In situ observation of damage evolution in TiC during hydrogen and deuterium ion irradiation at low temperatures

In situ observation of damage evolution in TiC during hydrogen and deuterium ion irradiation at low temperatures

ELSEVIER Journal of Nuclear Materials 239 (1996) 279-283 In situ observation of damage evolution in TiC during hydrogen and deuterium ion irradiatio...

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ELSEVIER

Journal of Nuclear Materials 239 (1996) 279-283

In situ observation of damage evolution in TiC during hydrogen and deuterium ion irradiation at low temperatures K. Hojou a. *, H. Otsu a, S. Furuno a, N. Sasajima b, K. Izui a Japan Atomic Energy Research Institute, Tokai-mura, lbaraki-ken 319-11, Japan h School of Engineering, Nagoya Universi~, Chikusa-ku, Nagoya 464, Japan

Abstract The processes of damage evolution in TiC crystals irradiated with 25 keV H~- ions at 12 K and 28 K and 25 keV D~ions at 18 K and the effect of annealing after the irradiation were examined by in situ observation with an electron microscope equipped with an ion accelerator. Amorphization was confirmed in TiC irradiated with H~ ions to a fluence of about 1 X 1021 H~-/m 2 at 12 K, while no amorphization occurred in TiC irradiated with D~- ions to a fluence of 4.5 X 1021 D~-/m 2 at 18 K. No amorphization in D~- irradiation is considered to be due to the difficulty of producing chemical bonding species which suppress the recovery of irradiation induced defects in comparison to H~ irradiation.

1. Introduction Metallic carbides such as TiC, NbC and WC have attracted much attention due to structural properties such as high melting point, extreme hardness and high electrical conductivity. Among these compounds, titanium carbide (TIC) has been widely applied to improve properties of material surfaces. In particular, tool steels coated with TiC have longer lifetimes than uncoated steels. Furthermore, TiC has been considered to be one of the candidate materials as first wall materials for fusion-reactors. This wall will be subjected to hydrogen and deuterium ion bombardment. However, studies on microstructural change in TiC crystals due to ion irradiation have not been much reported [1-3]. We have observed the dynamic behavior of the damage process in TiC crystals irradiated with helium ions at temperatures from 12 K to 1473 K [4]. As a result, it was found that TiC did not become amorphous by helium ion irradiation at these temperatures, contrary to the case of SiC [5] or graphite [6,7], and bubbles were formed after the irradiation to the fluence above 1 × 10 21 H e + / m 2 at 12 K and 5 X l0 2° H e + / m 2 at room temperature [4].

* Corresponding author. Fax: + 81-292 82 5922.

The present paper reports the results of the in situ TEM observation of TiC crystals irradiated with 25 keV hydrogen and deuterium ions at very low and room temperatures. Annealing effects on the irradiated TiC are also reported.

2. Experimental method TiC samples of 3 mm in diameter were made with an ultrasonic cutter. These were dimpled into 10-20 I.tm thickness at the center part of the discs. Then, thin TiC samples for electron microscopic observation were prepared by 2 keV Ar ÷ ion etching. The system used in this study consists of a 200 kV transmission electron microscope (TEM) equipped with a thermal field-emission electron source (JEOL) and a 40 kV ion accelerator (Origin). Irradiation was performed with 25 keV hydrogen and deuterium molecule ions at a flux of 1.3 x 1018 (H~-, D ~ ) / m Z s , respectively. The half of the energy of two atom molecule (H z, D 2) is shared by two individual atoms (H, D). That is, 25 keV H~- and D~- ions with a flux of 1.3 X 10t8/m2s correspond to 12.5 keV H ÷ and D ÷ ions with a flux of 2.6 × 10 Is (H +, D + ) / m 2 s . The penetration depths of 12.5 keV H ÷ ion and D ÷ ions in TiC were estimated to be about 240 nm by TRIM

0022-3115/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PH $ 0 0 2 2 - 3 1 1 5 ( 9 6 ) 0 0 4 3 0 - 8

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K. H~jou et al. / Journal ol Nuclear Matermls 239 (1996) 279-283

computer code. For irradiation and annealing experiments, two kinds of specimen holders of side entry type were used: One was the liquid helium cooled holder of double tilt of 636-He M type (Gatan) and the other was the high temperature holder of single tilt of 628-Ta type for heating up to 1520 K (Gatan). Irradiation was performed at 12 K, 18 K, 28 K and 293 K. After irradiation at low temperatures, the specimens were annealed at 190 K and 300 K with the cooling stage in the electron microscope. Following this annealing, the same samples were transferred to the high temperature holder and annealed from 300 K to 1518 K. The temperature was raised stepwise by 100 degrees at intervals of 20 min.

