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Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film
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Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film L. Wang , X.P. Wang , L.F. Zhang , Y.X. Gao , R. Liu , R. Gao , Y. Jiang , T. Hao , T. Zhang , Q.F. Fang , C.S. Liu PII: DOI: Reference:
S2352-1791(19)30055-9 https://doi.org/10.1016/j.nme.2019.100710 NME 100710
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Nuclear Materials and Energy
Received date: Revised date: Accepted date:
26 August 2019 23 October 2019 23 October 2019
Please cite this article as: L. Wang , X.P. Wang , L.F. Zhang , Y.X. Gao , R. Liu , R. Gao , Y. Jiang , T. Hao , T. Zhang , Q.F. Fang , C.S. Liu , Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film, Nuclear Materials and Energy (2019), doi: https://doi.org/10.1016/j.nme.2019.100710
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Highlights: He bubble size grows significantly up in He-charged FeCrNi film subjecting to the annealing from about 1 nm in diameter under 300 K to 5 nm under 1073 K. It is speculated that migration coarsening (MC) plays a main role in He bubble growth in He-charged FeCrNi films.
Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film L. Wang1, 2, X. P. Wang1, , L. F. Zhang1, 2, Y. X. Gao1, R. Liu1, R. Gao1, Y. Jiang1, T. Hao1, , T. Zhang 1 , Q.F. Fang1, C.S. Liu1 1
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of
Sciences, Hefei 230031, China 2
University of Science and Technology of China, Hefei 230026, China
*Corresponding authors, X. P. Wang: [email protected]; T. Hao: [email protected] Telephone: +86/0551/65591125; Fax: +86/0551/65591434
Abstract: He-charged FeCrNi films were prepared by radio frequency (RF) magnetron sputtering technique, and helium desorption behavior, growth process and evolution mechanism of helium bubbles in FeCrNi film were studied by thermal desorption spectroscopy (TDS) spectra combined with microstructure analysis under different annealing treatments. Three different He desorption peaks were observed in TDS spectra, which can be related to the release of interstitial helium atoms from the near-surface regions, helium-vacancy clusters with different sizes in FeCrNi film. TEM analysis showed that helium bubbles can be uniformly distributed in He-charged FeCrNi nanocrystalline films. Under annealing treatments at different temperatures, the size of helium bubbles obviously increases with increasing annealing temperature, corresponding to the significantly decrease in the number density of helium bubbles in FeCrNi films, and the growth mechanism of helium bubbles was suggested to originate from the migration and coalescence (MC) mechanism.
Key words: Magnetron sputtering, FeCrNi film, Thermal desorption, He bubbles. * Corresponding author, [email protected] (X. P. Wang);
[email protected] (T. Hao)
1. Introduction FeCrNi-based austenitic stainless steels have been widely used as the structural materials in nuclear reactors, because of their good mechanical properties, low corrosion and high resistance to radiation swelling [1, 2]. However, neutron irradiation in nuclear reactors not only causes vacancy-type defects, but also generates a large amount of helium (He) atoms by the (n, α) nuclear reaction [2]. The further study had indicated as well that He atoms are extremely difficult to be dissolved in metals and are easy to migrate owing to its low migration energy (~ 0.078 eV) [3]. As a result, He atoms easily migrated or diffused can aggregate and form high-density He bubbles in metals, resulting in the service performance degradation of materials, such as lower elongation, shorter creep fracture time as well as fatigue life [4, 5]. Therefore, the effect of He in FeCrNi-based austenitic stainless steels has become a very important and challenging research topic in nuclear structure materials. On the microscopic perspective, the radiation damage of materials is essentially caused by the continuous accumulation of irradiation defects, macroscopically resulting in swelling, hardening and embrittlement of materials. Hence the irradiation resistance of materials usually depends on whether they have self-healing ability to eliminate irradiation defects. Previous studies have shown that high density solid-solid interfaces in ultrafine-grained materials (especially nanocrystalline materials) can also act as helium absorption centers, diluting and absorbing helium defects, thus effectively inhibiting the nucleation and growth of helium bubbles. For example, ultrafine-grained W-TiC (100 nm~1µm) [6], nanocrystalline TiNi alloy (23~31 nm) [7] and Au (~23 nm) [8] exhibit stronger ion irradiation resistance than corresponding coarse-grained materials. Moreover, nanocrystalline Fe (~49 nm) [9] and Fe/TiO2 (~100 nm) [10] also exhibit stronger ability to inhibit He bubble growth than their coarse-grained counterparts. Therefore, grain refinement can be considered as one of the effective ways to inhibit the nucleation and growth of helium bubbles as well as He embrittlement problem. As for He behavior in nanocrystalline FeCrNi-based structural materials, how to
refine the grain and uniformly introduce a certain concentration of He atoms into metal matrix is the precondition. In general, the main methods of introducing helium into metals include tritium decay [11-14], neutron irradiation [15-19] and helium ion implantation [20-22]. The first two methods can introduce helium uniformly into materials, but the radioactive characteristics make it difficult to operate and control. Although He ion implantation method can easily control the implanted concentration of helium, it still has some disadvantages. For low energy He ion implantation, most of helium atoms are only introduced into the surface layer of the materials. And for high energy He ion plantation, it can produce displacement damage and influence He diffusion, agglomeration and bubble nucleation in metal materials. Remarkably, in recent years the magnetron sputtering technique as an economic, quick, convenient and safe method, had been developed to introduce helium atoms into metal materials. The magnetron sputtering method can introduce helium atoms uniformly into materials to some extent, and it would not lead to serious displacement damage. The above advantages in magnetron sputtering method are helpful for studying He behavior in FeCrNi films. Originally, Mattox et al. [23] had successfully fabricated Au films with He concentration up to 40 at% under pure helium atmosphere by cathode sputtering technology, which provided an effective method for introducing helium in general laboratory environment. So far, this method has been extended to other He-charged metal films, such as Al film [24], Cu film [25] and Ti film [26]. In our previous studies, He-charged FeCrNi and W films had been successfully prepared by magnetron sputtering technique, in which He concentration can be conveniently controlled by adjusting He/Ar partial pressure ratio [27, 28]. On the basis of our previous research, the helium-free and helium-charged FeCrNi nanocrystalline films were fabricated using a similar RF magnetron sputtering technique in this study, and helium desorption behavior and growth process of helium bubbles in FeCrNi films were thus carefully studied by thermal desorption spectroscopy (TDS) spectrum combined with microstructure analysis.
