- Email: [email protected]
Effect of Titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 °C
ARTICLE IN PRESS
Vacuum 80 (2006) 1003–1006 www.elsevier.com/locate/vacuum
Effect of Titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 1C X.Y. Zhanga,, Q. Wan, X.D. Baib, Z.X. Jina, X.Y. Liua a
Department of Engineering Physics, Building Zijing 15#, Room 836 B, Building Zijing 15], Room 836 B, Tsinghua University, 100084, PR China b Department of Materials Science and Engineering, Tsinghua University,100084, PR China Received 5 July 2005; received in revised form 4 January 2006; accepted 5 January 2006
Abstract The beneficial effect of titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 1C was investigated. Titanium ions were implanted by a MEVVA source at an energy of 40 keV with dose 5 1016, 1 1017, and 2 1017 ion/cm2 at the maximum temperature 130 1C. Weight gain curves of the as-received and implanted zircalloy-4 were measured after oxidation in air at 500 1C for 100 min. It was found that improvement was achieved in the oxidation behaviour of titanium ion implanted samples compared with that of the as-received one. The valence of the oxides in the scale was analyzed by X-ray photoemission spectroscopy. Glancing angle X-ray diffraction was used to examine the phase transformation in the oxide films and is showed that the addition of titanium transformed the phase from monoclinic zirconia to hexagonal zirconia. Finally, the mechanism of improvements oxidation behaviour is discussed. r 2006 Elsevier Ltd. All rights reserved. PACS: 81.05.Bx; 81.40.Wx; 81.65. b; 81.65.Kn Keywords: Zircalloy-4; Oxidation; Titanium ion implantation
1. Introduction Because of their low thermal neutron capture cross section, good corrosion resistance and reasonable mechanical properties, zircalloys are often applied as fuel cladding and core structural material in reactors. It is well known that certain modification methods such as ion beam surface processing [1,2] can significantly improve corrosion protection. To investigate and improve the corrosion resistance of zircalloys, ion implantation offers the possibility to introduce a controlled concentration of an element into a thin surface layer. Recent studies by Xu et al. [3] and Bai and coworkers [4,5] demonstrated that the performances of zircalloy-4 during aqueous corrosion and air oxidation were greatly enhanced by implanting with an oxygen-active element, yttrium. Peng et al. [6] and Chen et al. [7] also gave the result that La and Ce ion implantation could improve the air oxidation performance of zircalloy-4. Corresponding author. Tel.: +86 10 62772856; fax: +86 10 62772507.
E-mail address: [email protected] (X.Y. Zhang). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.007
Ti and Zr are in the same column in the periodic table, and as they have many similar properties, it was decided to study Ti in zircalloys. In this study, 40 keV titanium ion implantation was used in an attempt to change the high temperature oxidation behaviour of zircalloy-4. The main results reported in the paper are the oxidation behaviour, the measurement of weight gain curves and the phase analysis for the surfaces of as-received zircalloy-4 and titanium-implanted zircalloy-4 samples oxidized in air at 500 1C. 2. Experimental procedure 2.1. Preparation of specimens Samples were cut to 10 mm 10 mm 0.5 mm from a sheet of fully annealed zircalloy-4 by a wire cut machine. The composition of additional elements in zircalloy-4 is 1.4 wt% Sn, 0.23 wt% Fe, 0.1 wt% Cr, and p6 10 7 wt% Ni. Then, 700–1200 grade emery papers were used to mechanically polish, and a solution of 5 vol% HF, 35 vol%
ARTICLE IN PRESS 1004
X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
HNO3, and 60 vol% H2O was used to chemically polish samples. Subsequently, samples were rinsed by tap water for more than 5 min, and finally cleaned in an ultrasonic cleaner using deionized water.
