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In situ TEM study of the stability of nano-oxides in ODS steels under ion-irradiation
Journal of Nuclear Materials 428 (2012) 176–182
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
In situ TEM study of the stability of nano-oxides in ODS steels under ion-irradiation M-L. Lescoat a,⇑, J. Ribis a, A. Gentils b, O. Kaïtasov b, Y. de Carlan a, A. Legris c a
CEA, DEN, Service de Recherches Métallurgiques Appliquées, 91191 Gif-Sur-Yvette, France CSNSM, CNRS/IN2P3, Univ Paris-Sud, Bât. 108, 91405 Orsay Cedex, France c UMET, CNRS/UMR 8207, Bât. C6, Univ Lille 1, 59655 Villeneuve d’Ascq, France b
a r t i c l e
i n f o
Article history: Available online 28 December 2011
a b s t r a c t Oxide Dispersion Strengthened (ODS) ferritic–martensitic steels are considered for nuclear applications as structural components for fusion or fission reactors. To ensure good performances in service, the stability under irradiation of the microstructure and especially of Y–Ti–O nanoclusters have to be assessed. In situ Transmission Electron Microscopy has been performed to follow the Y–Ti–O nano-oxides dispersed in a Fe18Cr1W0.3Ti + 0.6Y2O3 ODS material under ion-irradiation at 500 °C. Microstructural examinations using bright and dark field mode showed that Y–Ti–O nano-precipitates (5 nm) are still present after irradiation up to 45 dpa. However, some larger oxides seem to be more affected by irradiation at 45 dpa (creation of point defects, interface and shape modification). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to their excellent mechanical properties, Oxide Dispersion Strengthened (ODS) ferritic/martensitic steels are promising candidates for fuel cladding in sodium cooled fast reactors and for structural applications in fusion reactors. The dispersion of nano-sized oxides associated with the ferritic matrix confers very good creep strength at high temperature and resistance to radiation swelling at high dose [1–3]. In reactor, these materials will be subject to drastic irradiation and temperature conditions, namely more than 150 dpa and 400 to 650 °C for sodium fast reactors. To ensure good performance in service, the microstructure of such ODS steels have to remain stable under irradiation at high temperature. Energetic ions can be used to investigate the effects of neutron irradiation in reactor components. Although the damage state depends on the particle type and damage rate, simulations with ion irradiations yield answer on basic mechanisms with the considerable advantage to enable easy variations of irradiation parameters without residual activity of the samples. The historical survey of the studies on ODS steels reported in the literature points out that there is no common conclusion regarding the stability of oxide particles under ion-irradiation. Some studies have shown that Y–Ti–O nano-clusters are stable under low damage dose ion irradiation (0.7, 1, 5, 10 dpa) at 300 °C [4] and 500 °C [5]. No modification of size, shape or chemical composition was detected at 20 dpa at 200 °C, 500 °C, 700 °C [6], 60 dpa, 650 °C [7] or 150 dpa, 670 °C [8]. But other studies highlighted an
instability of nano-clusters under irradiation. Indeed, some observed a decrease of the average size of nano-clusters after ion irradiation; even for doses as low as 1 dpa, 525 °C [9] and 1.4 dpa, 443 °C [10]. Likewise, oxide particles were found to dissolve slightly at 20 dpa, 380 °C [11] and to be amorphous with modified shapes at 33 dpa, 400 °C [12]. Oxide size modifications were found to be strongly temperature dependant [13,14] and decrease as their number density increase under heavy ion irradiation up to 150 dpa at 600 °C and 700 °C. To us, the matters with such investigations is first that nanoclusters are hardly characterized after irradiation because of their very fine size (close to the resolution limit of conventional techniques) and because of irradiation defects. Then, direct comparison of average mean sizes or number densities before and after irradiation could possibly raise questionable conclusions, since the nanoclusters distribution varies from areas to areas in such materials. In this context, in situ TEM studies seem to be of the most interest to assess the changes in the dispersoïds population induced by irradiation. However, very few in situ studies of the stability of ODS steel under ion irradiations have been performed so far to our knowledge [15,16]. In the present work, in situ Transmission Electron Microscopy (TEM) experiments are carried out to evaluate the stability of the Y–Ti–O nanoclusters of a Fe18Cr1W0.3Ti + 0.6Y2O3 ODS steel under ion irradiation up to 45 dpa at 500 °C.
2. Experimental 2.1. Specimen material
⇑ Corresponding author. Tel.: +33 169081178. E-mail address: [email protected] (M-L. Lescoat). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.12.009
The Fe18Cr1W0.3Ti + 0.6Y2O3 ODS ferritic steel is one of the new alloys developed at CEA/SRMA for nuclear applications [3] (CEA
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177
reference F20 and below referred to as ‘‘Fe18Cr–Y2O3’’). It has been prepared at the Plansee Society by mechanical alloying of two Aubert & Duval pre-alloyed powders in an attritor under hydrogen. A shear bar was hot extruded at 1100 °C, hot rolled 20% at 650 °C and annealed for 1 h at 1050 °C at the CEA. The chemical composition of the extruded powder is given in Table 1.
