Damage structure in Al2O3 single crystal irradiated with He-ions

366

Journal of Nuclear Materials 155--157 (1%X) 366 -371 North-Holland. Amsterdam DAMAGE

STRUCTURE

IN Al,O,

Y. KATANO, H. OHNO and

SINGLE

CRYSTAL

IRRADIATED

WITH

He-IONS

H. KATSUTA

Depurrment of Fuels and Materials Research, Japun Alomic Energy Research Institute, Tokai-mura, lharuki-ken 31 Y-1 I. Japan

The microstructures of single crystal alumina examined

in an electron

microscope.

irradiated with He-ions at 1023 to 1223 K to a dose of 1 x IO”’ He/m’ were In as-irradiated samples. high densities of defect clusters with 6-7 nm in an average size

were formed and these were almost independent of the irradiation temperatures. The clusters were revealed to be dislocation loops of interstitial type. In the sample annealed for 1 h at 1223 K after the He-irradiation at 1023 K, five distinguishable features appeared: high density of small cavities with 7 nm in an average size. highly coalesced cavity channels, small sized spherical precipitates of aluminium. large size alu~nium islands. and dislocation loops with an average size of 70 nm. These features were revealed to be strongly dependent on the helium irradiation damage and also on the distances of damaged layers from foil surface during annealing.. 03

1. Introduction Alumina is a candidate material for such as insulators and rf windows of fusion reactors because of its good structural and electrical properties. Although much work on radiation effects of alumina has been done by irradiations with fission neutrons [I -31, energetic electrons [4-91. or ions [lO,ll], the structural stability and microstructural development of alumina which contains a large amount of helium have not yet been well studied. Helium is calculated to be produced in alumina at a level of 500 appm per 1 MW . y/m2 via (n, a) reaction with fusion neutrons [12]. Therefore, it is of practical importance to elucidate the behavior of helium in alumina and the influences on microstructural development. In the present work, damage structures of single crystal alumina irradiated with energetic helium ions were investigated. 2. Experimental

procedure

The material used in this study was single crystal alumina (a - Al,O,) supplied by Rare Metallic Co., Ltd. The impurities are listed in table 1. Disk sample of 0.2 mm thick and 3 mm in diameter with the basal plane paratlel to the foil surface were thinned to 15 pm by both mechanical grinding and polishing with diamond paste. The thinned specimens were irradiated with 0.4 MeV He-ions at temperatures of 1023 to 1223 K using a Van de Graaff accelerator in JAERI. The current density was 1 mA/m2 and the dose was 1 X 10zO He/m2. The irradiations are predicted to inject helium to a peak concentration of 4 x 10’ appm and to produce a

400 kcV He+-

AP$,

I I I IOZ”Hehr?i

Depth lpm)

Fig. 1. Depth profiles of injected helium and displacement damage

in alumina.

displacement damage of 0.3 dpa at peak, by using a computer code TRIM85 [13]. In this calculation, displacement energy of 52 eV obtained by simply averaging 18 and 76 eV [4] for Al and 0, respectively, was assumed. The calculated depth-profiles for the implanted He and the deposit energy are shown in fig. 1. The front surface of the irradiated samples was carefully removed by about 0.8 pm by ion-thinning and then the samples were back-thinned to an electrontransparency by ion-thinning. The ion-thinning was carried out with 6 keV Ar-ions incident on the surface at an angle of 20 O. The microstructures were examined with an H-800 electron microscope operated at 200 kV. The thickness of the observed area was about 180-250 nm. The parameters for loops and cavities were analyzed by a Zeiss particle comparater. 3. Results

Table 1 Impurities in wt.% of single crystal alumina c

S

Fe

K

Ca

Na

Mg

0.0024 < 0.001 i 0.01 < 0.001 < 0.01 i 0.001 < 0.003 ~22-3115/8%/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

Microstructures of ahtmina irradiated with He-ions at temperatures of 1023 to 1223 K are shown in fig. 2. compared with that of a nonirradiated one. The temperature dependence of the density and the average size for

367

Y. Katano et al. / Damage structure in AI,Oj

Fig.

