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Irradiation creep and microstructural changes of ODS steels of different Cr-contents during helium implantation under stress
Journal of Nuclear Materials 437 (2013) 432–437
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Irradiation creep and microstructural changes of ODS steels of different Cr-contents during helium implantation under stress J. Chen a,⇑, P. Jung a, J. Henry b, Y. de Carlan b, T. Sauvage c, F. Duval c, M.F. Barthe c, W. Hoffelner a a
Department of Nuclear Energy and Safety, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland CEA/DEN, SRMA, F-91191 Gif-sur-Yvette Cedex, France c CEMHTI/CNRS, Université d’Orléans, 3A Rue de la Férollerie, 45071 Orléans Cedex 2, France b
a r t i c l e
i n f o
Article history: Received 26 November 2012 Accepted 23 February 2013 Available online 13 March 2013
a b s t r a c t Irradiation creep and microstructural changes of two ferritic ODS steels with 12% and 14% Cr have been studied by homogeneously implantation with helium under uniaxial tensile stresses from 40 to 300 MPa. The maximum dose was about 1.2 dpa (5000 appm-He) with displacement damage rates of 1 105 dpa/ s at a temperature of 300 °C. Irradiation creep compliances were measured to be 4.0 106 dpa1 MPa1 and 10 106 dpa1 MPa1 for 12 and 14Cr ODS, respectively. Subsequently, microstructural evolution was studied in detail by TEM observations, showing dislocation loops and bubbles distributed homogenously in the matrix. Some bubbles were attached to ODS particles. Finally, the effects of Cr content on irradiation creep and microstructural changes are discussed, including earlier results of a 19Cr ODS and a PM2000 ferritic steel. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The excellent high temperature strength, low irradiation-induced swelling and potential resistance to helium embrittlement of oxide dispersion strengthened (ODS) Fe–Cr steels have prompted investigations on their potential use as structural materials in advanced nuclear energy system such as fusion and Generation IV fission reactors worldwide in the last decade [1–13]. For instance, the application of ODS steels for blanket components in fusion reactors, and for cladding materials in Generation IV reactors may increase the safety margin and thermal efficiency, and provide a larger design tolerance. To fulfil different requirements, ODS steels with different Cr-contents are developed, ranging from 9–12 wt% Cr (i.e. typical ferritic/martensitic) to 20 wt% Cr (i.e. ferritic grade). They also may contain aluminum to improve corrosion resistance. To realize the use of ODS steels in nuclear energy systems, many efforts have been made to optimize production of ODS steels and to characterize their mechanical and corrosion properties [1–11]. Recently, investigations of helium effects on swelling [14] and fracture behavior [15] have been reported. However knowledge on irradiation damage is still very limited, and irradiation creep data for advanced 12 and 14Cr ODS steels are still missing. On the other hand, the newly developed 12Cr and 14Cr ODS differ in Cr content from 19Cr ODS [1] and an industrial ODS steel PM2000 with 20% Cr [7] of which irradiation creep data ⇑ Corresponding author. Address: Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland. Tel.: +41 56 310 2280; fax: +41 56 310 4595. E-mail address: [email protected] (J. Chen). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.02.071
[16,17] and related microstructural changes [18] have been reported recently. Therefore in the present work, in situ irradiation creep of 12 and 14Cr ODS steels was investigated during Heimplantation, while microstructural changes were studied by transmission electron microscopy (TEM) after implantation. These results are compared to those from 19% and 20% Cr steels in order to reveal possible effects of Cr content on irradiation creep and microstructural changes. 2. Experimental The advanced ferritic 12 Cr ODS steel was supplied by KOBELCO Research Institute, Japan, in plates of 6.5 33 600 mm3. The 14Cr ODS developed in the frame of the EU project GETMAT by CEA, France was in rods of 36 mm diameter. Compositions are given in Table 1. The detailed production process has been reported in Ref. [19,20]. Here a brief description is given. The 12Cr- and 14Cr- ODS ferritic steels, both were produced by powder metallurgy: Iron-base gas atomized powders were mechanically alloyed with 0.2–0.3% Y2O3 particles under hydrogen atmosphere in a high-energy ball mill. Then, the ODS steel powders were encapsulated in a soft steel can, consolidated by hot extrusion at 1100 °C (12Cr) and 1150 °C (14Cr), followed by a hot and cold rolling procedure and a final thermal treatment at 1200 °C (12Cr) and 1050 °C (14Cr) for 1 h and air-cooling for recrystallization. TEM micrographs showing the grain shapes and sizes (a), dislocation network (b) and the ODS particle distribution (c) are given in Figs. 1 and 2 for the 12 and 14Cr ODS, respectively. These pictures show that both steels in the as-received condition show grains elongated
J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437 Table 1 The main chemical composition (wt%) of 12Cr and 14Cr ODS ferritic steels.