3. Experimental results and discussion 3.1. Hydrogen ion irradiation at room temperature (RT)

The TiC specimens used consisted of grains with several b~m size and were homogeneous in appearance within the grains. Fig. l ( a ) - ( f ) show damage evolution in the specimens irradiated with hydrogen molecule ions of the flux of 1.3 × 1018 H + / m 2 s at RT. Many small defect clusters of about I - 2 nm in size were formed at the fluence near 3.8 × l0 en H + / m 2 and the number density was about 4 X 10t~'/m -'. As the irradiation proceeded the clusters grew and increased in number up to the fluence of about 1.5 X 10 21 H + / m e. Then, many small bubbles of about 1 nm in size were formed at the fluence near 2 X 10 21 H + / m 2, as shown in Fig. I(d). The density of

the bubbles increased up to the fluence of about 3.5 × 102~ H ~ / m 2. The bubble size did not change to these irradiation fluence. Bubble formation is more difficult in the case of hydrogen ion irradiation than helium ion irradiation. That is. the critical fluence at which bubbles can be observed is about ten times larger in the case of 25 keV H,~ than in the case of 20 keV He*. 3.2. Amorphization in TiC irradiated with hydrogen and deuterium ions at low temperature 3.2.1. Hydrogen irradiation at 12 K and 28 K The process of damage evolution in TiC irradiated with 25 keV H ,+ ions with a flux of 1.3X 10 j~ H _ ~ / m e s a t 12 K is shown in Fig. 2(a)-(d). Amorphization occurs by the irradiation with 25 keV H~' ions to a fluence of about I × 102~ H ~ / m ~' at 12 K, as is clearly recognized from the halos in the diffraction pattern in Fig. 2(b). With increasing irradiation fluence. the halo patterns became more clear which indicates further evolution of amorphization. No bubbles were found after the irradiation to a fluence of 3.4 × 1021 H ~ / m ' - at 12K. By the irradiation at 28 K, amorphization was found also to occur at a fluence of about 3 × 10 21 H£-/m 2 which is three times larger than in the case of 12 K irradiation, as shown in Fig. 3(a) (e). 3.2.2. Deuterium ion irradiation at 18 K No amorphization was recognized after 25 keV D j ion irradiation at 18 K to the fluence of 4.5 × 10 21 D + / m 2, as

O

. . . . . . . . . . . . .

a

Fig. 1. Processes of bubble formation in TiC crystal during irradiation with 25 keV H + ions of 1.3 × l0 is H ~ / m 2 s at room temperature. Fluence: (a) 3.8 × 102o H+/m 2, (b) 7.6 × 102o H ~ / m 2, (c) 1.5 X II)2' H~/,n', (d) 2.3 X 102' H ~ / m 2. (e) 3.4 × 1(12' H ~ / m 2, (f) 4.5× 1()21 H 2 / m 2.

K. Hojou et aL / Journal of Nuclear Materials 239 (1996) 279-283

281

Fig. 2. Processes of amorphization in TiC crystal during irradiation with 25 keV H~- ions of 1.3 x 10 t8 H + / m 2 s at 12 K. Fluence: (a) 3.8 X 1020 H~-/m 2, (b) 1.3 x 10 2' H~-/m 2, (c) 2.3 X 10 21 H~-/m 2, (d) 3.4 × 10 2' H~-/m 2. shown in Fig. 4(a)-(e). Small defect clusters of 1-3 nm in size were formed at a fluence larger than 2.3 X 10 20 D ~ - / m 2. The general aspect of these defect clusters is similar to the case of room temperature irradiation, except that onset of defect formation is somewhat hastened in comparison to the case of room temperature irradiation. Small bubbles became visible among these defect clusters after the irradiation to a fluence of 2.3 X 1021 D ~ - / m 2.

Remarkable phenomena revealed in the present experiments are that amorphization occurred by hydrogen ion irradiation at 12 K and 28 K, while no amorphization occurred by deuterium ion irradiation at 18 K. There clearly exist an isotope effect on amorphization due to irradiation (H~- and D~-) at very low temperature. The origin of these effects is considered in the following way. Amorphization is considered to occur by stabilizing the

Fig. 3. Processes of amorphization in TiC crystal during irradiation with 25 keV H~- ions of 1.3 X 1018 H~-//m 2 s at 28 K. Fluence: (a) 3.8 x 10 20 H + / m 2, (b) 7.6 X l0 2° n ~ / m 2, (c) 1.5 x l0 2~ n~-/m 2, (d) 2.3 x 1021 n~-/m 2, (e) 4.5 X 10 21 n 2+/ m 2 ..

