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Structure of complex oxide nanoparticles in a Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS RAF steel
Journal of Nuclear Materials 442 (2013) S158–S163
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Structure of complex oxide nanoparticles in a Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS RAF steel P. Unifantowicz a,⇑, T. Płocin´ski b, C.A. Williams c, R. Schäublin a, N. Baluc a a
Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, 5232 Villigen PSI, Switzerland Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Wołoska Street, 02-507 Warsaw, Poland c University of Oxford, Department of Materials, Parks Road OX1 3PH, UK b
a r t i c l e
i n f o
Article history: Available online 24 April 2013
a b s t r a c t One of the most crucial steps in the development of oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steels is the engineering of their microstructure, which includes control of the type and size of oxide nanoparticles. In this work, the composition and crystal structure of oxide particles grown in the Fe–14Cr–2W–0.3Ti– 0.3Y2O3 ODS RAF steel were characterized using advanced spectroscopic and microscopic techniques. The electron energy loss spectroscopic mapping has shown presence of numerous fine Y–Ti–O oxides but also larger Cr–Ti–O and Cr–N particles among those extracted from the bulk samples. In addition, atom probe tomography of the as-compacted ODS RAF samples revealed a uniform spatial distribution of fine oxides containing mainly Y, Ti, and O. The orthorhombic YTiO3, having distorted perovskite structure, was identified in all analyzed oxides using HR-STEM and diffraction pattern analysis. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel is one of the most promising candidate materials for structural components of the future nuclear fusion reactor because of its good radiation resistance, thermal conductivity, higher strength, and higher operation temperature relative to the base steel [1,2]. A key factor in the development of ODS RAF steel is the control and engineering of its microstructure to meet high demands with respect to the operating temperature and neutrons and ions irradiation dose. The microstructure of the RAF ODS steels should comprise submicron-sized a-Fe grains with a uniform dispersion of thermally stable oxide nanoparticles. These steels are produced by powder metallurgy, which includes preparation of powders by alloying of the elements constituting steel matrix and mixing them with Y2O3 or other Y-containing particles, subsequent consolidation of the powders and heat treatment. Properly tailored manufacturing parameters should allow designing the microstructure to improve mechanical properties, particularly ductility. The microstructure control includes monitoring oxide nanoparticles type, size and number density and uniformity of distribution but also preventing formation of large chromium oxi-
⇑ Corresponding author. Tel.: +41 76 756 3807. E-mail address: [email protected] (P. Unifantowicz). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.048
des and carbides which can decrease the ferrite stabilizing and anti-corrosive effect of Cr in the a-Fe matrix. Numerous studies indicate that the Y2O3 particles lattice does not stay intact after mechanical alloying realized by means of high energy ball-milling. Various oxides nano-precipitates containing mainly Y and Ti [3–7] but also Y and Cr [8] or Y and Al [9] were reported. According to recent studies, the oxides nanoparticles build shells containing solute elements including V and Cr (in the case of Eurofer-ODS [10]) or Cr (in the case of 14Cr model alloy [11]). The most desirable from the perspective of mechanical behavior are fine Y–Ti–O oxides particles. There are five types of Y–Ti–O oxides identified in RAF ODS steels: (1) Y2Ti2O7 with pyrochlore structure in the case of particles measuring above about 15 nm [12], (2) nonstoichiometric oxides in the case of smaller particles [7], (3) Y2TiO5 with orthorhombic lattice in 12Cr ODS [13,14], (4) a so called pyroortho phase being a mixture of Y2O3 and pyrochlore phases, [14] and (5) YTiO3 as indicated by small angle neutron spectroscopy [15] and transmission electron microscopy [16]. However, the presence of the fifth type in ODS RAF steel has not been fully proven. In this work a combination of advanced spectroscopic and microscopic tools such as EELS, atom probe tomography (APT) and HR-STEM were employed to identify the composition and crystal structure of the oxide nanoparticles in a Fe–14Cr–2W– 0.3Ti–0.3Y2O3 ODS RAF steel. The outcome of these investigations brings new insight into the characteristics and genesis of formation of the complex oxides in this type of steel.
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P. Unifantowicz et al. / Journal of Nuclear Materials 442 (2013) S158–S163 Table 1 Concentrations of elements in the bulk ODS RAF steel. Element
Fe
Cr
W
Co
Ti
Y
Mn
Ni
Si
Mo
Cu
N
O
C
at.%
Bal.
