Micromechanical testing of nuclear materials...damage profile. • 0.26 dpa –300 C and 750 C • Microcantilevers manufactured in the interphase, fibre and matrix of both irradiated

Advanced micromechanical testing

techniques of irradiated materials for

Nuclear applications

Dr Anna Kareer

Department of Materials, University of Oxford

RaDIATE Collaboration Meeting, Geneva, 17-21 December 2018

Radiation damage threats to structural

materials

1) Reduction in ductility and

work hardening capabilities

2) Segregation/precipitation

leading to embrittlement

3) Plastic instability and

prompt necking after yield

Microstrucuture

of neutron

irradiated Fe-

9Cr15 cm

Typical uniaxial tensile behaviour of

structural steels after irradiation

18/12/2018 A,Kareer | RaDIATE collaboration meeting 2018

Current reactor

maximum operating

conditions

Overview of operating temperatures and

expected damage levels for Generation IV

reactor designs.

Estimated operating temperature windows for

various structural materials in nuclear systems

with damage levels of 10-50 dpa.

Zinkle, S. J. & Busby, J. T. Structural materials for fission & fusion energy

Structural materials represent the key for containment of nuclear fuel. Mater.

Today 12, 12–19 (2009).

Materials issues in Future reactors


Mechanical testing of reactor irradiated

materials

Advanced test reactor (ATR)

Idaho national laboratory

Culham Materials Research Facility

(MRF) – Hot cell facility

• Low dose rates (200 dpa of damage in a test reactor would

take decades to achieve)

• Radioactive material handling / specialist facilities required for

active material

• Not efficient process for rapid characterisation of

mechanical response (expensive and time consuming)


• Much higher dose rates (104 that

of reactor irradiation)

• Negligible levels of activity

• Relatively cheap

• However only small volumes of

damaged material obtained…0

10

20

30

40

0

2

4

6

8

10

0 500 1000 1500 2000

Dis

pla

cem

ents

per

ato

m (

dp

a) H

elium

Co

ncen

tratio

n (ap

pm

)

Depth (nm)

Damage

Helium

Need for small scale mechanical testing to predict bulk properties

Mechanical testing of heavy ion irradiated

material

H + and Fe +

• Use ion irradiation to simulate neutron damage

420oC

5MeV

Ion irradiated damaged

microstructure


Nanoindentation: surface indentation vs.

cross-sectional indentation

Kiener, Daniel, et al. "Application of small-scale testing for investigation of ion-beam-

irradiated materials." Journal of Materials Research 27.21 (2012): 2724-2736.

• Measure hardness/yield strength and modulus

• Measure the relative increase in hardness due to

irradiation damage

• Plastic zone extends into regions of unirradiated

martial

• Indentation size effects for small indentation depth

Surface indentation

Cross-sectional

indentation

𝐻 = 3𝜎𝑦


Nanoindentation of Fe++ irradiated candidate

steels: Surface indentation

• Measure hardness/yield strength and

modulus as a function of depth –

dynamic indentation mode

• Measurement affected by the

indentation depth → Indentation size

effect unless large damaged layer02468

10121416182022

0 5 10

DP

A

Depth / um

High energy 70MeV Fe++


Bulk σy = 630MPa

10µm

Nanoindentation of Fe++ irradiated candidate

steels: Cross-section indentation

02468

10121416182022

0 2 4 6 8 10

DP

A

Depth / um

70MeV Fe++


High temperature Nanoindentation

• Variable temperature indenter (-

100~950 °C)

• Heats indenter tip and sample separately

allow isothermal contact

• High vacuum - prevents oxidation (10-7

mbar)

• Extremely low thermal drift even at high

temperature


High temperature Nanoindentation: He+

implanted Tungsten – Fusion applications

JSKL Gibson, SG Roberts, DEJ Armstrong MSEA 625, 380-384

To simulate damage in fusion

divertor helium ions

implanted into pure tungsten

at 850oC


Spherical indentation/macroscopic

compression in T91

5 µm

5 µm10 µm

20 µm

0 0.05 0.1 0.15 0.2

Strain

0

0.5

1

1.5

2

2.5

3

3.5

4

Str

ess

/ G

Pa

20um indenter

10 um indenter

5um indenter

Macroscopic compression

Hertzian contact

E = 210 GPa


Microcantilver testing of ODS steel: Stress

strain behaviour

Load

4 µm

• FIB machined micro-cantilevers in ODS and non ODS

• 20 x 4 x 5 micron with triangular cross section

• 0 dpa or 0.026 dpa Surrey at beam centre

• Stress-strain behaviour calculated using FEA model to

account for non-fixed end

C. Jones, D.E.J.Armstrong. S.G. Roberts18/12/2018 A,Kareer | RaDIATE collaboration meeting 2018

Macroscopic mechanical behaviour: ODS

steels

4 µm

C. Jones, D.E.J.Armstrong. S.G. Roberts

• Samples 1x1x12mm

• Samples tested in 4-

point bending

• Strain calculated using

digital image

correlation (DIC).


