Copyright © 2014

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Interaction of high flux plasma with RAFM steels:

experimental and computational assessment

(Eurofer 97 and advanced grades improved by

thermomechanical treatment)

Dmitry Terentyev, Lorenzo Malerba, Athina Puype,

Anastasia Bakaeva

SCK-CEN, Belgian Nuclear Research Centre

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SCK•CEN

SCK•CEN in short

Cradle of Belgian nuclear research, applications and energy

development in Belgium

Major international player in the field of nuclear R&D

~700 staff, >50% with academic degree + 70 PhD students

Annual turnover: 140 M€

45% government support

55% contract work

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Outline

Part I

Non-equilibrium D trapping under high flux plasma exposure:

lessons learned by studying Tungsten

Computational model to consider Non-equilibrium trapping

and its connection to diffusion limited trapping

Essential information from atomistic scale

Part II

Standard and improved high-Cr grades

Available improved 9-Cr batches

Preliminary tests including high flux plasma exposure

Work plan

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Part I

Part I

Non-equilibrium D trapping under high flux plasma exposure:

lessons learned by studying Tungsten

Computational model to consider Non-equilibrium trapping and its

connection to diffusion limited trapping

Essential information from atomistic scale

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Experimental data

Typical penetration depth ~

several µm

Intensive blistering with

pronounced by-modal size

distribution

TDS spectra: 3 release stages

Intensity of the high T stage

correlates with blister pattern

400 600 800 1000 12000

1

2

3

4

D2 r

ele

ase

, 10

13 s

-1

Temperature, K

W

5*1025

m-2

(1 shot)

400 600 800 1000 12000

1

2

3

4

D2 r

ele

ase

, 10

13 s

-1

Temperature, K

W

1027

m-2

(20 shots)

[1] M. Balden, S. Lindig, A. Manhard, J.-H. You, Journal of Nuclear Materials 414 (2011) 69-72.

[1]

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Depth distribution & sub-surface trapping

Tanabe et.al.

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Depth distribution

Three zones defining distribution

~nm

~ µm

~ mm

Tanabe et.al.

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High heat flux plasma exposures

PILOT-PSI (NL, DIFFER)

Stationary Loads & ELM-like pulses

B field = 0.4-1.6 T

Water cooling 20 C

(passive control of T surface)

Flux 1023-1024 D/m2

Exposure time (sec)

Temperature at the centre of sample

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High flux plasma changes sub-surface

Compressive stresses in sub-surface region generate:

Dense dislocation network

Limited within 10-15 µm

Poly-crystalline (ITER-specification)

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High flux plasma changes sub-surface

Compressive stresses in sub-surface region generate:

Dense dislocation network

Limited within 10-15 µm

Poly-crystalline (ITER-specification)

Recrystallized at 1600C (1h)

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High flux plasma changes sub-surface

Compressive stresses in sub-surface region generate:

Dense dislocation network

Limited within 10-15 µm

Single crystal (110 orientation)

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Experimental data

SEM edge on image

TEM image: 100-200 nm of top surface

Dislocation density 1012-1014 m-2

Random grain

boundaries

100

µm

RLAGB ~ µm

~ µ

m

R ~ 100 nм

Low angle grain

boundaries (LAGB

Dislocations

TEM sample

NRA range

Mean spacing 100±20 nm

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Experimental data

Select large grain (pre-characterized) material:

Recrystallized W

Perform controlled deformation:

Tensile test on macro sample

Sub-threshold plasma exposure:

PILOT PSI, T=470K, flux ~1025 D2/m2/sec

Exposure flux: 10, 70, 500 sec i.e. up to 5×1027

PIE sequence: SEM; TEM; TDS; SEM

0 5 10 15 20 25 30 35 400

50

100

150

200

250

P YS =1.93 (kN)

P TS =4.40 (kN)

P FS =4.32 (kN)

delta u=16.91 (mm)

delta t =17.04 (mm)

sigma YS=111 (MPa)

sigma TS=253 (MPa)

sigma FS=310 (MPa)

epsilon u=34 (%)

epsilon t =34 (%)S

tre

ss (

MP

a)

Strain (%)

Sample 1, tested on 20140127,

600°C (air atmosphere), 0.2 mm/min

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

0

100

200

300

400

500

Dis

loca

tio

n d

en

sity 1

01

2 m

-2

Str

ess (

MP

a)

Strain %

Density

Stress

L=0.2 µm ; k=1

6 samples 10 mm

( ) ( )d d p pM µb

dρ/dεp = M( 1/bL − kρ)

Strain hardening model by

Meckin and Cocks [1]

[1] H. Mecking, U.F. Kocks, Acta Metal. 29 (1981) 1865; [2] H. Sheng, G. Van Oost, et.al., J. Nucl. Mat. 444 (2014) 214.

