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Positron annihilation study on interaction between hydrogen and defects in AISI 304 stainless steel
ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 308–312 www.elsevier.com/locate/radphyschem
Positron annihilation study on interaction between hydrogen and defects in AISI 304 stainless steel Y.Q. Chen, Y.C. Wu, Z. Wang, S.J. Wang Department of Physics, The Hubei Province Key Laboratory of Nuclear Solid State Physics, Wuhan University, Wuhan 430072, PR China
Abstract Interaction between hydrogen and defects in AISI 304 stainless steel was investigated by positron annihilation lifetime spectra (PALS) and coincidence Doppler broadening (CDB) measurements. PALS results show that long lifetime component t2 is about 260–270 ps, and does not change with current density, which is ascribed to the formation of three-dimensional vacancy cluster of 5–7 vacancies. Furthermore, the CDB ratio curves show that an obvious peak appears in the high-momentum region after hydrogen charging, and the peak site does not vary with current density. The mechanism of interaction between hydrogen with defects is discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Positron annihilation; Coincidence doppler broadening (CDB); Hydrogen; Defect; Stainless steel
1. Introduction Positron annihilation spectroscopy (PAS) has been widely used as a probe for the determination of defects at the atomic level in solids, including vacancies, vacancy clusters, dislocations and nanometer-scale voids (Dupaquier and Mills, 1995). Hydrogen damage or hydrogen embrittlement (HE) of iron and steels has been investigated by PAS methods, i.e., angular correlation, positron annihilation lifetime spectra (PALS), and Doppler broadening energy spectroscopy, and other techniques. For instance, positron-lifetime study has been made for the electrolytically hydrogen-charged 316 stainless steel, and the results suggested that during hydrogen charging single vacancies are produced and form clusters-like microvoids (Ohkubo et al., 2000). Wu et al. (1991) investigated hydrogen damage and hydroCorresponding author. Tel.: +86 27 68752370; fax: +86 27 68753880. E-mail address: [email protected] (Y.C. Wu).
gen-induced plastic deformation in high-purity iron, and the critical current density produced hydrogen damage was determined by PAS. Studies of lattice defects in hydrogen-charged iron by PAS and electron microscopy showed vacancy clusters on the order of 1 nm in coldrolled and hydrogen-charged iron (Cao et al., 1993). Takagi et al. (2002) reported the PALS measurement by slow positron beam in iron films, which were grown under different hydrogen pressures. It has been suggested that there existed hydrogen-induced vacancies in the thin films. Cizek et al. (2004a) studied hydrogeninduced defects in bulk niobium by X-ray diffraction and PAS, and they indicated that vacancy–hydrogen complexes were introduced into the samples due to hydrogen loading. Recently, Wu and Jean (2004a) and Wu et al. (2004b) studied hydrogen-induced defects and hydrogen damage by PALS and slow positron beam in AISI 304 and 316 stainless steels, and indicated that hydrogen damage starting from surface to the bulk had a significant difference with depth. Hydrogen damage was found to occur near the surface (o 0.2 mm). At the
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.057
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spectrum recorded 1.5 107 events, accumulated at a rate of about 120/s.
3. Results and discussion Fig. 1 shows the lifetime component t1, t2, the second component intensity I2 and mean lifetime tm as a function of current density in AISI 304 stainless steel, respectively. The current density varies from 0 to 200 mA/cm2. t2 is about 260–270 ps, and does not changed with current density; however, its intensity I2
24 22 I2 (%)
surface, hydrogen damage is the greatest. Previous work found that hydrogen damage in AISI 304 and 316 stainless steels had a similar behavior; and strongly depended on the conditions of hydrogen charging. However, few experiments reported the researches for interaction between impurity atom (i.e. hydrogen) and defects or defects complex. Doppler broadening measurement can reveal the electron momentum distribution around the annihilation site. Especially, coincidence Doppler broadening (CDB) measurement developed by Lynn et al. (1977) decreases the background by 3 orders of magnitude, which enabled us to observe high momentum annihilations with core electrons. Hence, CDB can be analyzed to observe variation of momentum distribution in shape due to the contribution of core electrons, and to identify the interaction between impurity atoms and defects and type of defect complexes in metals and alloys. In this paper, we report the primary results of investigation of interaction between hydrogen and defects in AISI 304 stainless steel using PALS and CDB.
