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An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79 wt%Ti alloy
Journal Pre-proof An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79�wt%Ti alloy Fangfang Li, Lei Lu, Xiandong Meng, Hong Xiao, Hefei Ji, Rongguang Zeng, Xinjian Zhang, Xiaolin Wang, Yawen Zhao, Peng Shi PII:
S0925-8388(19)34370-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153124
Reference:
JALCOM 153124
To appear in:
Journal of Alloys and Compounds
Received Date: 9 July 2019 Revised Date:
3 November 2019
Accepted Date: 20 November 2019
Please cite this article as: F. Li, L. Lu, X. Meng, H. Xiao, H. Ji, R. Zeng, X. Zhang, X. Wang, Y. Zhao, P. Shi, An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79�wt%Ti alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153124. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79wt%Ti alloy Fangfang Li, Lei Lu, Xiandong Meng, Hong Xiao, Hefei Ji, Rongguang Zeng, Xinjian Zhang, Xiaolin Wang, Yawen Zhao*, Peng Shi** (China Academy of Engineering Physics, Mianyang 621900, China) Abstract: The chemical composition and microstructure of main inclusions in U-0.79wt%Ti alloy were investigated by Scanning Augur electron spectroscopy (SAM) and High resolution scanning transmission electron microscopy (HRSTEM). SAM analysis showed the predominant inclusion in U-0.79wt%Ti alloy was in the form of titanium carbonitrides. HRSTEM showed the high resolution atom image of titanium carbonitride inclusion and further verified the phase structure of titanium carbonitride inclusion was Ti(C,N). The after-hydriding analysis showed that the hydrogen attack preferentially occurred around most of the Ti(C,N) inclusions in U-0.79wt%Ti alloy, the initial hydride growth showed a parabolic-like kinetic behavior with a reaction order of 0.6. The inclusion-oxide interface can act as preferential transport channel for hydrogen to reach the metal. Key words: U-0.79wt%Ti alloy; inclusions; composition; microstructure; hydride; nucleation sites
1.
Introduction Uranium is an important nuclear material and widely used in many fields in
nuclear industry. However, it is very active. Uranium hydride corrosion, as one type of the corrosion of uranium, can result in the formation of uranium hydride (UH3), which is often harmful and undesired [1, 2]. The hydriding behavior of uranium has been extensively studied, according to previous works, the hydriding behavior of uranium could be affected by temperature, pressure, surrounding environment and gas purity [3, 4]. Besides, it could be also influenced by microstructural characters, such as surface condition, heat treatment methods and inclusions in uranium [5-7]. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected], [email protected] 1
Previous research has proved that the inclusions are preferentially susceptible to hydrogen attack [8, 9]. The carbide inclusion, as one kind of the main inclusions in uranium, is found to play a key role in controlling the location of hydride nucleation [10]. Arkush et al. [11] reported the uranium-hydrogen reaction occurred at the margins of large carbide inclusions in uranium. Siekhaus et al. [12] confirmed this by using SIMS. Jones et al. [13] presented a cross-section of a carbide inclusion in uranium after uranium hydride corrosion, and confirmed that severe corrosion occurred around the uranium carbide inclusion. U-Ti alloy is one kind of important uranium alloys. Titanium is a high melting point and chemically active metal, and can react easily with other kinds of elements at high temperature [14, 15]. Thus, during the preparation process of uranium-titanium alloy, the alloying alloy element titanium can readily react with some light elements in uranium, such as carbon and nitrogen to form titanium based inclusions [16]. Besides, in previous work, the hydriding kinetic of U-0.7wt%Ti alloy was reported to be effected by the impurities in U-0.7wt%Ti alloy, with the addition of silicon, the hydrogen corrosion of U-0.7wt%Ti alloy can be accelerated by promoting the hydriding nucleation [17]. Compared with the inclusions in uranium, titanium-based inclusions are guessed to be the predominant type of inclusion in U-Ti alloy. It is proposed the hydriding initiation of U-Ti alloy could also be influenced by the titanium-based inclusions. Hence, it is also necessary to investigate the inclusion composition in U-Ti alloy to better understand the hydriding behavior. However, few works have been done to character the inclusions in U-Ti alloy in previous reports. Therefore, in this paper, the chemical composition and microstructure of the inclusion in U-0.79wt%Ti alloy were characterized by SAM and HRSTEM. Besides, the effect of these inclusions on the initial nucleation sites for hydrogen corrosion of U-0.79wt%Ti alloy was also investigated.
2.