2. Experimental procedures 2.1 Material preparation Nanocrystalline FeCrNi films were prepared on Si single crystal substrates with a (111) preferred orientation using a similar RF magnetron sputtering route reported in Ref. [26]. A 304 stainless steel wafer (60 mm 3 mm) was used as the sputter target, and the sputtering power and sputtering time were controlled at 70 W and 4 h, respectively. During the sputtering process, pure Ar and a mixed atmosphere of He/Ar=4 were used to prepare the He free and He-charged FeCrNi films. In order to investigate the mechanism of He bubble growth, the He-charged FeCrNi films were annealed at 673 K, 873 K and 1073 K for 2 h, 4 h and 8 h to form He bubbles with different sizes. 2.2 Material analysis method The crystalline structure of FeCrNi films deposited on Si substrates was analyzed by X-ray diffractometer (XRD, A Philips X’pert PRO). Surface and cross-sectional morphology of the prepared films were observed by field emission scanning electron microscopy (FESEM, FEI Sirion 200) with an acceleration voltage of 5 keV. The element composition of deposited FeCrNi films were evaluated by an energy dispersive X-ray spectrometer (EDS) analysis using EDX XFlash 3001 detector and the Oxford Instruments INCA Energy system. The accelerating voltages were 10 keV. The desorption characteristics of helium in FeCrNi film was measured by a TDS instrument (TPD Workstation, Hiden Analytical Ltd.) with pre-vacuum up to 10-8 Pa. During He desorption analysis in the temperature range from room temperature (RT) to 1273 K, the mass spectrum of helium was collected by a quadrupole mass spectrometer with a heating rate of 10 K/min. The formation and distribution of helium bubbles in the as-deposited and the annealed He-charged FeCrNi films were observed and analyzed using a JEOL-2100 high resolution transmission electron microscopy (HRTEM) at an accelerated voltage of 200 kV.
3. Result and discussion 3.1 Microstructure
Fig.1 XRD patterns of He free FeCrNi films, as-deposited He-charged FeCrNi film and He-charged FeCrNi films annealed at 673K, 773K and 873K for 2h.
Fig.1 gives the XRD patterns of FeCrNi films deposited on the single crystal Si substrates in a pure Ar atmosphere and a mixed atmosphere of He/Ar=4, and two diffraction peaks in the XRD spectra under both conditions are corresponding to the crystallographic plane (110) and (211) of α-Fe, which indicates that the crystal structure of FeCrNi films prepared under pure Ar and He/Ar=4 belongs to typical
BCC structure, and the effect of He addition on the crystal structure of FeCrNi films can be neglected. In general, 304 steel is one of typical austenitic stainless steels with FCC structure. However, in this study the metastable BCC phase was produced by using the magnetron sputtering method, and the similar phenomenon was reported in references [29, 30]. As reported in the above references, by far the most effective way of producing single-phase BCC 304 SS is through rapid quenching method. In this study, the formation of FeCrNi films with BCC structure is probably due to a fast cooling process of the films on substrates (about 40 ºC) during sputtering deposition. Meanwhile, it was also found that the metastable BCC -phase transforms into the non-magnetic -phase after the annealing above 773 K [31]. To identify whether there was the phase transition or not in our experiment, the XRD spectra of He-charged FeCrNi films annealed at 673 K, 773 K and 873 K for 2h were also shown in Fig.1. As shown in Fig.1, the annealed FeCrNi film at 673 K had started to transform into the -phase with FCC structure, thus He-charged FeCrNi films after the annealing above 673 K had a FCC structure.
Fig.2 Surface morphology of FeCrNi films deposited on Si substrates under different He/Ar ratios: (a) 0, (b) 4:1；Cross-sectional morphology of FeCrNi films deposited on Si substrates under different He/Ar ratios: (c) 0 and (d) 4:1.
Fig.2 shows the surface and cross-section morphology of FeCrNi films deposited at room temperature (RT) under He/Ar=0 and He/Ar=4, respectively. Spherical nanocrystalline particles were observed from surface morphology, as shown in Fig. 2(a) and Fig. 2(b), in which most of the grains aggregated into clusters. Statistical
analysis of the surface SEM images demonstrates that the average grain size of the FeCrNi film is about 100 nm prepared in pure Ar atmosphere, and further decreases to about 75 nm for the film deposited in a mixed atmosphere of He/Ar=4. Compared with pure FeCrNi film, the decrease of the grain size in He-charged FeCrNi film is possibly associated with a large number of defects (vacancies, dislocations, etc.), which may result from the introduction of helium atoms during the sputtering process. According to the cross-section morphology shown in Fig. 2(c) and Fig. 2(d), FeCrNi films deposited in both pure Ar atmosphere and a mixed atmosphere of He/Ar=4 exhibit columnar crystal structures. The thickness of the FeCrNi films is about 2~2.5 µm, and the difference in sputtering atmosphere does not significantly affect the film thickness.
Samples
Cr (wt.%)
Ni (wt.%)
Fe (wt.%)
Alloy target
18
8
74
(Film) He/Ar=0
16
20
64
(Film) He/Ar=4
16
21
64
Table 1 Element composition of FeCrNi alloy target and FeCrNi films fabricated under varying He/Ar ratios.
Table.1 shows the results of EDS analysis for FeCrNi target, He free and He-charged FeCrNi films. He free FeCrNi film contains almost the same proportion of Fe, Cr and Ni elements compared with He-charged FeCrNi film. However, Ni contents in both two films are higher than that of FeCrNi alloy target. The possible reason is that the sputtering rate of Ni atoms is higher than that of Fe atoms and Cr atoms, resulting in an increase in Ni content in both two films. 3.2 He desorption behavior
Fig.3 Thermal desorption spectroscopy of FeCrNi films deposited on Si substrates under different He/Ar ratios of 0 and 4:1, respectively.
