The Enhanced Swelling Resistance of W/Cu Nanocomposites by Vacancy-Type Defects Self-Recovery

Abstract

In this study, the swelling resistance of W/Cu nanocomposites is investigated after helium irradiation at RT and 400 °C. The results show that W/Cu nanocomposites with interface structure present better resistance to helium swelling as compared with W monolayer. The PAS results reveal that the unique interfacial structure of W/Cu nanocomposites effectively improves the recovery of vacancy-type defects under He⁺ irradiation, which results in good resistance to irradiation swelling. This result shows that introducing interface structure can effectively enhance the swelling resistance of materials and sheds light on the design of radiation-tolerant materials for advanced nuclear reactor applications.

1. Introduction

Structural materials are used in nuclear reactors and are subjected to high levels of irradiation that cause the formation and accumulation of defects inside the structural materials. The defects include vacancies, interstitials, voids, and bubbles, and can lead to volume swelling and blistering that result in embitterment and instability of reactor components and a reduction in service life [1,2]. Meanwhile, helium atoms are also produced by neutron transmutation in reactors. Due to their low solubility and migration energy, helium atoms can precipitate and aggregate into helium bubbles and grow into voids at high excess volume sites (e.g., grain boundaries) [3]. Moreover, helium can also accelerate swelling by stabilizing void nuclei and can promote high-temperature embrittlement by enhancing void growth, which seriously degrade the physical and mechanical properties of structural materials [4,5]. Hence, managing helium behavior and delaying the conversion of “helium bubble to void” is crucial to the development of new structural materials for fusion and fission reactors. One of the most promising strategies is to induce precipitates, dispersoids, and high-density grain boundaries or heterointerfaces, which can act as sinks for point defects and traps for helium atoms in structural materials, such as nano-ferritic alloys (NFAs) [6,7] and multilayered nanocomposites [8].

A multilayered nanocomposite, which can stablize helium bubbles and can absorb and annihilate defects via interface, is one of the newly designed radiation damage-tolerant structural materials [9]. Atomic simulations have shown that a multilayered nanocomposite with bcc–fcc interface possessed inexhaustible sinks for radiation-induced point defects and high He “solubility”, which resulted in efficient Frenkel pair recombination and stable storage of helium at the interface [10,11,12]. Demkowicz et al. and Chen et al. verified that helium atoms could efficiently be trapped and stored at Cu/Nb, Cu/V, Cu/Mo, W/Cr, and W/Ni interfaces by neutron reflectometry [13,14,15,16]. Meanwhile, previous research has revealed that multilayered nanocomposites such as W/Cu, Cu/Nb, W/Ni, and Fe/W multilayered systems possess microstructural stability against intense irradiation [17,18]. This evidence indicates that a multilayered nanocomposite is expected to be an advanced irradiation-resistant material. To be used as reactor material, resistance to irradiation-induced swelling is also an important factor for evaluating radiation-resistant structural materials. The W/Cu system, an incoherent BCC–FCC interface, possesses good point defects sink and helium atoms with highly morphological stability, is a component candidate material for plasma facing material in a fusion reactor [19]. However, studies on the helium-induced swelling of multilayered nanocomposite are sparse [20]. The influence of interface structure on irradiation swelling resistance of structural materials is not very clear. Here, helium irradiation-induced swelling behaviors of multilayered W/Cu with single-layer thicknesses of 25 and 50 nm are investigated. The results show that multilayered W/Cu nanocomposites with interface structure possess better resistance to helium irradiation-induced swelling.