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deformed recrystallized

deformed recrystallized

Exposure 70 sec

8/15

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Experimental data

Reference

Deformed

300 400 500 600 700 800 900 10000.0

3.0x1012

6.0x1012

9.0x1012

1.2x1013

1.5x1013

1.8x1013

D2 r

ele

ase

(s

-1)

Temperature (K)

Dose: 1E26

1/m2

Deformed

Reference

Absence of the high temperature peak !

… suppression or burst of blisters ?

Possible reasons:

- Shallower nucleation depth (burst)

- Internal stress (suppression of bubble

growth by loop punching)

To be associated with dislocations ?

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Experimental data

0

4

8

12

REF-10SEC

P1

P2

P2

P1

P3

400 600 800 1000 1200

0

4

8

12

D2

rele

ase

, 1

0-1

2, s

-1D

2 re

lea

se

, 1

0-1

2, s

-1

P3

D2

rele

ase

, 1

0-1

2, s

-1

Temperature (K)

REF-490SEC

0

3

6

9

12

15

REF-70SEC

P1

P2

P3

0

4

8

12

P3

P3

P3

P2

P2

P2

P1

P1

D2

rele

ase

, 1

01

2, s

-1

DEF-10SEC

P1

400 600 800 1000 1200

0

4

8

12

D2 r

ele

ase

, 1

01

2, s

-1

Temperature, K

DEF-490SEC

0

3

6

9

12

15

D

2 r

ele

ase

, 1

01

2, s

-1

DEF-70SEC

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Experimental data

Legend:

10 100 10000.0

2.5

5.0

7.5

10.0

0.0

2.5

5.0

7.5

10.0

Pea

k inte

nsity, 10

12, s

-1

Exposure time (sec)

Reference

Deformed

Stage I Stage II Stage III

I II III

reference material

plastically deformed material

Plastic deformation promotes nucleation of H bubbles

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Experimental data

Our understanding of sub-surface trapping:

Establishment of steady-state D profile

Compensation of compressive stresses by

plastic deformation

Dislocation network acts as trapping &

nucleation source

Nano-metric bubbles get to steady-state

Pathway to validate/rationalize the

hypothesis

Characterize trapping & nucleation on

dislocations: atomistic sim.

Pass atomistic data to Mean Field (kinetic rate

theory) for non-equilibrium nucleation

Combine with classical MF (diffusion limited)

Ld

surface

D Su

rface (

nm

)

Bu

lk (

mm

)

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Hypothesis about trapping on dislocations

3/15

dislocations

H

H

H

?

Trapping at dislocations

and low angle GBs

H clustering and migration along

dislocations, trapping at random

GBs

H-dislocation interaction will

determine retention at low

temperature exposure

Experimental and theoretical ‘facts’:

H-H do not form clusters: no self-trapping like for He

Binding (H-H)=0.02; (H2-H)=0.01, (H3-H)=0.02; (H4-H)=0.03 eV [1]

Migration energy = 0.4 eV

H atom covers 30 µm within 1 ms: all permeated H immediately get

trapped

Typical µ-structure of commercial W grades:

Dislocations, sub-grains, random grains

Porosity, Impurity

Possible mechanisms of bubble nucleation without vacancies:

[1] K. Heinola, T. Ahlgren et.al., Physical Review B 82 (2010) 094102.

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Interaction of Hydrogen with screw dislocation

4/15

<111>

<110>

<112>

<111> view: Differential displacements map <111>

add H

SD

SD

H

H ΔE = -0.6 eV

ΔE ?

W W

W

a b c

Migrations:

a – in core

b – in-out

c – out core

[1] D. Terentyev et.al. Nuclear Fusion Lett. 54 (2014) 042002.

Charge density map

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Clustering of Hydrogen on screw dislocation

5/15

0.186

0.891

0.163

1.615

0.152

1.637

0.146

1.681

a)

b)

c)

d)

Size of HN

cluster

SD

length=b

H + SD -0.53

H + (H-SD) -0.59

H + (H2-SD) -0.62

H + (H3-SD) -0.39

H + (H4-SD) -0.36

Screw dislocation can trap several hydrogen atoms simultaneously, but incremental binding

energy decreases with number of trapped H atoms (from 0.6 – down to 0.25 eV).