309
20 18
2. Experiment
300
τ2 (ps)
280 260 240 220 120
τ1 (ps)
116 112 108 104 150 τm (ps)
AISI 304 (Fe/Cr18/Ni8) stainless steel foils were purchased from Goodfellow Corporation (Berwyn, PA). The obtained foils were cut into square samples of a size 25 mm 25 mm 0.3 mm. As-received samples were annealed at 1073 K for 15 min under a vacuum of 103 Pa. The electrolyte solution (0.5 mol/L H2SO4) was prepared from regent-grade chemicals and deionized water. Under cathodic polarization, the annealed samples were charged electrolytically with hydrogen for 1 h at different current density at room temperature. Platinum rod was used as the anode. Experiments were conducted before and after hydrogen charging in all samples. The PALS experiments were performed using a conventional fast–fast time coincidence system. Each of the lifetime spectra collected 1 106 counts. The positron source 22Na with an activity 20 mCi was deposited on a titanium foil and sandwiched between two pieces of the samples. The 22Na source emitted a continous energy spectrum with Emax ¼ 0.544 MeV, the depth of mean penetration in metals and alloys is about 10–100 mm. Therefore, the difference of hydrogen damage at the surface (o 0.2 mm) may be neglected, and this damage may be assumed to be homogeneous when we used 22Na as source. The lifetime resolution was 270 ps at a counting of 300 cps. The obtained PALS data were fitted with two lifetimes using the PATFIT program. The CDB spectrometer consisted of two pure germanium detectors arranged at 1801 to each other at a distance of 20 cm from target. The energy resolution of the Ge detectors is 1.76 and 1.64 keV (FWHM) at 1.33 MeV. Each coincidence
16
145 140 135 130 0
50
100
150
200
current density (mA/cm2) Fig. 1. I2, t1, t2 and tm as a function of current density in 0.5 mol/L H2SO4 solution for 1 h in annealed AISI 304 stainless steel.
ARTICLE IN PRESS Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
increases with current density, which indicate that the size of hydrogen-induced defects does not change too much, but number of defects increases with current density. t1 increases from 106 ps at annealed state to 118 ps at current density of 200 mA/cm2. The value of tm increases rapidly with current density, and tends to the saturation when the current density is more than 100 mA/cm2. Existing experiments indicated that hydrogen in iron and steels might induce vacancies, dislocations, vacancy clusters, microvoids, hydrogen bubbles and so on. The type, number and size of hydrogen-induced defects depend on the conditions of hydrogen charging. Generally speaking, vacancies and dislocations belong to the structural defects (positron lifetime value is approximately 140–200 ps), microvoids and hydrogen bubbles belong to hydrogen damage (positron lifetime value is approximately 400–500 ps), and while vacancy clusters are intermediate state (positron lifetime value is approximately 200–400 ps). These are all possible sites for positron trapping in hydrogen-charged samples. While it is very difficult to analyze the exact site for contributing to the lifetime data, our current best approach is to qualitatively understand the difference between a large-size defect (e.g. vacancy cluster) and a small-size defect (e.g. single vacancy and dislocation). However, in the fitting procedure, three state-trapping model fitting is tried, treating free state, a small-size defect and large-size defect existing; the fitted data are poor and unacceptable. Two state-trapping model fitting was tried, and it could give satisfactory results (see Fig. 1). Data of Fig. 1 show that t2 is about 260–270 ps before and after hydrogen charging, which is larger than the dislocation lifetime (165 ps) and mono-vacancy lifetime (175 ps) (Park et al., 1986) in Fe and steels. This long lifetime component is considered to be corresponding to three-dimensional vacancy cluster of 5–7 vacancy number (Van Veen et al., 1990), which suggests that single vacancies were produced and formed clusters during hydrogen charging. The increase of I2 shows that the number of vacancy clusters increases with current density. In our data fitting, we assumed that two state trapping model is satisfied, thus free state lifetime (1/tf ¼ I1/t1+I2/t2) should be equal to the bulk positron lifetime tb, i.e., the lifetime of free positrons in defectfree material. However, tf does not keep constant because the contribution of t1 contains annihilation of positrons in free state and small-size defects, The increase of t1 in Fig. 1 indicates that hydrogeninduced defects also produce small-size defects (e.g. dislocation and single vacancy). Somieski and Krause-Rehberg (1995) found that several iron alloys/ steels were damaged by mechanical stresses. The mean positron lifetime showed a clear dependence on the stress parameters. In the decomposition of lifetime
spectra, a second lifetime of about 220–270 ps, present in all cases, was ascribed to positron trapping in vacancy clusters. The curve shapes of the mean lifetime plotted versus the strain rate looked alike in different iron alloys. Similarly, previous work (Wu et al., 1991, 2004b; Wu and Jean, 2004a) showed that iron, 304 and 316 steels were damaged by hydrogen charging. The S parameter of Doppler broadening spectra as a function of current density showed similar behaviors, which imply that the types of hydrogeninduced defects in iron and steels were alike. The PALS study of the neutron-irradiated VVER-type reactor pressure vessel (RPV) steels also showed irradiationinduced vacancy cluster (5–6 vacancies) (Kocik et al., 2002). Those results supported that hydrogen induced defects produced vacancy clusters (5–7 vacancies). The variation of tm contains the contributions of both small-size defects and vacancy clusters. In AISI 304 (Fe/Cr18/Ni8) stainless steel, there exist Cr and Ni alloying elements and other’s impurities. Hydrogen-induced defects should contain vacancy–impurity complexes, which will be analyzed in the near future. Furthermore, we carried out the CDB experiments before and after hydrogen charging. The momentum distribution of CDB spectra spans several orders of magnitude. If we chose a logarithmic to display the distribution, many small, but significant differences can be obscured. However, ratio curves could be used to highlight differences between the Doppler profiles of different materials (Lynn et al., 1979). We chose the spectrum of the annealed AISI 304 stainless steel as reference spectrum and normalized other CDB spectrums to the reference one. The results were shown in Fig. 2.