Experimental method
2.1 Material and Characterization 2
The specimens used in this work were cut into disk with the size of 10 mm × 2 mm from a nominally U-0.79wt%Ti alloy rod with carbon impurity content below 70 ppm. The samples were initially abraded and polished and then cleaned with acetone, ethanol and distilled water to remove the surface oxide layer. Before hydriding experiment, surface morphology and chemical composition of the main inclusions in U-0.79wt%Ti alloy were characterized by a SAM system. For surface sputtering, a 2 kV Ar+ ion beam rastered over a 2.0 × 2.0 mm2 area was used. Specified inclusions sectioning for TEM analyzing was performed by a focused ion beam FIB instrument. The oxide layer on sample surfaces before TEM observations was removed by ion milling. The atomic-scale structure of inclusion was investigated by HRSTEM (operated at 300 kV). 2.2 Sample hydriding After polishing and cleaning, the specimens were then transferred into a self-designed Hot-Stage Microscope (HSM) system described in detail in our previous report [18]. The reaction chamber was first pumped to a base pressure of approximately 10-2 Pa, the sample was heated in vacuum to 200 ℃and kept for 30 min in order to activate the surface and decompose the chemisorbed hydroxyl species on top of the thin oxide over-layer. Then the chamber was cooled down to the reaction temperature of 70 ℃. Subsequently, the sample was exposed to hydrogen purified by a LaNi5 bed with a pressure of 1×105Pa. During the hydrogen exposure, the sample surface was in situ monitored by the optical microscope. The reaction was terminated by evacuating the chamber shortly after the appearance and growth of hydriding nucleation sites. After hydriding experiment, a vertical section through a selected inclusion with hydride formations was performed for analysis by FIB milling. 3.
Results and Discussion
3.1 Composition and Structure of the main inclusions in U-0.79wt%Ti alloy Fig. 1 shows the SEM image of a selected area with few inclusions and representative Auger spectras recorded from the substrate (point 1) and a selected inclusion (point 2) in the investigated U-0.79wt%Ti alloy. Two types of inclusions in U-0.79wt%Ti alloy could be identified according to the different contrast. The 3
predominant inclusions in U-0.79wt%Ti alloy are in darker contrast with varied size and shape, possibly triangular, quadrangular, and also irregular shape, whose sizes are 1-10µm in diameter. The other type of inclusions, which are in low contrast as shown by point 3 in Fig. 1a, are mainly UO2 based on the results of AES analysis. SEM/EDS analyses are usually adopted to characterize the chemical composition of inclusions [19, 20] . However, this method is not very efficient for analyzing non-metallic inclusions combined with carbon and oxygen, due to its analysis volume and the ineluctably atmosphere adsorbed carbon and oxygen. Compared with SEM/EDS, SAM with Ar+ sputtering Gun is more sensitive and more accurate for low Z elements, including carbon and oxygen [21]. Hence, this method was adopted here in order to obtain more actual result. Before acquiring the AES spectra, the surface of U-0.79wt%Ti alloy was cleaned by sputtering with Ar+ to remove the atmosphere adsorbed C and O. The AES spectra recorded form point 1 (Fig. 1b) shows the chemical composition of the substrate is metal U with very low amounts of Ti. The AES spectra from point 2 (Fig. 1c) shows that both carbon and titanium are present in the inclusion. The peak 275eV is attributed to C KLL transition, and the peaks located at 421eV and 390eV are assigned to Ti L3M2,3M4,5 and L3M2,3M2,3 transition, respectively. It is clearly seen from Fig. 1c that the Auger peak shape and energy of C KLL are very different from those of free carbon with a shift of 7eV towards higher energy, indicating that the inclusion is in a form of carbide [22]. In Fig. 1c, it can also be seen that the differentiated peak intensity of L3M2,3M2,3 transition is higher than that of L3M2,3M4,5 transition. The intensity ratio of the Ti L3M2,3M2,3 and L3M2,3M4,5 transitions changes compared with that of pure metal Ti, where the L3M2,3 M4,5 transition peak is the mainly characteristic peak [23]. Considering that there is a close overlap between the transition peaks of Ti L3M2,3M2,3 and N KLL (389eV), which contributes to the increase in intensity of the Ti L3M2,3M2,3 transition peak and there may be nitrogen incorporated in the process of inclusions formation. Hence, we think that the inclusion located at point 2 should contain Ti, C and N elements from TiCxNy. In this case, the nitrogen contribution to the combined peak can be calculated by subtracting the 4