2.2. Characterizations of the unirradiated material The unirradiated ODS material was characterized at CEA/SRMA by Transmission Electron Microscopy. TEM disks were punched from 60 lm-thick mechanically polished foils and electro polished using a 10% perchloric acid–90% ethanol solution at 10 °C in a Struers Tenupol thinning device. To facilitate identification of nano-precipitates, extraction replicas were also prepared. The ODS alloy was mechanically polished up to a mirror finish and then electrochemically etched using a 1% tetra methyl ammonium chloride–10% acetyl acetone–methanol solution. After carbon deposition, the carbon layer containing the precipitates was carefully removed using the same procedure as for etching and put in a TEM copper grid. Electron Energy Loss Spectrometry (EELS) and energy filtered TEM (EFTEM) were performed on a JEOL 2100 (LaB6) operating at 200 kV and equipped with a Gatan Imaging Filter (GIF). Both 2-windows (10 eV slits) and 3-windows (30 eV slits) EFTEM methods were used with doubly binned images of 512 512 pixels. High Resolution TEM (HRTEM) and Energy Dispersive Spectrometry (EDS) in Scanning TEM (STEM) mode were performed on a JEOL 2010 (FEG) operating at 200 kV.
2.3. Ion-irradiations and in situ TEM characterizations In situ experiments were performed at the CSNSM-JANNuS Orsay facility [17] equipped with a FEI Tecnai G2 20 TEM working at 200 kV and coupled with the IRMA ion implantor. Irradiations were performed at 500 °C and in a sequenced mode to limit shadowing effects from the GATAN heating double tilt holder. During irradiation, the specimen was tilted of a = 68° to be normal to the ion line. At given doses, irradiation was then stopped and the sample set back normal to the electron beam (a = 0°) to perform TEM observations. 150 keV Fe+ auto-ions were used to reach rapidly high dose without surface contamination that could occur under long exposure to the ion beam at high temperature despite the good vacuum inside the microscope column (better than 107 Torr). The depth profiles of the displacement damage (Fig. 1) were estimated using a full detailed cascade calculation of the SRIM-2008.04 code [18] in a Fe matrix (displacement energy of 40 eV). The damage is not homogeneous within the specimen and the dose in displacement per atom (dpa) is hereafter taken as the maximum dose reached at the Bragg peak located at 30 nm from the irradiation surface. From SRIM calculations of the ion range distribution, the concentration of additional injected ions in the matrix is around 2 1021 ions cm3 (i.e. 2%) at the Bragg peak and for the highest dose investigated (1.2 1016 at cm2). The chemical changes locally induced by implantation will thus be neglected in this paper. The irradiation conditions investigated are given in Table 2.
Table 1 Chemical composition (wt%) of the ODS material after mechanical alloying (powder). Fe
Cr
W
Ti
Mn
Si
Ni
C
N
O
Y203
wt%
17.75
0.95
0.26
0.31
0.30
0.19
0.03
0.02
0.11
0.56
Fig. 1. Depth profiles of displacement damages in Fe calculated by SRIM.
3. Results 3.1. Characterization of the unirradiated material The microstructure is made of very fine grains elongated along the hot extrusion direction (about 1 lm long and 300 nm large) and equiaxed in the transverse direction (300 nm) [19]. The micrograph of Fig. 2a shows the precipitation in the unirradiated material. The precipitates are dispersed throughout the grains with an average number density over 1022 m3. The particle sizes (diameters) range from few nanometers (Fig. 2b) to 50 nm; 80% of the precipitates being smaller than 10 nm. The histogram of size distributions (Fig. 2c) was compiled by measurements on 1040 nano-precipitates (<10 nm) from BF TEM micrographs and shows that the distribution of nano-clusters sizes is centered on 3 nm. Whereas some nano-precipitates are hardly identified in bright field TEM due to the low contrast between matrix and precipitates, EFTEM 2-windows analyses on Fe–M give a clear view of precipitates distribution over large areas [9]. Comparison of Fe jump ratio images (Fig. 2d) acquired in two different regions of the unirradiated material shows that the size distribution of nano-precipitates (Fe depleted zone) differs from area to area, pointing out the interest of in situ TEM experiments to evaluate the modification induced in the dispersion by irradiation. The nano-precipitates were finely characterized using TEM on extraction replica (Fig. 3). EFTEM elemental maps (Fig. 3a) show that nano-precipitates correlate with O and Ti enriched areas. Yttrium being hardly imaged by EFTEM, an EELS spectrum was acquired in this region (Fig. 3b). The signal shows two edges at 2080 eV and 2155 eV, related to Y–L3,2 ionization transitions. Ti (Ka,b) and Y (Ka,b) peaks obtained with EDX analyses (Fig. 3c) also confirm that typical nano-precipitates are Y–Ti–O clusters. The crystallographic structure of Y–Ti–O nano-precipitates was investigated using HRTEM. Fig. 3d shows the HRTEM micrograph from a typical nano-precipitate. Evaluation of the HRTEM image by means of fast Fourier transformation (Fig. 3e) indicates that interplanar distances and angle between the systems of plane are consistent 2Þ atomic planes of Y2Ti2O7 (pyrochlore, with the (0 0 4) and ð2 2 a = 10.09 Å) in [1 1 0] zone axis; with d222 = 2.91 Å, d004 = 2.52 Å 2Þ and (0 0 4) planes. and 54.7° between ð2 2 Results from EFTEM, EELS and EDS combined with HRTEM analysis point out that most of the nano-precipitates have Y2Ti2O7 composition. This conclusion is consistent with other studies by
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Table 2 Summary of the ion irradiation conditions investigated. Temperature (°C)
Energy (keV)
Flux (at cm2 s1)
Fluence (at cm2)
Maximum dose (dpa)
500 500 500 500
150 150 150 150
2.85 1012 3.75 1012 2.85 1012 3.75 1012
1.0 1015 6.0 1015 1.0 1016 1.2 1016
4 23 38 45
TEM on Fe14Cr–Y2O3 ODS alloys [20] or by lattice Monte Carlo simulations [21] which indicate that nano-sized precipitates (2–5 nm) are likely to be Y–Ti–O oxides. Similarly, it is in agreement with small angle neutron scattering [22] and atom probe tomography [23] performed on various nanostructured ferritic alloys which
suggest that the smallest clusters are Y–Ti–O transition oxides with chemical composition close to Y2Ti2O7 [24]. Synchrotron radiation X-ray diffraction and absorption experiments performed on the unirradiated Fe18Cr–Y2O3 material [25] show that additional secondary phases coexist with these Y2Ti2O7 nano-precipitates, namely Y2O3 (BCC, a = 10.63 Å and FCC, a = 5.30 Å), TiO2 (Orthorhombic, a = 4.90 Å, b = 9.46 Å, c = 2.96 Å) and Cr23C6 (FCC, a = 10.61 Å). These phases are likely to be the very large precipitates (>50 nm) that are not considered in this paper.
