Microstructural evolution of reactor internals stainless steel under xenon irradiation studied by GIXRD and positron annihilation technique

Annals of Nuclear Energy 96 (2016) 176–180

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Microstructural evolution of reactor internals stainless steel under xenon irradiation studied by GIXRD and positron annihilation technique Chaoliang Xu ⇑, Lu Zhang, Wangjie Qian, Jinna Mei, Xiangbing Liu, Guodong Zhang Suzhou Nuclear Power Research Institute, Suzhou, Jiangsu province 215004, China

a r t i c l e

i n f o

Article history: Received 15 January 2016 Received in revised form 29 April 2016 Accepted 30 April 2016 Available online 24 June 2016 Keywords: Stainless steel Irradiation Vacancy-type defects Ferrite phase

a b s t r a c t Specimens of reactor internals stainless steel were irradiated with 6 MeV Xe ions to peak displacement damage 2, 7 and 15 dpa. The grazing incidence X-ray diffraction (GIXRD), positron annihilation lifetime spectroscopy (PALS) and doppler broadening spectroscopy (DBS) were carried out to study the phase transition and vacancy-type defects variations after irradiation. A new ferrite phase diffraction peak a (1 1 0) after irradiated to 7 dpa and another two ferrite phase diffraction peak a(2 0 0) and a(2 1 1) after irradiated to 15 dpa were observed by GIXRD. The results of PLAS indicate that the mono-vacancies, dislocations and vacancy clusters were introduced by Xe irradiation. It is observed that the short lifetime parameter s1 increases from 2 to 7 dpa and then decreases at 15 dpa. Meanwhile, a small increment of S parameter after irradiated to 2 and 7 dpa and a remarkable increase after irradiated to 15 dpa are observed. The ultrafine ferrite particles formed at 15 dpa, which may trap the migrating vacancies and decrease the recombination of vacancies and interstitials, may explain this phenomenon. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Irradiation assisted stress corrosion cracking (IASCC) has been considered as the most important failure mode of reactor internals stainless steel and got more attention during reactor long-term service in recent decades. Previous works showed that radiation induced segregation (RIS) and irradiation hardening are the most important mechanisms (Etienne et al., 2010a,b; Was, 2007). RIS is primarily controlled by the Kirkendall vacancy mechanism, so the investigations of vacancy can provide important insights into stainless steel solute redistribution (Simonen and Bruemmer, 1996). But at present the real nature of the vacancy or openvolume defects and its evolution with fluence still require close examination. Irradiation hardening is a consequent of defects and precipitates impeding dislocation line. Thus the studies of vacancy and precipitate can obtain more information of other type of defects and get a better understands of hardening. That is to say, the understanding of microstructural evolution is the basis to obtain further analysis of IASCC. Previous studies have investigated the microstructural evolution of stainless steel after irradiation mainly by TEM and atom probe tomography technique (APT) (Etienne et al., 2010a; Fukuda et al., 1998; Renault et al., 2012). TEM and APT have

⇑ Corresponding author. E-mail address: [email protected] (C. Xu). http://dx.doi.org/10.1016/j.anucene.2016.04.052 0306-4549/Ó 2016 Elsevier Ltd. All rights reserved.

advantages in the field of interstitial-type defects and solute distribution, but the analysis on vacancy-type defects and phase transition are limited. In present studies, the phase transition and vacancy-type defects evolution of the reactor internals stainless steel after Xe irradiation were investigated by grazing incidence X-ray diffraction (GIXRD), positron annihilation lifetime spectroscopy (PALS) and doppler broadening spectroscopy (DBS) measurement. The interaction of the precipitate and vacancytype defects was also discussed.