The TDS method has been widely used to explore the trapping states of He atoms in materials, including interstitial helium atoms, He clusters and He bubbles and so on [32]. To investigate the possible existence states of He in He-charged FeCrNi films, the TDS profiles of the FeCrNi films sputtered on Si substrates under two different He/Ar ratios (0, 4:1) at room temperature are shown, within the range from RT to 1273 K in Fig.3. It is understandable that for the FeCrNi film prepared in pure Ar atmosphere, no He desorption peak was detected in the TDS curve since it was essential He free. For the case of FeCrNi film prepared under He/Ar=4, three wide peaks at different temperature regions were observed in TDS profile. For clear description, the three He desorption peaks are labeled as P1 peak, P2 peak and P3 peak, which are corresponding to 633 K, 893 K and 1046 K, respectively. It is necessary to point out that once desorption temperature increased up to about 1200 K, a rapidly increased background related to He desorption was observed, which actually corresponds to another He desorption peak although it does not appear completely due to the limitation of temperature range for the present TDS instrument. Generally, the higher helium desorption temperature corresponds to the higher binding energy of helium atoms in the material. As well-known，for different existence of He atoms in materials, such as interstitial helium atoms, helium-vacancy complexes and helium bubbles, the binding energy is quite different, thus the desorption temperature is accordingly different. Among different He defects, the
binding energy of He atoms at the interstitial sites is the smallest, which corresponds to the lowest desorption temperature in TDS profile. Accordingly, He bubbles contain more He atoms and vacancies (at least several hundreds of He atoms and vacancies) than HemVn complexes, the release of He atoms from He bubbles is more difficult. Thus, the necessary desorption temperature of He atoms from He bubbles will be the highest owing to its largest binding energy. Based on the first-order dissociation kinetics theory, the desorption activation energy Ed per He atom can be calculated according to the following equation [33]: Ed = [ln (νTp/β) - 3.64] kBTp
(1)
Where Ed is the desorption activation energy per He atom, ν is the jumping frequency of atoms (usually on the order of 1013/s), kB is the Boltzmann constant, Tp is the corresponding desorption peak temperature (K), and β is the heating rate (K/s). Thus, according to the desorption peak positions shown in Fig.3, the He desorption activation energies for different desorption peaks can be obtained. Due to the desorption activation energy is the sum of the binding energy and the migration energy, and the migration energy of He in 304ss is a constant (0.078 eV), so the binding energy can also be obtained, as listed in Table. 2. It is reasonable that the binding energy of He gradually increases with desorption peak temperature increasing, which corresponds to different trapping states of He in FeCrNi films. Peak
Peak position (K)
Desorption energy (eV)
Binding energy (eV)
P1
633
1.90.1
1.820.1
P2
893
2.70.1
2.620.1
P3
1046
3.20.05
3.120.05
Table 2 He desorption activation energy for the three different desorption peaks
In other He irradiated Fe-based materials [34], four He desorption peaks were also reported in the temperature range of RT~1150 K，and the peak positions in the TDS spectra centered around 610 K, 758 K, 1025 K, 1100 K, respectively. The He trapping mechanisms corresponding to the four peaks were interpreted as the desorption process of different He defects from Fe matrix, i.e. interstitial helium atoms from the
near-surface region of samples, helium-single vacancy complexes HenV (2 < n < 6), helium multi-vacancy complexes HenVm (1 < n, 2 < m) [34-37] and He bubbles, respectively [34]. These He desorption behaviors are quite similar to our present results: the observed three peaks at lower temperature should correspond to P1, P2 and P3 peaks in Fig.3, respectively, while the fourth peak centered at the highest temperature of 1100 K can be associated with the higher desorption peak above 1200 K in this work. For the P1 peak, the low desorption temperature denotes that He atoms are rather weakly trapped in the trapping sites. According to Table 2 and TDS analysis in Refs. [37], the He desorption mechanisms of P1 peak may be similarly ascribed to interstitial He atoms desorbed from the neighborhood of surface. Compared with P1 peak, P2 and P3 peak at higher temperatures are related to the desorption of He atoms from HenVm complexes with different He-to-vacancy ratio (n/m) for each He trapping mechanism [36]. The binding energy of helium atoms to HenVm clusters decreases with increasing He-to-vacancy ratio and the binding energy of vacancy to HenVm clusters increases with increasing He-to-vacancy ratio. Therefore, He-to-vacancy ratio in HenVm complexes can determine the thermal stability of complexes. According to Refs. [36], HenVm complexes are relatively stable when the He-to-vacancy ratio is about 1.8, and HenVm clusters emit or absorb vacancies when helium-to-vacancy ratio is less or larger than 1.8. Based on this consideration as well as the analysis of desorption activation energy in Table 2, it is speculated that P2 peak corresponds to He desorption from helium-single vacancy complexes HenV (2 < n < 6) [35-38], while P3 peak is generally attributed to He desorption from helium multi-vacancy complexes HenVm (1 < n, 2 < m) [36, 37]. The He desorption is also observed above 1200 K in Fig. 3, which should result from the release of He atoms from helium bubbles as ascribed in Ref. [34, 36]. By comparing with the intensity of different He desorption processes shown in Fig.3, it is found that the He desorption above 1200 K seems to be extremely obvious although only the lower side of desorption peak was observed owing to the limitation of measurement temperature. This result indicates that the number or the concentration of He bubbles is quite high in FeCrNi films prepared by the magnetron sputtering method.
Moreover, it is interesting to note that the desorption temperatures for different He defects in FeCrNi nanocrystalline film are higher than that in coarse-grained Fe and FeCr-based materials, which implies that He-vacancy clusters possibly have higher binding energy in FeCrNi films. R. Sugan et al. had investigated the thermal desorption of He atoms in coarse-grained Fe and Fe-5Cr irradiated by 8 keV He ions [38], and found that the desorption temperature and desorption activation energy for HenVm were about 950 K and 2.9 eV, respectively. The obtained values are smaller to some degrees than the corresponding result for HenVm clusters in our work: 1046 K for desorption peak temperature and desorption activation energy for 3.2 eV, as shown in Fig.3 and Table. 2. The increase of desorption temperature and activation energy in FeCrNi nanocrystalline film possibly originates from the capture of high density grain boundaries and dislocations in nanocrystalline materials, thus inhibiting the aggregation of helium-vacancy cluster as well as the growth of helium bubbles [39]. 3.3 Formation and evolution of He bubbles
Fig.4 Bright field TEM images of bubbles in FeCrNi films prepared on Si substrates under a He/Ar ratio of 4:1
Generally, He bubble formation and evolution can be investigated using TEM observation. Fig. 4(a) shows the bright field TEM images of He-charged FeCrNi film prepared under a He/Ar ratio of 4:1, and a large number of He bubbles (~1 nm) were found to be distributed uniformly in the film. The formation of He bubbles in as-deposited films may attribute to high mobility of ionized He atoms and abundant
vacancies in the depositing FeCrNi films, which are in favor of aggregation of these He atoms and formation of He bubbles during the magnetron sputtering. To clearly analyze the morphology and distribution of He bubbles, the local high-resolution TEM photograph are presented in Fig. 4(b). It can be seen that many He bubbles were also distributed near the grain boundary and formed as a necklace-like line. This phenomenon is close to the result obtained by Zheng et al. [40]. It was reported that the preferential nucleation of He bubbles easily occurred in some particular defect regions related with dislocation and grain boundary [41, 42]. As for He-charged FeCrNi film, vacancies can be easily produced and trapped by grain boundaries during the sputtering process. Such vacancies exhibit high affinity with He, which may drive helium atoms to move toward grain boundaries, and further aggregate to form helium bubbles [41]. Furthermore, some amorphous regions containing some helium bubbles were also observed, as shown in the zone A and zone B of the Fig. 4(b). The formation of amorphization may be possibly induced by the aggregation of He bubbles in the grain interiors [43, 44].
Fig.5 Under-focused (-800nm) TEM images of He-charged FeCrNi films prepared at He/Ar=4 and annealed at 673 K for 2h (a), 4h (b) and 8h (c); at 873K for 2h (d), 4h (e) and 8h (f); at 1073K for 2h (g), 4h (h) and 8h (i).
