Nature Materials - Atomic Structure of Nanoclusters in Oxide-dispersion-strengthened Steels

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LETTERSPUBLISHED ONLINE: 23 OCTOBER 2011 | DOI: 10.1038/NMAT3150

Atomic structure of nanoclusters in

oxide-dispersion-strengthened steelsA. Hirata1, T. Fujita1, Y. R. Wen1, J. H. Schneibel2, C. T. Liu2 and M. W. Chen1,3*

Oxide-dispersion-strengthened steels are the most promisingstructural materials for next-generation nuclear energy sys-tems because of their excellent resistance to both irradiationdamage and high-temperature creep14. Although it has been

known for a decade that the extraordinary mechanical prop-erties of oxide-dispersion-strengthened steels originate fromhighly stabilized oxide nanoclusters with a size smaller than

5 nm, the structure of these nanoclusters has not been clarifiedand remains as one of the most important scientific issues innuclear materials research27. Here we report the atomic-scalecharacterization of the oxide nanoclusters using state-of-the-

art Cs-corrected transmission electron microscopy. This studyprovides compelling evidence that the nanoclusters have a de-fective NaCl structure with a high lattice coherencywith thebccsteel matrix. Plenty of point defects as well as strong structural

affinity of nanoclusters with the steel matrix seem to be themost important reasons for theunusual stability of the clustersat high temperatures and in intensiveneutron irradiation fields.

The safety, reliability, economics, and efficiency of next-generation fission and future fusion energy systems will ultimatelydepend on developing new high-performance structural materialsthat can provide extended service for at least 60 years underextremely harsh environments where the materials are exposedto high temperatures, large time-varying stresses, chemicallyreactive surroundings, and intense neutron radiation13. Oxide

dispersion strengthened (ODS) steels have been strenuouslydeveloped as a promising structural material for next-generationnuclear energy systems because of their excellent resistanceto irradiation damage and high-temperature creep as well asextraordinary structural and chemical stability in extremely harshenvironments27. Small Y- or YTi oxide nanoprecipitates thatare uniformly dispersed in the steel matrix with a very highnumber density are responsible for reducing the creep rates bysix orders of magnitude at 650900 C, and contribute to theexcellent tensile ductility (RA > 40%) and strength (>2GPa) ofthe ODS steels79. They also present extremely high stability attemperatures as high as 1,400 C (0.91 Tm, where Tm standsfor the melting temperature) and in intense neutron irradiationfields. This unusual stability of the oxide clusters cannot be readily

explained by thermodynamics and traditional materials theories.To understand such extraordinary mechanical properties owing tothe highly stabilized oxide nanoprecipitates, a knowledge of thestructure and chemistry of the oxide nanoclusters is necessary inthe research field of the ODS steels.

The oxide nanoprecipitates in the ODS steels have beencharacterized mainly using transmission electron microscopy(TEM) by many researchers1018. These analyses have beenperformed for relatively large nanoparticles with sizes greater

1WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan, 2Center for Advanced Structural Materials, City University of

Hong Kong, Kowloon, Hong Kong, 3State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong

University, Shanghai 200030, China. *e-mail: [email protected].

a b

200 nm 200 nm

Figure 1 | Typical microstructure of the ODS steel. a, BF- and b,

HAADF-STEM images obtained from a 14YWT ODS steel.

than 5 nm. The types of crystal structures of the large YTiOnanoparticles have been suggested as Y2Ti2O7 (refs 1114) andY2TiO5 (ref. 11; Y/Ti1), and can be found inthe published crystaldatabases19. Atom probe tomography (APT), on the other hand,reveals the chemical composition of oxide nanoclusters with a sizeless than 5 nm (refs 20,21). Although the nanoclusters are basicallycomposed of Ti, O and Y, as well as significant amounts of Fe andCr (refs 2025), the Y/Ti ratio is much smaller than 1, within therange from 0.1to 0.6, anddepends on thematerial compositionandtimetemperature history of the material process22. In any case, the

low Y/Ti ratio is not consistent with those of the reported YTiOoxides, such as Y2Ti2O7 and Y2TiO5, which has been an importantchemical feature of the nanoclusters. However, the crystallographicstructure of the nanoclusters has been debated for many years andhas not been clarified by definite structure characterization4,2027.As the formation of highly dense oxide nanoclusters is the mosteffective way to achieve good mechanical properties28, it is thusvitally important to understandthe structural and chemical featuresof the oxide nanoclusters. Because the nanoclusters are very small(25 nm), embedded in the magnetic bcc-Fe matrix, and mayhave a coherent relation with the matrix, it is thus extremelydifficult to obtain the structural information by conventionalTEM, which is limited by a low spatial resolution and the lackof capability for atomic-scale chemical analysis. In this study, we

systematically characterized the atomic structure and chemistryof the nanoclusters in an ODS steel using the newly developedstate-of-the-art Cs-corrected TEM and scanning TEM (STEM) withultra-high spatial resolutions of0.10 nm.