2. Experiments The material used in this study is austenite stainless steel Z6CND17.12 used for reactor baffle-former bolts. The specimens used in our experiments were cut from bar with solution treatment at 1060 ± 10 °C for 90 min followed by air cooling. The chemical composition shows in Table 1. The specimens before irradiation were polished to mirror-like with mechanical method. The specimens were irradiated with 6 MeV Xe ions to 6.6
1014, 2.3
1015 and 5
1015 ions/cm2 at the ECR-320 kV High-voltage Platform in the Institute of Modern Physics. The theoretical calculation results of the displacement damage for those fluences by the Monte-Carlo code SRIM 2012 (Biersack and Haggmark, 1980) are 2, 7 and 15 dpa, taking the density of 7.8 g/cm3 and threshold displacement energy of 40 eV for Fe, Cr and Ni sub-lattices (ASTM, 2003). The distribution of

C. Xu et al. / Annals of Nuclear Energy 96 (2016) 176–180 Table 1 Chemical composition of the stainless steel Z6CND17.12.

a

Elementsa Wt.%

C 0.038

Si 0.340

Mn 1.24

P 0.008

S 0.003

Elements Wt.%

Cr 17.28

Ni 11.65

Cu 0.46

Mo 2.49

Co 0.010

Balance of composition is Fe.

14

2

200

1.5

150

1.0

100

0.5

50

0.0

Xe concentration/appm

Damage/dpa

6MeV Xe irradiated to 6.6x10 /cm 2.0

0 0

200

400

600

800

1000

1200

1400

1600

Depth/nm Fig. 1. Distribution of displacement damage and Xe concentration versus depth in the stainless steel irradiated with 6 MeV Xe ions to 6.6
1014 ions/cm2 according to SRIM 2012.

displacement damage and Xe concentration versus depth are shown in Fig. 1. In SRIM calculation process, the vacancy file obtained by the Kinchin–Pease quick calculation model was used to calculate the displacement damage values. GIXRD was carried out at Beijing Synchrotron Radiation Facility, Institute of High Energy Physics. X-rays was generated by a bending magnet, focused and monochromated to a wavelength of 0.154 nm. The X-ray scanning range was from 20 to 90° with a resolution of 0.05 degree. According to the penetration distance of X ray calculation by t0 = 1/lmq (where lm is the mass absorption factor (127 cm2/g) and q is the density (7.86 g/cm3)), the estimated penetration distance is about 10 lm. Thus, according to the penetration depth (about 0.7 lm), the incident angle of 4° was chosen in order to correspond to the microstructure at Xe irradiation damage region. PALS was measured by means of BaF2 lifetime spectrometer. The positron source 22Na with an activity 20 lCi was deposited on a titanium foil and sandwiched between two pieces of the specimens. The size of specimens in this measurement was 15
15
1 mm3. At least 106 counts were collected in each spectrum. Besides the source components, all measured PALS were fitted by the PATFIT program (Kirkegaard and Eldrup, 1974) with two lifetime components. The 22Na source emits a continuous energy spectrum with Emax = 0.544 MeV, and the depth of mean penetration in alloys is about 10–100 lm (Chen et al., 2007). The deeper penetration depth of positrons will lead to an increase of the bulk contribution in the results comparing to that of irradiation damage parts. Generally, the obtained lifetime parameter s and its intensity parameter I show the size and the concentration of the vacancy type defects. The increase of positron lifetime means increase of the size of the vacancy type defects and increase I means an increase of the concentration of these types of defects. The s1 is weighted average of annihilation lifetimes of the free positrons and the positrons trapped at mono-vacancies and dislocations; and s2 is the annihilation lifetimes of vacancy clusters.