12Cr 14Cr
Cr
W
Ti
Mn
Si
Y2O3
Fe
12 13.5
2.0 0.9
0.26 0.4
0.27
0.32
0.23 0.3
Bal. Bal.
along the rolling direction. The ODS particles with partially spherical and partially facetted shape are distributed quite uniformly. Sizes of dispersed ODS particles in the 12% Cr steel range mostly from 1.7 to 2.7 nm, but with maximum sizes up to 18 nm, while the sizes of ODS particles in the 14Cr ODS steel range from 2.2 up to 12.5 nm. The average sizes of grains and of diameters of the ODS particles in the as-received condition are summarized in Table 2. The dislocation density varies quite significantly from grain to grain, and is higher in 12Cr ODS than in 14Cr ODS. Dogbone shaped creep samples of 300 lm thickness were cut parallel to the rolling direction by spark erosion. The samples were mechanical polished on both sides to 100 lm with grad 2400 paper. The final samples had an overall size of 28 mm in length, 8 mm in width and 0.1 mm in thickness, with a gauge volume of 10 2 0.1 mm3. In situ creep under He-implantation was performed at the cyclotron of CEMHTI/CNRS. Details of the experimental set up are described in Refs. [21,22]. With 28 MeV 4He2+ ions passing through firstly a magnet scanning system, then a vacuum window of a
433
25 lm thick hastelloy foil and finally a degrader wheel with 24 Al-foils of variable thicknesses, the gauge area of the 0.1 mm thick samples was 3D-homogeneously implanted under constant uniaxial stress. Typical implantation rates were 0.04 appm/s with an average beam current density of 10 lA/cm2. The concurrent production of displacement damage was calculated by TRIM and SRIM for a displacement threshold energy of 40 eV and a binding energy of 3 eV, giving per implanted He-atom 275 displacements on the front side and 193 on the back side, averaging to 234 displaced lattice atoms. Accordingly, a displacement rate of about 9 106 dpa/ s (displacements per atom per second) is derived. The displacement damage distribution along the specimen depth is illustrated in Fig. 3a. As temperature variations due to beam fluctuation limited the precision of strain measurement by LVDT (Linear Variable Differential Transformer) beam was switched off about every 2 h to measure the change of specimen length. The implantation was performed at a temperature of 300 °C and was continued until the strain rate became constant (stationary creep). Then the implantation of the same specimen was continued at a different stress in the range of 40–300 MPa. To minimize systematic errors from dose effects on the microstructure, e.g. by accumulation of irradiation defects, applied stress was changed alternatively to higher and lower values (as indicated in Fig. 4). The temperature distribution along the gauge region was monitored by an infrared pyrometer under 45° from the backside of the specimens. The specimen geometry and measured temperature distribution along the longitudinal
Fig. 1. Microstructure of as received 12Cr ODS steel showing the grain shape and size (a), dislocation network (b) and the ODS particle distribution (c).
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J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437
Fig. 2. Microstructure of as received 14Cr ODS steel showing the grain shape and size (a), dislocation network (b) and the ODS particle distribution (c).
Table 2 Microstructural parameters of the used ODS steels. Material
12Cr 14Cr 19Cr PM2000
Cr (wt%)
12 13.7 18.4 20
Y2O3 (wt%)
0.23 0.30 0.37 0.50
direction during beam-on period are shown in Fig. 3b. Typical temperature variations across the gauge section, as well as fluctuation during irradiation time are ±10 °C. Finally, TEM specimens were prepared from the implanted gauge sections (see Ref. [18] for details) and TEM examinations were performed with a JEM 2010 at PSI. Foil thickness was determined by the carbon contamination spike method. 3. Results and discussion 3.1. Irradiation creep Because of different thermal expansion coefficient of specimen holder, support parts and sample itself, the measured length change by LVDT, also during beam-off periods depends on the temperature distribution in the chamber. Therefore, in order to
Grain size (lm)3
1.3 1.3 8.0 0.5 0.5 2 0.2 0.2 0.7 103 103 >104
ODS particle Size (nm)
Density(m3)
2.2 4.8 2.1 2.8
1.6 1023 4.5 1022 1.2 1024 5.1 1020
improve data precision, all strains have to be taken at a constant chamber temperature. An example is illustrated in Fig. 4, where irradiation creep was performed at a nominal temperature of 300 °C under a tensile stress of 45 MPa. The first reference point (marked as ‘‘0’’ in Fig. 4) was measured at equilibrium room temperature before irradiation. Then the length of the specimen was monitored after typical beam times of 2 h during beam-off period of typically 15 min, (marked as ‘‘1’’, ‘‘2’’ up to ‘‘8’’ in Fig. 4) during which chamber temperature dropped continuously. The second reference point (marked as ‘‘8a’’ in Fig. 4) was measured again at equilibrium room temperature after irradiation (after a waiting time of several hours). The strain measurements were taken at 32 °C and a precision of 1 105 was reached. Fig. 5 shows the strain eðrÞ ¼ Dl0l of 12Cr ODS steel (a) and 14Cr ODS steel (b) during implantation at 300 °C as a function of the displacement dose. Each stress change caused, aside from elastic
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J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437
4x10-3
12Cr ODS, 300°C 3x10-3
2x10-3
92 MPa 43 MPa 195 MPa 295 MPa 95 MPa
strain
1x10-3
0 6x10-3
14Cr ODS, 300°C 5x10-3 4x10-3 3x10-3
92 MPa 45 MPa 190 MPa 291 MPa 92 MPa
2x10-3 1x10-3 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
displacement-dose (dpa) Fig. 5. Strain of 12Cr ODS (a) and 14Cr ODS (b) steels during He-implantation at 300 °C as a function of displacement dose at different stresses. Fig. 3. Displacement profile along depth of specimen (a); and specimen geometry and temperature distribution along longitudinal direction during beam-on period (b).