In order to systematically study the growth mechanism of He bubbles in
FeCrNi-based austenitic stainless steel, the samples were annealed at different temperature with a constant time or at a constant temperature with different times. Based on the analysis of XRD spectra in Fig.1, the annealing temperature should be set at above 673K to make the crystal structure of FeCrNi film become FCC structure. The annealing process was performed in a high vacuum (<10-5 Pa) quartz to avoid oxidation. Fig. 5 gives TEM images of the annealed He-charged FeCrNi films at 673 K, 873 K and 1073 K for 2 h, 4 h and 8 h, respectively. As shown in Fig. 5(a-c), a large amount of small He bubbles (about 1 nm) marked with white dots were distributed in He-charged films annealed 673 K, and the size and number density of these bubbles only increase a little with increasing annealing time. These results indicate that He bubbles grow slowly at 673 K. When annealing temperature reached to 873 K, compared with that of at 673 K, He bubble started to grow rapidly accompanied by the dramatic decrease of number density. And the He bubble size increased and number density decreased when the annealing time increased from 2 h to 8 h, as shown in Fig. 5(d-f). The above results implied that the growth of He bubble in FeCrNi films can be sufficiently started to realize at 873 K. When the annealing temperature increased up to 1073 K, He bubble size further grew up compared with the case of 873 K, as well as it increases significantly with increasing the annealing time as shown in Fig. 5(g-i). It is evident that He bubbles accumulate disorderly not only in the interiors of the grain but also along grain boundaries, especially in Fig. 5(d) and Fig. 5(g), where the grain boundaries were marked with red arrows and some He bubbles accumulated along the grain boundary like a necklace. The relationship between the average size of He bubbles and annealing temperature or annealing time is depicted in Fig. 6(a), in which the radius of He bubbles was calculated by counting more than 300 helium bubbles in TEM photographs for each annealed sample.
Fig.6. (a) The average He bubble radius of He-charged FeCrNi films annealed at 673K, 873K and 1073K versus the annealing time; (b) Temperature dependence of helium bubble radius of He-charged FeCrNi annealed for 8h.
It has been pointed out that there are two main growth mechanisms of He bubbles in metals: one is the migration and coalesce (MC) mechanism, which occurs mainly through volume diffusion, surface diffusion and vapor transport [45]; another mechanism is Ostwald ripening (OR) mechanism, in which helium atoms are dissolved into the matrix from the smaller bubbles and reprecipitated in larger bubbles [46]. To further study the growth mechanism of He bubbles in FeCrNi nanocrystalline film, the growth of the mean bubble radius for MC and OR is quoted in Eq. (2) [47]. (2) Where
is implanted He concentration for MC or He concentration dissolved in the
matrix for OR,
is the diffusivity, ta is the annealing time and n has different
values depending on the operating diffusion mechanism [45, 47]. And in order to facilitate the calculation of the apparent activation energy, we kept the annealing time constant to analyze the relation between the average bubble radius and the annealing temperature. Therefore, we kept the annealing time constant and introduced the first Fick’s law of diffusion in Eq. (2), an Arrhenius equation can be deduced as follows. (3) Where D0 is the pre-exponential constant in m2/s, Q is the activation energy in eV, k is the Boltzmann constant in eV/K and r0 is the pre-exponential constant in nm that is the product of the constants in Eq. (2). The exponent value (n) is inherent in the experimental data when the apparent activation (Ea = Q/n) energy is obtained from Eq.
(3). Hence, the average radii can be calculated using the following equation [48, 49]: (4) Where A is the pre-exponential constant in nm that is the product of the constant r0 in the equation Eq. (3), C is a constant. As Fig. 6(a) shown that He bubbles tend to grow slow after the annealing for 8h and He bubble size reach a constant at a given temperature. Thus， the relationship between the average He bubble size and the annealing temperature for films annealed for 8h was discussed, as shown in Fig. 6(b). Based on the above formula, the apparent activation energy is deduced as 0.240.02 eV by means of a multiple non-linear curve fitting method. The apparent activation energy is very low, which is close to the calculated value of 0.29±0.015 eV deduced from the data in the low-temperature regime of He bubbles coarsening by MC in 304L reported by J. Chen et al. [50], which is far less than the activation energy calculated by OR mechanism [49]. Based on the above discussions, we speculate that MC is the prevailing coarsening mechanism of He bubbles in FeCrNi films during the annealing process within the range of 1073 K. He bubble growth mechanism still needs to be proved by more experiments, especially in situ TEM tests. For example, K. Ono et al. [51] had investigated the formation and migration of helium bubbles in Fe and Fe-9Cr alloy by in situ TEM. In their study, the Brown type motion of He bubbles was observed dynamically in the irradiated Fe and Fe-9Cr alloy. This demonstrated that the growth of He bubble in Fe and Fe-9Cr alloy should be accorded with the MC mechanism in a certain temperature range. If possible, the observation of He bubbles will be carried out by in situ TEM in subsequent experiments.
4. Conclusion FeCrNi films with and without He atoms were prepared by using RF magnetron sputtering method under a He/Ar mixed atmosphere and a pure Ar atmosphere. The results of XRD and SEM analysis showed that the He-charged FeCrNi film had a bcc
columnar structure. For the as-deposited He-charged FeCrNi film, the TDS spectra had roughly three He desorption peaks in the temperature range from RT-1273 K: P1 peak (633 K), P2 peak (893 K), P3 peak (1046 K), which corresponded to the release of interstitial helium atoms from the surface of films, helium-single vacancy complexes HenV (2 < n < 6) and helium multi-vacancy complexes HenVm (1 < n, 2 < m), respectively. And the desorption of He bubbles would occur at the temperature higher than 1273 K. TEM observations showed that the helium bubbles were uniformly distributed in grain interiors of the as-deposited He-charged FeCrNi film, and denser and a necklace-like line of He bubbles located near grain boundaries, the He bubble size was about 1 nm. At the same time, TEM images of the annealed FeCrNi films showed that helium bubbles mainly began to grow remarkably at 873 K and the size of He bubbles increased and the number density of He bubbles decreased with the annealing temperature and time increasing. Moreover, the apparent activation energy was about 0.24 eV, which was inferred that the growth of helium bubbles in FeCrNi films may mainly be governed by the MC mechanism instead of the OR mechanism. This study about the growth mechanism of helium bubbles in FeCrNi films provided the experimental foundation and theoretical basis for better understanding the He bubble growth behavior and further inhibiting the growth of helium bubbles in iron-based nuclear materials. It is of positive significance for the future development of structural materials in nuclear reactor.
Acknowledgments: This work was financially supported by the National Key Research and Development Program
of
China
(Grant
No.
2017YFE0300403,
2017YFA0402800,
2017YFE0300402) and the National Natural Science Foundation of China (Grant Nos. 5171181, 11374299, 11575231, 11674319, 51771184, 11775255).
Conflict of interest We declare that we have no any commercial or associative conflicts of interest to this work (the manuscript entitled as "Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film").
