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Scripta Materialia 61 (2009) 411–414

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Initial observation of grain boundary solute segregationin a zirconium alloy (ZIRLO) by three-dimensional atom probe

Daniel Hudson* and George D.W. Smith

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Received 20 February 2009; revised 17 April 2009; accepted 18 April 2009Available online 24 April 2009

Three-dimensional atom probe has been used to measure solute segregation to grain boundaries in ZIRLO, a Zr–Nb–Sn–O–Fealloy. It was found that Fe segregates strongly, along with a substantial amount of Nb. No statistically significant segregation of Sn,O, C or N was detected. The results are discussed in the context of the likely effect of heat treatment and irradiation on solute dis-tribution in Zr alloys.� 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Zirconium alloy; ZIRLO; Grain boundary segregation; 3-D atom probe; LEAP

In this paper, we demonstrate the ability ofthree-dimensional (3-D) atom probe to directly measurethe segregation of iron and other solutes to grain bound-aries in a zirconium alloy, and we discuss the possibleimplications of the observed behaviour. The maximumsolubility of iron in alpha zirconium is approximately200 at. ppm [1]. The majority of iron in zirconium alloysis rejected from the matrix, forming second-phase parti-cles (SPPs). In ZIRLO, a Zr–Nb–Sn–O–Fe alloy, theprincipal iron-containing phase is Zr(Nb,Fe)2 [2]. Thisintermetallic phase is relatively insensitive to radiationdamage compared to iron-containing intermetallics inZircaloy-4 [3]. It has been observed that the redistribu-tion of iron from SPPs into the zirconium matrix by fastneutron recoil reduces the corrosion resistance of Zirca-loy-type alloys both in service and in laboratory tests [3].However, neutron irradiation in ZIRLO still inducesredistribution of the alloying elements in service [4,5].This redistribution is now seen as a primary contributorto enhanced corrosion in pressurized water reactors. It istherefore of interest to determine the fate of iron atomsembedded in a zirconium matrix. It might be expectedthat much of any remaining iron in solid solution wouldbe rejected to the grain boundaries. Yueh and Cox [6] re-port using cathodoluminescence (CL) imaging to indi-rectly observe iron segregation to grain boundariesand cracks in the oxide films grown on zirconium alloys.We are not aware of any direct measurements of grain

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* Corresponding author. E-mail: daniel.hudson@materials.ox.ac.uk

boundary segregation in zirconium alloys prior to thepresent work.

The 3-D atom probe is a microscope that allows thereconstruction of 3-D ‘‘atom maps”. These reconstruc-tions can be interrogated and interpreted to determinethe nanoscale chemistry of the material. The atom proberequires the fabrication of sharp, conductive, needlessuch that a very high electric field is produced at the nee-dle tip on application of an electric potential. This po-tential is held just below that required to cause theionization of atoms at the apex of a needle. A furtherpulse of energy can then be applied to allow controlledionization, and subsequent radial projection of ions to-wards a position-sensitive detector. These ejected ionsundergo time-of-flight mass spectrometry and are recon-structed into atom maps by tomography.

Specimens for atom probe analysis were created usinga multi-stage process. Standard ZIRLO sheet was sup-plied by Westinghouse Electric Company (compositiongiven in Table 1 [7]). The sheet had undergone numeroushot- and cold-rolling stages. Between each rolling stagethe material was annealed at around 595 �C with theintention of fully recrystallizing the material. Suitablemethods for performing this process are described else-where [8]. The final anneal created a fine, equiaxedmicrostructure. The sheet was cut into small rods1 mm � 1 mm � 15 mm using a diamond saw. Theserods were electropolished in an electrolyte consistingof 25% perchloric acid (60%) and 75% acetic acid(100%) with an applied potential of 20 V DC until thecentre of the rod was etched away, creating two pointed

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Table 1. The nominal composition of Westinghouse ZIRLO reported by Natesan and Soppet [7] is compared with the bulk electron probemicroanalysis (EPMA) of material from this study. 3-D atom probe data for the bulk composition of the matrix and is also given. The matrix data istaken from a region more than 10 nm away from the grain boundary. All values are in atomic percent (at.%) and the uncertainty quoted is onestandard deviation.