Ratio to annealed stainless steel
310
1.3 10mA/cm2, 1h 50mA/cm2, 1h 100mA/cm2, 1h 200mA/cm2, 1h
1.2 1.1 1.0 0.9 0.8 0.7 0
5
10
15
20
25
30
35
PL (10-3m0C) Fig. 2. CDB ratio curves in AISI 304 stainless steel charged for 1 h at different current density.
ARTICLE IN PRESS Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
In Fig. 2, the CDB ratio curves show that the enhancement in the low-momentum region after hydrogen charging denotes that the positrons are trapped in hydrogen-induced defects, consistent with the results of positron lifetime (see Fig. 1). In the high momentum region, the curve of the sample with smaller hydrogencharging current density is higher than one with larger hydrogen-charging current density. It indicates that more positrons annihilate with valence electrons in sample with larger hydrogen-charging current density because more defects were produced. Especially in the high-momentum region, it is interesting to observe that an obvious peak appears and the peak site does not vary with current density after hydrogen charging. It has been suggested that hydrogen might interact with vacancy and become the center of vacancy accumulation, leading to formation of vacancy–hydrogen complexes, which results in change of chemical environment of defects. Of course, Cr and Ni alloy elements and other’s impurities might also form vacancy–impurity complexes, which affect the chemical environment of positron annihilation sites. Recently, Cizek et al. (2004b) investigated a variety of neutron-irradiated reactor pressure vessel steels of VVER-type using CDB. They observed the irradiationinduced migration of Cu atoms towards positron annihilation sites in dislocations, which turned out to be removed by annealing at 475 1C. In RPV steels contained Cr and Ni alloy, they did not observe that defect complexes of Cr or Ni alloying elements that induced an increase of high momentum components. Toyama et al. (2004) also investigated that irradiationinduced defects and Cu precipitation in Fe-based Model alloy for RPV steel. Vacancy–multi Cu atoms complexes and Cu precipitates were formed after irradiation but no enrichment of Ni or Mn atoms by past-irradiation annealing at 400 1C was observed. In addition, previous work found that hydrogen damage in iron, AISI 304 and 316 stainless steels had similar behavior, which implied that action of hydrogen results in change of chemical environment of defects, independent of alloying elements or impurities. More detail investigation is in progress and will be reported by performing annealing experiments in the future. Regarding the mechanism of interaction between hydrogen and defects, models of bivacancy–hydrogen complexes and four vacancy–hydrogen complexes were reported by Johnston et al. (1970). In the Au hardened in hydrogen gas, the interaction between hydrogen and vacancies can promote the formation of four vacancy. Ishizaki et al. (2004) reported that vacancy–hydrogen complexes and vacancy–helium complexes were introduced in Fe irradiated with H or He ions. The study of Cizek et al. (2004a) also indicated that vacancy–hydrogen complexes were introduced into bulk niobium due to hydrogen loading. Most probably these are vacancies surrounded by 4 hydrogen atoms.
311
4. Conclusion Interaction between hydrogen and defects in AISI 304 stainless steel was investigated by PALS and CDB measurements. The PALS results show that long lifetime component t2 is about 260–270 ps, and does not change with current density, which is considered to be corresponding to the formation of three-dimensional vacancy cluster of 5–7 vacancy number. The increase of I2 shows that the number of vacancy clusters increases with current density. Furthermore, the CDB ratio curves (ratio to annealed sample) show that the enhancement in the low-momentum region after hydrogen charging denotes that the positrons are trapped in hydrogeninduced defects, consistent with the PALS results. It is interesting to observe that an obvious peak appears in the high-momentum region after hydrogen charging, and the peak site does not vary with current density. The mechanism of interaction between hydrogen with vacancy is discussed.