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Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film L. Wang , X.P. Wang , L.F. Zhang , Y.X. Gao , R. Liu , R. Gao , Y. Jiang , T. Hao , T. Zhang , Q.F. Fang , C.S. Liu PII: DOI: Reference:
S2352-1791(19)30055-9 https://doi.org/10.1016/j.nme.2019.100710 NME 100710
To appear in:
Nuclear Materials and Energy
Received date: Revised date: Accepted date:
26 August 2019 23 October 2019 23 October 2019
Please cite this article as: L. Wang , X.P. Wang , L.F. Zhang , Y.X. Gao , R. Liu , R. Gao , Y. Jiang , T. Hao , T. Zhang , Q.F. Fang , C.S. Liu , Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film, Nuclear Materials and Energy (2019), doi: https://doi.org/10.1016/j.nme.2019.100710
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Highlights: He bubble size grows significantly up in He-charged FeCrNi film subjecting to the annealing from about 1 nm in diameter under 300 K to 5 nm under 1073 K. It is speculated that migration coarsening (MC) plays a main role in He bubble growth in He-charged FeCrNi films.
Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film L. Wang1, 2, X. P. Wang1, , L. F. Zhang1, 2, Y. X. Gao1, R. Liu1, R. Gao1, Y. Jiang1, T. Hao1, , T. Zhang 1 , Q.F. Fang1, C.S. Liu1 1
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of
Sciences, Hefei 230031, China 2
University of Science and Technology of China, Hefei 230026, China
*Corresponding authors, X. P. Wang: [email protected]; T. Hao: [email protected] Telephone: +86/0551/65591125; Fax: +86/0551/65591434
Abstract: He-charged FeCrNi films were prepared by radio frequency (RF) magnetron sputtering technique, and helium desorption behavior, growth process and evolution mechanism of helium bubbles in FeCrNi film were studied by thermal desorption spectroscopy (TDS) spectra combined with microstructure analysis under different annealing treatments. Three different He desorption peaks were observed in TDS spectra, which can be related to the release of interstitial helium atoms from the near-surface regions, helium-vacancy clusters with different sizes in FeCrNi film. TEM analysis showed that helium bubbles can be uniformly distributed in He-charged FeCrNi nanocrystalline films. Under annealing treatments at different temperatures, the size of helium bubbles obviously increases with increasing annealing temperature, corresponding to the significantly decrease in the number density of helium bubbles in FeCrNi films, and the growth mechanism of helium bubbles was suggested to originate from the migration and coalescence (MC) mechanism.
Key words: Magnetron sputtering, FeCrNi film, Thermal desorption, He bubbles. * Corresponding author, [email protected] (X. P. Wang);
[email protected] (T. Hao)
1. Introduction FeCrNi-based austenitic stainless steels have been widely used as the structural materials in nuclear reactors, because of their good mechanical properties, low corrosion and high resistance to radiation swelling [1, 2]. However, neutron irradiation in nuclear reactors not only causes vacancy-type defects, but also generates a large amount of helium (He) atoms by the (n, α) nuclear reaction [2]. The further study had indicated as well that He atoms are extremely difficult to be dissolved in metals and are easy to migrate owing to its low migration energy (~ 0.078 eV) [3]. As a result, He atoms easily migrated or diffused can aggregate and form high-density He bubbles in metals, resulting in the service performance degradation of materials, such as lower elongation, shorter creep fracture time as well as fatigue life [4, 5]. Therefore, the effect of He in FeCrNi-based austenitic stainless steels has become a very important and challenging research topic in nuclear structure materials. On the microscopic perspective, the radiation damage of materials is essentially caused by the continuous accumulation of irradiation defects, macroscopically resulting in swelling, hardening and embrittlement of materials. Hence the irradiation resistance of materials usually depends on whether they have self-healing ability to eliminate irradiation defects. Previous studies have shown that high density solid-solid interfaces in ultrafine-grained materials (especially nanocrystalline materials) can also act as helium absorption centers, diluting and absorbing helium defects, thus effectively inhibiting the nucleation and growth of helium bubbles. For example, ultrafine-grained W-TiC (100 nm~1µm) [6], nanocrystalline TiNi alloy (23~31 nm) [7] and Au (~23 nm) [8] exhibit stronger ion irradiation resistance than corresponding coarse-grained materials. Moreover, nanocrystalline Fe (~49 nm) [9] and Fe/TiO2 (~100 nm) [10] also exhibit stronger ability to inhibit He bubble growth than their coarse-grained counterparts. Therefore, grain refinement can be considered as one of the effective ways to inhibit the nucleation and growth of helium bubbles as well as He embrittlement problem. As for He behavior in nanocrystalline FeCrNi-based structural materials, how to
refine the grain and uniformly introduce a certain concentration of He atoms into metal matrix is the precondition. In general, the main methods of introducing helium into metals include tritium decay [11-14], neutron irradiation [15-19] and helium ion implantation [20-22]. The first two methods can introduce helium uniformly into materials, but the radioactive characteristics make it difficult to operate and control. Although He ion implantation method can easily control the implanted concentration of helium, it still has some disadvantages. For low energy He ion implantation, most of helium atoms are only introduced into the surface layer of the materials. And for high energy He ion plantation, it can produce displacement damage and influence He diffusion, agglomeration and bubble nucleation in metal materials. Remarkably, in recent years the magnetron sputtering technique as an economic, quick, convenient and safe method, had been developed to introduce helium atoms into metal materials. The magnetron sputtering method can introduce helium atoms uniformly into materials to some extent, and it would not lead to serious displacement damage. The above advantages in magnetron sputtering method are helpful for studying He behavior in FeCrNi films. Originally, Mattox et al. [23] had successfully fabricated Au films with He concentration up to 40 at% under pure helium atmosphere by cathode sputtering technology, which provided an effective method for introducing helium in general laboratory environment. So far, this method has been extended to other He-charged metal films, such as Al film [24], Cu film [25] and Ti film [26]. In our previous studies, He-charged FeCrNi and W films had been successfully prepared by magnetron sputtering technique, in which He concentration can be conveniently controlled by adjusting He/Ar partial pressure ratio [27, 28]. On the basis of our previous research, the helium-free and helium-charged FeCrNi nanocrystalline films were fabricated using a similar RF magnetron sputtering technique in this study, and helium desorption behavior and growth process of helium bubbles in FeCrNi films were thus carefully studied by thermal desorption spectroscopy (TDS) spectrum combined with microstructure analysis.