Sn Nb Fe Cr Ni Si Hf C O N H Ga

Nominal 0.76 0.96 0.18 – – – 0.002 0.1 0.62 0.03 – –EPMA (bulk) 0.75 ± 0.02 0.91 ± 0.04 0.15 ± 0.03 – – – – – – – – –3DAP (matrix) 0.69 ± 0.02 0.52 ± 0.02 0.003 ± 0.001 – – – – 0.055 ± 0.006 0.39 ± 0.02 – – 0.17 ± 0.01

412 D. Hudson, G. D. W. Smith / Scripta Materialia 61 (2009) 411–414

needles. These were then polished further using a singledrop of the same solution under an optical microscopeat a lower voltage (1–3 V). The specimens were thenwashed in ethanol and kept under vacuum. Focusedion beam (FIB) milling was used to perform the laststage of sharpening. The end of each specimen wassharpened by applying a series of annular milling pat-terns along the axis until the required end geometrywas achieved, i.e. a tip with radius between 30 and80 nm and a very low shank angle with no other sharpedges within 10 lm of the tip apex. FIB milling was con-ducted using a FEI FIB 200 commercial instrument.

Specimens were analyzed using an Imago ScientificInstruments LEAP� 3000 X HR, a 3-D atom probe withenhanced field of view [9]. Both electronic pulsing andlaser pulsing methods were used to remove ions fromvarious specimens. For the laser-pulsed atom probespecimens, the tips were cooled to a temperature of60 K. A 0.6 nJ, 10 ps laser pulse was used with a repeti-tion rate of 200 kHz. The standing voltage on the tipwas varied automatically in order to maintain an evap-oration rate of 15 ions for every 1000 pulses. This rela-tively high rate produces a very low background signal.During voltage pulsing the tips were cooled to 90 K. Thepulse amplitude was kept at 15% of the standing voltageapplied to the specimen. The voltage pulsing repetitionrate was also 200 kHz. Zirconium-based atom probe tipshave a tendency to fracture during analysis [10]. Thisproblem is alleviated to some extent by the use of laserpulsing [11]. During voltage pulsing the automaticstanding voltage control was used to maintain a muchlower evaporation rate of 2 ions in every 1000 pulses.

Figure 1 shows atom maps of a ZIRLO specimencontaining a grain boundary. The feature is unambigu-ously identifiable as a boundary as its image projectionis 2-D, as opposed to dislocation networks that maybe present in the material. Solute segregated to these de-fects would be visible in the atom probe as distinct 1-Darrangements. The maps have been oriented so that theview is along the plane of the grain boundary. Localmagnification effects occur at interfaces observed inthe atom probe [12] and limit the spatial resolution thatcan be achieved mapping species in these regions. Forexample, field-focusing can occur in the region of inter-faces, leading to an apparent increase in total atom den-sity in that region. This is evident in Figure 1a, whichshows a map of all the ions detected, regardless of theiridentity. This acts as a further confirmation of the pres-ence of the boundary. The 3-D atom maps in Figure 1band c demonstrate the spatial distribution of iron andniobium, respectively. These maps were reconstructedfrom voltage-pulsed atom probe data. The extent ofthe non-uniformity in the distribution of these species

is much greater than that due to the field focussing ef-fects seen in Figure 1a. Segregation of iron and niobiumto the boundary region is clearly evident, as is the nio-bium present in solid solution.

