Microstructural characterization and strengthening mechanisms of a 12Cr-ODS steel

Author’s Accepted Manuscript Microstructural characterization and strengthening mechanisms of a 12Cr-ODS steel Jingjie Shen, Yanfen Li, Feng Li, Huilong Yang, Zishou Zhao, Sho Kano, Yoshitaka Matsukawa, Yuhki Satoh, Hiroaki Abe www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(16)30783-3 http://dx.doi.org/10.1016/j.msea.2016.07.030 MSA33861

To appear in: Materials Science & Engineering A Received date: 17 June 2016 Revised date: 6 July 2016 Accepted date: 8 July 2016 Cite this article as: Jingjie Shen, Yanfen Li, Feng Li, Huilong Yang, Zishou Zhao, Sho Kano, Yoshitaka Matsukawa, Yuhki Satoh and Hiroaki Abe, Microstructural characterization and strengthening mechanisms of a 12Cr-ODS s t e e l , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.07.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microstructural characterization and strengthening mechanisms of a 12Cr-ODS steel Jingjie Shen

a,*

, Yanfen Li

b,*

, Feng Li a, Huilong Yang b, Zishou Zhao a, Sho Kano c, Yoshitaka

Matsukawa b, Yuhki Satoh b, Hiroaki Abe b,c a

Graduate School of Engineering, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan

b c

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan

School of Engineering, The University of Tokyo, 2-22 Shirakata Shirane, Tokai-mura, Ibaraki

319-1188, Japan [email protected] (J.J. Shen) [email protected] (Y.F. Li) *

Corresponding authors.

Abstract The microstructure of the 12Cr-ODS steel was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques. The results showed that the microstructure consisted of fine and elongated grains, high density of dislocations, and large concentration of nm-scale oxide particles, which were identified as five types of crystal structures by high resolution TEM (HRTEM) imaging, such as monoclinic and cubic Y2O3, hexagonal and orthorhombic Y2TiO5, and cubic Y2Ti2O7. The dominant {001}<110> component and weaker <111> parallel to normal direction texture were observed. Besides, the yield stress was experimentally measured and quantitatively estimated. The findings indicated that theoretical calculation was in accordance with the experimental measurements, and strengthening contributions from solid-solution atoms and grain boundaries were linearly additive, whereas those from dislocations and oxide dispersoids were averaged by root mean square summation. Keywords: oxide dispersion strengthened steels, microstructure, nm-scale oxide particles, HRTEM, strengthening mechanisms 1. Introduction 1

Oxide dispersion strengthened (ODS) steels have been developing as candidate materials for fuel cladding tubes in fast breeder reactors and blanket applications for fusion reactors. The steels exhibit remarkable tensile, creep strength and excellent irradiation damage tolerance in comparison with other conventional heat resistance steels and austenitic steels [1-6]. These exceptional properties are closely related to the addition of high density of nm-scale oxide dispersoids by mechanical alloying. Consequently, a great number of studies on the nm-scale oxide particles, such as crystal structure, chemistry composition, and orientation relationship with matrix, have been investigated by means of transmission electron microscopy (TEM), and/or atom probe tomography (APT) [7-10]. They revealed that there were several kinds of particles in Ti-doped ODS steels, such as Y2O3 with cubic structure [11], Y2Ti2O7 with pyrochlore structure [11-13], Y2TiO5 with either hexagonal [12] or orthorhombic structure [14], YTiO3 [15], TiCr2O4 [16], Ti(C,O,N) [10, 17], and M23C6 [17]. The advantage of those nm-scale oxide particles is described as follows: (1) to inhibit the migration of dislocations and grain boundaries to enhance strength; (2) to provide sinks for the irradiation induced defects; and (3) to supply nucleation sites for helium bubbles to suppress bubble formation at grain boundaries [18, 19]. All of the above attribute to retardation of loss of toughness at lower irradiation temperatures and to potential degradation of creep strength at higher temperatures. To fabricate bulk ODS steels, mechanical alloying and subsequent consolidation with hot isotropic pressing and/or hot extrusion are the common processes, which generally produce fine grain microstructure with high density of dislocations and nm-scale oxide particles. Therefore, grain boundary strengthening, dislocation strengthening and oxide dispersion strengthening were considered as predominant strengthening mechanisms in the ODS steels [20-22]. However, few attempts have been done to clarify the strengthening mechanisms in ODS steels by taking all the relevant strengthening contributors into consideration, and to provide reasonable models to calculate the yield stress in comparison with the experimental results. For instance, Song et al. [20] reported the hardening mechanisms of a 12Cr-ODS steel (Fe-12Cr-1.1W-0.2V-0.14Ta0.002B-0.3Y2O3) were composed of grain boundary hardening and dispersion hardening, whereas the dislocation hardening was not considered. In addition, Schneibel et al. [21] summarized the calculated yield stress of PM2000 (Fe-20Cr-5.5Al-0.5Ti-0.5Y2O3) and 14YWT (Fe-14Cr-3W-0.4Ti-0.25Y2O3) alloys in literature, which were highly overestimated with an error ranging from 7% to 111%. 2