titanium contribution, based on the peak intensity of the clear Ti L3M2,3 M4,5 (421eV) transition [24]. According to the semi-quantitative results from AES, the average composition of TiCxNy inclusions is Ti0.42C0.49N0.09. Fig. 2 depicts the HRSTEM results of the TiCxNy inclusion. According to the EDS microanalysis (Fig. 2b), the inclusion is composed mainly of the elements Ti, C and N, which is in agreement with the AES results. The analysis of the FFT image (insert in Fig. 2a), interplanar distances and angles have shown that the inclusion possess a cubic structure (Fm-3m space group) with the unit cell parameters a = 0.4247 nm, which is in accordance with the previous report [25]. To resolve the atomic configuration of the inclusions, high resolution HAADF STEM imaging was performed to characterize the atom-scale microstructure of the inclusion, in which the contrast intensity of an atomic column is dominated by the atomic number. The HAADF images viewed along [011] directions were displayed in Fig. 2a. The bright dots in this image represent the positions of Ti columns, which is similar with the projection image of TiC along [011] in Fig. 2c, no obvious lattice distortion of Ti atoms array has been induced. Unfortunately, due to the small Z number, the carbon and nitrogen atoms did not show distinguishable contrast. The unit cell parameter of the inclusion is close to that of TiC(a = 0.4324) [26]. The interplanar distances of (200) can be measured and calculated to 2.12Å from Fig. 2a. The value of interplanar distances misfit can be derived by equation ε = (dexpr.–dTiC)/((d expr. + dTiC)/2) × 100%. Taking into account that d(200)expr. = 0.212 nm and d(200)TiC = 0.216 nm we have ε = 1.8%. The little shrink of its unit cell may be induced by the doped N [27]. It can be deduced that the inclusion is still kept the skeleton of TiC, and N atoms may replace C atoms randomly in the crystal lattice. According to the results above, we can presume that the chemistry composition of this inclusion is Ti(C,N) with a cubic structure. 3.2 Influence of inclusions on the hydriding nucleation of U-0.79wt%Ti alloy To resolve the nucleation and initial growth of hydride, an optical microscope was used to in situ record the dynamic change of surface morphology during the hydrogen exposure process. 5
The optical images of the sample surface were recorded every 30 seconds. Two typical images collected prior to hydrogen exposure, namely at t= 0 s, and after hydrogen exposure, namely at 270 s, are shown in Fig. 3. As shown in Fig. 3a, at t0 moment, in the investigated visual field, a number of Ti(C,N) inclusions could be identified according to their brighter contrast. These Ti(C,N) inclusions are different in size and shape. Compared with Fig. 3a, after hydriding reaction for 270s (Fig. 3b), the hydriding spots could be identified according to the darker contrast because of their volume expansion during hydride growth. To reveal the relationship between the inclusions and the hydride sites, the photomicrographs are analyzed with ImageJ software and the result is shown in Fig. 3c, in which the inclusions are colored in red, while the hydride areas in blue. It could be clearly noticed that, most of the Ti(C,N) inclusions are surrounded by hydride after hydrogen exposure. More importantly, the diameter of hydrides around the Ti(C,N) inclusions is usually larger than in some other area, which possibly means that more hydrogen were gathered and consumed near those inclusions. Hence, the hydriding experiment indicates that the Ti(C,N) inclusions in U-0.79wt%Ti alloy are usually the preferential nucleation and growth centers during hydrogen exposure. Besides, the area growth of an individual hydride site, marked 1# in Fig. 3c, is analyzed quantitatively as a function of time from the in situ recording images, and the results are shown in Fig. 3d. Generally, the reaction rate equation can be written as: r=−
=−
(1)
Where k is the reaction rate constant, n is the reaction order. Here, r is the hydride growth rate, c is the hydride area. In the case of the increasing hydride area versus time, Eq. (1) can be integrated analytically as: = (1 − )( + A)
(2)
A line of best fit to the data points is also presented in Fig.3d, which gives the expression (R2 Value of 0.998): c = (0.0315 + 0.686)
.
(3)
Based on Eq. (2) and Eq. (3), the values of k and n from the fit are respectively 6
0.0875 and 0.64. Combining Eq. (1) and Eq. (3), the growth rate of the hydride is: r = 0.0885(0.0315t + 0.686)
.