2. Experimental Procedures

W/Cu nanocomposites with single layer thicknesses of 25 nm (denoted W/Cu-25) and 50 nm (denoted W/Cu-50) were deposited on 0.5 mm silicon substrates at room temperature by DC magnetron sputtering. During preparation, the argon pressure was 0.5 Pa and the base pressure was greater than 8 × 10⁻⁶ Pa. The sputtering rates of W and Cu were 4.22 and 4.32 nm/min, respectively. The W/Cu-25 (total 20 layers) and W/Cu-50 (total 10 layers) nanocomposites with the same total thickness of 500 nm were prepared successfully. At the surface of the samples, a nickel layer with a thickness of 10 nm was deposited to prevent oxidization of the Cu layer. The Cu and W monolayers with a thickness of 500 nm were also prepared in the same conditions, as reference samples. The helium irradiation was carried out on an LC-4 ion implanter with 100 keV, 2.2 × 10¹² ions/cm²∙s (0.35 μA/cm²), at room temperature (RT) and 400 °C. Under these conditions, samples with a total dose of 2 × 10¹⁶ ions/cm² were prepared. Figure 1 shows the helium contents and damage profiles of He irradiated W monolayer and W/Cu nanocomposite estimated by SRIM-2008 with the “Quick” K-P option, wherein the displacement energy was 90 eV for W and 30 eV for Cu [21,22,23]. According to the SRIM simulation, the maximum depths of helium distribution and damage were ~410 nm and 380 nm, respectively, for the W monolayer, and ~500 nm and ~470 nm, respectively, for the W/Cu-50 sample, that is, helium irradiated swelling came entirely from the sample without the contribution of silicon substrates.

The surface morphologies of the irradiated samples were analyzed in Kelvin probe force microscopy (KPFM) mode using an atomic force microscope (AFM, BrukerInc.,USA, Dimension Icon). The positron annihilation Doppler broadening (DB) spectra measurements were performed by a slow-positron beam facility at the Institute of High Energy Physics. Slow positrons were generated using a 1.85 GBq ²²Na radiation source with an energy range from 0.5 to 25 keV. The detective depth was about ~358 nm for the W monolayer and ~500 nm for the W/Cu nanocomposites. This is calculated by the empirical equation [24] as follows:

$$Z\left(E\right)=\left(\frac{4\times {10}^4}{\rho }\right)·E^{1.6}$$

where Z(E) is the depth below the surface, expressed in nm; E is the incident energy (keV) of the slow positron; and ρ is the material density in the unit of kg/m³ (pure W is 19,250 kg/m³ and pure Cu is 8960 kg/m³). The microstructure analysis of the nanocomposites was performed on an FEI Tecnai G² F20 microscope and the focus ion beam (FIB, Helios Nanolab 600) was used to prepare the samples. For the statistical analysis of the bubbles, the Image Pro-Plus software (IPP) was used to measure the size and density of bubbles from the TEM images, and the TEM samples’ thicknesses were estimated using the convergent-beam technique [25].

3. Results and Discussion

Prior to He⁺ irradiation, a copper mask (Figure 2a) with a hole size of ~10 μm was used to cover the sample. Figure 2b,c show typical AFM images of W/Cu-50 after He⁺ irradiation at RT. Patterns with a size of ~10 × 10 μm formed on the surface of the sample due to irradiation swelling. Similar patterns were also observed on the W and Cu monolayers, and W/Cu-25 nanocomposite after He⁺ irradiation at RT and 400 °C. Figure 2d shows the height square wave along the yellow dot line in Figure 2b. The wave peaks and troughs correspond to the irradiation and non-irradiation regions, respectively. The swelling height can be represented by the relative height, Δh, measured from the wave trough to the peaks. After He⁺ irradiation, the maximum swelling height of ~20 nm was observed in the W monolayer at RT, and was reduced to ~5.4 nm at 400 °C. Figure 2e shows the swelling heights of the W and Cu monolayers, W/Cu-25, and W/Cu-50 after He⁺ irradiation at RT and 400 °C. Obviously, the swelling height of the W/Cu-25 and W/Cu-50 (nm) is lower than that of the W monolayer. It indicates that the introduction of the interface structure improves the irradiation swelling resistance of the W monolayer under He⁺ irradiation at RT. This higher swelling resistance of the W/Cu nanocomposites may be due to two factors. First, as compared with the W monolayer, the Cu monolayer presents a lower swelling height after irradiated at RT (Figure 2e). Thus, introducing the nano-Cu layer with better He⁺ irradiation swelling resistance improves the swelling resistance of W/Cu nanocomposites. Second, the interface structure effectively absorbs the vacancy and helium atoms, and therefore, impedes the formation of voids or helium bubbles [9,11,15]. With an increase in the irradiation temperature, the swelling heights of the samples were reduced. Especially, the swelling height of the W monolayer presented a drastic reduction by 75% (~15 nm). For the W/Cu-25 and W/Cu-50 samples, the swelling decreased by 44.3% (~4 nm) and 37.8% (~3 nm), respectively. This suggests that the swelling of both W monolayer and W/Cu nanocomposites was inhibited under 400 °C He⁺ irradiation. This agrees with previous studies that have shown that high temperatures enhanced self-recovery diffusion mechanisms, and thereby, suppressed accumulation of vacancies under He⁺ irradiation [6]. Moreover, the swelling of W/Cu nanocomposites presented lower sensitivity to irradiation temperature as compared with the W monolayer. This implies that the interface structure may play a similar function as irradiation temperature that improves the self-recovery under ion irradiation.