Ti ions were implanted into zircalloy-4 by a metal vapour vacuum arc (MEVVA) implanter. Samples were loaded onto an aluminum sample holder and put into the chamber at a vacuum level of 8.0 10 4 Pa. The implantation doses were 5 1016/cm2, 1 1017/cm2 and 2 1017 / cm2. The ion extraction voltage was 40 keV and the beam current density was 10 mA/cm2 for the implantation. During implantation, the samples were not cooled. The maximum temperature of the samples, which was mostly determined by the experimental conditions such as extracted voltage, beam current density, implantation time and properties of implanted ions and base materials, was 130 1C. To investigate the high temperature oxidation behaviour of the zircalloy-4 samples, the oxidation was carried out in air at 500 1C for 100 min, and the weight gain curves were obtained. X-ray photoemission spectroscopy (XPS) was employed to analyze the composition and the valence of the surface layer of the zircalloy-4 samples. In addition, glancing angle X-ray diffraction (GAXRD) was used to examine the phase transformation resulting from the titanium ion implantation in the oxide films.
0.14
(1) 0.12 wight gain (mg/cm2)
2.2. Experiment details
0.16
0.10
(4)
0.08
(3)
0.06
(2) (1)as-received (2)5E16 (3)1E17 (4)2E17
0.04 0.02 0.00 -0.02 0
20
40 60 Time (min)
80
100
Fig. 1. Oxidation weight gain curves of (1) as-received zircalloy-4, and after (2) 5 1016, (3) 1 1017 and (4) 2 1017 ion/cm2 titanium ion implantation. Zircalloy-4 was oxidized in air at 500 1C for 100 min.
3. Results and discussion 3.1. Weight gain curve of zircalloy-4 samples Some researchers have reported that the oxidation resistance of metals could be improved by implanting some reactive elements, such as yttrium, cerium and other rare earths [8–11]. It has been assumed that the effects of these elements are similar, in a coating, or in an alloy. However, the exact mechanisms involved remain unclear up to now. Weight gain curves of the specimens implanted with different doses were measured, and Fig. 1 demonstrates the oxidation curve of un-implanted zicaloy-4 and zircalloy-4 implanted with Ti ions to doses 5 1016, 1 1017 and 2 1017 ions/cm2 at 5001C in air. From Fig. 1, it is clear that the oxidation weight gain increases with the oxidation time. Comparing the curves for different ion doses, the oxidation weight gain curves of implanted specimens are all lower than that of un-implanted zircalloy4, and the higher the dose, the higher is the weight gain curve. 3.2. Distribution of implanted ions The distribution of implanted ions was simulated by Trim96 [12] as shown in Fig. 2. The number of ions used in the simulation was 99,999. It was found that the implanted
Fig. 2. The range distribution of implanted titanium ions from a Trim calculation.
Ti ions were mainly concentrated about 40–70 nm deep from the surface. The real distribution of Ti ions may be deeper from the surface because of the temperature rise during implantation.
3.3. Composition and valance of oxides Chemical composition of surface layer ions was measured by XPS. Because of system errors, results of binding energy position need to be adjusted by comparing with the standard binding energy position of carbon absorbed on the surface, which is 285.0 eV. Fig. 3 shows the spectra of carbon. It is easy to calculate the adjustment of energy
ARTICLE IN PRESS X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
positions of 1 1016, 5 1016 and 1 1017 /cm2, are 4.8, 4.6 and 4.1 eV, respectively. Table 1 lists the adjusted values of binding energy position of Zr 3d5/2 and Ti 3d5/2, and Fig. 4 shows the adjusted spectra. Studying the XPS spectra for the different implantation doses, it is clear that there is no significant change in shape and peak position, but a slight lateral excursion for the curves of different implantation doses because of trivial incidental experiment errors. It is also shown from the XPS spectra that the binding energies coincide well with the standard values of TiO2 (458.8) and ZrO2 (182.2). Therefore, it appears that the oxides of Ti and Zr exist in the form of TiO2 and ZrO2.
