In-situ microstructural observation of radiation damage in nickel produced by energetic heavy particles

597

Journal of Nuclear Materials 122 & 123 (1984) 597-601 North~Ho~d, Amsterdam

IN-SITU MICROSTR~CTURA~OBSERVATION Of RABIATION DAMAGE IN NICKEL PRODUCED BY ENERGETIC HEAVY PARTICLES* Shiori ISHINO, Koji FUKUYA,** Takeo MUROGA, Naoto S~KINURA and Hiroshi KAWANISHI Department of Nuclear engineering,The Univeristy of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Microstr~c~uralchanges during 300 and 400 keV Ar+ irradiation in pure nickel between 300 and 773 K have been observed in-situ in an electron microscope. Some of the observations are recorded on a video tape. Various phenomena characteristicof cascade damage have been observed. Clustering of point defects is influenced strongly by the presence of point defects s’tnks: surfaces, pre-existing dislocations,loops and cavities. Wedge-shapedspecimens are utilized to sort out the complex behavior of microst~ctural evofutfon. Of great interest is the fact that under certain conditions, metastable defect clusters with a very short lifetime are formed durina irradiation at 773 K. The implication of these observationsto fusion neutron damage modeling is discussed. similar to that produced by 14 MeV neutrons.

I. INTROBUCTION Because of the nonexistenceof intense fusion

Cascade damage has been known to have a

neutron sources, it is of great importance for

pronounced effect on void swelling by the for-

radiation damage studies of fusion reactor mate-

mation of vacancy clusters. The structure of

rials to establish correlation of data obtained

cascades and subcascades has been studied

with existing radiation sourcest ffssion reac-

extensively by observing collapsed vacancy

tors, accelerators, HYEMs and so on, to the case

loops.4 However, recent observations indicate

of fusion reactors. The correlation should be

that there are other types of phenomena which

based on physical modeling of microstructural

are characteristicsof cascade damage, espe-

and microchcmical evolution during irradiation.

cially when the cascade is produced near point

However, the data, except those obtained by

defect sinks.3,B*S Some of these phenomena

HVEMs, are "snap shots.fi Continuous observation

have been treated by computer simulation calcw-

of damage evolution during heavy particle irra-

lations.7 The present study will proceed along

diation is highly desirable, since cascade

this line in more detail by using pure

damage produced by energetic heavy particles is

specimens. Emphasis will be placed on vacancy

one of the characteristicfeatures of fusion

loop for~tion near defect sinks such as sur-

neutron radiation damage.

nickel

faces and dislocations. The results will be

We have built a facility for in-situ observa-

compared with those obtained in pure aluminum

tion of radiation damage by heavy particles by

as well as those by computer simulations.

combinfng a 400 kV heavy ion acceleratorwith a

These results will provide a useful basis for

200 kV electron microscope.I-3 The facility

analyzing the results obtained by 14 MeV neu-

provides heavy ions with a range of PKA energies

tron irradiationsusing the RTNS-II facility.

*This work supported by Grant-fn-Aid for Pusfon Research from the Ministry of Education, Science and Culture. **Present address: Toshiba Corporation* Nuclear engineering Laboratory, Kawasaki, Japan. 0022-3115/84/%03.00 (North-Holland

0 Elsevier Science Publishers Physics Publishing Division)

B.V.

598

S. Ishino et al. / Observation of radiation damage in nickel

2. EXPERIMENTAL

TECHNIQUES

3. RESULTS AND DISCUSSION

2.1 Specimen The material

3.1 Cascade-dislocation used in this study is MARZ

Figure 1 shows microstructural

Grade nickel foil of 50 pm thickness with 99.995% purity,

purchased

search Corporation. weight ppm:

