Core-Shell Structures of Oxygen-rich Nanofeatures in Oxide-dispersion

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Core/shell structures of oxygen rich nanofeatures in oxide dispersionstrengthened FeCr alloysEmmanuelle A. MarquisCitation:Appl. Phys. Lett. 93, 181904 (2008); doi: 10.1063/1.3000965View online: http://dx.doi.org/10.1063/1.3000965View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v93/i18Published by theAIP Publishing LLC.Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journalTop downloads: http://apl.aip.org/features/most_downloaded

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Core/shell structures of oxygen-rich nanofeatures in oxide-dispersionstrengthened FeCr alloys

Emmanuelle A. Marquisa

Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom

Received 13 September 2008; accepted 24 September 2008; published online 4 November 2008

With the significant improvement in high temperature creep properties and resistance to radiation

damage by addition of nanoscale oxide features, oxide-dispersion strengthened ODS ferritic/martensitic alloys are potential candidates for structural applications in nuclear fusion reactors. The

structure of the oxygen-rich nanofeatures was analyzed by atom-probe tomography in three ODS

alloys: MA957, ODS Fe12 wt %Cr, and ODS Eurofer-97. Although field evaporation and

reconstruction of the precipitates suffer from artefacts, a core/shell structure is found even for very

small precipitates. Precipitate cores are Y rich while shell regions are enriched in Ti, Cr, or V

depending on alloy composition. 2008 American Institute of Physics.DOI:10.1063/1.3000965

In the search for structural materials for fusion power

plants, oxide-dispersion-strengthened ODS reduced activa-tion ferritic/martensitic steels appear as potential candidates

for blanket structures. The nanosized oxygen-rich structures

stabilize grain boundaries, improve creep resistance,and pro-

vide a high density of trapping sites for helium,1

thereby

increasing the possible operating temperature of these alloys

to relevant values for the current design of nuclear plants,

i.e., 650 C and higher.2

The superiority of ODS steel over

their non-ODS counterpart is well understood and large ef-

forts in Europe, USA, and Japan have been undertaken to

establish the relationship between microstructure, mechani-

cal properties, and irradiation behavior.1

Although the oxide

nanofeatures NFs appear as the major component improv-ing the overall material behavior, detailed knowledge of their

internal structure and behavior is yet to be gained.1

Atom-probe tomography APT has been previouslyused to analyze NFs, principally in ODS Ti-containing steels.Larsonet al.

3followed by Miller et al.

47found fine disper-

sions of Y-, O-, and Ti-rich precipitates with sizes between 3

and 5 nm in various alloys. Their results, particularly on NF

chemical compositions, illustrate the complexity of analyz-

ing ODS alloys. Indeed, particle compositions are highly

sensitive to alloy chemistry, processing conditions, and

choice of atom-probe tomography analysis parameters. More

recent analytical transmission electron microscopy TEMmeasurements

8as well as APT observations

9in an ODS

Eurofer-97 alloy revealed that 10 nm diameter NFs havecore-shell structures with Y-rich cores and V- and Cr-

enriched shells. The current work presents detailed APTanalysis of the internal structure and composition of nano-

scale oxygen-rich features in three different ODS FeCr

based alloys. Particular emphasis is placed on analyzing the

effects of evaporation artifacts affecting data interpretation.

After thermomechanical processing, an ODS martensitic

Eurofer-97 alloyof nominal composition 9Cr, 1.5W, 0.4Mn,0.2V, 0.11C, and 0.3Y2O3 in wt % and described in Ref. 10was annealed at 1050 C for 30 min, air cooled and tem-

pered at 750 C for 2 h.11

An ODS ferritic/martensitic

Fe 12 wt % Cr model alloy containing 0.25 wt % Y2O3was

annealed at 750 C for 4 h following hot-isostatic pressing

at 1100 C.12

The third alloy, a commercial vendor MA957

with nominal composition 14Cr, 0.9Ti, 0.3Mo, and

0.25Y2O3, was hot extruded at 1150 C. APT sampleswere prepared by electropolishing using a solution of 2%

percholoric acid in 2-butoxyethanol. Field ion imaging

FIM was performed on a 3DAP microscope with samplesmaintained at 50 K in 105 torr of helium. Analyses were

performed using a LEAP-3000XSI or a LEAP-3000HR mi-

croscope. During analysis, samples were maintained between

30 and 40 K and either voltage pulsing 20% pulse fraction,200 kHz repetition rate or laser pulsing wavelength of532 nm, pulse width of 10 ps, repetition rate of 200 kHz,

energy of0.4 nJ and spot size 5 m was used for fieldevaporation.