Acknowledgments This research is supported by the Natural Science Foundation of China (10475062, 10575077) and Natural Science Foundation of Hubei Province (2004ABA006).
References Cao, B.S., Ichinose, H., Yamamoto, S., Li, H., Ishida, Y., 1993. Characterization of hydrogen-induced defects in iron by positron-annihilation. Philos. Mag. A. 67 (5), 1177–1186. Cizek, J., Procha´zka, I., Bec˘va´rˇ , F., Kuzˇel, R., Cieslar, M., 2004a. Hydrogen-induced defects in bulk niobium. Phys. Rev. B. 69 (22), 2241061–22410613. Cizek, J., Bec˘va´rˇ , F., Procha´zka, I., Koc˘ı´ k, J., 2004b. 2DCoincidence Doppler broadening measurement on neutronirradiated VVER-type reactor pressure vessel steels. Mater. Sci. Forum 445–446, 63–65. Dupaquier, A., Mills, Jr., A.J. (Eds.), 1995. Positron Spectroscopy of Solids. IOS Press, Amsterdam. Ishizaki, T., Xu, Q., Yoshiie, T., Nagata, S., 2004. The recovery of gas—vacancy complexes in Fe irradiated with high energy H or He irons. Mater. Trans. 45 (1), 9–12. Johnston, I.A., Dobson, P.S., Smallman, R.E., 1970. The heterogeneous nucleation of tetrahedra in quenched gold. Proc. Roy. Soc. Lond. A. 315, 231–238. Kocik, J., Keilova´, E., Cˇı´ zˇek, J., Procha´zka, I., 2002. TEM and PAS study of neutron irradiated VVER-type RPV steels. J. Nucl. Mater. 303 (1), 52–64. Lynn, K.G., Macdonald, J.R., Boie, R.A., Feldman, L.C., et al., 1977. Positron-annihilation momentum profiles in aluminum–core contribution and independent-particle model. Phys. Rev. Lett. 38 (5), 241–244.
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Lynn, K.G., Dickman, J.E., Brown, W.L., Robbinsm, M.F., Bonerup, E., 1979. Vacancies studied by positron-annihilation with high-momentum core electrons. Phys. Rev. B. 20 (9), 3566–3572. Ohkubo, H., Sugiyama, S., Fukuzato, K., Takenaka, M., Tsukuda, N., Kuramoto, E., 2000. Positron-lifetime study of electrically hydrogen charged Ni, austenitic stainless steel and Fe. J. Nucl. Mater. 283, 858–862. Park, Y.K., Waber, J.T., Meshii, M., et al., 1986. Dislocation studies on deformed single-crystals of high-purity iron using positron-annihilation-determination of dislocation densities. Phys. Rev. B. 34 (2), 823–836. Somieski, B., Krause-Rehberg, R., 1995. Application of the positron lifetime spectroscopy as a method of nondestructive testing, Mater. Mater. Sci. Forum 175–178, 989–992. Takagi, K., Furukawa, N., Kanazawa, N., Suzuki, I.R., Ohaira, T., 2002. Formation of hydrogen-induced vacancies during growth of the Fe layer studied by slow positron beam. Surf. Sci. 514 (1–3), 298–302.
Toyama, T., Nagai, Y., Tang, T., et al., 2004. Irradiationinduced defects and Cu precipitation in ternary Fe-based model alloy for nuclear reactor pressure vessel steels study positron annihilation and 3D atom probe. Mater. Sci. Forum 445–446, 195–197. van Veen, A., Schut, H., Vries, J.de, Hakvoort, R.A., Ijpma, M.R., 1990. Positron beams for solids and surfaces. In: Schultz, P.J., Massoumi, G.R., Simpson, P.J. (Eds.), AIP Conference Proceedings. American Institute of Physics, New York pp. 171, 218. Wu, Y.C., Jean, Y.C., 2004a. Hydrogen-induced defects of AISI 316 stainless steel studied by variable energy Doppler broadening energy spectra. Phys. Stat. Solidi A. 201 (5), 917–922. Wu, Y.C., Tian, Z.Z., Chang, X.R., Hsiao, C.M., 1991. Positron-annihilation study on hydrogen damage in iron of high-purity. Scr. Metall. Mater. 25 (6), 1431–1434. Wu, Y.C., Zhang, X.H., Jean, Y.C., et al., 2004b. Positron annihilation study on hydrogen-induced defects in AISI 304 stainless steel. Mater. Sci. Forum. 445–446, 213–215.