3.2. In situ ion irradiations After irradiation at 500 °C, the diffraction contrast from defects and bend contours drastically harden the imaging of nano-precipi-
Fig. 2. Microstructure of the unirradiated Fe18Cr–Y2O3. (a) TEM image of the dispersion. (b) High magnification TEM image of a typical nano-precipitate. (c) Particle size (diameter) distribution. (d) Fe–M jump-ratio images (2-windows) from two different areas.
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Fig. 3. Characterizations of typical nano-precipitates on extraction replica from unirradiated Fe18Cr–Y2O3. (a) 3-windows EFTEM elemental maps. (b) EELS spectrum on the same region. (c). EDX spectrum on the nano-precipitate showed in the STEM image. (d) High-resolution TEM image. (e) FFT image.
tates in Bright Field (BF) TEM. However, TEM in Dark Field (DF) mode was found effective to improve the contrast from nanoclusters and was thus used to follow Y–Ti–O nano-precipitates under irradiation. The micrographs obtained before irradiation (BF) and at 23 dpa (DF) are given Fig. 4a. The 1–4 numbered large oxides prove that the same region is followed under irradiation. After irradiation, clusters are still observable in DF but image processing was performed to optimize the precipitate–matrix contrast. The processed images are given Fig. 4b. The processed image at 23 dpa clearly shows nano-clusters that coincide with the 5–6 nm Y–Ti–O circled before irradiation, thus indicating that Y–Ti–O nano-precipitates are apparently stable up to 23 dpa at 500 °C. As images to be compared before and after irradiation are either BF or DF micrographs, determination of size evolution by direct measurements is not relevant here. Looking at higher doses (up to 45 dpa), it has also been possible to follow Y–Ti–O nano-precipitates under irradiation by in situ TEM. Fig. 5a shows the same region of Fe18Cr–Y2O3 before irradiation (BF TEM) and at 45 dpa (DF TEM). Similarly, image processing was performed to enhance the contrast from nano-precipitates. The processed images are framed aside TEM micrographs. Direct comparisons of processed image indicate that 4–6 nm Y–Ti–O are still present at 45 dpa (see circled clusters). Fig. 5b shows the DF
image of a larger region of the Fe18Cr–Y2O3 irradiated at 45 dpa. On this DF image, numerous clusters are detectable and precipitates as small as 4 nm are clearly highlighted in the processed image close by (see circled nanoclusters). These experiments thus confirm that Y–Ti–O nano-precipitates are apparently stable under irradiation up to 45 dpa at 500 °C. However, beside these Y–Ti–O nano-precipitates, some other large oxides (>20 nm) seem to undergo specific behaviors that differ from oxide to oxide, probably depending of their nature, size and local environment. Fig. 6a and b illustrates the behavior of two large oxides under irradiation up to 4 and 38 dpa respectively. Looking at the DF images Fig. 6a, irradiation defects appear in the oxide at the first stages of irradiation. The precipitate/matrix interface seems also damaged, evolving from a spherical shape before irradiation to a less regular shape at 4 dpa. At higher dose, some oxides seem to undergo shape modification under irradiation. The precipitate Fig. 6b is faceted before irradiation but appears modified at 38 dpa, tending to a more isotropic spherical shape. 4. Discussion In this study, we have shown by in situ TEM that the Y–Ti–O nano-precipitates dispersed in Fe18Cr–Y2O3 are apparently stable
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Fig. 4. In situ stability of typical nano-precipitates in Fe18Cr–Y2O3 under irradiation up to 23 dpa. (a) BF TEM image before irradiation and DF TEM image at 23 dpa (b) Processed images, before irradiation and at 23 dpa.
Fig. 5. In situ stability of typical nano-precipitates in Fe18Cr–Y2O3 under irradiation up to 45 dpa. (a) BF TEM image before irradiation and DF TEM image at 45 dpa with processed images framed aside. (b) DF and processed image of another region at 45 dpa.