Microstructures of single crystal alumina: nonirradiated (a). irradiated with 0.4 MeV He-ions to a dose of 1 x 1O20/m2 at 1023 K (b), 1123 K (c) and 1223 K(d).

defect clusters or loops are presented in fig. 3. The fractional distributions of loop size are also shown in this figure. The microstructures in the He-irradiations at temperatures of 1023 to 1223 K are characterized by the

defect clusters of high density (3-4 X 10Z2/m3) with average size of 6-7 nm. It is noted that in the range of these temperatures the formation of defect cluster is almost independent of the irradiated temperature. But the cluster size increased with increasing temperatures and the density decreased at temperatures above 1223 K. In the sample irradiated at 1223 K, several defect clusters were observed to grow to dislocation loops of some 10 nm in size on prismatic poioj planes, as shown in fig. 4. The analyses through loop invisibility criteria of gv b = 0 and an inside/outside technique revealed that these dislocation loops were all of pure edge type and were interstitial in character with b = 4 [lOjO]. Hence, the defect clusters observed in the irradiations at these temperatures may presumably be mostly small dislocation loops formed by aggregation of radiation-produced interstitials. Although resolution was not clear as shown in fig. 4a, fine and white contrasts like cavities smaller than 3 nm are observed in a fairly high density in the sample irradiated at 1223 K. But no cavities were discernible in the sample irradiated at temperatures below 1123 K. These are considered to be cavities or aggIomerates of vacancies and helium atoms. In the sample annealed for 1 h at 1223 K after He irradiation at 1023 K, the drastic changes were observed

I’I-;ii:.;;: 0

10 "?

EJoO

30

20

Loop

0

10

diamter (nm) Temperature I"C I 800 !

'Y

900 /

30

20

1000

5

1

-

%

-5

et--


E -g

1022

8” 5

= 8

2

0

5

1

,dIi 773

873

i4 973

1073

Temperature

Fig. 3. Distributions irradiated

with He-ions ture dependence

1173

o

1273

(K)

of loop sizes in single crystal

alumina at 1023 and 1223 K, and the temperaof loop densities and the sizes.

368

Y. Kutmo et al. / Dumuge .rtructure

rn

Al#,

Fig. 4. A micrograph showing the dislocation loops lying on (lOi0) prismatic planes under g = [ lOi and Z nearly equal to [OOOl] (a). and the corresponding weak beam dark field micrograph (b). in single crystal alumina sample irradiated with He-ions at 1223 K.

Fig. 5. Damage

structures in single crystal alumina sample annealed for 1 h at 1223 K after He-irradiation at 1023 K. Damaged layers are, (a) distant from the foil surface and (b) exposed to the surface, as illustrated in fig. 6.

369

Y. Katano et al. / Damage structure in Al,O_,

Fig. 6. Stereo pair of micrographs showing damage structure from the foil of single crystal alumina He-irradiation

at 1023 K, and a schematic

in the microstructures as shown in figs. 5 and 6 of stereo pair. It is evident that the features observed in fig. 6 are strongly dependent of the distance from the foil ege. This implies that microstructural changes during annealing were affected by the concentration of injected helium and the displacement damage, and also by the distance of the damaged depths from the foil surface, as illustrated in fig. 6. In the region just above the area where large cavity channels appeared in fig. 6, conspicuous cavities or white contrast features were formed as shown in fig. 5a. These had a high density of 5 X 1022/m3 with an average size of 7 nm and had an estimated swelling of 1.2%. The size of cavities are seen to be larger in regions near to the ion-range as observed by using a stereo-mi-

illustration

as a function

annealed of depth.

for 1 h at 1223 K after

croscopy technique. However, around the ion-range which was exposed to the foil surface, cavities coalesced into large cavity channels of 300 nm long and 30 nm in diameter at maxima as shown in figs. 5b and 6. Then, the cavity sizes are seen to have a bimodal distribution. The annealing also grew the dislocation loops to an average size of 70 nm, as observed in fig. 5a. In addition, these grown loops are partly seen to be decorated with the relatively large cavities. The spherical and black contrast features or precipitates of 5 nm in an average size were formed in the bulk in the annealed sample. These precipitates had a fairly high density of 1.2 x 102’/m3 and a volume fraction of about 0.1%. No association was observed between these precipitates and cavities or dislocation loops. However,

370

Y. KUmlO

et

(11.