13.8
1.65
0.65
0.30
0.13
0.03
0.01
0.10
0.01
0.01
0.4
0.3
0.1
2. Materials and methods The ODS RAF steel ingot fabrication involved high-energy ball milling of Fe, Cr, W, Ti, and Y2O3 powders with purities of: 99.5, 99, 99.9, 99.9, and 99.99 wt.%, respectively. The powders were added in a weight ratio corresponding to nominal composition: Fe–14Cr–2W–0.3Ti–0.3Y2O3. Mechanical alloying of the initial powders was done in a RetschÒ planetary ball mill using effective rotational bowl speed of 300 rpm, ball to powder weight ratio of 10:1, total time of 40 h, and H2 atmosphere. The powder was encapsulated in a soft steel can and degassed for 2 h in a vacuum at 1073 K and then consolidated by hot isostatic press at a maximum pressure of 200 MPa and a temperature of 1423 K. The inductively coupled plasma and LecoÒ techniques were used to determine the concentrations of elements in the bulk sample. The composition of the consolidated steel is given in Table 1. The nominal composition of ODS RAF steel investigated in this work was selected based on the previously optimized composition of the ODS steel produced by basically the same route at CRPP [17,18]. The standard TEM disc was prepared from the consolidated steel for general microstructure. The procedure consisted of polishing discs of ODS RAF with 1-mm diameter embedded in a 3 mm austenite steel ring, mechanical polishing of the discs to about 50 lm thickness, and then electro-polishing using voltage of 20.5 V and 7%HCl04/93%C2H5OH solution cooled to about 243 K. The use of a 1 mm sample allowed to minimize the ferromagnetic effect of the sample on the electron beam [19]. The APT specimens were prepared using standard electropolishing methods and a 25% HCl/75%CH3COOH solution. APT analysis was carried out using an Imago LEAP 3000 HR instrument in laser pulsing analysis mode with a laser energy of 0.3 nJ, 8 lm beam spot size, and frequency of 200 kHz. The size distribution of the particles was found using the maximum separation method [20] based on Y and Ti containing ions using a maximum separation distance of 1.3 nm. To enable a straightforward interpretation of HR-STEM images, reinforcement particles were separated from the steel matrix by their extraction on amorphous carbon film. This method helped resolve the lattice of nanoparticles, which is not a trivial task in the case of standard bulk samples. In the case of standard TEM discs, the fine particles give a relatively weak diffraction signal compared with the signal from the matrix. The method for particle extraction involved mirror polishing of the bulk steel sample followed by short electrolytic polishing of the surface and deposition of carbon by arc sputtering. The C layers were detached by electrolytic etching in the solution used for TEM discs and collected on a Cu grid. Images for oxide particle lattice imaging were taken using a Cscorrected scanning TEM Hitachi HD2700 operating at 200 keV equipped with EELS spectrometer. Its advantage is a good sensitivity for light elements in comparison with EDS and relatively fast mapping, enabling compositional checks of the extracted particles. Typical magnification was 300,000 and the size of spectrum images was of 64 60 pixels 1340 eV with a spectrum dispersion of 0.1 and 0.5 eV in case of low loss (5/145 eV) and core loss (310–980 eV) energy ranges, respectively. The following characteristic EEL edges were used: Y N2,3 and Ti M4,5 in low loss with maxima at about 36 and 47 eV, respectively, and also N K, O K, and Cr
L2,3 in core loss energy range with maxima at about 401, 532, and 575 eV, respectively. To identify the crystal structure of the oxides, fast Fourier transformation (FFT) was applied to atomic HR-STEM images. The characteristic distances in reciprocal space were measured together with their ratios and angles between the vectors. Then, the inverse Fourier transformation was applied to filter out the noise from the lattice image. The obtained patterns were compared with the geometries of standard electron diffraction patterns for various Y–Ti–O and Y–O oxides obtained from a crystallographic database [21]. The electron diffraction patterns matching the vectors geometry in FFT images were simulated in jEMSÒ software [22]. 3. Results 3.1. General microstructure The microstructure of bulk ODS RAF steel sample, following HIP is shown in Fig. 1a. Fine grains have a high density of dislocations due to stresses related to austenite to ferrite transformation during cooling after HIP. The average crystallite size found from peak broadening of an XRD spectrum is about 250 nm. Some larger particles were also observed. A higher magnification bright field TEM image in Fig. 1b shows numerous very fine precipitates about 5 nm in diameter. In addition, less numerous larger particles were found. 3.2. Particles composition 3.2.1. APT APT was employed to characterize the composition and spatial distribution of the smallest nanoparticles in the ODS RAF steel. The average radius of gyration of the nanoparticles was 2.5 ± 0.1 nm and the number density about 4 ± 0.5 1023 m 3. This density was in agreement with that obtained by of dark field TEM images analyses in steel samples prepared in similar way [16]. In addition, larger oxides >10 nm radius were occasionally seen in the analysis volume. Some larger oxides had the same core composition as the small (2 nm) nanoparticles, whereas others were also enriched in N. The graph in Fig. 2b shows a proximity histogram indicating the distribution of solute elements across a 3% Y + Ti iso-concentration surface, including all nanoparticles in the analyzed volume. Along with Y, Ti, and O, the nanoparticles were enriched in Cr and appreciable levels of Fe. Although iron was clearly detected in the particles core, it was considered to be an artifact of analysis, similar like in APT analyses in [23], therefore it was not shown in the histogram. From Fig. 2b the ratio of Y, Ti, and O at the core of the particles is approximately Y:Ti1.75:O3.5, which does not correspond to any known Y–Ti–O oxide. 3.2.2. EELS and HR-STEM To bring about oxides particles crystal structure, the reinforcing particles extracted on a carbon film were investigated by EELS and HR-STEM. The extraction allowed analysis of particles in a broader size range because the particles were collected from a surface of about 1 mm2, thus corresponding to a much larger volume in comparison with the volume of the APT sample. In addition, the
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Fig. 1. BF TEM images showing: (a) typical microstructure of Fe–14Cr–2W–0.3Ti–0.3Y2O3 sample and (b) nanoprecipitates in the steel matrix.
Fig. 2. (a) APT maps of Y, Ti and O distributions in the analyzed volume and (b) proximity histogram showing the distribution of solute elements across a 3% Y + Ti isoconcentration surface, including all the nanoparticles in the analyzed volume.
Fig. 3. STEM Z-contrast image of particles extracted on carbon foil with EELS maps corresponding to the present elements.