Charge density maps [e-/Å3] Binding energy [eV]

[1] D. Terentyev et.al. Nuclear Fusion Lett. 54 (2014) 042002.

Pure W One H added

Two H added Three H added

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Nucleation of stable Hydrogen cluster on dislocation

6/15

A: Dislocation + cluster HN

B: dislocation + (Jog—HN) + Jog+

SD

SD

SD

SD

J-

J+

W

W

W

W

+

+

0 2 4 6 8 10-2

0

2

4

6

8

10

(A-B

), e

V

Number of hydrogen atoms

Z = 1B

Z = 2B

Loop punching: growth of bubbles due to excess pressure via the emission of dislocation loops

Jog punching: formation of vacancy jog to accommodate HN cluster + emission of interstitial jog

No attraction of H in

bulk – no way to form

critical concentration.

Clustering at

dislocation core leads

to jog emission at

NH=8

Classical loop punching mechanism

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Description of the model

H source

defined from

steady state

solution

H im

pla

nta

tio

n

λd

H … …

~ 10-100 μm; time ~ 1000 seconds

Fu

ll-s

cale

P

hysi

cs

of

mo

del

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Computational model results & prototype tool

Estimation of experimental conditions for H bubble

formation and surface blistering

1E22 1E23 1E24 1E25 1E26

400

500

600

700

800

900

1000

1100

1200

Tem

pera

ture

(K

)

Dislocation-mediated retention

Dislocation density = 0.5x1012

Incident flux (m-2s

-1)

Loop punching (blistering)

Xu, H.Y. JNM 2014. 447(1–3): p. 22-27.

Zayachuk,Y. et al Nucl. Fus. 2013. 53(1): p. 013013

hydrogen atom

interstitial jog

dislocation line

vacancy jog

Steady state solution for

diffusion in a field of

traps

𝐶 𝑥 = 𝐶0 exp −𝑥𝑘𝐷

𝑘𝐷 = 𝜌 for dislocations

𝑘𝐺𝐵~𝑑 for grain

boundaries

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Model of H retention

Two zoned retention

Dynamic retention in sub

surface region

Trapping on natural defects

in the bulk area

Depth

Co

nce

ntr

ati

on

C s

ub

su

rface

C bulk

10 μm

10 - 1000 μm

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Release of trapped H

Release from bulk and

diffusion to the surface

Concentration of H is

defined by trapping and

growth at low angle

grain boundaries

Binding energy of 2.0 eV

Depth

Co

nce

ntr

ati

on

CS =

1.1

*10

-5

CB = 3.92*10-7

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Part II

Part II

Standard and improved high-Cr grades

Available improved 9-Cr batches

Preliminary tests including high flux plasma exposure

Copyright © 2014

SCK•CEN

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 1 10 100 1000

Radiation effects in structural materials

Dose received (dpa)

Ho

mo

log

ical

Tem

pera

ture

(T

/TM)

> 0.1 dpa, <0.35 TM

Radiation hardening and embrittlement

>> 10 dpa, T>0. 5 TM

If He>100appm

He embrittlement at GB

(intergranular fracture)

>10 dpa, 0.3TM<T<0.6TM

Phase instabilities from

radiation-induced

segregation and

precipitation

Volumetric void swelling

(dimensional instability)

>10 dpa, 0.35TM<T<0.45TM

Irradiation creep

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Standard and improved high-Cr grades for Fusion Appl.

One principle problem of 9-Cr steels: low temperature

embrittlement

Second one: creep, limiting application up to 650C

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Standard and improved high-Cr grades for Fusion Appl.

