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Ti atomic ratio on microstructure of oxide dispersion strengthened alloys
Materials Characterization 134 (2017) 35–40
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Effect of Y/Ti atomic ratio on microstructure of oxide dispersion strengthened alloys
MARK
Chenyang Lua,b, Zheng Lua,⁎, Rui Xiea, Zhengyuan Lia, Chunming Liua, Lumin Wangb a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China b Department of Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, MI 48109, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxide dispersion strengthened alloys Mechanical alloying Hot isostatic pressing Nano oxides Electron energy loss spectroscopy Advanced transmission electron microscopy
Oxide dispersion strengthened (ODS) alloys are the leading candidate materials for advanced fission and fusion reactor components because of their excellent high temperature creep strength and radiation tolerance. The promising performance of ODS alloys relies on a high-density population of nano oxides. Here, four 9Cr-ODS alloys with varying initial Y/Ti atomic ratio were processed using a combined route of mechanical alloying (MA) and hot isostatic pressing (HIP). Microstructural characterizations of the consolidated alloys were examined by advanced transmission electron microscopy (TEM). Great differences in oxide distribution behavior were observed. 9Cr-ODS alloy with a Y/Ti ratio of 0.4 presented the finest and the highest number density of nano oxides among the four alloys. The effects of Y/Ti ratio on oxide formation were explored, and the precipitated mechanisms of the nano oxides were interpreted.
1. Introduction The safe, economic, environmental friendly and advanced fourthgeneration fission reactor and fusion reactor have been considered as the most ideal energy resources and the best solutions to meet the rapidly increasing global energy needs. The development of advanced nuclear energy systems demands the structural materials that can sustain the extreme internal reactor environments, such as high pressure, elevated temperature, high-dose neutron irradiation, complex chemical corrosion and so on. Oxide dispersion strengthened (ODS) alloys are among the most promising candidate materials for the fuel claddings in advanced fission reactor and the first wall/blanket applications in fusion reactor because of their excellent high temperature performance and radiation tolerance [1–3]. The superior properties of ODS alloys over conventional ferritic steels are attributed to high number density of nano oxides that homogenously distribute in the ferritic matrix. Three significant advantages are introduced: (1) nano oxides impede dislocation motion, retard grain recrystallization and improve creep resistance at high temperature; (2) nano oxides act as efficient trapping sites for absorbing transmutant helium (He) atoms and dispersing the helium atoms into high density nanoscale helium bubbles, sequentially minimizing the risk of helium embrittlement in the material; and (3) nano oxides act as stable sinks for enhancing the recombination of the excess vacancies and interstitials survived from cascade collision,
⁎
Corresponding author. E-mail addresses: [email protected] (C. Lu), [email protected] (Z. Lu).
http://dx.doi.org/10.1016/j.matchar.2017.10.004 Received 3 August 2017; Received in revised form 2 October 2017; Accepted 2 October 2017 Available online 03 October 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
improving the damage tolerance of the material [1,2,4–6]. Generally, ODS alloys are produced by mechanical alloying (MA) and various hot consolidated processing, including hot isostatic pressing (HIP), hot extrusion (HE) and spark plasma sintering (SPS) [7–11]. The major elements Y, O and Ti in nano oxides are introduced into the ODS alloys by mechanically alloying Y2O3 and Ti powders with metallic or pre-alloy powders. Optimizing the recipe of the alloy composition and the metallurgical process are the important issues for developing high-performance ODS alloys. Initial Y/Ti atomic ratio is a very sensitive parameter that controls the size, distribution, chemical composition of Y-Ti-O nano oxides, as well as their coherence with the matrix in ODS alloys. These characteristics of nano oxides are very crucial to the mechanical properties and also the radiation performance of ODS alloys. Ukai et al. firstly found that the addition of 0.3 (wt%) Ti can produce an excellent refinement and high density of Y-Ti-O nanooxides in Fe-Cr ferritic matrix [12]. Dou et al. studied the effects of Ti concentration addition on the high-Cr ODS steels [13]. Zhong et al. found that Y/Ti ratio has a high influence on the recrystallization behavior of Fe-14Cr ODS steel [14]. In this study, Fe-9Cr (wt%) was designed as the base or matrix alloy, and then 0.3% Ti and varied concentration of Y2O3 were added into the metallic powders. This approach allowed us to study the effect of Y/Ti atomic ratio on the formation behavior of nano-oxides along with the microstructural evolution of the matrix. Advanced scanning transmission electron microscope (STEM),
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3. Results
Table 1 Chemical compositions (mass%) of ODS alloys in this study.
9CrY 0.1-9CrYT 0.4-9CrYT 1.0-9CrYT
Fe
Cr
W
Mn
Ta
V
Ti
Y2O3
Y/Ti (atom)
Bal. Bal. Bal. Bal.