/

Damu‘qe

other black but weak contrast features wfith irregular shape were formed with large sizes of 20-50 nm in an apparently low density beyond the edge of the ion-range, that is. near the bottom surface of the foil. as observed in stereo-microscopy. Furthermore, these features were formed in the localized area where pronounced cavity coalescence into channel was observed. The morphology of these features is similar to those observed in electron-irradiated alumina by Pells [5] and it suggests that these features would be also aluminium islands. 4. Discussions In the alumina injected helium to 4 x lo3 appm at temperatures below 1123 K, helium bubbles were hardly formed and the temperature independent formation of interstitial loops was predominant. The results of loop formation at temperatures above 1073 K in this work were similar to those by Rechtin [lo] who analyzed the radiation effects with 0.6 MeV He-ions of almost the same dose at low temperature of 500 K. Beside the temperature independence in loop formation behavior over a wide range of temperatures of 500-1223 K. the loop growth in the alumina irradiated with He-ions is sluggish compared to the results obtained in the irradiations with 0.3 MeV [7] or 1 MeV [5] electrons. In these electron irradiations. the irradiated temperatures were lower by 150-200 K than the highest temperature in this work and the electron irradiation would produce only isolated defects. Therefore, it seems that the coexistence of helium atoms with atomic displacement cascades by energetic knock-on atoms produced by the He-ion irradiation would suppress the loop growth. This might be due to the increase of nucleation of defect clusters associated with helium atoms and displacement cascades. Another possibility is that the mobilities of defects would become small because of strong trapping by helium atoms. The present low rate of loop growth suggests that the trapping of point defects by helium would be effective even at high temperatures up to 1123 K. The trapping effect would be diminished with raising temperatures to 1223 K and radiation-produced defects. such as vacancies. would aggregate to form cavities under the influence of helium atoms. One can observe this phenomenon slightly in fig. 4a. The smaller and spherical precipitates observed in the bulk in the annealed sample are believed to be Al metal colloids from the morphology [8]. The precipitate contrasts and the diffraction spots from these Al precipitates have been reported to disappear on annealing at temperatures higher than the Al melting point. But, the precipitates in the present sample were formed during annealing at 1223 K, e.g., much higher than the Al melting point. The fact indicates that the Al metal precipitates would be formed and be stabilized under the strong influence of highly injected helium atoms and

.ciruc’ture

11, AI,O,

radiation-induced defect clusters such as cavities and loops. We have observed the phenomena that the cavity coalesced into large channels and aluminium islands appeared in the localized and almost coincided area around the damage peak exposed to the foil surface. These suggests that increased flow of radiation-produced defects to the foil surface might enhance the cavity growth and probably oxygen interstitials would migrate swiftly to the surface and the vaporised during annealing. Then. exess oxygen vacancies would precipitate more effectively in the region with helium containing cavities than in other regions distant from the foil surface. Accordingly, surplus Al interstitiala would precipitate as aluminium islands near the bottom-surface. This would be due to that Al defects were formed in large number density than oxygen point defects because of small displacement energy [4] and in large depth:, during displacement cascade evolution. On the other hand, the smaller and spherical precipitates formed in the bulk in the sample could presumably nucleate in embryos during the He-irradiation and would grow to the present size and be stabilized even at 1223 K. 5. Summary Helium bubbles hardly formed in the single crystal alumina as-irradiated with He-ions to 1 X 10zO/m’ below 1223 K. and the dislocation loops with high density of 334 x 1022/m3 and with average size of 6-7 nm were observed predominantly. The formation of the dislocation loops were almost independent of irradiation temperatures. On annealing for 1 h at 1223 K after the He irradiation at 1023 K to the same dose, the microstructures changed dramatically. Two types of precipitates and two types of cavities appeared and loops grew to an average size of 70 nm. These features showed the strong dependence of both the depth-profiles of He-irradiated damage and the distances of the damaged layers from the foil surface during annealing in the vacuum. The present results seem to exhibit particular behavior of defect species under influences of helium atoms, which requires further studies to clarify in detail. The authors wish to thank Mr. H. Mitamura assistances in TEM examination of sample.

for his

References

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