Radiation Physics and Chemistry 76 (2007) 308–312 www.elsevier.com/locate/radphyschem
Positron annihilation study on interaction between hydrogen and defects in AISI 304 stainless steel Y.Q. Chen, Y.C. Wu, Z. Wang, S.J. Wang Department of Physics, The Hubei Province Key Laboratory of Nuclear Solid State Physics, Wuhan University, Wuhan 430072, PR China
Abstract Interaction between hydrogen and defects in AISI 304 stainless steel was investigated by positron annihilation lifetime spectra (PALS) and coincidence Doppler broadening (CDB) measurements. PALS results show that long lifetime component t2 is about 260–270 ps, and does not change with current density, which is ascribed to the formation of three-dimensional vacancy cluster of 5–7 vacancies. Furthermore, the CDB ratio curves show that an obvious peak appears in the high-momentum region after hydrogen charging, and the peak site does not vary with current density. The mechanism of interaction between hydrogen with defects is discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Positron annihilation; Coincidence doppler broadening (CDB); Hydrogen; Defect; Stainless steel
1. Introduction Positron annihilation spectroscopy (PAS) has been widely used as a probe for the determination of defects at the atomic level in solids, including vacancies, vacancy clusters, dislocations and nanometer-scale voids (Dupaquier and Mills, 1995). Hydrogen damage or hydrogen embrittlement (HE) of iron and steels has been investigated by PAS methods, i.e., angular correlation, positron annihilation lifetime spectra (PALS), and Doppler broadening energy spectroscopy, and other techniques. For instance, positron-lifetime study has been made for the electrolytically hydrogen-charged 316 stainless steel, and the results suggested that during hydrogen charging single vacancies are produced and form clusters-like microvoids (Ohkubo et al., 2000). Wu et al. (1991) investigated hydrogen damage and hydroCorresponding author. Tel.: +86 27 68752370; fax: +86 27 68753880. E-mail address: [email protected] (Y.C. Wu).
gen-induced plastic deformation in high-purity iron, and the critical current density produced hydrogen damage was determined by PAS. Studies of lattice defects in hydrogen-charged iron by PAS and electron microscopy showed vacancy clusters on the order of 1 nm in coldrolled and hydrogen-charged iron (Cao et al., 1993). Takagi et al. (2002) reported the PALS measurement by slow positron beam in iron films, which were grown under different hydrogen pressures. It has been suggested that there existed hydrogen-induced vacancies in the thin films. Cizek et al. (2004a) studied hydrogeninduced defects in bulk niobium by X-ray diffraction and PAS, and they indicated that vacancy–hydrogen complexes were introduced into the samples due to hydrogen loading. Recently, Wu and Jean (2004a) and Wu et al. (2004b) studied hydrogen-induced defects and hydrogen damage by PALS and slow positron beam in AISI 304 and 316 stainless steels, and indicated that hydrogen damage starting from surface to the bulk had a significant difference with depth. Hydrogen damage was found to occur near the surface (o 0.2 mm). At the
0969-806X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2006.03.057
ARTICLE IN PRESS Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
spectrum recorded 1.5 107 events, accumulated at a rate of about 120/s.
3. Results and discussion Fig. 1 shows the lifetime component t1, t2, the second component intensity I2 and mean lifetime tm as a function of current density in AISI 304 stainless steel, respectively. The current density varies from 0 to 200 mA/cm2. t2 is about 260–270 ps, and does not changed with current density; however, its intensity I2
24 22 I2 (%)
surface, hydrogen damage is the greatest. Previous work found that hydrogen damage in AISI 304 and 316 stainless steels had a similar behavior; and strongly depended on the conditions of hydrogen charging. However, few experiments reported the researches for interaction between impurity atom (i.e. hydrogen) and defects or defects complex. Doppler broadening measurement can reveal the electron momentum distribution around the annihilation site. Especially, coincidence Doppler broadening (CDB) measurement developed by Lynn et al. (1977) decreases the background by 3 orders of magnitude, which enabled us to observe high momentum annihilations with core electrons. Hence, CDB can be analyzed to observe variation of momentum distribution in shape due to the contribution of core electrons, and to identify the interaction between impurity atoms and defects and type of defect complexes in metals and alloys. In this paper, we report the primary results of investigation of interaction between hydrogen and defects in AISI 304 stainless steel using PALS and CDB.