Thus, in this study, the comprehensive microstructural characterization on a 12Cr-ODS steel was performed to achieve the structural parameters, and then the strengthening mechanisms were elucidated based on the comparison between the theoretical calculation and experimental measurements. 2. Experimental procedure 2.1 Material The 12Cr-ODS steel with a nominal composition of Fe-12Cr-2W-0.3Ti-0.25Y2O3 fabricated by KOBELCO was utilized in this study. The chemical composition is listed in Table 1. This alloy was prepared by mechanically alloying a mixture of base metal and Y2O3 powders in a milling attritor at a speed of 250 r.p.m. for 48 hours under an argon atmosphere. The resultant powders were, then, degassed for 3 hours at 673 K in a vacuum condition (< 0.1 Pa), canned in mild steel and consolidated into a 30-mm diameter bar by hot extrusion at 1423 K. The bar was hot forged into a sheet at 1423 K and then annealed at 1373 K for 1 hour. Finally, the sheet was machined and cold rolled with 40% thickness reduction, and followed by annealing at 1323 K for 1 hour with air cooling. 2.2 Microstructural characterization A Rigaku Ultimate IV system with Cu Kα radiation was used for X-ray diffraction (XRD) measurement, which was operated at 40 kV and 40 mA. In order to calibrate and eliminate the broadening effect caused by instruments, the specimen was synchronously examined with a silicon wafer. The diffraction peaks were fitted by Lorentzian function, from which the Bragg angle θ and the full width at the half maximum intensity (FWHM) were obtained. The lattice parameter a was decided by the Cohen-Wagner extrapolation plot (ahkl vs. cos2θ/sinθ) [23], and the micro-strain was determined by means of Williamson-Hall equation [24]. cos θ

0.9

2

sin θ

(1)

where B is the integral widths that were attained by eliminating the instrumental broadening, θ is the diffraction angle,

is the wavelength of X-ray, D is the grain size, and

matrix. Hence, the dislocation density ρ was derived as [25]: ρ

14.4

2 2

(2)

3

is the strain in

1

where b is the magnitude of the Burgers vector that was assumed as 2 a<111> in this work. Microstructure and crystallographic texture were characterized by a JEOL JXA 8530 fieldemission scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) device. Texture intensity was determined by orientation distribution function (ODF), which was computed by the series expansion method using TSL OIM analysis software. And the orientation was depicted in the form of the Euler angles (φ1, Ф, φ2) that were calculated under the assumption of orthonormal sample symmetry, i.e., 0°

{φ1, Ф, φ2}

90° [26].

Features of nm-scale oxide particles were observed utilizing JEM-ARM200F and JEM-2100 TEMs operating at 200 kV. 3-mm diameter TEM discs were mechanically thinned down to ~ 80 μm and electrochemically polished by a Tenupol-5 device in a solution of 5 vol. % perchloric acid and 95 vol. % acetic acid at room temperature. In order to measure the chemical composition of nanoparticles, high angle angular dark field (HAADF) studies and energy dispersive spectroscopy (EDS) mapping were performed in STEM mode by JEM-ARM200F. Besides, high resolution TEM (HRTEM) imaging by JEM-2100 and corresponding fast Fourier transformation (FFT) power spectrum indexing were carried out in the light of Pearson’s Crystal Data. The lattice spacing and inter-planar angles were measured by Gatan DigitalMicrograph and ImageJ software. To obtain the number density of nanoparticles, the thickness of the observed TEM foils were measured at two beam condition.

2.3 Uniaxial tensile test Dog-bone shaped tensile specimens with a gauge section of 5

1.2

0.5 mm were prepared

by electrical discharge machining. Tensile tests were carried out at room temperature with a strain rate of 6.67

10-4 s-1. The load and displacement values were recorded to get strain-stress

curve, from which the yield strength, 0.2% offset yield strength, was obtained. 3. Results and discussion 3.1 Microstructural characterization 3.1.1 Dislocation density The XRD profile and corresponding extrapolation plots are shown in Fig. 1. The diffraction peaks were indexed in Fig. 1a, and the lattice parameter a was also estimated as ~ 0.28742 nm in Fig. 1b. Besides, the micro-strain was decided based on the plot as shown in Fig. 1c. According 4

to Eq. (3), the dislocation density was approximately determined as ~ 6.8

1014 m-2, which was

in the same order as 14YWT alloy [27] and much higher than that of conventional alloys, indicating recovery slightly occurred during the final annealing presumably owing to inhibition of oxide particles. 3.1.2 Microtexture EBSD analysis of microstructure and texture of the specimen is shown in Fig. 2. The inverse pole figure (IPF) image exhibited the elongated grains whose long axes were parallel to the extrusion direction (or cold rolling direction). The grains were preferentially oriented to near <001> and <111>, which were denoted by red and blue colors in Fig. 2a, respectively. Moreover, the existence of preferential orientations was also confirmed in the φ2 = 45° section of ODF map. In Fig. 2b, {001}<110> component had particularly strong peak intensity which was more than 40 times higher than the theoretical estimation based on powder diffraction. The weaker <111> parallel to normal direction texture (γ-fiber) had an intensity of approximate 2 times random. This is a common feature of the ODS ferritic steel due to the heavy deformation processes in manufacture, such as hot extrusion and cold rolling [26, 28]. In addition, the grain size ranged from ~ 1 μm to ~ 8 μm, which was simply specified as the mean diameter of grains taken as circles. 3.1.3 Chemical composition, size distribution, and number density of nanoparticles Fig. 3 shows HAADF micrograph and EDS element mapping of nm-scale oxide particles. Segregation of Y, Ti and O in the nanoparticles was evident. The heavy element (Y) attributes large angle scattering of incoming electrons resulting in high contrast features in HAADF. Moreover, the EDS mapping image revealed that some nanoparticles contained Y and O, others were rich in Y, Ti, and O. Oxygen was observed in matrix as well, probably owing to oxidation of Fe-Cr matrix during electrochemical polishing or subsequent process. Fig. 4 shows the bright-field TEM image of dislocations and nanoparticles in matrix, and size distribution of the nanoparticles. Average diameter and number density of the oxide particles were determined as 3.5

0.7 nm and ~ 1.3

1023 m-3, respectively. The volume fraction of nm-

scale oxides was, then, estimated as ~ 0.3%.

5

3.1.4 Crystallographic structures of nanoparticles HRTEM imaging was performed to identify the crystal structures of the Y-O and Y-Ti-O enriched nanoparticles. Results are shown in Fig. 5 - 9 and Tables 2 - 6. Fig. 5 shows an HRTEM lattice image of an oxide particle together with the corresponding FFT power spectrum diffraction pattern. The measured inter-planar distances and angles are listed in Table 2. These values were well consistent with those of the monoclinic-Y2O3, which was indexed with [3̅ 1̅ 2̅ ] zone axis. Fig. 6 shows an HRTEM lattice image of a cubic-Y2O3 particle with indexation of the corresponding FFT image. Table 3 lists the measured and calculated inter-planar distances and angles shown in Fig. 6. The results were well in agreement with the cubic-Y2O3, which was ̅̅̅] zone axis. indexed with [5113 The monoclinic Y2O3 was rarely reported in ODS steels, except Refs. [29, 30], in which it was supplied as the starting powder for EUROFER 97 and a model ODS steel (Fe-12Cr0.4Y2O3). In the present work, the cubic Y2O3 powder was supplied. The structure transformation presumably proceeded during mechanical alloying process according to previous studies [31-33], which manifested that cubic Y2O3 could transform into a monoclinic modification by grinding with steel balls, on the contrary, the transformation from a monoclinic structure to a cubic structure would be achieved by annealing. Fig. 7 and 8 show the HRTEM lattice images of orthorhombic-, hexagonal-Y2TiO5 and the corresponding FFT images with indices. As shown in Table 4 and 5, the measured inter-planar distances and angles were reasonable in accordance with the [011̅] (Fig. 7b) and [0001] (Fig. 8b) zone axes. Fig. 9 shows an HRTEM lattice image of a cubic-Y2Ti2O7 particle with indexing of the corresponding FFT image. The results of measured and calculated inter-planar distances and angles are listed in Table 6, which were well in agreement with the cubic-Y2Ti2O7 indexed with [011] zone axis. Ti addition in ODS steels was extremely effective to reduce the size of oxide particles [34], and two types of combination were identified, i.e., Y2O3 + TiO2 → Y2TiO5, and Y2O3 + 2TiO2 → Y2Ti2O7 [35]. Therefore, Y2TiO5 and Y2Ti2O7 have been demonstrated as the main oxide particles in Ti-doped ODS steels [8, 11-14, 18]. In summary, the crystal structures and lattice parameters are listed in Table 7. There existed five types of nm-scale oxide particles in this 12Cr-ODS steel, such as monoclinic and cubic 6