(4)
In addition, the area growth versus reaction time of another two individual hydride sites, marked 2# and 3# in Fig. 3c are also analyzed quantitatively using the method above. The fitted values of k and n for site 2# and 3# are 0.0928 and 0.0898, while the values of n are 0.6 and 0.61, respectively, and the results are consistent with that of site 1#. It can be concluded from the obtained reaction order (n≈0.6) that the reaction is not a 1st order reaction, which is thought to be obeyed by the uranium-hydrogen system [28]. From the fitted plots in Fig. 3d, it can be seen that the area growth of hydride site is parabolic with respect to time, which is similar to that reported by Ji et al. [29]. The parabolic-like kinetic behavior shows that the reaction is possibly a diffusion controlled process. A consensus on the initiation of uranium hydride corrosion is generally described as follows [8, 30-32] : the hydrogen first adsorbs and dissociates on the oxide surface of uranium metal, and then penetrates through the oxide layer, followed by the aggregation of H atoms at the oxide-metal interface. When the dissolved H atoms exceed a critical concentration, uranium hydride will form. In our experiments, a preferential hydride nucleation near those Ti(C,N) inclusions was observed. In order to better understand the underlying mechanism, a vertical FIB cross-section was performed through a selected Ti(C,N) inclusion-hydride site, as shown in Fig.4. The cross-section view in Fig. 4 and the phenomenon in Fig. 3b provide clearly evidence for the trace of hydrogen during hydriding progress. The inclusion-oxide interface takes precedence to attract hydrogen and provides transfer channel for it over the other sites on the surface exposed to hydrogen. As is known, the reaction rates of Ti(C,N) inclusions and the metal matrix with oxygen are different which can induce the different oxide thickness over inclusion and metal [33, 34] . Subsequently, there would result in a discontinuity of the outer oxide-layer near the inclusion boundary which may have significant influence over the corrosion initiation [35, 36]. The transfer of hydrogen through the inclusion-oxide interface is considered to be 7
faster than that through the oxide layer [37, 38] . The hydrogen transferred through the inclusion-oxide interface would accumulate rapidly to the solubility limit, and then initiate the hydrogenation reaction. Beside, for the different oxide thickness over inclusion and metal there would generate stress at the inclusion-oxide interface, which may also promote the lattice distortion at the margin of inclusion and accelerate the diffusion of hydrogen. Therefore, it would be concluded that the Ti(C,N) inclusions preferred to act as hydrogen nucleation sites is closely related with the outer discontinuous oxide-layer over the metal, where a fast diffusion channel for hydrogen can be provided by the inclusion-oxide interface. Aside from this, it is also widely accepted that, hydrogen are easily trapped by the defects in the metal matrix, including grain boundaries, vacancies, and also interfaces. The trapping or retention of hydrogen in this kind of boundaries has been identified in some similar material systems. For example, Chen et al. [39] reported a direct observation of trapping of hydrogen atoms near the vanadium carbides particles in a ferrite steel interface with using APT (Atom Probe Tomography). Similar results have also been given by other researchers [40]. One common feature could be induced, even the interface between carbide and metal matrix are coherent or semi-coherent, and the interface could help to trap the hydrogen atoms. In the case of uranium, the segregation of hydrogen near the particles could increase the concentration of hydrogen, and drive the formation of uranium hydride. This is another possible reason for the preferential hydride nucleation near the inclusions. To summarize, the above results show a close relationship with the Ti(C,N) inclusions in U-0.79wt%Ti alloy and hydride attack, which is similar for that of uranium carbide inclusions and uranium hydride corrosion in the literatures. The inclusion-oxide interface provides preferential pathways for hydrogen to reach the metal. 4.
Conclusions This study focuses on a detailed characterization of titanium inclusions in
U-0.79wt%Ti alloy and discusses the influence of these inclusions on the initial hydriding nucleation sites. SAM and HRSTEM analysis showed that titanium 8
carbonitrides were the predominant inclusion types in U-0.79wt%Ti alloy, the phase structure of titanium inclusion was Ti(C,N). Preferentially hydride attack around most of the Ti(C,N) inclusions showed that the hydride nucleation and growth sites had a great relationship with Ti(C,N) inclusions. The area growth of hydride site showed a parabolic-like kinetic behavior, indicating a diffusion controlled hydriding process. The vertical cross-section through a selected Ti(C,N) inclusion-hydride site showed clear evidence that the hydride growth occurred at the margins of the Ti(C,N) inclusion below the surface of bulk metal. The inclusion-oxide interface can act as faster transport channel for hydrogen to reach the metal.
Acknowledgements This work was supported by the National Science Foundation for Young Scientists of China [No. 51701195 and 51541109]. The authors also express their appreciation to Wenyuan Wang for helpful discussions on the results analysis and to Xian’e Tang for technical assistance on the experimental measurements.
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Figures caption
Fig. 1 SEM image of inclusions in U-0.79wt%Ti alloy (a). The AES spectras of the specimen substrate (b) and a selected inclusion (c) shown in the SEM image.
Fig. 2 (a) High-resolution HAADF images of the Ti(C,N) inclusion viewed along the [011] zone axis and corresponding FFT. (b) The EDX spectrum of (a) analyzed by nanoprobe. (c) The projected position of Ti atoms in TiC structure along [011], based on the configuration of Ti atoms in TiC (Nakamura [26]).
Fig. 3 Optical images of U-Ti specimen collected at different times, (a) t=0s, (b) t=270s, after the beginning of hydrogen exposure. (c): To make it clear, the Ti(C,N) inclusions and larger size hydrides in (b) were marked by red and blue respectively. (d): plots of three individual hydride areas versus reaction time, solid dots: experimental data for different time, solid curves: fitting of the experiment data based on Eq. (2).
Fig. 4 SEM image of a FIB etched cross section through a Ti(C,N) inclusion after hydrogen exposure. The distance of hydrogen diffusion along the two different interfaces (metal–oxide and metal-Ti(C,N)) matched.
Figure list:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Highlights: ·
The microstructural and composition characteristics of main inclusion were identified as Ti(C,N) with a combined analysis with SAM and HRTEM.
·
An Enhanced hydrogen corrosion and preferential hydride nucleation near the inclusions was observed.
