Journal of Nuclear Materials xxx (2013) xxx–xxx

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Journal of Nuclear Materials

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Effects of titanium concentration and tungsten additionon the nano-mesoscopic structure of high-Cr oxide dispersionstrengthened (ODS) ferritic steels

Peng Dou a,⇑, Akihiko Kimura a, Ryuta Kasada a, Takanari Okuda b, Masaki Inoue c, Shigeharu Ukai d,Somei Ohnuki d, Toshiharu Fujisawa e, Fujio Abe f

a Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb Kobelco Research Institute, 1-5-5 Takatsukadai, Nishi-ku, Kobe, Hyogo 651-2271, Japanc Advanced Nuclear System R&D Directorate, Japan Atomic Energy Agency, 4002 Narita, O-arai, Ibaraki 311-1393, Japand Graduate School of Engineering, Hokkaido University, N13, W8, Kita-ku, Sapporo 060-8628, Japane Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japanf Structural Metals Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

a r t i c l e i n f o

Article history:Available online xxxx

0022-3115/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jnucmat.2013.04.090

⇑ Corresponding author. Address: City University ofKowloon, Hong Kong. Tel.: +852 3442 7468; fax: +85

E-mail address: doup@tsinghua.edu.cn (P. Dou).

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a b s t r a c t

To study the effects of titanium concentration and tungsten addition on the nano-mesoscopic structure ofhigh-Cr oxide dispersion strengthened (ODS) ferritic steels, the spatial and size distributions, shape, andcoherency of the oxide particles in SOC-5 (Fe–15.95Cr–0.09Ti–0.34Y2O3) and SOC-P3 (Fe–13.32Cr–1.9W–0.16Ti–0.33Y2O3) were studied by diffraction contrast techniques, including weak beam electron micros-copy. When the titanium concentration is increased from 0.09 to 0.16 wt.% with 1.9 wt.% W added, thegrain size decreases considerably, while the number density and mean size of the oxide particles signif-icantly increases and decreases, respectively. �63.5% and �96% of the oxide particles in SOC-5 and SOC-P3 (diameter <4.5 nm), respectively, are coherent with the bcc steel matrix. In SOC-5, �36% of the oxideparticles (4.5–10 nm in diameter) are semi-coherent; misfit moiré fringes are seen across only �12% ofthe oxide particles with misfit moiré fringe spacing of 1.12 nm, indicating the misfit strain is 0.11. How-ever, only �4% of the oxide particles in SOC-P3 (4.5–10 nm in diameter) are semi-coherent; misfit moiréfringes are seen across only �2% of the oxide particles with misfit moiré fringe spacing of 0.98 nm, indi-cating the misfit strain is 0.126. The mean misfit strain of coherent oxide particles in SOC-5 is �0.017.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Future fusion energy will require new high-performancestructural alloys with outstanding properties sustainable underlong-term service in ultra-severe environments, including highoperating temperatures (�773–1273 K), large time-varying stres-ses, intense neutron radiation field leading to high level of neutrondamage producing up to 200 dpa and 2000 appm of helium, andchemically-reactive environments (e.g., highly corrosive coolants)[1,2]. The stringent requirements of structural materials for fusionreactors include superior high-temperature strength; excellentcreep resistance; high resistance to irradiation damage andswelling; compatibility with cooling media (high resistance to cor-rosion in coolants such as lithium, lithium-lead, lithium-tin, Flibe(LiF–BeF2 or Li2BeF4), Flinak (LiF–NaF–KF), supercritical pressur-ized water (SCPW) and He); a low ductile-to-brittle transition

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temperature with extremely low susceptibility to thermal agingembrittlement; high resistance to hydrogen embrittlementand/or helium embrittlement; high resistance to stress corrosioncracking (SCC) in hot pressurized water; high thermal conductiv-ity; low thermal expansion coefficient; low residual activation;excellent workability; and good weldability [1–5].