Figure 3 shows the microstructure of the W monolayer and the W/Cu-25 nanocomposite after He⁺ irradiation at RT and 400 °C. Due to similar swelling resistance of the W/Cu-25 and W/Cu-50 samples, the microstructure of W/Cu-25 is presented here for comparaison with the W monolayer. Additionally, a similar microstructure of W/Cu-50 with W/Cu-25 under He⁺ irradiation can also be found in our previous study [20] and in [19]. After He⁺ irradiation at RT and 400 °C, no obvious voids are formed in the W monloayer, as shown in Figure 3a,d; some dislocation loops and visible helium bubbles with a size of ~1 nm were observed (see Figure 3b–f). After irradiation at 400 °C, the size of the bubbles almost does not change, and the bubble density increases from 1.2 ± 0.24 × 10¹⁸ cm⁻³ (RT) to 1.8 ± 0.36 × 10¹⁸ cm⁻³. Figure 3g–i show the BF--TEM image of the W/Cu-25 nanocomposite after He⁺ irradiation at RT. In Figure 3g, the W/Cu-25 clearly presents a layered structure with an ~25 nm Cu layer and an ~25 nm W layer. The Cu and W layers present a “bright” and “dark” contrast, respectively, due to different atomic numbers. After He⁺ irradiation, no obvious voids are formed and the boundary between the W and Cu layers is still clearly visible. Even if irradiated at 400 °C, no obvious structural changes or a new layer present at the W/Cu interfacial region (see Figure 3j). This was consistent with a previous report that magnetron--sputtered Cu/W multilayer possessed highly morphological stability of the multilayered structure and radiation tolerance [19]. Visible helium bubbles, with sizes of ~1 nm and ~2 nm, were also observed in the W and Cu monolayers, respectively, as shown in Figure 3h. As compared with RT irradiation, helium bubble size increases slightly and the density reduces from 2.2 ± 0.44 × 10¹⁸ cm⁻³ (RT) to 1.84 ± 0.36 × 10¹⁸ cm⁻³ after 400 °C irradiation. Meanwhile, high--density helium bubbles which are in the form of a “bright line” present at the W/Cu interface, which indicate that high--density helium bubbles are formed at the W/Cu interface due to its strong sink strength and traps for helium [9,26,27]. It should be noted that the density of the helium bubbles is higher and the helium bubble sizes are larger in the W/Cu-25 than that in the W monolayer. Hence, the higher swelling resistance of the W/Cu-25 and W/Cu-50 samples does not result from a reduction in helium bubbles, although some helium atoms are absorbed by the W/Cu interface. Other invisible vacancy--like defects may play a dominant role in the improvement of the swelling resistance of W/Cu multilayered nanocomposites.