177

DBS was conducted by the slow positron beam with incident positron energies from 0.25 to 24 keV, and a high purity-Ge detector with an energy resolution of 1.64 keV at the 511 keV peak was used. 5
105 annihilation events with a count rate of 1000 cps in the 511 keV peak were collected. The spectra were characterized by two standard line-shape parameters S and W. The S parameter is roughly proportional to the number of positrons that annihilated with valence electrons at open-volume defects. It can be said that the parameter S is an increasing function of the concentration of vacancy clusters if a narrow distribution of cluster sizes can be assumed and saturation trapping does not take place (Lambrecht and Malerba, 2011). 3. Results and analysis 3.1. GIXRD Fig. 2 is the GIXRD patterns of unirradiated stainless steel and stainless steel irradiated to 2, 7 and 15 dpa. The GIXRD of unirradiated specimen shows three face-centered-cubic austenite diffraction peaks of c(1 1 1), c(2 0 0) and c(2 2 0). No other diffraction peaks were observed. With increasing the irradiation fluence, a new diffraction peak corresponding to the a(1 1 0) appears in specimen to 7 dpa and a more remarkable diffraction peak a(1 1 0) and two new a phase peaks a(2 0 0) and a(2 1 1) were observed in specimen irradiated to 15 dpa. This indicates that the content of a phase increases with increasing irradiation damage. Previous studies have indicated that the formation of ferrite phase was due to ion irradiation in austenitic steel by TEM (Margolin et al., 2010; Van Renterghem et al., 2011; Sakamoto et al., 1990) and XRD (Chai et al., 2012; Sakamoto et al., 1994; Mottu et al., 2004). They suggested that defects or lattice disorder produced by irradiation will induce high residual stresses within the lattice, leading to the structure transformation (Mottu et al., 2004; Johnson et al., 1987). However, previous studies have indicated that the radiation-induced magnetic behavior in austenitic stainless steels is related to the formation of magnetic phases (a phase) (Gussev et al., 2014). The formation of a magnetic phase in austenitic stainless steels appeared to be sensitive to alloy silicon and manganese which accelerated ferrite accumulation and consider as the results of RIS. Maziasz (1989) also proved that the major driving force of transformation of austenite matrix to ferrite was RIS. Morisawa expected that the phase transformation was induced by RIS in irradiated and hydrogen treated stainless steel (Morisawa et al., 2002). Therefore, considering the analysis above, we can conclude that, besides the formation of residual stresses, RIS may be another reason for the formation of ferrite phase in irradiated stainless steel. The formation of ferrite phase is a combined effect of residual stresses and RIS. Previous study has proved that ferrite phase produced by irradiation is about several nanometers in size and 1022 m3 in density by TEM (Tan and Busby, 2013). Therefore, these ultrafine ferrite phases may act as a nucleus of defects in irradiation process and cause a different defects evolution behavior between 7 and 15 dpa. This will be discussed by PALS and DBS in next two parts. 3.2. PALS Positrons will be captured by vacancy-type defects and the positron lifetime and relative intensity will change with respect to the size and concentration of vacancy-type defects. Fig. 3 is the variations of positron lifetime s and relative intensity I versus irradiation damage. As observed from Fig. 3, the short lifetime parameter s1 and long lifetime parameter s2 increase after irradia-

178

C. Xu et al. / Annals of Nuclear Energy 96 (2016) 176–180

γ(200)

30

40

γ(111)

un-irradiated

γ(220)

50

60

70

80

90

2dpa

γ(200)

Intensity

Intensity

γ(111)

γ(220)

30

40

50

2θ/degree

γ(111)

α(110)

Intensity

40

45

70

γ(111)

7dpa

50

γ(200)

Intensity

γ(111)

60

γ(220)

α(110)

40

90

15dpa

γ(200) γ(220) α(110) α(200)

30

80

2θ/degree

50

60

70

80

90

2θ/degree

30

40

50

60

α(211)

70

80

90

2θ/degree

Fig. 2. GIXRD patterns of the Xe irradiated stainless steel as a function of ion fluence.