High-Cr ODS ferritic steels are promising structural materialsfor nuclear reactors because they exhibit high resistance tocorrosion in SCPW [4], lead-bismuth eutectic (LBE) [4], and Flinak(LiF–NaF–KF) [5]; superior mechanical properties (e.g., tensile andcreep strength at high temperature) [6,7]; high irradiationresistance to hardening, swelling, and embrittlement due toself-healing mechanism [8–10]; high resistance to hydrogenand/or helium embrittlement [11,12]; high resistance to SCC inhot pressurized water [13]; and good weldability [14]. To improvethe corrosion resistance to SCPW, two high-Cr ODS steels (SOC-5(Fe–15.95Cr–0.09Ti–0.34Y2O3) and SOC-P3 (Fe–13.32Cr–1.9W–0.16Ti–0.33Y2O3)) have been newly developed for fusion reactorapplication. It was found that SOC-P3 has excellent strength andoptimal creep resistance, which are much better than that of SOC-5.

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To clarify the underlying mechanism for the superior mechani-cal strength of SOC-P3 owing to the highly stabilized oxide parti-cles and understand the physics process for the formation ofnanometer-scale Y–Ti–O features with different Ti contents andW addition so as to achieve optimum chemical composition andthe improvement of ODS steels, the spatial and size distributions,shape, and coherency of the oxide particles in SOC-5 and SOC-P3were studied by diffraction contrast techniques, including weakbeam electron microscopy.

2. Experimental

Two of the newly developed high-Cr ODS ferritic steels, i.e.,SOC-5 and SOC-P3, which have chemical compositions ofFe–0.04C–0.01Si–0.01Mn–15.95Cr–0.09Ti–0.34Y2O3 and Fe–0.046C–0.03Si–0.03Mn–13.32Cr–0.16Ti–1.9W–0.33Y2O3, respectively, wereused in the present research. The fabrication procedure of theODS steels is detailed in Ref. [15].

Disc type transmission electron microscopy (TEM) specimenswith 3 mm diameter were punched from the sheets parallel andperpendicular to the extrusion axis, mechanically thinned to�100 lm and then electro-polished in a TENUPOL device usingHClO4 + 95%CH3COOH as electrolyte at around 293 K. The nano-mesoscopic structures of the ODS steels, especially the morphologyand coherency of oxide particles, were characterized by TEM usinga JEOL JEM 2010 transmission electron microscope (acceleratingvoltage 200 kV) equipped with a double tilt specimen holder. Foilthickness was determined by the convergent beam electron dif-fraction method.

Specifically, the metal/oxide interface structures of the nano-particles in SOC-5 and SOC-P3 were studied by diffraction contrasttechniques, including weak beam electron microscopy. The detailsof the principle and process for the judgment of particle coherencyfrom Ashby and Brown contrast [16] and McIntyre and Browncontrast [17], and the methods of measuring the lattice misfitand misfit strain in the matrix surrounding the coherent andsemi-coherent particles from diffraction contrast images aredescribed in Section 2.2 and Section 2.3 of Ref. [2], respectively.The shear moduli of SOC-5 and SOC-P3 were all assumed to beequal to that of PM 2000 (65 GPa at room temperature) [18]. TheYoung’s moduli, E, of the matrix of SOC-5 and SOC-P3, were thenestimated to be 169 GPa, because t is 0.3. The bulk modulus ofY2TiO5 particle, K, is 134.6 GPa [19].

