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Irradiation effects in H and He implanted B2 iron aluminide
Fusion Engineering and Design 143 (2019) 207–211
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Irradiation effects in H and He implanted B2 iron aluminide Meijuan Hu, Pei Zhang, Jingwen Ba, Guikai Zhang, Meng Liu, Tao Tang
⁎
T
Institute of Materials, China Academy of Engineering Physics, P.O. Box 9071-12, No. 9 Huafengxincun, Jiangyou 621907, Sichuan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: FeAl Ion implantation Hydrogen Helium Embrittlement
In order to study the irradiation effect of hydrogen and helium implantation, B2-FeAl alloys were exposed to hydrogen and helium beams with energy of 250 and 540 keV under doses ranging from 1 × 1016 to 1 × 1017 ions cm−2. The phase and surface morphology of FeAl alloy before and after implantation were characterized by X-ray diffraction and scanning electron microscope, respectively. The structure disorder induced by ion implantation was found, and irradiated damage appeared as large amount of dark zone only for high fluence of implantation. Using transmission electron microscopy, the microstructure and implantation structure defects such as gas bubbles and voids were investigated in high dose implanted FeAl. The formation process of bubbles and voids can be deduced by the analysis of TEM images, and the growth of the voids might be promoted by the synergistic effect of H and He implantation. It was speculated that the synergistic effect of tritium and helium might also be found in occasions of tritium irradiation.
1. Introduction Iron aluminides have been among most studied intermetallics due to the excellent oxidation and corrosion resistance [1,2]. Combined with advantages of low density and low material cost, they are being actively considered for high-temperature structural applications such as structural members in aircraft, heating elements, nuclear reactor components and et al [3,4]. However, the poor ductility and brittle fracture of FeAl intermetallics greatly limit the large-scale practical applications. These drawbacks of iron aluminides are mainly caused by sensitivity to hydrogen or water in the ambient atmosphere, namely the environmental embrittlement [5–7]. On the other hand, grain-boundary weakness and vacancy hardening also play important roles in inducing embrittlement with high content of Al or at elevated temperatures [1]. In tritium-related system, FeAl/Al2O3 composite coating has been selected as one of the prior developed tritium permeation barriers (TPB). FeAl intermediate layer can effectively relieve the great thermal mismatch between Al2O3 and metal substrate [8–11]. When exposed to higher temperature environments of tritium-containing atmosphere, more vacancies will be generated and subsequently become to new hydrogen/tritium-trapping sites in FeAl, leading to hydrogen embrittlement and the failure of tritium permeation properties [3]. Moreover, helium induced by tritium decay would definitely bring other different consequences such as helium embrittlement to FeAl. Unfortunately, in addition to hydrogen embrittlement, the helium embrittlement has not been paid enough attention while the FeAl intermetallics using in TPBs.
⁎
Aiming to simulate the irradiated FeAl sample exposed in tritiumcontaining atmosphere, helium and hydrogen were introduced into FeAl in this work in order to investigate the hydrogen and helium embrittlement in FeAl. Considering convenience of the actual operation, implantation was chosen as a relatively effective way to obtain FeAl sample contained both hydrogen and helium without long period of tritium-aging. In this paper, the discussion was based on the changes of structural and microstructural defects analysis of B2-FeAl specimens implanted with 250 keV H and 540 keV He under different doses, using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, the effect of the helium and hydrogen implantation has been briefly discussed. 2. Experimental 2.1. Materials synthesis FeAl alloys were produced by repeated induction melting under argon of 99.999% purity at the stoichiometric ratio of Fe55Al45. The alloys were produced using Fe of 99.98% purity and Al (99.9%). After melting, the ingots were cut into required dimensions for the further characterization. The surfaces were then polished with different sizes of sand paper (from 200# to 1000#) and diamond paste.
Corresponding author. E-mail address: [email protected] (T. Tang).
https://doi.org/10.1016/j.fusengdes.2019.03.199 Received 30 October 2018; Received in revised form 10 February 2019; Accepted 31 March 2019 Available online 06 April 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 143 (2019) 207–211
M. Hu, et al.
Table 1 Co-implantation parameters of FeAl samples. Sample number
Ion
Energy (keV)
Depth of implantation (μm)
Ion fluence (Ions/cm2)
Beam dimension (cm2)
Beam current (μA)
Ion flux (Ions•cm−2 s-1)
1
H He H He
250 540 250 540
1.39 1.23 1.39 1.23
1 × 1016 1 × 1016 1 × 1017 1 × 1017
1.7 × 1.3 1.6 × 1.4 1.7 × 1.3 1.6 × 1.4
1.8 2.1 1.8 2.1
5.56 × 1012 2.78 × 1012 5.56 × 1012 2.78 × 1012
2
2.2. Irradiation condition and SRIM calculation
followed by final thinning and polishing at 5 keV and 2 keV.
Co-implantation was carried out at room temperature using heavy ions research facility in Lanzhou (HIRFL). Thermocouples were mounted on the specimen surface to measure the surface temperature. The increment in temperature during implantation was less than 5 °C according to the measurement. He ions (He+) with the energy of 540 keV and H ions (H+) with 250 keV were implanted into samples at fluences of 1.0 × 1016 and 1.0 × 1017 ions/cm2. The implanted fluence was measured by a current integrator, and the detailed fluence parameters of implantation are shown in Table 1. All fluences were accumulative. According to stopping and range of ions in matter (SRIM) simulations (The calculation type ˜ detailed calculation with full Damage Cascades, Edisp_Fe/Al ˜25 eV, material density ˜ 5.542 g/cm3 or 7.787 × 1022 atoms/cm3) [12,13], the profiles of H and He ion distribution and damage with depth are shown in Fig. 1. Each fluence corresponds to the estimated H or He concentrations/displacement levels of 0.67 at.% /0.058 dpa (H, 1.0 × 1016 ions/cm2), 6.70 at.% /0.58 dpa (H, 1.0 × 1017 ions/cm2), 0.578 at.% /0.583 dpa (He, 1.0 × 1016 ions/cm2) and 5.78 at.% /5.83 dpa (He, 1.0 × 1017 ions/cm2), respectively, at the concentration/damage peak. The maximum of ions projected range was about 1.65 μm, and the H concentration and damage peaks were at the depth around 1.42 and 1.38 μm, while that of He peaks were 1.22 and 1.28 μm, respectively.
2.4. Characterization techniques After implantation, the X-ray diffraction (XRD) was performed with the Cu Kɑ (λ=1.5406 Å) line in a D8 Discover diffractometer. θ-2θ scans were performed with 2θ typically ranging from 10° to 90° at 0.02° per step. The surface morphologies of the implanted specimen were carried out on dual beam SEM/FIB system with electron beam mode, which was also used for preparing the cross-sections of the implanted bulk specimen. The observations on FIB cross-section specimens were performed using a Libra 200FE transmission electron microscope (TEM) operated at 200 kV. 3. Results and discussion 3.1. Original structure defects Considering the original structure plays important role in FeAl embrittlement, the characterization microstructure of the synthesized FeAl alloy was conducted, using optical micrograph (OM) and TEM observation. The synthesized FeAl alloy presents a microstructure of equiaxed grains with a relative uniform size (Fig. 2a). The average grain size determined by the linear intersect method was about 20 ± 5 μm. Besides, abundant of dislocations are distributed uniformly in FeAl (Fig. 2b), suggesting the original structure feature of FeAl. However, once intruded by hydrogen or helium, such a high density of two-dimensional (2D) defects, such as dislocations or grain boundaries might become to the hydrogen/helium-trapping sites, absorbing a much greater concentration of hydrogen and helium. It can be proposed that hydrogen/helium embrittlement is a result of an accumulation of these dislocations and grain boundaries causing crack nucleation [1].
2.3. TEM sample preparation Transmission electron microscopy (TEM) samples were prepared using the focused ion beam (FIB) ‘lift-out’ technique in a dual-beam instrument. To protect the specimen top surface from Ga ion sputtering/damage, the specimen was coated with 200 nm Pt using electron beam deposition followed by 3 mm Pt deposited using Ga ion beam. Note that the thick Pt layer is deposited on the top surface of specimen to help prevent the specimen from bending in the final FIB thinning and polishing procedure. The FIB specimen was cut using 30 keV Ga ions,
3.2. Phase characterization Fig. 3 shows the recorded XRD patterns of FeAl alloys before and after different fluences of hydrogen and helium ion implantation. As can be seen, all the diffraction peaks match well with the cubic iron aluminide (JCPDS No. 45-0983) before ion implantation, while a broad diffraction peak around 20° appeared after the implantation. The intensity of the broad peak also increased with the implantation fluence, indicating structure disorder of FeAl alloy induced by H and He ion implantation. On the other hand, all the intensity of FeAl diffraction peaks decreased when applied 1 × 1016 ions cm−2 of irradiation, only (211) plane exhibited abnormal intensity increase, accompanied with a small shoulder peak in the small angle direction. For FeAl implanted by a higher fluence of 1 × 1017 ions cm-2 implantation, the diffraction peak decreased even disappeared, confirming the increase of structure disorder degree. Particularly, the diffraction peak of (200) plane shifted to small angle direction with the increase of implantation fluence. This change in the peak position of (200) by the H and He implantation might be due to the development of the shoulder peak of (200) peak, which can be observed in XRD patterns of (200) of un-implanted specimen, indicating the increase of (200) lattice spacing and the lattice expansion of FeAl along (200) plane. From this point of view, it can be deduced that the implanted H and He ions in FeAl crystal lattice might
Fig. 1. H and He concentration (at.%) and damage (dpa) distribution with depth obtained from SRIM 2013 for 250 keV H and 540 keV He in FeAl with fluence of 1 × 1017/cm2. 208
Fusion Engineering and Design 143 (2019) 207–211
M. Hu, et al.
Fig. 2. (a) Optical and (b) TEM micrographs of synthesized FeAl alloy.
might be caused by large amount of ions implantation. While for low dose of implantation, no obvious damage area can be observed on the surface. Combined with the XRD analysis, the degree of implantation damage is greatly dependent on the fluence of implantation. When injected by more H and He atoms, the tendency of structure disorder in FeAl become higher, accompanied with more structure defects, which were observed as the dark area in the surface of FeAl.
3.4. Implantation induced defects In order to investigate the structure defects induced by H and He implantation, TEM analysis was conducted on FeAl sample implanted with high fluence of H and He. The specimen was stored 6 months after implantation for sufficient interactions between implanted atoms and structural defects. Fig. 5 shows the cross section view of the damaged structures from FIB fabricated specimen. As shown in Fig. 5(a), an obvious dark stripe around 70–100 nm wide was formed under the surface in depth of 100 nm, and a wider damaged zone consisted with bulges and cavities can be observed starting from the slim stripe mentioned above and extending to a deeper position. This damaged area located from 0.1 to 1.5 μm in depth, and the damage distribution is consistent with the SRIM calculation result. For undamaged matrix phase far away from the damaged zone, clear lattice fringes were presented and the corresponding selected area electron diffraction (SAED) pattern can be well indexed along [11¯ 3] zone axis of FeAl. However, even though the damaged zone of implantation appeared to be consisted with disordered structures as shown in Fig. 5(a) and (d), the magnification image of damaged zone still showed obvious lattice fringes with basal distances of 0.279 nm, which can be assigned to (100) plane of FeAl. Besides, the corresponded SAED pattern is nearly the same as that of the matrix phase, consisted with bright rhombic
Fig. 3. XRD patterns of FeAl alloys before and after different fluence of hydrogen and helium ion implantation.
distribute along the preferential orientation such as (200) plane, causing the increase of the specific lattice spacing in FeAl crystal, indicating the change in crystalline structure from cubic structure to tetragonal structure.
3.3. Surface morphology characterization Fig. 4 presents the surface morphologies of FeAl alloys after different fluence of hydrogen and helium ion implantation. When applied high fluence (1 × 1017/cm2) of hydrogen and helium ions, large amount of dark area appeared on the surface of FeAl sample, which
Fig. 4. SEM images of FeAl samples implanted with 540 keV He ions and 250 keV H ions at different fluences of (a) 1 × 1017/cm2 and (b) 1 × 1016/cm2. 209
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Fig. 5. Cross-sectional TEM micrographs of FeAl sample stored 6 months after 1 × 1017/cm2 He and H ions implantation: (a) overall image from surface to matrix; magnification images and corresponding SAED patterns (inset) of matrix phase (b) and implantation damaged zone (c); (d–f) different microstructures of implantation damaged zone.
While the helium atoms are “insoluble” to the metal matrix, which are most easily trapped by vacancies and formed the helium-vacancy complex clusters. In this case, the small helium bubbles become to the accumulation result of He-V clusters, which means the nucleation of helium bubble. For single implantation of hydrogen, it is reported that presence of H did not affect the microstructures. No cavities and no obvious dislocation structure were formed, suggesting that H has little effect on cavity nucleation, where the cavities can be highly pressurized bubbles or under-pressurized voids [17]. While for dual implantation of H and He, a synergistic effect was found in many material systems, namely the existence of H has an important role for promoting cavity’s growth [17–20]. In order to release the increased pressure, gaseous atoms of He and H would push on each other and collide with the surrounding lattice matrix atoms then push the matrix atoms aside and
spots, confirming the crystalline nature of the damaged zone, which means the structure disordering and/or amorphization by the H and He implantation can not be observed from the TEM results. On the other hand, gas bubbles and voids were also found in the implantation zone of FeAl, as displayed in Fig. 5(e) and (f). The tiny gas bubbles ranging from 5 to 15 nm were distributed uniformly with high densities. Compared with the obscured bubbles, it is easy to observe a few larger size of voids with diameters of 30 to 100 nm scattered in the damage zone. In order to compare the difference of implantation effect between co-implantation and single implantation, single He and H implantation were conducted on FeAl sample. However, no bubble or void structure was found from the cross-sectional TEM images of single He and H implantation, as shown in Fig. 6. It is known that high dose of He and H ions implantation is easy to produce gas bubbles in metals [14–16].
Fig. 6. Cross-sectional TEM micrographs of FeAl sample after single He (a) and H (b) ion implantation at fluence of 1 × 1017/cm2. 210
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further increase bubble’s size, which appears as the process to the growth of the bubbles. In this condition, the bubbles observed in irradiated FeAl can be inferred as the precursors for void growth, which might be promoted by the presence of H [21]. Therefore, according to the above deduction, the “nucleation and growth” process of bubbles to a larger size of voids in Fig. 5(e) and 5(f) might be the example of the above statement. Once the pressure of gas atoms accumulated in the voids exceeded the tolerance of the matrix material, hydrogen and helium will diffuse along the linear defects of grain boundaries or dislocations and released from the surface, which might cause the brittle fracture in FeAl [22–24]. In addition to the aging effect of tritium, suppose the synergy behaviors between hydrogen isotopes and He are similar (The synergistic effect was also found in D-He implantation [20]). Even though tritium induces embrittlement in FeAl when in the application of TPBs, the helium generated from the tritium-aging might cause an even worse effect on FeAl due to the formation of helium bubbles. On the other hand, although the synergistic effect of H and He is mostly found in implantation research, the synergistic effect of tritium and helium should also be extended in occasions of tritium irradiation due to the following reasons: (1) Defects such as vacancies or dislocations are the common products of ion implantation, trapping gaseous atoms in the matrix; (2) When applied in tritium-related systems, the ambient atmosphere of high temperature and the irradiation of tritium would induce high content of vacancies and high densities of dislocations in FeAl, as displayed in the characterization of synthesized FeAl. The structure defects induced by tritium irradiation are nearly similar to that of implantation. (3) With the similar structural environment and implanted/generated insoluble gas atoms, the synergistic effect of T and He should be similar to that of H and He. Besides, the growth of bubbles and voids promoted/accelerated by the presence of tritium might lead to severe problems of FeAl structure, such as poor ductility and brittle fracture. From this point of view, helium-induced embrittlement might play as a more serious obstacle for FeAl application in tritium-related system and should be thoroughly investigated in the near future.
[2] [3] [4]
[5] [6] [7] [8]
[9]
[10]
[11]
[12]
[13]
[14] [15] [16]
[17]
4. Conclusion
[18]
B2-FeAl alloys were synthesized and co-implanted with 250 keV H and 540 keV He under fluence of 1 × 1016 and 1 × 1017 ions cm−2, respectively. XRD, SEM and TEM were then used to characterize the structure and defects of irradiated FeAl samples. Results showed a clear implantation effect with variable appearance of structure disorder, dark damaged stripe, gas bubbles and voids. Even though the structure disorder was found based on XRD analysis, SAED analysis confirmed that the irradiated area still remains crystalline. The damage defects of high fluence implantation were observed as bubbles and voids by TEM technique. The large void is deduced to be the evolution result of tiny bubbles, and the growth of voids might be promoted by the synergistic effect of H and He dual implantation. A hypothesis is proposed that the helium embrittlement might bring even worse effect to FeAl in tritiumrelated systems based on the discussion.