1005
500 1C for 100 min. M represents monoclinic, T represents tetragonal and H represents hexagonal zirconia. From Fig. 5, it is clear that the oxide in the scale is almost entirely monoclinic zirconia (M-ZrO2) in the oxidized as-received sample whereas in Ti implanted samples, tetragonal and hexagonal zirconia forms. With increasing implantation dose, proportion of H-zirconia decreased and that of T-zirconia (T-ZrO2) and M-ZrO2
3.4. GAXRD spectra at 0.31 incident angle of zircalloy-4 Because depth distribution of implanted titanium ions was in the range of about 40–70 nm as mentioned above, GAXRD was used to investigate the mechanism responsible for the improvement of the oxidation behaviour of zircalloy-4 after Ti implantation. Diffraction angle at 0.31 corresponds to a theoretical penetration depth of about 55 nm. Fig. 5 shows the GAXRD spectra of as-received and Ti implanted zircalloy-4 samples oxidized in air at
Fig. 3. XPS spectra of the C 1 s absorbed on implanted samples: (1) implanted by 5 1016 Ti+ ion/cm2 at 40 keV, (2) implanted by 1 1017 Ti+ ion/cm2 at 40 keV and (3) implanted by 2 1017 ion/cm2 Ti+ ion/cm2 at 40 keV.
Fig. 4. XPS spectra of adjusted (a) Ti3d5/2 peak and (b) Zr 3d5/2 peak in the oxide scale after the Ti+ implantation to doses (1) 5 1016, (2) 1 1017 and (3) 2 1017 ion/cm2.
Table 1 The 3d5/2 bonding energy of Ti3d5/2 and Zr3d5/2 Elements
Experimental data (eV) Adjusted data (eV)
Ti 3d5/2
Zr 3d5/2
5E16
1E17
2E17
5E16
1E17
2E17
463.5 458.7
463.3 458.7
462.8 458.7
187.0 182.1
186.8 182.2
186.3 182.2
ARTICLE IN PRESS 1006
X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
zirconia with the amount of Ti [13–15], or because combination of Ti and O could depress formation of Hzirconia which is mainly induced from oxygen vacancies produced during ion implantation. This needs further investigation. 4. Conclusions Titanium ion implantation was employed to investigate the oxidation behaviour changes of zircalloy-4 in air at 500 1C for 100 min. Following conclusions could be made:
Fig. 5. GAXRD spectra of oxidized (1) as-received zircalloy-4, and after (2) 5 1016, (3) 1 1017 and (4) 2 1017 ion/cm2 40 keV Ti+ implantation. M, T and H represent monoclinic, tetragonal and hexagonal zirconia, respectively.
became higher. After implantation by 2 1017 ions/cm2, more M-zirconia was found than in other two implanted samples. It is well known that when the oxide film grows on the matrix, T-ZrO2 emerges first. Owing to its low stabilization, T-ZrO2 transforms into the M-ZrO2 promptly, and then the stress in the oxide film that arises owing to the lattice misfit becomes larger and larger. When the stress has accumulated to some critical value, the oxide film will crack and fresh oxygen can easily enter the matrix and react with zirconium. However, when titanium was implanted into zircalloy-4, oxygen may react with it, and Ti4+ may substitute Zr4+ to form TiO2. It has been reported that titania could stabilise T-ZrO2 in a ceramic study [13,14], and the T-ZrO2 formed after implantation may improve the oxidation corrosion behaviour of zircalloy-4 [6]. In addition, Ce ion implantation, according to Chen et al. [7], has resulted in appearance of ZrO1 x and ZrO0.35, formation of H-zirconia causes volume of zirconia to decrease rapidly and greatly, which also could improve oxidation corrosion behaviour of zircalloy-4. From Fig. 5, it is easy to see that Ti ion implantation not only stabilised the T-ZrO2, but also, led to the formation of H-zirconia. Consequently, the oxidation corrosion resistance could be improved as discussed in Section 3.1. The result that diffraction peak intensity of Hzirconia decreased with increasing of implantation dose and the weight gain of samples increased also coincides with this explanation. To explain the decrease of the diffraction peak intensity of H-zirconia with increased implantation dose, it is proposed that this is an effect related to the amount of implanted Ti in zircalloy-4. This may be due to the dependence of phase transformation of TiO2 (R-A) in
(1) The oxidation weight gain of samples increases with increase of oxidation time, and the oxidation weight gain of implanted specimens is lower than that of asreceived zircalloy-4. Furthermore, oxidation weight gain increases with an increase in implantation dose of titanium. (2) Mechanism for the improvement in the oxidation corrosion behaviour of zircalloy-4 by titanium implantation was the stabilization of T-ZrO2 and formation of H-zirconia. Occurrence of T-ZrO2 and H-zirconia leads to a slower oxidation rate. The oxidation weight gain may increase with the increase of implantation dose because of the decrease in the fraction of H-zirconia. (3) Further investigation is needed to confirm that the level of H-zirconia decreases with an increase in implantation dose.