Macroscopic mechanical behaviour: ODS steels

4 µm


MaterialAverage elastic

modulus /GPa

Average yield

stress /MPa

Non ODS 281 ± 6 421 + 35− 32

ODS 299 ± 6 842 + 63− 47

No oxide

particles

Oxide

Particles

15um


Microcantilever bend testing of proton

irradiated ODS steels

4 µm

ODSNon-ODS

Non - ODS ODS

Modulus / GPa Yield Stress/ GPa Modulus / GPa Yield stress/ GPa

Unirradiated 208±5 1.05±0.07 216±20 1.30±0.4

Irradiated 224±6 1.57±0.15 196±12 1.44±0.3



Cantilever depth (μm)

Yie

ld S

tres

s (G

Pa

)

Bulk yield stress non

oxide containing

Bulk yield stress oxide

containing

Size effect observed,

Minimal effect above 5μm

thickness


irradiated ODS steels: Size effects




irradiated structural steels: Size effects

4 µm

The apparent

strength of the

microcantilevers

appeared to

drastically deviate

from macroscale

values (i.e. source

limited) for beam

depths ≤ 4.5 µm



0.0 0.5 1.0 1.5 2.00.0

0.5

1.0

1.5

Fracture

Lo

ad

, m

NBending displacement, m

• Si ions.

• Three energies 500 keV, 1 MeV, 2 MeV.

• Damaged zone ~1.4 µm deep.

• Dose ratio 3 : 2 : 1 relatively uniform damage profile.

• 0.26 dpa – 300°C and 750°C

• Microcantilevers manufactured in the interphase, fibre and matrix of both irradiated and unirradiated material

Ion irradiation of SiC-SiC composites for

nuclear applications – fracture experiments

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Dam

age level, a

.u.

Depth, m

Fiber

5 µm

Interphase

5 µm5 µm

Matrix

2 µm

Damage profile

500 nm

Dr Y. Zayachuk18/12/2018 A,Kareer | RaDIATE collaboration meeting 2018


nuclear applications – fracture experiments

Dr Y. Zayachuk

0

1

2

3

4

5

Matrix

Fiber

Fra

ctu

re toughness, M

Pa*m

1/2

Interphase

0

1

2

3

4

5

6Single-layered interphase

Unirradiated

0.26 dpa, 300oC

2.6 dpa, 750oC

Fra

ctu

re t

ou

gh

ne

ss, M

Pa

*m1/2

Interphase

Fiber

Matrix

Unirradiated Irradiated

Following the irradiations:

Interphase – toughness increases, doesn’t

noticeably change with the increase of

dose.

Fiber – toughness progressively increases

with the increase of dose.

Matrix – no clear trend.

Fracture toughness measurements –

cantilevers with straight notch.

Matrix – 4.1 MPa*m1/2

Fiber – 2.1 MPa*m1/2

Interphase – 0.8 MPa*m1/2.



nuclear applications – fracture behaviour at

temperature

Dr Y. Zayachuk

0.02 0.04 0.06 0.08 0.10 0.12 0.140

5

10

15

20

25

30

600oC

RT

Fra

ctu

re s

tre

ss, G

Pa

Fracture strain

Single-layered interphase grade

Cantilevers in the matrix

• Room temperature:

Fracture stress – 21 GPa;

Fracture strain – 13%.

• 600°C:

Fracture stress – 12 GPa;

Fracture strain – 5%.

Hot nanoindenter (Micro Materials

NanoTest Xtreme) – vacuum tests at

600°C .


Microcantilever fracture experiments: load –

unload in Tungsten

2 um

load partial-unload

chevron notch

stable crack growth & plasticity- softening of the LD curve

- displacement jumps

loading/unloading rate: 5 μN/s

B. Li, D.E.J.Armstrong, S.G.Roberts18/12/2018 A,Kareer | RaDIATE collaboration meeting 2018

Microcantilever fracture experiments:

Tungsten: Post-mortem fractography

B. Li, D.E.J.Armstrong, S.G.Roberts

2 um

22/06/16400 nm

Stable crack growth

Cleavage fracture surface

Curved crack front


Dislocation based CPFEM to simulate

Nanoindentation in implanted Tungsten

S. Das, F. Hoffmann, E.K.Tarleton


Summary

• By simulating reactor irradiation with charged particle accelerators –

provide rabid screening of radiation damage in candidate materials for

nuclear applications

• Micromechanical testing enables a large amount of data to be obtained

from small volumes of material (either active material or where only

small volumes are available or shallow irradiated layers)

• Nanoindentation is a rapid screening tool to measure the

hardness/modulus of irradiated material, microcantilevers can be used to

obtain mechanical properties beyond the yield point (full flow curve) and

fracture behaviour

• Combining micromechanical tests with dislocation based modelling

enables a full characterisation of the mechanical properties of irradiated

materials


Thank you!

Documents

Micromechanical testing of nuclear materials...damage profile. • 0.26 dpa –300 C and 750 C • Microcantilevers manufactured in the interphase, fibre and matrix of both irradiated