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Fig. 6. Irradiation-induced modifications of two large oxides in Fe18Cr–Y2O3. (a) DF TEM images before irradiation and at 4 dpa. (b) BF TEM images before irradiation and at 38 dpa.
up to 45 dpa under ion irradiation at 500 °C. However, it appears that larger oxides (>20 nm) are more likely affected by irradiation, some of them showing evidences of interface and shape modifications. These experimental observations are in agreement with the results from Allen et al. [13,14]. By TEM on Fe9Cr–0.36Y2O3 irradiated with 5 MeV Ni ions at 500 °C, they showed that the smallest size particles (5–10 nm) did not seem to change, whereas larger (15–20 nm) were lost at 150 dpa; the range of particles sizes becoming narrower with increasing irradiation dose. This tendency is also consistent with the results from Rogozhkin et al. [26] who studied by atom-probe tomography the stability of the Eurofer ODS under neutron irradiation at 330 °C. They found that bigger yttrium oxide particles (>10 nm) are not stable and partly dissolved at 32 dpa, whereas ultra-fine nanoclusters (1–3 nm) have comparable size before and after irradiation (but modified composition). To better understand the phenomena induced by irradiation, displacement cascades were simulated by SRIM full cascade damage calculations [17] using a single 150 keV Fe+ ion and displacement energies of 57 eV for Y and O (from Rechtin and Wiedersich [27] and Zinkle and Kinoshita [28]). Two situations were considered; first the case of isolated oxide particles and secondly the case of interconnected nano-oxide particles dispersed in Fe. First, in the case of isolated oxides embedded in Fe, different scenarios are possible (Fig. 7). Displacement cascades can create point defects and Frenkel pairs both in the matrix (Fig. 7a) and the single oxide (Fig. 7b). The damage cascades created by irradiation are then likely ejecting atoms from the oxide to the matrix (Fig. 7c) or from the matrix to the oxide (Fig. 7d), what could possibly lead to a dissolution of the phase if radiation-induced recoils dissolution of the precipitate is predominant [29]. However, to describe the reality of the behavior of ODS materials under irradiation; using isolated oxide particles is not relevant. A model dispersoïd-matrix material with interconnected nano-oxides, as illustrated in Fig. 8, would be more representative. Considering such a system, the stability of Y–Ti–O nano-precipitates observed experimentally could be explained in two ways: First, isolated nano-precipitates could be intrinsically stable with ballistically ejected atoms that would back diffuse to rejoin the original particle. Then, the stability could be cooperative. As shown in Fig. 8, the ODS
Fig. 7. Illustrations of displacement cascades for single Y–Ti–O oxides in Fe.
Fig. 8. Illustration of displacement cascades for a typical ODS system with interconnected Y–Ti–O nano-oxides dispersed in Fe.
microstructure is described as a complex system where interchanges and competition between ballistic ejections and radiation induced diffusion could lead to a steady state regime with no apparent evolution of the Y–Ti–O nano-precipitates under irradiation. At the present time, complementary experiments are needed to clarify these points and to go further on the description of the behavior of ODS steels under irradiation: – Additional characterizations of the smallest nano-oxides (few nanometers) are under progress to evaluate if any chemical modification or radiation-induced segregation has been induced in the 45 dpa irradiated materials. – As shown by the depth profiles of displacement damages (Fig. 1), the dpa is highly depth dependant. To assess the effect of ion irradiation on the precipitates, the depth of the oxides has to be
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considered, requiring 3D TEM work. Moreover, it has to be mentioned that large oxides at the surface of the specimen could be affected by preferential sputtering during ion irradiation. – Different behaviors were identified experimentally for the oxides, functions of their size, nature or environment. The authors suggested in another study [20] that Y–Ti–O nano-oxides are likely coherent with the ferritic matrix, whereas larger oxides are more probably incoherent. In this context, further work is engaged to better understand the connections between size, coherency and stability under irradiation and to evaluate if interfaces play a role on the behavior of oxides under irradiation. – More complete ion irradiation conditions (dose and temperature) will be investigated. Different irradiation temperatures would allow us to discern the contribution of cascade mixing and radiation-induced diffusion in the precipitate stability under irradiation. – The use of model materials with low precipitate density is as well considered in order to compare the behavior of isolated phases (large inter-particle distance) with cooperative ones (smaller inter-particle distance).
5. Conclusions In situ ion irradiations have been used to evaluate the stability of Y–Ti–O nano-oxides in a Fe18Cr1WO.3Ti + 0.6Y2O3 ODS steel at 500 °C. Thanks to in situ TEM, it has been possible to follow the same Y–Ti–O nano-precipitates under irradiation up to 45 dpa, what has never been reported so far to our knowledge. Dark Field TEM along with image processing allowed us to image nano-precipitates with acceptable contrast, despite the very fine size of the clusters and the presence of irradiation defects. From these experiments, the microstructure did not appear to evolve consistently under ion irradiation at 500 °C. Y–Ti–O nanoprecipitates as small as 5 nm are apparently stable up to 45 dpa. However, some larger oxides seemed more affected by irradiation, showing evidences of interface and shape modifications.
Acknowledgments This study was made in the frame of a tripartite agreement between the CEA, AREVA NP and EDF and thanks to the funding of the EMIR network for the in situ TEM experiment. The authors would like to thank the CSNSM-JANNuS Orsay facility.
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Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
In situ TEM study of the stability of nano-oxides in ODS steels under ion-irradiation M-L. Lescoat a,⇑, J. Ribis a, A. Gentils b, O. Kaïtasov b, Y. de Carlan a, A. Legris c a
CEA, DEN, Service de Recherches Métallurgiques Appliquées, 91191 Gif-Sur-Yvette, France CSNSM, CNRS/IN2P3, Univ Paris-Sud, Bât. 108, 91405 Orsay Cedex, France c UMET, CNRS/UMR 8207, Bât. C6, Univ Lille 1, 59655 Villeneuve d’Ascq, France b
a r t i c l e
i n f o
Article history: Available online 28 December 2011
a b s t r a c t Oxide Dispersion Strengthened (ODS) ferritic–martensitic steels are considered for nuclear applications as structural components for fusion or fission reactors. To ensure good performances in service, the stability under irradiation of the microstructure and especially of Y–Ti–O nanoclusters have to be assessed. In situ Transmission Electron Microscopy has been performed to follow the Y–Ti–O nano-oxides dispersed in a Fe18Cr1W0.3Ti + 0.6Y2O3 ODS material under ion-irradiation at 500 °C. Microstructural examinations using bright and dark field mode showed that Y–Ti–O nano-precipitates (5 nm) are still present after irradiation up to 45 dpa. However, some larger oxides seem to be more affected by irradiation at 45 dpa (creation of point defects, interface and shape modification). Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to their excellent mechanical properties, Oxide Dispersion Strengthened (ODS) ferritic/martensitic steels are promising candidates for fuel cladding in sodium cooled fast reactors and for structural applications in fusion reactors. The dispersion of nano-sized oxides associated with the ferritic matrix confers very good creep strength at high temperature and resistance to radiation swelling at high dose [1–3]. In reactor, these materials will be subject to drastic irradiation and temperature conditions, namely more than 150 dpa and 400 to 650 °C for sodium fast reactors. To ensure good performance in service, the microstructure of such ODS steels have to remain stable under irradiation at high temperature. Energetic ions can be used to investigate the effects of neutron irradiation in reactor components. Although the damage state depends on the particle type and damage rate, simulations with ion irradiations yield answer on basic mechanisms with the considerable advantage to enable easy variations of irradiation parameters without residual activity of the samples. The historical survey of the studies on ODS steels reported in the literature points out that there is no common conclusion regarding the stability of oxide particles under ion-irradiation. Some studies have shown that Y–Ti–O nano-clusters are stable under low damage dose ion irradiation (0.7, 1, 5, 10 dpa) at 300 °C [4] and 500 °C [5]. No modification of size, shape or chemical composition was detected at 20 dpa at 200 °C, 500 °C, 700 °C [6], 60 dpa, 650 °C [7] or 150 dpa, 670 °C [8]. But other studies highlighted an
instability of nano-clusters under irradiation. Indeed, some observed a decrease of the average size of nano-clusters after ion irradiation; even for doses as low as 1 dpa, 525 °C [9] and 1.4 dpa, 443 °C [10]. Likewise, oxide particles were found to dissolve slightly at 20 dpa, 380 °C [11] and to be amorphous with modified shapes at 33 dpa, 400 °C [12]. Oxide size modifications were found to be strongly temperature dependant [13,14] and decrease as their number density increase under heavy ion irradiation up to 150 dpa at 600 °C and 700 °C. To us, the matters with such investigations is first that nanoclusters are hardly characterized after irradiation because of their very fine size (close to the resolution limit of conventional techniques) and because of irradiation defects. Then, direct comparison of average mean sizes or number densities before and after irradiation could possibly raise questionable conclusions, since the nanoclusters distribution varies from areas to areas in such materials. In this context, in situ TEM studies seem to be of the most interest to assess the changes in the dispersoïds population induced by irradiation. However, very few in situ studies of the stability of ODS steel under ion irradiations have been performed so far to our knowledge [15,16]. In the present work, in situ Transmission Electron Microscopy (TEM) experiments are carried out to evaluate the stability of the Y–Ti–O nanoclusters of a Fe18Cr1W0.3Ti + 0.6Y2O3 ODS steel under ion irradiation up to 45 dpa at 500 °C.
2. Experimental 2.1. Specimen material
⇑ Corresponding author. Tel.: +33 169081178. E-mail address: [email protected] (M-L. Lescoat). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.12.009
The Fe18Cr1W0.3Ti + 0.6Y2O3 ODS ferritic steel is one of the new alloys developed at CEA/SRMA for nuclear applications [3] (CEA
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reference F20 and below referred to as ‘‘Fe18Cr–Y2O3’’). It has been prepared at the Plansee Society by mechanical alloying of two Aubert & Duval pre-alloyed powders in an attritor under hydrogen. A shear bar was hot extruded at 1100 °C, hot rolled 20% at 650 °C and annealed for 1 h at 1050 °C at the CEA. The chemical composition of the extruded powder is given in Table 1.
2.2. Characterizations of the unirradiated material The unirradiated ODS material was characterized at CEA/SRMA by Transmission Electron Microscopy. TEM disks were punched from 60 lm-thick mechanically polished foils and electro polished using a 10% perchloric acid–90% ethanol solution at 10 °C in a Struers Tenupol thinning device. To facilitate identification of nano-precipitates, extraction replicas were also prepared. The ODS alloy was mechanically polished up to a mirror finish and then electrochemically etched using a 1% tetra methyl ammonium chloride–10% acetyl acetone–methanol solution. After carbon deposition, the carbon layer containing the precipitates was carefully removed using the same procedure as for etching and put in a TEM copper grid. Electron Energy Loss Spectrometry (EELS) and energy filtered TEM (EFTEM) were performed on a JEOL 2100 (LaB6) operating at 200 kV and equipped with a Gatan Imaging Filter (GIF). Both 2-windows (10 eV slits) and 3-windows (30 eV slits) EFTEM methods were used with doubly binned images of 512 512 pixels. High Resolution TEM (HRTEM) and Energy Dispersive Spectrometry (EDS) in Scanning TEM (STEM) mode were performed on a JEOL 2010 (FEG) operating at 200 kV.
2.3. Ion-irradiations and in situ TEM characterizations In situ experiments were performed at the CSNSM-JANNuS Orsay facility [17] equipped with a FEI Tecnai G2 20 TEM working at 200 kV and coupled with the IRMA ion implantor. Irradiations were performed at 500 °C and in a sequenced mode to limit shadowing effects from the GATAN heating double tilt holder. During irradiation, the specimen was tilted of a = 68° to be normal to the ion line. At given doses, irradiation was then stopped and the sample set back normal to the electron beam (a = 0°) to perform TEM observations. 150 keV Fe+ auto-ions were used to reach rapidly high dose without surface contamination that could occur under long exposure to the ion beam at high temperature despite the good vacuum inside the microscope column (better than 107 Torr). The depth profiles of the displacement damage (Fig. 1) were estimated using a full detailed cascade calculation of the SRIM-2008.04 code [18] in a Fe matrix (displacement energy of 40 eV). The damage is not homogeneous within the specimen and the dose in displacement per atom (dpa) is hereafter taken as the maximum dose reached at the Bragg peak located at 30 nm from the irradiation surface. From SRIM calculations of the ion range distribution, the concentration of additional injected ions in the matrix is around 2 1021 ions cm3 (i.e. 2%) at the Bragg peak and for the highest dose investigated (1.2 1016 at cm2). The chemical changes locally induced by implantation will thus be neglected in this paper. The irradiation conditions investigated are given in Table 2.
Table 1 Chemical composition (wt%) of the ODS material after mechanical alloying (powder). Fe
Cr
W
Ti
Mn
Si
Ni
C
N
O
Y203
wt%
17.75
0.95
0.26
0.31
0.30
0.19
0.03
0.02
0.11
0.56
Fig. 1. Depth profiles of displacement damages in Fe calculated by SRIM.
3. Results 3.1. Characterization of the unirradiated material The microstructure is made of very fine grains elongated along the hot extrusion direction (about 1 lm long and 300 nm large) and equiaxed in the transverse direction (300 nm) [19]. The micrograph of Fig. 2a shows the precipitation in the unirradiated material. The precipitates are dispersed throughout the grains with an average number density over 1022 m3. The particle sizes (diameters) range from few nanometers (Fig. 2b) to 50 nm; 80% of the precipitates being smaller than 10 nm. The histogram of size distributions (Fig. 2c) was compiled by measurements on 1040 nano-precipitates (<10 nm) from BF TEM micrographs and shows that the distribution of nano-clusters sizes is centered on 3 nm. Whereas some nano-precipitates are hardly identified in bright field TEM due to the low contrast between matrix and precipitates, EFTEM 2-windows analyses on Fe–M give a clear view of precipitates distribution over large areas [9]. Comparison of Fe jump ratio images (Fig. 2d) acquired in two different regions of the unirradiated material shows that the size distribution of nano-precipitates (Fe depleted zone) differs from area to area, pointing out the interest of in situ TEM experiments to evaluate the modification induced in the dispersion by irradiation. The nano-precipitates were finely characterized using TEM on extraction replica (Fig. 3). EFTEM elemental maps (Fig. 3a) show that nano-precipitates correlate with O and Ti enriched areas. Yttrium being hardly imaged by EFTEM, an EELS spectrum was acquired in this region (Fig. 3b). The signal shows two edges at 2080 eV and 2155 eV, related to Y–L3,2 ionization transitions. Ti (Ka,b) and Y (Ka,b) peaks obtained with EDX analyses (Fig. 3c) also confirm that typical nano-precipitates are Y–Ti–O clusters. The crystallographic structure of Y–Ti–O nano-precipitates was investigated using HRTEM. Fig. 3d shows the HRTEM micrograph from a typical nano-precipitate. Evaluation of the HRTEM image by means of fast Fourier transformation (Fig. 3e) indicates that interplanar distances and angle between the systems of plane are consistent 2Þ atomic planes of Y2Ti2O7 (pyrochlore, with the (0 0 4) and ð2 2 a = 10.09 Å) in [1 1 0] zone axis; with d222 = 2.91 Å, d004 = 2.52 Å 2Þ and (0 0 4) planes. and 54.7° between ð2 2 Results from EFTEM, EELS and EDS combined with HRTEM analysis point out that most of the nano-precipitates have Y2Ti2O7 composition. This conclusion is consistent with other studies by
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Table 2 Summary of the ion irradiation conditions investigated. Temperature (°C)
Energy (keV)
Flux (at cm2 s1)
Fluence (at cm2)
Maximum dose (dpa)
500 500 500 500
150 150 150 150
2.85 1012 3.75 1012 2.85 1012 3.75 1012
1.0 1015 6.0 1015 1.0 1016 1.2 1016
4 23 38 45
TEM on Fe14Cr–Y2O3 ODS alloys [20] or by lattice Monte Carlo simulations [21] which indicate that nano-sized precipitates (2–5 nm) are likely to be Y–Ti–O oxides. Similarly, it is in agreement with small angle neutron scattering [22] and atom probe tomography [23] performed on various nanostructured ferritic alloys which
suggest that the smallest clusters are Y–Ti–O transition oxides with chemical composition close to Y2Ti2O7 [24]. Synchrotron radiation X-ray diffraction and absorption experiments performed on the unirradiated Fe18Cr–Y2O3 material [25] show that additional secondary phases coexist with these Y2Ti2O7 nano-precipitates, namely Y2O3 (BCC, a = 10.63 Å and FCC, a = 5.30 Å), TiO2 (Orthorhombic, a = 4.90 Å, b = 9.46 Å, c = 2.96 Å) and Cr23C6 (FCC, a = 10.61 Å). These phases are likely to be the very large precipitates (>50 nm) that are not considered in this paper.