P. Unifantowicz et al. / Journal of Nuclear Materials 442 (2013) S158–S163
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Fig. 4. (a, d, g) High resolution STEM images with images obtained by fast Fourier transformation in the inclusions; (b, e, h) inverse FFT images with marked planes indices and (c, f, i) simulated electron diffraction patterns for YTiO3 particles in orientations: [1 3 5], [4 3 5] and [3 2 2] respectively.
extraction method allowed the avoidance of the strong signal from the steel matrix, which keeps the crystal lattice from resolving particles as fine as several nanometers in diameter. Only very few
small Y–Ti–O nanoparticles were observed in the extracted foil, although a high density of Y–Ti–O nano-particles was evident from the bright field TEM imaging and APT proxigrams of the bulk sam-
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ples. This is most likely due to their removal during the extraction, possibly caused by weaker electrostatic forces holding particles on the carbon film. The dark field HR-STEM image in Fig. 3 shows a typical agglomeration of particles found on the carbon film. The EELS maps, shown in the same figure, were obtained using characteristic EEL edges for light (O, N, C) and metallic elements, which were previously confirmed by EDS mapping. A series of maps in various regions of the foil showed the presence of other particles besides the Y–Ti–O, including Cr–Ti–O and Cr–N, which accounted for 13% and 7% of the particles, respectively. The mean diameter of the extracted Y–Ti–O particle was about 20 nm, which is significantly larger than those found in TEM BF images and in APT maps. In addition, chromium nitrides, usually several 100 nm in diameter, were found. The presence of chromium nitrides indicates air intake during powder milling or handling. 3.3. Crystal structure of the extracted oxide particles EELS mapping was done to cross-check the composition of selected areas of interest containing appreciable number of extracted particles. High resolution STEM images of several oxide nanoparticles of Y–Ti–O type are shown in Fig. 4a, d and g. The patterns obtained by applying FFT to HR-STEM bright field images of the Y–Ti–O nanoparticles are shown in Fig. 4a, d and g. Pairs of vectors were drawn in reciprocal space corresponding to characteristic distances between lattice planes to find the pattern of the searched oxide among all possible diffraction patterns of the known candidates, including c-Y2Ti2O7, o-Y2TiO5, o-YTiO3, and c-Y2O3. The lines and indices corresponding to lattice planes in asfiltered inverse FFT images are shown in Fig. 4b, e and h. The FFT patterns of the three oxide particles were in a very good agreement with the modeled electron diffraction patterns of the orthorhombic YTiO3 oxide, which are shown in Fig. 4c, f and i. The patterns corresponded to [1 3 5], [4 3 5] and [3 2 2] zone axis orientations, respectively. The measured and nominal values for the given lattice orientations for the three particles are summarized in Table 2. The measured d1, d2, and d3 values correspond to the distances in real space obtained by analysis of FT images and the nominal values correspond to the vectors marked in yellow in the modeled diffraction patterns. The a values are the angles between g vectors with absolute values equal to the respective values of 1/d. As a result of the geometrical analyses, all three particles were identified as the orthorhombic YTiO3 oxide with the following cell parameters: a = 5.338, b = 7.613 and c = 5.690 Å [24]. The maximum standard deviations of the measured d-distances and angles with respect to nominal values were in range from 2.4% to 3.9% and 0.6% to 2.8%, respectively, which indicates a good accuracy of scale. On the other hand, these differences could arise from slightly nonstandard unit cell volume.
In the case of [3 2 2] orientation, similar lengths of d vectors were found as for the Y2TiO5 oxide in [2 7 1] orientation, however its simulated pattern geometry was significantly different than that obtained using FFT. The orthorhombic YTiO3 oxide lattice has Pnma space group and can be described as a distorted perovskite, in which Y3+ cations are surrounded by mutually tilted octahedra made up of Ti3+ and O2 ions [25]. Its prototype is SrTiO3 with cubic perovskite structure. 4. Discussion The lack of chromium in the smallest extracted Y–Ti–O oxides (shown in EELS maps) and its presence in oxides analyzed by APT may be misleading. To explain this discrepancy, one can consider the following. In previous EELS analyses of a steel with the same nominal composition, prepared essentially the same way [26] Y and Ti were found in shells around the Cr–Ti–O precipitates but no Cr shells could be evidenced. This study supports the opinion that Cr presence inside the particles in this work may be an artifact of APT. On the other hand, the shells could have been removed with the steel matrix during electrolytic etching. In the mentioned work on EELS analysis [26] Cr was not identified in Y–Ti–O oxides particles, in ODS RAF steel by using EFTEM. Thus, although the APT and EELS results presented here could not be directly compared, previous results indicate that Y–Ti–O particles with diameters between several and several tens of nanometers are likely the same oxide. The difference in elements ratio between the APT and TEM observations may arise from different particles or existing artifacts of both techniques leading to different metal-to-oxygen ratios. The