Plastic flow instability

Dynamic absorption of defects

Drastic softening of matrix

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Development of new improved Eurofer grades

Composition C Mn Si Cr W N B Zr V Ta

Target 1 0.020 0.400 0.030 8.500 2.500 0.070 0.500 0.500

Target 2 0.030 0.400 0.030 9.000 1.000 0.070 0.500 0.400

Target 3 0.030 0.400 0.030 9.000 1.000 0.070 0.250 0.150

Target 4 0.050 0.400 0.030 9.000 1.000 0.020 0.300 0.200

Target 5 0.050 0.400 0.030 9.000 1.000 0.020 0.005 0.300 0.200

Target 6 0.050 0.400 0.030 9.000 1.000 0.040 0.005 0.300 0.200

Target 7 0.070 0.400 0.030 9.000 2.000 0.070 0.300 0.200

Target 8 0.030 0.400 0.030 9.000 2.000 0.070 0.300 0.200

Target 9 0.050 0.400 0.030 9.000 2.000 0.070 0.300 0.200

Target 10 0.020 9.000 1.000 0.020 0.250 0.150

Target 11 0.020 8.000 1.000 0.020 0.250 0.150

Selection of compositions (casting done by OCAS)

Comp. for investigation on Low Temperature EUROFER

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How to reduce DBTT and enhance creep resistance

Modify precipitate distribution

Change ratio MC to MX

Modify H-treatment

Modify Composition

V and Nb:

Stabilize grains

Reduce grain coarsening

Increase ductility

9Cr remains optimum

Less Cr – poor corrosion resist

More Cr – hardening (α’) and

δ-ferrite

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Advanced Thermo-Mechanical Treatment: TMT

Wt% C Cr Mn N2 Ta V W

0.09Ta 0.11 9.13 0.40 0.011 0.09 0.23 1.04

0.11Ta 0.11 9.24 0.39 0.011 0.11 0.23 1.07

Experimental Procedure Composition of the 9wt% Cr RAFM steel batches 2014

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Advanced Thermo-Mechanical Treatment: TMT

1

2

0.09Ta

0.11Ta

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Effect of Heat Treatment

0

5

10

15

20

25

30

35

40

45

50

0.09wt%Ta_LFRT 0.11wt%Ta_LFRT

DA

V,E

q [

mic

ron

]

Effect of Ta content on grain size

TMT+HT

TMT

PAG

Block

Substructure

PAG

Block

Substructure

M23C6

0.5 µm M23C6

0.5 µm

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Precipitation of Vanadium and Tantalum

-10

10

30

50

70

90

110

130

150

170

02468

1012141618202224

850 900 950 1000 1050 1100

DA

V,

Eq

[n

m]

DA

V,

Eq

[m

icro

ns]

Annealing temperature [°C]

Martensite structure and carbide

measurements

Carbides

Block size

00.010.020.030.040.050.060.070.080.09

0.10.11

TMT Anneal Q&T

wt%V & wt%Ta precipitated

Wt% Cr Ta V

0.09Ta 9.3 0.086 0.22

Fraction of elements that precipitate

at different stages of Q&T treatment

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Mechanical properties of improved grades

300

350

400

450

500

550

600

650

700

-100 0 100 200 300 400 500 600

σU

TS

[M

Pa]

Temperature [°C]

Ultimate tensile strength

880°C

920°C

980°C

1050°C

0

2

4

6

8

10

-200 -160 -120 -80 -40 0 40 80

Dia

l en

erg

y [

J]

Temperature [°C]

I196C - 880°C - KLST specimens

DBTT

Q&T: 880°C tested at -140°C

Inte

rmed

iate

level

Q&T: 880°C tested at -120°C

Up

per

level

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Preliminary results

400 500 600 700 800

1E10

1E11

1E12

1E13

1E14

D2

re

lea

se

(1

/s/c

m2)

Temperature (K)

Exp 430-450 K

T91 (T1) ref

Eurofer 97(e3) ref

E97 non-exposed

High Flux D plasma exposure at Pilot PSI (NL)

Surface temperature: 430-450K

Flux ~1024 D/m2

Fluence ~5×1025 D/m2

Reference measurements

T91; E97

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Preliminary results

Exposure at 430-450K: Blisters and individual grains

NRA/TEM analysis to be done to confirm sub-surface retention & µ-structure

modification

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Preliminary results

Exposure at 430-450K: reference vs. advanced grade

Chemical analysis to be done to confirm observations of Carbides at surface

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Preliminary results

400 600 800 1000 1200

0.0

2.0x1011

4.0x1011

6.0x1011

8.0x1011

1.0x1012

1.2x1012

1.4x1012

1.6x1012

D2 r

ete

ntion (

1/s

/m2)

Temperature (K)

E97 exp. 960K

E97 non exposed

High Flux D plasma exposure at Pilot PSI (NL)

Surface temperature: 950-970 K

Flux ~1024 D/m2

Fluence ~5×1025 D/m2

Reference measurements

E97;