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Ductile–brittle transition behaviors for the W-films-deposited stainless steel before and after hydrogen ion irradiation
Surface & Coatings Technology 193 (2005) 117 – 122 www.elsevier.com/locate/surfcoat
Ductile–brittle transition behaviors for the W-films-deposited stainless steel before and after hydrogen ion irradiation Lei. Jia-ronga,b, P. Zoua,b, B. Yangb, N.K. Huangb,* b
a Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang, 621900, China Key Laboratory for Radiation Physics and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, 610064, PR China
Available online 13 January 2005
Abstract Tungsten films were deposited on stainless steel with DC magnetron sputtering followed by argon ion beam bombardment. The ductile– brittle transition behavior of these specimens before and after hydrogen ion irradiation has been investigated by means of instrumented impact test at a series of temperature and SEM morphology observation. The results show that with decreasing temperature, the type of fracture was changed from ductile to brittle. The results on E p and fracture time show that deposition of W film does not decrease the impact toughness of substrate. While measurements on E i, X max and fracture morphologies show that it can effectively improve the impact toughness of stainless steel. Hydrogen ion irradiation led to a small decrease of impact toughness for the W-film-deposited stainless steel. D 2004 Elsevier B.V. All rights reserved. Keywords: W films; Ion beam technique; Ductile–brittle transition
1. Introduction Tungsten has a higher energy threshold for physical sputtering, it does not form hydrides and it has the highest melting point among all metals, the lowest vapor pressure, good thermal conductivity and high temperature strength. All of these features make it useful as coatings of the plasma wall in fusion reactor. Nevertheless, there remain several problems to be solved before its application as plasma facing component, for example, its brittle nature and consequent hard machining, erosion and plasma contamination, uncertainty in tritium retention and hydrogen effect [1,2]. High Z refractory metals are usually produced by a powder metallurgy. W is very brittle below a ductile–brittle transition temperature (DBTT) which is usually near or a little above room temperature (RT) and hence machining is very difficult at RT. Due to the lack of ductility, the utilization of tungsten as a structure material does not seem easy and reliable [2]. Improvement of the brittle nature of W by alloying is in * Corresponding author. Tel.: +86 28 85412230; fax: +86 28 85410252. E-mail address: [email protected] (N.K. Huang). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.043
progress, and an alternative way is using the tungsten as a thin layer on some structural materials. W coatings on the fusion reactor wall materials are developed. There are many methods for W film preparation such as CVD, plasma spray and others with ion, electron and laser beams [3,4]. It is found that W films with columnar grain structure have a good resistant-cracking on stainless steel and some coating methods can form a suitable transition layer between film and substrate that can improve the mismatch degree of thermal coefficients of W and substrate materials in our work. In this paper, we report the ductile–brittle transition behaviors of stainless steel with W films deposited on its surface before and after hydrogen ion irradiation in order to study the effect of hydrogen ion irradiation on the impact toughness of stainless steel with or without W films deposited by ion beam technology.
2. Experimental The standard Charpy V notched substrates made of stainless steel for instrumented impact test are shown in
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similar shapes at corresponding test temperature and similar trends of fracture with temperature, and the typical ductile and brittle features were also at 20 and 122 8C, respectively. Fig. 3 shows the curves of crack initiation energy E i [7] at different test temperature for different specimens. Among them, curve (a) represents the E i for stainless steel, curves (b) and (c) for the W film deposited stainless steel before and after hydrogen ion irradiation, respectively. The trends
Fig. 1. Main dimension of Charpy V specimen.
Fig. 1. They were polished, degreased in benzene using ultrasonic cleaning, rinsed in de-ionized water and finally dried. W films on substrates were prepared in our multifunction deposition equipment [5]. The chamber had a base pressure of 610 4 Pa, but this pressure increased to 0.1 Pa during DC magnetron sputtering deposition. As soon as the deposited W film became about 12 nm thick, Ar+ ion beam with 30 keV was then used to bombard it at room temperature. Ar+ ion dose was about (0.6–1)1016 ions/ cm2, and the pressure during ion bombardment was about 510 3 Pa. This thickness of W films was selected based on TRIM-92 code calculation in order to get the transition layer between the film and the substrate. After that, deposition was continued to get another layer with the same thickness on the bombarded surface, and followed by Ar+ bombardment at the same conditions as first in order to get the mixed transition between the layers. Such a process was repeated up to the thickness of W films to be about 300 nm. Some specimens were then irradiated by H+ ion of 10 keV and 11018 ions/cm2. The pressure during H+ ion bombardment was about 510 3 Pa. Instrumented Charpy impact tests were done in the machine of Tinius Olson equipped with WDEM-84 instrument, where the data were collected by GRC830ISB system. The temperature point selected was usually maintained for half an hour, low temperature was obtained by cooling liquid nitrogen, and high temperature by heating automatically in the machine. The temperature tolerance is F1 8C. The micrographs on the fracture surface of specimens produced by Charpy impact tests were obtained with AMRAY 1845 type scan electron microscopy.