·
The enhanced hydrogen corrosion was attributed to the fast diffusion channel provided by the mismatch of the oxide layer at the inclusions boundaries.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34370-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.153124
Reference:
JALCOM 153124
To appear in:
Journal of Alloys and Compounds
Received Date: 9 July 2019 Revised Date:
3 November 2019
Accepted Date: 20 November 2019
Please cite this article as: F. Li, L. Lu, X. Meng, H. Xiao, H. Ji, R. Zeng, X. Zhang, X. Wang, Y. Zhao, P. Shi, An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79�wt%Ti alloy, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153124. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
An enhanced hydrogen corrosion by the Ti(C,N) inclusions in U-0.79wt%Ti alloy Fangfang Li, Lei Lu, Xiandong Meng, Hong Xiao, Hefei Ji, Rongguang Zeng, Xinjian Zhang, Xiaolin Wang, Yawen Zhao*, Peng Shi** (China Academy of Engineering Physics, Mianyang 621900, China) Abstract: The chemical composition and microstructure of main inclusions in U-0.79wt%Ti alloy were investigated by Scanning Augur electron spectroscopy (SAM) and High resolution scanning transmission electron microscopy (HRSTEM). SAM analysis showed the predominant inclusion in U-0.79wt%Ti alloy was in the form of titanium carbonitrides. HRSTEM showed the high resolution atom image of titanium carbonitride inclusion and further verified the phase structure of titanium carbonitride inclusion was Ti(C,N). The after-hydriding analysis showed that the hydrogen attack preferentially occurred around most of the Ti(C,N) inclusions in U-0.79wt%Ti alloy, the initial hydride growth showed a parabolic-like kinetic behavior with a reaction order of 0.6. The inclusion-oxide interface can act as preferential transport channel for hydrogen to reach the metal. Key words: U-0.79wt%Ti alloy; inclusions; composition; microstructure; hydride; nucleation sites
1.
Introduction Uranium is an important nuclear material and widely used in many fields in
nuclear industry. However, it is very active. Uranium hydride corrosion, as one type of the corrosion of uranium, can result in the formation of uranium hydride (UH3), which is often harmful and undesired [1, 2]. The hydriding behavior of uranium has been extensively studied, according to previous works, the hydriding behavior of uranium could be affected by temperature, pressure, surrounding environment and gas purity [3, 4]. Besides, it could be also influenced by microstructural characters, such as surface condition, heat treatment methods and inclusions in uranium [5-7]. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected], [email protected] 1
Previous research has proved that the inclusions are preferentially susceptible to hydrogen attack [8, 9]. The carbide inclusion, as one kind of the main inclusions in uranium, is found to play a key role in controlling the location of hydride nucleation [10]. Arkush et al. [11] reported the uranium-hydrogen reaction occurred at the margins of large carbide inclusions in uranium. Siekhaus et al. [12] confirmed this by using SIMS. Jones et al. [13] presented a cross-section of a carbide inclusion in uranium after uranium hydride corrosion, and confirmed that severe corrosion occurred around the uranium carbide inclusion. U-Ti alloy is one kind of important uranium alloys. Titanium is a high melting point and chemically active metal, and can react easily with other kinds of elements at high temperature [14, 15]. Thus, during the preparation process of uranium-titanium alloy, the alloying alloy element titanium can readily react with some light elements in uranium, such as carbon and nitrogen to form titanium based inclusions [16]. Besides, in previous work, the hydriding kinetic of U-0.7wt%Ti alloy was reported to be effected by the impurities in U-0.7wt%Ti alloy, with the addition of silicon, the hydrogen corrosion of U-0.7wt%Ti alloy can be accelerated by promoting the hydriding nucleation [17]. Compared with the inclusions in uranium, titanium-based inclusions are guessed to be the predominant type of inclusion in U-Ti alloy. It is proposed the hydriding initiation of U-Ti alloy could also be influenced by the titanium-based inclusions. Hence, it is also necessary to investigate the inclusion composition in U-Ti alloy to better understand the hydriding behavior. However, few works have been done to character the inclusions in U-Ti alloy in previous reports. Therefore, in this paper, the chemical composition and microstructure of the inclusion in U-0.79wt%Ti alloy were characterized by SAM and HRSTEM. Besides, the effect of these inclusions on the initial nucleation sites for hydrogen corrosion of U-0.79wt%Ti alloy was also investigated.
2.
