The use of both neutron and ion irradiation to show the

microstructural origins of strong flux-sensitivity of void swelling in

model Fe-Cr-Ni alloys

T. Okita, N. Sekimura and T. IwaiUniversity of Tokyo, Tokyo, Japan

F.A. GarnerPacific Northwest National Laboratory, Richland, WA, USA

Outline of this presentation

• Neutron irradiation experiment on Fe-15Cr-16Ni conducted in FFTF fast neutron reactor at ~ 426 ˚C at seven dose rates between 8.9 x 10-9 to 1.7 x 10-6 dpa/sec.

• Ion irradiation experiment on the identical Fe-15Cr-16Ni conducted with 4 MeV Ni3+ ions at 300, 400, 500, and 600 ˚C at three different dose rates between 1.0 x10-3 to 1.0 x 10-4 dpa/sec.

Neutron irradiation ; FFTF/MOTA

FFTF CORE ABOVE CORE

BELOW CORE

Distance from midplane (cm)

dp

a/s

ec in

sta

inle

ss s

tee

ls 1 2 3 4 5 6 7 8BC

0 25 50 12075 100 150-100 -75 -50 -25

10-7

10-5

10-6

10-8

10-9

Irradiation at seven positions in, below and above the core.

2 cycles of irradiation in FFTF cycles #11 and #12 to achieve two dose levels at each dose rate.

Materials Open Test Assembly

Multiple specimens at each condition.

Dose Rate, dpa/sec Dose, dpa Temperature, ˚C

#11 #12 #11 #11 & #12 #11 #12

1.7 x 10-6 1.4 x 10-6 43.8 67.8 427 408

7.8 x 10-7 9.5 x 10-7 20.0 32.4 390 387

5.4 x 10-7 8.4 x 10-7 14.0 28.8 430 424

3.1 x 10-7 3.0 x 10-7 8.05 11.1 411 410

9.1 x 10-8 2.1 x 10-7 2.36 6.36 430 431

2.7 x 10-8 6.6 x 10-8 0.71 1.87 434 437

8.9 x 10-9 2.2 x 10-8 0.23 0.61 436 444

Neutron irradiation conditions

Constant time experiment 2.59 x 10 7 sec (Cycle #11)

1.76 x 10 7 sec (Cycle #12)

426 ± 18 ˚C

Previous studies for the effect of dose rates

For austenitic alloys, there are not enough studies, with sufficiently detailed databases for high dose rate irradiation.

Typically, the effects of dose rate have been investigated covering less than one order difference in dose rate.

Microstructural response depending on such a small difference in dose rate might lie within the experimental error bands, however.

Alloys Authors ReactorsDifference in

dose rate

Austenitic alloys

Muroga et al. RTNS-II < 1 order

Muroga et al. RTNS-II, JOYO > 2 orders

Neustroev et al. BOR-60 < 1 order

Garner et al. EBR-II, FFTF < 1 order

Lewthwaite et al. DFR > 1 order

Garner et al. BOR-60 > 1 order

Garner et al. BN-350 < 1 order

Porter et al. EBR-II < 1 order

Kruglov et al. BR-10 < 1 order

Kozlov et al. BN-600 < 1 order

Seran et al. Rapsodie < 1 order

Seran et al. Pheonix < 1 order

Grossbeck et al. BR-2 < 1 order

Cole et al. EBR-II < 1 order

Walters et al. EBR-II < 1 order

Schneider et al. Rapsodie < 1 order

Allen et al. EBR-II > 1 order

Garner et al. EBR-II < 2 orders

A533B

Fe-Cu alloys

Nanstad et al. MTR, HFIR > 1 order

Kitao JMTR < 1 order

Yanagida KUR < 1 order

Ferritic alloys Garner et al. EBR-II, FFTF < 1 order

Nickel Alloys Garner et al. EBR-II < 1 order

Previous studies for the effect of dose ratesAlloys Authors Reactors

Difference in dose rate

Austenitic alloys

Muroga et al. RTNS-II < 1 order

Muroga et al. RTNS-II, JOYO > 2 orders

Neustroev et al. BOR-60 < 1 order

Garner et al. EBR-II, FFTF < 1 order

Lewthwaite et al. DFR > 1 order

Garner et al. BOR-60 > 1 order

Garner et al. BN-350 < 1 order

Porter et al. EBR-II < 1 order

Kruglov et al. BR-10 < 1 order

Kozlov et al. BN-600 < 1 order

Seran et al. Rapsodie < 1 order

Seran et al. Pheonix < 1 order

Grossbeck et al. BR-2 < 1 order

Cole et al. EBR-II < 1 order

Walters et al. EBR-II < 1 order

Schneider et al. Rapsodie < 1 order

Allen et al. EBR-II > 1 order

Garner et al. EBR-II < 2 orders

A533B

Fe-Cu alloys

Nanstad et al. MTR, HFIR > 1 order

Kitao JMTR < 1 order

Yanagida KUR < 1 order

Ferritic alloys Garner et al. EBR-II, FFTF < 1 order

Nickel Alloys Garner et al. EBR-II < 1 order

It has been possible to achieve wider ranges of dose rates from comparison of results obtained in two or three different reactors.