309
20 18
2. Experiment
300
τ2 (ps)
280 260 240 220 120
τ1 (ps)
116 112 108 104 150 τm (ps)
AISI 304 (Fe/Cr18/Ni8) stainless steel foils were purchased from Goodfellow Corporation (Berwyn, PA). The obtained foils were cut into square samples of a size 25 mm 25 mm 0.3 mm. As-received samples were annealed at 1073 K for 15 min under a vacuum of 103 Pa. The electrolyte solution (0.5 mol/L H2SO4) was prepared from regent-grade chemicals and deionized water. Under cathodic polarization, the annealed samples were charged electrolytically with hydrogen for 1 h at different current density at room temperature. Platinum rod was used as the anode. Experiments were conducted before and after hydrogen charging in all samples. The PALS experiments were performed using a conventional fast–fast time coincidence system. Each of the lifetime spectra collected 1 106 counts. The positron source 22Na with an activity 20 mCi was deposited on a titanium foil and sandwiched between two pieces of the samples. The 22Na source emitted a continous energy spectrum with Emax ¼ 0.544 MeV, the depth of mean penetration in metals and alloys is about 10–100 mm. Therefore, the difference of hydrogen damage at the surface (o 0.2 mm) may be neglected, and this damage may be assumed to be homogeneous when we used 22Na as source. The lifetime resolution was 270 ps at a counting of 300 cps. The obtained PALS data were fitted with two lifetimes using the PATFIT program. The CDB spectrometer consisted of two pure germanium detectors arranged at 1801 to each other at a distance of 20 cm from target. The energy resolution of the Ge detectors is 1.76 and 1.64 keV (FWHM) at 1.33 MeV. Each coincidence
16
145 140 135 130 0
50
100
150
200
current density (mA/cm2) Fig. 1. I2, t1, t2 and tm as a function of current density in 0.5 mol/L H2SO4 solution for 1 h in annealed AISI 304 stainless steel.
ARTICLE IN PRESS Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
increases with current density, which indicate that the size of hydrogen-induced defects does not change too much, but number of defects increases with current density. t1 increases from 106 ps at annealed state to 118 ps at current density of 200 mA/cm2. The value of tm increases rapidly with current density, and tends to the saturation when the current density is more than 100 mA/cm2. Existing experiments indicated that hydrogen in iron and steels might induce vacancies, dislocations, vacancy clusters, microvoids, hydrogen bubbles and so on. The type, number and size of hydrogen-induced defects depend on the conditions of hydrogen charging. Generally speaking, vacancies and dislocations belong to the structural defects (positron lifetime value is approximately 140–200 ps), microvoids and hydrogen bubbles belong to hydrogen damage (positron lifetime value is approximately 400–500 ps), and while vacancy clusters are intermediate state (positron lifetime value is approximately 200–400 ps). These are all possible sites for positron trapping in hydrogen-charged samples. While it is very difficult to analyze the exact site for contributing to the lifetime data, our current best approach is to qualitatively understand the difference between a large-size defect (e.g. vacancy cluster) and a small-size defect (e.g. single vacancy and dislocation). However, in the fitting procedure, three state-trapping model fitting is tried, treating free state, a small-size defect and large-size defect existing; the fitted data are poor and unacceptable. Two state-trapping model fitting was tried, and it could give satisfactory results (see Fig. 1). Data of Fig. 1 show that t2 is about 260–270 ps before and after hydrogen charging, which is larger than the dislocation lifetime (165 ps) and mono-vacancy lifetime (175 ps) (Park et al., 1986) in Fe and steels. This long lifetime component is considered to be corresponding to three-dimensional vacancy cluster of 5–7 vacancy number (Van Veen et al., 1990), which suggests that single vacancies were produced and formed clusters during hydrogen charging. The increase of I2 shows that the number of vacancy clusters increases with current density. In our data fitting, we assumed that two state trapping model is satisfied, thus free state lifetime (1/tf ¼ I1/t1+I2/t2) should be equal to the bulk positron lifetime tb, i.e., the lifetime of free positrons in defectfree material. However, tf does not keep constant because the contribution of t1 contains annihilation of positrons in free state and small-size defects, The increase of t1 in Fig. 1 indicates that hydrogeninduced defects also produce small-size defects (e.g. dislocation and single vacancy). Somieski and Krause-Rehberg (1995) found that several iron alloys/ steels were damaged by mechanical stresses. The mean positron lifetime showed a clear dependence on the stress parameters. In the decomposition of lifetime
spectra, a second lifetime of about 220–270 ps, present in all cases, was ascribed to positron trapping in vacancy clusters. The curve shapes of the mean lifetime plotted versus the strain rate looked alike in different iron alloys. Similarly, previous work (Wu et al., 1991, 2004b; Wu and Jean, 2004a) showed that iron, 304 and 316 steels were damaged by hydrogen charging. The S parameter of Doppler broadening spectra as a function of current density showed similar behaviors, which imply that the types of hydrogeninduced defects in iron and steels were alike. The PALS study of the neutron-irradiated VVER-type reactor pressure vessel (RPV) steels also showed irradiationinduced vacancy cluster (5–6 vacancies) (Kocik et al., 2002). Those results supported that hydrogen induced defects produced vacancy clusters (5–7 vacancies). The variation of tm contains the contributions of both small-size defects and vacancy clusters. In AISI 304 (Fe/Cr18/Ni8) stainless steel, there exist Cr and Ni alloying elements and other’s impurities. Hydrogen-induced defects should contain vacancy–impurity complexes, which will be analyzed in the near future. Furthermore, we carried out the CDB experiments before and after hydrogen charging. The momentum distribution of CDB spectra spans several orders of magnitude. If we chose a logarithmic to display the distribution, many small, but significant differences can be obscured. However, ratio curves could be used to highlight differences between the Doppler profiles of different materials (Lynn et al., 1979). We chose the spectrum of the annealed AISI 304 stainless steel as reference spectrum and normalized other CDB spectrums to the reference one. The results were shown in Fig. 2.