2. Experimental procedures 2.1 Material preparation Nanocrystalline FeCrNi films were prepared on Si single crystal substrates with a (111) preferred orientation using a similar RF magnetron sputtering route reported in Ref. [26]. A 304 stainless steel wafer (60 mm 3 mm) was used as the sputter target, and the sputtering power and sputtering time were controlled at 70 W and 4 h, respectively. During the sputtering process, pure Ar and a mixed atmosphere of He/Ar=4 were used to prepare the He free and He-charged FeCrNi films. In order to investigate the mechanism of He bubble growth, the He-charged FeCrNi films were annealed at 673 K, 873 K and 1073 K for 2 h, 4 h and 8 h to form He bubbles with different sizes. 2.2 Material analysis method The crystalline structure of FeCrNi films deposited on Si substrates was analyzed by X-ray diffractometer (XRD, A Philips X’pert PRO). Surface and cross-sectional morphology of the prepared films were observed by field emission scanning electron microscopy (FESEM, FEI Sirion 200) with an acceleration voltage of 5 keV. The element composition of deposited FeCrNi films were evaluated by an energy dispersive X-ray spectrometer (EDS) analysis using EDX XFlash 3001 detector and the Oxford Instruments INCA Energy system. The accelerating voltages were 10 keV. The desorption characteristics of helium in FeCrNi film was measured by a TDS instrument (TPD Workstation, Hiden Analytical Ltd.) with pre-vacuum up to 10-8 Pa. During He desorption analysis in the temperature range from room temperature (RT) to 1273 K, the mass spectrum of helium was collected by a quadrupole mass spectrometer with a heating rate of 10 K/min. The formation and distribution of helium bubbles in the as-deposited and the annealed He-charged FeCrNi films were observed and analyzed using a JEOL-2100 high resolution transmission electron microscopy (HRTEM) at an accelerated voltage of 200 kV.
3. Result and discussion 3.1 Microstructure
Fig.1 XRD patterns of He free FeCrNi films, as-deposited He-charged FeCrNi film and He-charged FeCrNi films annealed at 673K, 773K and 873K for 2h.
Fig.1 gives the XRD patterns of FeCrNi films deposited on the single crystal Si substrates in a pure Ar atmosphere and a mixed atmosphere of He/Ar=4, and two diffraction peaks in the XRD spectra under both conditions are corresponding to the crystallographic plane (110) and (211) of α-Fe, which indicates that the crystal structure of FeCrNi films prepared under pure Ar and He/Ar=4 belongs to typical
BCC structure, and the effect of He addition on the crystal structure of FeCrNi films can be neglected. In general, 304 steel is one of typical austenitic stainless steels with FCC structure. However, in this study the metastable BCC phase was produced by using the magnetron sputtering method, and the similar phenomenon was reported in references [29, 30]. As reported in the above references, by far the most effective way of producing single-phase BCC 304 SS is through rapid quenching method. In this study, the formation of FeCrNi films with BCC structure is probably due to a fast cooling process of the films on substrates (about 40 ºC) during sputtering deposition. Meanwhile, it was also found that the metastable BCC -phase transforms into the non-magnetic -phase after the annealing above 773 K [31]. To identify whether there was the phase transition or not in our experiment, the XRD spectra of He-charged FeCrNi films annealed at 673 K, 773 K and 873 K for 2h were also shown in Fig.1. As shown in Fig.1, the annealed FeCrNi film at 673 K had started to transform into the -phase with FCC structure, thus He-charged FeCrNi films after the annealing above 673 K had a FCC structure.
Fig.2 Surface morphology of FeCrNi films deposited on Si substrates under different He/Ar ratios: (a) 0, (b) 4:1；Cross-sectional morphology of FeCrNi films deposited on Si substrates under different He/Ar ratios: (c) 0 and (d) 4:1.
Fig.2 shows the surface and cross-section morphology of FeCrNi films deposited at room temperature (RT) under He/Ar=0 and He/Ar=4, respectively. Spherical nanocrystalline particles were observed from surface morphology, as shown in Fig. 2(a) and Fig. 2(b), in which most of the grains aggregated into clusters. Statistical
analysis of the surface SEM images demonstrates that the average grain size of the FeCrNi film is about 100 nm prepared in pure Ar atmosphere, and further decreases to about 75 nm for the film deposited in a mixed atmosphere of He/Ar=4. Compared with pure FeCrNi film, the decrease of the grain size in He-charged FeCrNi film is possibly associated with a large number of defects (vacancies, dislocations, etc.), which may result from the introduction of helium atoms during the sputtering process. According to the cross-section morphology shown in Fig. 2(c) and Fig. 2(d), FeCrNi films deposited in both pure Ar atmosphere and a mixed atmosphere of He/Ar=4 exhibit columnar crystal structures. The thickness of the FeCrNi films is about 2~2.5 µm, and the difference in sputtering atmosphere does not significantly affect the film thickness.
Samples
Cr (wt.%)
Ni (wt.%)
Fe (wt.%)
Alloy target
18
8
74
(Film) He/Ar=0
16
20
64
(Film) He/Ar=4
16
21
64
Table 1 Element composition of FeCrNi alloy target and FeCrNi films fabricated under varying He/Ar ratios.
Table.1 shows the results of EDS analysis for FeCrNi target, He free and He-charged FeCrNi films. He free FeCrNi film contains almost the same proportion of Fe, Cr and Ni elements compared with He-charged FeCrNi film. However, Ni contents in both two films are higher than that of FeCrNi alloy target. The possible reason is that the sputtering rate of Ni atoms is higher than that of Fe atoms and Cr atoms, resulting in an increase in Ni content in both two films. 3.2 He desorption behavior
Fig.3 Thermal desorption spectroscopy of FeCrNi films deposited on Si substrates under different He/Ar ratios of 0 and 4:1, respectively.