Experimental method
2.1 Material and Characterization 2
The specimens used in this work were cut into disk with the size of 10 mm × 2 mm from a nominally U-0.79wt%Ti alloy rod with carbon impurity content below 70 ppm. The samples were initially abraded and polished and then cleaned with acetone, ethanol and distilled water to remove the surface oxide layer. Before hydriding experiment, surface morphology and chemical composition of the main inclusions in U-0.79wt%Ti alloy were characterized by a SAM system. For surface sputtering, a 2 kV Ar+ ion beam rastered over a 2.0 × 2.0 mm2 area was used. Specified inclusions sectioning for TEM analyzing was performed by a focused ion beam FIB instrument. The oxide layer on sample surfaces before TEM observations was removed by ion milling. The atomic-scale structure of inclusion was investigated by HRSTEM (operated at 300 kV). 2.2 Sample hydriding After polishing and cleaning, the specimens were then transferred into a self-designed Hot-Stage Microscope (HSM) system described in detail in our previous report [18]. The reaction chamber was first pumped to a base pressure of approximately 10-2 Pa, the sample was heated in vacuum to 200 ℃and kept for 30 min in order to activate the surface and decompose the chemisorbed hydroxyl species on top of the thin oxide over-layer. Then the chamber was cooled down to the reaction temperature of 70 ℃. Subsequently, the sample was exposed to hydrogen purified by a LaNi5 bed with a pressure of 1×105Pa. During the hydrogen exposure, the sample surface was in situ monitored by the optical microscope. The reaction was terminated by evacuating the chamber shortly after the appearance and growth of hydriding nucleation sites. After hydriding experiment, a vertical section through a selected inclusion with hydride formations was performed for analysis by FIB milling. 3.
Results and Discussion
3.1 Composition and Structure of the main inclusions in U-0.79wt%Ti alloy Fig. 1 shows the SEM image of a selected area with few inclusions and representative Auger spectras recorded from the substrate (point 1) and a selected inclusion (point 2) in the investigated U-0.79wt%Ti alloy. Two types of inclusions in U-0.79wt%Ti alloy could be identified according to the different contrast. The 3
predominant inclusions in U-0.79wt%Ti alloy are in darker contrast with varied size and shape, possibly triangular, quadrangular, and also irregular shape, whose sizes are 1-10µm in diameter. The other type of inclusions, which are in low contrast as shown by point 3 in Fig. 1a, are mainly UO2 based on the results of AES analysis. SEM/EDS analyses are usually adopted to characterize the chemical composition of inclusions [19, 20] . However, this method is not very efficient for analyzing non-metallic inclusions combined with carbon and oxygen, due to its analysis volume and the ineluctably atmosphere adsorbed carbon and oxygen. Compared with SEM/EDS, SAM with Ar+ sputtering Gun is more sensitive and more accurate for low Z elements, including carbon and oxygen [21]. Hence, this method was adopted here in order to obtain more actual result. Before acquiring the AES spectra, the surface of U-0.79wt%Ti alloy was cleaned by sputtering with Ar+ to remove the atmosphere adsorbed C and O. The AES spectra recorded form point 1 (Fig. 1b) shows the chemical composition of the substrate is metal U with very low amounts of Ti. The AES spectra from point 2 (Fig. 1c) shows that both carbon and titanium are present in the inclusion. The peak 275eV is attributed to C KLL transition, and the peaks located at 421eV and 390eV are assigned to Ti L3M2,3M4,5 and L3M2,3M2,3 transition, respectively. It is clearly seen from Fig. 1c that the Auger peak shape and energy of C KLL are very different from those of free carbon with a shift of 7eV towards higher energy, indicating that the inclusion is in a form of carbide [22]. In Fig. 1c, it can also be seen that the differentiated peak intensity of L3M2,3M2,3 transition is higher than that of L3M2,3M4,5 transition. The intensity ratio of the Ti L3M2,3M2,3 and L3M2,3M4,5 transitions changes compared with that of pure metal Ti, where the L3M2,3 M4,5 transition peak is the mainly characteristic peak [23]. Considering that there is a close overlap between the transition peaks of Ti L3M2,3M2,3 and N KLL (389eV), which contributes to the increase in intensity of the Ti L3M2,3M2,3 transition peak and there may be nitrogen incorporated in the process of inclusions formation. Hence, we think that the inclusion located at point 2 should contain Ti, C and N elements from TiCxNy. In this case, the nitrogen contribution to the combined peak can be calculated by subtracting the 4