The microstructure of the 14YWT-ODS steel (nominal com-position: Fe14Cr3W0.4Ti (wt.%) with 0.25 wt.% Y2O3) wasfirst surveyed using bright field-STEM (BF-STEM) and high angleannular dark field (HAADF) STEM techniques. Figure 1a shows thetypical microstructure of the ODS steel used in this study. In the

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5 nm

a

c

e

b

d

f

Fe-L edge

Cr-L edge

N-K edge

O-K edge

HAADFSTEM

Ti-L edge

Figure2 | EELS chemical mapping of the nanoclusters. a, Fe-L, b, O-K,

c, Cr-L, d, Ti-L, and e, N-K EELS elemental maps obtained from a

nanoparticle with a size of 23 nm. f, The corresponding HAADF-STEM

image.

BF-STEM image, the steel matrix with grain sizes of100200 nmcan be clearly identified. The bright and dark contrast of indi-vidual grains comes from the diffraction variations originatingfrom the different crystallographic orientations of each grain. TheHAADF-STEM image, on the other hand, mainly shows the masscontrast, where the regions with lower-density and/or includinglighter elements (for example oxides in the steel) are imaged withdarker contrast and vice versa. From Fig. 1b of the HAADF-STEMimage, we can see a large number of dark dots, as indicated bythe arrowheads, with sizes ranging from 2 nm to 50 nm, whichcorrespond to Ti-rich oxide nanoprecipitates in the polycrystallinematrix (see Supplementary Fig. S2). Each grain with a bcc structurehas homogeneous contrast, although a small residual diffractioncontrast is still visible, showing a slight difference in the contrast

from grain to grain as well as the bright contrast of a few nanoparti-cles. Therefore, the HAADF-STEM technique enables us to directlyobserve thedistribution of thenanoprecipitatesin theODS steel.

As the most important question on the microstructure of ODSsteels is the nature of the small precipitates, we systematicallycharacterized nanoclusters that are smaller than 5 nm and havea composition that features low Y/Ti atomic ratios (0.10.6),as shown in Supplementary Fig. S3. Energy-dispersive X-rayspectroscopy (EDS) analysis confirms that these dark regionscorrespond to the nanoclusters as the enriched Ti and Y elementscan be detected (see Supplementary Fig. S4). The detailed elementdistribution in the nanoclusters was investigated by electron-energy-loss spectroscopy (EELS) in the STEM mode. Elementalmaps were derived from EELS spectrum images, where the EELSspectra were acquired from each pixel (0.3 nm 0.3 nm). In the

d

a

b c

1 nm

Exp.bcc+NaCl

Exp.

bcc+

NaCl bcc

[110]bcc

bcc

Figure 3 | HAADF-STEM characterization of the nanocluster.

a, Experimental HAADF-STEM image of a nanocluster. b, Enlarged

experimental image from blue box area in a, showing a coherent lattice

structure in the interface region. c, Simulated image using a model with acoherent bcc and NaCl structures. d, Simulated image using a model with

only the bcc matrix for comparison. Images of lower panels of bd,

corresponding to the yellow box area in a, are enlarged views of the upper

panels of b,c and d, respectively. The real-space STEM image of the

interface region with periodically enhanced atomic columns can be

explained well by the coherent bcc/NaCl model, providing straightforward

evidence that the nanoclusters have a NaCl structure and are coherent with

the bcc steel matrix.

STEMEELS maps (Fig. 2), there are two nanoclusters: one is justat a grain boundary and another is located inside a grain. Theenrichments of O, Ti, and N are clearly seen, together with thescarcity of Fe and Cr-rich shells surrounding the Ti(O, N) cores.

Furthermore, the EDS analysis shows that the Y element is alsoenriched in the particle and the Y/Ti ratio is about 0.20 (seeSupplementary Fig. S4), consistentwith APT measurements21.

The HAADF-STEM image of a nanocluster in a thin regionnear the TEM specimen edge is shown in Fig. 3a with the electronincidence parallel to the [110]bcc direction. The sample thicknessof the thin area was estimated to be 37nm by means of anEELS log-ratio technique (see Supplementary Fig. S5)29. As theimaged region is very thin, a nanocluster with dark contrastand distorted lattices can be clearly observed because of weakerinterferencefromthe surrounding bccmatrix. Thesize of theclusteris 23 nm, which is comparable to the nanoclusters detected byAPT (refs 20,21). It seems that the central part of the cluster is muchmore defective whereas the cluster/matrix interface region showsexcellent lattice coherence with a particular periodicity composed

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4nm

Electron

4 nm

Model 2

Ti(Y, Fe, Cr)O cluster

bcc-Fe

4nm

Model 1 Model 2 Model 3

Model 1 Model 2 Model 3 Exp.

1 nm

[110]bcc [001]bcc

[110]bcc

c d

e

a

b

Figure 4 | Structure modelling of the nanocluster from [110]bcc direction. a, 3D external view of a structure model (Model 2) where a 3.0 nm (Ti,Y,Fe,Cr)O

nanocluster is embedded in the bcc matrix. bd, The projections of Model 1, Model 2, and Model 3 are depicted, respectively, in b,c, and d, where green,

red, blue, purple and orange circles denote Ti, Y, Fe, Cr and O atoms respectively. The corresponding simulated HAADF-STEM images from the three

models are shown in the lower panels of bd. e, The experimental HAADF-STEM image. Model 2 is the structure model which is most consistent with theexperimental data.

of brighter and darker atomic contrasts arising from the metal(brighter) and oxygen (darker) atomic columns of the nanoclustercoherently overlapping the bcc lattice of the matrix (Fig. 3b). Theexcellent lattice coherency at the interface may be associated withan enrichment of Cr that mediates the lattice mismatch betweenthe bcc Fe and oxide.

To understand the atomic structure of the nanoclusters, weconstructed three possible structural models in which a nanoclusteris embedded in the bcc-Fe matrix (Fig. 4). Based on fast Fouriertransform (FFT) analysis of the atomic-resolution STEM images(see Fig. 5, Supplementary Figs S7 and S8), the crystal structure

of the nanoclusters is consistent with a NaCl-type TiO structure(the details will be discussed later), which is also confirmed by theperiodicity of the atomic columns with enhanced intensity at theinterface between the nanocluster and bcc matrix(Fig. 3). Based onAPT measurements21, the chemical composition of the nanoclusterwas set tobe (Ti43.9Y6.9Fe3.4Cr1.1)O44.7. Considering vacancies play akey role in stabilizing nanoclusters27,30, we introduced vacancies atboth the O and Ti sites in the TiO cluster model. To be consistentwith the experimental images, the concentration of vacancies wasvaried over a wide range. We found that the best match between thesimulated and the experimental images requires10 at% vacancies.Figure 4a shows a three-dimensional external view of the structuremodel. The spherical Ti(Y,Fe,Cr)O cluster is positioned at thecentre of a bcc matrix box with a dimension of 4 nm4 nm4nm.The orientation relationship between bcc and Ti(Y,Fe,Cr)-O is

determined as (110)bcc (002)oxide and [110]bcc [110]oxide based onthe FFT analysis of the STEM images (see Fig. 5, SupplementaryFigs S7 and S8). In this simulation, the electron incidence is parallelto the [110]bcc direction. The nanocluster in Model 1 (Fig. 4b)has an unrelaxed perfect NaCl structure that is coherent with thebcc matrix. It is apparent that the simulated image is dissimilarto the experimental one as the nanocluster in the experimentalimage is much darker, accompanied with obvious lattice distortioncompared to the simulated one. The model structure was thenrelaxed using a molecular dynamics (MD) simulation with bothmany-body and two-body interatomic potentials. During the

relaxation, the neighbouring atoms of vacancies move towards theopen space, leading to a certain lattice distortion. In particular,the defect density in the central part of the nanocluster seemsslightly higher than that at the cluster/matrix interface region.Model 2 (Fig. 4c) is constructed on the basis of the MD relaxedstructure. As shown in Fig. 4c, thesimulated image based on Model2 is phenomenologically consistent with the experimental one incontrast variation of the atomic image (Fig. 4e), verifying that thevacancies indicated byab initio calculations27 and positron-lifetimespectroscopy30 are compatible with our STEM observations. Tofurther verify the reliability of Model 2, we constructed Model 3,in which the relaxed, defective NaCl nanocluster has an incoherentrelation with the bcc matrix (Fig. 4d). The lattice constant of theincoherent cluster is 15% larger than that of the coherent one.Interestingly, the simulated image based on Model 3 is also similar




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NATURE MATERIALS DOI: 10.1038/NMAT3150 LETTERS

Exp.

Model 2

Exp.

bcc matrix

Sim.

bcc matrix

Inner part

Outer rim part

Inner part

Outer rim part

A

B

A

B

Model 3

Model 1

Inner part

Outer rim part

Inner part

Outer rim part

A

B

A

B

a b

c d

B

B

A

A

B

B

A

A

Figure5 | Structure characterization of the nanocluster from the [110]bcc direction. FFT patterns of inner and outer rim parts (indicated in the images)obtained from: a, experimental image; b, simulated image from Model 1; c, Model 2; and d, Model 3. FFT patterns from the bcc matrix are also shown. The

well-defined FFT patterns of the bcc matrix and NaCl structure can be observed from the coherent interface regions ( A, A and A), directly demonstrating

that the nanoclusters have a NaCl structure with a high lattice coherency with the bcc steel matrix. The nearly identical FFT patterns taken from central

parts of the clusters in B and B indicate that the nanoclusters contain a large number of lattice defects.

to the experimental image, although it seems to be lacking insome details compared with the image from Model 2 and theexperimental STEM image. However, the small difference cannot beconvincinglyobserved on thebasis of theSTEM imagesalone.

To unambiguously distinguish the difference between theseSTEM images, FFT analysis was performed for the experimentaland the simulated HAADF-STEM images (Fig. 5). The outer rimparts of both images provide clear FFT patterns coming from

the NaCl structure (also see Supplementary Fig. S7), whereas theinner parts give relatively diffuse FFT patterns owing to a moredistorted atomic structure. Moreover, the FFT analysis for bothModel 1 and Model 3, which fail to explain the experimentaldata, provides more rigorous evidence that Model 2 gives thebest matching and should be the most dependable structure modelof the nanocluster. In Model 2, 111-type spots of NaCl (seeSupplementary Fig. S7) are clearly seen in the FFT pattern fromthe outer rim part, whereas the FFT pattern from the centrepart shows a much more diffuse pattern. Furthermore, from the[111]bcc direction, an experimental HAADF image is also fairlyconsistent with Model 2 (see Supplementary Fig. S8). Theseobservations provide compelling evidence that the defective NaCl-type Ti(Y,Fe,Cr)-O is the most promising structure model of the

oxide nanoclusters in the ODS steel.Vacancieshave recently been suggestedto be an importantfactorin stabilizing the oxide nanoclusters27,30. This feature is consistentwith the fact that the NaCl-type TiO structure is capable ofaccommodating a large number of vacancies31. Moreover, TiO witha NaCl-type structure is a strongly nonstoichiometric interstitialcompound and thus exhibits a very broad compositional rangefrom Ti40 at% O to Ti55 at.% O in the equilibrium TiOphase diagram. Interestingly, the TiN, Ti(O,N), Fe(O,N), YNsystems also form a NaCl-type structure19, indicating that theNaCl-type structure has great chemical flexibility as well as the highvacancy capability to form nanoclusters with multiple constituentelements. On the basis of the present structural and chemicalcharacterization, the unusual stability of the nanoclusters at hightemperature and under intense neutron irradiation seems to be

associated with the defective structure and the strong structuralaffinity with the bcc matrix. The full lattice coherency between theoxide nanoclusters and the steel matrix gives rise to a very lowinterface energy between the two disparate materials, oxide andmetal. The low interface energy, along with the very low solubilityof O and Y in bcc Fe, can effectively prevent the coarsening ofthe oxide precipitates32. Moreover, the intrinsic defective structureof the nanoclusters can naturally tolerate the radiation-induced

damage2

. Therefore, the ODS steels with a large number ofdefective nanoprecipitates represent a novel material state, whichis intrinsically distinct from conventional nano-phase materials,which are usually metastable and can rapidly become coarsenedat high temperatures.

MethodsAlloy synthesis. The ODS steel used in this study was prepared by mechanicalalloying of the alloy powders, Fe14Cr3W0.4Ti (wt.%), with 0.25wt. % Y2O3powders, followed by canning in an evacuated jacket and hot extrusion at 850 C.The hot-extruded ODS/MA ingot was then annealed for 1 h at 1,000 C as astandard heat treatment for the formation of nanoclusters and nanoparticles.Details of the material processing have been given by Hoelzer et al.33. This alloy isdesignated as 14YWT, where 14 indicates the Cr concentration in wt.% and YWTrepresents the alloying additions of Y2O3, W and Ti.

TEM characterization. Specimens for TEM characterization were prepared usingan electropolishing method with a solution composed of HClO 4, dibutyl-ethanoland ethanol (1:6:2) at 30 C. Microstructure characterization of the ODSsteel was performed using a JEM-2100F TEM (JEOL, 200 kV) equipped withdouble spherical aberration (Cs) correctors for both the probe-forming andimage-forming lenses. Elemental mappings were obtained using EELS and EDS.EELS and EDS measurements were carried out using Gatan GIF Tridiem andJEOL JED-2300T, respectively. Quantitative EDS chemical analysis was carried outusing theoretical k-factors. High angle annular dark field (HAADF) images, forwhich the contrast is basically proportional to the square of the atomic number,were acquired using an annular-type STEM detector while BF-STEM images weresimultaneously recorded using a STEM BF detector. The collecting angle in thisstudy ranges from 100 to 267 mrad, which is high enough for HAADF-STEM (seeSupplementary Fig. S1).

Structure modelling and image simulation. The structure models for imagesimulation were constructed by MD simulation. First we prepared a cubic bcc-Fe

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LETTERS NATURE MATERIALS DOI: 10.1038/NMAT3150

structure including a spherical NaCl-type TiYFeO structure as an initialstructure. Then the structure was annealed at 1,273K for structure relaxationusing many-body generalized embedded-atom potentials for FeFe and FeTipairs, two-body BornMayerHuggins potentials for TiO, TiTi and OO pairs,and two-body Lennard-Jones potentials for FeY, TiY, OY, YY and FeOpairs. The parameters for Lennard-Jones potentials were determined based onatomic distances in the binary and ternary oxide structures. The annealing timeis 23 fs, and further increasing the time does not cause any obvious structurechanges. As there were insufficient Cr potentials and Cr does not affect theHAADF contrast much, we replaced some Ti atoms with Cr atoms after the MDsimulation to form a (Ti43.9Y6.9Fe3.4Cr1.1)O44.7 nanocluster for the HAADFSTEMsimulations. The HAADFSTEM simulations were performed using the WinHREM software (HREM Research). The algorithm of the code has been verifiedto be reliable for simulating HAADFSTEM images with a large atomic cell andalso for simulating Cs-corrected STEM images34. In the calculations, the probeconvergence angle is 25 mrad and the HAADF detector inner and outer angles are100 and 267 mrad, respectively.

Received 2 June 2011; accepted 19 September 2011;

published online 23 October 2011

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AcknowledgementsThis work was sponsored by Global COE for Materials Research and Education, World

Premier International (WPI) Research Center Initiative for Atoms, Molecules and

Materials, MEXT, Japan. We thank Okunishi of JEOL for his technical assistance and

D.T. Hoelzerat ORNL,USA forproviding14YWTsamplesfor thisstudy.

Author contributionsM.W.C. and C.T.L. planned this project. A.H. and M.W.C. designed research, analysed

data, constructed models and wrote the paper. A.H. contributed to STEM experiments

and image simulation. T.F. contributed to EELS analysis. Y.R.W. contributed to TEM

specimen preparation. J.H.S. and C.T.L. contributed to sample preparation. All authors

discussed the results and commented on the manuscript.

Additional informationThe authors declare no competing financial interests. Supplementary information

accompanies this paper on www.nature.com/naturematerials.Reprints and permissions

information is available online at http://www.nature.com/reprints. Correspondence and

requests for materials should be addressed to M.W.C.


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Nature Materials - Atomic Structure of Nanoclusters in Oxide-dispersion-strengthened Steels

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