identification of oxide lattice as orthorhombic YTiO3 is supported by its unique crystal lattice, which has smaller number of symmetry translations compared with the pyrochlore oxides and, thus, a smaller number of symmetry translations, making it distinguishable from the other Y–Ti–O oxides. Previously Alinger [15] who used SANS to identify oxides in ODS steel indicated growth of YTiO3 and suggested presence of nonequilibrium oxides. One possible reason for the occurrence of this rare oxide is that bulk YTiO3 formation occurs in a reducing atmosphere while bulk Y2Ti2O7 pyrochlore is more stable at higher oxygen pressures (as indicated in [27]). Thus, one should take into account the influence of milling atmosphere, considering that H2 can change the local chemistry of interfaces created during mechanical alloying. This may cause Ti ions to change the oxidation state from +4 in TiO2 considering it a Ti donor to +3 and leading to formation of the YTiO3 phase. Another possible reason for the formation of YTiO3 is its crystallographic relation towards the Fe-based matrix, considering that half of the b parameter of the oxide: 2.84 Å corresponds approximately to the lattice constant of a-Fe of 2.87 Å, which could result in a semi-coherent crystallographic relationship between the oxide and Fe matrix, as indicated in [28]. A characteristic feature of the perovskite structure is that it
Table 2 Results of quantitative analysis of the HR-STEM images and standard electron diffraction patterns matching the orthorhombic YTiO3 lattice. Crystal zone axis Planes indices Distances – d (Å) Measured Nominal St. dev. (%) Angles – a (°) Measured Nominal St. dev. (%)
[1 3 5]
[4 3 5]
1,2, 1
2, 1,1
3,1,0
2,1,1
d1 2.762 2.717 1.1 a(g1 g3) 43.3 41.8 2.5
d2 2.519 2.379 3.9 a(g2 g3) 50.3 52.4 2.8
d3 1.848 1.783 2.5 a(g1 g2) 93.6 94.2 0.4
d1 2.418 2.379 1.1 a(g1 g3) 61.5 62 0.6
[3 3 2] 1, 3,1 d2 2.190 2.123 2.4 a(g2 g3) 51.5 51.5 0
1, 2,2 d3 2.037 2.034 0.1 a(g1 g2) 113 113.5 0.3
1,1,0 d1 3.906 3.880 0.4 a(g1 g3) 63.8 63.5 0.3
1,1, 3 d2 2.053 2.123 2.4 a(g2 g3) 27.3 28.4 2.7
0,2, 3 d3 1.854 1.892 1.5 a(g1 g2) 91.1 91.9 0.6
P. Unifantowicz et al. / Journal of Nuclear Materials 442 (2013) S158–S163
allows flexibility of tilts and rotations of the TiO6 octahedra and therefore an extensive creation of vacancies or substitutional atoms is possible [29]. This flexibility of the oxide lattice can be of advantage considering enhanced helium trapping, which is one of the functions of the reinforcement nanoparticles in ODS RAF. 5. Conclusions The oxide nanoparticles reinforcing the Fe–14Cr–2W–0.3Ti– 0.3Y2O3 ODS RAF steel were characterized in terms of composition and crystal structure. TEM BF imaging revealed numerous 5 nm particles and also larger ones. APT analysis showed that the nanoparticles are made up of Y, Ti, and O. Numerous larger, (20 nm) Y–Ti–O particles and a considerable amount of much larger Cr–N and Cr–Ti–O particles were found among the extracted particles as indicated by EELS mapping. HRSTEM and quantitative analysis of images obtained by Fourier transformation for 15 nm and larger particles allowed identification of the YTiO3 oxide with orthorhombic lattice, representing a distorted perovskite structure. Acknowledgements The Paul Scherrer Institute is acknowledged for the overall use of its facilities. This work, supported by the European Communities under the contract of Association between EURATOM/Confédération Suisse, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was also performed within the framework of the Integrated European Project ‘‘ExtreMat‘‘ (contract NMP-CT2004-500253) with financial support by the European Community. It only reflects the view of the authors and the European Community is not liable for any use of the information contained therein. Financial support was also provided by the European project ‘‘FEMaS’’ (FP7 Grant Agreement No. 224752).
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References [1] R. McClintock, D.T. Hoelzer, M.A. Sokolov, R.K. Nanstad, J. Nucl. Mater. 386–388 (2009) 307–311. [2] S. Ukai, M. Fujiwara, J. Nucl. Mater. 307–311 (2002) 749–757. [3] T. Okuda, M. Fujiwara, J. Mat. Sci. Lett. 14 (1995) 1600–1603. [4] M. Klimiankou, R. Lindau, A. Möslang, Micron 36 (2005) 1–8. [5] M.K. Miller, K.F. Russell, D.T. Hoelzer, J. Nucl. Mater. 351 (2006) 261. [6] E.A. Marquis, Appl. Phys. Lett. 93 (2008). [7] H. Sakasegawa, L. Chaffron, F. Legendre, L. Boulanger, T. Cozzika, M. Brocq, Y. de Carlan, J. Nucl. Mater. 384 (2009) 115–118. [8] V. de Castro, E.A. Marquis, S. Lozano-Perez, R. Pareja, M.L. Jenkins, Acta Mater. 59 (2011) 3927–3936. [9] L. Hsiung, M.J. Fluss, A. Kimura, Mater. Lett. 64 (2010) 1782–1785. [10] M. Klimenkov, R. Lindau, K. Möslang, J. Nucl. Mater. 386–366 (2009) 553– 556. [11] V. de Castro, T. Leguey, M.A. Auger, S. Lozano-Perez, M.L. Jenkins, J. Nucl. Mater. 417 (2011) 217–220. [12] M. Klimiankou, R. Lindau, K. Möslang, Nucl. Mater. 329–33 (2004) 347–351. [13] S. Yamashita, Ohtsuka, N. Akasaka, S. Ukai, S. Ohnuki, Phil. Mag. Lett. 84 (2004) 525–529. [14] C. Cayron, A. Montani, D. Venet, Y. de Carlan, J. Nucl. Mater. 399 (2010) 219– 224. [15] M.J. Alinger, G.R. Odette, D.T. Hoelzer, Acta Mater. 57 (2009) 392–406. [16] P. Unifantowicz, Z. Oksiuta, P. Olier, Y. de Carlan, N. Baluc, J. Fusion Eng. Des. 86 (2011). 2413–1416. [17] Z. Oksiuta, N. Baluc, Adv. Mater. Res. 59 (2009) 308–312. [18] P. Olier, Z. Oksiuta, J.F. Melat, D. Hamon, T. Leblond, Y. De Carlan, N. Baluc, Adv. Mater. Res. 59 (2009) 313–318. [19] R. Schaeublin, D. Gelles, M. Victoria, J. Nucl. Mater. 