titanium contribution, based on the peak intensity of the clear Ti L3M2,3 M4,5 (421eV) transition [24]. According to the semi-quantitative results from AES, the average composition of TiCxNy inclusions is Ti0.42C0.49N0.09. Fig. 2 depicts the HRSTEM results of the TiCxNy inclusion. According to the EDS microanalysis (Fig. 2b), the inclusion is composed mainly of the elements Ti, C and N, which is in agreement with the AES results. The analysis of the FFT image (insert in Fig. 2a), interplanar distances and angles have shown that the inclusion possess a cubic structure (Fm-3m space group) with the unit cell parameters a = 0.4247 nm, which is in accordance with the previous report [25]. To resolve the atomic configuration of the inclusions, high resolution HAADF STEM imaging was performed to characterize the atom-scale microstructure of the inclusion, in which the contrast intensity of an atomic column is dominated by the atomic number. The HAADF images viewed along [011] directions were displayed in Fig. 2a. The bright dots in this image represent the positions of Ti columns, which is similar with the projection image of TiC along [011] in Fig. 2c, no obvious lattice distortion of Ti atoms array has been induced. Unfortunately, due to the small Z number, the carbon and nitrogen atoms did not show distinguishable contrast. The unit cell parameter of the inclusion is close to that of TiC(a = 0.4324) [26]. The interplanar distances of (200) can be measured and calculated to 2.12Å from Fig. 2a. The value of interplanar distances misfit can be derived by equation ε = (dexpr.–dTiC)/((d expr. + dTiC)/2) × 100%. Taking into account that d(200)expr. = 0.212 nm and d(200)TiC = 0.216 nm we have ε = 1.8%. The little shrink of its unit cell may be induced by the doped N [27]. It can be deduced that the inclusion is still kept the skeleton of TiC, and N atoms may replace C atoms randomly in the crystal lattice. According to the results above, we can presume that the chemistry composition of this inclusion is Ti(C,N) with a cubic structure. 3.2 Influence of inclusions on the hydriding nucleation of U-0.79wt%Ti alloy To resolve the nucleation and initial growth of hydride, an optical microscope was used to in situ record the dynamic change of surface morphology during the hydrogen exposure process. 5
The optical images of the sample surface were recorded every 30 seconds. Two typical images collected prior to hydrogen exposure, namely at t= 0 s, and after hydrogen exposure, namely at 270 s, are shown in Fig. 3. As shown in Fig. 3a, at t0 moment, in the investigated visual field, a number of Ti(C,N) inclusions could be identified according to their brighter contrast. These Ti(C,N) inclusions are different in size and shape. Compared with Fig. 3a, after hydriding reaction for 270s (Fig. 3b), the hydriding spots could be identified according to the darker contrast because of their volume expansion during hydride growth. To reveal the relationship between the inclusions and the hydride sites, the photomicrographs are analyzed with ImageJ software and the result is shown in Fig. 3c, in which the inclusions are colored in red, while the hydride areas in blue. It could be clearly noticed that, most of the Ti(C,N) inclusions are surrounded by hydride after hydrogen exposure. More importantly, the diameter of hydrides around the Ti(C,N) inclusions is usually larger than in some other area, which possibly means that more hydrogen were gathered and consumed near those inclusions. Hence, the hydriding experiment indicates that the Ti(C,N) inclusions in U-0.79wt%Ti alloy are usually the preferential nucleation and growth centers during hydrogen exposure. Besides, the area growth of an individual hydride site, marked 1# in Fig. 3c, is analyzed quantitatively as a function of time from the in situ recording images, and the results are shown in Fig. 3d. Generally, the reaction rate equation can be written as: r=−
=−
(1)
Where k is the reaction rate constant, n is the reaction order. Here, r is the hydride growth rate, c is the hydride area. In the case of the increasing hydride area versus time, Eq. (1) can be integrated analytically as: = (1 − )( + A)
(2)
A line of best fit to the data points is also presented in Fig.3d, which gives the expression (R2 Value of 0.998): c = (0.0315 + 0.686)
.
(3)
Based on Eq. (2) and Eq. (3), the values of k and n from the fit are respectively 6
0.0875 and 0.64. Combining Eq. (1) and Eq. (3), the growth rate of the hydride is: r = 0.0885(0.0315t + 0.686)
.
(4)
In addition, the area growth versus reaction time of another two individual hydride sites, marked 2# and 3# in Fig. 3c are also analyzed quantitatively using the method above. The fitted values of k and n for site 2# and 3# are 0.0928 and 0.0898, while the values of n are 0.6 and 0.61, respectively, and the results are consistent with that of site 1#. It can be concluded from the obtained reaction order (n≈0.6) that the reaction is not a 1st order reaction, which is thought to be obeyed by the uranium-hydrogen system [28]. From the fitted plots in Fig. 3d, it can be seen that the area growth of hydride site is parabolic with respect to time, which is similar to that reported by Ji et al. [29]. The parabolic-like kinetic behavior shows that the reaction is possibly a diffusion controlled process. A consensus on the initiation of uranium hydride corrosion is generally described as follows [8, 30-32] : the hydrogen first adsorbs and dissociates on the oxide surface of uranium metal, and then penetrates through the oxide layer, followed by the aggregation of H atoms at the oxide-metal interface. When the dissolved H atoms exceed a critical concentration, uranium hydride will form. In our experiments, a preferential hydride nucleation near those Ti(C,N) inclusions was observed. In order to better understand the underlying mechanism, a vertical FIB cross-section was performed through a selected Ti(C,N) inclusion-hydride site, as shown in Fig.4. The cross-section view in Fig. 4 and the phenomenon in Fig. 3b provide clearly evidence for the trace of hydrogen during hydriding progress. The inclusion-oxide interface takes precedence to attract hydrogen and provides transfer channel for it over the other sites on the surface exposed to hydrogen. As is known, the reaction rates of Ti(C,N) inclusions and the metal matrix with oxygen are different which can induce the different oxide thickness over inclusion and metal [33, 34] . Subsequently, there would result in a discontinuity of the outer oxide-layer near the inclusion boundary which may have significant influence over the corrosion initiation [35, 36]. The transfer of hydrogen through the inclusion-oxide interface is considered to be 7