9 9 9 9
1.5 1.5 1.5 1.5
0.4 0.4 0.4 0.4
0.1 0.1 0.1 0.1
0.2 0.2 0.2 0.2
– 0.3 0.3 0.3
0.3 0.1 0.3 0.7
– 0.1 0.4 1.0
The bight field (BF) — STEM and corresponding high angle annular dark field (HAADF) STEM images for the various ODS alloys are illustrated in Fig. 1a–h. The average grain sizes of 9CrY, 0.1-9CrYT, 0.49CrYT and 1.0-9CrYT alloys are 2.0 μm, 1.0 μm, 1.6 μm and 1.8 μm, respectively. There are both sub-micron oxides and nano oxides distributed in the ferritic matrix. Sub-micron oxides with diameter 50–200 nm are normally observed on grain boundaries. Fig. 2(a) shows a representative HAADF image of some large oxides obtained from 0.19CrYT. EDS mapping has been conducted and indicated core/shell structure of large oxides, which Mn, Cr and O are enriched and Fe is depleted in cores and Ti are enriched in the shells, as shown in Fig. 2(b–f). HRTEM image shown in Fig. 2(g) represents the crystalline structure of a large oxide, the lattice structure of the oxide (core) is in good agreement with spinel structure MnCr2O4 [7] with a zone axis of [114]. In 9CrY alloy, submicron oxide has been also identified as MnCr2O4 but Ti enriched shell is not observed. The average size and number density of spinel oxides in four studied ODS alloys are summarized in Table 3. It is found that, with the addition of Ti, the average size of the submicron oxides is refined. The concentration of the initial of Y2O3 has no obvious influence on the size of the oxides but significantly affects the oxide density. Spinel structured sub-micron oxides distribute almost twice lower density in 1.0-9CrYT than in 0.1-9CrYT and 0.4-9CrYT. Optimizing the nano oxide distribution is the critically important issue to control mechanical properties and radiation resistance of ODS alloys. HAADF imaging has been employed for capturing the distributions of nano oxides in this study. Nano oxides exhibit dark contrast compared to the matrix because of the oxygen enrichment in oxides resulting into the lower density areas than the matrix. Advanced spherical aberration STEM allows us to directly characterize the smallest nano oxides with 1–2 nm diameter, which were not able to observe clearly by conventional TEM. Fig. 3 shows the HAADF micrographs taken under a relative high magnification, revealing homogeneous spatial distributions of nano oxides ranging from 1 to 20 nm in four ODS alloys. Obviously, the concentration of initial Y and Ti strongly influences the distribution of nano oxides. The TEM sample thickness was measured by EELS and the detail distribution of nano oxides are summarized in Table 3. More than 1000 nano oxides in each sample were analyzed. The average nano oxide size increases in order of 0.49CrYT, 0.1-9CrYT, 9CrY and 1.0-9CrYT, and the nano oxide density decreases in the same order. The size distribution histograms of nano oxides in four alloys are plotted in Fig. 3e. The highest fraction of the nano oxides in 0.4-9CrYT is found to be associated with 2 ± 0.5 nm (56%) while the fraction of nano oxides larger than 10 nm is very low. The highest fraction of nano oxides in 9CrY and 0.1-9CrYT are found in the size range of 3 ± 0.5 nm (33% and 43%, respectively). 1.0-9CrYT exhibits clearly much larger oxides than the others as shown in both histogram and HAADF images. A large number of nano oxides larger than 10 nm are observed in 1.0-9CrYT. Many previous studies indicated that the portion of smallest nano oxides (normally smaller than 3 nm), so-called nanoclusters (NC), may have a different structure with others (larger than 3 nm), which was thought to be the most effectiveness for enhancing radiation resistance and also the creep resistance in ODS alloys [1,18]. Thus, it is necessary to specifically study the distributions of the NCs. The percentage of NCs is estimated to be 90%, 83% 63% and 20% in the alloy of 0.4-9CrYT, 0.1-9CrYT, 9CrY and 1.0-9CrYT, respectively. It is very challenge to study the structure of NC using HRTEM because there are too many interferences coming from the ferritic matrix where the nano size oxide embed in. A typical NC in 0.4-9CrYT ODS was characterized by HR-HAADF as shown in Fig. 4(a). This NC with a diameter of 2.5 nm presents darker contrast compared to the surrounding ferritic matrix because of the enrichment of O elements. The image was taken under the incident electron beam paralleled to the bcc
equipped with energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) was employed for characterizing the distribution, structure and chemical composition of oxides.
2. Experimental Three alloys of Fe-9Cr-1.5W-0.4Mn-0.1Ta-0.2V-0.3Ti-xY2O3 (mass %), x = 0.1, 0.3 and 0.7, were produced. The Y/Ti atomic ratios in the three alloys are about 0.1, 0.4 and 1.0, respectively. An ODS alloy without Ti addition was also produced for comparison. Designed compositions of all the developed alloys with their identifying names in this study are shown in Table 1. High pure Fe, Cr, W, Mn, Ta, V, Ti and nanoscale Y2O3 powders were used as starting materials. The powder purities are shown in Table 2. The powder mixture was mechanical alloyed in an argon-gas atmosphere using a planetary high-energy ball miller (Fritsch P5) with a chrome-steel bowl and steel balls with a ballto-powder weight ratio of 10:1 and a rotation speed of 200 rpm. The diameter of the steel balls is 10 mm. A steady-state microstructure of supersaturation powders ready for consolidation was achieved after 50 h ball milling. The as-milled powders were canned and degassed at 673 K for 4 h. Then the can was sealed and the as-milled powders were consolidated by HIP at 1373 K under a pressure of 200 MPa for 2 h. For minimizing the aberrations induced by the ferromagnetism materials, TEM samples were prepared by focused ion beam (FIB) liftout method by using a FEI Helios Nanolab Dualbeam workstation. Final thinning was performed using 5 kV Ga+ with a ± 7° incidence angle between the Ga+ beam and the sample surface for 5 min. Additionally, a “flash polishing” technique was conducted to remove FIB-induced damage, which was also able to further thin the foil thickness for achieving a better electron transparency [15–17]. Flash polishing was conducted using an electropolishing apparatus with an electrolytic solution of 4% perchloric acid in 96% ethanol. An electric potential of 6–14 V was applied to the FIB-polished specimen at a temperature range between − 30 and − 50 °C. The polishing period was controlled between 0.05 and 0.2 s with an accurate timer. A JEOL JEM 3011 transmission electron microscopy (TEM) operating at 300 keV was used for imaging high-resolution TEM (HRTEM) images. A double spherical aberration (CS)-corrected JEOL JEM 3100 Analytical Electron Microscope was employed to accomplish the STEM imaging with an inner angle of 59 mrad and camera length of 15 cm. EDS was conducted at JEOL 3100 for roughly examining the chemical composition of large particles. Ultra-fast EELS system attached to the JEOL 3100 was employed to examine the chemical composition of finer particles. The EELS experiments were performed with a probe size about 0.2 nm. The spectra were calibrated using the Fe-L edges for ODS alloy. For EELS measurements, an energy resolution of 2 eV and dispersion of 1 eV/ pixel were employed.
Table 2 The purity of original powders, wt%.
Purity
Fe
Cr
W
Mn
Ta
V
Ti
Y2O3
99.9
99.95
≥ 99.8
99.5
99.5
99.99
99.0
99.9
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Fig. 1. TEM micrographs for various alloys. (a) BF image of 9CrY; (b) HAADF image of 9CrY; (c) BF image of 0.1-9CrYT; (d) HAADF image of 0.1-9CrYT; (e) BF image of 0.4-9CrYT; (f) HAADF image of 0.4-9CrYT; (g) BF image of 1.0-9CrYT; (h) HAADF image of 1.0-9CrYT.
Fig. 2. Spinel structured Mn(Ti)Cr2O4 in 0.1-9CrYT, (a) HAADF image; (b–f) EDS maps; (g) HRTEM image.
the ODS alloys [7]. The structure of the NCs in ODS alloy is still in a debate: (1) non-equilibrium phases or Guinier-Preston-zone transition phases with the solute atoms occupying or nearing the lattice sites of bcc Fe matrix; (2) NaCl rock salt structure; (3) amorphous [2,19,20]. It is worth to note that Brandes et al. observed very similar NCs in 14YWT and stated that the smallest NCs were amorphous characterized by nano-diffraction [19]. However, based on our observation, it is very hard to define that NCs are amorphous or coherent with the bcc matrix. A recent molecular dynamic (MD) simulation result from Michael Higgins indicated that the oxides smaller than 3 nm had an amorphous or disorder structure in Y-O ODS system [21], which may ambitiously support Brandes's observation. However, the further study in Y-Ti-O ODS system by experiment and simulation is eagerly needed. The chemical composition of the nano oxides was analyzed by EELS. Fig. 4(b) shows the EELS mapping result obtained in 0.4-9CrYT. The spectrums of the oxide and the matrix in the energy range between 300 eV and 800 eV are shown in Fig. 4(c) and (d), showing the Ti-L2,3 peak appears in oxide spectrum but misses in the matrix spectrum. The signals from the Fe-L2,3 peak show weaker contrast in the oxide, indicating there is less iron content. The signals from the Y-L2,3, Ti-L2,3
Table 3 Statistical analysis of oxides in ODS alloys. Mn(Ti)Cr2O4
9CrY 0.1-9CrYT 0.4-9CrYT 1.0-9CrYT
Y-Ti-O nano-oxides
Average size (nm)
Density (m− 3)
Average size (nm)
Density (m− 3)
103 73 71 83
6.9 × 1019 8.1 × 1019 8.1 × 1019 4.9 × 1019
4.2 3.9 3.0 6.5
5.9 × 1022 8.2 × 1022 13.3 × 1022 4.5 × 1022
[111]matrix direction. It is clearly to show the atomic planes of the matrix in Fig. 4, but there is no specific lattice structure has been observed in the NC. It suggests that they are not Y2Ti2O7, Y2TiO5 or Y2O3. Similar NCs were observed in 14YWT and other ODS alloys [19,20]. In this study, we obtain the NCs by tuning the Y/Ti ratio in 9Cr ODS alloys. The best recipe to produce the finest size and highest density of NCs in 9Cr ODS alloys is presented. The excellent NCs distribution can largely enhance the radiation resistance and mechanical properties of 37
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Fig. 3. HAADF images of nano oxides in (a) 9CrY; (b) 0.1-9CrYT; (c) 0.4-9CrYT; (d) 1.0-9CrYT; (e) size distributions of nano oxides in four ODS alloys.
Fig. 4. (a) High-resolution HAADF image of a 2.5 nm NC; (b) EELS mapping of a NC in 0.4-9CrYT, Fe-L2,3; Y-L2,3; Ti-L2,3; O-K. (c) EELS spectrum of matrix; (d) EELS spectrum of oxide in (b).
