The characterisation of second phases in the Zr-Nb and Zr ... · A. Harte Second phases in the...

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The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical reviewDOI:10.1016/j.jnucmat.2018.03.030

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Citation for published version (APA):Harte, A., Griffiths, M., & Preuss, M. (2018). The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fealloys: A critical review. Journal of Nuclear Materials. https://doi.org/10.1016/j.jnucmat.2018.03.030

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https://doi.org/10.1016/j.jnucmat.2018.03.030

A.Harte SecondphasesintheZr-NbandZr-Nb-Sn-Fealloys:Acriticalreview 1

ThecharacterisationofsecondphasesintheZr-NbandZr-Nb-Sn-Fealloys:Acriticalreview

1AllanHarte,2MalcolmGriffiths,1MichaelPreuss1SchoolofMaterials,TheUniversityofManchester,OxfordRoad,Manchester,UK,M193PL2Dept.ofMechanicalandMaterialsEngineering,Queen'sUniversity,NicolHall,60UnionStreetKingston,Ontario,CanadaK7L3N6Correspondingauthor:AllanHarte,mailto:[email protected] Abstract

ThenatureandevolutionoftheFeenvironmentinZr-NbandZr-Nb-Sn-Fe systems is essential to alloy performance during corrosion,hardening and irradiation-induced growth. Unfortunately, there isambiguity in the literature regarding the characterisation ofsecondaryphases in thesesystems.Thepresence,ornot,ofFe inβ-Nb phase has been a source of disagreement. In ternary ZrNbFeintermetallics, identical compositions have been designated asZr(Nb,Fe)2or(Zr,Nb)3Fe.WeshowthatwhileZr(Nb,Fe)2iscommonlyreported, it is not always justified. The cubic phase (Zr,Nb)2Fe iseasily identified, but its composition is more variable after lowtemperature heat treatments. We demonstrate the need forcorrelative approaches in the assessment of phase composition,crystallography and local Fe environment under different heattreatment regimes. Irradiation effects allow us to draw cluesregarding phase designation, but there is diverse behaviour underirradiationduetoinitialphasecomposition,irradiationdoserateandtemperature.

TableofContents

1 Introduction...........................................................................................................................................................................2

2 Nbterminalsolidsolubilityinα-Zr.............................................................................................................................2

3 Theβ-Nbphaseand“wheretheirongoes”.............................................................................................................4

4 AmbiguityintheidentificationofbinaryZr-Fephases......................................................................................7

5 TheidentificationofternaryZr-Nb-Fephases.......................................................................................................75.1 The(Zr,Nb)3Fephase.............................................................................................................................................115.2 ThehexagonalZr(Nb,Fe)2andcubic(Zr,Nb)2Fephases........................................................................135.2.1 Thehexagonalphase.....................................................................................................................................145.2.2 Thecubicphase...............................................................................................................................................15

6 IrradiationEffects............................................................................................................................................................166.1 Thehexagonalphase.............................................................................................................................................166.2 Thecubicphase........................................................................................................................................................18

7 Conclusions.........................................................................................................................................................................18

Accepted15thMarch2018JournalofNuclearMaterials


1 IntroductionThe corrosion and mechanical performance of Zralloys is intrinsically related to the nature,morphology and behaviour of second phases andtheunderlyingmicrostructure [1–9]. It is thereforeessential tocharacterisesecondphaseswithclarityand consistency in order to accurately assess theirevolving behaviour during the lifetime of nuclearcomponents.Feisknowntobeacriticalelementformanyperformancecriteriaandsoitsinitialstateinsecondary phases and its evolution under alloyoperatingconditionsmustbestudiedwithaccuracyandprecision,asonlythencanweelucidateatrulymechanistic understanding of deleteriousphenomena. Such analysis lends itself to spatiallyresolved techniques, such as electron microscopyand related practises. Whilst synchrotron X-rayradiation studies have provided data on whethersecondary phases exist in commercial alloys [10],widerquantitativeX-raydataonphasestructure israre and, in X-ray studies, phase composition iscalculated from alloy composition and not directlydetermined.Themainsourceofdataforthepresentreview will therefore arise from electronmicroscopyandrelatedtechniques.In commercial Zr-Nb and Zr-Nb-Sn-Fe alloys thereare several phases that may exist, depending onalloy composition and thermomechanical history.The phases commonly reported in these alloys areα-Zr,β-Zr,β-NbandternaryZr-Nb-Fephases,oftendesignatedZr(Nb,Fe)2,(Zr,Nb)3Feor(Zr,Nb)2Fe.Thesecondary phases arise from a lack of Nb solidsolubility in α-Zr and this topic is addressed insection 2.Whilst the composition and structure ofthe solid solution phases are generally consistent,the content of impurity Fe in β-Nb has been thecauseforconflictingreportsandwillbeassessedinsection3of this review.The second sectionof thisreview concerns the ternary intermetallics. Thesesystems contain a significant amount of all threeconstituents, but the data reported by differentauthorsoftendoesnotmatchwhat is statedas thestoichiometric composition. Such conflicts havebeenthesourceofconfusioninthecharacterisationof equilibriumphases in commercial Zr-Nb-(Sn)-Fealloys and we aim to highlight and discuss thereasons for the ambiguity that currently exists. All

compositions from literature sources that areanalysed in this work are presented in Figure 1,withalloyandheattreatmentdetailsanalysedinthetext and provided explicitly in the supplementarymaterial.Pleasenotethatallphasecompositionsareprovidedinatomicpercent(at.%),unlessotherwisestated.

2 Nbterminalsolidsolubilityinα-ZrInafundamentalstudyontheprecipitationkineticsof the Zr-Nb system, parallel X-ray diffraction andthermoelectric powermeasurements were used todemonstrate the slow nucleation of secondaryphases in such alloys,made evenmore sluggishbytheadditionofsubstitutionalelementSnandfasterbyaddingFeordeformationpriortoannealing[11].Thisprecipitationoftheβ-NbphaseandtheternaryZrNbFephases in bulkα-Zr is primarily due to thelow terminal solid solubility (TSS) of Nb in the αphase. The value of this TSS is, however, cause forsomedisagreement.In2000,Canayetal.performedsystematic electron microprobe chemical analysison various ternary Zr-Nb-Sn alloys withcomposition Nb 1-3 at.% and Sn 0.8-1.6 at.% andquaternaryZr-Nb-Sn-FealloyswithcompositionNb1at.%,Sn0.7-0.8at.%andFe0.2-0.7at.%[12].Heattreatments were performed above the Zr-Nbmonotectoid temperature and for times ranging120-710 hr to achieve equilibrium conditions. TheconcentrationofNbfoundintheα-Zrwasverylow,<0.1at.%inmostcaseswherethealloycompositionrepresented that of a commercial alloy. Nbconcentrations inα-Zronly reached0.3-0.7at.% inalloyswithhighcontentsofNb,Snandthatcontainno Fe. However, it is very difficult to produce a ZralloywithnoFeatall,ashighlightedinasystematicTEMstudyofthephasespresentinZr-Nb-Fealloys[13].Inthisotherstudy,theauthorsdemonstratedasignificant effect of Fe on Nb TSS in Zr alloys andbecauseFeisalmostalwayspresentasanimpurityinZralloysitisverydifficulttodetermineNbTSSinthebinaryalloy.Interestingly,theauthorsobservedno β-Nb phases within α-Zr when the alloy Nbcontentis0.29wt.%anddidobserveβ-NbwhentheNb content was 0.49 wt.%. This leads to theconclusionthatNbTSSinα-Zris intherange0.29-0.49 wt.% at temperatures below the Zr-Nbmonotectoid temperature (<600 °C), a valuemuchgreaterthanthatsuggestedbyCanayetal.


Figure 1Ternary Zr-Nb-Fediagramgenerated fromdata after [3,20–27,29,41–44,46,62–64]. All compositionsaregiveninat.%.Markerstyleandcolourrelatetothephasedesignatedbytheauthorsofthatpublication.

In the ternary or quaternary alloys containing Sn,the α-phaseNb content in Zr-Sn-Nb alloys appearstobeafunctionoftheSnconcentration,whereasinbinary Zr-Nb alloys the Nb content in the α-phasecouldbeashighas0.49wt.%.InalloyswithahighSncontentsuchasExcelalloy(Zr-3.5%Sn-0.8%Nb-0.8%Mo,wt.%)thereisdebatablynodetectableNbin the alpha phase (see discussion by Griffiths in[14]).Althoughvaluesfortheα-phaseNbcontentof0.23wt.% have been reported for Excel alloy [15],these values are not supported by the spectrashown in the paper and the accuracy of thequantitativeanalysisatsuchlowvaluesisthereforequestionable.Nbcontent in theα-ZrphaseofExcelhasonlybeenconvincinglyshowninalloysthatarewater quenched from between 690 and 876 °C,resultingintheretentionofNbintheα-Zrphasetobetween 0.37 and 0.59 wt.% [16]. With lower Sn

contents in ZIRLOTM and E635 alloys (1 wt.% Sn),theNbcontentintheα-phaseismeasurablygreaterthan for the Excel alloy (3.5 wt.% Sn). It appearsthat Sn, which is an α-stabiliser, reduces thesolubilityofNb in theα-phase. At levelsofSn thatapplytocommercialZr-Nb-Sn-Fealloys(1wt.%Sn),theSnandNbco-existinsolidsolution[13].Theco-existence of Nb and Sn in solid solution in alloyssuch as E635 and ZIRLOTM (1 wt.% Sn) and thereductioninNbinsolidsolutioninExcelalloywithhigherSncontent isonereasonwhyExcelbehavesmore like the Zircaloys in terms of irradiationgrowth compared with other Nb-containing alloys[17].ThequestionofNbTSSinα-Zrasafunctionoftemperaturethereforeremainstobedefinedforthebinary phase. It should be noted that all of thechemical composition work alluded to here wasperformed in a standardless manner, which is not


suitableforNbquantificationduetoissuesofX-rayfluorescenceofZrbyNbKβ.Aquantitativeenergy-dispersive X=ray spectroscopy (EDS) study isthereforenecessarytoresolvethismatter,oruseofa method that does not suffer from fluorescenceeffects, such as atom probe tomography. Thesupersaturation of Nb in α-Zr has beendemonstrated by the irradiation-inducedprecipitation of small Nb-rich phases in alloys Zr-1NbandZr-1Nb-0.1Fe(wt.%)[18].

3 Theβ-Nbphaseand“wheretheirongoes”Thecompositionofβ-Nbphaseandwhetherornotit contains any Fe is subject to disagreement. Weaim to assess and clarify the reasons for thecontradictory data in this paper. Thedisagreementseems to be especially prevalent in commercialbinary alloys and so we will begin there. Themicrostructural evolution of commercial Zr-2.5Nb(wt.%) under conventional heat treatment regimesisasfollows.Afterquenching,cold-workandstress-relief,binaryalloyZr-2.5Nb(wt.%)containsbothα-Zr andβ-Zrphases. Furtherheat treatmentsbelowthe Zr-Nb monotectoid temperature (~600 °C)result in the decomposition of themetastable β-Zrphase,whichoccursviaa two-stageprocess. InZr-2.5Nbpressuretubingattemperatures<550°Cthefirststagecomprisesdecompositionofβ-ZrtoaNb-enriched β-Zr phase and a Nb-depleted hexagonalω-phase, where the Nb-enriched β-Zr phase takestwo forms -oneatα-Zr/β-Zrgrainboundariesand

onetheuntransformedβ-Zrphase[19].Thesecondstage is the transformation of the Nb-depleted ω-phaseintoα-ZrandtheNb-enrichedβ-Zrintoβ-Nbphase. At temperatures close to the monotectoidtemperature the formation of the ω phase issuppressed close to grain boundaries [10]. Inadvanced stages of decomposition the β-Zr phasetransformstotheequilibriumstate,β-Nb.Theβ-Nbcan exist together with enriched β-Zr, and is alsofoundinisolatedgrainsandasprecipitatesatgrainboundaries. Eventually the equilibrium state isachievedsuchthat,inthecaseofthebinaryphases,thereexistsBCCβ-NbandHCPα-ZrthatcontainsNbin solid solution (<1 wt.%). Interpretation of thebinary phase states is often confounded by thepresenceofFethat,likeNb,isaβ-stabiliser.TheFeandNb is enriched in themetastable phase states,and forms discrete ZrNbFe precipitates in advanceof thecompletedecompositionof theβ-Zrphasetoβ-Nb [10]. The compositional range for β-Nb isgivenfromseveralstudiesintheternarydiagramofFigure 1 using data from [20–26], an analysis ofwhichisshowninFigure2.ThediagraminFigure1represents the phases identified at roomtemperature,someofwhichmaynotbeequilibriumphases. Figure 2a shows the full compositionsreportedfortheNb-richprecipitates,demonstratingpredominantly zero concentration of Fe, but, insomecases,upto4at.%Fe(seeFigure1).Figure2bshowstheZr:Nbatomicratioofthereportedwithamodalvalueof~0.125.

Figure2Ananalysisofthecompositionalrangeofphasesdesignatedβ-Nb.a)AfrequencyhistogramoftheZr,NbandFecompositionofthosephases(NB:b)plottedonlogscaleasmostcontain0at.%Fe).b)AfrequencyhistogramofthenumberofphasesreportedwithagivenratioofZr:Nb.


The presence of Fe in β-Nb is reported in modelalloys that have significant Zr contents after longheattreatmentsathightemperaturesabovetheZr-Nbmonotectoidtemperature(~600°C)andabovethe lowest temperature for Zr-Fe β-phase stability(~ 775 °C). One example is the Zr60.6Nb36.2Fe3.2phase in alloy Zr41Nb32Fe27after heating at 900 °Cfor500hr[21].Thisdesignationofβ-Nbshouldbedistinguished from the β-Nb that exists after heat-treatments below the Zr-Nb monotectoid. The Fecontent in β-Nb below the Zr-Nb monotectoid isreportedincommercialalloysbutonlyconvincinglyinphasesthatwereZrNbFeternaryphasespriortoirradiation that had undergone irradiation-inducedFe depletion resulting in a composition such asZr42Nb50Fe8,at.%[27].Again, thedescriptionof thephaseasβ-Nbissomewhatambiguousgiventhatβ-Nb has a Nb composition~85%-90% according tothebinaryZr-Nbphasediagram[28].ThephasesinFigures1and2thatarereportedwithaZr:Nbratio~0.1, nominally β-Nb, with some non-zero Fecontent are phases inmodel alloys that have beenquenched from longheat treatmentsat550-900 °C[21,25], i.e. mostly above the Zr-Nb monotectoidtemperature. Under commercial alloy heattreatments and compositions, the vast majority ofstudies report that there is no Fe detected in β-Nb[3,13,20,22,24,26,27,29–32]. Some early work didreportFe inβ-Nb incommercialalloysandwewillnow take the opportunity to assess the validity ofthose claims in order to alleviate any confusion onthetopic.In 1991, Northwood et al. reported on themicrostructure of Zr-2.5Nb (wt.%) [33].Whilst theauthors noted that their standardless Cliff-Lorimerapproach to chemical quantification by energydispersiveX-rayspectroscopy(EDS)couldresult inanerrorinFemeasurementofasmuchas±25at.%,theydidnotprovide any evidence to support theirclaim that “there is a small amountofFeandCr inmost of the β particles”. In fact, the only “typical”EDS spectrum that they provided for the β-Nbparticles contained no Fe peak at all. Such acontradictionmay be due to themethod bywhichthedatawascollected;boththinfoilsandextractedparticles were analysed but the authors were notclear in regard to which method their statementsconcerningFewerebased.WhilsttheEDSspectrumprovided in Figure 6 of the publication byNorthwoodetal.isfromatypicalextractedparticleandcontainsnoFe,Fedetectionmaybeexpectedinthinfoils;arecentpublicationfromFrancisetal.hasclearly shown Fe segregation to the α-Zr/β-Nb

interface in low-Sn ZIRLOTM [34], Figure 3.Therefore,dependingon theposition fromwhichasingleEDSspectrumisacquiredandthesizeof thesamplingelectronprobe,Femaybedetectedinthefoil at the interphase region. In such cases it isimportant tonote thatFewillbe segregated to thefull 3D interfacial region and so some Fe may bedetectedintheinterfacialregionaboveorbelowtheprecipitate, depending on where the thin foilsurfaces cut the precipitate. Here, 3D tomographictechniques(EDS,atomprobetomography)wouldbeof use. Another source of an Fe X-ray signal isscattering from the pole-piece in an electronmicroscope. The magnitude of this Fe artefactincreases proportionally with the number ofbackscattered electrons generated and, thus, thevolume of material under examination. Therefore,errors of this kindwill beminimised for extractedparticles.Likewise,theFeartefactcanbeaccountedfor with spatially resolved mapping protocols andquantificationproceduresforremovingbackgroundsignals.In thin foils, site-specific segregationhighlights theneed for spatially resolved chemical information,especially as Fe scattering from the pole piece ofTEMs can result in Fe peak artefacts. In Zr-2.5Nb(wt.%), a line scan was used to demonstrate Feenrichmentattheinterphaseregionα-Zr/β-Zr[19].Asenrichedβ-Zrultimatelytransformstoβ-Nb,thisobservation is consistentwith that of Francis et al.[34].Similarly,alinescanwasusedtodemonstrateFeenrichmentatα-Zr/α-ZrboundariesinZr-2.5Nb-0.1Fe (wt.%) in the early 90s by Perovic et al.[35,36]. However, much of the early work by A.Perovic,V.PerovicandtheirassociateshasbeenanadditionalsourceofconfusionregardingFeinβ-Nb.Forinstance,inthegroup’sfirstpublication[35],Feisreported inβ-Nbbutallβ-Nbshownisontheα-Zr/α-Zr boundaries. As these boundaries are alsoshown tobe rich inFe, their superpositionwithβ-Nb may be misleading. Further, when quoting thecontentofFeintheβ-Nbparticles,0.86wt.%Feandslightly more in cold-worked stress-relievedmaterial,theauthorsquotedthemaximumFevaluefor all of the particles studied and not the mean.Theydidnotprovideany indicationof thenumberofparticlesstudiedorthecompositionofindividualparticles. In such circumstances a β-Nb particlecontaining up to 85 wt% Nb is listed in the samegroupingasa “β-Nb”particle containingup to0.86wt% Fe. Given the difficulty in accurate chemicalanalysis of small particles there is no way ofknowing the true compositions of those phases


identified as β-Nb. The heat-treatment was 6h at500°C–insufficienttoattainequilibrium[10].Thissame maximum value was also quoted in a laterpublicationbythegroup[37]onirradiatedmaterialand, whilst no Fe is detected in β-Nb post-irradiation, the authors state that the β-Nb in thisirradiated state contains < 0.15wt.% Fe accordingtotheirproposeddetectionlimit.Thisismisleadingon two counts: first, if no Fe is detected abovebackgroundthennoFeshouldbereported;second,

such impressive detection limits are rarely quotedinthewidercommunity,whereminimumdetectionlimitsof0.5wt.%oreven1wt.%areoftenattestedto, especially for elements that have associatedsystem artefacts, such as Fe. Spatially resolvedchemicaldata is thereforenecessary forconvincinganalyses, and, in the case of β-Nb, electron beamdrift-correction is necessary as particles are oftensmall and their interfacial region may containsegregatedelements.

Figure3Fesegregationtotheinterfacialregionα-Zr/β-Nb,inthesamebodyofworkas[34].Partsa),b)andc)showtheZr,NbandFemaps.Partd)showsthesethreemapsoverlaid.Parte)showsa linescanthroughthecentreoftheparticle.Partsf),g)andh)showtheEDSspectrafromtheinterphaseregion,theα-Zrmatrixandthecentreoftheparticle,respectively.

One possible motivation for those early reports ofFe in β-Nb is to answer the question of whathappens to the Fe after β-Zr decomposition, i.e.“where the iron goes”. Fe is a β-Zr stabiliser [38]and, indeed, Fe is reported in β-Zr, exemplified inFigure 1 with an Fe mean ± 1.96*std of 2.8 ± 2.1

at.%. The variability of Fe in β-Zr is thereforesignificant,butthereisclearlysomeFeinβ-Zr.Afterthedecompositionofβ-Zr toNb-enrichedβ-Zr andω-phase(whichisbothNb-andFe-depletedrelativetotheparentβ-Zr),thereisafurthertransformationof Nb-enriched β-Zr to β-Nb, which, as has been


discussed, contains no Fe under commercial alloyheattreatmentconditions.So,thequestionremainsconcerningthefinalresidenceofFe.IntheZircaloy-2, Fe (and alsoNi) is found at the vastmajority ofhigh-angle grain boundaries [39] and whilst wehave already discussed the presence of Fe atinterphase regions and at grain boundaries in theZr-Nbsystem,itremainstobedeterminedwhetherthis is sufficient to account for the amount ofimpurityFepresentinZr-2.5Nb(wt.%).Underβ-Zrdecomposition, thepossibilityofFedispersing intoα-Zr and/or present as binary Zr-Fe intermetallicshas been suggested by some [35], and ternaryZrNbFe intermetallics have been demonstrated byothers [13,19]. The existence of ternary ZrNbFeintermetallicshasbeenshowninquaternaryZr-Nb-Sn-Fe alloys such as ZIRLOTM [34,40], NSF-2 [41]and E635 [20,24,27,42–44], and has also beenshown in Zr-1.25Nb (wt.%) [29] and in Zr-2.5Nb(wt.%), either standard pressure tubing containingFeasanimpurity,typically<0.15wt.%[45],orwithadded 0.1 and 0.5Fe (wt.%) [46]. An analysis oftheseternaryphasesistofollowinsection5.AstheFe content in binary Zr-Nb alloys is very low, suchternary phases are scarcely reported but theymaysimplybesmallenoughinnumberandsizetohavebeen missed in earlier studies with crude probesizes and coarse detection limits. Modern TEMequipmentaffordsmuchimproveddetectabilityandthe oversights and corresponding ambiguities thathave existed in the past should no longer be anissue.

4 AmbiguityintheidentificationofbinaryZr-Fephases

In an early characterisation of Zr-2.5Nb (wt.%)pressure tubing it was suggested that Fe exists inthe form of ZrFe binary intermetallics, and an EDSspectrumwaspublishedtothateffectintheenergyrange 5-19 keV [47]. Whilst binary ZrFeintermetallicsareknown inZr-Sn-Fe-Cr typealloys[48],andhavebeendemonstratedinZr-Nballoysoflow Nb and Fe content [13], Zr2Fe and Zr2Si bothhavebodycentredtetragonalallotropesandsomaybe confused with one another when characterisedby electron diffraction alone; indeed, it was laterreportedthatFe-coatedzirconiumsilicidesmightbemistakenforbinaryZrFephasesifSiisnotexplicitlylooked for in the EDS spectrum [49] and it isthereforeunfortunatethat theauthorsof the initialstudy [47] did not publish the full spectrum toincludetheSiKαandKβemissionlinesat1.7and1.8keV. Further, the precipitate assumedly Zr2Fe in

Figure 2 of that publication has a periphery thatdiffracts differently to the main body of theprecipitate, suggesting that it is indeedcoatedwithanother phase or region. Themis-identification ofthe Zr2Fe then impacts the future interpretation oftheMössbauersignalsassignedtothisphaseinthatstudy.ThepresenceofbinaryZrFeintermetallicsinZr-2.5Nb (wt.%) was reported again at a similartime [33], but the diffraction patterns provided inthat study did not correlatewellwith the indexingand no chemical analysis was provided. Woo andGriffiths havewell-characterised the appearance ofvarious phases in “binary” Zr-Nb systems withimpurities of Fe, and, to a lesser extent, Cr, Ni, CuandW[13]:binaryZrFeandZrSiphasesarepresentatlowNbandFecontents(<0.2wt.%Nband<24wt. ppm Fe) and higher Nb and Fe alloyspredominantly nucleate ZrNbFe intermetallics andtheβ-Nbphase. It is therefore advisable topublishthefull0-20keVEDSspectrumwhenreportingtheexistence of binary ZrFe phases in such alloys toremove the suspicion of silicides. In recent work,heavy ionKr2+ irradiation of alloy Zr-2.5Nb (wt.%)has resulted in the irradiation-induceddecompositionoftheβ-Zrphasetoω-phaseandhasshown the presence of Fe-rich phases post-irradiation[50],thoughttobebinaryZrFeinnature.However, as yet, a quantitative analysis on theseprecipitates remains to be performed, and theirnucleationmayberelatedspecificallytoirradiationand not necessarily as an irradiation-assistedevolution towards the phases of thermodynamicequilibrium. It is likely that their nucleation isirradiation-induced,asbinaryZr3Fe isknowntoberelatively resistant to irradiation [51,52] and isknown to nucleate in Zircaloy-4 after neutronirradiation [53] and in Zircaloy-2 after protonirradiation [54]. As ternary intermetallics are themostlikelycandidateforwheretheFeresidesinZr-2.5Nb (wt.%) after β-Zr decomposition, theircharacterisation will be assessed in the followingsection.

5 The identification of ternary Zr-Nb-Fephases

Similarity in the composition of phases can easilyresult in confusion regarding stoichiometricdesignation. The early characterisation ofintermetallicZrNbFephasestookplaceintheUSSR,where researchers identified a phase with acomposition of approximately Zr52Nb10Fe38 (at.%)[55,56].Thisphasewasreferredtoasthe“T”phase,possibly due to the Russian “троичный” for


“ternary”, and the crystal structure was notdetermined at that time. A wider compositionalrange of ternary ZrNbFephases has been reportedsince. Some insight into the nature of the ZrNbFephases can be obtained from work on the binaryZrFe alloys. In a comprehensive assessment ofbinary ZrFe phases, Stein et al. have noted fourphases,ofwhichweshallbeconcernedwith three:ZrFe2 (cubic or hexagonal), Zr2Fe (tetragonal orface-centred cubic) and Zr3Fe (orthorhombic) [57].ItisconceivablethattheZrNbFephasediagrammayalso contain a similar variation of phases with theaddedcomplicationthattheslowerdiffusingNbwillresult in metastable states that would complicatethefinalstructurewhenagivenalloyissubjectedtomultipleheattreatments.Intheternaryequivalents,

wemayexpectNbtoreplacesomeoftheZrduetothe similarities between these two elements intermsoftheiratomicmass(Zr91.22andNb92.91gmol-1)andatomicradii(Zr0.206andNb0.198nm,theoretical). Surprisingly,however,most reportsofternaryZrNbFephasesstatethatNbreplacessomeof the Fe to produce the phase Zr(Nb,Fe)2 and thisphase is commonly referred to as a Laves phase.This replacement of Fe by Nb instead of Zr by Nbhas lead to conflicting reports in terms of phasedesignation for similar phase compositions. In oneof theearliest reports identifying thephases in theZr-Nb-FeternarysystembyKanematsu[58]theNbwas assumed to substitute for Zr in the equivalentphasesoftheZrFebinary.

Figure4AnanalysisofthecompositionalrangereportedfortheternaryZr-Nb-Fephasesa)hexagonalandb)cubic.DatatakenfromFigure1.

The compositions commonly reported for theternary ZrNbFe phases are usually designated aseitherhexagonalZr(Nb,Fe)2orcubic(Zr,Nb)2Fe.Thechemical range of these designations is shown infrequency histograms in Figure 4. This figuredemonstrateswhy the hexagonal and cubic phasesareeasytodistinguish;thecubicphaseinFigure4bhas well-defined and separate constituentcompositions,however,forthehexagonalphasesinFigure 4a there is significant overlap incompositions. This overlap in the hexagonal phasecomposition has resulted in ambiguity in itsdesignationaseitherZr(Nb,Fe)2or (Zr,Nb)3Fe [19].This ambiguity is shown in Figure 1; the reported(Zr,Nb)3Fe composition is at the very boundary ofthe very large range of compositions reported as

Zr(Nb,Fe)2byothers.Thesetwophasedesignationsare chosen by authors to reflect their binaryequivalents, Zr3FeandZrFe2. It isnoteworthy thatZr2Fe and ZrFe2 are the two main intermetallicphases above 775 °C,whereas Zr3Fe and ZrFe2 arethe two main intermetallic phases below 775 °Caccording to Stein et al. [57]. When assessingwhetherornottheZrNbFephasesidentifiedinanyparticularstudyareconsistentwiththeZr-Fephasediagramoneneeds to knowwhether the final heattreatmentisaboveorbelow775°C,orsomesimilarphase boundary temperature for the Zr-Nb-Feternary.WhilstZr(Nb,Fe)2hasbeenreportedinthemajority of cases, many of those compositionsreportedalsofitwellwith(Zr,Nb)3Festoichiometry.In Figure 5a and Figure 5b we plot frequency


histograms of some atomic ratios for all hexagonalphases reported. In Figure 5a we pair Nb with Feand Cr to assess the suitability of Zr(Nb,Fe)2designationand inFigure5bwepairNbwithZr toassess the suitability of (Zr,Nb)3Fedesignation.Wecanseethattheratio(Nb+Fe+Cr):Zrisfrequently<2 and that the ratio (Zr+Nb):(Fe+Cr) is most

frequentlybetween2and3;hencetheconfusioninstoichiometric designation. It is likely, and oftenstated,thatmostresearchersprefertolabelaphasecontaining Zr, Nb and Fe as hexagonal LavesZr(Nb,Fe)2 because stacking faults are also oftenobserved, however it is not clear that this is adefiningconditionforthisphase.

Figure 5 An analysis of the ratio of ternary constituents in analogy to binary equivalents by pairingNbwitheitherthelighttransitionelementsFe,CrorwithZr;forthehexagonalphaseNbcoupledwitha)Fe,Crandb)ZrandforthecubicphasewithNbcoupledwitha)Fe,Crandb)Zr.DatatakenfromFigure1.

An analysis of the relationships between phasecomposition and alloy composition or heattreatment type may help us to deduce correctstoichiometry. In Figure6weplot the compositionof a phase against the atomic ratio Fe:Nb in thealloy. In part a), b) and c) the Zr, Nb and FecompositionforthephasedesignatedbyauthorsashexagonalZr(Nb,Fe)2 isplotted in fourways: thosealloys heat treated either above or below the

monotectoid temperature for β-Zr decomposition(~600°C)andthosethathaveaphasecompositionratio(Zr+Nb):(Fe+Cr)either<or≥2.5(limitchosenas half way between 2 and 3), i.e. how well thephase fits with stoichiometry (Zr,Nb)3Fe but wasdesignatedZr(Nb,Fe)2by theauthors (note thatCr,whenreported, isaddedtoFeforsimplicity).FromFigure 6 b) we can see that the Nb content in thephase is inversely proportional to the Fe:Nb in the


alloyandfromFigure6c)thattheFecontentinthephase is directly proportional to the Fe:Nb in thealloy. From Fe:Nb in part a) we see that the ZrcontentinthephasedoesnotchangewithrespecttotheFe:Nbinthealloy.Therefore,thephaseFe:Nbisdirectly proportional to the alloy Fe:Nb. However,suchatrendisonlyobservablewhentheratioFe:Nb

inthealloyexceeds~0.5,whichismuchlargerthanincommercialalloysystems.Ithasbeennotedthat“…the stoichiometry of the ternary intermetallicphases was nearly independent of the alloycompositions” [59] but this likely refers to theZr:(Fe+Nb)ratio,whichisindeedconstantasshownhereinFigure6(a).

Figure 6 An analysis of the variation in phase compositionwith respect to alloy Fe:Nb ratio, both above andbelowthemonotectoidtemperatureforβ-Zrphasedecomposition;forthehexagonalphaseina)Zr,b)Nbandc)Feandforthecubicphaseind)Zr,e)Nbandf)Fe.DatatakenfromFigure1.

Whilst we may suggest that a constant Zr phasecontentsuggestsaconstantZr:(Nb+Fe+Cr)ratioandtherefore the appropriate designation Zr(Nb,Fe)2,theZrcontentremainssignificantlyscatteredintherange29-43at.%Zrwithoneoutlierat55.2at.%Zr.This is a significant point because Laves phases, afamily ofwhich Zr(Nb,Fe)2 is often reported to be,commonly have strict stoichiometriesAB2with theA composition varying by 2-3 at.% [60]. Althoughthe Zr compositional range in Zr-Fe systems thisvariation is known to be wider than this, the Zrcomposition range for the reported Zr(Nb,Fe)2 isextraordinarily wide. This may reflect a tendencyformatrixoverlapwhenanalysing thecompositionofsmallparticlesembedded inaZr-richmatrixbutonlyaccentuates theuncertainty in thedata that isreported.

From the data in Figure 6, 32% of compositionshave a ratio (Zr+Nb):(Fe+Cr) ≥ 2.5, i.e. have an Fecontent ≤ 28.5 at.% and therefore fit well withstoichiometry (Zr,Nb)3Fe. This is more prevalentbelow themonotectoid temperature than above it;34% of reported compositions fit well with(Zr,Nb)3Fe below the monotectoid and above themonotectoidonly15%,highlightingtheimportanceof investigations both above and below thistemperature. In Figure 6 parts d), e) and f) thisanalysis is repeated for a different phase: cubic(Zr,Nb)2Fe,designatedbyauthors inanalogy to thebinary Zr2Fe. For this phase, only 3% of phasedesignations better fit (Zr,Nb)3Fe (4% below themonotectoid and 0% above), and so the ambiguityin stoichiometry designation is not so much of aproblem with the cubic phase. The (Zr,Nb)2Fe isrelatively easy to identify as it is cubic, asopposedtohexagonal,andcontainscharacteristicallylowNb


content. The Zr(Nb,Fe)2 is always reported ashexagonaland the (Zr,Nb)3Fehasbeenreportedasboth orthorhombic and hexagonal (which is alsoorthorhombic). Therefore, when an investigatorobserves a hexagonal structure with a reasonablyhighNbcontenttheymaysimplyassumeZr(Nb,Fe)2phase.Whilstthismaynotnecessarilybeincorrect,there are conflicting reports. The following is adetailed and critical assessment of phasedesignationinrelationtothesereports.

5.1 The(Zr,Nb)3FephaseItisimportantthatauthorsprovideasmanydetailsas possible in regards to their analysis, asinvestigations of small volumes are easilyunrepresentative and are prone to errors. Secondphases in commercial Zr alloys generally have adiameterontheorderof<100nm.Assuch,analysistechniques with high spatial resolution arenecessary for their characterisation andtransmission electron microscopy (TEM) has suchcapability.However,samplepreparation iskey; thematrix in which the second phases are imbeddedmay contribute to the chemical data obtained byenergydispersiveX-rayspectroscopy(EDS)ordatarelating to crystallography by the recording ofelectrondiffractionpatterns(DPs).Toalleviatethisissue, one must be specific in the number of

particles studied and their locationwith respect tothe foil edge, such as those studied by Tian et al.,shownhere inFigure7a forbothβ-NbandternaryZr-Nr-Fe phases in an α-Zr matrix [26] and,similarly, other examples that explicitly show theparticle being reported upon, such as the ternaryZrNbFeparticle in Figure 7b [19]. These examples,whilstexplicit,wouldbenefitfromspatiallyresolveddata in the form of line scans or mapping, see forexample the maps shown in [45], used to identifyboth the β-Nb and (Zr,Nb)3(Fe,Cr) phases in Zr-2.5Nb pressure tubing heat-treated below themonotectoid temperature. Oversights in thispractise make it difficult to assess the validity ofcompositionssuchas“50Zr-30Nb-20Fe(%)”[29],inwhichtheparticlenumberandlocationdetailswerenotgivenandthepercentagetypewasnotprovided.Ifatomicpercentthenthisreferstoastoichiometrythat could be written (Zr,Nb)4Fe, which has nobinary equivalent and has not been reportedelsewhere. Thismaypoint towards an excess of Zrsignal fromthematrix. If thecompositionisweightpercentthenwhenconvertedtoatomicpercentthestoichiometry is closer to (Zr,Nb)3Fe. Thisstoichiometric designation has been reported tosomeextentandwillbeassessedhere.

Figure7a)Tianetal.showthenumberofparticlesstudiedandtheirlocationwithrespecttothefoiledgewhenreporting their quantified compositions [26]. b) Griffiths et al. explicitly show the ternary particle underdiscussionanditslocationwithrespecttotheα/βinterface.


To avoid excess matrix contribution to chemicalquantification, Ng-Yelim, Woo and Carpenter usedextractionreplicastoremoveallmatrixcontributionforthecharacterisationofsecondphaseparticlesin,amongst others, alloy Zr-2.5Nb (wt.%) withadditions of either 0.1Fe or 0.5Fe (wt.%) [46]. Theauthorscomparedtheircompositionalanalysis toastoichiometric alloy of Zr1.5Nb1.5Fe1 (presumablyat.%), which they determined by EDS asZr37.5Nb37.5Fe25.0 (at.%).Theauthorsdesignatedthisphase(Zr,Nb)3Fe.Whilstthisassignmentmayseemappropriate as an analogue to the binaryorthorhombic Zr3Fe [57,61], no crystallographicanalysis was provided for this stoichiometry alloy.Theonlyreportedelectrondiffractionworkwasforthe extracted particles in the commercial-typealloys, which were found to have the hexagonalcrystalstructurewith latticeparametersa=0.54,c= 0.88 nm and the following mean compositions:Zr35.7Nb40.9Fe23.4 (at.%) for the 0.1Fe alloy andZr37.8Nb40.2Fe22.0(at.%)forthe0.5Fealloy(notethattheCrcontenthasbeenaddedtotheFecontentforclarity) [46]. Whilst the compositions of theextracted particles almost match that of theZr1.5Nb1.5Fe1, there was no direct crystallographiccomparison shown and there was no report ofwhether the Zr1.5Nb1.5Fe1 was thoroughly analysedformultiplephases.InagreementwithNg-Yelimetal. [46],Motta et al. reported hexagonal (Zr,Nb)3FeinanalloywiththesamestoichiometryZr1.5Nb1.5Fe1(presumablyat.%)viaEDSandelectrondiffraction,althoughnospectraorpatternswereshowninthatparticular publication [62]. The lattice parametersreportedforthehexagonalphasefoundinthatalloywereingoodagreementwiththoseoftheextractedparticlesinNg-Yelimetal.:a=0.54andc=0.88nm[62].Nikulina et al. reported two phases in alloy E635(Zr-1Nb-1Sn-0.4Fe (wt.%)), which were attributedto the following stoichiometries: the (Zr,Nb)3FesimilartothatofNg-Yelimetal.andZr(Nb,Fe)2[42].Although the authors only gave a singlecompositional range to include both phases andgave no indication of the number of particlesstudied or their location with respect to the foiledge, and sowe cannot discuss their compositionshere, they did provide two crystallographicidentificationsbyelectrondiffraction:orthorhombic(Zr,Nb)3Fewitha=0.88,b=0.33,c=1.10nmandhexagonal Zr(Nb,Fe)2 with a = 0.51, c = 0.83 nm.Therefore,incomparisontoNg-Yelimetal.[46]andMottaetal. [62],Nikulinaetal.gavethe(Zr,Nb)3Festoichiometrybutwithadifferent crystal structure

andadifferentstoichiometriccompoundZr(Nb,Fe)2withthesamestructureasNg-Yelimetal.’s[46]andMottaetal.’s[62](Zr,Nb)3Fe.The binary phase Zr3Fe is Re3B-type orthorhombicwith lattice parameters a = 0.332, b = 1.100, c =0.882nm[57],inexcellentagreementwithNikulinaet al.’s structuredetermination for (Zr,Nb)3Fe [42].However, the two phase designations by Nikulinahighlights the confusion between the designation(Zr,Nb)3Fe and Zr(Nb,Fe)2. The confusion of(Zr,Nb)3Fe and Zr(Nb,Fe)2 would occur for acompositionofsomethingsurroundingZr33Nb42Fe25(at.%),whichisattheedgeoftherangereportedinthewiderliterature(seethecentralregionofFigure1)and32%ofreportsofZr(Nb,Fe)2intheliteraturewould fitwellwith thedesignation (Zr,Nb)3Fe (seeFigure6a)forphase(Zr+Nb):Fe>2.5).Importantly,therehavebeenreports thatshowNbtohaveverylow solubility in the binary phase Zr3Fe (0-2 at.%,depending on alloy and heat treatment) [63].Therefore, we may not expect (Zr,Nb)3Fe to beorthorhombic, although orthorhombic (Zr,Nb)3FehasbeennotedbythesamegroupbothinE635[42]and in E125 Zr-2.5Nb (wt.%) [20].However, as nodiffraction characterisationwas evidenced in thoseworks we may assume that this phase was notstudiedindetail.The lattice parameter of hexagonal (Zr,Nb)3FereportedbyNg-Yelimetal.[46]andMottaetal.[62]isinagreementwiththehexagonalZr(Nb,Fe)2phaseofNikulinaetal. [42], and indeedwithmanyothergroups for the reported Zr(Nb,Fe)2 phase[20,21,23,27,44,64], a more in-depth discussion ofwhich is to follow. It may be the case that thehexagonal [46,62] and orthorhombic [42] latticesare confusedbecause ahexagonal crystal is indeedorthorhombic. However, the lattice parametersnoted by the relevant groups are significantlydifferentforthistobealikelysourceofconfusion.Itmay be the case that the phase designationZr(Nb,Fe)2isassignedtoaphaseifitsimplyappearshexagonal, even though it may fit better withstoichiometricdesignation(Zr,Nb)3Fe.However,thedesignation as (Zr,Nb)3Fe has not been reportedsinceAverinetal.in2000inZr-2.5Nbandhasneverbeen explicitly characterised with scrutiny bymodernmethods. The very existence of this phase,its compositional range, its structure, its formationunderdifferentheattreatments, itsthermodynamicmetastability in comparison to other, more wellknown, phases and its behaviour under irradiationallremainopenquestionstodate.


Insummary,itiseitherthecasethatthereisasinglehexagonalternaryphasewithawidecompositionalrange or it is the case that there are two phases,possibly both hexagonal, with narrowercompositional ranges and with the Nb associatedwitheithertheFe,asinZr(Nb,Fe)2,orwiththeZr,asin (Zr,Nb)3Fe. It is noteworthy thatKobylyansky etal. describe certain precipitates as (Zr,Nb)2Fe andalso state that the Fe content is 25 at% afterirradiation in Table 11 of [43], which may alsoindicate that the phase has reached equilibrium attheirradiationtemperature(about330°C),andisagood example of how such confusion instoichiometric designation can arise. Indeed, thecompositionofLavesphasesisknowntobenarrow,but heat treatments are likely to play a significantroleinthecompositionsobtainedinternaryphases.Whilst most characterisation of ternary ZrNbFephaseshave reported stoichiometriesofZr(Nb,Fe)2and (Zr,Nb)2Fe (see the following section for adiscussion of this latter phase), the effect of heattreatments above and below the monotectoidtemperature for β-Zr decomposition has not beenstudied in detail with respect to varying alloycomposition. We show this effect in Figure 6 andmore detailed, systematic, correlative chemical-structural studies of the same alloys above andbelow the monotectoid are necessary for betterphase identification in Zr-Nb-Fe ternary systems.Thequestionremainsas towhether the (Zr,Nb)3Fephase,reportedpriorto2000,existsandhassimplybeendesignatedZr(Nb,Fe)2byrecenttradition.Itisthe present authors’ opinion that there is a habitwithin the literature to simply assign a phase asZr(Nb,Fe)2,even if thecompositiondoesnotreflectthis stoichiometry. Therefore, a line of enquiry isneededtodirectlyresolvethisissue,preferablywithinvestigationintoFe,NbandZr localenvironmentsinorder toassesswhetherweshouldassociate theNb with the Fe or the Zr for designation asZr(Nb,Fe)2or(Zr,Nb)3Fe,respectively.

5.2 The hexagonal Zr(Nb,Fe)2 and cubic(Zr,Nb)2Fephases

In more recent years, extensive study of modelternary Zr-Nb-Fe alloys by Argentinian groups[21,23,63–65]andFrenchgroups [3,22,25,66]havereported the existence of two dominant ternaryZrNbFe phases: the hexagonal Zr(Nb,Fe)2 and theface-centredcubic(Zr,Nb)2Fe.Thesecomprehensivecharacterisation studies have included many alloycompositions andhave been assessedwith variousanalytical techniques, sometimes combining

Mössbauer spectroscopy for Fe coordinationenvironment with X-ray diffraction for theidentification of crystal structure and themeasurement of lattice parameters and chemicalquantification by wavelength dispersivespectroscopyintheelectronmicroprobeorenergy-dispersive X-ray spectroscopy in the TEM. Whilstthesestudieshavebeencomprehensiveintherangeof alloys considered, the work is often in a singletemperature range (either above or below themonotectoid for β-Zr decomposition). Further,because the testmatrix is so large in thesestudies,the compositions reported are a singlemeasurementforanalloy/heattreatmentcondition;no indication is given in regards to compositionalvariation or error in the measurement itself.Althoughitisimportanttoobtaindatafromalargenumber of particles for good statistics to assessphasecompositionalrange(seetheworkofHarteetal. in [67] for the compositional range ofprecipitatesinZircaloy-2asafunctionofprotonandneutron irradiation dose), it is more important toperform accurate and precise measurementsthrough good practise; doing so will allow propercomparisonsbetweendifferentstudiesandreliableconclusions. Regardless of the lack of anycompositional range in these reports, the volumefraction of reported Zr(Nb,Fe)2 relative to(Zr,Nb)2Fe has been shown as dependent on alloycomposition [64] and the importance of longannealing times to reach equilibrium phasecompositions has been demonstrated, especially atlowannealingtemperatures[3,22],e.g.upto20,000hrat400°C.This is importantwhenslowdiffuserssuch as Nb are involved in the phase composition[66].Longheat treatmentsare rarelyperformed incommercial alloys due to the relative importanceplaced upon control of grain size and stress relief.Therefore,non-equilibriumcompositionsshouldbeexpected incommercialsystems.Ofcoursethiscanresult in confusion in regards to stoichiometricassignment.Due to the ambiguity in stoichiometric designationof compositions surrounding Zr33Nb042Fe25 (at.%),already discussed in the introduction to section 5and within section 5.1 as either Zr(Nb,Fe)2 or(Zr,Nb)3Fe, we will refer to the phases commonlydesignated as Zr(Nb,Fe)2 as hexagonal and(Zr,Nb)2Feascubic,withtheunderstandingthatthehexagonal phase is sometimes ambiguous in itsstoichiometry.


5.2.1 ThehexagonalphaseThe differences in the compositional range for thephasesdesignatedashexagonalandcubichasbeendemonstrated in Figure 1 and Figure 4 and theconflict between designation of a hexagonal phaseasZr(Nb,Fe)2or(Zr,Nb)3FeisshowninFigure1andFigure 6. However, Figure 1 and Figure 6demonstrate the relative consistency of the Zrcontent relative to that of Nb and Fe, suggesting agreater likelihood of Zr(Nb,Fe)2 in most cases. InFigure6wecanalsoseethattheFeandNbcontentof the hexagonal phase is correlated to the atomicratio Fe:Nb in the alloy, and, likewise, the cubicphasecontentofZrandNbratioiscorrelatedtothealloy Zr:Nb. In Figure 5 we present frequencyhistograms of the two different possible Nbcouplings forboth thehexagonal and cubicphases.In Figure 5 a) and b) for the hexagonal phase weshow the phase atomic ratio (Nb+Fe+Cr):Zr and(Zr+Nb):(Fe+Cr) to demonstrate that the ratio(Nb+Fe+Cr):Zrisgenerally<2andthattheaverage(Zr+Nb):(Fe+Cr) lies between 2 and 3. The cubicphaseshowsbetteradherencetostoichiometrybutthere remains significant variation betweenmeasurements.Asummaryofthemean±1.96*stdisgiveninTable1.ForthehexagonalphasewecanseefromTable1that the variation in Nb and Fe measurement isalmost three times that for Zr, reinforcing thestoichiometric formula Zr(Nb,Fe)2. There are somestudies thatunfortunately add confusion throughalackofclarity inassigningaphaseasZr(Nb,Fe)2or(Zr,Nb)3Fe; several possible crystallographicsolutions, hexagonal, tetragonal and orthorhombichavebeenprovided in a singlepublicationwithnoindexed diffraction patterns and only a singlecompositional range reported [27,30]. Otherpublications have simply stated that there are“mainly” Zr(Nb,Fe)2 in alloy E635 but have notprovided any characterisation or referred to anyminor fraction of secondary phase [31]. There hasalso been confusion in phase designation in alloyNSF-2afteranincompletereportofthecompositionof a phase and its designation as (Zr,Nb)Fe2 [41].Thisphasewasindexedashexagonalbuttherewasconfusioninthereportingofthecompositionandsono further analysis can take place here. There isevidence for the existence of the hexagonal phaseover a wide alloy compositional range; Zr(Nb,Fe)2has been reported in alloys containing as littlealloying elements as alloy Zr99.78Nb0.17Fe0.05 (at.%)[3]tothosecontainingasmuchasalloyZr40Nb50Fe10or Zr30Nb5Fe65 (at.%) [23] and there are several

studies that show the presence of the hexagonalphase in commercial alloys with various Feadditions [24,27] and in a wide range of modelalloys [3,21–23,64]. This alloy compositional rangeis reflected in thewide compositional range of thephaseitself.The hexagonal phase has been widely reported as(C14)MgZn2-typecrystallography,which isaLavesphase structure and is often referred to as such.However,Steinetal.’sdata indicatethat theparentZrFe2phaseinequilibriumattemperatures<600°Cis cubic [57].Wemay study the variation in latticeparameter as a function of chemical content fromliterature data in which parallel chemical andcrystallographicanalyseswereperformed.InFigure8aandFigure8bweplotphasechemicalcontentasafunctionofunitcellvolumeforthehexagonalandthe cubic phase, respectively. Stark differences areclearbetweenthetwophasesinthatthehexagonalphase unit cell volume is much more sensitive tochangesincomposition.Wemayexpecttheunitcellto increase with an increase of Nb content anddecrease in Fe due to an atomic radii argument.Whilst thismaybe true forboth the assignmentofthehexagonalphaseasZr(Nb,Fe)2or(Zr,Nb)3Fe,theextent to which this is the case is a subject forfurtherwork.ToresolvethepositionsoftheZr,NbandFe in thehexagonalphase,Toffolon et al. usedRietveld refinement of XRD data on a hexagonalternary phase of composition Zr37.0Nb26.5Fe36.5(at.%), which fits well with the Zr(Nb,Fe)2stoichiometricassignment[25].Theirworkshowedthat the Nb and Fe are interchangeable on the 2aand6hWyckoffpositions.Itwouldbeinterestingtorepeatthisexperimentforahexagonalphasewithastoichiometry that fits well with the designation(Zr,Nb)3Fe.


Table1TheaveragecompositionsoftheternaryZrNbFephaseswhendesignatedasthehexagonalorcubicphase.DatatakenfromFigure1.

Meancomposition±1.96*std[at.%] Phasedesignation Zr Nb Fe Cr #MeasurementsHexagonal 35.6±5.0 32.2±14.4 30.8±13.0 1.1±5.4 112Cubic 57.9±10.9 10.1±10.8 31.8±5.2 - 43

Figure8Ananalysisofthevariationinunitcellvolumewithchangingphasecompositionfora)thehexagonalphaseandb)thecubicphase.DatatakenfromFigure1.

5.2.2 ThecubicphaseThe FCC (Zr,Nb)2Fe phase dates back to thecompositional range noted by Alekseeva andKorotkova in 1989 [55,56] but the alloycompositions studied at that time have sincebeenreassessedanddeterminedtoconsistofthe two phases hexagonal and cubicdesignatedZr(Nb,Fe)2and(Zr,Nb)2Fe[21],asopposed to theoriginally-proposedsingle “T”phase.However,thename“T”phasehasbeenretainedby someRussian groups todescribethe (Zr,Nb)2Fe phase alone, which is presentin higher volume fractions when the Fe:Nbratio in the alloy is relatively high [3,22], forinstance, in E635 with 0.65Fe (wt.%) asopposedtotheusual0.35Fe(wt.%)[27].Dueto the dependence of the cubic phase’sexistence on the Fe:Nb ratio in the alloy,chemicalheterogeneityintheas-castmaterialwill also play a role in and this aspect ofprocessing in relation to Zr-Nb-Feintermetallicphaseshasnotbeeninvestigated

to date. Therefore, triple melting isrecommendedforthoroughmixing,especiallyfor alloys in which long heat treatments arenotpossible.Inthefirstinstanceofstructurerecognition,acubicphaseinternaryZr-Nb-Fesystemswas reported in 1999 byMössbauerspectroscopyaloneasaphasesimilartoZr2Febut containing some Nb [63]. This was laterconfirmed by correlative analyticalMössbauer, X-ray diffraction and electron-based chemical quantification techniques[21,65]as(Zr,Nb)2Fe.The identification of this cubic phase byelectronmicroscopy techniques is somewhateasierthanthatofthehexagonalphase,astheamount of its constituents are so different tooneanother.ThisisdemonstratedinFigure1andFigure4suchthatthenumberofatomsofNb < Fe < Zr. In Table 1 we show thecomposition for thiscubicphaseas themean± 1.96*std, which demonstrates the largervariabilityintheZrandNbcontentrelativeto


the Fe, reinforcing the stoichiometry(Zr,Nb)2Fe. In Figure 5b and Figure 5c weshow the pairing ofNbwith either the Fe orthe Zr, respectively, to producestoichiometries of approximately (Nb,Fe)3Zr4or (Zr,Nb)2Fe, respectively, only the latter ofwhichhasabinaryequivalentandisthereforemost likely. In comparison to the hexagonalphase in Figure 5a and Figure 5b, thestoichiometric assignment for this phase ismuch clearer. However, it was initiallyreportedthatthetetragonalZr2Fehasalatticeparameter approximately half that of FCC(Zr,Nb)2Fe[63].Assuch,the(Zr,Nb)2Fephaseis sometimes referred to as (Zr,Nb)4Fe2 toavoid any assumption that this FCC ternaryphase holds any similarity to the tetragonalbinary equivalent [3,22,25,66]. There is anFCCallotropeofZr2Fewithverysimilarlatticeparameters to FCC (Zr,Nb)2Fe: a = 1.221 nmfor FCC Zr2Fe [57] and a = 1.21-1.23 nm forFCC (Zr,Nb)2Fe [21,23,41,44]. Such similarityin latticeparametermaybeexpected ifNb istoreplacesomeZratZrlatticesites,whichisreinforced by the different Fe environmentobserved in (Zr,Nb)2Fe relative toZr2Fe [63].However,duetothesimilarityinatomicmassof Zr andNb, Rietveld refinement of powderX-ray diffraction spectra cannot distinguishbetweenthetwo[25].Thetrendsincompositionforthecubicphaseare somewhat different to the hexagonalphase. In Figure 6d we show that there is apositive correlation between the cubic phasecomposition of Zr and the alloy Zr:Nb ratio,and, correspondingly, a negative correlationwith Nb content in Figure 6e. Interestingly,the data from the cubic phase heat treatedabove and below the monotectoid do notfollow on from one another as the data doesfor the hexagonal phase in Figure 6a-c). Thisdemonstrates that there is a dependence ofcubicphasecompositiononalloycompositionbut also that the effect of heat treatmenttemperature on the phase composition ismore significant for the cubic phase than forthe hexagonal phase. Although the result ofthis temperature effect is to retaincompositional range for the variable Zr andNbelements forbothheat treatmentregimesabove and below themonotectoid, the lowertemperatureheattreatmentsdonotdothisaswell and the ‘fixed’ Fe content is more

variable. This is easily explained from athermodynamic equilibrium standpoint,suggesting that the organisation of atomswithin the cubic phase is more well definedthan those in the hexagonal. For alloys ofsimilar Zr:Nb ratio but heat treated above orbelow the monotectoid, those below themonotectoid tend to have lower Zr contentand higher Nb, whichmight be explained byNb as a slow diffuser at lower temperatures.Above the monotectoid the Fe content isreasonably constant at 32-33 at.%but belowthe monotectoid the Fe content is quitevariableat25-37at.%.Wemayconcludethatthe effect of heat treatment temperature ismore severe for the cubic phase than for thehexagonal, but with the ambiguity indesignation of hexagonal as Zr(Nb,Fe)2 or(Zr,Nb)3Fe and will the latter of these beingpresent predominantly below themonotectoid, it may be the case that thehexagonalismorepronetometastablephasesbelowthemonotectoid.Theeffectof changesinchemistryonunitcellvolumeforthecubicphaseisassessedinFigure8forthosestudiesthat report both. In doing this, we observelittlechangeinthecompositionwithchanginglatticeparameter,especiallyincomparisontothehexagonalphase.TheexactpositionofZrand Nb in the lattice of the cubic phasethereforeremainsanopenquestion,althoughit is likely valid that the Zr andNb positionsareinterchangeable.