3. Results and discussion

3.1. Effect of titanium concentration and tungsten addition on grainmorphology

The grain morphologies of SOC-5 and SOC-P3 are shown inFig. 1a–d, respectively. The mean intercept grain diameters ofSOC-5 (Fig. 1b) and SOC-P3 (Fig. 1d) measured in a plane perpen-dicular to the extrusion axes are 1.18 lm (uncertainty: +0.3 lmand �0.2 lm) and 0.78 lm (uncertainty: +0.1 lm and �0.3 lm),respectively. For SOC-5 (Fig. 1a), the average grain length, averagegrain width, and grain aspect ratios (GARs) measured in a planeparallel to the extrusion axes are 3.96 lm (uncertainty: +0.45 lmand �0.4 lm), 1.16 lm (uncertainty: +0.35 lm and �0.25 lm),and 3.4 (uncertainty: +0.45 and �0.4), respectively. However, forSOC-P3 (Fig. 1c), the average grain length, average grain width,and GAR are 3.28 lm (uncertainty: +0.4 lm and �0.35 lm),0.75 lm (uncertainty: +0.15 lm and �0.25 lm), and 4.3 (uncer-tainty: +0.35 and �0.4), respectively. An average grain size de-creases significantly when the Ti concentration is increased from0.09 to 0.16 wt.% and 1.9 wt.% W is added.

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3.2. Effect of titanium concentration and tungsten addition on oxideparticle morphology

Fig. 2 shows the bright field images of the oxide particles dis-persed in the matrix of SOC-5 (Fig. 2a) and SOC-P3 (Fig. 2b).�63.5% of the oxides in SOC-5 and almost all the particles inSOC-P3 (�96%) are very small with diameter <4.5 nm. The verysmall oxide particles keep their shape under different diffractionconditions. Thus, it can be concluded that they are spherical. InSOC-5, the relatively larger particles (diameter P4.5 nm), whichconstitute �36% of the oxides in the ODS steel, appear to adopt acubical shape, Fig. 2a. Weak beam dark field images of the oxideparticles in SOC-5 and SOC-P3 are shown in Fig. 3a and b, respec-tively. According the contrast features, the oxide particles were di-vided into three groups, showing lobe–lobe contrast, black/whitedot contrast, and oxide particles surrounded by fringes, whichare thought to be misfit moiré fringes.

When determining the number density and size distributionof the oxide particles in SOC-5 and SOC-P3, all the three groupsof particles were counted and measured together with no dis-tinction made among them. The length of the no contrast linewas taken as the diameter of the oxide particles exhibitinglobe–lobe contrast [16]. The diameter of the oxide particlesshowing black/white dot contrast was regarded as the diameterof the dots [3,20]. The diameter of the semi-coherent oxide par-ticles surrounded by misfit moiré fringes was defined as theaverage length along and perpendicular to the misfit moiréfringes [3]. The total number density was estimated based onthe bright field images shown in Fig. 2 and other bright-fielddown-zone images.

The size distribution histograms of the oxide particles in bothODS steels are presented in Fig. 4. SOC-5 has bimodal oxide particlesize. There are two local maximum numbers of oxide particles at1–2 nm and 4–5 nm. For the oxide particles in SOC-5 and SOC-P3, the mean diameters are 3.85 and 2.8 nm, the total number den-sities are 7.07 � 1022 and 1.46 � 1023 m�3, the inter-particle spac-ings are 60.6 and 49.5 nm, and the volume fractions are 0.211% and0.168%, respectively. When the Ti concentration is increased from0.09 to 0.16 wt.% with 1.9 wt.% W added, the mean diameter andinter-particle spacing of oxide particles decrease considerablywhile the number density increases very remarkably, which maylead to more intensive suppressing of dislocation loop growth[21] and, consequently, still better irradiation resistance due tothe significantly enhanced self-healing mechanism resulting fromthe presence of much higher proportion of metal/oxide interfaceto bulk [9,10].

In general, many factors influence the spatial and size distri-butions of Y–Ti complex oxide particles (e.g., the amount ofexcess oxygen [22–24], the Ti concentration [22–25], theprocessing temperature history (especially the consolidationtemperature) [26], and the W addition [24]). It was found thatincreasing Ti concentration is very effective for producing fineroxide particles with higher number density [24,25], and more-over, the effect of W on the microstructure is smaller than thatof excess O and Ti, but still produces a favorable effect in reduc-ing the size and increasing the number density of the oxide par-ticles [24].