3. Results and discussion Fig. 2 shows the original recorded curves of the instrumented impact test for stainless steel at different test temperature. It can be seen that the type of fracture was changed from ductile to brittle at test temperature from 20 to 122 8C, the curve at 20 or 122 8C is characteristic of the typical ductile or brittle fracture, respectively [6]. The curves (not shown here) for the W film deposited stainless steel before and after hydrogen ion irradiation had
Fig. 2. Original recorded curves of the instrumented impact test for stainless steel at: (a) 20 8C; (b) 70 8C; (c) 122 8C.
L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
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Fig. 3. Crack initiation energy E i curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
Fig. 4. Crack propagation energy E p curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
Fig. 5. Impact absorbing energy E t curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
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Fig.6. Fracture time vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
of these curves are similar each other. The W film deposited stainless steel had the highest values E i as shown in Fig. 3(b) at the whole test temperature range except at 140 8C. E i value decreases for the W film deposited stainless steel after hydrogen ion irradiation shown in curve (c), but they are still higher than those for stainless steel substrate except at 20 8C. Usually, the larger the E i value is, the more difficult the crack initiation is. The results show that stainless steel covered with W film on its surface had better property of resistant crack initiation even after hydrogen ion irradiation. Possibly, most natural micro-cracks on the surface of stainless steel were eliminated due to the covered W films. Fig. 4 shows the curves of crack propagation energy E p at different temperature. There are almost the same at temperature of 120 to 50 8C except at 20 8C. E p for the specimens with W films is smaller than that for substrate,
and became much more smaller after hydrogen ion irradiation at 20 8C. Deposition of W film does not seem to affect crack propagation in stainless steel at this temperature range. Fig. 5 shows the curves of impact absorbing energy E t at different test temperature. Usually, larger E t value represents a better impact toughness of material. An impact absorbing energy E t consists of crack initiation energy E i and crack propagation energy E p, i.e., E t=E i+E p. E t value is always larger for the samples with W films than that for substrate at the temperature of 120 to 50 8C even after hydrogen ion irradiation except 140 and 20 8C. Hydrogen ion irradiation led to decreasing of the E t, but E t for the W film deposited stainless steel after hydrogen ion irradiation is still higher than that for substrate. Obviously, this indicates that deposition of W film on stainless steel can improve the impact toughness of stainless steel and decrease
Fig. 7. Deflection under maximum load vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
the harmful effect of hydrogen ion irradiation on the impact toughness of stainless steel. The other parameters obtained from instrumented impact tests can also be used to review the ductile–brittle behaviors of the specimens. Fig. 6 shows curves of the fracture time at different test temperature. Usually, the longer the fracture time is, the better the impact toughness is. The fracture time does not have the temperature dependence in the range of 140 to 50 8C. But it become divergent at 20 8C, where
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the fracture time for the specimen with W film was longer than that for substrate, but it decreased rapidly and became shorter than that for substrate after hydrogen ion irradiation. Based on it, hydrogen seems to be a kind of harmful impurity in the specimens at this temperature. Fig. 7 shows deflection X max curves under the maximum of load at different test temperature. Usually, the larger the deflection X max is, the better the impact toughness is. Specimens with W film had larger X max value than stainless steel. After hydrogen ion irradiation, the deflection X max for the W film deposited stainless steel decreased at 70 to 50 8C, but it was still larger than that for substrate at the whole test temperature. Fig. 8 shows the fracture morphologies at 70 8C for specimens. It can be found that all of these microstructures are characteristic of dimple fracture in a net-like distribution, and there are a lot of small dimples within the large dimples, which can be seen on amplification at higher times (not shown here). The difference between these specimens is that the dimples in the W film deposited stainless steel are larger and deeper than those in the substrate, but these dimples became smaller and shallower after hydrogen ion irradiation. This observation shows that deposition of W film on stainless steel improve impact toughness of stainless steel, and hydrogen ion irradiation leads to a little decrease in impact toughness.