307–311 (2002) 197. [20] J. M. Hyde, C.A. English In: G.E. Lucas, M.A. Kirk Jr., R.G. Elliman, Proceedings of the MRS 2000 Fall Meeting, Symposium R, vol. 650, MRS, Boston, 2001 R6.6.1. [21] www.crystallography.net. [22] P.A. Stadelmann, Ultramicroscopy 21 (1987) 131. [23] C.A. Williams, G.D.W. Smith, E.A. Marquis, Ultramicroscopy 125 (2013) 10– 17. [24] J.R. Hester, K. Tomimoto, H. Noma, F.P. Okamura, J. Akimitsu, Acta Cryst. B53 (1997) 739–744. [25] W. Knafo, C. Meingast, A.V. Boris, P. Popovich, N.N. Kovaleva, P. Yordanov, A. Maljuk, R.K. Kremer, Phys. Rev. B 054431 (2009) 5p. [26] P. Unifantowicz, R. Schäublin, C. Hébert, T. Płocin´ski, G. Lucas, N. Baluc, J. Nucl. Mater. 422 (2012) 131–136. [27] X. Huang, H. Zhao, W. Shen, W. Qiu, W. Wu, J. Phys. Chem. Solids 67 (2006) 2609–2613. [28] A. Ramar, N. Baluc, R. Schäublin, J. Nucl. Mater. 386–388 (2009) 515–519. [29] J.B. Goodenough, Struct. Bond, Springer, 2001. ISBN 3-540-67522-1.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Structure of complex oxide nanoparticles in a Fe–14Cr–2W–0.3Ti–0.3Y2O3 ODS RAF steel P. Unifantowicz a,⇑, T. Płocin´ski b, C.A. Williams c, R. Schäublin a, N. Baluc a a
Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, 5232 Villigen PSI, Switzerland Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Wołoska Street, 02-507 Warsaw, Poland c University of Oxford, Department of Materials, Parks Road OX1 3PH, UK b
a r t i c l e
i n f o
Article history: Available online 24 April 2013
a b s t r a c t One of the most crucial steps in the development of oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steels is the engineering of their microstructure, which includes control of the type and size of oxide nanoparticles. In this work, the composition and crystal structure of oxide particles grown in the Fe–14Cr–2W–0.3Ti– 0.3Y2O3 ODS RAF steel were characterized using advanced spectroscopic and microscopic techniques. The electron energy loss spectroscopic mapping has shown presence of numerous fine Y–Ti–O oxides but also larger Cr–Ti–O and Cr–N particles among those extracted from the bulk samples. In addition, atom probe tomography of the as-compacted ODS RAF samples revealed a uniform spatial distribution of fine oxides containing mainly Y, Ti, and O. The orthorhombic YTiO3, having distorted perovskite structure, was identified in all analyzed oxides using HR-STEM and diffraction pattern analysis. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel is one of the most promising candidate materials for structural components of the future nuclear fusion reactor because of its good radiation resistance, thermal conductivity, higher strength, and higher operation temperature relative to the base steel [1,2]. A key factor in the development of ODS RAF steel is the control and engineering of its microstructure to meet high demands with respect to the operating temperature and neutrons and ions irradiation dose. The microstructure of the RAF ODS steels should comprise submicron-sized a-Fe grains with a uniform dispersion of thermally stable oxide nanoparticles. These steels are produced by powder metallurgy, which includes preparation of powders by alloying of the elements constituting steel matrix and mixing them with Y2O3 or other Y-containing particles, subsequent consolidation of the powders and heat treatment. Properly tailored manufacturing parameters should allow designing the microstructure to improve mechanical properties, particularly ductility. The microstructure control includes monitoring oxide nanoparticles type, size and number density and uniformity of distribution but also preventing formation of large chromium oxi-
⇑ Corresponding author. Tel.: +41 76 756 3807. E-mail address: [email protected] (P. Unifantowicz). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.04.048
des and carbides which can decrease the ferrite stabilizing and anti-corrosive effect of Cr in the a-Fe matrix. Numerous studies indicate that the Y2O3 particles lattice does not stay intact after mechanical alloying realized by means of high energy ball-milling. Various oxides nano-precipitates containing mainly Y and Ti [3–7] but also Y and Cr [8] or Y and Al [9] were reported. According to recent studies, the oxides nanoparticles build shells containing solute elements including V and Cr (in the case of Eurofer-ODS [10]) or Cr (in the case of 14Cr model alloy [11]). The most desirable from the perspective of mechanical behavior are fine Y–Ti–O oxides particles. There are five types of Y–Ti–O oxides identified in RAF ODS steels: (1) Y2Ti2O7 with pyrochlore structure in the case of particles measuring above about 15 nm [12], (2) nonstoichiometric oxides in the case of smaller particles [7], (3) Y2TiO5 with orthorhombic lattice in 12Cr ODS [13,14], (4) a so called pyroortho phase being a mixture of Y2O3 and pyrochlore phases, [14] and (5) YTiO3 as indicated by small angle neutron spectroscopy [15] and transmission electron microscopy [16]. However, the presence of the fifth type in ODS RAF steel has not been fully proven. In this work a combination of advanced spectroscopic and microscopic tools such as EELS, atom probe tomography (APT) and HR-STEM were employed to identify the composition and crystal structure of the oxide nanoparticles in a Fe–14Cr–2W– 0.3Ti–0.3Y2O3 ODS RAF steel. The outcome of these investigations brings new insight into the characteristics and genesis of formation of the complex oxides in this type of steel.