Y2O3, hexagonal and orthorhombic Y2TiO5, and cubic-Y2Ti2O7. Monoclinic Y2O3 probably formed due to mechanical alloying was firstly detected in this alloy. Besides, cubic Y2O3, as the starting powder, was reasonably determined because of partial combination with Ti. In addition, Y2TiO5 and cubic-Y2Ti2O7 were mostly mentioned in ODS steels owing to Ti addition. 3.2 Strengthening mechanisms Based on the microstructural characteristics of this material, the strength can be contributed by solid-solution strengthening (σss), grain boundary strengthening (σg), dislocation strengthening (σdis) and dispersion strengthening from nm-scale oxide particles (σp). It is widely accepted that the yield stress can be estimated by a sum of strengthening contributions, that is, linear summation as follows: σy

σ0

σss

σg

σdis

σp

(3)

where σ0 is friction stress of single crystal pure iron, which was evaluated to be 50 - 60 MPa [36] and here equals to 53.9 MPa [37, 38]. The engineering strain-stress curve of this steel tested at room temperature is shown in Fig. 10. The value of yield strength was obtained as 1068 MPa. 3.2.1 Solid-solution strengthening The solid-solution strengthening is generally divided into interstitial strengthening and substitutional strengthening. Utilizing the Thermo-Calc software, the equilibrium concentration of carbon and nitrogen in matrix at room temperature was calculated as 6.5 10-11 and 7.5 10-11 in wt. %, respectively. Moreover, by Ti addition in this steel, coarse precipitates enriched Ti(C,O) were observed [17], thereby much smaller amount of carbon and nitrogen could be dissolved as interstitial atoms. Consequently, the strengthening contribution from carbon and nitrogen was negligible in this work. On the other hand, substitutional strengthening of chromium and tungsten was usually written as [39, 40]: σss

0.00689kCn

(4)

where k is the strengthening coefficient, such as the value 1400 for chromium, and 11000 for tungsten, C is the equilibrium concentration of substitutional elements in atomic percent, and n 7

equals to 0.75. According to Eq. (4), the strengthening component from substitutional solidsolution was evaluated as 67 MPa for chromium, and 51.8 MPa for tungsten, respectively. 3.2.2 Grain boundary strengthening It has been well known that the grain size of a polycrystalline material plays an important role on mechanical properties. The grain boundary strengthening is usually expressed by HallPetch equation [41, 42]: σg

kHP /√

(5)

where kHP is the Hall-Petch coefficient, and D is grain diameter, which was determined as ~ 2.8 2.2 μm by EBSD analysis in section 3.1.2. Takaki et al. [36] proposed kHP should be 600 MPa·μm1/2 in low-carbon (0.005 ~ 0.2% C) steels. On the contrary, Funakawa et al. [43] studied the effect of Cr content on the Hall-Petch coefficient in the ultra-low carbon steels, and reported kHP decreased by ~ 20 MPa·μm1/2 with per 1 wt.% Cr addition. As a result, kHP = 600 MPa·μm1/2 is not applicable in the high Cr ODS steels, which may be one reason for the overestimation of yield stress in Ref. [21]. Besides, Kim et al. [22] demonstrated that kHP of the 14YWT steels at room temperature was estimated as ~ 338 MPa·μm1/2 by fitting yield stresses dependent on the reverse square root of grain size, which seems consistent with the result in Ref. [43]. Therefore, kHP = 360 MPa·μm1/2 was used in the present study. 3.2.3 Dislocation strengthening For the dislocation strengthening, it can be estimated by Bailey-Hirsch relationship [44], which is widely accepted as: σdis

√ρ

(6)

where M is the Taylor factor that was recommended as 3.06 for most polycrystalline bcc metals [45],

is a constant taken as 0.38 [40, 46], G is the shear modulus for iron and equals to 81.6

GPa [37], b is the Burgers vector and ρ is the dislocation density, which were determined by XRD in section 3.1.1. 3.2.4 Dispersion strengthening

8

The Orowan by-pass mechanism, assuming the oxide particles are impenetrable, has been accepted to explain the dispersion strengthening, and it is expressed by Ashby-Orowan equation [47, 48]. σp

0.8 √1 -

2

ln ( 2 )

2

√ (√ - 1) 3 4

p

2

√

3

(7)

(8) (9)

p

where υ is the Poisson’s ratio that equals to 0.3, L is the average inter-particle spacing, x is the average diameter of particles on the slip planes, f is the volume fraction, and dp is the average diameter of oxide particles, which were determined by TEM observation in section 3.1.3. 3.2.5 Comparison of measured and predicted yield strength The values of strengthening contributions from both experimental measurements and theoretical calculations are listed in Table 8 and shown in Fig. 11. As can be obviously seen, the result of linear summation was overestimated with an error of ~ 30%, comparing with the experimentally-obtained value. Nevertheless, an alternative model has been well accepted to estimate the yield stress, which was written as below [37, 49]: σy

σ0

σss

σg √σ2dis

σ2p

(10)

where the dislocation strengthening and dispersion strengthening were calculated by root mean square summation instead of simply linear addition. This is because of the narrow intervals of dislocations and nanoparticles, resulting from the large concentration of dislocations and nanoparticles in matrix [37]. For instance, the dislocation density was determined to be ~ 6.8 1014 m-2, giving the average dislocation spacing of ~ 38 nm by assumption of ρ-1/2. According to Eq. (8), the average inter-particle spacing was ~ 44 nm. It is clear that the distribution of dislocations and nanoparticles in matrix was comparable, and the motion of dislocations would be impeded by both dislocations and oxide particles. Evidently, the dislocation strengthening and dispersion strengthening ought to be mixed together, and it introduces new smaller effective spacing [49], which can be derived in the following. 9

The total density of both dislocations and nanoparticles is given by: ρtot

ρdis

ρp

(11)

As the inter-particle spacing is proportional to ρ-1/2, the effective spacing

eff

of both

dislocations and nanoparticles is expressed by: 1

1

2 eff

2 dis

where

1

dis

(12)

2 p

and

p

are the spacing of dislocations and nanoparticles, respectively.

For dislocation and dispersion strengthening, the strength addition is proportional to square root of density and inversely proportional to spacing respectively. Hence, the root mean square summation makes sense: σ2dis

p

σ2dis

σ2p

(13)

Therefore, the strengthening contributions from dislocations and nanoparticles cannot be linear additive. As shown in Table 8 and Fig. 11, the estimation upon Eq. (10) is better consistent with the experimental result with an error of ~ 4%, suggesting it is more reasonable to evaluate the yield stress by Eq. (10). Also, it is evident that the increment order of strengthening components is σdis > σp > σg > σss. The dominant contributions to the yield strength are dislocation strengthening and dispersion strengthening, which account for ~ 65%.

4. Conclusions The 12Cr-ODS steel with a nominal composition of Fe-12Cr-2W-0.3Ti-0.25Y2O3 was fabricated by powder metallurgy including mechanical alloying, hot extrusion, hot forging, cold rolling, and subsequent heat treatment. The microstructure was characterized by XRD, HRTEM and EBSD techniques. Furthermore, the yield strength was experimentally examined and quantitatively calculated based on various strengthening mechanisms, respectively. The main conclusions can be drawn as follows: (1) The microtexture was composed of dominant {001}<110> component and weaker <111> parallel to normal direction texture. (2) The microstructure consisted of fine and elongated grains, high density of dislocations (~ 6.8

1014 m-2), and large concentration of nm-scale oxide particles (average diameter: 3.5

nm, number density: ~ 1.3

1023 m-3).

10

0.7

(3) The monoclinic Y2O3 was identified in this alloy for the first time, and the rest types were determined as cubic Y2O3, hexagonal and orthorhombic Y2TiO5, and cubic Y2Ti2O7, respectively. This fundamental information would be necessary for further research, such as interfacial structure, shape transitions, nucleation-growth-coarsening kinetics, irradiation resistance of nm-scale oxide particles, and so on. (4) Good agreement was obtained between the experimental measurement and theoretical calculation, suggesting that strengthening contributors from solid-solution atoms and grain boundaries were linear additive, while the dislocation strengthening and oxide dispersion strengthening were estimated by root mean square summation.

Acknowledgements This research was sponsored in part by a project “R&D of nuclear fuel cladding materials and their environmental degradations for the development of safety standards” entrusted to Tohoku University by Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. The authors would like to thank the support of China Scholarship Council. References [1] S. Ukai, M. Harada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, T. Nishida, M. Fujiwara, K. Asabe, Tube manufacturing and mechanical properties of oxide dispersion strengthened ferritic steel, J. Nucl. Mater. 204 (1993) 74-80. [2] G.R. Romanoski, L.L. Snead, R.L. Klueh, D.T. Hoelzer, Development of an oxide dispersion strengthened, reduced-activation steel for fusion energy, J. Nucl. Mater. 283-287 (2000) 642-646. [3] T. Muroga, M. Gasparotto, S.J. Zinkle, Overview of materials research for fusion reactors, Fusion Eng. Des. 61-62 (2002) 13-25. [4] J. Saito, T. Suda, S. Yamashita, S. Ohnuki, H. Takahashi, N. Akasaka, M. Nishita, S. Ukai, Void formation and microstructural development in oxide dispersion strengthened ferritic steels during electron-irradiation, J. Nucl. Mater. 258-263 (1998) 1264-1268. [5] K. Asano, Y. Kohno, A. Kohyama, T. Suzuki, H. Kusanagi, Microstructural evolution of an oxide dispersion strengthened steel under charged particle irradiation, J. Nucl. Mater. 155-157 (1988) 928-934.

11

[6] R.L. Klueh, P.J. Maziasz, I.S. Kim, L. Heatherly, D.T. Hoelzer, N. Hashimoto, E.A. Kenik, K. Miyahara, Tensile and creep properties of an oxide dispersion-strengthened ferritic steel, J. Nucl. Mater. 307-311 (2002) 773-777. [7] M. Klimiankou, R. Lindau, A. Moslang, HRTEM study of yttrium oxide particles in ODS steels for fusion reactor application, J. Cryst. Growth 249 (2003) 381-387. [8] M. Klimiankou, R. Lindau, A. Möslang, TEM characterization of structure and composition of nanosized ODS particles in reduced activation ferritic-martensitic steels, J. Nucl. Mater. 329333 (2004) 347-351. [9] M.K. Miller, K.F. Russell, D.T. Hoelzer, Characterization of precipitates in MA/ODS ferritic alloys, J. Nucl. Mater. 351 (2006) 261-268. [10] M.K. Miller, C.M. Parish, Q. Li, Advanced oxide dispersion strengthened and nanostructured ferritic alloys, Mater. Sci. Technol. 29 (2013) 1174-1178. [11] J. Ribis, Y. de Carlan, Interfacial strained structure and orientation relationships of the nanosized oxide particles deduced from elasticity-driven morphology in oxide dispersion strengthened materials, Acta Mater. 60 (2012) 238-252. [12] S. Yamashita, S. Ohtsuka, N. Akasaka, S. Ukai, S. Ohnuki, Formation of nanoscale complex oxide particles in mechanically alloyed ferritic steel, Philos. Mag. Lett. 84 (2004) 525-529. [13] M. Ohnuma, J. Suzuki, S. Ohtsuka, S.W. Kim, T. Kaito, M. Inoue, H. Kitazawa, A new method for the quantitative analysis of the scale and composition of nanosized oxide in 9Cr-ODS steel, Acta Mater. 57 (2009) 5571-5581. [14] H. Kishimoto, R. Kasada, O. Hashitomi, A. Kimura, Stability of Y-Ti complex oxides in Fe16Cr-0.1Ti ODS ferritic steel before and after heavy-ion irradiation, J. Nucl. Mater. 386-388 (2009) 533-536. [15] P. Unifantowicz, T. Płociński, C.A. Williams, R. Schäublin, N. Baluc, Structure of complex oxide nanoparticles in a Fe-14Cr-2W-0.3Ti-0.3Y2O3 ODS RAF steel, J. Nucl. Mater. 442 (2013) S158-S163. [16] C.Y. Lu, Z. Lu, R. Xie, C.M. Liu, L.M. Wang, Microstructure of a 14Cr-ODS ferritic steel before and after helium ion implantation, J. Nucl. Mater. 455 (2014) 366-370. [17] Y. Li, J. Shen, F. Li, H. Yang, S. Kano, Y. Matsukawa, Y. Satoh, H. Fu, H. Abe, T. Muroga, Effects of fabrication processing on the microstructure and mechanical properties of oxide dispersion strengthening steels, Mater. Sci. Eng. A 654 (2016) 203-212. 12

[18] Y. Wu, E.M. Haney, N.J. Cunningham, G.R. Odette, Transmission electron microscopy characterization of the nanofeatures in nanostructured ferritic alloy MA957, Acta Mater. 60 (2012) 3456-3468. [19] G.R. Odette, P. Miao, D.J. Edwards, T. Yamamoto, R.J. Kurtz, H. Tanigawa, Helium transport, fate and management in nanostructured ferritic alloys: In situ helium implanter studies, J. Nucl. Mater. 417 (2011) 1001-1004. [20] M. Song, C. Sun, J. Jang, C.H. Han, T.K. Kim, K.T. Hartwig, X. Zhang, Microstructure refinement and strengthening mechanisms of a 12Cr ODS steel processed by equal channel angular extrusion, J. Alloys Compd. 577 (2013) 247-256. [21] J.H. Schneibel, M. Heilmaier, W. Blum, G. Hasemann, T. Shanmugasundaram, Temperature dependence of the strength of fine- and ultrafine-grained materials, Acta Mater. 59 (2011) 13001308. [22] J.H. Kim, T.S. Byun, D.T. Hoelzer, C.H. Park, J.T. Yeom, J.K. Hong, Temperature dependence of strengthening mechanisms in the nanostructured ferritic alloy 14YWT: Part IIMechanistic models and predictions, Mater. Sci. Eng. A 559 (2013) 111-118. [23] J. Pešička, R. Kužel, A. Dronhofer, G. Eggeler, The evolution of dislocation density during heat treatment and creep of tempered martensite ferritic steels, Acta Mater. 51 (2003) 4847-4862. [24] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22-31. [25] G.K. Williamson, R.E. Smallman, Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray Debye-Scherrer spectrum, Philos. Mag. 1 (1956) 34-46. [26] M.Y. Huh, O. Engler, Effect of intermediate annealing on texture, formability and ridging of 17%Cr ferritic stainless steel sheet, Mater. Sci. Eng. A 308 (2001) 74-87. [27] J. Bentley, D.T. Hoelzer, TEM characterization of tensile-tested 14YWT nanostructured ferritic alloys, Microsc. Microanal. 14 (2008) 1416-1417. [28] S. Ukai, M. Fujiwara, Perspective of ODS alloys application in nuclear environments, J. Nucl. Mater. 307-311 (2002) 749-757. [29] V. de Castro, T. Leguey, M.A. Monge, A. Muñoz, R. Pareja, D.R. Amador, J.M. Torralba, M. Victoria, Mechanical dispersion of Y2O3 nanoparticles in steel EUROFER 97: process and optimisation, J. Nucl. Mater. 322 (2003) 228-234.

13

[30] V. de Castro, E.A. Marquis, S. Lozano-Perez, R. Pareja, M.L. Jenkins, Stability of nanoscale secondary phases in an oxide dispersion strengthened Fe-12Cr alloy, Acta Mater. 59 (2011) 3927-3936. [31] S. Begin-Colin, G. Le Caër, M. Zandona, E. Bouzy, B. Malaman, Influence of the nature of milling media on phase transformations induced by grinding in some oxides, J. Alloys Compd. 227 (1995) 157-166. [32] M.H. Cho, D.H. Ko, K. Jeong, S.W. Whangbo, C.N. Whang, S.C. Choi, S.J. Cho, Structural transition of crystalline Y2O3 film on Si (111) with substrate temperature, Thin Solid Films 349 (1999) 266-269. [33] C. Degueldre, S. Conradson, W. Hoffelner, Characterisation of oxide dispersionstrengthened steel by extended X-ray absorption spectroscopy for its use under irradiation, Comput. Mater. Sci. 33 (2005) 3-12. [34] S. Ukai, M. Harada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, K. Asabe, T. Nishida, M. Fujiwara, Alloying design of oxide dispersion strengthened ferritic steel for long life FBRs core materials, J. Nucl. Mater. 204 (1993) 65-73. [35] T. Okuda, M. Fujiwara, Dispersion behaviour of oxide particles in mechanically alloyed ODS steel, J. Mater. Sci. Lett. 14 (1995) 1600-1603. [36] S. Takaki, D. Akama, N. Nakada, T. Tsuchiyama, Effect of grain boundary segregation of interstitial elements on Hall-Petch coefficient in steels, Mater. Trans. 55 (2014) 28-34. [37] N. Kamikawa, K. Sato, G. Miyamoto, M. Murayama, N. Sekido, K. Tsuzaki, T. Furuhara, Stress-strain behavior of ferrite and bainite with nano-precipitation in low carbon steels, Acta Mater. 83 (2015) 383-396. [38] F.B. Pickering, Physical Metallurgy and the Design of Steels, Applied Science Publishers, London, 1978. [39] C.E. Lacy, M. Gensamer, The tensile properties of alloyed ferrites, Trans. Am. Soc. Met. 32 (1944) 88. [40] Q. Li, Modeling the microstructure-mechanical property relationship for a 12Cr-2W-V-MoNi power plant steel, Mater. Sci. Eng. A 361 (2003) 385-391. [41] E.O. Hall, The deformation and ageing of mild steel, Proc. Phys. Soc. B 64 (1951) 747-753. [42] N.J. Petch, The cleavage strength of polycrystals, J. Iron Steel Inst. 174 (1953) 25-28.

14

[43] Y. Funakawa, T. Ujiro, Change in yield strength of ultra low carbon steels with Cr addition, Tetsu to Hagane 96 (2010) 162-171. [44] J.E. Bailey, P.B. Hirsch, The dislocation distribution, flow stress, and stored energy in coldworked polycrystalline silver, Philos. Mag. 5 (1960) 485-497. [45] R.E. Stoller, S.J. Zinkle, On the relationship between uniaxial yield strength and resolved shear stress in polycrystalline materials, J. Nucl. Mater. 283-287 (2000) 349-352. [46] A.S. Keh, Work hardening and deformation sub-structure in iron single crystals deformed in tension at 298°K, Philos. Mag. 12 (1965) 9-30. [47] M.F. Ashby, Oxide dispersion strengthening, Godon and Breach, New York, 1958. [48] T. Gladman, Precipitation hardening in metals, Mater. Sci. Technol. 15 (1999) 30-36. [49] E. Hornbogen, E.A. Starke, Theory assisted design of high strength low alloy aluminum, Acta Metall. Mater. 41 (1993) 1-16.

Fig. 1. (a) XRD spectrum of the specimen with Si wafer, (b) plot of lattice parameter a versus cos2θ/sinθ, and (c) plot of Williamson-Hall equation.

15

Fig. 2. (a) IPF image of the specimen and (b) corresponding φ2

45° section of ODF map. Black

lines in (a) denote high angle grain boundaries (> 15°), and ■ represents {001}<110> component.

Fig. 3. The HAADF micrograph and EDS element mapping of nanoparticles.

16

Fig. 4. (a) The bright-field TEM micrograph of dislocations and nanoparticles, and (b) size distribution of nanoparticles. Red and yellow triangles represent dislocations and nanoparticles, respectively.

Fig. 5. (a) The HRTEM lattice micrograph of an Y2O3 particle with monoclinic structure, and (b) corresponding FFT power spectrum with indexing.

17

Fig. 6. (a) The HRTEM lattice micrograph of an Y2O3 particle with cubic structure, and (b) corresponding FFT power spectrum with indexing.

Fig. 7. (a) The HRTEM lattice micrograph of an Y2TiO5 particle with orthorhombic structure, and (b) corresponding FFT power spectrum with indexing.

18

Fig. 8. (a) The HRTEM lattice image of an Y2TiO5 particle with hexagonal structure, and (b) corresponding FFT power spectrum with indexing.

Fig. 9 (a) The HRTEM lattice micrograph of an Y2Ti2O7 particle with cubic structure, and (b) corresponding FFT power spectrum with indexing.

19

1200

Stress (MPa)

1000 800 600 400 200 0

0

4

8

12

RD

16

Strain (%) Fig. 10. The engineering strain-stress curve of the 12Cr-ODS steel tested at room temperature.

Fig. 11. Comparison of calculated and measured yield stress. Strengthening components from friction stress, solid-solution strengthening, grain boundary strengthening, dislocation strengthening, and dispersion strengthening were denoted by σ0, σss, σg, σdis, and σp, respectively.

20

Table 1 Chemical composition of the 12Cr-ODS steel (wt. %). Element Composition

Cr 12.01

W 1.91

Ti 0.31

Y 0.20

C 0.033

N 0.008

Fe Balance

Table 2 Calculated and measured inter-planar distances (d) and angles ( ) shown in Fig. 5. d1 (1̅ 11) 3.12 3.11

d (Å), (°) Calculated Measured

d2 (2̅ 03) 2.73 2.75

d3 (112̅) 2.67 2.73

12

13

66.1 65

63.3 63.5

23

129.4 128.5

Table 3 Calculated and measured inter-planar distances (d) and angles ( ) shown in Fig. 6. d (Å), (°) Calculated Measured

d1 (512) 1.94 1.96

d3 (32̅1) 2.84 2.84

d2 (231) 2.84 2.89

12

13

42.95 43

42.95 42

23

85.9 85

Table 4 Calculated and measured inter-planar distances (d) and angles ( ) shown in Fig. 7. d (Å), (°) Calculated Measured

d1 (400) 2.52 2.59

d3 (2 ̅ ̅ ) 2.91 2.85

d2 (211) 2.91 2.90

12

13

55.8 55.5

55.8 55

23

111.6 110.5

Table 5 Calculated and measured inter-planar distances (d) and angles ( ) shown in Fig. 8. d1(1̅100) 3.13 3.11

d (Å), α (°) Calculated Measured

d2(1̅ 010) 3.13 3.17

d3(011̅ 0) 3.13 3.13

12

13

23

60 60

60 60

120 120

Table 6 Calculated and measured inter-planar distances ( ) and angles ( ) shown in Fig. 9. d (Å), (°) Calculated Measured

d1(400) 2.55 2.57

d2(22̅2) 2.94 2.90

d3(222̅) 2.94 2.95

12

54.7 54

13

54.7 55

23

109.4 109

Table 7 Summary of identified oxide particles in the 12Cr-ODS steel. Oxide particles Y2O3 Y2TiO5 Y2Ti2O7

Crystal system Monoclinic Cubic Orthorhombic Hexagonal Cubic

a (Å) 13.50 10.6 10.35 3.61 10.09

b (Å) 3.47 10.6 3.7 3.61 10.09 21

c (Å) 8.46 10.6 11.25 11.84 10.09

(°) 90 90 90 90 90

β (°) 99.77 90 90 90 90

γ (°) 90 90 90 120 90

Table 8 Experimentally-obtained yield stress and calculated strengthening contributions (MPa). σy,exper 1068

σ0 53.9

σss 118.8

σg 215

σdis 615.8

22

σp 380

σy,cal (Eq. 3) 1383.5

σy,cal (Eq. 10) 1111.3