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K. Hojou et aL / Journal of Nuclear Materials 239 (1996) 279-283

Fig. 4. Damage evolution in TiC crystal during irradiation with 25 keV I)2 ions of 1.3 X Ill ~s D+/m 2 s at 18 K. Fluence: (a) 2.3 x 102" Df/m ~', (b) 3.8 × ill 2'' D ~ / m 2, (c) 2.3 × I(I 2' D ~ / , n 2, (d) 3.4 × I(I -~' D_~/m 2. (el 4.5 × Ill -~' D + / m -'.

irradiation-induced detects. The stabilization of defects is possible by producing resistant species for recovery of defects or recombination of Frenkel pairs. One of such species may be produced by chemical bonding between implanted atoms and target atoms, T i - H and C - H bonding in the present case. On the other hand, the chemical reactivity at low temperature of hydrogen is known to be

l I) to 100 times larger than that of deuterium due to tunneling effects [8]. Therefore, it seems more difficult for deuterium to form chemical bondings effective tbr stabilizing detects than for hydrogen. Accordingly, deuterium ion irradiation results in no amorphization on the contrary to hydrogen ion irradiation. This is considered to be the main reason of the isotope

Fig. 5. Bubble growth during annealing at 190 K to 1518 K after H + ion irradiation at 12 K to a fluence of 3.4 × 1021 H+/m 2. Annealing temperature: (a) 190 K, (b) 293 K, (c) 1273 K, (d) 1423 K, (e) 1518 K.

K. Hojou et al. / Journal of Nuclear Materials 239 (1996) 279-283 effects (between H and D) on amorphization in TiC at low temperature irradiation, actually observed in the present experiments. 3.3. Annealing after 25 keV H2+ ion irradiation After irradiation at 12 K to the fluence of 3.4 X 10 21 + 9 H z / m - , the annealing experiment was performed from 190 K to 1518 K, as shown in Fig. 5(a)-(e). The temperature was raised stepwise by 100 degrees at intervals of 20 rain. The remarkable features revealed by the in situ observation were as follows: bubbles of about 1 nm in size which were not visible by the irradiation at 12 K were observed after the annealing at room temperature. The amorphous phase was stable during annealing at room temperature, as shown Fig. 5(b). By the annealing between 1423 K and 1518 K, some of the small bubbles began to grow to several 10 nm by coalescence with each other, as shown in Fig. 5(e).

4. Conclusions The results obtained in the present experiments are summarized as follows. (1) Amorphization occurs in TiC irradiated with 25 keV H~- ions at fluences exceeding about 1 × 10 21 H ~ - / m 2 at 12 K and about 3 X 1021 H ~ - / m 2 at 28 K. (2) No amorphization occurs in TiC irradiated with 25

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keV D ] ions to a fluence of 4.5 x 10 21 D ~ - / m 2 at the low temperature of 18 K. (3) The reason why amorphization occurs for H~- irradiation at low temperature and not for D ~ can be attributed to the difference in chemical reactivity between H and D. That is, hydrogen atoms make chemical bonding species more easily than deuterium atoms due to the tunneling effect. These chemical species will disturb the recovery of irradiation-induced defects, thus resulting in amorphization. (4) By annealing at 293 K after hydrogen ion irradiation at 12 K to a fluence of 3.4 × 1021 H ~ - / m 2 the small bubbles were observed and then by annealing over 1423 K, bubbles were found to grow and coalesce.

References [1] M. Iseki and Z. Kabeya, J. Nucl. Mater. 133&134 (1985) 722. [2] D. Foumier, M.O. Ruault and R.G. Saint-Jacques, Nucl. Instrum. Meth. BI9&20 (1987)559. [3] K. Hojou, S. Furuno, H. Otsu, K. Izui and T. Tsukamoto, J. Nucl. Mater. 155-157 (1988) 298. [4] K. Hojou, H. Otsu, S. Furuno, K. lzui and T. Tsukamoto, J. Nucl. Mater. 212-215 (1994) 281. [5] K. Hojou and K. Izui, J. Nucl. Mater. 133&134 (1985) 709. [6] K.N. Kushita and K. Hojou, Ultramicroscopy 35 (1991) 289. [7] K. Niwase, M. Sugimoto, T. Tanabe and F.E. Fujita, J. Nucl. Mater. 155-157 (1988) 303. [8] V.A. Benderskii, D.E. Makarov and C.A. Wight, in: Chemical Dynamics at Low Temperatures, eds. I. Prigogine and S.A. Rice (Wiley, 1994) p. 191.