faster than that through the oxide layer [37, 38] . The hydrogen transferred through the inclusion-oxide interface would accumulate rapidly to the solubility limit, and then initiate the hydrogenation reaction. Beside, for the different oxide thickness over inclusion and metal there would generate stress at the inclusion-oxide interface, which may also promote the lattice distortion at the margin of inclusion and accelerate the diffusion of hydrogen. Therefore, it would be concluded that the Ti(C,N) inclusions preferred to act as hydrogen nucleation sites is closely related with the outer discontinuous oxide-layer over the metal, where a fast diffusion channel for hydrogen can be provided by the inclusion-oxide interface. Aside from this, it is also widely accepted that, hydrogen are easily trapped by the defects in the metal matrix, including grain boundaries, vacancies, and also interfaces. The trapping or retention of hydrogen in this kind of boundaries has been identified in some similar material systems. For example, Chen et al. [39] reported a direct observation of trapping of hydrogen atoms near the vanadium carbides particles in a ferrite steel interface with using APT (Atom Probe Tomography). Similar results have also been given by other researchers [40]. One common feature could be induced, even the interface between carbide and metal matrix are coherent or semi-coherent, and the interface could help to trap the hydrogen atoms. In the case of uranium, the segregation of hydrogen near the particles could increase the concentration of hydrogen, and drive the formation of uranium hydride. This is another possible reason for the preferential hydride nucleation near the inclusions. To summarize, the above results show a close relationship with the Ti(C,N) inclusions in U-0.79wt%Ti alloy and hydride attack, which is similar for that of uranium carbide inclusions and uranium hydride corrosion in the literatures. The inclusion-oxide interface provides preferential pathways for hydrogen to reach the metal. 4.
Conclusions This study focuses on a detailed characterization of titanium inclusions in
U-0.79wt%Ti alloy and discusses the influence of these inclusions on the initial hydriding nucleation sites. SAM and HRSTEM analysis showed that titanium 8
carbonitrides were the predominant inclusion types in U-0.79wt%Ti alloy, the phase structure of titanium inclusion was Ti(C,N). Preferentially hydride attack around most of the Ti(C,N) inclusions showed that the hydride nucleation and growth sites had a great relationship with Ti(C,N) inclusions. The area growth of hydride site showed a parabolic-like kinetic behavior, indicating a diffusion controlled hydriding process. The vertical cross-section through a selected Ti(C,N) inclusion-hydride site showed clear evidence that the hydride growth occurred at the margins of the Ti(C,N) inclusion below the surface of bulk metal. The inclusion-oxide interface can act as faster transport channel for hydrogen to reach the metal.
Acknowledgements This work was supported by the National Science Foundation for Young Scientists of China [No. 51701195 and 51541109]. The authors also express their appreciation to Wenyuan Wang for helpful discussions on the results analysis and to Xian’e Tang for technical assistance on the experimental measurements.
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Figures caption
Fig. 1 SEM image of inclusions in U-0.79wt%Ti alloy (a). The AES spectras of the specimen substrate (b) and a selected inclusion (c) shown in the SEM image.
Fig. 2 (a) High-resolution HAADF images of the Ti(C,N) inclusion viewed along the [011] zone axis and corresponding FFT. (b) The EDX spectrum of (a) analyzed by nanoprobe. (c) The projected position of Ti atoms in TiC structure along [011], based on the configuration of Ti atoms in TiC (Nakamura [26]).
Fig. 3 Optical images of U-Ti specimen collected at different times, (a) t=0s, (b) t=270s, after the beginning of hydrogen exposure. (c): To make it clear, the Ti(C,N) inclusions and larger size hydrides in (b) were marked by red and blue respectively. (d): plots of three individual hydride areas versus reaction time, solid dots: experimental data for different time, solid curves: fitting of the experiment data based on Eq. (2).
Fig. 4 SEM image of a FIB etched cross section through a Ti(C,N) inclusion after hydrogen exposure. The distance of hydrogen diffusion along the two different interfaces (metal–oxide and metal-Ti(C,N)) matched.
Figure list:
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Highlights: ·
The microstructural and composition characteristics of main inclusion were identified as Ti(C,N) with a combined analysis with SAM and HRTEM.
·
An Enhanced hydrogen corrosion and preferential hydride nucleation near the inclusions was observed.
·
The enhanced hydrogen corrosion was attributed to the fast diffusion channel provided by the mismatch of the oxide layer at the inclusions boundaries.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: