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Formation of aluminide coatings by low-temperature heat treatment of Al coating electrodeposited from ionic liquid
Journal of Nuclear Materials 412 (2011) 274–277
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Formation of aluminide coatings by low-temperature heat treatment of Al coating electrodeposited from ionic liquid Li Yan a, Xu Bajin a, Ling Guoping a,⇑, Liu Kezhao b, Chen Chang’an b, Zhang Guikai b a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China National Key Laboratory of Surface Physics, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 December 2010 Accepted 22 March 2011 Available online 30 March 2011
a b s t r a c t In this study, we proposed a two-step approach to prepare aluminide coatings, namely electrodepositing Al from AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid at room temperature and subsequent heat treatment at low temperature. The adherence of the coating was checked by a simple mechanical scratch test. The surface and cross-sectional morphologies, phase structures and chemical compositions of the coatings after heat treatment were characterized by scanning electronic microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX), respectively. The deposited Al coatings were in homogenous and controllable thickness with excellent adherence to the substrate. The coatings were brittle Fe2Al5 and FeAl3 phase after 5 min heat treatment at 670 °C, which transformed into ductile FeAl phase after 16 h heat treatment. The advantages of this method in eliminating the brittle Fe2Al5, cracks and pores in the aluminide coatings were discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction It is highly demanded to reduce the tritium permeation through blanket structural materials and cooling tubes in the International Thermonuclear Experimental Reactor (ITER), considering the radiological danger and loss of valuable tritium. An effective solution is to form coatings on the surface of structural materials as tritium permeation barriers (TPB). Aluminide coating, which forms Al2O3 on its surface through oxidization, is identified as the most promising TPB coating [1,2]. Many methods were developed to produce aluminide coatings, such as hot dip aluminizing (HDA), chemical vapour deposition (CVD) and vacuum plasma spray (VPS) [3–5]. The main drawback of those methods was the presence of diverse imperfections of the coating, like cracks, pores and separations of the coatings, which were responsible for the low permeation reduction factors measured in the Pb–17Li melt [5–7]. None of these processes have reached technological maturity yet. Therefore new approaches are still needed to be developed to satisfy the demand of TPB. Liu and his colleagues [8] proposed to prepare TPB by a combined process using a double glow plasma technology, but it was not compatible with the ITER test module geometry. In this paper, we proposed a two-step approach, including electrodepositing Al coatings on structural materials from ionic liquid and subsequent heat treatment at low temperature, to prepare aluminide coatings. Electrodeposition is such a widely applied ⇑ Corresponding author. Tel.: +86 571 87952648; fax: +86 571 87952358. E-mail address: [email protected] (G. Ling). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.03.035
and highly developed technology that it permits to coat complex geometries homogeneously, even on the inner surface of tubes. Al can only be deposited from nonaqueous media, such as organic solvents, high-temperature molten salts and ionic liquids [9,10]. Since organic solvents are volatile and inflammable and electrodeposition from molten salts requires high temperature, ionic liquid is the most promising media for the electrodeposition of Al [11]. Recently, Konys and his colleagues proposed to prepare aluminide coatings for TPB by electrodepositing a thick Al coating from ionic liquids and subsequent high-temperature heat treatment at 980 °C [12]. Basic studies on electrodepositing Al coatings from ionic liquids on stainless steels were reported in our early work [13]. In this paper, further studies were carried out by low-temperature heat treatment of Al coating with a low thickness. The advantages of this method in eliminating the brittle Fe2Al5, cracks and pores in the coating were discussed. This method may provide another approach for the preparation of TPB coatings after further oxidization at low temperature of the prepared aluminide coating into alpha-Al2O3, which is under execution.
2. Experimental The substrate alloys used in this work were commercial type HR-2 austenitic stainless steels and SUS430 ferritic stainless steels. The compositions are listed in Table 1. All specimens were ground using a SiC abrasive paper to a 600 grade finish and ultrasonically degreased in ethanol and acetone for 15 min each prior to the electrodeposition process.
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Table 1 Chemical compositions of the substrate alloys (wt.%). Element
SUS430
HR-2
C Si Mn Cr Ni S P N Fe
60.12 60.75 61.00 16.00–18.00 60.06 60.03 60.04
60.045 61.00 8.00–10.00 19.00–21.50 5.50–8.00 –
Balance
0.20–0.36 Balance
Experimental processes were performed in a nitrogen-filled glove-box. The electrodeposition experiments were carried out in a molar ratio 2:1 AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid at room temperature with a pure Al (99.999%) anode at the direct current density ranging from 5 mA/ cm2 to 30 mA/cm2 for 5–40 min. The Al coated specimens were heat treated at 640–670 °C in a furnace for 5 min to 100 h. After the heat treatment, all the samples were cooled down in the furnace to room temperature. The adherence of the coating was checked by a simple mechanical scratch with a steel knife deliberately imposed across the coated Al surface. X-ray diffraction (XRD) is used to examine the phase constitution of the coating after heat treatment. The surface and cross-sectional morphologies of the coatings were characterized by field emission gun scanning electron microscopy (FEG-SEM). The chemical composition of the coating was identified by means of an energy dispersive X-ray (EDX) analyzer coupled to the SEM instrument.
Fig. 2. Surface SEM images of the Al coating electrodeposited on SUS430 (a) 10 mA/ cm2, 40 min; (b) 30 mA/cm2, 40 min.
3. Results and discussion 3.1. Electrodeposition of Al coating Fig. 1 presents the typical cross-section of Al coating on SUS430 after the scratch test. The deposited Al coating was homogeneous in thickness. A deep groove through the Al coating was observed. Neither peeling off nor rupture of the coating neighboring to the groove were observed, which indicated that the Al coating exhibited good adherence to the substrate. The surface morphologies of as-deposited Al coating are shown in Fig. 2. When the current density was in the range of 10–30 mA/cm2, the deposited Al coatings were dendrite-free and compact. The average size of the Al grains on the surface was about 6–4 lm.
Fig. 3. Plots of the thickness of Al coating versus electrodeposition time at the current density of 20 mA/cm2.
Fig. 3 shows the relationship between the thickness of Al coating and electrodeposition time at the current density of 20 mA/cm2. A linear relationship was found, indicating that the thickness of the deposited Al coating was easily controllable. Efficiency of electrodeposition was around 94%. Experiments were carried out on hollow cylinders 10 mm, 12 mm and 14 mm in internal diameter, with a length of 50 mm. All the milk-white Al coatings cohered closely to the inner surface of the tubes. One of the samples is shown in Fig. 4. The results show that it is viable to deposit Al coating on the inner surface of containers and segments. 3.2. Characterization of aluminide coating Fig. 1. Cross-sectional image of the Al coating deposited on SUS430 after scratch test (20 mA/cm2, 45 min).
Fig. 5 presents the XRD patterns measured from the surface of the coatings after heat treatment at 670 °C. Characteristic peaks
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Fig. 4. Image of Al coating electrodeposited on the inner surface of a tube (/ 12 mm 50 mm, 10 mA/cm2, 40 min).
Fig. 5. XRD patterns of the coatings on HR-2 after heat treatment at 670 °C for (a) 5 min; (b) 4 h; (c) 16 h.
of Fe2Al5 and FeAl3 were observed in the pattern of the sample after 5 min heat treatment (Fig. 5a). After 4 h heat treatment, the coating mainly consisted of Fe2Al5, FeAl2 and FeAl phase, seeing Fig. 5b. Comparing to Fig. 5a, it was obviously showed that the peak intensities of the Fe2Al5 phase decreased and the FeAl3 peaks disappeared while the FeAl peaks appeared, which was attributed to the transformation of FeAl3 and Fe2Al5 phase into FeAl phase. The peaks detected for the coating after 16 h heat treatment (Fig. 5c) were mainly FeAl characteristic peaks and a few Fe3Al peaks. Cross-sectional micrographs of the coatings after heat treatment at 670 °C are presented in Fig. 6. After 5 min heat treatment, the coating was 8 lm in thickness (Fig. 6a), which was equal to the thickness of original deposited Al coating before heat treatment. Fig. 6b showed four different areas within the coating after 4 h heat treatment, including a layer with needle-like precipitation and three homogeneous layers outside. Thickness of the inner layer, the transition layer and the outer layer was around 2 lm, 8 lm and 5 lm, respectively. The coating after 16 h heat treatment was a homogenous 8 lm layer with a 3 lm precipitation layer (Fig. 6c). It is noticeable that all the coatings in Fig. 6 were compact without any defects, like cracks and pores. EDX results of various layers in Fig. 6 were given in Table 2. The composition contained Al, Fe and the alloying elements of the substrate Cr, Mn and Ni. Since Cr, Mn and Ni are solid-soluted in the substrate forming single bcc phase, the contents of Fe, Cr, Mn and Ni should be considered simultaneously in the EDX analysis. The Al to (Fe, Cr, Mn, Ni) atomic ratio of the layer in Fig. 6a (Spectrum A) was 2.2, close to the atomic ratio of Fe2Al5. The Al to (Fe, Cr, Mn, Ni) atomic ratios of the three layers in Fig. 6b were
Fig. 6. Cross-sectional SEM images of the coatings on HR-2 after heat treatment at 670 °C for (a) 5 min; (b) 4 h; (c) 16 h.
Table 2 EDX results of the spectrums of Fig. 6 in at.%. Element Spectrum Spectrum Spectrum Spectrum Spectrum
A B C D E
Al
Fe
Cr
Ni
Mn
(Fe, Cr, Ni, Mn)
68.30 66.45 61.93 32.36 40.74
18.97 20.84 24.54 39.09 40.65
6.51 6.35 7.53 16.14 9.69
3.28 3.09 2.32 7.43 2.04
2.94 3.27 3.68 4.98 6.89
31.70 33.55 38.07 67.64 59.27
close to Fe2Al5 (Spectrum B), FeAl2 (Spectrum C) and FeAl (Spectrum D), respectively. Since the needle-like precipitation in Fig. 6b was too small, EDX measurement could not be obtained. It was reported that the needle-like precipitation was Fe3Al phase [6,14]. The precipitation of Fe3Al is possible, considering all the samples were cooled down in the furnace and the element Cr in the substrate may have effects on the existence temperature of Fe3Al. The Al to (Fe, Cr, Mn, Ni) atomic ratio of the layer in Fig. 6c (Spectrum E) was 0.68, in the atomic ratio range of FeAl. Accordingly, the coating after 5 min heat treatment at 670 °C was Fe2Al5 and FeAl3. After 4 h heat treatment, the structure of the coating was Fe3Al/FeAl/FeAl2/Fe2Al5. After 16 h heat treatment
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the tendency of forming pores. With controllable thickness of Al coating and low-temperature heat treatment, the pores were reduced markedly. 4. Conclusions
Fig. 7. Interface of the coatings and the SUS430 substrate after heat treatment at 640 °C for 100 h.
the coating was ductile FeAl and Fe3Al precipitation. The brittle Fe2Al5 layer disappeared completely. 3.3. Discussion As reported, the Fe2Al5 layer formed by HDA is about 100 lm in thickness or even higher, which needs rather a high temperature and long time for a complete transformation into FeAl [15,16]. By the approach proposed in this paper, the thickness of Fe2Al5 layer is only about 10 lm (Fig. 6a), much lower than that formed by HDA. According to the thermodynamic data, the transformation from Fe2Al5 phase into low Al content intermetallics is thermodynamically favored. The time needed for a transformation of such a thin Fe2Al5 layer is much shorter. When the aluminized sample was lifted up in HDA process, there was a high thermal stress in the coating, which lead to the formation of cracks in the brittle thick Fe2Al5 layer [6,17]. As mentioned in Section 2, by the method of electrodeposition, the deposition of Al coating was carried out at room temperature. And in the heat treatment process, the samples were cooled down to room temperature in the furnace. Even though Fe2Al5 was formed after 5 min heat treatment, there would not form cracks in the brittle coating (Fig. 6a). After 16 h heat treatment at 670 °C, the aluminide coating was almost ductile FeAl. Cracks did not form even under natural cooling from 670 °C to room temperature in air. It is inevitable to form pores in the high-temperature heat treatment of HDA, as known as the Kirkendall Effect. The band of pores would adversely affect the tritium permeation rate and the mechanical properties of the structural materials [16,18]. An additional hot isostatic pressing process was needed to eliminate the pores [17,19]. Generally, the number of pores will increase with higher temperature and higher concentration gradient of Al. By this new approach, no pores were observed after heat treatment at the interface of the aluminide coating (see Fig. 6), even after 100 h heat treatment at 640 °C (as shown in Fig. 7). Since the deposited Al coating was in low thickness, it formed iron aluminide coating after less than 5 min heat treatment, making the concentration gradient of Al between the aluminide coating and substrate much lower. The equilibrium concentration of vacancies in the FeAl phase is much lower at lower temperature [20], which reduces
A new two-step approach was proposed in this work to prepare aluminide coating on stainless steel by electrodepositing Al coating from ionic liquid at room temperature and subsequent heat treatment at low temperature. Scratch test showed that the Al coating exhibited high adherence to the substrate. Al coatings can be easily deposited on the inner surface of tubes. Suitable current density for electrodepositing Al at room temperature was 10–30 mA/cm2. The thickness of Al coating was controllable, which was determined by the time of electrodeposition and current density. The coating was brittle Fe2Al5 and FeAl3 after 5 min heat treatment at 670 °C, but it transformed into ductile FeAl after 16 h heat treatment. Since the coatings were heat treated at low temperature and cooled down in a furnace to room temperature, no cracks formed in the coating. There were no pores observed in the coatings after heat treatment. The thin Al coating and the low-temperature of heat treatment reduced the tendency of forming pores. Acknowledgment This research is funded by the Ministry of Science and Technology of the People’s Republic of China under the ITER research program (2010GB112001). References [1] G.W. Hollenberg, E.P. Simonen, G. Kalinin, A. Terlain, Fusion Eng. Des. 28 (1995) 190–208. [2] T. Terai, J. Nucl. Mater. 248 (1997) 153–158. [3] G. Benamati, C. Chabrol, A. Perujo, E. Rigal, H. Glasbrenner, J. Nucl. Mater. 271 (1999) 391–395. [4] D.L. Smith, J. Konys, T. Muroga, V. Evitkhin, J. Nucl. Mater. 307 (2002) 1314– 1322. [5] H. Glasbrenner, J. Konys, Z. Voss, O. Wedemeyer, J. Nucl. Mater. 307 (2002) 1360–1363. [6] J. Konys, A. Aiello, G. Benamati, L. Giancarli, Fusion Sci. Technol. 47 (2005) 844– 850. [7] A. Aiello, A. Ciampichetti, G. Benamati, J. Nucl. Mater. 329 (2004) 1398–1402. [8] H.B. Liu, J. Tao, J. Xu, Z.F. Chen, X.J. Sun, Z. Xu, J. Nucl. Mater. 378 (2008) 134– 138. [9] T. Shikama, R. Knitter, J. Konys, T. Muroga, K. Tsuchiya, A. Moesslang, H. Kawamura, S. Nagata, Fusion Eng. Des. 83 (2008) 976–982. [10] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 8 (2009) 621–629. [11] A. Lisenkov, M.L. Zheludkevich, M.G.S. Ferreira, Electrochem. Commun. 12 (2010) 729–732. [12] J. Konys, W. Krauss, N. Holstein, Fusion Eng. Des. 85 (2010) 2141–2145. [13] Y. Li, G.P. Ling, K.Z. Liu, C.A. Chen, G.K. Zhang, J. Zhejiang Univ. Eng. Sci. 43 (2009) 1316–1321. [14] W. Deqing, Appl. Surf. Sci. 254 (2008) 3026–3032. [15] H. Glasbrenner, O. Wedemeyer, J. Nucl. Mater. 257 (1998) 274–281. [16] E. Serra, H. Glasbrenner, A. Perujo, Fusion Eng. Des. 41 (1998) 149–155. [17] H. Glasbrenner, K. Stein-Fechner, J. Konys, J. Nucl. Mater. 283–287 (2000) 1302–1305. [18] Y.Y. Chang, C.C. Tsaur, J.C. Rock, Surf. Coat. Technol. 200 (2006) 6588–6593. [19] H. Glasbrenner, J. Konys, Fusion Eng. Des. 58–9 (2001) 725–729. [20] D. Gupta, Diff. Proces. Adv. Technol. Mater., Springer-Verlag GmbH & Co. KG, New York, 2005.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Formation of aluminide coatings by low-temperature heat treatment of Al coating electrodeposited from ionic liquid Li Yan a, Xu Bajin a, Ling Guoping a,⇑, Liu Kezhao b, Chen Chang’an b, Zhang Guikai b a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China National Key Laboratory of Surface Physics, China Academy of Engineering Physics, Mianyang 621900, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 15 December 2010 Accepted 22 March 2011 Available online 30 March 2011
a b s t r a c t In this study, we proposed a two-step approach to prepare aluminide coatings, namely electrodepositing Al from AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid at room temperature and subsequent heat treatment at low temperature. The adherence of the coating was checked by a simple mechanical scratch test. The surface and cross-sectional morphologies, phase structures and chemical compositions of the coatings after heat treatment were characterized by scanning electronic microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX), respectively. The deposited Al coatings were in homogenous and controllable thickness with excellent adherence to the substrate. The coatings were brittle Fe2Al5 and FeAl3 phase after 5 min heat treatment at 670 °C, which transformed into ductile FeAl phase after 16 h heat treatment. The advantages of this method in eliminating the brittle Fe2Al5, cracks and pores in the aluminide coatings were discussed. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction It is highly demanded to reduce the tritium permeation through blanket structural materials and cooling tubes in the International Thermonuclear Experimental Reactor (ITER), considering the radiological danger and loss of valuable tritium. An effective solution is to form coatings on the surface of structural materials as tritium permeation barriers (TPB). Aluminide coating, which forms Al2O3 on its surface through oxidization, is identified as the most promising TPB coating [1,2]. Many methods were developed to produce aluminide coatings, such as hot dip aluminizing (HDA), chemical vapour deposition (CVD) and vacuum plasma spray (VPS) [3–5]. The main drawback of those methods was the presence of diverse imperfections of the coating, like cracks, pores and separations of the coatings, which were responsible for the low permeation reduction factors measured in the Pb–17Li melt [5–7]. None of these processes have reached technological maturity yet. Therefore new approaches are still needed to be developed to satisfy the demand of TPB. Liu and his colleagues [8] proposed to prepare TPB by a combined process using a double glow plasma technology, but it was not compatible with the ITER test module geometry. In this paper, we proposed a two-step approach, including electrodepositing Al coatings on structural materials from ionic liquid and subsequent heat treatment at low temperature, to prepare aluminide coatings. Electrodeposition is such a widely applied ⇑ Corresponding author. Tel.: +86 571 87952648; fax: +86 571 87952358. E-mail address: [email protected] (G. Ling). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.03.035
and highly developed technology that it permits to coat complex geometries homogeneously, even on the inner surface of tubes. Al can only be deposited from nonaqueous media, such as organic solvents, high-temperature molten salts and ionic liquids [9,10]. Since organic solvents are volatile and inflammable and electrodeposition from molten salts requires high temperature, ionic liquid is the most promising media for the electrodeposition of Al [11]. Recently, Konys and his colleagues proposed to prepare aluminide coatings for TPB by electrodepositing a thick Al coating from ionic liquids and subsequent high-temperature heat treatment at 980 °C [12]. Basic studies on electrodepositing Al coatings from ionic liquids on stainless steels were reported in our early work [13]. In this paper, further studies were carried out by low-temperature heat treatment of Al coating with a low thickness. The advantages of this method in eliminating the brittle Fe2Al5, cracks and pores in the coating were discussed. This method may provide another approach for the preparation of TPB coatings after further oxidization at low temperature of the prepared aluminide coating into alpha-Al2O3, which is under execution.
2. Experimental The substrate alloys used in this work were commercial type HR-2 austenitic stainless steels and SUS430 ferritic stainless steels. The compositions are listed in Table 1. All specimens were ground using a SiC abrasive paper to a 600 grade finish and ultrasonically degreased in ethanol and acetone for 15 min each prior to the electrodeposition process.
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Table 1 Chemical compositions of the substrate alloys (wt.%). Element
SUS430
HR-2
C Si Mn Cr Ni S P N Fe
60.12 60.75 61.00 16.00–18.00 60.06 60.03 60.04
60.045 61.00 8.00–10.00 19.00–21.50 5.50–8.00 –
Balance
0.20–0.36 Balance
Experimental processes were performed in a nitrogen-filled glove-box. The electrodeposition experiments were carried out in a molar ratio 2:1 AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid at room temperature with a pure Al (99.999%) anode at the direct current density ranging from 5 mA/ cm2 to 30 mA/cm2 for 5–40 min. The Al coated specimens were heat treated at 640–670 °C in a furnace for 5 min to 100 h. After the heat treatment, all the samples were cooled down in the furnace to room temperature. The adherence of the coating was checked by a simple mechanical scratch with a steel knife deliberately imposed across the coated Al surface. X-ray diffraction (XRD) is used to examine the phase constitution of the coating after heat treatment. The surface and cross-sectional morphologies of the coatings were characterized by field emission gun scanning electron microscopy (FEG-SEM). The chemical composition of the coating was identified by means of an energy dispersive X-ray (EDX) analyzer coupled to the SEM instrument.
Fig. 2. Surface SEM images of the Al coating electrodeposited on SUS430 (a) 10 mA/ cm2, 40 min; (b) 30 mA/cm2, 40 min.
3. Results and discussion 3.1. Electrodeposition of Al coating Fig. 1 presents the typical cross-section of Al coating on SUS430 after the scratch test. The deposited Al coating was homogeneous in thickness. A deep groove through the Al coating was observed. Neither peeling off nor rupture of the coating neighboring to the groove were observed, which indicated that the Al coating exhibited good adherence to the substrate. The surface morphologies of as-deposited Al coating are shown in Fig. 2. When the current density was in the range of 10–30 mA/cm2, the deposited Al coatings were dendrite-free and compact. The average size of the Al grains on the surface was about 6–4 lm.
Fig. 3. Plots of the thickness of Al coating versus electrodeposition time at the current density of 20 mA/cm2.
Fig. 3 shows the relationship between the thickness of Al coating and electrodeposition time at the current density of 20 mA/cm2. A linear relationship was found, indicating that the thickness of the deposited Al coating was easily controllable. Efficiency of electrodeposition was around 94%. Experiments were carried out on hollow cylinders 10 mm, 12 mm and 14 mm in internal diameter, with a length of 50 mm. All the milk-white Al coatings cohered closely to the inner surface of the tubes. One of the samples is shown in Fig. 4. The results show that it is viable to deposit Al coating on the inner surface of containers and segments. 3.2. Characterization of aluminide coating Fig. 1. Cross-sectional image of the Al coating deposited on SUS430 after scratch test (20 mA/cm2, 45 min).
Fig. 5 presents the XRD patterns measured from the surface of the coatings after heat treatment at 670 °C. Characteristic peaks
276
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Fig. 4. Image of Al coating electrodeposited on the inner surface of a tube (/ 12 mm 50 mm, 10 mA/cm2, 40 min).
Fig. 5. XRD patterns of the coatings on HR-2 after heat treatment at 670 °C for (a) 5 min; (b) 4 h; (c) 16 h.
of Fe2Al5 and FeAl3 were observed in the pattern of the sample after 5 min heat treatment (Fig. 5a). After 4 h heat treatment, the coating mainly consisted of Fe2Al5, FeAl2 and FeAl phase, seeing Fig. 5b. Comparing to Fig. 5a, it was obviously showed that the peak intensities of the Fe2Al5 phase decreased and the FeAl3 peaks disappeared while the FeAl peaks appeared, which was attributed to the transformation of FeAl3 and Fe2Al5 phase into FeAl phase. The peaks detected for the coating after 16 h heat treatment (Fig. 5c) were mainly FeAl characteristic peaks and a few Fe3Al peaks. Cross-sectional micrographs of the coatings after heat treatment at 670 °C are presented in Fig. 6. After 5 min heat treatment, the coating was 8 lm in thickness (Fig. 6a), which was equal to the thickness of original deposited Al coating before heat treatment. Fig. 6b showed four different areas within the coating after 4 h heat treatment, including a layer with needle-like precipitation and three homogeneous layers outside. Thickness of the inner layer, the transition layer and the outer layer was around 2 lm, 8 lm and 5 lm, respectively. The coating after 16 h heat treatment was a homogenous 8 lm layer with a 3 lm precipitation layer (Fig. 6c). It is noticeable that all the coatings in Fig. 6 were compact without any defects, like cracks and pores. EDX results of various layers in Fig. 6 were given in Table 2. The composition contained Al, Fe and the alloying elements of the substrate Cr, Mn and Ni. Since Cr, Mn and Ni are solid-soluted in the substrate forming single bcc phase, the contents of Fe, Cr, Mn and Ni should be considered simultaneously in the EDX analysis. The Al to (Fe, Cr, Mn, Ni) atomic ratio of the layer in Fig. 6a (Spectrum A) was 2.2, close to the atomic ratio of Fe2Al5. The Al to (Fe, Cr, Mn, Ni) atomic ratios of the three layers in Fig. 6b were
Fig. 6. Cross-sectional SEM images of the coatings on HR-2 after heat treatment at 670 °C for (a) 5 min; (b) 4 h; (c) 16 h.
Table 2 EDX results of the spectrums of Fig. 6 in at.%. Element Spectrum Spectrum Spectrum Spectrum Spectrum
A B C D E
Al
Fe
Cr
Ni
Mn
(Fe, Cr, Ni, Mn)
68.30 66.45 61.93 32.36 40.74
18.97 20.84 24.54 39.09 40.65
6.51 6.35 7.53 16.14 9.69
3.28 3.09 2.32 7.43 2.04
2.94 3.27 3.68 4.98 6.89
31.70 33.55 38.07 67.64 59.27
close to Fe2Al5 (Spectrum B), FeAl2 (Spectrum C) and FeAl (Spectrum D), respectively. Since the needle-like precipitation in Fig. 6b was too small, EDX measurement could not be obtained. It was reported that the needle-like precipitation was Fe3Al phase [6,14]. The precipitation of Fe3Al is possible, considering all the samples were cooled down in the furnace and the element Cr in the substrate may have effects on the existence temperature of Fe3Al. The Al to (Fe, Cr, Mn, Ni) atomic ratio of the layer in Fig. 6c (Spectrum E) was 0.68, in the atomic ratio range of FeAl. Accordingly, the coating after 5 min heat treatment at 670 °C was Fe2Al5 and FeAl3. After 4 h heat treatment, the structure of the coating was Fe3Al/FeAl/FeAl2/Fe2Al5. After 16 h heat treatment
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the tendency of forming pores. With controllable thickness of Al coating and low-temperature heat treatment, the pores were reduced markedly. 4. Conclusions
Fig. 7. Interface of the coatings and the SUS430 substrate after heat treatment at 640 °C for 100 h.
the coating was ductile FeAl and Fe3Al precipitation. The brittle Fe2Al5 layer disappeared completely. 3.3. Discussion As reported, the Fe2Al5 layer formed by HDA is about 100 lm in thickness or even higher, which needs rather a high temperature and long time for a complete transformation into FeAl [15,16]. By the approach proposed in this paper, the thickness of Fe2Al5 layer is only about 10 lm (Fig. 6a), much lower than that formed by HDA. According to the thermodynamic data, the transformation from Fe2Al5 phase into low Al content intermetallics is thermodynamically favored. The time needed for a transformation of such a thin Fe2Al5 layer is much shorter. When the aluminized sample was lifted up in HDA process, there was a high thermal stress in the coating, which lead to the formation of cracks in the brittle thick Fe2Al5 layer [6,17]. As mentioned in Section 2, by the method of electrodeposition, the deposition of Al coating was carried out at room temperature. And in the heat treatment process, the samples were cooled down to room temperature in the furnace. Even though Fe2Al5 was formed after 5 min heat treatment, there would not form cracks in the brittle coating (Fig. 6a). After 16 h heat treatment at 670 °C, the aluminide coating was almost ductile FeAl. Cracks did not form even under natural cooling from 670 °C to room temperature in air. It is inevitable to form pores in the high-temperature heat treatment of HDA, as known as the Kirkendall Effect. The band of pores would adversely affect the tritium permeation rate and the mechanical properties of the structural materials [16,18]. An additional hot isostatic pressing process was needed to eliminate the pores [17,19]. Generally, the number of pores will increase with higher temperature and higher concentration gradient of Al. By this new approach, no pores were observed after heat treatment at the interface of the aluminide coating (see Fig. 6), even after 100 h heat treatment at 640 °C (as shown in Fig. 7). Since the deposited Al coating was in low thickness, it formed iron aluminide coating after less than 5 min heat treatment, making the concentration gradient of Al between the aluminide coating and substrate much lower. The equilibrium concentration of vacancies in the FeAl phase is much lower at lower temperature [20], which reduces