Note that the amount of excess oxygen and fabrication proce-dure (including the processing temperature) of SOC-5 are similarto those of SOC-P3. However, the Ti concentrations of SOC-5 andSOC-P3 are 0.09 and 0.16 wt.%, respectively, and moreover, theconcentration of W of SOC-P3 is 1.9 wt.%, while there is no W inSOC-5. Therefore, the experimental fact that SOC-5 has oxide par-ticles with larger size and much lower number density, relative toSOC-P3, is due to the much lower concentration of Ti and the ab-sence of W.

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Fig. 1. Grain morphologies of SOC-5 and SOC-P3 in: (a) longitudinal section of SOC-5, (b) transverse section of SOC-5, (c) longitudinal section of SOC-P3, and (d) transversesection of SOC-P3.

P. Dou et al. / Journal of Nuclear Materials xxx (2013) xxx–xxx 3

3.3. Effect of titanium concentration and tungsten addition on thecoherency of oxide particles

In Fig. 3, the image contrast of the oxides seems to depend onthe particle size. For the very small oxide particles (diameter<4.5 nm) in SOC-5 and SOC-P3, some are evidently coherent withthe bcc steel matrix according to Ashby and Brown contrast behav-ior [16] because they appear as small lobe-lobe contrast with a nocontrast line perpendicular to the g vector. However, others are ob-served as dot contrast. Based on equation: PS ¼ gjejr3

0n�2g (where g

(g ¼ d�1hkl), e, r0, and ng denote the modulus of the active diffraction

vector, the misfit strain, the constrained particle radius, and thecorresponding extinction distance, respectively) [17], an estimateof the maximum possible value of the parameter PS (assuming aparticle size of 4.5 nm and a misfit strain of 0.1) yields values from0.006 to 0.02 for all reasonable diffraction conditions (e.g., forFig. 3, g ¼ d�1

hkl is 0.2027�1 nm�1, and ng calculated to be 50.1 nm[27]). In general, the modulus of the misfit strain of coherent pre-cipitates is less than 0.05 (|e| 6 0.05). Hence, 0.1 is a conservative

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assumption and, therefore, PS is far below McIntyre and Brown’scritical value of PS 6 0.2 [17].

Therefore, the strain field contrast behavior of the very smalloxide particles in SOC-5 and SOC-P3, which is shown in Fig. 3,agrees well with the contrast features of tiny precipitates proposedby McIntyre and Brown [17] and mentioned in Section 2.2 of Ref.[2]. Evidently, whether the very small oxide particles appear assmall lobes with no contrast line perpendicular to the g vector oras black/white dots only depends on their depth in TEM foils. Itis reasonable to assume that the very small oxide particles appear-ing as black/white dots are also coherent with the matrix. Theassumption has already been confirmed by high-resolution trans-mission electron microscopy (HRTEM) analyses on both ODS steels,and will be detailed in another paper.

Although 180 and 200 very small oxide particles in SOC-5 andSOC-P3, respectively, have been analyzed by HRTEM, no semi-coherent or incoherent oxide has been identified because all thevery small oxide particles were found to be coherent with thematrix. HRTEM observations indicate that the relatively larger

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Fig. 2. Bright field images of the oxide particles dispersed in the matrix of SOC-5 (a) and SOC-P3 (b) under near dynamical two-beam imaging conditions and in slightlyunder-focused regime.

Fig. 3. Weak beam dark field images of the oxide particles dispersed in the matrixof SOC-5 (a) and SOC-P3 (b).

Fig. 4. Size distributions of the oxide particles dispersed in the matrix of SOC-5 andSOC-P3. Black bars denote SOC-5 and white bars denote SOC-P3.

4 P. Dou et al. / Journal of Nuclear Materials xxx (2013) xxx–xxx

particles (4.5–10 nm in diameter) in SOC-5 and SOC-P3 tend to besemi-coherent. According to the statistics results based on Fig. 4,coherent oxides constitute �63.5% and �96% while semi-coherentoxides constitute �36% and �4% of the particles in SOC-5 andSOC-P3, respectively. Misfit moiré fringes were seen across semi-coherent particles constituting only �12% and �2% of the oxides

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in SOC-5 (Fig. 3a) and SOC-P3 (Fig. 3b), respectively. According tothe results of HRTEM analyses, the large particles (diameter>10 nm) in SOC-5 and SOC-P3 tend to be incoherent. Thus, thecoherency of the oxides in both ODS steels is size-dependent.