strain-rate/dose-rate (dpa-1)
0.504
14Cr ODS under 45 MPa at 300°C
length change (mm)
0.502
0.500
1
0
2
3 4 5 6
0.498
0.494 22
26
28
30
32
300°C
0.003
0.002
12 Cr-ODS 14Cr-ODS 19Cr-ODS PM2000
0.001
0
100
200
300
stress (MPa)
8a
24
Fe-Cr ODS ferritic steels
0.000
7 8
0.496
0.004
34
36
38
40
42
chamber temperature (°C) Fig. 4. Length changes monitored during beam-off periods by LVDT as a function of the chamber temperature. All strains were measured at 32 °C. Numbers 1–8 are successive strain measurements after typically 2 h irradiation periods each. Numbers 0 and 8 are reference measurements at equilibrium room temperature before and after the irradiation period, respectively.
strain, also a short transient stage before stationary creep was reached. Those transient strains are similar to observations in 19Cr ODS [16] and PM2000 [17]. They are ascribed to irradiationinduced relaxation. It is worth to notice that in both steels a contraction of the specimen against the applied tensile stress occurred at the beginning of irradiation when the applied stress was reduced (e.g. from 300 to 100 MPa), but already after a dose of less than 0.03 dpa, creep proceeds again in the stress direction.
Fig. 6. Irradiation creep rates per displacement rates of 12Cr ODS and 14Cr ODS steels as a function of tensile stress under He-implantation at 300 °C. Linear fits give irradiation creep compliances. The 19Cr ODS [11] and PM2000 [12] data are included for comparison.
According to results from PM2000 [17], it is fair to assume that thermal creep is negligible at 300 °C. Irradiation-induced creep rates, e0 ; i.e. strain-rate per dose-rate, k (in unit of dpa1) were obtained by fitting straight lines to the stationary parts of the curves in Fig. 5. These values are plotted in Fig. 6 as a function of the applied stress for both steels. The data from PM2000 and 19Cr ODS are also included for comparison. The data can be fitted by linear stress dependence up to 300 MPa (solid line):
e0 ðrÞ ¼ B0 r þ e00
ð1Þ
with creep compliances B0, i.e. creep rate per dose rate (assuming linear dependence) and stress, of 4.0 106 dpa1 MPa1 and
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J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437 Table 3 Result on irradiation creep and microstructural changes. Materials
r0.2 at 300 °C (MPa)
Creep compliance (106 dpa1 MPa1)
Offset e00 (103 dpa1)
Loop size (nm)
Bubble size (nm)
Bubble density (1023 m3)
12Cr ODS 14Cr ODS 19Cr ODS PM2000
992 1087 965 716
4.0 10. 4.0 5.7
1.2 0.72 1.5 2.0
5.3 7.5 n.d. 3.6
1.4 1.8 1.1 1.0
n.d. n.d. 1.0 4.5
n.d. = not determined.
Fig. 7. (a) In-focus and (b) under-focus bright-field images of the same spot showing the ODS particles (dark small dots, 2 nm) and bubble (bright small dots, 0.75 nm) distributions after implantation at 300 °C to 5000 appm-He in a 12Cr ODS sample, respectively. (c) Under-focus bright-field image showing the ODS particles (dark spots) and bubbles (small bright spots) in 14Cr ODS steel. Notice that the bubble distribution is quite homogenous in both materials.
10 106 dpa1 MPa1 for 12Cr and 14Cr ODS steels at a temperature of 300 °C, respectively. Extrapolating the lines in Fig. 6 to zero stress gives ordinate-offsets e00 of 1.19 103 and 0.72 103 dpa1 for 12Cr and 14Cr ODS steels, respectively. e00 represents stress-independent dimensional change rates, for example from isotropic volume swelling:
DV ¼ 3 e00 kt V
ð2Þ
where t is time. The measured irradiation creep compliances B0 (slope of lines in Fig. 6) and ordinate offsets e00 including the earlier results of 19Cr ODS steel and PM2000 are summarized in Table 3 together with loop size, bubble size and density. The yield stresses (r0.2) at 300 °C from Ref. [14] are included. No remarkable effects of Cr content, grain size and dispersoid size on irradiation creep properties are observed. The apparently slightly higher B0 value of the 14Cr ODS cannot be unambiguously related to any material property (Table 2) or microstructural evolution (Table 3). Only the transient elastic strains after stress changes (Fig. 5) are significantly larger in the 14Cr ODS than for all other Cr contents (see also Refs. [16] and [17]). Even when those transient strains can not yet be safely related to a underlying mechanism (cf. Ref. [23]), they may indicate a sensitivity of the material to irradiation, which in turn will also enhance the compliance for irradiation creep. 3.2. TEM observation TEM was conducted after in situ creep tests. In both ODS steels, bubble formation during implantation is observed. Fig. 7 shows an in-focus bright field image of 12Cr ODS steel (a), and