However, difficulties remain in such comparative studies because of simultaneous changes in other important irradiation parameters, such as the neutron energy spectrum or temperature history.

Dose Rate, dpa/sec Dose, dpa Temperature, ˚C

#11 #12 #11 #11 & #12 #11 #12

1.7 x 10-6 1.4 x 10-6 43.8 67.8 427 408

7.8 x 10-7 9.5 x 10-7 20.0 32.4 390 387

5.4 x 10-7 8.4 x 10-7 14.0 28.8 430 424

3.1 x 10-7 3.0 x 10-7 8.05 11.1 411 410

9.1 x 10-8 2.1 x 10-7 2.36 6.36 430 431

2.7 x 10-8 6.6 x 10-8 0.71 1.87 434 437

8.9 x 10-9 2.2 x 10-8 0.23 0.61 436 444

Neutron irradiation conditions in this study

• Fast neutron irradiation in FFTF reactor

More than two orders difference in dose rates achieved in one reactor.

Active temperature control system at ± 5 ˚C, using a variation of He / Ar gas ratio in the gas gap.

Cavity microstructure in Fe-15Cr-16Ni

100 nm

1 cycle

2 cycles

0.23 dpa 8.05 dpa 43.8 dpa

0.61 dpa 11.1 dpa 67.8 dpa

8.9 x 10-9 dpa/sec 3.1 x 10-7 dpa/sec 1.7 x 10-6 dpa/sec

Cumulative Dose (dpa)

Fe-15Cr-16Ni, SA408 - 444 ˚C

• Lower dose rate enhances swelling by shortening the incubation dose.

• The steady state swelling rate is not affected by the difference in dose rate.

Enhanced swelling at lower dose rate

Sw

elli

ng

(%

)

17 x 10-7

dpa/sec

0

10

20

30

0 10 20 30 40 50 60 70

387 ˚C

390 ˚C

7.8

5.4

3.10.9

- Density change and microscopy data -

1%/dpa

Cumulative Dose (dpa)

Sw

elli

ng

(%

)

Enhanced swelling at lower dose rate

0123451 %/dpa9.1x 10-8

dpa/sec

02468102.70.89• The steady state swelling

rate is also observed at dose rates as low as < 10-7

dpa/sec and irradiated less than 1 dpa.

• The incubation dose of swelling can therefore vary from < 1dpa to > 45 dpa when the dose rate varies over more than two orders of magnitude.

Fe-15Cr-16Ni, SA430 - 444 ˚C

Cumulative Dose (dpa)

Sw

elli

ng

(%

)

Enhanced swelling at lower dose rate

0123451 %/dpa9.1x 10-8

dpa/sec

02468102.70.89

Incubation Dose

• The steady state swelling rate is also observed at dose rates as low as < 10-7

dpa/sec and irradiated less than 1 dpa.

• The incubation dose of swelling can therefore vary from < 1dpa to > 45 dpa when the dose rate varies over more than two orders of magnitude.

Fe-15Cr-16Ni, SA430 - 444 ˚C

Dose Rate (dpa/sec)

Incu

ba

tio

n D

ose

(d

pa)

Strong effect of dose rate on incubation dose

• The incubation dose of swelling is almost linearly proportional to dose rate.

0.1

1

10

100

10 -8 10 -7 10 -6 10-5

(dpa/sec)0.99

390 / 387 ℃

Fe-15Cr-16Ni, SA408 - 444 ˚C

Enhanced dislocation evolution at lower dose rate

• Lower dose rate enhances dislocation evolution.

• This effect arises primarily from the enhanced loop growth at lower dose rate. Cumulative Dose (dpa)T

ota

l Dis

loc

atio

n D

en

sit

y (

x1

014 m

-2) 10

8

6

4

2

00 10 20 30 40 50 60 70

17 x 10-7 dpa/sec

5.4

7.8

3.1

0.9

0.27

0.089

387 ˚C

390 ˚C

Fe-15Cr-16Ni, SA408 - 444 ˚C

Dose Rate (dpa/sec)

Lo

op

De

nsi

ty (

m-3)

Dose rate dependence of loop density

At relatively low doses, the loop density is proportional to (dpa/sec)1/2.

This is agreed with the previous analysis that the saturated loop density is proportional to (dpa/sec)1/2.

Dislocation loop density is not proportional to (dpa/sec)1/2 at doses higher than ~ 10 dpa, because loop unfaulting had occurred.

1023

1022

1021

1020

10-9 10-8 10-7 10-6

(dpa/sec)1/2

0.23 dpa

0.61 0.71

1.872.36

6.36

8.05

11.1

14.0

Fe-15Cr-16Ni, SA410 - 444 ˚C

Effects of dose rate on dislocation evolution

• Loop line lengths seem to increase with dose below 10 dpa, and decrease thereafter, because of loop unfaulting and network dislocation formation above 10 dpa.

• There seem to be little effect of dose rate on these remaining loop line lengths, because some loops at lower dose rate grow large enough to be unfaulted and become network dislocation.