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Fig. 5. (a) HRTEM image of a Y2Ti2O7 in 1.0-9CrYT; (b) HRTEM image of a Y2O3 in 0.1-9CrYT.
and O because very high density metastable Y-Ti-O NCs are preferred to re-precipitate. Compared with the pyrochlore-structure Y2Ti2O7, the precipitation of Y-Ti-O NCs needs lower formation energy because of their coherent structure with ferritic matrix. High density of coherency nano oxides can significantly improve the material ductility, creep resistance and radiation resistance [7,18,26,27], which is the preferred oxide type for designing ODS alloy. Barnard predicted the oxide evolution along with the O concentration in ODS alloys by computer simulation [23]. He found that when the O concentration was below the threshold, only Y2O3 appeared as precipitate phase in matrix, and all Ti remained in solution [23]. In the processing of ODS alloy, O concentration is mainly controlled by the addition of Y2O3. In 0.1-9CrYT, low concentration of oxygen may suppress the formation of Y-Ti-O nano oxides, but be in form of larger Y2O3. Although there are still many NCs and Y2Ti2O7 formed in 0.19CrYT as shown above, the formation of larger Y2O3 may increase the average size of the nano oxides and decrease the oxide density. This computer simulation results do qualitatively support our TEM observation, suggesting the low concentration of initial Y2O3 addition will jeopardize the oxide distribution in ODS alloy. It is well to know that, higher density and finer size of nano oxides are beneficial to improve the properties of ODS alloys. According to the result from this study, the optimal concentrations of initial Y2O3 and Ti are around 0.3 wt% and 0.3 wt%, i.e. Y/Ti = 0.4, in 9Cr-ODS alloys.
and O-K show strong contrast in the oxide, indicating the nano oxides are Y-Ti-O type. The diameter of the oxide shown in Fig. 4(b) is about 10 nm. It is difficult to obtain EELS mapping from the oxide smaller than 3 nm due to relatively large thickness of the specimen and large excitation volume. The structure of the oxide larger than 3 nm has been observed by HRTEM. Pyrochlore-structured Y2Ti2O7 oxides are found in all Ti-added ODS alloys. Fig. 5(a) shows a HRTEM image of Y2Ti2O7 oxide observed in 1.0-9CrYT ODS alloy with an incident electron is parallel to the [124]oxide zone axis. Y2O3 are observed in 0.1-9CrYT and 9CrY ODS alloy but not in the other ODS alloys. Fig. 5(b) shows a HRTEM image of Y2O3 observed in 0.1-9CrYT ODS alloy with an incident beam of [301]oxide zone axis. No orthorhombic and hexagonal Y2TiO5 is observed in these alloys as reported in other studies [22]. 4. Discussion Ukai et al. found that the addition of Ti refined the nano oxides in ODS alloys [12]. Barnard et al. predicted that the particle refinement by small addition of Ti was due to the increased opportunity and driving force for nucleation of Y-Ti-O oxides over Y2O3 [23]. Ti refinenment effect has been directly observed by advanced STEM characterization in this study. Comparing the 9CrY and 0.4-9CrYT, which both includes 0.3% Y2O3, 0.4-9CrYT exhibits much finer and higher density of oxides than 9CrY because of the Ti addition. However, with the fixed 0.3% Ti, tuning the concentration of initial Y2O3 can also significantly affect the oxide distribution. As shown in Fig. 2, overdose (1.0-9CrYT) or insufficient (0.1-9CrYT) Y2O3 addition causes the coarsening of nano oxides in ODS alloys. Thus, studying how to control the oxide behavior by tuning the initial element addition and sequentially understanding the formation mechanisms of nano oxides are very important for controlling the microstructures and optimizing the fabrication processing of ODS alloys. It is well known that initial Y2O3 powders can decompose into Y and O atoms during MA and simultaneously dissolve into the ferritic matrix to form a supersaturated solid solution. High energy MA process can introduce high density dislocations and point defects in the matrix, which provide preferred precipitate sites for the nano oxides. During the high temperature consolidation, the addition of Ti will combine with Y and O and re-precipitate as Y-Ti-O nano oxides in the matrix [24,25]. Previous atom probe tomography (APT) studies showed that, Y-Ti-O NCs were normally composed with a low atomic ratio in a very fine size, while larger oxides (Y2Ti2O7 and Y2TiO5) were composed with a higher Y/Ti atomic ratio, indicating that the oxide size and oxide type formed in ODS alloy were actually dominated by the Y/Ti ratio [1]. In this study, high concentration Y and O in the 1.0-9CrYT alloy sufficiently associates with Ti to form a great number of stable pyrochlorestructure Y2Ti2O7 oxides. But in 0.4-9CrYT alloy, the formation of Y2Ti2O7 oxides is suppressed obviously by reduced concentration of Y
5. Summary Four 9Cr ODS alloys were produced by MA and HIP with varied initial Y2O3 and Ti additions. The microstructures of ODS alloys were characterized by HRTEM, STEM, EDS and EELS. High density of Y-Ti-O NCs and Y2Ti2O7 oxides, accompanied with large spinel Mn(Ti)Cr2O4 are observed in all Ti-added ODS alloys. Y2O3 are found in 0.1-9CrYT and 9CrY ODS alloys. 0.4-9CrYT presents the highest number density and the finest size of Y-Ti-O nano oxides among the four alloys. It is estimated that 90% of all the nano oxides in 0.4-9CrYT were coherent Y-Ti-O NCs. The optimal recipe of 9Cr-ODS alloy for obtaining the best oxide distribution has been studied and presented with 0.3% Ti and 0.3% Y2O3 additions (Y/Ti = 0.4). Acknowledgements This research is supported by the National Natural Science Foundation of China (51471049). References [1] G.R. Odette, D.T. Hoelzer, Irradiation-tolerant nanostructured ferritic alloys: transforming helium from a liability to an asset, JOM 62 (2010) 84–92.
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Effect of Y/Ti atomic ratio on microstructure of oxide dispersion strengthened alloys
MARK
Chenyang Lua,b, Zheng Lua,⁎, Rui Xiea, Zhengyuan Lia, Chunming Liua, Lumin Wangb a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China b Department of Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, MI 48109, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxide dispersion strengthened alloys Mechanical alloying Hot isostatic pressing Nano oxides Electron energy loss spectroscopy Advanced transmission electron microscopy
Oxide dispersion strengthened (ODS) alloys are the leading candidate materials for advanced fission and fusion reactor components because of their excellent high temperature creep strength and radiation tolerance. The promising performance of ODS alloys relies on a high-density population of nano oxides. Here, four 9Cr-ODS alloys with varying initial Y/Ti atomic ratio were processed using a combined route of mechanical alloying (MA) and hot isostatic pressing (HIP). Microstructural characterizations of the consolidated alloys were examined by advanced transmission electron microscopy (TEM). Great differences in oxide distribution behavior were observed. 9Cr-ODS alloy with a Y/Ti ratio of 0.4 presented the finest and the highest number density of nano oxides among the four alloys. The effects of Y/Ti ratio on oxide formation were explored, and the precipitated mechanisms of the nano oxides were interpreted.
1. Introduction The safe, economic, environmental friendly and advanced fourthgeneration fission reactor and fusion reactor have been considered as the most ideal energy resources and the best solutions to meet the rapidly increasing global energy needs. The development of advanced nuclear energy systems demands the structural materials that can sustain the extreme internal reactor environments, such as high pressure, elevated temperature, high-dose neutron irradiation, complex chemical corrosion and so on. Oxide dispersion strengthened (ODS) alloys are among the most promising candidate materials for the fuel claddings in advanced fission reactor and the first wall/blanket applications in fusion reactor because of their excellent high temperature performance and radiation tolerance [1–3]. The superior properties of ODS alloys over conventional ferritic steels are attributed to high number density of nano oxides that homogenously distribute in the ferritic matrix. Three significant advantages are introduced: (1) nano oxides impede dislocation motion, retard grain recrystallization and improve creep resistance at high temperature; (2) nano oxides act as efficient trapping sites for absorbing transmutant helium (He) atoms and dispersing the helium atoms into high density nanoscale helium bubbles, sequentially minimizing the risk of helium embrittlement in the material; and (3) nano oxides act as stable sinks for enhancing the recombination of the excess vacancies and interstitials survived from cascade collision,
⁎
Corresponding author. E-mail addresses: [email protected] (C. Lu), [email protected] (Z. Lu).
http://dx.doi.org/10.1016/j.matchar.2017.10.004 Received 3 August 2017; Received in revised form 2 October 2017; Accepted 2 October 2017 Available online 03 October 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
improving the damage tolerance of the material [1,2,4–6]. Generally, ODS alloys are produced by mechanical alloying (MA) and various hot consolidated processing, including hot isostatic pressing (HIP), hot extrusion (HE) and spark plasma sintering (SPS) [7–11]. The major elements Y, O and Ti in nano oxides are introduced into the ODS alloys by mechanically alloying Y2O3 and Ti powders with metallic or pre-alloy powders. Optimizing the recipe of the alloy composition and the metallurgical process are the important issues for developing high-performance ODS alloys. Initial Y/Ti atomic ratio is a very sensitive parameter that controls the size, distribution, chemical composition of Y-Ti-O nano oxides, as well as their coherence with the matrix in ODS alloys. These characteristics of nano oxides are very crucial to the mechanical properties and also the radiation performance of ODS alloys. Ukai et al. firstly found that the addition of 0.3 (wt%) Ti can produce an excellent refinement and high density of Y-Ti-O nanooxides in Fe-Cr ferritic matrix [12]. Dou et al. studied the effects of Ti concentration addition on the high-Cr ODS steels [13]. Zhong et al. found that Y/Ti ratio has a high influence on the recrystallization behavior of Fe-14Cr ODS steel [14]. In this study, Fe-9Cr (wt%) was designed as the base or matrix alloy, and then 0.3% Ti and varied concentration of Y2O3 were added into the metallic powders. This approach allowed us to study the effect of Y/Ti atomic ratio on the formation behavior of nano-oxides along with the microstructural evolution of the matrix. Advanced scanning transmission electron microscope (STEM),
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3. Results
Table 1 Chemical compositions (mass%) of ODS alloys in this study.
9CrY 0.1-9CrYT 0.4-9CrYT 1.0-9CrYT
Fe
Cr
W
Mn
Ta
V
Ti
Y2O3
Y/Ti (atom)
Bal. Bal. Bal. Bal.
9 9 9 9
1.5 1.5 1.5 1.5
0.4 0.4 0.4 0.4
0.1 0.1 0.1 0.1
0.2 0.2 0.2 0.2
– 0.3 0.3 0.3
0.3 0.1 0.3 0.7
– 0.1 0.4 1.0
The bight field (BF) — STEM and corresponding high angle annular dark field (HAADF) STEM images for the various ODS alloys are illustrated in Fig. 1a–h. The average grain sizes of 9CrY, 0.1-9CrYT, 0.49CrYT and 1.0-9CrYT alloys are 2.0 μm, 1.0 μm, 1.6 μm and 1.8 μm, respectively. There are both sub-micron oxides and nano oxides distributed in the ferritic matrix. Sub-micron oxides with diameter 50–200 nm are normally observed on grain boundaries. Fig. 2(a) shows a representative HAADF image of some large oxides obtained from 0.19CrYT. EDS mapping has been conducted and indicated core/shell structure of large oxides, which Mn, Cr and O are enriched and Fe is depleted in cores and Ti are enriched in the shells, as shown in Fig. 2(b–f). HRTEM image shown in Fig. 2(g) represents the crystalline structure of a large oxide, the lattice structure of the oxide (core) is in good agreement with spinel structure MnCr2O4 [7] with a zone axis of [114]. In 9CrY alloy, submicron oxide has been also identified as MnCr2O4 but Ti enriched shell is not observed. The average size and number density of spinel oxides in four studied ODS alloys are summarized in Table 3. It is found that, with the addition of Ti, the average size of the submicron oxides is refined. The concentration of the initial of Y2O3 has no obvious influence on the size of the oxides but significantly affects the oxide density. Spinel structured sub-micron oxides distribute almost twice lower density in 1.0-9CrYT than in 0.1-9CrYT and 0.4-9CrYT. Optimizing the nano oxide distribution is the critically important issue to control mechanical properties and radiation resistance of ODS alloys. HAADF imaging has been employed for capturing the distributions of nano oxides in this study. Nano oxides exhibit dark contrast compared to the matrix because of the oxygen enrichment in oxides resulting into the lower density areas than the matrix. Advanced spherical aberration STEM allows us to directly characterize the smallest nano oxides with 1–2 nm diameter, which were not able to observe clearly by conventional TEM. Fig. 3 shows the HAADF micrographs taken under a relative high magnification, revealing homogeneous spatial distributions of nano oxides ranging from 1 to 20 nm in four ODS alloys. Obviously, the concentration of initial Y and Ti strongly influences the distribution of nano oxides. The TEM sample thickness was measured by EELS and the detail distribution of nano oxides are summarized in Table 3. More than 1000 nano oxides in each sample were analyzed. The average nano oxide size increases in order of 0.49CrYT, 0.1-9CrYT, 9CrY and 1.0-9CrYT, and the nano oxide density decreases in the same order. The size distribution histograms of nano oxides in four alloys are plotted in Fig. 3e. The highest fraction of the nano oxides in 0.4-9CrYT is found to be associated with 2 ± 0.5 nm (56%) while the fraction of nano oxides larger than 10 nm is very low. The highest fraction of nano oxides in 9CrY and 0.1-9CrYT are found in the size range of 3 ± 0.5 nm (33% and 43%, respectively). 1.0-9CrYT exhibits clearly much larger oxides than the others as shown in both histogram and HAADF images. A large number of nano oxides larger than 10 nm are observed in 1.0-9CrYT. Many previous studies indicated that the portion of smallest nano oxides (normally smaller than 3 nm), so-called nanoclusters (NC), may have a different structure with others (larger than 3 nm), which was thought to be the most effectiveness for enhancing radiation resistance and also the creep resistance in ODS alloys [1,18]. Thus, it is necessary to specifically study the distributions of the NCs. The percentage of NCs is estimated to be 90%, 83% 63% and 20% in the alloy of 0.4-9CrYT, 0.1-9CrYT, 9CrY and 1.0-9CrYT, respectively. It is very challenge to study the structure of NC using HRTEM because there are too many interferences coming from the ferritic matrix where the nano size oxide embed in. A typical NC in 0.4-9CrYT ODS was characterized by HR-HAADF as shown in Fig. 4(a). This NC with a diameter of 2.5 nm presents darker contrast compared to the surrounding ferritic matrix because of the enrichment of O elements. The image was taken under the incident electron beam paralleled to the bcc
equipped with energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) was employed for characterizing the distribution, structure and chemical composition of oxides.
2. Experimental Three alloys of Fe-9Cr-1.5W-0.4Mn-0.1Ta-0.2V-0.3Ti-xY2O3 (mass %), x = 0.1, 0.3 and 0.7, were produced. The Y/Ti atomic ratios in the three alloys are about 0.1, 0.4 and 1.0, respectively. An ODS alloy without Ti addition was also produced for comparison. Designed compositions of all the developed alloys with their identifying names in this study are shown in Table 1. High pure Fe, Cr, W, Mn, Ta, V, Ti and nanoscale Y2O3 powders were used as starting materials. The powder purities are shown in Table 2. The powder mixture was mechanical alloyed in an argon-gas atmosphere using a planetary high-energy ball miller (Fritsch P5) with a chrome-steel bowl and steel balls with a ballto-powder weight ratio of 10:1 and a rotation speed of 200 rpm. The diameter of the steel balls is 10 mm. A steady-state microstructure of supersaturation powders ready for consolidation was achieved after 50 h ball milling. The as-milled powders were canned and degassed at 673 K for 4 h. Then the can was sealed and the as-milled powders were consolidated by HIP at 1373 K under a pressure of 200 MPa for 2 h. For minimizing the aberrations induced by the ferromagnetism materials, TEM samples were prepared by focused ion beam (FIB) liftout method by using a FEI Helios Nanolab Dualbeam workstation. Final thinning was performed using 5 kV Ga+ with a ± 7° incidence angle between the Ga+ beam and the sample surface for 5 min. Additionally, a “flash polishing” technique was conducted to remove FIB-induced damage, which was also able to further thin the foil thickness for achieving a better electron transparency [15–17]. Flash polishing was conducted using an electropolishing apparatus with an electrolytic solution of 4% perchloric acid in 96% ethanol. An electric potential of 6–14 V was applied to the FIB-polished specimen at a temperature range between − 30 and − 50 °C. The polishing period was controlled between 0.05 and 0.2 s with an accurate timer. A JEOL JEM 3011 transmission electron microscopy (TEM) operating at 300 keV was used for imaging high-resolution TEM (HRTEM) images. A double spherical aberration (CS)-corrected JEOL JEM 3100 Analytical Electron Microscope was employed to accomplish the STEM imaging with an inner angle of 59 mrad and camera length of 15 cm. EDS was conducted at JEOL 3100 for roughly examining the chemical composition of large particles. Ultra-fast EELS system attached to the JEOL 3100 was employed to examine the chemical composition of finer particles. The EELS experiments were performed with a probe size about 0.2 nm. The spectra were calibrated using the Fe-L edges for ODS alloy. For EELS measurements, an energy resolution of 2 eV and dispersion of 1 eV/ pixel were employed.