Acknowledgements Special thanks to the Ministry of Science and Technology of China (MSTC no.G2000067207-1). We would also like to thank Tsinghua University for their financial support. References [1] McCafferty E, Natishan PM, Hubler GK. Nucl Instr Meth B 1991;56–57:639. [2] Etoh Y, Shimada S, Kikuchi K. J Nucl Sci Technol 1992;29(12):1173. [3] Xu J, Bai X, Jin A, Fan Y. J Mater Sci Lett 2000;19:1633. [4] Bai X, Xu J, He F, Fan Y. Nucl Instr Meth B 2000;160:49. [5] He F, Bai X, Xu J, Wang S, An J, Sun Z, et al. J Mater Sci Lett 1999;18:715. [6] Peng DQ, Bai XD, Chen XW, Zhou QG. Nucl Instr Meth B 2003;201:589–94. [7] Chen XW, Bai XD, Yu HR, Zhou QG, Chen BS. Nucl Instr Meth B 2002;196:293–8. [8] Stringer J. Mater Sci Eng A 1989;120:129. [9] Saito Y, Onay B, Maruyanma T. J Phys IV 1993;3:217. [10] Stroosnijder MF, Mevrel R, Bennett MJ. Mater High Temp 1994;12:53. [11] Przybylski K, Yurek GJ. Mater Sci Forum 1989;43:1. 298. [12] Ziegler JF. The stopping and range of ions in matter, vol. 2–6. New York: Pergamon Press; 1977–85. [13] Hunter BA, Howard CJ. J. Solid State Chem 1999;146:363–8. [14] Yang J, Ferreira JMF. Mater Res Bull 1998;33(3):389–94. [15] Zou H, Lin YS. Appl Catal A: Gen 2004;265:35–42.
Vacuum 80 (2006) 1003–1006 www.elsevier.com/locate/vacuum
Effect of Titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 1C X.Y. Zhanga,, Q. Wan, X.D. Baib, Z.X. Jina, X.Y. Liua a
Department of Engineering Physics, Building Zijing 15#, Room 836 B, Building Zijing 15], Room 836 B, Tsinghua University, 100084, PR China b Department of Materials Science and Engineering, Tsinghua University,100084, PR China Received 5 July 2005; received in revised form 4 January 2006; accepted 5 January 2006
Abstract The beneficial effect of titanium ion implantation on the oxidation behaviour of zircalloy-4 at 500 1C was investigated. Titanium ions were implanted by a MEVVA source at an energy of 40 keV with dose 5 1016, 1 1017, and 2 1017 ion/cm2 at the maximum temperature 130 1C. Weight gain curves of the as-received and implanted zircalloy-4 were measured after oxidation in air at 500 1C for 100 min. It was found that improvement was achieved in the oxidation behaviour of titanium ion implanted samples compared with that of the as-received one. The valence of the oxides in the scale was analyzed by X-ray photoemission spectroscopy. Glancing angle X-ray diffraction was used to examine the phase transformation in the oxide films and is showed that the addition of titanium transformed the phase from monoclinic zirconia to hexagonal zirconia. Finally, the mechanism of improvements oxidation behaviour is discussed. r 2006 Elsevier Ltd. All rights reserved. PACS: 81.05.Bx; 81.40.Wx; 81.65. b; 81.65.Kn Keywords: Zircalloy-4; Oxidation; Titanium ion implantation
1. Introduction Because of their low thermal neutron capture cross section, good corrosion resistance and reasonable mechanical properties, zircalloys are often applied as fuel cladding and core structural material in reactors. It is well known that certain modification methods such as ion beam surface processing [1,2] can significantly improve corrosion protection. To investigate and improve the corrosion resistance of zircalloys, ion implantation offers the possibility to introduce a controlled concentration of an element into a thin surface layer. Recent studies by Xu et al. [3] and Bai and coworkers [4,5] demonstrated that the performances of zircalloy-4 during aqueous corrosion and air oxidation were greatly enhanced by implanting with an oxygen-active element, yttrium. Peng et al. [6] and Chen et al. [7] also gave the result that La and Ce ion implantation could improve the air oxidation performance of zircalloy-4. Corresponding author. Tel.: +86 10 62772856; fax: +86 10 62772507.