Spectra of the dataset shown in Figure 1 are availableas Supplementary online material. The main species de-tected were identified as containing Zr, Nb, Sn, C, N, Oand Fe. The background noise levels seen in the massspectra were low; approximately 10�4 of the height ofthe largest signal in the mass spectra. All alloying ele-ments in solid solution were within the expected solubil-ity limits. The compositional data for the matrix(defined as material inside the grain interiors and morethan 10 nm away from the interface) is summarized inTable 1. Only 0.003 ± 0.001 at.% iron is seen in solidsolution in the matrix away from the interface. How-ever, as expected, appreciable solid solubilities are foundfor niobium (0.52 ± 0.02 at.%), tin (0.69 ± 0.02 at.%)and oxygen (0.39 ± 0.02 at.%). Comparison of the ma-trix compositional information with the nominal com-position from Table 1 shows that locally the amountof carbon and oxygen in the specimen is lower than ex-pected. No hafnium-containing peaks were identified inthis atom probe study, or in any other atom probe inves-tigations of zirconium-based alloys reported to date.The concentration of this element, 0.002 at.%, is aroundthe noise floor of most atom probe microscopes. There isoverlap in the spectrum between ZrO+ and Fe+ at m/e = 56. By comparing the isotopic abundance of theZrO+ peaks it is estimated that 28 ± 9 atoms of ironare contained in this peak. Because the position of these28 atoms cannot be determined, they cannot be includedin the compositional analysis of the boundary. This ne-glected data constitutes about 2% of the iron solute inthe region of interest. Gallium is introduced in to thesample during FIB milling preparation. The overall levelof implantation is low, around 0.2%, as seen in Table 1.This level of damage is unlikely to have significantly af-fected the structure of the grain boundary observed.Hydrogen uptake during electrochemical sample prepa-ration was also minimal.

The relative proportions of the various atomic speciespresent at different locations across the boundary can bedetermined using cumulative concentration profiles[13,14]. Cumulative profiles are calculated across thegrain boundary by selecting cylindrical regions perpen-dicular to the interface within the 3-D atom map recon-struction. These profiles are plots of the cumulativenumber of atoms of a particular solute against the totalnumber of atoms detected along this region normal tothe interface. The gradient of each curve at a given pointdescribes the instantaneous concentration at that posi-tion in the cross-section. This method excludes effects

Figure 1. 3-D atom maps of a grain boundary region of ZIRLO (a)showing all ions, (b) showing the segregation of iron, and (c) thesegregation of and niobium at the interface. The grain boundaryoccupies a larger area than would be expected in the reconstructionspace; this is due to local magnification effects arising from the atomprobe specimen geometry.

D. Hudson, G. D. W. Smith / Scripta Materialia 61 (2009) 411–414 413

relating to the boundary geometry. The disadvantage ofthis approach is that it reduces the sample size and so in-creases the statistical uncertainty related to an observa-tion. Cumulative compositional profiles, taken acrossthe boundary from Figure 1, are shown in Figure 2.From Figure 2, the concentrations of iron and niobiumare seen to be substantially enriched in the grain bound-ary. The other elements (Sn, O, C, and N) appear not tosegregate to the boundary. The constant oxygen concen-tration across the interface suggests that this grainboundary was not acting as a short-circuit route for oxi-dation, despite its proximity to the zirconium–air inter-face after specimen preparation.

Quantitative measurement of the number of ‘‘mono-layers” of excess solute present at the boundary can beobtained by calculation of the Gibbsian boundary ex-cess, CI, using these cumulative compositional profiles.Several profiles were taken at different positions on theinterface and their results averaged in order to improvethe significance of the measurement. For an interface

such as a grain boundary we would expect to find thatthe material in the grains on either side shares a com-mon composition. For such a boundary the number ofexcess atoms detected at the interface, Nexcess, can read-ily be found by measuring the distance between the twoparallel regions of the profile that represent the concen-tration of solute within either grain. Nexcess for iron isrepresented graphically in Figure 2. An estimate of theamount of solute at the boundary can be made using:

C ¼ N excess

edA; ð1Þ

where C is the excess amount of solute in terms of atomsper square meter, ed is the atom probe detector effi-ciency, and A is the surface area of the region of interestover which the cumulative composition profile was ta-ken. For the LEAP ed � 0.38. For the grain boundarycase where a cumulative profile is created with equal dis-tance on either side of the interface, this can be moreusefully expressed as:

Ci ¼Nðca � cmÞ

edA; ð2Þ

where N is the total number of atoms in the cross-sec-tion, Ca is the average concentration of a solute, i, overthe entire profile, and Cm is the concentration of a sol-ute, i, in the matrix region.