an under-focus bright field image of the same spot (b), and under-focus bright field image of 14Cr ODS steel after irradiation creep test at 300 °C. The helium bubbles have a Gaussian distribution around 1.4 ± 0.3 nm and 1.8 ± 0.4 nm in 12 and 14Cr ODS steels, respectively. A careful look at the images (a) and (b) show that in the 12Cr ODS steel most bubbles are trapped to ODS particles which are contrasted as small dark dots in (a). The bubbles appeared as even smaller bright dots in Fig. 7b which are associated to ODS particles. In the 14Cr ODS steel the bubble distribution is somewhat different as shown in Fig. 7c. Some bubbles are attached to ODS particles but some are not. The bubbles attached to ODS particles are of similar size as those in the matrix. The bubble size and density at 300 °C is controlled by diffusion of the helium atoms, while the influence of the ODS particles is minor, due to their much lower density. At higher temperatures, when bubbles will coarsen, their size and density may be influenced by the dipersoids. It may be expected that in a temperature regime where bubble growth is dominated by a migration and coalescence mechanism, ODS particles may prevent further growth. This conjecture deserves further investigation. Dislocation loops as observed by TEM after irradiation creep are illustrated in Fig. 8 for 12Cr (a) and 14Cr (b) ODS steels. The images were taken with electron-beam direction close to [0 0 1] under imaging conditions, namely with a diffraction vector g = (2 0 0) indicated by the arrow in Fig. 8. Both ½h1 1 1i and h1 0 0i loops are observed. The loops were approximately circular similar as reported in Ref. [13]. The small edge-on loop has a typical coffee bean contrast with a no-contrast center line which length is used to measure its size. In the case of an inclined loop, an elliptical shape appears on the viewing screen (TEM image). The length of its major axis is a measurement of the loop size. The preliminary
J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437
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Fig. 8. Week-beam dark-field images showing the dislocation loops in 12Cr (a) and 14Cr (b) ODS steels irradiated at 300 °C up to 1.3 dpa.
measurement gives average loop diameters of 5.3 ± 2.0 and 7.5 ± 2.0 nm for 12 and 14Cr ODS steels, respectively. The loop number density has not been quantitatively analyzed, but it is clearly seen that loop density is higher in 14Cr compared to 12Cr. Microstructural results are summarized in Table 3. A detailed characterization of loops in PM2000 can be found in Ref. [18]. In materials without ODS particles, bubbles are attached to loops and form bubble-loop complexes at low temperatures (6400 °C) which is evident from experiment results [18] and is also predicted by Molecular Dynamic simulation [24]. The high density of fine ODS particles can reduce the number density of loops, therefore improving the resistance against low temperature helium embrittlement. 4. Conclusions (1) Irradiation creep rates of both 12Cr and 14Cr-ODS ferritic steels a temperature of 300 °C show linear stress dependence up to 300 MPa at. (2) Irradiation creep rate per dose rate and stress at a temperature of 300 °C amounts to 4.0 106 dpa1 MPa1 and 10 106 dpa1 MPa1 for 12Cr- and 14Cr-ODS, respectively. Irradiation creep properties are remarkably insensitive to Cr content, grain size and dispersoid size. (3) Dislocation loops and helium bubbles are distributed homogenously in the matrix. In the case of high density fine dispersoids, most bubbles are attached to ODS particles. This may suppress loop formation as well as growth of bubbles, thereby increasing the resistance of ODS ferritic steels against helium embrittlement.
Acknowledgements The authors appreciate the help of the cyclotron operation team at CEMHTI/CNRS, Orléans. Work was performed within the Swiss
Generation IV Program. It was partially financed by the EU-project GETMAT. References [1] A. Kimura, Mater. Trans. 46 (2005) 394. [2] A. Alamo, H. Regle, G. Pons, L.L. Bechade, Mater. Sci. Forum. 88–90 (1992) 183. [3] M. Klimiankou, R. Lindau, A. Moeslang, J. Schroeder, Powder Metall. 48 (2005) 277. [4] A. Ramar, Z. Oksiuta, N. Baluc, R. Schäublin, Fusion Eng. Des. 82 (2007) 2543– 2549. [5] S. Ukai, M. Fujiwara, J. Nucl. Mater. 307–311 (2002) 749–757. [6] N.Y. Iwata, A. Kimura, M. Fujiwara, N. Kawashima, J. Nucl. Mater. 367–370 (2007) 191. [7] Dispersion–Strengthened High-Temperature Materials, Material properties and applications, Prospectus from Plansee, 2003, 706 DE.04.03(1000)RWF. [8] A. Czyrska-Filemonowicz, B. Dubiel, Mater. Proc. Technol. 63 (1997) 53–64. [9] D.K. Mukhopadhyay, F.H. Froes, D.S. Gelles, J. Nucl. Mater. 258–263 (1998) 1209. [10] R.L. Klueh, J.P. Shingledecker, R.W. Swindeman, D.T. Hoelzer, J. Nucl. Mater. 341 (1998) 103. [11] Luke L. Hsiung, Michael J. Fluss, Scott J. Tumey, B. William Choi, Yves Serruys, Francois Willaime, Akihiko Kimura, Phys. Rev. B 82 (2010) 184103. [12] Y. de Carlan, J.-L. Bechade, P. Dubuisson, J.-L. Seran, P. Billot, A. Bougault, T. Cozzika, S. Doriot, D. Hamon, J. Henry, M. Ratti, N. Lochet, D. Nunes, P. Olier, T. Leblond, M.H. Mathon, J. Nucl. Mater. 386–388 (2009) 430. [13] P. Dubuisson, Y. de Carlan, V. Garat, M. Blat, J. Nucl. Mater. 428 (2012) 6. [14] K. Yutani, H. Kishimoto, R. Kasada, A. Kimura, J. Nucl. Mater. 367–370 (2007) 423. [15] A. Hasegawa, M. Ejiri, S. Nogami, M. Ishiga, R. Kasada, A. Kimura, K. Abe, S. Jitsukawa, J. Nucl. Mater. 386–388 (2009) 241. [16] J. Chen, M.A. Pouchon, A. Kimura, P. Jung, W. Hoffelner, J. Nucl. Mater. 386–388 (2009) 143. [17] J. Chen, P. Jung, M.A. Pouchon, T. Rebac, W. Hoffelner, J. Nucl. Mater. 373 (2009) 22. [18] J. Chen, P. Jung, W. Hoffelner, H. Ullmaier, Acta Mater. 56 (2008) 250. [19] Marta Serrano, Dolores Gomez-Briceño, Michael Rieth, GETMAT 8th ExeCo Meeting, 29–30 September 2011, Paris. [20] P. Olier, J. Malaplate, M.H. Mathon, D. Nunes, D. Hamon, L. Toualbi, Y. de Carlan, L. Chaffron, J. Nucl. Mater. 428 (2012) 40. [21] P. Jung, J. Viehweg, C. Schwaiger, Nucl. Instr. Meth. 154 (1978) 207. [22] P. Jung, A. Schwarz, H.K. Sahu, Nucl. Instr. Meth. A234 (1985) 331. [23] P. Jung, J. Nucl. Mater. 113 (1983) 133. [24] N. Gao, H. Van Swygenhoven, M. Victoria, J. Chen, J. Phys.: Condens. Matter. 23 (2011) 442201.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Irradiation creep and microstructural changes of ODS steels of different Cr-contents during helium implantation under stress J. Chen a,⇑, P. Jung a, J. Henry b, Y. de Carlan b, T. Sauvage c, F. Duval c, M.F. Barthe c, W. Hoffelner a a