3.2. In situ ion irradiations After irradiation at 500 °C, the diffraction contrast from defects and bend contours drastically harden the imaging of nano-precipi-
Fig. 2. Microstructure of the unirradiated Fe18Cr–Y2O3. (a) TEM image of the dispersion. (b) High magnification TEM image of a typical nano-precipitate. (c) Particle size (diameter) distribution. (d) Fe–M jump-ratio images (2-windows) from two different areas.
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Fig. 3. Characterizations of typical nano-precipitates on extraction replica from unirradiated Fe18Cr–Y2O3. (a) 3-windows EFTEM elemental maps. (b) EELS spectrum on the same region. (c). EDX spectrum on the nano-precipitate showed in the STEM image. (d) High-resolution TEM image. (e) FFT image.
tates in Bright Field (BF) TEM. However, TEM in Dark Field (DF) mode was found effective to improve the contrast from nanoclusters and was thus used to follow Y–Ti–O nano-precipitates under irradiation. The micrographs obtained before irradiation (BF) and at 23 dpa (DF) are given Fig. 4a. The 1–4 numbered large oxides prove that the same region is followed under irradiation. After irradiation, clusters are still observable in DF but image processing was performed to optimize the precipitate–matrix contrast. The processed images are given Fig. 4b. The processed image at 23 dpa clearly shows nano-clusters that coincide with the 5–6 nm Y–Ti–O circled before irradiation, thus indicating that Y–Ti–O nano-precipitates are apparently stable up to 23 dpa at 500 °C. As images to be compared before and after irradiation are either BF or DF micrographs, determination of size evolution by direct measurements is not relevant here. Looking at higher doses (up to 45 dpa), it has also been possible to follow Y–Ti–O nano-precipitates under irradiation by in situ TEM. Fig. 5a shows the same region of Fe18Cr–Y2O3 before irradiation (BF TEM) and at 45 dpa (DF TEM). Similarly, image processing was performed to enhance the contrast from nano-precipitates. The processed images are framed aside TEM micrographs. Direct comparisons of processed image indicate that 4–6 nm Y–Ti–O are still present at 45 dpa (see circled clusters). Fig. 5b shows the DF
image of a larger region of the Fe18Cr–Y2O3 irradiated at 45 dpa. On this DF image, numerous clusters are detectable and precipitates as small as 4 nm are clearly highlighted in the processed image close by (see circled nanoclusters). These experiments thus confirm that Y–Ti–O nano-precipitates are apparently stable under irradiation up to 45 dpa at 500 °C. However, beside these Y–Ti–O nano-precipitates, some other large oxides (>20 nm) seem to undergo specific behaviors that differ from oxide to oxide, probably depending of their nature, size and local environment. Fig. 6a and b illustrates the behavior of two large oxides under irradiation up to 4 and 38 dpa respectively. Looking at the DF images Fig. 6a, irradiation defects appear in the oxide at the first stages of irradiation. The precipitate/matrix interface seems also damaged, evolving from a spherical shape before irradiation to a less regular shape at 4 dpa. At higher dose, some oxides seem to undergo shape modification under irradiation. The precipitate Fig. 6b is faceted before irradiation but appears modified at 38 dpa, tending to a more isotropic spherical shape. 4. Discussion In this study, we have shown by in situ TEM that the Y–Ti–O nano-precipitates dispersed in Fe18Cr–Y2O3 are apparently stable
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Fig. 4. In situ stability of typical nano-precipitates in Fe18Cr–Y2O3 under irradiation up to 23 dpa. (a) BF TEM image before irradiation and DF TEM image at 23 dpa (b) Processed images, before irradiation and at 23 dpa.
Fig. 5. In situ stability of typical nano-precipitates in Fe18Cr–Y2O3 under irradiation up to 45 dpa. (a) BF TEM image before irradiation and DF TEM image at 45 dpa with processed images framed aside. (b) DF and processed image of another region at 45 dpa.