In the three alloys, the oxide NFs appear as dark regions

during FIM imaging arrowed in Fig. 1, suggesting that alower evaporation field than that of the surrounding matrix is

required to evaporate these oxygen-rich phases.13

In such a

case, a precipitate appearing at the surface of the specimen

will momentarily evaporate at a faster rate than the surround-

ing matrix until a lower local of curvature is reached, the

field is decreased at the precipitate, and an equiprobability of

field ionization is established over the emitter surface. The

dimple thereby created at the surface of the evaporating pre-

cipitate creates a focusing effect for the ions originating from

the precipitates and from the nearby surrounding matrix13

and, indeed, the detected atomic density at the particles in

the three alloys is up to four times higher than in the sur-rounding matrix. Although focusing of the ion trajectories

implies that the particles should also appear compressed in

aAuthor to whom correspondence should be addressed. Tel.: 44 1865

273711. FAX: 44 1865 273789. Electronic mail: emmanuelle.marquis@

materials.ox.ac.uk.

FIG. 1. FIM images ofa ODS Fe-12Cr alloy, b MA957, and c ODS

Eurofer 97 alloy.

APPLIED PHYSICS LETTERS 93, 181904 2008

0003-6951/2008/9318/181904/3/$23.00 2008 American Institute of Physics93, 181904-1

http://dx.doi.org/10.1063/1.3000965http://dx.doi.org/10.1063/1.3000965http://dx.doi.org/10.1063/1.3000965http://dx.doi.org/10.1063/1.3000965http://dx.doi.org/10.1063/1.3000965http://dx.doi.org/10.1063/1.3000965


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the lateral dimensions, the oxygen atoms are detected over awider lateral extent than the metal ions present in the pre-

cipitates. This effect is visible in both voltage and laser puls-

ing modes and particularly noticeable in Fig. 3c as indi-cated by the black arrows. Similar observations on the

evaporation behavior of O from NFs have been reported

previously.47

It is possible that chromatic aberrations

whereby ion trajectory depends on atom type such as de-

scribed in Ref. 14 may take place.

Another specific characteristic of the reconstructed par-

ticles is the systematic presence of a solute-rich area under-

neath in the evaporation sequence each Y-rich cloud, asshown in Figs. 2a2c. Concentration profiles calculated

along 2 nm diameter cylinders intercepting the particles andoriented parallel to the specimen axis, reveal not only a Ti,

Cr, or V rich area underneath the particles but a similar and

smaller region above the particles as well Figs. 2a2c.Since the crystallographic orientation of the APT samples

prepared from bulk materials is random, this systematic

evaporation sequence is interpreted as being an evaporation

artifact due to trajectory aberrations and evaporation field

variations as explained above. Moreover, the feature geom-

etry leading to such evaporation patterns is that of a core-

shell structure, with the shell region and possibly the core

region requiring a lower evaporation field than the matrix. In

the ODS Fe12Cr alloy, a Cr rich shell is observed that ex-

tends slightly further in the depth direction than the O atoms.In the ODS Eurofer-97 alloy, V and O are present in the shell

and a Cr enhancement is also observed in the shell and core

of the precipitates. Although the extent of the shell is asym-

metric in the three-dimensional 3D reconstruction with anapparent thicker layer underneath each precipitate, the actual

thickness is expected to be on average uniform around the

precipitates as shown previously by transmission electron

microscopy.8

Similar reconstructed uneven shell thicknesses

have been observed in larger precipitate structures in Al

alloys.15,16

The stretching in depth is enhanced by the actual

nonuniform evaporation sequence yielding the dimpled tip

shape and the assumed uniform evaporation over a hemi-

spherical tip shape used in the reconstruction algorithm. The

evaporation sequence error displaces the low evaporationfield regions core and shell toward the apex of the tip. Thepresence of a shell is observed independently of the size of

the nanofeatures. Figure 2 illustrates the case of small NFs

with sizes less than 4 nm where the evaporation and recon-

struction artifacts occur over distances as large as the NFs

themselves and the internal structure is significantly altered.

Figure3 illustrates the case for larger NFs 8 nm and largerwhere the shell structure is visible all around the core. As the

size increases, the distinction between core and shell be-

comes clearer, i.e., the length scale of the nanoparticles be-

come larger than that of trajectory aberrations.

The exact compositions of the cores and shells are diffi-

cult to determine and require accurate understanding of theevaporation field, evaporation sequence, and ion trajectories.

An estimate of the combined concentrations can, however,

FIG. 2. Color onlineSlices though 3D reconstructions showing the evapo-ration structures of small oxide NFs and profiles in a MA957 in voltagemode, b ODS Fe-12Cr alloy in voltage mode, and c ODS Eurofer 97alloy in laser mode. FIG. 3. Color onlineSlices though 3D reconstructions showing the evapo-

ration structures of larger oxide NFs and profiles in a MA957, b ODSFe-12Cr alloy, and c ODS Eurofer 97 alloy. All samples were analyzed inlaser pulsing mode.

181904-2 Emmanuelle A. Marquis Appl. Phys. Lett. 93, 181904 2008



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be obtained using a cluster search algorithm17 based on theevaporating species Y, YO plus TiO MA957 or CrO ODSFe12Cror CrO, and VOEuroferwith a separation distanceof 0.91.1 nmdepending on the alloy and dataset, a matrixinclusion distance of 0.50.6 nm and an erosion distance of

0.50.65 nm. Assuming that the presence of Fe inside the

particles is strictly due to some matrix atoms being focused

and their trajectories overlapping with that of the particles

atoms, a subtraction of the matrix concentration based on the

number of Fe atoms found in the precipitates is performed.

The resulting concentrations are shown in TableI.Since the

reconstructed oxygen atom distribution covers a wider vol-

ume than for the metal atoms, the concentration of this ele-

ment in the particles may be underestimated. Note that theconcentrations measured by the cylinder method are strongly

affected by the choice of the cylinder diameter and the value

of 2 nm was chosen as to avoid edge effects due to the small

precipitate sizes. The depletion of Cr in Fig. 2a is not rep-resentative and a variety of behaviors is actually observed

with an average Cr enrichment in the particles as found by a

cluster search algorithm method TableI. The M:O ratio iscomparable in all three alloys. In the three alloys as well,

other solutes, Mn, Al, Si, and C also partition to some NFs

and the overall concentrations are in general agreement with

previous works.57

Separating shell and core atoms is diffi-

cult without assumptions on the particle or shell structures,

or without further experimental observations using differenttechniques such as shell thickness from TEM observation.

The similarities between the structure of the oxide nano-

particles in all three alloys, i.e., a Y-rich core and solute-rich

shell, suggest that their structures may be governed by com-

mon fundamental thermodynamic properties, such as the in-

terfacial energy between the oxide phases and the surround-

ing matrix. The Y2O3oxidethe most stable of the Cr,V, Ti,Y oxides with the highest free energy of formation

18may,

however, have a high nucleation barrier and the presence of a

shell would decrease the interfacial energy, favoring the

nucleation of the oxide particles. This may account for the

higher number density of oxide particles obtained with addi-

tion of Ti.4 Note that a similar mechanism was described for

the formation of CuMnNi rich precipitates in RPV steels.19

Future work will involve characterizing the interfacial prop-

erties, quantifying the effect of precipitate size, irradiation,

and thermal annealing on the development and stability of

core/shell structures, as well as determining respective com-

positions and the role of solutes.

The author acknowledges G.R. Odette University ofCalifornia, Santa Barbara, USA for stimulating discussionsand for providing the MA957, Professor A. Cerezo and Pro-

fessor G.D.W. Smith University of Oxford for valuablesupport, the group of R. Pareja Universidad Carlos III,Madrid, Spainfor supplying the ODS Fe-12Cr alloy, and R.Lindau Forschungszentrum Karlsruhe, Germany for theODS Eurofer-97. This work was funded by Engineering and

Physical Sciences Research Council EPSRC under GrantNo. EP/077664.

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TABLE I. Measured concentrations in oxide core/shell NFs with sizes less than 5 nm.

Alloy O Ti Y Cr V M:O

MA957 42.83.6 37.93.0 14.61.2 4.61.0 1.3

ODS Fe12Cr 36.93.5 24.85.8 37.14.8 1.7

Eurofer 35.94.2 24.73 4.46.2 33.92.3 1.8

181904-3 Emmanuelle A. Marquis Appl. Phys. Lett. 93, 181904 2008

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Core-Shell Structures of Oxygen-rich Nanofeatures in Oxide-dispersion

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