Department of Nuclear Energy and Safety, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland CEA/DEN, SRMA, F-91191 Gif-sur-Yvette Cedex, France c CEMHTI/CNRS, Université d’Orléans, 3A Rue de la Férollerie, 45071 Orléans Cedex 2, France b
a r t i c l e
i n f o
Article history: Received 26 November 2012 Accepted 23 February 2013 Available online 13 March 2013
a b s t r a c t Irradiation creep and microstructural changes of two ferritic ODS steels with 12% and 14% Cr have been studied by homogeneously implantation with helium under uniaxial tensile stresses from 40 to 300 MPa. The maximum dose was about 1.2 dpa (5000 appm-He) with displacement damage rates of 1 105 dpa/ s at a temperature of 300 °C. Irradiation creep compliances were measured to be 4.0 106 dpa1 MPa1 and 10 106 dpa1 MPa1 for 12 and 14Cr ODS, respectively. Subsequently, microstructural evolution was studied in detail by TEM observations, showing dislocation loops and bubbles distributed homogenously in the matrix. Some bubbles were attached to ODS particles. Finally, the effects of Cr content on irradiation creep and microstructural changes are discussed, including earlier results of a 19Cr ODS and a PM2000 ferritic steel. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The excellent high temperature strength, low irradiation-induced swelling and potential resistance to helium embrittlement of oxide dispersion strengthened (ODS) Fe–Cr steels have prompted investigations on their potential use as structural materials in advanced nuclear energy system such as fusion and Generation IV fission reactors worldwide in the last decade [1–13]. For instance, the application of ODS steels for blanket components in fusion reactors, and for cladding materials in Generation IV reactors may increase the safety margin and thermal efficiency, and provide a larger design tolerance. To fulfil different requirements, ODS steels with different Cr-contents are developed, ranging from 9–12 wt% Cr (i.e. typical ferritic/martensitic) to 20 wt% Cr (i.e. ferritic grade). They also may contain aluminum to improve corrosion resistance. To realize the use of ODS steels in nuclear energy systems, many efforts have been made to optimize production of ODS steels and to characterize their mechanical and corrosion properties [1–11]. Recently, investigations of helium effects on swelling [14] and fracture behavior [15] have been reported. However knowledge on irradiation damage is still very limited, and irradiation creep data for advanced 12 and 14Cr ODS steels are still missing. On the other hand, the newly developed 12Cr and 14Cr ODS differ in Cr content from 19Cr ODS [1] and an industrial ODS steel PM2000 with 20% Cr [7] of which irradiation creep data ⇑ Corresponding author. Address: Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland. Tel.: +41 56 310 2280; fax: +41 56 310 4595. E-mail address: [email protected] (J. Chen). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.02.071
[16,17] and related microstructural changes [18] have been reported recently. Therefore in the present work, in situ irradiation creep of 12 and 14Cr ODS steels was investigated during Heimplantation, while microstructural changes were studied by transmission electron microscopy (TEM) after implantation. These results are compared to those from 19% and 20% Cr steels in order to reveal possible effects of Cr content on irradiation creep and microstructural changes. 2. Experimental The advanced ferritic 12 Cr ODS steel was supplied by KOBELCO Research Institute, Japan, in plates of 6.5 33 600 mm3. The 14Cr ODS developed in the frame of the EU project GETMAT by CEA, France was in rods of 36 mm diameter. Compositions are given in Table 1. The detailed production process has been reported in Ref. [19,20]. Here a brief description is given. The 12Cr- and 14Cr- ODS ferritic steels, both were produced by powder metallurgy: Iron-base gas atomized powders were mechanically alloyed with 0.2–0.3% Y2O3 particles under hydrogen atmosphere in a high-energy ball mill. Then, the ODS steel powders were encapsulated in a soft steel can, consolidated by hot extrusion at 1100 °C (12Cr) and 1150 °C (14Cr), followed by a hot and cold rolling procedure and a final thermal treatment at 1200 °C (12Cr) and 1050 °C (14Cr) for 1 h and air-cooling for recrystallization. TEM micrographs showing the grain shapes and sizes (a), dislocation network (b) and the ODS particle distribution (c) are given in Figs. 1 and 2 for the 12 and 14Cr ODS, respectively. These pictures show that both steels in the as-received condition show grains elongated
J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437 Table 1 The main chemical composition (wt%) of 12Cr and 14Cr ODS ferritic steels.