[19]
[20]
[21]
[22]
[23]
Acknowledgement The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21471137).
[24]
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Irradiation effects in H and He implanted B2 iron aluminide Meijuan Hu, Pei Zhang, Jingwen Ba, Guikai Zhang, Meng Liu, Tao Tang
⁎
T
Institute of Materials, China Academy of Engineering Physics, P.O. Box 9071-12, No. 9 Huafengxincun, Jiangyou 621907, Sichuan, China
A R T I C LE I N FO
A B S T R A C T
Keywords: FeAl Ion implantation Hydrogen Helium Embrittlement
In order to study the irradiation effect of hydrogen and helium implantation, B2-FeAl alloys were exposed to hydrogen and helium beams with energy of 250 and 540 keV under doses ranging from 1 × 1016 to 1 × 1017 ions cm−2. The phase and surface morphology of FeAl alloy before and after implantation were characterized by X-ray diffraction and scanning electron microscope, respectively. The structure disorder induced by ion implantation was found, and irradiated damage appeared as large amount of dark zone only for high fluence of implantation. Using transmission electron microscopy, the microstructure and implantation structure defects such as gas bubbles and voids were investigated in high dose implanted FeAl. The formation process of bubbles and voids can be deduced by the analysis of TEM images, and the growth of the voids might be promoted by the synergistic effect of H and He implantation. It was speculated that the synergistic effect of tritium and helium might also be found in occasions of tritium irradiation.
1. Introduction Iron aluminides have been among most studied intermetallics due to the excellent oxidation and corrosion resistance [1,2]. Combined with advantages of low density and low material cost, they are being actively considered for high-temperature structural applications such as structural members in aircraft, heating elements, nuclear reactor components and et al [3,4]. However, the poor ductility and brittle fracture of FeAl intermetallics greatly limit the large-scale practical applications. These drawbacks of iron aluminides are mainly caused by sensitivity to hydrogen or water in the ambient atmosphere, namely the environmental embrittlement [5–7]. On the other hand, grain-boundary weakness and vacancy hardening also play important roles in inducing embrittlement with high content of Al or at elevated temperatures [1]. In tritium-related system, FeAl/Al2O3 composite coating has been selected as one of the prior developed tritium permeation barriers (TPB). FeAl intermediate layer can effectively relieve the great thermal mismatch between Al2O3 and metal substrate [8–11]. When exposed to higher temperature environments of tritium-containing atmosphere, more vacancies will be generated and subsequently become to new hydrogen/tritium-trapping sites in FeAl, leading to hydrogen embrittlement and the failure of tritium permeation properties [3]. Moreover, helium induced by tritium decay would definitely bring other different consequences such as helium embrittlement to FeAl. Unfortunately, in addition to hydrogen embrittlement, the helium embrittlement has not been paid enough attention while the FeAl intermetallics using in TPBs.
⁎
Aiming to simulate the irradiated FeAl sample exposed in tritiumcontaining atmosphere, helium and hydrogen were introduced into FeAl in this work in order to investigate the hydrogen and helium embrittlement in FeAl. Considering convenience of the actual operation, implantation was chosen as a relatively effective way to obtain FeAl sample contained both hydrogen and helium without long period of tritium-aging. In this paper, the discussion was based on the changes of structural and microstructural defects analysis of B2-FeAl specimens implanted with 250 keV H and 540 keV He under different doses, using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, the effect of the helium and hydrogen implantation has been briefly discussed. 2. Experimental 2.1. Materials synthesis FeAl alloys were produced by repeated induction melting under argon of 99.999% purity at the stoichiometric ratio of Fe55Al45. The alloys were produced using Fe of 99.98% purity and Al (99.9%). After melting, the ingots were cut into required dimensions for the further characterization. The surfaces were then polished with different sizes of sand paper (from 200# to 1000#) and diamond paste.
Corresponding author. E-mail address: [email protected] (T. Tang).
https://doi.org/10.1016/j.fusengdes.2019.03.199 Received 30 October 2018; Received in revised form 10 February 2019; Accepted 31 March 2019 Available online 06 April 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 143 (2019) 207–211
M. Hu, et al.
Table 1 Co-implantation parameters of FeAl samples. Sample number
Ion
Energy (keV)
Depth of implantation (μm)
Ion fluence (Ions/cm2)
Beam dimension (cm2)
Beam current (μA)
Ion flux (Ions•cm−2 s-1)
1
H He H He
250 540 250 540
1.39 1.23 1.39 1.23
1 × 1016 1 × 1016 1 × 1017 1 × 1017
1.7 × 1.3 1.6 × 1.4 1.7 × 1.3 1.6 × 1.4
1.8 2.1 1.8 2.1
5.56 × 1012 2.78 × 1012 5.56 × 1012 2.78 × 1012
2
2.2. Irradiation condition and SRIM calculation
followed by final thinning and polishing at 5 keV and 2 keV.
Co-implantation was carried out at room temperature using heavy ions research facility in Lanzhou (HIRFL). Thermocouples were mounted on the specimen surface to measure the surface temperature. The increment in temperature during implantation was less than 5 °C according to the measurement. He ions (He+) with the energy of 540 keV and H ions (H+) with 250 keV were implanted into samples at fluences of 1.0 × 1016 and 1.0 × 1017 ions/cm2. The implanted fluence was measured by a current integrator, and the detailed fluence parameters of implantation are shown in Table 1. All fluences were accumulative. According to stopping and range of ions in matter (SRIM) simulations (The calculation type ˜ detailed calculation with full Damage Cascades, Edisp_Fe/Al ˜25 eV, material density ˜ 5.542 g/cm3 or 7.787 × 1022 atoms/cm3) [12,13], the profiles of H and He ion distribution and damage with depth are shown in Fig. 1. Each fluence corresponds to the estimated H or He concentrations/displacement levels of 0.67 at.% /0.058 dpa (H, 1.0 × 1016 ions/cm2), 6.70 at.% /0.58 dpa (H, 1.0 × 1017 ions/cm2), 0.578 at.% /0.583 dpa (He, 1.0 × 1016 ions/cm2) and 5.78 at.% /5.83 dpa (He, 1.0 × 1017 ions/cm2), respectively, at the concentration/damage peak. The maximum of ions projected range was about 1.65 μm, and the H concentration and damage peaks were at the depth around 1.42 and 1.38 μm, while that of He peaks were 1.22 and 1.28 μm, respectively.
2.4. Characterization techniques After implantation, the X-ray diffraction (XRD) was performed with the Cu Kɑ (λ=1.5406 Å) line in a D8 Discover diffractometer. θ-2θ scans were performed with 2θ typically ranging from 10° to 90° at 0.02° per step. The surface morphologies of the implanted specimen were carried out on dual beam SEM/FIB system with electron beam mode, which was also used for preparing the cross-sections of the implanted bulk specimen. The observations on FIB cross-section specimens were performed using a Libra 200FE transmission electron microscope (TEM) operated at 200 kV. 3. Results and discussion 3.1. Original structure defects Considering the original structure plays important role in FeAl embrittlement, the characterization microstructure of the synthesized FeAl alloy was conducted, using optical micrograph (OM) and TEM observation. The synthesized FeAl alloy presents a microstructure of equiaxed grains with a relative uniform size (Fig. 2a). The average grain size determined by the linear intersect method was about 20 ± 5 μm. Besides, abundant of dislocations are distributed uniformly in FeAl (Fig. 2b), suggesting the original structure feature of FeAl. However, once intruded by hydrogen or helium, such a high density of two-dimensional (2D) defects, such as dislocations or grain boundaries might become to the hydrogen/helium-trapping sites, absorbing a much greater concentration of hydrogen and helium. It can be proposed that hydrogen/helium embrittlement is a result of an accumulation of these dislocations and grain boundaries causing crack nucleation [1].