5.4 x 10-7

dpa/sec

3.1

0.91

3.1

0.089

0.27

0.91

0.27

0.089

Loop Network

0

2

6

8

10

4

0 10 20 0 10 20 30

Cumulative Dose (dpa)

Dis

loc

ati

on

De

ns

ity

(x

101

4 m

-2) 5.4 x 10-7

dpa/sec

Effects of dose rate on dislocation evolution

• The rate of network dislocation evolution is enhanced at lower dose rates, caused by the enhanced nucleation and growth of dislocation loops.

• Therefore, the total dislocation density, which includes both network dislocations and loop line length is important to understand the dose rate effects on microstructural evolution in the higher dose region.

5.4 x 10-7

dpa/sec

3.1

0.91

3.1

0.089

0.27

0.91

0.27

0.089

Loop Network

0

2

6

8

10

4

0 10 20 0 10 20 30

Cumulative Dose (dpa)

Dis

loc

ati

on

De

ns

ity

(x

101

4 m

-2) 5.4 x 10-7

dpa/sec

Cumulative Dose (dpa)

Ca

vit

y D

en

sity

(x1

022 m

-3)

Enhanced cavity nucleation at lower dose rates

At a given dose rate, cavity density increases with dose.

Low dose rates enhance cavity nucleation.

Both the absolute value and the rate of increase in cavity density are higher at lower dose rate.

17 x 10-7 dpa/sec

0

0.5

1.0

1.5

2.0

0 10 20 30 40 50 60 70

387 ˚C

390 ˚C

3.1

5.4

7.8

0.9

0.27

0.089

Fe-15Cr-16Ni, SA408 - 444 ˚C

Cavity Diameter (nm)

Cav

ity

Den

sity

(x1

021 m

-3)

Effect of dose rate on cavity size distributionat 7.2 ± 0.8 dpa

• Larger cavities can be observed only at the lower dose rate, indicating that cavity growth is also enhanced at low dose rate.

• A higher density of small cavities can be observed at the lower dose rate, indicating continuous operation of cavity nucleation.

Higher Dose RateLower Dose Rate8.05 dpa

3.1 x 10-7 dpa/sec

6.36 dpa

0.91 / 1.5 x 10-7 dpa/sec

0 543211020300102030400

Cumulative Dose (dpa)

Av

era

ge

Ca

vity

Dia

me

ter

(nm

)

Average cavity diameter is not a good measure of dose rate effects

At similar cumulative dose levels, cavities with larger diameter caused by enhanced cavity growth at lower dose rate are offset by the small cavities caused by continuous cavity nucleation, resulting in little effect of dose rate on average diameter.

50

40

30

20

10

00 10 20 30 40 50 60 70

17 x 10-7 dpa/sec

5.4

7.8

3.10.9

0.27

0.089

387 ˚C

390 ˚C

Fe-15Cr-16Ni, SA408 - 444 ˚C

01020300102030400 54321

Interpretation of cavity size distribution

2 cycles irradiation6.36 dpa, 0.91 / 2.1 x 10-7 dpa/sec

1 cycle irradiation2.36 dpa, 0.91 x 10-7 dpa/sec

Cavity Diameter (nm)

Ca

vit

y D

en

sity

(x1

021m

-3)

Recent Cavities

Earlier Cavities

Recent cavitiesCavities nucleated during the 2nd cycle of irradiation

Earlier cavitiesCavities nucleated during the 1st cycle of irradiation

Cumulative Dose (dpa)Dia

me

ters

of

“Ea

rlie

r C

avit

ies

” (n

m)

Enhanced cavity growth at lower dose rates

It is clearly observed that cavity growth is strongly enhanced at lower dose rates.

At lower dose rate, accelerated dislocation evolution provides sufficient vacancies, resulting in enhancements of both cavity nucleation and growth.

Earlier cavitiesCavities nucleated during the 1st cycle

0

10

20

30

40

50

0 10 20 30 40 50 60 70

1.7 x 10-7

dpa/sec

5.4

7.8

3.10.9

0.27

0.089

387 ˚C

390 ˚C

Fe-15Cr-16Ni, SA408 - 444 ˚C

Irradiation Dose (dpa)

Sw

elli

ng

(%

)

Effects of dose rate on swelling in ion-irradiated Fe-15Cr-16Ni

300˚C101

100

10-1

10-2

10-3

10-1 100 101

400˚C 500˚C 600˚C

4 MeV Ni3+ irradiation shows the dose rate effect operates at all temperatures.

1.0 x 10-4 dpa/sec

4.0 x 10-4 dpa/sec

1.0 x 10-3 dpa/sec

10-1 100 101 10-1 100 101 10-1 100 101 102

Summary

• Lower dose rate increases swelling by shortening the incubation dose for swelling. The incubation dose is proportional to dose rate.

• The steady state swelling rate is not affected by the difference in dose rate.

• Lower dose rate enhances network dislocation formation. This is caused by enhanced loop growth and unfaulting.

• At lower dose rate, enhanced dislocation evolution increases sink strength of interstitials. This is the major reason to enhance cavity nucleation and growth at lower dose rate.