97

I1(%)

96 95 94

τ2(ps)

360 340 320

τ1(ps)

164

160

156 0

2

4

6

8

10

Damage(dpa)

12

14

16

Fig. 3. The variations of positron lifetime parameters of s1, s2 and intensity parameters I1 versus irradiation damage.

tion, indicating the introduction of open-volume defects by ion irradiation. With the irradiation damage increase, s1 increases from 2 dpa to 7 dpa and then decreases at 15 dpa, while s2

increases continuously with damage to 15 dpa. The intensity parameter I1 increases gradually with the irradiation damage. It has been known that the annihilation lifetime of free positrons, mono-vacancy and dislocation are 110, 175 and 165 ps separately in steels and the annihilation lifetime of vacancy clusters have much longer annihilation lifetimes (Zhu et al., 2005; Zeman et al., 2007). In our case, for the unirradiated specimens s1 = 156 ps and s2 = 313 ps were obtained. With irradiation damage increase to 15 dpa, s1 and s2 increase to 165 ps and 366 ps separately. This suggests that the simple open-volume defects such as mono-vacancies and dislocations and different size vacancy clusters were introduced in unirradiated specimen (produced by fabrication and mechanical polishing) and specimens irradiated by 6 MeV Xe ions. In general positrons in specimens containing open-volume defects are subjected to competition between annihilation in the free state and trapping into localized states at the open-volume defects; the higher concentration and smaller size of openvolume defects will cause the larger trapping rate (Wang et al., 2013). As suggested by Fig. 3, small variations of the relative intensity I1 (increase) and I2 (decrease, I1 + I2 = 100%, not show in Fig. 3) are observed, while the obtained s1 and s2 increase continuously with irradiation damage (except s1 at 15 dpa). The increase of s1 indicates a reduction of positrons to be trapped by simple openvolume defects, compared to the positrons annihilating in a free state, indicating an increase in the number density of these defects from 2 to 7 dpa. On the other hand, the fact that the intensity I1 increase indicates that the positrons annihilating trapped at the vacancy clusters decrease and thus their intensity reduce

C. Xu et al. / Annals of Nuclear Energy 96 (2016) 176–180

synchronously. According to the analysis of average size of vacancy clusters by Lambrecht and Almazouzi (2011), the vacancy clusters is considered to be corresponding to vacancy cluster of about 8–15 vacancy number with irradiation damage increasing. It should be noticed the decrease of s1 after irradiated to 15 dpa. The decrease of s1 indicates the average size reduction of open-volume defects, which will not happen at present condition. So there may be new open-volume defects turning up from 7 to 15 dpa. According to the analysis of GIXRD, the ferrite phase is gradually formed during ion irradiation and its content shows a prominent increase after irradiated to 15 dpa. These ultrafine ferrite particles have an average size of nanometer and would interact with vacancies and become the nucleation centers of vacancy clusters. Previously Slugen (2002) indicate the positron lifetime of precipitated phase in steel is around 99–116 ps. So it is reasonable to believe that the lifetime of ultrafine ferrite phase in earlier stage is closed to the precipitated phase and cause the short lifetime component decrease at 15 dpa. On the other hand, Sato et al. (2012) indicate that the presence of gas atoms in the vacancy clusters shortens the lifetime of the vacancy clusters. Olsen et al. (2007) and Yoshiie et al. (2009) also show that when the vacancy clusters contain many gas atoms, the positron annihilation lifetime of the vacancy clusters decreases to less than 200 ps. Shivachev et al. (2002) calculate the positron lifetime of defects in nickel containing gas atoms and find that the values of positron lifetime in vacancy clusters are lowered to about 150 ps when the number of gas atoms more than 15. Therefore, the lifetime s1 decrease may be affected by the implantation of Xe atoms when irradiated to 15 dpa. However, considering the low gas atoms concentration of Xe irradiation (100 appm/dpa), Xe irradiation will not introduce large amount of gas atoms. Thus we now cannot confirm the influence of gas atom on short lifetime parameter s1. More detailed studies will be conducted later. 3.3. DBS The evolution of the S parameter as a function of incident positron energy and mean implantation depth for unirradiated specimen and specimens irradiated to 2, 7 and 15 dpa is shown in Fig. 4. The mean depth of the annihilating positron from the surface is calculated using the established equation z ¼ ð4
104 ÞE1:6 =q, where z is expressed in units of nm, q is the