The TDS method has been widely used to explore the trapping states of He atoms in materials, including interstitial helium atoms, He clusters and He bubbles and so on [32]. To investigate the possible existence states of He in He-charged FeCrNi films, the TDS profiles of the FeCrNi films sputtered on Si substrates under two different He/Ar ratios (0, 4:1) at room temperature are shown, within the range from RT to 1273 K in Fig.3. It is understandable that for the FeCrNi film prepared in pure Ar atmosphere, no He desorption peak was detected in the TDS curve since it was essential He free. For the case of FeCrNi film prepared under He/Ar=4, three wide peaks at different temperature regions were observed in TDS profile. For clear description, the three He desorption peaks are labeled as P1 peak, P2 peak and P3 peak, which are corresponding to 633 K, 893 K and 1046 K, respectively. It is necessary to point out that once desorption temperature increased up to about 1200 K, a rapidly increased background related to He desorption was observed, which actually corresponds to another He desorption peak although it does not appear completely due to the limitation of temperature range for the present TDS instrument. Generally, the higher helium desorption temperature corresponds to the higher binding energy of helium atoms in the material. As well-known，for different existence of He atoms in materials, such as interstitial helium atoms, helium-vacancy complexes and helium bubbles, the binding energy is quite different, thus the desorption temperature is accordingly different. Among different He defects, the
binding energy of He atoms at the interstitial sites is the smallest, which corresponds to the lowest desorption temperature in TDS profile. Accordingly, He bubbles contain more He atoms and vacancies (at least several hundreds of He atoms and vacancies) than HemVn complexes, the release of He atoms from He bubbles is more difficult. Thus, the necessary desorption temperature of He atoms from He bubbles will be the highest owing to its largest binding energy. Based on the first-order dissociation kinetics theory, the desorption activation energy Ed per He atom can be calculated according to the following equation [33]: Ed = [ln (νTp/β) - 3.64] kBTp
(1)
Where Ed is the desorption activation energy per He atom, ν is the jumping frequency of atoms (usually on the order of 1013/s), kB is the Boltzmann constant, Tp is the corresponding desorption peak temperature (K), and β is the heating rate (K/s). Thus, according to the desorption peak positions shown in Fig.3, the He desorption activation energies for different desorption peaks can be obtained. Due to the desorption activation energy is the sum of the binding energy and the migration energy, and the migration energy of He in 304ss is a constant (0.078 eV), so the binding energy can also be obtained, as listed in Table. 2. It is reasonable that the binding energy of He gradually increases with desorption peak temperature increasing, which corresponds to different trapping states of He in FeCrNi films. Peak
Peak position (K)
Desorption energy (eV)
Binding energy (eV)
P1
633
1.90.1
1.820.1
P2
893
2.70.1
2.620.1
P3
1046
3.20.05
3.120.05
Table 2 He desorption activation energy for the three different desorption peaks
In other He irradiated Fe-based materials [34], four He desorption peaks were also reported in the temperature range of RT~1150 K，and the peak positions in the TDS spectra centered around 610 K, 758 K, 1025 K, 1100 K, respectively. The He trapping mechanisms corresponding to the four peaks were interpreted as the desorption process of different He defects from Fe matrix, i.e. interstitial helium atoms from the
near-surface region of samples, helium-single vacancy complexes HenV (2 < n < 6), helium multi-vacancy complexes HenVm (1 < n, 2 < m) [34-37] and He bubbles, respectively [34]. These He desorption behaviors are quite similar to our present results: the observed three peaks at lower temperature should correspond to P1, P2 and P3 peaks in Fig.3, respectively, while the fourth peak centered at the highest temperature of 1100 K can be associated with the higher desorption peak above 1200 K in this work. For the P1 peak, the low desorption temperature denotes that He atoms are rather weakly trapped in the trapping sites. According to Table 2 and TDS analysis in Refs. [37], the He desorption mechanisms of P1 peak may be similarly ascribed to interstitial He atoms desorbed from the neighborhood of surface. Compared with P1 peak, P2 and P3 peak at higher temperatures are related to the desorption of He atoms from HenVm complexes with different He-to-vacancy ratio (n/m) for each He trapping mechanism [36]. The binding energy of helium atoms to HenVm clusters decreases with increasing He-to-vacancy ratio and the binding energy of vacancy to HenVm clusters increases with increasing He-to-vacancy ratio. Therefore, He-to-vacancy ratio in HenVm complexes can determine the thermal stability of complexes. According to Refs. [36], HenVm complexes are relatively stable when the He-to-vacancy ratio is about 1.8, and HenVm clusters emit or absorb vacancies when helium-to-vacancy ratio is less or larger than 1.8. Based on this consideration as well as the analysis of desorption activation energy in Table 2, it is speculated that P2 peak corresponds to He desorption from helium-single vacancy complexes HenV (2 < n < 6) [35-38], while P3 peak is generally attributed to He desorption from helium multi-vacancy complexes HenVm (1 < n, 2 < m) [36, 37]. The He desorption is also observed above 1200 K in Fig. 3, which should result from the release of He atoms from helium bubbles as ascribed in Ref. [34, 36]. By comparing with the intensity of different He desorption processes shown in Fig.3, it is found that the He desorption above 1200 K seems to be extremely obvious although only the lower side of desorption peak was observed owing to the limitation of measurement temperature. This result indicates that the number or the concentration of He bubbles is quite high in FeCrNi films prepared by the magnetron sputtering method.
Moreover, it is interesting to note that the desorption temperatures for different He defects in FeCrNi nanocrystalline film are higher than that in coarse-grained Fe and FeCr-based materials, which implies that He-vacancy clusters possibly have higher binding energy in FeCrNi films. R. Sugan et al. had investigated the thermal desorption of He atoms in coarse-grained Fe and Fe-5Cr irradiated by 8 keV He ions [38], and found that the desorption temperature and desorption activation energy for HenVm were about 950 K and 2.9 eV, respectively. The obtained values are smaller to some degrees than the corresponding result for HenVm clusters in our work: 1046 K for desorption peak temperature and desorption activation energy for 3.2 eV, as shown in Fig.3 and Table. 2. The increase of desorption temperature and activation energy in FeCrNi nanocrystalline film possibly originates from the capture of high density grain boundaries and dislocations in nanocrystalline materials, thus inhibiting the aggregation of helium-vacancy cluster as well as the growth of helium bubbles [39]. 3.3 Formation and evolution of He bubbles
Fig.4 Bright field TEM images of bubbles in FeCrNi films prepared on Si substrates under a He/Ar ratio of 4:1
Generally, He bubble formation and evolution can be investigated using TEM observation. Fig. 4(a) shows the bright field TEM images of He-charged FeCrNi film prepared under a He/Ar ratio of 4:1, and a large number of He bubbles (~1 nm) were found to be distributed uniformly in the film. The formation of He bubbles in as-deposited films may attribute to high mobility of ionized He atoms and abundant
vacancies in the depositing FeCrNi films, which are in favor of aggregation of these He atoms and formation of He bubbles during the magnetron sputtering. To clearly analyze the morphology and distribution of He bubbles, the local high-resolution TEM photograph are presented in Fig. 4(b). It can be seen that many He bubbles were also distributed near the grain boundary and formed as a necklace-like line. This phenomenon is close to the result obtained by Zheng et al. [40]. It was reported that the preferential nucleation of He bubbles easily occurred in some particular defect regions related with dislocation and grain boundary [41, 42]. As for He-charged FeCrNi film, vacancies can be easily produced and trapped by grain boundaries during the sputtering process. Such vacancies exhibit high affinity with He, which may drive helium atoms to move toward grain boundaries, and further aggregate to form helium bubbles [41]. Furthermore, some amorphous regions containing some helium bubbles were also observed, as shown in the zone A and zone B of the Fig. 4(b). The formation of amorphization may be possibly induced by the aggregation of He bubbles in the grain interiors [43, 44].
Fig.5 Under-focused (-800nm) TEM images of He-charged FeCrNi films prepared at He/Ar=4 and annealed at 673 K for 2h (a), 4h (b) and 8h (c); at 873K for 2h (d), 4h (e) and 8h (f); at 1073K for 2h (g), 4h (h) and 8h (i).