E-mail address: [email protected] (X.Y. Zhang). 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.01.007
Ti and Zr are in the same column in the periodic table, and as they have many similar properties, it was decided to study Ti in zircalloys. In this study, 40 keV titanium ion implantation was used in an attempt to change the high temperature oxidation behaviour of zircalloy-4. The main results reported in the paper are the oxidation behaviour, the measurement of weight gain curves and the phase analysis for the surfaces of as-received zircalloy-4 and titanium-implanted zircalloy-4 samples oxidized in air at 500 1C. 2. Experimental procedure 2.1. Preparation of specimens Samples were cut to 10 mm 10 mm 0.5 mm from a sheet of fully annealed zircalloy-4 by a wire cut machine. The composition of additional elements in zircalloy-4 is 1.4 wt% Sn, 0.23 wt% Fe, 0.1 wt% Cr, and p6 10 7 wt% Ni. Then, 700–1200 grade emery papers were used to mechanically polish, and a solution of 5 vol% HF, 35 vol%
ARTICLE IN PRESS 1004
X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
HNO3, and 60 vol% H2O was used to chemically polish samples. Subsequently, samples were rinsed by tap water for more than 5 min, and finally cleaned in an ultrasonic cleaner using deionized water.
Ti ions were implanted into zircalloy-4 by a metal vapour vacuum arc (MEVVA) implanter. Samples were loaded onto an aluminum sample holder and put into the chamber at a vacuum level of 8.0 10 4 Pa. The implantation doses were 5 1016/cm2, 1 1017/cm2 and 2 1017 / cm2. The ion extraction voltage was 40 keV and the beam current density was 10 mA/cm2 for the implantation. During implantation, the samples were not cooled. The maximum temperature of the samples, which was mostly determined by the experimental conditions such as extracted voltage, beam current density, implantation time and properties of implanted ions and base materials, was 130 1C. To investigate the high temperature oxidation behaviour of the zircalloy-4 samples, the oxidation was carried out in air at 500 1C for 100 min, and the weight gain curves were obtained. X-ray photoemission spectroscopy (XPS) was employed to analyze the composition and the valence of the surface layer of the zircalloy-4 samples. In addition, glancing angle X-ray diffraction (GAXRD) was used to examine the phase transformation resulting from the titanium ion implantation in the oxide films.
0.14
(1) 0.12 wight gain (mg/cm2)
2.2. Experiment details
0.16
0.10
(4)
0.08
(3)
0.06
(2) (1)as-received (2)5E16 (3)1E17 (4)2E17
0.04 0.02 0.00 -0.02 0
20
40 60 Time (min)
80
100
Fig. 1. Oxidation weight gain curves of (1) as-received zircalloy-4, and after (2) 5 1016, (3) 1 1017 and (4) 2 1017 ion/cm2 titanium ion implantation. Zircalloy-4 was oxidized in air at 500 1C for 100 min.
3. Results and discussion 3.1. Weight gain curve of zircalloy-4 samples Some researchers have reported that the oxidation resistance of metals could be improved by implanting some reactive elements, such as yttrium, cerium and other rare earths [8–11]. It has been assumed that the effects of these elements are similar, in a coating, or in an alloy. However, the exact mechanisms involved remain unclear up to now. Weight gain curves of the specimens implanted with different doses were measured, and Fig. 1 demonstrates the oxidation curve of un-implanted zicaloy-4 and zircalloy-4 implanted with Ti ions to doses 5 1016, 1 1017 and 2 1017 ions/cm2 at 5001C in air. From Fig. 1, it is clear that the oxidation weight gain increases with the oxidation time. Comparing the curves for different ion doses, the oxidation weight gain curves of implanted specimens are all lower than that of un-implanted zircalloy4, and the higher the dose, the higher is the weight gain curve. 3.2. Distribution of implanted ions The distribution of implanted ions was simulated by Trim96 [12] as shown in Fig. 2. The number of ions used in the simulation was 99,999. It was found that the implanted
Fig. 2. The range distribution of implanted titanium ions from a Trim calculation.