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P. Unifantowicz et al. / Journal of Nuclear Materials 442 (2013) S158–S163 Table 1 Concentrations of elements in the bulk ODS RAF steel. Element
Fe
Cr
W
Co
Ti
Y
Mn
Ni
Si
Mo
Cu
N
O
C
at.%
Bal.
13.8
1.65
0.65
0.30
0.13
0.03
0.01
0.10
0.01
0.01
0.4
0.3
0.1
2. Materials and methods The ODS RAF steel ingot fabrication involved high-energy ball milling of Fe, Cr, W, Ti, and Y2O3 powders with purities of: 99.5, 99, 99.9, 99.9, and 99.99 wt.%, respectively. The powders were added in a weight ratio corresponding to nominal composition: Fe–14Cr–2W–0.3Ti–0.3Y2O3. Mechanical alloying of the initial powders was done in a RetschÒ planetary ball mill using effective rotational bowl speed of 300 rpm, ball to powder weight ratio of 10:1, total time of 40 h, and H2 atmosphere. The powder was encapsulated in a soft steel can and degassed for 2 h in a vacuum at 1073 K and then consolidated by hot isostatic press at a maximum pressure of 200 MPa and a temperature of 1423 K. The inductively coupled plasma and LecoÒ techniques were used to determine the concentrations of elements in the bulk sample. The composition of the consolidated steel is given in Table 1. The nominal composition of ODS RAF steel investigated in this work was selected based on the previously optimized composition of the ODS steel produced by basically the same route at CRPP [17,18]. The standard TEM disc was prepared from the consolidated steel for general microstructure. The procedure consisted of polishing discs of ODS RAF with 1-mm diameter embedded in a 3 mm austenite steel ring, mechanical polishing of the discs to about 50 lm thickness, and then electro-polishing using voltage of 20.5 V and 7%HCl04/93%C2H5OH solution cooled to about 243 K. The use of a 1 mm sample allowed to minimize the ferromagnetic effect of the sample on the electron beam [19]. The APT specimens were prepared using standard electropolishing methods and a 25% HCl/75%CH3COOH solution. APT analysis was carried out using an Imago LEAP 3000 HR instrument in laser pulsing analysis mode with a laser energy of 0.3 nJ, 8 lm beam spot size, and frequency of 200 kHz. The size distribution of the particles was found using the maximum separation method [20] based on Y and Ti containing ions using a maximum separation distance of 1.3 nm. To enable a straightforward interpretation of HR-STEM images, reinforcement particles were separated from the steel matrix by their extraction on amorphous carbon film. This method helped resolve the lattice of nanoparticles, which is not a trivial task in the case of standard bulk samples. In the case of standard TEM discs, the fine particles give a relatively weak diffraction signal compared with the signal from the matrix. The method for particle extraction involved mirror polishing of the bulk steel sample followed by short electrolytic polishing of the surface and deposition of carbon by arc sputtering. The C layers were detached by electrolytic etching in the solution used for TEM discs and collected on a Cu grid. Images for oxide particle lattice imaging were taken using a Cscorrected scanning TEM Hitachi HD2700 operating at 200 keV equipped with EELS spectrometer. Its advantage is a good sensitivity for light elements in comparison with EDS and relatively fast mapping, enabling compositional checks of the extracted particles. Typical magnification was 300,000 and the size of spectrum images was of 64 60 pixels 1340 eV with a spectrum dispersion of 0.1 and 0.5 eV in case of low loss (5/145 eV) and core loss (310–980 eV) energy ranges, respectively. The following characteristic EEL edges were used: Y N2,3 and Ti M4,5 in low loss with maxima at about 36 and 47 eV, respectively, and also N K, O K, and Cr
L2,3 in core loss energy range with maxima at about 401, 532, and 575 eV, respectively. To identify the crystal structure of the oxides, fast Fourier transformation (FFT) was applied to atomic HR-STEM images. The characteristic distances in reciprocal space were measured together with their ratios and angles between the vectors. Then, the inverse Fourier transformation was applied to filter out the noise from the lattice image. The obtained patterns were compared with the geometries of standard electron diffraction patterns for various Y–Ti–O and Y–O oxides obtained from a crystallographic database [21]. The electron diffraction patterns matching the vectors geometry in FFT images were simulated in jEMSÒ software [22]. 3. Results 3.1. General microstructure The microstructure of bulk ODS RAF steel sample, following HIP is shown in Fig. 1a. Fine grains have a high density of dislocations due to stresses related to austenite to ferrite transformation during cooling after HIP. The average crystallite size found from peak broadening of an XRD spectrum is about 250 nm. Some larger particles were also observed. A higher magnification bright field TEM image in Fig. 1b shows numerous very fine precipitates about 5 nm in diameter. In addition, less numerous larger particles were found. 