12Cr 14Cr
Cr
W
Ti
Mn
Si
Y2O3
Fe
12 13.5
2.0 0.9
0.26 0.4
0.27
0.32
0.23 0.3
Bal. Bal.
along the rolling direction. The ODS particles with partially spherical and partially facetted shape are distributed quite uniformly. Sizes of dispersed ODS particles in the 12% Cr steel range mostly from 1.7 to 2.7 nm, but with maximum sizes up to 18 nm, while the sizes of ODS particles in the 14Cr ODS steel range from 2.2 up to 12.5 nm. The average sizes of grains and of diameters of the ODS particles in the as-received condition are summarized in Table 2. The dislocation density varies quite significantly from grain to grain, and is higher in 12Cr ODS than in 14Cr ODS. Dogbone shaped creep samples of 300 lm thickness were cut parallel to the rolling direction by spark erosion. The samples were mechanical polished on both sides to 100 lm with grad 2400 paper. The final samples had an overall size of 28 mm in length, 8 mm in width and 0.1 mm in thickness, with a gauge volume of 10 2 0.1 mm3. In situ creep under He-implantation was performed at the cyclotron of CEMHTI/CNRS. Details of the experimental set up are described in Refs. [21,22]. With 28 MeV 4He2+ ions passing through firstly a magnet scanning system, then a vacuum window of a
433
25 lm thick hastelloy foil and finally a degrader wheel with 24 Al-foils of variable thicknesses, the gauge area of the 0.1 mm thick samples was 3D-homogeneously implanted under constant uniaxial stress. Typical implantation rates were 0.04 appm/s with an average beam current density of 10 lA/cm2. The concurrent production of displacement damage was calculated by TRIM and SRIM for a displacement threshold energy of 40 eV and a binding energy of 3 eV, giving per implanted He-atom 275 displacements on the front side and 193 on the back side, averaging to 234 displaced lattice atoms. Accordingly, a displacement rate of about 9 106 dpa/ s (displacements per atom per second) is derived. The displacement damage distribution along the specimen depth is illustrated in Fig. 3a. As temperature variations due to beam fluctuation limited the precision of strain measurement by LVDT (Linear Variable Differential Transformer) beam was switched off about every 2 h to measure the change of specimen length. The implantation was performed at a temperature of 300 °C and was continued until the strain rate became constant (stationary creep). Then the implantation of the same specimen was continued at a different stress in the range of 40–300 MPa. To minimize systematic errors from dose effects on the microstructure, e.g. by accumulation of irradiation defects, applied stress was changed alternatively to higher and lower values (as indicated in Fig. 4). The temperature distribution along the gauge region was monitored by an infrared pyrometer under 45° from the backside of the specimens. The specimen geometry and measured temperature distribution along the longitudinal
Fig. 1. Microstructure of as received 12Cr ODS steel showing the grain shape and size (a), dislocation network (b) and the ODS particle distribution (c).
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Fig. 2. Microstructure of as received 14Cr ODS steel showing the grain shape and size (a), dislocation network (b) and the ODS particle distribution (c).