Ti ions were mainly concentrated about 40–70 nm deep from the surface. The real distribution of Ti ions may be deeper from the surface because of the temperature rise during implantation.
3.3. Composition and valance of oxides Chemical composition of surface layer ions was measured by XPS. Because of system errors, results of binding energy position need to be adjusted by comparing with the standard binding energy position of carbon absorbed on the surface, which is 285.0 eV. Fig. 3 shows the spectra of carbon. It is easy to calculate the adjustment of energy
ARTICLE IN PRESS X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
positions of 1 1016, 5 1016 and 1 1017 /cm2, are 4.8, 4.6 and 4.1 eV, respectively. Table 1 lists the adjusted values of binding energy position of Zr 3d5/2 and Ti 3d5/2, and Fig. 4 shows the adjusted spectra. Studying the XPS spectra for the different implantation doses, it is clear that there is no significant change in shape and peak position, but a slight lateral excursion for the curves of different implantation doses because of trivial incidental experiment errors. It is also shown from the XPS spectra that the binding energies coincide well with the standard values of TiO2 (458.8) and ZrO2 (182.2). Therefore, it appears that the oxides of Ti and Zr exist in the form of TiO2 and ZrO2.
1005
500 1C for 100 min. M represents monoclinic, T represents tetragonal and H represents hexagonal zirconia. From Fig. 5, it is clear that the oxide in the scale is almost entirely monoclinic zirconia (M-ZrO2) in the oxidized as-received sample whereas in Ti implanted samples, tetragonal and hexagonal zirconia forms. With increasing implantation dose, proportion of H-zirconia decreased and that of T-zirconia (T-ZrO2) and M-ZrO2
3.4. GAXRD spectra at 0.31 incident angle of zircalloy-4 Because depth distribution of implanted titanium ions was in the range of about 40–70 nm as mentioned above, GAXRD was used to investigate the mechanism responsible for the improvement of the oxidation behaviour of zircalloy-4 after Ti implantation. Diffraction angle at 0.31 corresponds to a theoretical penetration depth of about 55 nm. Fig. 5 shows the GAXRD spectra of as-received and Ti implanted zircalloy-4 samples oxidized in air at
Fig. 3. XPS spectra of the C 1 s absorbed on implanted samples: (1) implanted by 5 1016 Ti+ ion/cm2 at 40 keV, (2) implanted by 1 1017 Ti+ ion/cm2 at 40 keV and (3) implanted by 2 1017 ion/cm2 Ti+ ion/cm2 at 40 keV.
Fig. 4. XPS spectra of adjusted (a) Ti3d5/2 peak and (b) Zr 3d5/2 peak in the oxide scale after the Ti+ implantation to doses (1) 5 1016, (2) 1 1017 and (3) 2 1017 ion/cm2.
Table 1 The 3d5/2 bonding energy of Ti3d5/2 and Zr3d5/2 Elements
Experimental data (eV) Adjusted data (eV)
Ti 3d5/2
Zr 3d5/2
5E16
1E17
2E17
5E16
1E17
2E17
463.5 458.7
463.3 458.7
462.8 458.7
187.0 182.1
186.8 182.2
186.3 182.2
ARTICLE IN PRESS 1006
X.Y. Zhang et al. / Vacuum 80 (2006) 1003–1006
zirconia with the amount of Ti [13–15], or because combination of Ti and O could depress formation of Hzirconia which is mainly induced from oxygen vacancies produced during ion implantation. This needs further investigation. 4. Conclusions Titanium ion implantation was employed to investigate the oxidation behaviour changes of zircalloy-4 in air at 500 1C for 100 min. Following conclusions could be made:
Fig. 5. GAXRD spectra of oxidized (1) as-received zircalloy-4, and after (2) 5 1016, (3) 1 1017 and (4) 2 1017 ion/cm2 40 keV Ti+ implantation. M, T and H represent monoclinic, tetragonal and hexagonal zirconia, respectively.