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Fig. 6. Irradiation-induced modifications of two large oxides in Fe18Cr–Y2O3. (a) DF TEM images before irradiation and at 4 dpa. (b) BF TEM images before irradiation and at 38 dpa.
up to 45 dpa under ion irradiation at 500 °C. However, it appears that larger oxides (>20 nm) are more likely affected by irradiation, some of them showing evidences of interface and shape modifications. These experimental observations are in agreement with the results from Allen et al. [13,14]. By TEM on Fe9Cr–0.36Y2O3 irradiated with 5 MeV Ni ions at 500 °C, they showed that the smallest size particles (5–10 nm) did not seem to change, whereas larger (15–20 nm) were lost at 150 dpa; the range of particles sizes becoming narrower with increasing irradiation dose. This tendency is also consistent with the results from Rogozhkin et al. [26] who studied by atom-probe tomography the stability of the Eurofer ODS under neutron irradiation at 330 °C. They found that bigger yttrium oxide particles (>10 nm) are not stable and partly dissolved at 32 dpa, whereas ultra-fine nanoclusters (1–3 nm) have comparable size before and after irradiation (but modified composition). To better understand the phenomena induced by irradiation, displacement cascades were simulated by SRIM full cascade damage calculations [17] using a single 150 keV Fe+ ion and displacement energies of 57 eV for Y and O (from Rechtin and Wiedersich [27] and Zinkle and Kinoshita [28]). Two situations were considered; first the case of isolated oxide particles and secondly the case of interconnected nano-oxide particles dispersed in Fe. First, in the case of isolated oxides embedded in Fe, different scenarios are possible (Fig. 7). Displacement cascades can create point defects and Frenkel pairs both in the matrix (Fig. 7a) and the single oxide (Fig. 7b). The damage cascades created by irradiation are then likely ejecting atoms from the oxide to the matrix (Fig. 7c) or from the matrix to the oxide (Fig. 7d), what could possibly lead to a dissolution of the phase if radiation-induced recoils dissolution of the precipitate is predominant [29]. However, to describe the reality of the behavior of ODS materials under irradiation; using isolated oxide particles is not relevant. A model dispersoïd-matrix material with interconnected nano-oxides, as illustrated in Fig. 8, would be more representative. Considering such a system, the stability of Y–Ti–O nano-precipitates observed experimentally could be explained in two ways: First, isolated nano-precipitates could be intrinsically stable with ballistically ejected atoms that would back diffuse to rejoin the original particle. Then, the stability could be cooperative. As shown in Fig. 8, the ODS
Fig. 7. Illustrations of displacement cascades for single Y–Ti–O oxides in Fe.
Fig. 8. Illustration of displacement cascades for a typical ODS system with interconnected Y–Ti–O nano-oxides dispersed in Fe.
microstructure is described as a complex system where interchanges and competition between ballistic ejections and radiation induced diffusion could lead to a steady state regime with no apparent evolution of the Y–Ti–O nano-precipitates under irradiation. At the present time, complementary experiments are needed to clarify these points and to go further on the description of the behavior of ODS steels under irradiation: – Additional characterizations of the smallest nano-oxides (few nanometers) are under progress to evaluate if any chemical modification or radiation-induced segregation has been induced in the 45 dpa irradiated materials. – As shown by the depth profiles of displacement damages (Fig. 1), the dpa is highly depth dependant. To assess the effect of ion irradiation on the precipitates, the depth of the oxides has to be
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considered, requiring 3D TEM work. Moreover, it has to be mentioned that large oxides at the surface of the specimen could be affected by preferential sputtering during ion irradiation. – Different behaviors were identified experimentally for the oxides, functions of their size, nature or environment. The authors suggested in another study [20] that Y–Ti–O nano-oxides are likely coherent with the ferritic matrix, whereas larger oxides are more probably incoherent. In this context, further work is engaged to better understand the connections between size, coherency and stability under irradiation and to evaluate if interfaces play a role on the behavior of oxides under irradiation. – More complete ion irradiation conditions (dose and temperature) will be investigated. Different irradiation temperatures would allow us to discern the contribution of cascade mixing and radiation-induced diffusion in the precipitate stability under irradiation. – The use of model materials with low precipitate density is as well considered in order to compare the behavior of isolated phases (large inter-particle distance) with cooperative ones (smaller inter-particle distance).
5. Conclusions In situ ion irradiations have been used to evaluate the stability of Y–Ti–O nano-oxides in a Fe18Cr1WO.3Ti + 0.6Y2O3 ODS steel at 500 °C. Thanks to in situ TEM, it has been possible to follow the same Y–Ti–O nano-precipitates under irradiation up to 45 dpa, what has never been reported so far to our knowledge. Dark Field TEM along with image processing allowed us to image nano-precipitates with acceptable contrast, despite the very fine size of the clusters and the presence of irradiation defects. From these experiments, the microstructure did not appear to evolve consistently under ion irradiation at 500 °C. Y–Ti–O nanoprecipitates as small as 5 nm are apparently stable up to 45 dpa. However, some larger oxides seemed more affected by irradiation, showing evidences of interface and shape modifications.
Acknowledgments This study was made in the frame of a tripartite agreement between the CEA, AREVA NP and EDF and thanks to the funding of the EMIR network for the in situ TEM experiment. The authors would like to thank the CSNSM-JANNuS Orsay facility.
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