4. Conclusion The ductile–brittle transition behavior of stainless steel with W films deposited on its surface before and after hydrogen ion irradiation has been investigated by means of instrumented impact test at a series of temperature and SEM morphology observation. The main conclusions of this study are given below. (1)
(2)
(3)
(4) Fig. 8. Fracture morphologies of the specimens at test temperature of 70 8C: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
The original recorded curves of the instrumented impact test show that the type of fracture was changed from ductile to brittle at temperature from 20 to 122 8C. Stainless steel or with W film on it before and after hydrogen ion irradiation had similar tendency of fracture behavior with test temperature. The crack propagation energy E p and fracture time manifest that deposition of W film does not affect the impact toughness of stainless steel. While the results of the crack initiation energy E i and deflection X max under the maximum load at different temperature as well as fracture morphologies show that deposition of W film can effectively improve the impact toughness of stainless steel. Hydrogen ion irradiation led to a little bad effect on impact toughness for the W-film-deposited stainless steel, but it does not basically influence the impact toughness of stainless steel in the present experimental conditions.
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Acknowledgements This work was supported by National Natural Science Foundation of China (59781002), Specialized Research Fund for the Doctoral program of Higher Education (98061001), and Foundation of Ion Beam Lab. Shanghai Institute of Metallurgy, Chinese Academy of Science.
References [1] R.D. Watson, J. Nucl. Mater. 176–177 (1990) 110. [2] T. Tanabe, M. Akiba, Y. Ueda, K. Ohya, M. Wada, V. Philipps, Fusion Eng. Des. 39–40 (1998) 275.
[3] A.A. Pisarev, A.V. Varara, S.K. Zhdanov, J. Nucl. Mater. 220–222 (1995) 926. [4] D.Z. Wang, J.X. Chen, H.L. Zhang, N.K. Huang, Nucl. Instrum. Methods B171 (2000) 465. [5] N.K. Huang, M.H. Wang, Y. Shen, Meas. Sci. Technol. 3 (1993) 879. [6] T. Kobayashi, Eng. Frecture Mech. 19 (1) (1984) 67. [7] T. Tani, M. Magumo, Metall. Mater. Trans. 26A (1995) 391.
Ductile–brittle transition behaviors for the W-films-deposited stainless steel before and after hydrogen ion irradiation Lei. Jia-ronga,b, P. Zoua,b, B. Yangb, N.K. Huangb,* b
a Institute of Nuclear Physics and Chemistry, Chinese Academy of Engineering Physics, Mianyang, 621900, China Key Laboratory for Radiation Physics and Technology of Education Ministry of China, Institute of Nuclear Science and Technology, Sichuan University, Chengdu, 610064, PR China
Available online 13 January 2005
Abstract Tungsten films were deposited on stainless steel with DC magnetron sputtering followed by argon ion beam bombardment. The ductile– brittle transition behavior of these specimens before and after hydrogen ion irradiation has been investigated by means of instrumented impact test at a series of temperature and SEM morphology observation. The results show that with decreasing temperature, the type of fracture was changed from ductile to brittle. The results on E p and fracture time show that deposition of W film does not decrease the impact toughness of substrate. While measurements on E i, X max and fracture morphologies show that it can effectively improve the impact toughness of stainless steel. Hydrogen ion irradiation led to a small decrease of impact toughness for the W-film-deposited stainless steel. D 2004 Elsevier B.V. All rights reserved. Keywords: W films; Ion beam technique; Ductile–brittle transition
1. Introduction Tungsten has a higher energy threshold for physical sputtering, it does not form hydrides and it has the highest melting point among all metals, the lowest vapor pressure, good thermal conductivity and high temperature strength. All of these features make it useful as coatings of the plasma wall in fusion reactor. Nevertheless, there remain several problems to be solved before its application as plasma facing component, for example, its brittle nature and consequent hard machining, erosion and plasma contamination, uncertainty in tritium retention and hydrogen effect [1,2]. High Z refractory metals are usually produced by a powder metallurgy. W is very brittle below a ductile–brittle transition temperature (DBTT) which is usually near or a little above room temperature (RT) and hence machining is very difficult at RT. Due to the lack of ductility, the utilization of tungsten as a structure material does not seem easy and reliable [2]. Improvement of the brittle nature of W by alloying is in * Corresponding author. Tel.: +86 28 85412230; fax: +86 28 85410252. E-mail address: [email protected] (N.K. Huang). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.043
progress, and an alternative way is using the tungsten as a thin layer on some structural materials. W coatings on the fusion reactor wall materials are developed. There are many methods for W film preparation such as CVD, plasma spray and others with ion, electron and laser beams [3,4]. It is found that W films with columnar grain structure have a good resistant-cracking on stainless steel and some coating methods can form a suitable transition layer between film and substrate that can improve the mismatch degree of thermal coefficients of W and substrate materials in our work. In this paper, we report the ductile–brittle transition behaviors of stainless steel with W films deposited on its surface before and after hydrogen ion irradiation in order to study the effect of hydrogen ion irradiation on the impact toughness of stainless steel with or without W films deposited by ion beam technology.