became higher. After implantation by 2 1017 ions/cm2, more M-zirconia was found than in other two implanted samples. It is well known that when the oxide film grows on the matrix, T-ZrO2 emerges first. Owing to its low stabilization, T-ZrO2 transforms into the M-ZrO2 promptly, and then the stress in the oxide film that arises owing to the lattice misfit becomes larger and larger. When the stress has accumulated to some critical value, the oxide film will crack and fresh oxygen can easily enter the matrix and react with zirconium. However, when titanium was implanted into zircalloy-4, oxygen may react with it, and Ti4+ may substitute Zr4+ to form TiO2. It has been reported that titania could stabilise T-ZrO2 in a ceramic study [13,14], and the T-ZrO2 formed after implantation may improve the oxidation corrosion behaviour of zircalloy-4 [6]. In addition, Ce ion implantation, according to Chen et al. [7], has resulted in appearance of ZrO1 x and ZrO0.35, formation of H-zirconia causes volume of zirconia to decrease rapidly and greatly, which also could improve oxidation corrosion behaviour of zircalloy-4. From Fig. 5, it is easy to see that Ti ion implantation not only stabilised the T-ZrO2, but also, led to the formation of H-zirconia. Consequently, the oxidation corrosion resistance could be improved as discussed in Section 3.1. The result that diffraction peak intensity of Hzirconia decreased with increasing of implantation dose and the weight gain of samples increased also coincides with this explanation. To explain the decrease of the diffraction peak intensity of H-zirconia with increased implantation dose, it is proposed that this is an effect related to the amount of implanted Ti in zircalloy-4. This may be due to the dependence of phase transformation of TiO2 (R-A) in
(1) The oxidation weight gain of samples increases with increase of oxidation time, and the oxidation weight gain of implanted specimens is lower than that of asreceived zircalloy-4. Furthermore, oxidation weight gain increases with an increase in implantation dose of titanium. (2) Mechanism for the improvement in the oxidation corrosion behaviour of zircalloy-4 by titanium implantation was the stabilization of T-ZrO2 and formation of H-zirconia. Occurrence of T-ZrO2 and H-zirconia leads to a slower oxidation rate. The oxidation weight gain may increase with the increase of implantation dose because of the decrease in the fraction of H-zirconia. (3) Further investigation is needed to confirm that the level of H-zirconia decreases with an increase in implantation dose.
Acknowledgements Special thanks to the Ministry of Science and Technology of China (MSTC no.G2000067207-1). We would also like to thank Tsinghua University for their financial support. References [1] McCafferty E, Natishan PM, Hubler GK. Nucl Instr Meth B 1991;56–57:639. [2] Etoh Y, Shimada S, Kikuchi K. J Nucl Sci Technol 1992;29(12):1173. [3] Xu J, Bai X, Jin A, Fan Y. J Mater Sci Lett 2000;19:1633. [4] Bai X, Xu J, He F, Fan Y. Nucl Instr Meth B 2000;160:49. [5] He F, Bai X, Xu J, Wang S, An J, Sun Z, et al. J Mater Sci Lett 1999;18:715. [6] Peng DQ, Bai XD, Chen XW, Zhou QG. Nucl Instr Meth B 2003;201:589–94. [7] Chen XW, Bai XD, Yu HR, Zhou QG, Chen BS. Nucl Instr Meth B 2002;196:293–8. [8] Stringer J. Mater Sci Eng A 1989;120:129. [9] Saito Y, Onay B, Maruyanma T. J Phys IV 1993;3:217. [10] Stroosnijder MF, Mevrel R, Bennett MJ. Mater High Temp 1994;12:53. [11] Przybylski K, Yurek GJ. Mater Sci Forum 1989;43:1. 298. [12] Ziegler JF. The stopping and range of ions in matter, vol. 2–6. New York: Pergamon Press; 1977–85. [13] Hunter BA, Howard CJ. J. Solid State Chem 1999;146:363–8. [14] Yang J, Ferreira JMF. Mater Res Bull 1998;33(3):389–94. [15] Zou H, Lin YS. Appl Catal A: Gen 2004;265:35–42.