Re-

The specimens

CR:~. at 1173 K for 1

hr in a vacuum of 3x10-5 Pa and then cut out into discs of 3 mm in diameter. electropolished Tenupole

with a solution

electropolishing

by a apparatus

of 20 vol.% of perchloric

facility

elsewhere.I,6

2.3 Irradiation

us to

goniometer

microscope,

is observed

in self-ion-

at room temperature,3

in

has been

to be of vacancy type because with electrons

is irradiated

observation

placed inside the electron with 300 or 400 keV

on the specimen

The beam intensity

simulation

subse-

annihilates

also suggests

growth of vacancy clusters especially

the equivalent

temperatures

The ion

at an angle of

enhanced

near dislocations,

in the case of cascade

damage, at

where the vacancy

becomes finite but not too high.7

fact, in the present experiment,

on a high temperature

Ar+ ions at 300, 573, 673 and 773 K.

enon is most prominent

at 673 K, being much

less at 573 K because of low vacancy mobility as well as at 773 K because of the high emission rate of vacancies interesting

from clusters.

to note that according

It is

to tempera-

ture scaling by T/Tm or by T/Emv, 673 K in

cal-

using the EDEP-1 code are given in

Table 1. Dislocations fraction

are imaged under two beam dif-

conditions

reflections.

using (111) or (220) type

The thickness

of the specimen

estimated

by counting

Table 1.

Range and damage parameter

I nc

r

ent

equal thickness

is

fringes. for nickel. eV

8 x 1O-4

8.6 x 10-4

Depth of damage peak, nm

65

90

Mean range, nm

110

150

Peak damage rate in d s/s per 1 mA/m B

In

such a phenom-

ranges from 0.18 to

The range and damage parameters

2.0 mA/m2.

aluminum

quent irradiation

mobility

changes.

and microscopic

mounted

single-tilt

culated

facil-

A video

parts of the specimen and to

record rapid microstructural

70-90".

irradiated

A computer

system has been added, enabled

beam is incident

apparent only on one side of the dislocation.

estimated

acid

The details of the in-situ observation ity have been described

The specimen,

and a loop denuded zone becomes

the cluster.

2.2 In-situ observation

observe thicker

the

after starting

which the nature of the cluster

in acetic acid.

recording

disloca-

tion line immediately

A similar phenomenon

They are then

for TEM observations

twin-jet

Loops are formed near a pre-existing

irradiation,

are annealed

at 673 K with

a peak dose rate of 5x10-4 dpa/s to 0.1 dpa.

are, in

C:15, O:lZ, N:5, Mg:2, Si:17,

Cr:2, Fe:20, Sn:1.2,

changes

during a 400 keV Ar+ irradiation

from Materials

Major impurities

interaction

Fig. 1.

Microstructural changes of Ni irradiated with 400 keV Ar+ at 673 K.

S. Ishino et al. / Observation

nickel corresponds

to room temperature

num, where Tm is the melting

1.1 and 0.62 eV are assumed

in alumi-

temperature

and Emv is a vacancy migration

energy,

of radiation

for nickel and alumi-

that the phenomenon

is caused by cascade-dislocation that the clusters vacancy

shown in Fig. 1 interaction

around the dislocation

type formed due to preferential

tion of interstitials a mechanism copper.8

However,

to dislocations irradiation

absorp-

by the dislocation.

has been confirmed

and

are of

Such

in HVEM-irradiated

small loops being attached

in nickel after 14 MeV neutron

at 473 K using RTNS-II have been con-

firmed to be of interstitial

type.9

low dose neutron

in copper and other

metals

irradiation

has indicated

dislocations

Similarly,

that the loops around the

are of interstitial

type.10

fore, the nature of the small clusters

There-

still

of surface sinks

As a measure free zone width irradiation

is plotted

the thickness

importance

as a function of

distance

tion is 34.9 nm.

evolution

depends strongly especially

This indicates

on

an increasing

of the effect of the surface at

Here,

from the (2001 reflecin part

is thinner than 150 nm,

do not have an area1 density proportional the specimen

thickness.

to

Instead, the area1

density shows a maximum

at the region about 70

nm thick, and decreases

gradually

about 150 nm.

to zero at

The loops are considered

of the vacancy type produced effect of the surface

to be

by a strong sink

for interstitials.

