for them forming perfect
loops and then growing and gliding is prolonged.
29