Ratio to annealed stainless steel
310
1.3 10mA/cm2, 1h 50mA/cm2, 1h 100mA/cm2, 1h 200mA/cm2, 1h
1.2 1.1 1.0 0.9 0.8 0.7 0
5
10
15
20
25
30
35
PL (10-3m0C) Fig. 2. CDB ratio curves in AISI 304 stainless steel charged for 1 h at different current density.
ARTICLE IN PRESS Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
In Fig. 2, the CDB ratio curves show that the enhancement in the low-momentum region after hydrogen charging denotes that the positrons are trapped in hydrogen-induced defects, consistent with the results of positron lifetime (see Fig. 1). In the high momentum region, the curve of the sample with smaller hydrogencharging current density is higher than one with larger hydrogen-charging current density. It indicates that more positrons annihilate with valence electrons in sample with larger hydrogen-charging current density because more defects were produced. Especially in the high-momentum region, it is interesting to observe that an obvious peak appears and the peak site does not vary with current density after hydrogen charging. It has been suggested that hydrogen might interact with vacancy and become the center of vacancy accumulation, leading to formation of vacancy–hydrogen complexes, which results in change of chemical environment of defects. Of course, Cr and Ni alloy elements and other’s impurities might also form vacancy–impurity complexes, which affect the chemical environment of positron annihilation sites. Recently, Cizek et al. (2004b) investigated a variety of neutron-irradiated reactor pressure vessel steels of VVER-type using CDB. They observed the irradiationinduced migration of Cu atoms towards positron annihilation sites in dislocations, which turned out to be removed by annealing at 475 1C. In RPV steels contained Cr and Ni alloy, they did not observe that defect complexes of Cr or Ni alloying elements that induced an increase of high momentum components. Toyama et al. (2004) also investigated that irradiationinduced defects and Cu precipitation in Fe-based Model alloy for RPV steel. Vacancy–multi Cu atoms complexes and Cu precipitates were formed after irradiation but no enrichment of Ni or Mn atoms by past-irradiation annealing at 400 1C was observed. In addition, previous work found that hydrogen damage in iron, AISI 304 and 316 stainless steels had similar behavior, which implied that action of hydrogen results in change of chemical environment of defects, independent of alloying elements or impurities. More detail investigation is in progress and will be reported by performing annealing experiments in the future. Regarding the mechanism of interaction between hydrogen and defects, models of bivacancy–hydrogen complexes and four vacancy–hydrogen complexes were reported by Johnston et al. (1970). In the Au hardened in hydrogen gas, the interaction between hydrogen and vacancies can promote the formation of four vacancy. Ishizaki et al. (2004) reported that vacancy–hydrogen complexes and vacancy–helium complexes were introduced in Fe irradiated with H or He ions. The study of Cizek et al. (2004a) also indicated that vacancy–hydrogen complexes were introduced into bulk niobium due to hydrogen loading. Most probably these are vacancies surrounded by 4 hydrogen atoms.
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4. Conclusion Interaction between hydrogen and defects in AISI 304 stainless steel was investigated by PALS and CDB measurements. The PALS results show that long lifetime component t2 is about 260–270 ps, and does not change with current density, which is considered to be corresponding to the formation of three-dimensional vacancy cluster of 5–7 vacancy number. The increase of I2 shows that the number of vacancy clusters increases with current density. Furthermore, the CDB ratio curves (ratio to annealed sample) show that the enhancement in the low-momentum region after hydrogen charging denotes that the positrons are trapped in hydrogeninduced defects, consistent with the PALS results. It is interesting to observe that an obvious peak appears in the high-momentum region after hydrogen charging, and the peak site does not vary with current density. The mechanism of interaction between hydrogen with vacancy is discussed.