3.2. Particles composition 3.2.1. APT APT was employed to characterize the composition and spatial distribution of the smallest nanoparticles in the ODS RAF steel. The average radius of gyration of the nanoparticles was 2.5 ± 0.1 nm and the number density about 4 ± 0.5 1023 m 3. This density was in agreement with that obtained by of dark field TEM images analyses in steel samples prepared in similar way [16]. In addition, larger oxides >10 nm radius were occasionally seen in the analysis volume. Some larger oxides had the same core composition as the small (2 nm) nanoparticles, whereas others were also enriched in N. The graph in Fig. 2b shows a proximity histogram indicating the distribution of solute elements across a 3% Y + Ti iso-concentration surface, including all nanoparticles in the analyzed volume. Along with Y, Ti, and O, the nanoparticles were enriched in Cr and appreciable levels of Fe. Although iron was clearly detected in the particles core, it was considered to be an artifact of analysis, similar like in APT analyses in [23], therefore it was not shown in the histogram. From Fig. 2b the ratio of Y, Ti, and O at the core of the particles is approximately Y:Ti1.75:O3.5, which does not correspond to any known Y–Ti–O oxide. 3.2.2. EELS and HR-STEM To bring about oxides particles crystal structure, the reinforcing particles extracted on a carbon film were investigated by EELS and HR-STEM. The extraction allowed analysis of particles in a broader size range because the particles were collected from a surface of about 1 mm2, thus corresponding to a much larger volume in comparison with the volume of the APT sample. In addition, the
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Fig. 1. BF TEM images showing: (a) typical microstructure of Fe–14Cr–2W–0.3Ti–0.3Y2O3 sample and (b) nanoprecipitates in the steel matrix.
Fig. 2. (a) APT maps of Y, Ti and O distributions in the analyzed volume and (b) proximity histogram showing the distribution of solute elements across a 3% Y + Ti isoconcentration surface, including all the nanoparticles in the analyzed volume.
Fig. 3. STEM Z-contrast image of particles extracted on carbon foil with EELS maps corresponding to the present elements.
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Fig. 4. (a, d, g) High resolution STEM images with images obtained by fast Fourier transformation in the inclusions; (b, e, h) inverse FFT images with marked planes indices and (c, f, i) simulated electron diffraction patterns for YTiO3 particles in orientations: [1 3 5], [4 3 5] and [3 2 2] respectively.
extraction method allowed the avoidance of the strong signal from the steel matrix, which keeps the crystal lattice from resolving particles as fine as several nanometers in diameter. Only very few
small Y–Ti–O nanoparticles were observed in the extracted foil, although a high density of Y–Ti–O nano-particles was evident from the bright field TEM imaging and APT proxigrams of the bulk sam-
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ples. This is most likely due to their removal during the extraction, possibly caused by weaker electrostatic forces holding particles on the carbon film. The dark field HR-STEM image in Fig. 3 shows a typical agglomeration of particles found on the carbon film. The EELS maps, shown in the same figure, were obtained using characteristic EEL edges for light (O, N, C) and metallic elements, which were previously confirmed by EDS mapping. A series of maps in various regions of the foil showed the presence of other particles besides the Y–Ti–O, including Cr–Ti–O and Cr–N, which accounted for 13% and 7% of the particles, respectively. The mean diameter of the extracted Y–Ti–O particle was about 20 nm, which is significantly larger than those found in TEM BF images and in APT maps. In addition, chromium nitrides, usually several 100 nm in diameter, were found. The presence of chromium nitrides indicates air intake during powder milling or handling. 3.3. Crystal structure of the extracted oxide particles EELS mapping was done to cross-check the composition of selected areas of interest containing appreciable number of extracted particles. High resolution STEM images of several oxide nanoparticles of Y–Ti–O type are shown in Fig. 4a, d and g. The patterns obtained by applying FFT to HR-STEM bright field images of the Y–Ti–O nanoparticles are shown in Fig. 4a, d and g. Pairs of vectors were drawn in reciprocal space corresponding to characteristic distances between lattice planes to find the pattern of the searched oxide among all possible diffraction patterns of the known candidates, including c-Y2Ti2O7, o-Y2TiO5, o-YTiO3, and c-Y2O3. The lines and indices corresponding to lattice planes in asfiltered inverse FFT images are shown in Fig. 4b, e and h. The FFT patterns of the three oxide particles were in a very good agreement with the modeled electron diffraction patterns of the orthorhombic YTiO3 oxide, which are shown in Fig. 4c, f and i. The patterns corresponded to [1 3 5], [4 3 5] and [3 2 2] zone axis orientations, respectively. The measured and nominal values for the given lattice orientations for the three particles are summarized in Table 2. The measured d1, d2, and d3 values correspond to the distances in real space obtained by analysis of FT images and the nominal values correspond to the vectors marked in yellow in the modeled diffraction patterns. The a values are the angles between g vectors with absolute values equal to the respective values of 1/d. As a result of the geometrical analyses, all three particles were identified as the orthorhombic YTiO3 oxide with the following cell parameters: a = 5.338, b = 7.613 and c = 5.690 Å [24]. The maximum standard deviations of the measured d-distances and angles with respect to nominal values were in range from 2.4% to 3.9% and 0.6% to 2.8%, respectively, which indicates a good accuracy of scale. On the other hand, these differences could arise from slightly nonstandard unit cell volume.