The size dependence of the particle coherency was analyzedfrom the micrographs shown in Fig. 3 and other weak beam darkfield images, and the statistics results of the proportion of coher-ent, semi-coherent, and incoherent particles are present in Table1. The coherency of the oxide particles with the bcc steel matrixis enhanced very significantly with the titanium concentrationincreased from 0.09 to 0.16 wt.% and 1.9 wt.% W is added.Incidentally, most of the very small particles in both ODS steelswere identified as hexagonal Y2TiO5 oxide while most of the rela-

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Table 1Coherency of oxide particles dispersed in the matrix of SOC-5 and SOC-P3.

Steels SOC-5 Uncertainty SOC-P3 Uncertainty

Coherent Diameter range (nm) <4.5 – <4.5 –Number ratio (%) 63.5 – 96 –Misfit strain e 0.017 +0.002–0.002 – –

Semi-coherent Diameter range (nm) 4.5–10 – 4.5–10 –Number ratio (%) 36 (12%) – 4 (2%) –Spacing of misfit moiré fringes (nm) 1.12 +0.02–0.02 0.98 +0.02–0.01Lattice misfit d 0.181 +0.003–0.003 0.207 +0.002–0.004Misfit strain e 0.11 +0.002–0.002 0.126 +0.001–0.003

Incoherent Diameter range (nm) >10 – >10 –Number ratio (%) 0.5 – / –

P. Dou et al. / Journal of Nuclear Materials xxx (2013) xxx–xxx 5

tive larger particles in SOC-5 and SOC-P3 (diameter >4.5 nm) wereidentified as orthorhombic or hexagonal Y2TiO5 oxide by theHRTEM analyses.

3.4. Effect of titanium concentration and tungsten addition on themisfit strain of oxide particles

For SOC-5, measurements and calculations of misfit strain, e,were done on 30 coherent particles, appearing as small black lobesin two beam dynamic bright field images or nearly strong twobeam bright field images, which are shown in Fig. 2a and otherimages. The mean e of coherent particles in SOC-5 is �0.0145.However, the e obtained by Ashby–Brown technique is alwaystoo low by as much as 15% because of the system error due tothe two-beam over-estimation of extinction distance [16]. There-fore, the real mean e of coherent oxide particles in SOC-5 is �0.017.

However, for coherent particles in SOC-P3, at least three differ-ent values of misfit strain have been obtained. Because the propor-tion of the coherent particles exhibiting lobe-lobe contrast with nocontrast line perpendicular to the g vector is extremely low whilealmost all the coherent particles appear as black/white dots, Figs.2b and 3b, it is very difficult to get statistic results of the propor-tion of coherent oxide particles with different specific value of mis-fit strain from diffraction contrast images alone.

The misfit moiré fringe spacing (DM) of semi-coherent particles inSOC-5 is 1.12 nm (Figs. 2a and 3a), according to which the modulusof the lattice misfit, d, was calculated to be�0.181, based on Eq. (4) inRef. [2] since dð1�10Þ is 0.2027 nm, and then the modulus of the e wascalculated to be �0.11, based on Eq. (2) in Ref. [2]. The DM of semi-coherent particles in SOC-P3 is 0.98 nm (Fig. 3b), according to whichthe modulus of the d was calculated to be�0.207, and then the mod-ulus of the e was calculated to be�0.126. The e of coherent particlesin SOC-5 and SOC-P3, and the moduli of the d and e of semi-coherentparticles in both ODS steels are also listed in Table 1.