2.3. TEM sample preparation Transmission electron microscopy (TEM) samples were prepared using the focused ion beam (FIB) ‘lift-out’ technique in a dual-beam instrument. To protect the specimen top surface from Ga ion sputtering/damage, the specimen was coated with 200 nm Pt using electron beam deposition followed by 3 mm Pt deposited using Ga ion beam. Note that the thick Pt layer is deposited on the top surface of specimen to help prevent the specimen from bending in the final FIB thinning and polishing procedure. The FIB specimen was cut using 30 keV Ga ions,
3.2. Phase characterization Fig. 3 shows the recorded XRD patterns of FeAl alloys before and after different fluences of hydrogen and helium ion implantation. As can be seen, all the diffraction peaks match well with the cubic iron aluminide (JCPDS No. 45-0983) before ion implantation, while a broad diffraction peak around 20° appeared after the implantation. The intensity of the broad peak also increased with the implantation fluence, indicating structure disorder of FeAl alloy induced by H and He ion implantation. On the other hand, all the intensity of FeAl diffraction peaks decreased when applied 1 × 1016 ions cm−2 of irradiation, only (211) plane exhibited abnormal intensity increase, accompanied with a small shoulder peak in the small angle direction. For FeAl implanted by a higher fluence of 1 × 1017 ions cm-2 implantation, the diffraction peak decreased even disappeared, confirming the increase of structure disorder degree. Particularly, the diffraction peak of (200) plane shifted to small angle direction with the increase of implantation fluence. This change in the peak position of (200) by the H and He implantation might be due to the development of the shoulder peak of (200) peak, which can be observed in XRD patterns of (200) of un-implanted specimen, indicating the increase of (200) lattice spacing and the lattice expansion of FeAl along (200) plane. From this point of view, it can be deduced that the implanted H and He ions in FeAl crystal lattice might
Fig. 1. H and He concentration (at.%) and damage (dpa) distribution with depth obtained from SRIM 2013 for 250 keV H and 540 keV He in FeAl with fluence of 1 × 1017/cm2. 208
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Fig. 2. (a) Optical and (b) TEM micrographs of synthesized FeAl alloy.
might be caused by large amount of ions implantation. While for low dose of implantation, no obvious damage area can be observed on the surface. Combined with the XRD analysis, the degree of implantation damage is greatly dependent on the fluence of implantation. When injected by more H and He atoms, the tendency of structure disorder in FeAl become higher, accompanied with more structure defects, which were observed as the dark area in the surface of FeAl.
3.4. Implantation induced defects In order to investigate the structure defects induced by H and He implantation, TEM analysis was conducted on FeAl sample implanted with high fluence of H and He. The specimen was stored 6 months after implantation for sufficient interactions between implanted atoms and structural defects. Fig. 5 shows the cross section view of the damaged structures from FIB fabricated specimen. As shown in Fig. 5(a), an obvious dark stripe around 70–100 nm wide was formed under the surface in depth of 100 nm, and a wider damaged zone consisted with bulges and cavities can be observed starting from the slim stripe mentioned above and extending to a deeper position. This damaged area located from 0.1 to 1.5 μm in depth, and the damage distribution is consistent with the SRIM calculation result. For undamaged matrix phase far away from the damaged zone, clear lattice fringes were presented and the corresponding selected area electron diffraction (SAED) pattern can be well indexed along [11¯ 3] zone axis of FeAl. However, even though the damaged zone of implantation appeared to be consisted with disordered structures as shown in Fig. 5(a) and (d), the magnification image of damaged zone still showed obvious lattice fringes with basal distances of 0.279 nm, which can be assigned to (100) plane of FeAl. Besides, the corresponded SAED pattern is nearly the same as that of the matrix phase, consisted with bright rhombic
Fig. 3. XRD patterns of FeAl alloys before and after different fluence of hydrogen and helium ion implantation.
distribute along the preferential orientation such as (200) plane, causing the increase of the specific lattice spacing in FeAl crystal, indicating the change in crystalline structure from cubic structure to tetragonal structure.
3.3. Surface morphology characterization Fig. 4 presents the surface morphologies of FeAl alloys after different fluence of hydrogen and helium ion implantation. When applied high fluence (1 × 1017/cm2) of hydrogen and helium ions, large amount of dark area appeared on the surface of FeAl sample, which
Fig. 4. SEM images of FeAl samples implanted with 540 keV He ions and 250 keV H ions at different fluences of (a) 1 × 1017/cm2 and (b) 1 × 1016/cm2. 209
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Fig. 5. Cross-sectional TEM micrographs of FeAl sample stored 6 months after 1 × 1017/cm2 He and H ions implantation: (a) overall image from surface to matrix; magnification images and corresponding SAED patterns (inset) of matrix phase (b) and implantation damaged zone (c); (d–f) different microstructures of implantation damaged zone.