Table 2 Microstructural parameters of the used ODS steels. Material
12Cr 14Cr 19Cr PM2000
Cr (wt%)
12 13.7 18.4 20
Y2O3 (wt%)
0.23 0.30 0.37 0.50
direction during beam-on period are shown in Fig. 3b. Typical temperature variations across the gauge section, as well as fluctuation during irradiation time are ±10 °C. Finally, TEM specimens were prepared from the implanted gauge sections (see Ref. [18] for details) and TEM examinations were performed with a JEM 2010 at PSI. Foil thickness was determined by the carbon contamination spike method. 3. Results and discussion 3.1. Irradiation creep Because of different thermal expansion coefficient of specimen holder, support parts and sample itself, the measured length change by LVDT, also during beam-off periods depends on the temperature distribution in the chamber. Therefore, in order to
Grain size (lm)3
1.3 1.3 8.0 0.5 0.5 2 0.2 0.2 0.7 103 103 >104
ODS particle Size (nm)
Density(m3)
2.2 4.8 2.1 2.8
1.6 1023 4.5 1022 1.2 1024 5.1 1020
improve data precision, all strains have to be taken at a constant chamber temperature. An example is illustrated in Fig. 4, where irradiation creep was performed at a nominal temperature of 300 °C under a tensile stress of 45 MPa. The first reference point (marked as ‘‘0’’ in Fig. 4) was measured at equilibrium room temperature before irradiation. Then the length of the specimen was monitored after typical beam times of 2 h during beam-off period of typically 15 min, (marked as ‘‘1’’, ‘‘2’’ up to ‘‘8’’ in Fig. 4) during which chamber temperature dropped continuously. The second reference point (marked as ‘‘8a’’ in Fig. 4) was measured again at equilibrium room temperature after irradiation (after a waiting time of several hours). The strain measurements were taken at 32 °C and a precision of 1 105 was reached. Fig. 5 shows the strain eðrÞ ¼ Dl0l of 12Cr ODS steel (a) and 14Cr ODS steel (b) during implantation at 300 °C as a function of the displacement dose. Each stress change caused, aside from elastic
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4x10-3
12Cr ODS, 300°C 3x10-3
2x10-3
92 MPa 43 MPa 195 MPa 295 MPa 95 MPa
strain
1x10-3
0 6x10-3
14Cr ODS, 300°C 5x10-3 4x10-3 3x10-3
92 MPa 45 MPa 190 MPa 291 MPa 92 MPa
2x10-3 1x10-3 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
displacement-dose (dpa) Fig. 5. Strain of 12Cr ODS (a) and 14Cr ODS (b) steels during He-implantation at 300 °C as a function of displacement dose at different stresses. Fig. 3. Displacement profile along depth of specimen (a); and specimen geometry and temperature distribution along longitudinal direction during beam-on period (b).
strain-rate/dose-rate (dpa-1)
0.504
14Cr ODS under 45 MPa at 300°C
length change (mm)
0.502
0.500
1
0
2
3 4 5 6
0.498
0.494 22
26
28
30
32
300°C
0.003
0.002
12 Cr-ODS 14Cr-ODS 19Cr-ODS PM2000
0.001
0
100
200
300
stress (MPa)
8a
24
Fe-Cr ODS ferritic steels
0.000
7 8
0.496
0.004
34
36
38
40
42
chamber temperature (°C) Fig. 4. Length changes monitored during beam-off periods by LVDT as a function of the chamber temperature. All strains were measured at 32 °C. Numbers 1–8 are successive strain measurements after typically 2 h irradiation periods each. Numbers 0 and 8 are reference measurements at equilibrium room temperature before and after the irradiation period, respectively.
strain, also a short transient stage before stationary creep was reached. Those transient strains are similar to observations in 19Cr ODS [16] and PM2000 [17]. They are ascribed to irradiationinduced relaxation. It is worth to notice that in both steels a contraction of the specimen against the applied tensile stress occurred at the beginning of irradiation when the applied stress was reduced (e.g. from 300 to 100 MPa), but already after a dose of less than 0.03 dpa, creep proceeds again in the stress direction.
Fig. 6. Irradiation creep rates per displacement rates of 12Cr ODS and 14Cr ODS steels as a function of tensile stress under He-implantation at 300 °C. Linear fits give irradiation creep compliances. The 19Cr ODS [11] and PM2000 [12] data are included for comparison.
According to results from PM2000 [17], it is fair to assume that thermal creep is negligible at 300 °C. Irradiation-induced creep rates, e0 ; i.e. strain-rate per dose-rate, k (in unit of dpa1) were obtained by fitting straight lines to the stationary parts of the curves in Fig. 5. These values are plotted in Fig. 6 as a function of the applied stress for both steels. The data from PM2000 and 19Cr ODS are also included for comparison. The data can be fitted by linear stress dependence up to 300 MPa (solid line):
e0 ðrÞ ¼ B0 r þ e00
ð1Þ
with creep compliances B0, i.e. creep rate per dose rate (assuming linear dependence) and stress, of 4.0 106 dpa1 MPa1 and
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J. Chen et al. / Journal of Nuclear Materials 437 (2013) 432–437 Table 3 Result on irradiation creep and microstructural changes. Materials
r0.2 at 300 °C (MPa)
Creep compliance (106 dpa1 MPa1)
Offset e00 (103 dpa1)
Loop size (nm)
Bubble size (nm)
Bubble density (1023 m3)
12Cr ODS 14Cr ODS 19Cr ODS PM2000
992 1087 965 716
4.0 10. 4.0 5.7
1.2 0.72 1.5 2.0
5.3 7.5 n.d. 3.6
1.4 1.8 1.1 1.0
n.d. n.d. 1.0 4.5
n.d. = not determined.
Fig. 7. (a) In-focus and (b) under-focus bright-field images of the same spot showing the ODS particles (dark small dots, 2 nm) and bubble (bright small dots, 0.75 nm) distributions after implantation at 300 °C to 5000 appm-He in a 12Cr ODS sample, respectively. (c) Under-focus bright-field image showing the ODS particles (dark spots) and bubbles (small bright spots) in 14Cr ODS steel. Notice that the bubble distribution is quite homogenous in both materials.