2. Experimental The standard Charpy V notched substrates made of stainless steel for instrumented impact test are shown in
118
L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
similar shapes at corresponding test temperature and similar trends of fracture with temperature, and the typical ductile and brittle features were also at 20 and 122 8C, respectively. Fig. 3 shows the curves of crack initiation energy E i [7] at different test temperature for different specimens. Among them, curve (a) represents the E i for stainless steel, curves (b) and (c) for the W film deposited stainless steel before and after hydrogen ion irradiation, respectively. The trends
Fig. 1. Main dimension of Charpy V specimen.
Fig. 1. They were polished, degreased in benzene using ultrasonic cleaning, rinsed in de-ionized water and finally dried. W films on substrates were prepared in our multifunction deposition equipment [5]. The chamber had a base pressure of 610 4 Pa, but this pressure increased to 0.1 Pa during DC magnetron sputtering deposition. As soon as the deposited W film became about 12 nm thick, Ar+ ion beam with 30 keV was then used to bombard it at room temperature. Ar+ ion dose was about (0.6–1)1016 ions/ cm2, and the pressure during ion bombardment was about 510 3 Pa. This thickness of W films was selected based on TRIM-92 code calculation in order to get the transition layer between the film and the substrate. After that, deposition was continued to get another layer with the same thickness on the bombarded surface, and followed by Ar+ bombardment at the same conditions as first in order to get the mixed transition between the layers. Such a process was repeated up to the thickness of W films to be about 300 nm. Some specimens were then irradiated by H+ ion of 10 keV and 11018 ions/cm2. The pressure during H+ ion bombardment was about 510 3 Pa. Instrumented Charpy impact tests were done in the machine of Tinius Olson equipped with WDEM-84 instrument, where the data were collected by GRC830ISB system. The temperature point selected was usually maintained for half an hour, low temperature was obtained by cooling liquid nitrogen, and high temperature by heating automatically in the machine. The temperature tolerance is F1 8C. The micrographs on the fracture surface of specimens produced by Charpy impact tests were obtained with AMRAY 1845 type scan electron microscopy.
3. Results and discussion Fig. 2 shows the original recorded curves of the instrumented impact test for stainless steel at different test temperature. It can be seen that the type of fracture was changed from ductile to brittle at test temperature from 20 to 122 8C, the curve at 20 or 122 8C is characteristic of the typical ductile or brittle fracture, respectively [6]. The curves (not shown here) for the W film deposited stainless steel before and after hydrogen ion irradiation had
Fig. 2. Original recorded curves of the instrumented impact test for stainless steel at: (a) 20 8C; (b) 70 8C; (c) 122 8C.