While the helium atoms are “insoluble” to the metal matrix, which are most easily trapped by vacancies and formed the helium-vacancy complex clusters. In this case, the small helium bubbles become to the accumulation result of He-V clusters, which means the nucleation of helium bubble. For single implantation of hydrogen, it is reported that presence of H did not affect the microstructures. No cavities and no obvious dislocation structure were formed, suggesting that H has little effect on cavity nucleation, where the cavities can be highly pressurized bubbles or under-pressurized voids [17]. While for dual implantation of H and He, a synergistic effect was found in many material systems, namely the existence of H has an important role for promoting cavity’s growth [17–20]. In order to release the increased pressure, gaseous atoms of He and H would push on each other and collide with the surrounding lattice matrix atoms then push the matrix atoms aside and
spots, confirming the crystalline nature of the damaged zone, which means the structure disordering and/or amorphization by the H and He implantation can not be observed from the TEM results. On the other hand, gas bubbles and voids were also found in the implantation zone of FeAl, as displayed in Fig. 5(e) and (f). The tiny gas bubbles ranging from 5 to 15 nm were distributed uniformly with high densities. Compared with the obscured bubbles, it is easy to observe a few larger size of voids with diameters of 30 to 100 nm scattered in the damage zone. In order to compare the difference of implantation effect between co-implantation and single implantation, single He and H implantation were conducted on FeAl sample. However, no bubble or void structure was found from the cross-sectional TEM images of single He and H implantation, as shown in Fig. 6. It is known that high dose of He and H ions implantation is easy to produce gas bubbles in metals [14–16].
Fig. 6. Cross-sectional TEM micrographs of FeAl sample after single He (a) and H (b) ion implantation at fluence of 1 × 1017/cm2. 210
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further increase bubble’s size, which appears as the process to the growth of the bubbles. In this condition, the bubbles observed in irradiated FeAl can be inferred as the precursors for void growth, which might be promoted by the presence of H [21]. Therefore, according to the above deduction, the “nucleation and growth” process of bubbles to a larger size of voids in Fig. 5(e) and 5(f) might be the example of the above statement. Once the pressure of gas atoms accumulated in the voids exceeded the tolerance of the matrix material, hydrogen and helium will diffuse along the linear defects of grain boundaries or dislocations and released from the surface, which might cause the brittle fracture in FeAl [22–24]. In addition to the aging effect of tritium, suppose the synergy behaviors between hydrogen isotopes and He are similar (The synergistic effect was also found in D-He implantation [20]). Even though tritium induces embrittlement in FeAl when in the application of TPBs, the helium generated from the tritium-aging might cause an even worse effect on FeAl due to the formation of helium bubbles. On the other hand, although the synergistic effect of H and He is mostly found in implantation research, the synergistic effect of tritium and helium should also be extended in occasions of tritium irradiation due to the following reasons: (1) Defects such as vacancies or dislocations are the common products of ion implantation, trapping gaseous atoms in the matrix; (2) When applied in tritium-related systems, the ambient atmosphere of high temperature and the irradiation of tritium would induce high content of vacancies and high densities of dislocations in FeAl, as displayed in the characterization of synthesized FeAl. The structure defects induced by tritium irradiation are nearly similar to that of implantation. (3) With the similar structural environment and implanted/generated insoluble gas atoms, the synergistic effect of T and He should be similar to that of H and He. Besides, the growth of bubbles and voids promoted/accelerated by the presence of tritium might lead to severe problems of FeAl structure, such as poor ductility and brittle fracture. From this point of view, helium-induced embrittlement might play as a more serious obstacle for FeAl application in tritium-related system and should be thoroughly investigated in the near future.
[2] [3] [4]
[5] [6] [7] [8]
[9]
[10]
[11]
[12]
[13]
[14] [15] [16]
[17]
4. Conclusion
[18]
B2-FeAl alloys were synthesized and co-implanted with 250 keV H and 540 keV He under fluence of 1 × 1016 and 1 × 1017 ions cm−2, respectively. XRD, SEM and TEM were then used to characterize the structure and defects of irradiated FeAl samples. Results showed a clear implantation effect with variable appearance of structure disorder, dark damaged stripe, gas bubbles and voids. Even though the structure disorder was found based on XRD analysis, SAED analysis confirmed that the irradiated area still remains crystalline. The damage defects of high fluence implantation were observed as bubbles and voids by TEM technique. The large void is deduced to be the evolution result of tiny bubbles, and the growth of voids might be promoted by the synergistic effect of H and He dual implantation. A hypothesis is proposed that the helium embrittlement might bring even worse effect to FeAl in tritiumrelated systems based on the discussion.
[19]
[20]
[21]
[22]
[23]
Acknowledgement The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21471137).
[24]
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