In the case of [3 2 2] orientation, similar lengths of d vectors were found as for the Y2TiO5 oxide in [2 7 1] orientation, however its simulated pattern geometry was significantly different than that obtained using FFT. The orthorhombic YTiO3 oxide lattice has Pnma space group and can be described as a distorted perovskite, in which Y3+ cations are surrounded by mutually tilted octahedra made up of Ti3+ and O2 ions [25]. Its prototype is SrTiO3 with cubic perovskite structure. 4. Discussion The lack of chromium in the smallest extracted Y–Ti–O oxides (shown in EELS maps) and its presence in oxides analyzed by APT may be misleading. To explain this discrepancy, one can consider the following. In previous EELS analyses of a steel with the same nominal composition, prepared essentially the same way [26] Y and Ti were found in shells around the Cr–Ti–O precipitates but no Cr shells could be evidenced. This study supports the opinion that Cr presence inside the particles in this work may be an artifact of APT. On the other hand, the shells could have been removed with the steel matrix during electrolytic etching. In the mentioned work on EELS analysis [26] Cr was not identified in Y–Ti–O oxides particles, in ODS RAF steel by using EFTEM. Thus, although the APT and EELS results presented here could not be directly compared, previous results indicate that Y–Ti–O particles with diameters between several and several tens of nanometers are likely the same oxide. The difference in elements ratio between the APT and TEM observations may arise from different particles or existing artifacts of both techniques leading to different metal-to-oxygen ratios. The identification of oxide lattice as orthorhombic YTiO3 is supported by its unique crystal lattice, which has smaller number of symmetry translations compared with the pyrochlore oxides and, thus, a smaller number of symmetry translations, making it distinguishable from the other Y–Ti–O oxides. Previously Alinger [15] who used SANS to identify oxides in ODS steel indicated growth of YTiO3 and suggested presence of nonequilibrium oxides. One possible reason for the occurrence of this rare oxide is that bulk YTiO3 formation occurs in a reducing atmosphere while bulk Y2Ti2O7 pyrochlore is more stable at higher oxygen pressures (as indicated in [27]). Thus, one should take into account the influence of milling atmosphere, considering that H2 can change the local chemistry of interfaces created during mechanical alloying. This may cause Ti ions to change the oxidation state from +4 in TiO2 considering it a Ti donor to +3 and leading to formation of the YTiO3 phase. Another possible reason for the formation of YTiO3 is its crystallographic relation towards the Fe-based matrix, considering that half of the b parameter of the oxide: 2.84 Å corresponds approximately to the lattice constant of a-Fe of 2.87 Å, which could result in a semi-coherent crystallographic relationship between the oxide and Fe matrix, as indicated in [28]. A characteristic feature of the perovskite structure is that it
Table 2 Results of quantitative analysis of the HR-STEM images and standard electron diffraction patterns matching the orthorhombic YTiO3 lattice. Crystal zone axis Planes indices Distances – d (Å) Measured Nominal St. dev. (%) Angles – a (°) Measured Nominal St. dev. (%)
[1 3 5]
[4 3 5]
1,2, 1
2, 1,1
3,1,0
2,1,1
d1 2.762 2.717 1.1 a(g1 g3) 43.3 41.8 2.5
d2 2.519 2.379 3.9 a(g2 g3) 50.3 52.4 2.8
d3 1.848 1.783 2.5 a(g1 g2) 93.6 94.2 0.4
d1 2.418 2.379 1.1 a(g1 g3) 61.5 62 0.6
[3 3 2] 1, 3,1 d2 2.190 2.123 2.4 a(g2 g3) 51.5 51.5 0
1, 2,2 d3 2.037 2.034 0.1 a(g1 g2) 113 113.5 0.3
1,1,0 d1 3.906 3.880 0.4 a(g1 g3) 63.8 63.5 0.3
1,1, 3 d2 2.053 2.123 2.4 a(g2 g3) 27.3 28.4 2.7
0,2, 3 d3 1.854 1.892 1.5 a(g1 g2) 91.1 91.9 0.6
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allows flexibility of tilts and rotations of the TiO6 octahedra and therefore an extensive creation of vacancies or substitutional atoms is possible [29]. This flexibility of the oxide lattice can be of advantage considering enhanced helium trapping, which is one of the functions of the reinforcement nanoparticles in ODS RAF. 5. Conclusions The oxide nanoparticles reinforcing the Fe–14Cr–2W–0.3Ti– 0.3Y2O3 ODS RAF steel were characterized in terms of composition and crystal structure. TEM BF imaging revealed numerous 5 nm particles and also larger ones. APT analysis showed that the nanoparticles are made up of Y, Ti, and O. Numerous larger, (20 nm) Y–Ti–O particles and a considerable amount of much larger Cr–N and Cr–Ti–O particles were found among the extracted particles as indicated by EELS mapping. HRSTEM and quantitative analysis of images obtained by Fourier transformation for 15 nm and larger particles allowed identification of the YTiO3 oxide with orthorhombic lattice, representing a distorted perovskite structure. Acknowledgements The Paul Scherrer Institute is acknowledged for the overall use of its facilities. This work, supported by the European Communities under the contract of Association between EURATOM/Confédération Suisse, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This work was also performed within the framework of the Integrated European Project ‘‘ExtreMat‘‘ (contract NMP-CT2004-500253) with financial support by the European Community. It only reflects the view of the authors and the European Community is not liable for any use of the information contained therein. Financial support was also provided by the European project ‘‘FEMaS’’ (FP7 Grant Agreement No. 224752).
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