L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
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Fig. 3. Crack initiation energy E i curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
Fig. 4. Crack propagation energy E p curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
Fig. 5. Impact absorbing energy E t curves vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
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Fig.6. Fracture time vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
of these curves are similar each other. The W film deposited stainless steel had the highest values E i as shown in Fig. 3(b) at the whole test temperature range except at 140 8C. E i value decreases for the W film deposited stainless steel after hydrogen ion irradiation shown in curve (c), but they are still higher than those for stainless steel substrate except at 20 8C. Usually, the larger the E i value is, the more difficult the crack initiation is. The results show that stainless steel covered with W film on its surface had better property of resistant crack initiation even after hydrogen ion irradiation. Possibly, most natural micro-cracks on the surface of stainless steel were eliminated due to the covered W films. Fig. 4 shows the curves of crack propagation energy E p at different temperature. There are almost the same at temperature of 120 to 50 8C except at 20 8C. E p for the specimens with W films is smaller than that for substrate,
and became much more smaller after hydrogen ion irradiation at 20 8C. Deposition of W film does not seem to affect crack propagation in stainless steel at this temperature range. Fig. 5 shows the curves of impact absorbing energy E t at different test temperature. Usually, larger E t value represents a better impact toughness of material. An impact absorbing energy E t consists of crack initiation energy E i and crack propagation energy E p, i.e., E t=E i+E p. E t value is always larger for the samples with W films than that for substrate at the temperature of 120 to 50 8C even after hydrogen ion irradiation except 140 and 20 8C. Hydrogen ion irradiation led to decreasing of the E t, but E t for the W film deposited stainless steel after hydrogen ion irradiation is still higher than that for substrate. Obviously, this indicates that deposition of W film on stainless steel can improve the impact toughness of stainless steel and decrease
Fig. 7. Deflection under maximum load vs. test temperature for: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
the harmful effect of hydrogen ion irradiation on the impact toughness of stainless steel. The other parameters obtained from instrumented impact tests can also be used to review the ductile–brittle behaviors of the specimens. Fig. 6 shows curves of the fracture time at different test temperature. Usually, the longer the fracture time is, the better the impact toughness is. The fracture time does not have the temperature dependence in the range of 140 to 50 8C. But it become divergent at 20 8C, where
121
the fracture time for the specimen with W film was longer than that for substrate, but it decreased rapidly and became shorter than that for substrate after hydrogen ion irradiation. Based on it, hydrogen seems to be a kind of harmful impurity in the specimens at this temperature. Fig. 7 shows deflection X max curves under the maximum of load at different test temperature. Usually, the larger the deflection X max is, the better the impact toughness is. Specimens with W film had larger X max value than stainless steel. After hydrogen ion irradiation, the deflection X max for the W film deposited stainless steel decreased at 70 to 50 8C, but it was still larger than that for substrate at the whole test temperature. Fig. 8 shows the fracture morphologies at 70 8C for specimens. It can be found that all of these microstructures are characteristic of dimple fracture in a net-like distribution, and there are a lot of small dimples within the large dimples, which can be seen on amplification at higher times (not shown here). The difference between these specimens is that the dimples in the W film deposited stainless steel are larger and deeper than those in the substrate, but these dimples became smaller and shallower after hydrogen ion irradiation. This observation shows that deposition of W film on stainless steel improve impact toughness of stainless steel, and hydrogen ion irradiation leads to a little decrease in impact toughness.
4. Conclusion The ductile–brittle transition behavior of stainless steel with W films deposited on its surface before and after hydrogen ion irradiation has been investigated by means of instrumented impact test at a series of temperature and SEM morphology observation. The main conclusions of this study are given below. (1)
(2)
(3)
(4) Fig. 8. Fracture morphologies of the specimens at test temperature of 70 8C: (a) stainless steel substrate; (b) the W film deposited stainless steel; (c) the W film deposited stainless steel after hydrogen ion irradiation.
The original recorded curves of the instrumented impact test show that the type of fracture was changed from ductile to brittle at temperature from 20 to 122 8C. Stainless steel or with W film on it before and after hydrogen ion irradiation had similar tendency of fracture behavior with test temperature. The crack propagation energy E p and fracture time manifest that deposition of W film does not affect the impact toughness of stainless steel. While the results of the crack initiation energy E i and deflection X max under the maximum load at different temperature as well as fracture morphologies show that deposition of W film can effectively improve the impact toughness of stainless steel. Hydrogen ion irradiation led to a little bad effect on impact toughness for the W-film-deposited stainless steel, but it does not basically influence the impact toughness of stainless steel in the present experimental conditions.
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L. Jia-rong et al. / Surface & Coatings Technology 193 (2005) 117–122
Acknowledgements This work was supported by National Natural Science Foundation of China (59781002), Specialized Research Fund for the Doctoral program of Higher Education (98061001), and Foundation of Ion Beam Lab. Shanghai Institute of Metallurgy, Chinese Academy of Science.
References [1] R.D. Watson, J. Nucl. Mater. 176–177 (1990) 110. [2] T. Tanabe, M. Akiba, Y. Ueda, K. Ohya, M. Wada, V. Philipps, Fusion Eng. Des. 39–40 (1998) 275.
[3] A.A. Pisarev, A.V. Varara, S.K. Zhdanov, J. Nucl. Mater. 220–222 (1995) 926. [4] D.Z. Wang, J.X. Chen, H.L. Zhang, N.K. Huang, Nucl. Instrum. Methods B171 (2000) 465. [5] N.K. Huang, M.H. Wang, Y. Shen, Meas. Sci. Technol. 3 (1993) 879. [6] T. Kobayashi, Eng. Frecture Mech. 19 (1) (1984) 67. [7] T. Tani, M. Magumo, Metall. Mater. Trans. 26A (1995) 391.