Acknowledgments This research is supported by the Natural Science Foundation of China (10475062, 10575077) and Natural Science Foundation of Hubei Province (2004ABA006).
References Cao, B.S., Ichinose, H., Yamamoto, S., Li, H., Ishida, Y., 1993. Characterization of hydrogen-induced defects in iron by positron-annihilation. Philos. Mag. A. 67 (5), 1177–1186. Cizek, J., Procha´zka, I., Bec˘va´rˇ , F., Kuzˇel, R., Cieslar, M., 2004a. Hydrogen-induced defects in bulk niobium. Phys. Rev. B. 69 (22), 2241061–22410613. Cizek, J., Bec˘va´rˇ , F., Procha´zka, I., Koc˘ı´ k, J., 2004b. 2DCoincidence Doppler broadening measurement on neutronirradiated VVER-type reactor pressure vessel steels. Mater. Sci. Forum 445–446, 63–65. Dupaquier, A., Mills, Jr., A.J. (Eds.), 1995. Positron Spectroscopy of Solids. IOS Press, Amsterdam. Ishizaki, T., Xu, Q., Yoshiie, T., Nagata, S., 2004. The recovery of gas—vacancy complexes in Fe irradiated with high energy H or He irons. Mater. Trans. 45 (1), 9–12. Johnston, I.A., Dobson, P.S., Smallman, R.E., 1970. The heterogeneous nucleation of tetrahedra in quenched gold. Proc. Roy. Soc. Lond. A. 315, 231–238. Kocik, J., Keilova´, E., Cˇı´ zˇek, J., Procha´zka, I., 2002. TEM and PAS study of neutron irradiated VVER-type RPV steels. J. Nucl. Mater. 303 (1), 52–64. Lynn, K.G., Macdonald, J.R., Boie, R.A., Feldman, L.C., et al., 1977. Positron-annihilation momentum profiles in aluminum–core contribution and independent-particle model. Phys. Rev. Lett. 38 (5), 241–244.
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Y.Q. Chen et al. / Radiation Physics and Chemistry 76 (2007) 308–312
Lynn, K.G., Dickman, J.E., Brown, W.L., Robbinsm, M.F., Bonerup, E., 1979. Vacancies studied by positron-annihilation with high-momentum core electrons. Phys. Rev. B. 20 (9), 3566–3572. Ohkubo, H., Sugiyama, S., Fukuzato, K., Takenaka, M., Tsukuda, N., Kuramoto, E., 2000. Positron-lifetime study of electrically hydrogen charged Ni, austenitic stainless steel and Fe. J. Nucl. Mater. 283, 858–862. Park, Y.K., Waber, J.T., Meshii, M., et al., 1986. Dislocation studies on deformed single-crystals of high-purity iron using positron-annihilation-determination of dislocation densities. Phys. Rev. B. 34 (2), 823–836. Somieski, B., Krause-Rehberg, R., 1995. Application of the positron lifetime spectroscopy as a method of nondestructive testing, Mater. Mater. Sci. Forum 175–178, 989–992. Takagi, K., Furukawa, N., Kanazawa, N., Suzuki, I.R., Ohaira, T., 2002. Formation of hydrogen-induced vacancies during growth of the Fe layer studied by slow positron beam. Surf. Sci. 514 (1–3), 298–302.
Toyama, T., Nagai, Y., Tang, T., et al., 2004. Irradiationinduced defects and Cu precipitation in ternary Fe-based model alloy for nuclear reactor pressure vessel steels study positron annihilation and 3D atom probe. Mater. Sci. Forum 445–446, 195–197. van Veen, A., Schut, H., Vries, J.de, Hakvoort, R.A., Ijpma, M.R., 1990. Positron beams for solids and surfaces. In: Schultz, P.J., Massoumi, G.R., Simpson, P.J. (Eds.), AIP Conference Proceedings. American Institute of Physics, New York pp. 171, 218. Wu, Y.C., Jean, Y.C., 2004a. Hydrogen-induced defects of AISI 316 stainless steel studied by variable energy Doppler broadening energy spectra. Phys. Stat. Solidi A. 201 (5), 917–922. Wu, Y.C., Tian, Z.Z., Chang, X.R., Hsiao, C.M., 1991. Positron-annihilation study on hydrogen damage in iron of high-purity. Scr. Metall. Mater. 25 (6), 1431–1434. Wu, Y.C., Zhang, X.H., Jean, Y.C., et al., 2004b. Positron annihilation study on hydrogen-induced defects in AISI 304 stainless steel. Mater. Sci. Forum. 445–446, 213–215.