The calculated value for the Gibbsian excess can berelated to known properties of the material to give anestimate of the excess solute at the interface in termsof an equivalent number of monolayers of material [15]:

Ui ¼Ci

qx; ð3Þ

where q is the average density of the material and x is theclosest matrix plane spacing. These properties are taken tobe the density and basal (0 0 0 1) plane spacing of a-Zr,q = 43.0 atom nm�3 and x = 0.323 nm [16], respectively.

It is estimated that the equivalent of 0.33 ± 0.03 mon-olayers of iron are segregated to the interface, where theuncertainty in the measurement represents one standarddeviation over seven interface cross-sections. Niobiumsegregated to a lesser extent, with 0.15 ± 0.04 monolay-ers of excess material at the grain boundary. There was amuch greater relative variation in the measured excessniobium than iron. The segregation of both iron andniobium was also seen in a longer LEAP run collectedusing laser pulsing. Data from that experiment is avail-able as a Supplementary online resource.

Enhancement factors for individual solutes at theinterface can be estimated by making the simplifyingassumption that the excess solute is concentrated in asingle atom layer at the plane of the interface. Theenhancement factor for iron is very sensitive to theuncertainty in the measurement of its matrix concentra-tion. However, it is of the order of 1000. The enhance-ment factor for niobium can be estimated with morecertainty, and is found to be approximately 3.5.

Using data from Ref. [17], the Goldschmidt atomicradii for atoms in conditions of 12-fold symmetry areZr 1.6 A, Sn 1.58 A, Nb 1.47 A, Fe 1.28 A, C 0.77 A,N 0.71 A and O 0.60 A. From this data the substitutional

Figure 2. Cumulative profiles showing the number of solute atomsdetected across the grain boundary; major and minor solutes are shownseparately. The iron profile has been redrawn in the top left to demonstratethe relation between Nexcess and such profiles. A cylindrical region is selectedperpendicular to the interface with equal lengths entering both grains.

414 D. Hudson, G. D. W. Smith / Scripta Materialia 61 (2009) 411–414

atoms with smaller mismatch with zirconium are seen tohave lower excess at the boundary. Iron, with a radiusmismatch greater than 15%, segregates most stronglyto the boundary. Although niobium (with a radius mis-match 8%) is seen to segregate, the relative proportionretained in solid solution is much greater than for iron.Tin, which has only a very small difference in radiusfrom zirconium (1%) does not show any detectableinterfacial segregation.

In stress relief annealed ZIRLO, interstitial solutessuch as carbon, nitrogen and oxygen tend to stay in so-lid solution. This is presumably related to their high so-lid solubilities in a-Zr, and is in marked contrast to theirbehaviour in body-centred cubic metals such as iron,where segregation of interstitial elements to grainboundaries is very pronounced.

Segregation of iron to the boundary could have oc-curred during the b ? a transition or during annealingwithin the a phase and more work is required to distin-guish the effect of the different thermal treatments ongrain boundary segregation. It is clearly of interest todetermine how the corrosion behaviour is influencedby the distribution of iron between grain boundaryand matrix. Zirconium alloys are generally resistant tostress-corrosion cracking [18], suggesting that excessiron at the grain boundary does not lead to enhancedcorrosion in these systems. Iron redistributed into solidsolution has been shown to lead to enhanced corrosion[3,5]. This would suggest that iron segregation to theboundary improves alloy performance compared to

there being a supersaturated solid solution in the matrix.However, it is likely that under irradiation, redistribu-tion of this boundary material into the matrix would oc-cur, and be detrimental to the corrosion properties.Future work will focus on comparisons of solute levelsin zirconium alloys with differing corrosion resistance,to further develop our understanding of the impact ofgrain boundary chemistry on corrosion resistance.

The authors would like to thank their collabo-rators from EDF Energy, Westinghouse, and the OpenUniversity and Manchester University, which make upthe MUZIC consortium. They would also like toacknowledge the assistance from, and discussions with,Dr. Michael Preuss, Dr. David Saxey and Prof. AlfredCerezo. This research was funded by the Engineeringand Physical Sciences Research Council (EPSRC) andUK MoD (DSTL) under Grant No. EP/E036384/1.

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.scriptamat.2009.04.032.

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