10 106 dpa1 MPa1 for 12Cr and 14Cr ODS steels at a temperature of 300 °C, respectively. Extrapolating the lines in Fig. 6 to zero stress gives ordinate-offsets e00 of 1.19 103 and 0.72 103 dpa1 for 12Cr and 14Cr ODS steels, respectively. e00 represents stress-independent dimensional change rates, for example from isotropic volume swelling:
DV ¼ 3 e00 kt V
ð2Þ
where t is time. The measured irradiation creep compliances B0 (slope of lines in Fig. 6) and ordinate offsets e00 including the earlier results of 19Cr ODS steel and PM2000 are summarized in Table 3 together with loop size, bubble size and density. The yield stresses (r0.2) at 300 °C from Ref. [14] are included. No remarkable effects of Cr content, grain size and dispersoid size on irradiation creep properties are observed. The apparently slightly higher B0 value of the 14Cr ODS cannot be unambiguously related to any material property (Table 2) or microstructural evolution (Table 3). Only the transient elastic strains after stress changes (Fig. 5) are significantly larger in the 14Cr ODS than for all other Cr contents (see also Refs. [16] and [17]). Even when those transient strains can not yet be safely related to a underlying mechanism (cf. Ref. [23]), they may indicate a sensitivity of the material to irradiation, which in turn will also enhance the compliance for irradiation creep. 3.2. TEM observation TEM was conducted after in situ creep tests. In both ODS steels, bubble formation during implantation is observed. Fig. 7 shows an in-focus bright field image of 12Cr ODS steel (a), and
an under-focus bright field image of the same spot (b), and under-focus bright field image of 14Cr ODS steel after irradiation creep test at 300 °C. The helium bubbles have a Gaussian distribution around 1.4 ± 0.3 nm and 1.8 ± 0.4 nm in 12 and 14Cr ODS steels, respectively. A careful look at the images (a) and (b) show that in the 12Cr ODS steel most bubbles are trapped to ODS particles which are contrasted as small dark dots in (a). The bubbles appeared as even smaller bright dots in Fig. 7b which are associated to ODS particles. In the 14Cr ODS steel the bubble distribution is somewhat different as shown in Fig. 7c. Some bubbles are attached to ODS particles but some are not. The bubbles attached to ODS particles are of similar size as those in the matrix. The bubble size and density at 300 °C is controlled by diffusion of the helium atoms, while the influence of the ODS particles is minor, due to their much lower density. At higher temperatures, when bubbles will coarsen, their size and density may be influenced by the dipersoids. It may be expected that in a temperature regime where bubble growth is dominated by a migration and coalescence mechanism, ODS particles may prevent further growth. This conjecture deserves further investigation. Dislocation loops as observed by TEM after irradiation creep are illustrated in Fig. 8 for 12Cr (a) and 14Cr (b) ODS steels. The images were taken with electron-beam direction close to [0 0 1] under imaging conditions, namely with a diffraction vector g = (2 0 0) indicated by the arrow in Fig. 8. Both ½h1 1 1i and h1 0 0i loops are observed. The loops were approximately circular similar as reported in Ref. [13]. The small edge-on loop has a typical coffee bean contrast with a no-contrast center line which length is used to measure its size. In the case of an inclined loop, an elliptical shape appears on the viewing screen (TEM image). The length of its major axis is a measurement of the loop size. The preliminary
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Fig. 8. Week-beam dark-field images showing the dislocation loops in 12Cr (a) and 14Cr (b) ODS steels irradiated at 300 °C up to 1.3 dpa.
measurement gives average loop diameters of 5.3 ± 2.0 and 7.5 ± 2.0 nm for 12 and 14Cr ODS steels, respectively. The loop number density has not been quantitatively analyzed, but it is clearly seen that loop density is higher in 14Cr compared to 12Cr. Microstructural results are summarized in Table 3. A detailed characterization of loops in PM2000 can be found in Ref. [18]. In materials without ODS particles, bubbles are attached to loops and form bubble-loop complexes at low temperatures (6400 °C) which is evident from experiment results [18] and is also predicted by Molecular Dynamic simulation [24]. The high density of fine ODS particles can reduce the number density of loops, therefore improving the resistance against low temperature helium embrittlement. 4. Conclusions (1) Irradiation creep rates of both 12Cr and 14Cr-ODS ferritic steels a temperature of 300 °C show linear stress dependence up to 300 MPa at. (2) Irradiation creep rate per dose rate and stress at a temperature of 300 °C amounts to 4.0 106 dpa1 MPa1 and 10 106 dpa1 MPa1 for 12Cr- and 14Cr-ODS, respectively. Irradiation creep properties are remarkably insensitive to Cr content, grain size and dispersoid size. (3) Dislocation loops and helium bubbles are distributed homogenously in the matrix. In the case of high density fine dispersoids, most bubbles are attached to ODS particles. This may suppress loop formation as well as growth of bubbles, thereby increasing the resistance of ODS ferritic steels against helium embrittlement.
Acknowledgements The authors appreciate the help of the cyclotron operation team at CEMHTI/CNRS, Orléans. Work was performed within the Swiss
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