A new two-step approach was proposed in this work to prepare aluminide coating on stainless steel by electrodepositing Al coating from ionic liquid at room temperature and subsequent heat treatment at low temperature. Scratch test showed that the Al coating exhibited high adherence to the substrate. Al coatings can be easily deposited on the inner surface of tubes. Suitable current density for electrodepositing Al at room temperature was 10–30 mA/cm2. The thickness of Al coating was controllable, which was determined by the time of electrodeposition and current density. The coating was brittle Fe2Al5 and FeAl3 after 5 min heat treatment at 670 °C, but it transformed into ductile FeAl after 16 h heat treatment. Since the coatings were heat treated at low temperature and cooled down in a furnace to room temperature, no cracks formed in the coating. There were no pores observed in the coatings after heat treatment. The thin Al coating and the low-temperature of heat treatment reduced the tendency of forming pores. Acknowledgment This research is funded by the Ministry of Science and Technology of the People’s Republic of China under the ITER research program (2010GB112001). References [1] G.W. Hollenberg, E.P. Simonen, G. Kalinin, A. Terlain, Fusion Eng. Des. 28 (1995) 190–208. [2] T. Terai, J. Nucl. Mater. 248 (1997) 153–158. [3] G. Benamati, C. Chabrol, A. Perujo, E. Rigal, H. Glasbrenner, J. Nucl. Mater. 271 (1999) 391–395. [4] D.L. Smith, J. Konys, T. Muroga, V. Evitkhin, J. Nucl. Mater. 307 (2002) 1314– 1322. [5] H. Glasbrenner, J. Konys, Z. Voss, O. Wedemeyer, J. Nucl. Mater. 307 (2002) 1360–1363. [6] J. Konys, A. Aiello, G. Benamati, L. Giancarli, Fusion Sci. Technol. 47 (2005) 844– 850. [7] A. Aiello, A. Ciampichetti, G. Benamati, J. Nucl. Mater. 329 (2004) 1398–1402. [8] H.B. Liu, J. Tao, J. Xu, Z.F. Chen, X.J. Sun, Z. Xu, J. Nucl. Mater. 378 (2008) 134– 138. [9] T. Shikama, R. Knitter, J. Konys, T. Muroga, K. Tsuchiya, A. Moesslang, H. Kawamura, S. Nagata, Fusion Eng. Des. 83 (2008) 976–982. [10] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 8 (2009) 621–629. [11] A. Lisenkov, M.L. Zheludkevich, M.G.S. Ferreira, Electrochem. Commun. 12 (2010) 729–732. [12] J. Konys, W. Krauss, N. Holstein, Fusion Eng. Des. 85 (2010) 2141–2145. [13] Y. Li, G.P. Ling, K.Z. Liu, C.A. Chen, G.K. Zhang, J. Zhejiang Univ. Eng. Sci. 43 (2009) 1316–1321. [14] W. Deqing, Appl. Surf. Sci. 254 (2008) 3026–3032. [15] H. Glasbrenner, O. Wedemeyer, J. Nucl. Mater. 257 (1998) 274–281. [16] E. Serra, H. Glasbrenner, A. Perujo, Fusion Eng. Des. 41 (1998) 149–155. [17] H. Glasbrenner, K. Stein-Fechner, J. Konys, J. Nucl. Mater. 283–287 (2000) 1302–1305. [18] Y.Y. Chang, C.C. Tsaur, J.C. Rock, Surf. Coat. Technol. 200 (2006) 6588–6593. [19] H. Glasbrenner, J. Konys, Fusion Eng. Des. 58–9 (2001) 725–729. [20] D. Gupta, Diff. Proces. Adv. Technol. Mater., Springer-Verlag GmbH & Co. KG, New York, 2005.