In order to systematically study the growth mechanism of He bubbles in
FeCrNi-based austenitic stainless steel, the samples were annealed at different temperature with a constant time or at a constant temperature with different times. Based on the analysis of XRD spectra in Fig.1, the annealing temperature should be set at above 673K to make the crystal structure of FeCrNi film become FCC structure. The annealing process was performed in a high vacuum (<10-5 Pa) quartz to avoid oxidation. Fig. 5 gives TEM images of the annealed He-charged FeCrNi films at 673 K, 873 K and 1073 K for 2 h, 4 h and 8 h, respectively. As shown in Fig. 5(a-c), a large amount of small He bubbles (about 1 nm) marked with white dots were distributed in He-charged films annealed 673 K, and the size and number density of these bubbles only increase a little with increasing annealing time. These results indicate that He bubbles grow slowly at 673 K. When annealing temperature reached to 873 K, compared with that of at 673 K, He bubble started to grow rapidly accompanied by the dramatic decrease of number density. And the He bubble size increased and number density decreased when the annealing time increased from 2 h to 8 h, as shown in Fig. 5(d-f). The above results implied that the growth of He bubble in FeCrNi films can be sufficiently started to realize at 873 K. When the annealing temperature increased up to 1073 K, He bubble size further grew up compared with the case of 873 K, as well as it increases significantly with increasing the annealing time as shown in Fig. 5(g-i). It is evident that He bubbles accumulate disorderly not only in the interiors of the grain but also along grain boundaries, especially in Fig. 5(d) and Fig. 5(g), where the grain boundaries were marked with red arrows and some He bubbles accumulated along the grain boundary like a necklace. The relationship between the average size of He bubbles and annealing temperature or annealing time is depicted in Fig. 6(a), in which the radius of He bubbles was calculated by counting more than 300 helium bubbles in TEM photographs for each annealed sample.
Fig.6. (a) The average He bubble radius of He-charged FeCrNi films annealed at 673K, 873K and 1073K versus the annealing time; (b) Temperature dependence of helium bubble radius of He-charged FeCrNi annealed for 8h.
It has been pointed out that there are two main growth mechanisms of He bubbles in metals: one is the migration and coalesce (MC) mechanism, which occurs mainly through volume diffusion, surface diffusion and vapor transport [45]; another mechanism is Ostwald ripening (OR) mechanism, in which helium atoms are dissolved into the matrix from the smaller bubbles and reprecipitated in larger bubbles [46]. To further study the growth mechanism of He bubbles in FeCrNi nanocrystalline film, the growth of the mean bubble radius for MC and OR is quoted in Eq. (2) [47]. (2) Where
is implanted He concentration for MC or He concentration dissolved in the
matrix for OR,
is the diffusivity, ta is the annealing time and n has different
values depending on the operating diffusion mechanism [45, 47]. And in order to facilitate the calculation of the apparent activation energy, we kept the annealing time constant to analyze the relation between the average bubble radius and the annealing temperature. Therefore, we kept the annealing time constant and introduced the first Fick’s law of diffusion in Eq. (2), an Arrhenius equation can be deduced as follows. (3) Where D0 is the pre-exponential constant in m2/s, Q is the activation energy in eV, k is the Boltzmann constant in eV/K and r0 is the pre-exponential constant in nm that is the product of the constants in Eq. (2). The exponent value (n) is inherent in the experimental data when the apparent activation (Ea = Q/n) energy is obtained from Eq.
(3). Hence, the average radii can be calculated using the following equation [48, 49]: (4) Where A is the pre-exponential constant in nm that is the product of the constant r0 in the equation Eq. (3), C is a constant. As Fig. 6(a) shown that He bubbles tend to grow slow after the annealing for 8h and He bubble size reach a constant at a given temperature. Thus， the relationship between the average He bubble size and the annealing temperature for films annealed for 8h was discussed, as shown in Fig. 6(b). Based on the above formula, the apparent activation energy is deduced as 0.240.02 eV by means of a multiple non-linear curve fitting method. The apparent activation energy is very low, which is close to the calculated value of 0.29±0.015 eV deduced from the data in the low-temperature regime of He bubbles coarsening by MC in 304L reported by J. Chen et al. [50], which is far less than the activation energy calculated by OR mechanism [49]. Based on the above discussions, we speculate that MC is the prevailing coarsening mechanism of He bubbles in FeCrNi films during the annealing process within the range of 1073 K. He bubble growth mechanism still needs to be proved by more experiments, especially in situ TEM tests. For example, K. Ono et al. [51] had investigated the formation and migration of helium bubbles in Fe and Fe-9Cr alloy by in situ TEM. In their study, the Brown type motion of He bubbles was observed dynamically in the irradiated Fe and Fe-9Cr alloy. This demonstrated that the growth of He bubble in Fe and Fe-9Cr alloy should be accorded with the MC mechanism in a certain temperature range. If possible, the observation of He bubbles will be carried out by in situ TEM in subsequent experiments.
4. Conclusion FeCrNi films with and without He atoms were prepared by using RF magnetron sputtering method under a He/Ar mixed atmosphere and a pure Ar atmosphere. The results of XRD and SEM analysis showed that the He-charged FeCrNi film had a bcc
columnar structure. For the as-deposited He-charged FeCrNi film, the TDS spectra had roughly three He desorption peaks in the temperature range from RT-1273 K: P1 peak (633 K), P2 peak (893 K), P3 peak (1046 K), which corresponded to the release of interstitial helium atoms from the surface of films, helium-single vacancy complexes HenV (2 < n < 6) and helium multi-vacancy complexes HenVm (1 < n, 2 < m), respectively. And the desorption of He bubbles would occur at the temperature higher than 1273 K. TEM observations showed that the helium bubbles were uniformly distributed in grain interiors of the as-deposited He-charged FeCrNi film, and denser and a necklace-like line of He bubbles located near grain boundaries, the He bubble size was about 1 nm. At the same time, TEM images of the annealed FeCrNi films showed that helium bubbles mainly began to grow remarkably at 873 K and the size of He bubbles increased and the number density of He bubbles decreased with the annealing temperature and time increasing. Moreover, the apparent activation energy was about 0.24 eV, which was inferred that the growth of helium bubbles in FeCrNi films may mainly be governed by the MC mechanism instead of the OR mechanism. This study about the growth mechanism of helium bubbles in FeCrNi films provided the experimental foundation and theoretical basis for better understanding the He bubble growth behavior and further inhibiting the growth of helium bubbles in iron-based nuclear materials. It is of positive significance for the future development of structural materials in nuclear reactor.
Acknowledgments: This work was financially supported by the National Key Research and Development Program
of
China
(Grant
No.
2017YFE0300403,
2017YFA0402800,
2017YFE0300402) and the National Natural Science Foundation of China (Grant Nos. 5171181, 11374299, 11575231, 11674319, 51771184, 11775255).
Conflict of interest We declare that we have no any commercial or associative conflicts of interest to this work (the manuscript entitled as "Helium desorption behavior and growth mechanism of helium bubbles in FeCrNi film").
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