Table 2 The purity of original powders, wt%.
Purity
Fe
Cr
W
Mn
Ta
V
Ti
Y2O3
99.9
99.95
≥ 99.8
99.5
99.5
99.99
99.0
99.9
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Fig. 1. TEM micrographs for various alloys. (a) BF image of 9CrY; (b) HAADF image of 9CrY; (c) BF image of 0.1-9CrYT; (d) HAADF image of 0.1-9CrYT; (e) BF image of 0.4-9CrYT; (f) HAADF image of 0.4-9CrYT; (g) BF image of 1.0-9CrYT; (h) HAADF image of 1.0-9CrYT.
Fig. 2. Spinel structured Mn(Ti)Cr2O4 in 0.1-9CrYT, (a) HAADF image; (b–f) EDS maps; (g) HRTEM image.
the ODS alloys [7]. The structure of the NCs in ODS alloy is still in a debate: (1) non-equilibrium phases or Guinier-Preston-zone transition phases with the solute atoms occupying or nearing the lattice sites of bcc Fe matrix; (2) NaCl rock salt structure; (3) amorphous [2,19,20]. It is worth to note that Brandes et al. observed very similar NCs in 14YWT and stated that the smallest NCs were amorphous characterized by nano-diffraction [19]. However, based on our observation, it is very hard to define that NCs are amorphous or coherent with the bcc matrix. A recent molecular dynamic (MD) simulation result from Michael Higgins indicated that the oxides smaller than 3 nm had an amorphous or disorder structure in Y-O ODS system [21], which may ambitiously support Brandes's observation. However, the further study in Y-Ti-O ODS system by experiment and simulation is eagerly needed. The chemical composition of the nano oxides was analyzed by EELS. Fig. 4(b) shows the EELS mapping result obtained in 0.4-9CrYT. The spectrums of the oxide and the matrix in the energy range between 300 eV and 800 eV are shown in Fig. 4(c) and (d), showing the Ti-L2,3 peak appears in oxide spectrum but misses in the matrix spectrum. The signals from the Fe-L2,3 peak show weaker contrast in the oxide, indicating there is less iron content. The signals from the Y-L2,3, Ti-L2,3
Table 3 Statistical analysis of oxides in ODS alloys. Mn(Ti)Cr2O4
9CrY 0.1-9CrYT 0.4-9CrYT 1.0-9CrYT
Y-Ti-O nano-oxides
Average size (nm)
Density (m− 3)
Average size (nm)
Density (m− 3)
103 73 71 83
6.9 × 1019 8.1 × 1019 8.1 × 1019 4.9 × 1019
4.2 3.9 3.0 6.5
5.9 × 1022 8.2 × 1022 13.3 × 1022 4.5 × 1022
[111]matrix direction. It is clearly to show the atomic planes of the matrix in Fig. 4, but there is no specific lattice structure has been observed in the NC. It suggests that they are not Y2Ti2O7, Y2TiO5 or Y2O3. Similar NCs were observed in 14YWT and other ODS alloys [19,20]. In this study, we obtain the NCs by tuning the Y/Ti ratio in 9Cr ODS alloys. The best recipe to produce the finest size and highest density of NCs in 9Cr ODS alloys is presented. The excellent NCs distribution can largely enhance the radiation resistance and mechanical properties of 37
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Fig. 3. HAADF images of nano oxides in (a) 9CrY; (b) 0.1-9CrYT; (c) 0.4-9CrYT; (d) 1.0-9CrYT; (e) size distributions of nano oxides in four ODS alloys.
Fig. 4. (a) High-resolution HAADF image of a 2.5 nm NC; (b) EELS mapping of a NC in 0.4-9CrYT, Fe-L2,3; Y-L2,3; Ti-L2,3; O-K. (c) EELS spectrum of matrix; (d) EELS spectrum of oxide in (b).