On the other hand, in a region with a thickness greater than 200 nm, interaction interstitial dislocation

arrangement,

which

is common to

Between the two regions, interaction

as described

recombination,

location

However,

enhances

mostly

if the dislocation the presence

formation.

by

were

of the dis-

the local interstitial

thereby enhancing

vacancy cluster

in

In this region, point

defects might have been annihilated

strength,

among

loops gives rise to a tangled

bulk specimens.2

not present.

at

of a

with 400 keV

The loops, appearing

of the specimen which

mutual

as shown in Fig. 2.

of the specimen,

high temperature.

defect-

irradiated

Ar+ to a peak dose of 0.1 dpa at 673 K.

3.1 is most prominent.

of surface sink strength,

temperature

Microstructural

specimen

dislocation-cascade

remains to be clarified. 3.2 Effect

even deep inside the speci-

Figure 3 shows the microstructure

wedge-shaped

the extinction

num, respectively. We believe

higher temperatures men.

in K

for which

599

damage in nickel

sink

the chance of Here again, the

small loops are formed only on one side of the dislocation.

6or-----l

Dependence thickness

200

400

600

of microstructural

changes

on the

at 773 K is surmnarized in Table 2.

800

IRRADIATIONTEMPERATURE (K) Fig. 2.

Fig. 3. Temperature zone width.

dependence

of defect-free

Microstructure of wedge-shaped specimen of nickel irradiated with Ar+ at 673 K.

600

S. ishino

Table 2. Region

et al. / Ohservation

Change of microstructure

Thickness,nm

I

<

Defect

80-100

thicker

IV

Pusslble -.

than

III

at 773 K.

s,echanls,,,

~_ ._ __

___.___ vacancy stage of defect

Annihilation of fnrerstirials at surfaces facilitates formation of metastsble vacancy clusters originating from a high vacancy concentration in a cascade.

Interstitial loop growth Lo 50s60nm, then diminishing I” size. finally disappearing. Form&Ion of cavities afLer Iloop size reaches maXll”U,“. Formation of unstable vacancy clusters at a dose when I-loop size reaches maxuwm.

< 120 -200

III

damage in flickel

during 300 keV Ar+ irradiation

structure

Forniatlon of unstable clusters at the early Irradiation. No stable

<100~150

II

of radiation

Super~satilratlon cavity formdtlon.

Of

“aCPllc,es

results

I,,

‘This provides new sinks for both 1 and V and the stability of I-loop 15 gradually lost by abs”rblr,g vacancies. 00s.~ of appearance of unstable vacancy clusters corresponds to maxlmun, vacancy supersaturation.

Growth of lnterstltlal loops, some of which interact with each other to form coarse tangllng.the others reach maximum in size and finally dlsCavities or Ar bubbles appear. and unstable vacancy clusters are observed as in Region II.

Some of the dlslo‘atlon-like contrast the coarse tangling may arise from surface marking, produced by prevention of annihllatlon of dislocations moved the surface by oxide layer. Other mechanisms are the sane as Region II.

Most of the interstitial loops grow to form dense dislocation tangling as shown I” an ordinary bulk specimen.

As

1”

a bulk

sprclmen,

formatlo,,

of

in

to

I-

loop prevails. Density of tangled dislocations depends on number density of I-loops. When the tangling is dense. vacancy supersaturation 1s kept low and less chance of cavity formation results.

Two things may be pointed out as interesting

They cannot be attributed

phenomena;

since they can also be observed

in region II.

The frequency

cluster

first, the interstitial

a certain

size and then become unstable

the growth of cavities. a function

a formation

of irradiation

of metastable

will be described 3.3 Metastable Metastable

time for two individ-

been observed

are only observed

taken every 0.5 set from

screen.