Mean implantation depth/nm 0

15

46

89

141

201

270

345

427

516

611

711

817

0.475

Unirradiated 2dpa 7dpa 15dpa

0.470

S parameter

0.465

0.460

179

density in units of kg/m3 (here we used the density of pure iron with a value of 7.86
103 kg/m3), and E is the incident positron energy in units of keV. The top axis in Fig. 4 gives the calculated mean depth from the surface of the annihilated positron. Fig. 4 shows a sharp decrease of S parameter with increasing positron energy at surface region and then approaches a relatively stable status with small decrease in unirradiated specimen. The presence of open-volume defects will cause the increase of S values. So this indicates that there has been high density of defects existing from surface to about 100 nm which may be a result of specimen fabrication and mechanical polishing as discussed in PALS above. In addition, because the number of positrons diffusing back to the surface region is remarkable at surface region and cause an uncertainty, we do not consider the data from surface to 100 nm (about 0 to 7 keV) in the follow analysis. For the irradiated specimens, as shown in Fig. 4, the S parameters nearly keep a stable status with mean implantation depth. Compared with unirradiated specimen, a small increment of S parameter is detected for 2 and 7 dpa specimen, while a remarkable increase is observed after irradiated to 15 dpa. According to the theory of positron annihilation (Zhu et al., 2015), the large S value would suggest that the atomic environment around the positron trapped site is electron poor, indicating that more vacancies in the irradiated specimen are produced with sufficient concentrations. Thus, the increase of S value suggests a prominent increase on the concentration of open-volume defects in the specimens. This phenomenon also indicates that the fluence dependence of vacancy-type defects and suggests that more vacancy-type defects are introduced by higher irradiation fluence. According to the previous studies by Iwai et al. (2004), S parameter of iron irradiated by ion at room temperature rises with increasing fluence and then it reaches a saturation value that depends on open-volume from vacancy-type defects. In the present work, the small variations of S value after irradiated to 2 and 7 dpa indicated that the S parameter has been saturated when irradiated to 2 dpa. Actually, Iwai et al. indicated that the S parameter saturation value is species dependence and is less than 1 dpa for He, C, O, Fe irradiation. The saturated S parameter values tend to be higher for heavier ion irradiation. So our studies are consistent with Iwai et al. It should be mentioned that the S parameter of specimen irradiated to 15 dpa show a remarkable increase. This phenomenon may be related to the appearance of ferrite particles. The ferrite particle is proven to exist by GIXRD analysis after Xe irradiation. These ferrite particles would become the nucleation centers of vacancy clusters. When the concentration of nuclei is high, vacancy clusters may be larger in number and smaller in size (Druzhkov and Perminov, 2010). Previous discuss above have indicate that these ferrite particles are about several nanometers in size and 1022 m3 in density. The ultrafine ferrite particles forming cluster nuclei trap the migrating vacancies. As a result, absorption of vacancies at sinks becomes less probable and vacancy clusters had an enhanced concentration, leading to a significant increase of the S-parameter after irradiated to 15 dpa.

0.455

4. Conclusion

0.450

Specimens of reactor internals stainless steel were irradiated by 6 MeV Xe ions to 2, 7 and 15 dpa at room temperature and then investigated with GIXRD, PALS and DBS. The formation of new ferrite phase diffraction peak after irradiated to 7 and 15 dpa was observed and the residual stresses and RIS may be its possible reason. The mono-vacancies, dislocations and vacancy clusters are produced by ion irradiation. The decrease of short lifetime parameter s1 in PALS and remarkable increase of S parameter after irradiated to 15 dpa in DBS are observed and the ultrafine ferrite

0.445

0

2

4

6

8

10

12

14

16

18

20

22

24

Positron energy/keV Fig. 4. S-parameter as a function of incident positron energy (mean implantation depth) in unirradiated and irradiated stainless steels.

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