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Fig. 5. (a) HRTEM image of a Y2Ti2O7 in 1.0-9CrYT; (b) HRTEM image of a Y2O3 in 0.1-9CrYT.
and O because very high density metastable Y-Ti-O NCs are preferred to re-precipitate. Compared with the pyrochlore-structure Y2Ti2O7, the precipitation of Y-Ti-O NCs needs lower formation energy because of their coherent structure with ferritic matrix. High density of coherency nano oxides can significantly improve the material ductility, creep resistance and radiation resistance [7,18,26,27], which is the preferred oxide type for designing ODS alloy. Barnard predicted the oxide evolution along with the O concentration in ODS alloys by computer simulation [23]. He found that when the O concentration was below the threshold, only Y2O3 appeared as precipitate phase in matrix, and all Ti remained in solution [23]. In the processing of ODS alloy, O concentration is mainly controlled by the addition of Y2O3. In 0.1-9CrYT, low concentration of oxygen may suppress the formation of Y-Ti-O nano oxides, but be in form of larger Y2O3. Although there are still many NCs and Y2Ti2O7 formed in 0.19CrYT as shown above, the formation of larger Y2O3 may increase the average size of the nano oxides and decrease the oxide density. This computer simulation results do qualitatively support our TEM observation, suggesting the low concentration of initial Y2O3 addition will jeopardize the oxide distribution in ODS alloy. It is well to know that, higher density and finer size of nano oxides are beneficial to improve the properties of ODS alloys. According to the result from this study, the optimal concentrations of initial Y2O3 and Ti are around 0.3 wt% and 0.3 wt%, i.e. Y/Ti = 0.4, in 9Cr-ODS alloys.
and O-K show strong contrast in the oxide, indicating the nano oxides are Y-Ti-O type. The diameter of the oxide shown in Fig. 4(b) is about 10 nm. It is difficult to obtain EELS mapping from the oxide smaller than 3 nm due to relatively large thickness of the specimen and large excitation volume. The structure of the oxide larger than 3 nm has been observed by HRTEM. Pyrochlore-structured Y2Ti2O7 oxides are found in all Ti-added ODS alloys. Fig. 5(a) shows a HRTEM image of Y2Ti2O7 oxide observed in 1.0-9CrYT ODS alloy with an incident electron is parallel to the [124]oxide zone axis. Y2O3 are observed in 0.1-9CrYT and 9CrY ODS alloy but not in the other ODS alloys. Fig. 5(b) shows a HRTEM image of Y2O3 observed in 0.1-9CrYT ODS alloy with an incident beam of [301]oxide zone axis. No orthorhombic and hexagonal Y2TiO5 is observed in these alloys as reported in other studies [22]. 4. Discussion Ukai et al. found that the addition of Ti refined the nano oxides in ODS alloys [12]. Barnard et al. predicted that the particle refinement by small addition of Ti was due to the increased opportunity and driving force for nucleation of Y-Ti-O oxides over Y2O3 [23]. Ti refinenment effect has been directly observed by advanced STEM characterization in this study. Comparing the 9CrY and 0.4-9CrYT, which both includes 0.3% Y2O3, 0.4-9CrYT exhibits much finer and higher density of oxides than 9CrY because of the Ti addition. However, with the fixed 0.3% Ti, tuning the concentration of initial Y2O3 can also significantly affect the oxide distribution. As shown in Fig. 2, overdose (1.0-9CrYT) or insufficient (0.1-9CrYT) Y2O3 addition causes the coarsening of nano oxides in ODS alloys. Thus, studying how to control the oxide behavior by tuning the initial element addition and sequentially understanding the formation mechanisms of nano oxides are very important for controlling the microstructures and optimizing the fabrication processing of ODS alloys. It is well known that initial Y2O3 powders can decompose into Y and O atoms during MA and simultaneously dissolve into the ferritic matrix to form a supersaturated solid solution. High energy MA process can introduce high density dislocations and point defects in the matrix, which provide preferred precipitate sites for the nano oxides. During the high temperature consolidation, the addition of Ti will combine with Y and O and re-precipitate as Y-Ti-O nano oxides in the matrix [24,25]. Previous atom probe tomography (APT) studies showed that, Y-Ti-O NCs were normally composed with a low atomic ratio in a very fine size, while larger oxides (Y2Ti2O7 and Y2TiO5) were composed with a higher Y/Ti atomic ratio, indicating that the oxide size and oxide type formed in ODS alloy were actually dominated by the Y/Ti ratio [1]. In this study, high concentration Y and O in the 1.0-9CrYT alloy sufficiently associates with Ti to form a great number of stable pyrochlorestructure Y2Ti2O7 oxides. But in 0.4-9CrYT alloy, the formation of Y2Ti2O7 oxides is suppressed obviously by reduced concentration of Y
5. Summary Four 9Cr ODS alloys were produced by MA and HIP with varied initial Y2O3 and Ti additions. The microstructures of ODS alloys were characterized by HRTEM, STEM, EDS and EELS. High density of Y-Ti-O NCs and Y2Ti2O7 oxides, accompanied with large spinel Mn(Ti)Cr2O4 are observed in all Ti-added ODS alloys. Y2O3 are found in 0.1-9CrYT and 9CrY ODS alloys. 0.4-9CrYT presents the highest number density and the finest size of Y-Ti-O nano oxides among the four alloys. It is estimated that 90% of all the nano oxides in 0.4-9CrYT were coherent Y-Ti-O NCs. The optimal recipe of 9Cr-ODS alloy for obtaining the best oxide distribution has been studied and presented with 0.3% Ti and 0.3% Y2O3 additions (Y/Ti = 0.4). Acknowledgements This research is supported by the National Natural Science Foundation of China (51471049). References [1] G.R. Odette, D.T. Hoelzer, Irradiation-tolerant nanostructured ferritic alloys: transforming helium from a liability to an asset, JOM 62 (2010) 84–92.
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