The specimen

peak damage rate of l.2xlO-3

dpa/sec at 773 K.

clusters vacancy

As

small dots with size less

than about 5 nm are generated and disappear

is in

with the

peak damage is 2.3 dpa.

shown by the circles,

with a typical

in a flashy way lifetime of one

A few of them survive

We believe

Figure 5 shows

and is being irradiated

The accumulated

5, the frequency

for 15-30 seconds.

that they are metastable

vacancy

formed by a sudden increase of concentration

triggered

by a cascade.

forma-

to be propor-

In the case of

of the occurrence

of

the event is of the order of 1014/m2 set, which

sec.

at 673 K or below.

the video monitor

Figure

vacancy cluster

No such event has

confirmed

tional to the beam current.

is roughly

at 773 K.

to argon clusters

of the metastable

tion is qualitatively

in the next subsection.

a series of photographs

region III,

A second point is

vacancy clusters which

defect clusters

during irradiation

due to

Change of loop size as

ual loops is shown in Fig. 4.

second.

loops grow to

l/100 of the flux of 8.7xlOI5/m2

The local conditions

for the clusters

to

be formed may require that a strong sink bias for interstitials

should exist, and that the

size of the cascade should be large enough so that the clustering vacancies

could surmount

the loss of

by thermal emission.

3.4 Correlation

to fusion reactor conditions

The results described

above show that phe-

nomena in heavy ion irradiation

can be similar

to those observed

in neutron irradiated

rials, indicating

that the basic cascade pro-

cesses are similar

mate-

in both irradiations,

that the ion irradiation applied to mechanistic

studies.

However,

have to be well aware of the limitations; example,

the distortion

and

can be successfully we for

of defect distributions,

S. Ishino et al. / Observation

of radiation

601

damage in nickel

0.2

200

0

600

400

pm

800

IRRADIATION TIME (set)

Change of individual

Fig. 4.

especially implanted ments.

loop size at 773 K.

at high temperature,

the effect of

atoms, helium and other alloying

In the case of alloys, segregation

alloying

elements

of radiation

is one of the central

effects

eleof

issues

and must be clarified

Fig. 5. Unstable clusters of point defect (circled constant) observed during bombardment width 300 keV Ar+ ions at 773 K in Ni. The specimen has been bombarded to 2.3 dpa. The displacement rate is 1.2~10'~ dpaJsec. The time interval between snap shots is 0.5 sec.

in

2. K. Fukuya, H. Kawanishi and S. Ishino, J. Nucl. Mater. 1038104 (1981) 1385. 3. T. Muroga, K. Fukuya, H. Kawanishi and

the near future.

S. Ishino, J. Nucl. Mater. 1349. 4.

1036104

(1981)

CONCLUSIONS

(1) Cascade-dislocation

in nickel

at 673 K is similar

to that in aluminum

at 300 K, suggesting

similar phenomena

occurs (2)

interaction

at the same homologous

The stability

depend on the total sink strength

itself as exemplified interstitial (3)

deter-

defect structure by the behavior

of

loops at 773 K.

At 773 K, metastable with a typical

vacancy clusters

lifetime

of 1 second have

been found by video recording

Yamada Conf. V, Point Defects and Defect Interactions in Metals, eds. J. Takamura, M. Doyama and M. Kiritani (Univ. of Tokyo Press, 19821 799.

temperature.

of the defect is shown to

mined by the evolved

4. B. L. Eyre and C. A. English, Proc.

techniques.

6. S. Ishino, H. Kawanishi, K. Fukuya and T. Muroga, IEEE Trans. Nucl. Sci. NS-30 (1983) 1255. 7. T. Muroga and S. Ishino,

J.

Nucl. Mater.

117 (1983) 36.

8. M. Suehiro, N. Yoshida and M. Kiritani, Proc. Yamada Conf.,

ibid., 795.

9. M. Kiritani, N. Yoshida and S. Ishino,

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5. C. A. English, J. Nucl. Mater. 108&109 (1982) 104.

this volume. and K. Grant-

1979).

10.

I. M. Robertson, M. L. Jenkins and C. A. English, J. Nucl. Mater. 108&109 (1982)

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