The yield stresses of SOC-5 and SOC-P3 at room temperature are910 and 1285 MPa, respectively. That is, the mechanical strengthwas successfully improved with the Ti concentration increasedfrom 0.09 to 0.16 wt.% and 1.9 wt.% W added. This is mainly attrib-uted to the increased grain boundary strengthening due to the sig-nificant reduction in grain size (Section 3.1), the enhanced particlestrengthening due to the remarkably finer oxides with much den-ser dispersion [28] (Section 3.2), and the improved solid solutionstrengthening due to the addition of 1.9 wt.% W. W (ferrite-formerelements) improves the creep strength, thermal stability, and cor-rosion resistance [5] of ODS alloys.

Increasing the Ti concentration from 0.09 to 0.16 wt.% with1.9 wt.% W added leads to excellent lattice coherency betweenthe oxide particles and the bcc steel matrix (Section 3.3 and Table1 in Section 3.4), which gives rise to very low interface energy. Thelow interface energy of coherent oxides, which reduces the Gibbs–Thomson effect at the interface, the very low solubility of O and Y

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in bcc Fe [29], and the extremely low diffusion coefficient of Y ele-ment in a-Fe matrix at various temperatures [30], can effectivelyprevent the coarsening of the Y–Ti–O particles. Moreover, if a sol-ute atom (e.g., Y, and/or Ti) can attach less easily to a coherentinterface, it could be imagined that this process is even more diffi-cult if the particle has a complex lattice structure composed ofthree or two distinct elements, as is the case for the orthorhom-bic/hexagonal Y2TiO5 complex oxide. This can also lead to theexcellent coarsening resistance of the oxide particles at high tem-perature. Finally, W improves the thermal stability of ODS steels bysuppressing the coarsening of oxides. Therefore, oxide particles inSOC-P3 can be expected to be very stable and exhibit still higherthermal stability than those in SOC-5, leading to even higher capa-bility for keeping attractive properties during long-term exposureto high temperature for nuclear applications.

Creep threshold stresses observed in precipitation-, or disper-sion-strengthened alloys are explained by mechanisms based onparticle shearing [31], bypass by climb [31,32], or detachment[33]. The former two are applicable to coherent [31,32,34] andsemi-coherent particles [35] with the lattice misfit and misfitstrain having significant influence on the creep threshold stresses[31,32,34,35] while the third is operative for incoherent particles[33]. Thus the proportion of coherent, semi-coherent, and incoher-ent particles in both ODS steels and the magnitude of lattice misfitand misfit strain of the oxides have been obtained in this work. Inaddition, the magnitude of lattice misfit and misfit strain of theoxide particles has been obtained by HRTEM and will be detailedin another paper with the quantitative analyses of the creepstrengthening in both ODS materials.

4. Conclusions

For the two newly-developed high-Cr ODS ferritic steels, i.e.,SOC-5 (Fe–15.95Cr–0.09Ti–0.34Y2O3) and SOC-P3 (Fe–13.32Cr–1.9W–0.16Ti–0.33Y2O3), the following was determined:

(1) When the titanium concentration is increased from 0.09 to0.16 wt.% with 1.9 wt.% W added, the grain size decreasesconsiderably, and the number density and mean diameterof the oxide particles significantly increase and decrease,respectively.

(2) �63.5% and �96% of the oxide particles in SOC-5 and SOC-P3(diameter <4.5 nm) are coherent with the bcc steel matrix,respectively. �36% and only �4% of the oxide particles inSOC-5 and SOC-P3 are semi-coherent, respectively.

(3) Misfit moiré fringes are seen across only �12% and �2% ofthe oxide particles in SOC-5 and SOC-P3, with the misfitmoiré fringe spacings of 1.12 and 0.98 nm, indicating themisfit strains are �0.11 and �0.126, respectively.

(4) The mean misfit strain of coherent oxide particles in SOC-5 is�0.017.

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Acknowledgements

Present study includes the result of ‘‘R&D of corrosion resistantsuper ODS steel for highly efficient nuclear systems’’ entrusted toKyoto University by the Ministry of Education, Culture, Sports, Sci-ence and Technology of Japan (MEXT).

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