Positron annihilation spectroscopy (PAS) is one of the more powerful and well-established techniques to characterize the vacancy-type defects of a material [28]. Here, the PAS is used to investigate the vacancy-type defects in irradiated samples, as shown in Figure 4. The S parameter, which describes the information of positron annihilation with valence electron, is defined as the ratio of γ counts in the central energy region around 511 keV (510.2–511.8 keV) to the total counts around 499.5–522.5 keV. Hence, an increase in the S parameter indicates the presence of vacancy-like defects. According to the SRIM simulation, the damage range is 0~300 nm (0~22 keV) for the W monolayer, and 0~350 nm (0~18.5 keV) for the W/Cu nanocomposites. Obviously, the S parameter values increase in the yellow range after He⁺ irradiation for all the samples, which indicate that the amount of vacancy-type defects are formed under He⁺ irradiation [28,29]. When the irradiation temperature increases to 400 °C, the S parameter values are reduced for all the samples. This coincided with previous studies that have shown that some of the mono-vacancies generated by irradiation were recovered, and helium atoms occupied the rest of the vacancies to form He_mV_n (m > n) [29,30]. Meanwhile, accompanying the reduction in the S parameters, the swelling heights are reduced for all the samples (Figure 2e). This indicates that the reductions in the S parameter values and swelling heights are primarily the result of the recovery of vacancy-type defects. Moreover, the W monolayer presents a greater reduction in S parameter value and swelling height as compared with the W/Cu-50 and -25 nanocomposites, which suggests more vacancy-type defects are recovered at 400 °C irradiation. Figure 4d shows the comparison of the S parameters of the W monolayer, W/Cu-50, and W/Cu-25. According to the SRIM simulation, the W/Cu nanocomposites generate more serious damage than that of the W monolayer. Due to the lower vacancy-formation energy of Cu (Cu, 1.09 eV and W, 3.16 eV) [31], theoretically, more vacancies or vacancy-type defects are produced in W/Cu nanocomposites. However, a smaller change in the S parameter is observed in irradiated W/Cu nanocomposites. This abnormal phenomenon implies that most of the vacancies are annihilated and recovered, and/or occupied by helium atoms in the form of He_mV_n (m > n) [29,30], which results in smaller changes in S parameter values under ion irradiation in W/Cu nanocomposites. In other words, the W/Cu nanocomposites with interface structure effectively improve vacancy recovery and the ability to absorb helium atoms during He⁺ ion irradiation, which is in agreement with previous studies [9,26,27]. Hence, introducing interface structure effectively improves the swelling resistance of W/Cu nanocomposites.

In addition, the ΔS of W/Cu-25 is smaller than that of the W/Cu-50 with RT and 400 °C irradiation (see Figure 5), which suggests that the concentration of vacancy-type defects is greater than that of W/Cu-50 after He⁺ irradiation. That is, W/Cu nanocomposites with more interface structures effectively improve the recovery of the vacancy-type defects. On the one hand, as compared with W/Cu-50, in this study, improved resistance to swelling is not observed in W/Cu-25. Under this irradiation condition, the interface structures in the W/Cu-50 are enough to mitigate the generation of the vacancy-type defects, no more interfaces are needed. On the other hand, the formation of helium bubbles (Figure 3) plays a dominant role in the helium irradiation-induced swelling. These may be the key factors for the similar swelling resistance between W/Cu-25 and W/Cu-50 nanocomposites.

4. Conclusions

In summary, in this study, we investigated the helium swelling resistance of nanocomposites at RT and 400 °C. As compared with the W monolayer, the W/Cu nanocomposites with interface structure present good helium swelling resistance. The PAS reveals that W/Cu nanocomposites with interface structure effectively improve the vacancy recovery and the ability to absorb helium atoms under He⁺ irradiation. In addition, the reduction in vacancy-type defects in the W/Cu nanocomposites results in good swell resistance as compare with the W monolayer. Moreover, as compared with W/Cu-25, the interface structure in W/Cu-50 is enough to mitigate the generation of vacancy-type defects and to improve the swelling resistance of the structural materials. Thus, introducing interface structure can effectively enhance the swelling resistance of structural materials. This result sheds light on the design of radiation-tolerant structural materials for advanced nuclear reactor applications.