6 IrradiationEffectsItmaybeexpectedthatsecondaryparticlesofthe same phasewill evolve under irradiationin a similar manner. Therefore, ambiguity inphasedesignationmaybepartlyresolvedbyastudy of the effects of irradiation on thatphase.However, this prospect is complicatedbyanumberoffactors,namelyvariablephasecomposition, irradiation dose rate andirradiationtemperature[68].Thefollowingisan assessment of the irradiation response ofthephasesdesignatedashexagonalorcubic.

6.1 ThehexagonalphaseThe ternary phase commonly designated ashexagonal Zr(Nb,Fe)2 has been reported todisplay three different modes of behaviourunderirradiation.Thefirstmodeisauniformamorphisation at low temperature neutron


irradiations[27](alloyE635,60°C,6dpa,7.6x1025nm-2,2x1018nm-2s-1,1.6x10-7dpas-1)or under proton irradiation at a higherirradiation temperature and at a dose ratehigherbytwoordersofmagnitude[69](alloyM5®, 350 °C, 2 x 10-5 dpa s-1). The secondmode is partial amorphisation at theperiphery of the particle, a “rare event”described in a single publication (alloyE635,350 °C, 4 dpa, 5 x1025 nm-2) [30]. The thirdand final mode reported is of noamorphisation whatsoever, but a loss of Feand a transformation from the hexagonalstructure to aBCC structure,which has beendemonstrated in alloyE635 (7 x1025 nm-2, 3x1017nm-2s-1)attemperaturesof315°C[20],330-350 °C [24,27] and in Low-Sn ZIRLOTMafterBWRirradiation(9x1025nm-2,18dpa)at a temperature 280-330 °C [70]. Fedissolution from the hexagonal secondaryphase without amorphisation nortransformation has been observed underhigher dose rate proton irradiation (2-7 dpa,~1x10-7dpas-1)at350°C[34].Whilstmostofthesereportsstatethattheinitialcompositionof the ternaryphasesmostcloselyresembledZr(Nb,Fe)2, even though there is no detailedcharacterisation, therehasbeena reportofabetter fit for the initial composition of theternaryphaseas(Zr,Nb)3FeinalloyZr-2.5Nb,with post-irradiation analysis demonstratinguniformdepletionofFeandatransformationwithin the whole particle from amonocrystalline state to a polycrystallinestate (1.6 x1025 nm-2,~2.6 dpa, 250-300 °C)[71].There are three well-defined irradiationtemperature regimes for theZr(Fe,Cr)2Lavesphase in the Zircaloys. The first is a uniformamorphisation at low temperature neutronirradiation [72] and under irradiation withions at higher temperatures [73,74]. Thesecond regime is at normal operatingirradiationtemperaturesandresultsinFelossand coupled amorphisation progressing fromthe particle periphery inwards withincreasing irradiationdose[72,73,75,76], thisprocessbeingquickeratthe lowerendoftheoperating temperature range at 270 °C [77].The final regime isathighneutronorprotonirradiationtemperaturesandresultsinFelosswithoutamorphisation,re-precipitationinthematrix and even particle growth

[53,67,75,78]. Whilst these temperatureregimesmay show resemblance to the threemodes of evolution in the hexagonal ZrNbFephase, it is important tonote that, for theZr-Nb-Fe alloys, the particle(s) that displayprogressive amorphisation and those thatdisplaytransformationfromHCPtoBCCwereirradiated in the same temperature anddoserateregime,yettheywerereportedtobethesame phase initially. Such a difference inbehaviour for the same phase could only bedue toeithera significantlywidevariation ininitial phase composition, which was notdetailedinthatpublication,or,alternatively,adifferent initial phase entirely. A lack ofstatistically relevant and quantitativecharacterisation of particles in the initialconditionandtheambiguityindesignationofthe hexagonal phase between Zr(Nb,Fe)2 and(Zr,Nb)3Fe (see section 5) compounds thisproblem.The characterisation of particles in the threetemperature regimes is much betterdocumented in the Zircaloys than in theZrNbFe alloys. Mechanistic explanations fordiffering behaviour as a function oftemperature have their foundation in thecompetitionbetween irradiationdamagerateand thermal annealing of that damage withregards to thehomologous temperature foraparticularphase, its stoichiometric rangeanditscapabilityforself-solubilityinthedefectivestate [53,67,79,80]. However, there is asignificant difference in the behaviour of theZircaloyZr(Fe,Cr)2andthehexagonalphaseinZrNbFe alloys; the latter undergoes astructuraltransformationandtheformerdoesnot. This may be explained by a greaterdiffusion coefficient in α-Zr for Cr relative toNb [81] and the β-stabilising nature ofNb inZr, but more work is required to assess theself-solubilityandstoichiometricrangeof theZrNbFesecondaryphases inboth their initialanddefectivestates.Thereisaninherentdifficultyinpost-mortemanalyses of phases; after irradiation there iscommonly Fe depletion from Fe-containingprecipitates and so the initial phasecomposition and therefore phase designationbecomes difficult, especially when there areconflicting or incomplete reports of phasecomposition prior to irradiation, i.e.


Zr(Nb,Fe)2 or (Zr,Nb)3Fe. Accurate, spatiallyresolvedchemicaldataonastatisticalnumberof particles should therefore be an essentialpart of any systematic study, anddevelopmentsininsituheavyionTEMstudieswill allow a direct assessment of individualparticles’evolutionduringirradiation.

6.2 ThecubicphaseUnder neutron irradiation the cubic phase(Zr,Nb)2Fe retains its crystalline core andshows a polycrystallinity at the particleperiphery that is likely Nb-rich platelets[14,43].WhilstEDSline-scanshavebeenusedto demonstrate Fe-depletion from only theparticle periphery coincident with the Nbplateletprecipitation[14],suchtrendscanbeattributed to particle edge effects. Particleextraction alleviates this issue andKobylyansky et al. have demonstrated adecrease in average Fe content in the cubicphase from an initial 29 at.% to 23.5 at.% inalloy NSF and 25.2 at.% in alloy E635,coincidentwithincreasesinZrandNbcontent[43]. As these particles were extracted fromthematrix,thisdecreaseinFecontentcannotbe attributed to the precipitation of Nb-richphases. The irradiation-inducedpolycrystallinity is reminiscent of a similarphenomenon observed by Griffiths et al. in aparticle designated (Zr,Nb)3Fe in alloy Zr-2.5Nb (wt.%) [71],which, in similarity to thepost-irradiation composition of the cubicphase initially (Zr,Nb)2Fe, also has an Fecontentof25at.%.ThedecreaseinFecontentin (Zr,Nb)2Fe and any recrystallization topolycrystallinity may indicate a range ofmetastable states after low temperature heattreatments. This idea is supported by thelarger variability in phase composition attemperatures below the monotectoidtemperature, < 600 °C, in Figure 6f of thisreview.

7 ConclusionsThepresenceofFe inβ-Nbphaseseemsonlyto be convincing in alloys that are eitherquenched from temperatures above themonotectoid for β-Zr decomposition or inmodel alloys inwhich the Fe content is high.Inothercases,itseemsthatFeiscontainedatthe interfacialregionbetweenα-Zrandβ-Nb.This highlights the importance of spatially

resolved chemical analysis and/or phaseextraction techniques in preparation for thestudyof individualphases, independent fromanymatrix inwhich theymay be embedded.AnyexcessofFeisthereforelikelytobefoundin interfacial regions or in a low volumefractionofternaryZr-Nb-Fephases.The ternary phase cubic (Zr,Nb)2Fe isreasonably well defined due to its strictadherence to stoichiometry, especially attemperatures above the monotectoid.However, approximately one third ofcompositions reported as hexagonalZr(Nb,Fe)2 fit well with the stoichiometry(Zr,Nb)3FeinanalogytothebinaryequivalentZr3Fe, and from the early 1990s to the year2000 this alternative stoichiometry wasreported in a few publications as having thesame structure as Zr(Nb,Fe)2. It is thereforeeither the case that Zr(Nb,Fe)2 has anextraordinarily wide stoichiometric range,whichwouldbeunusualforaLavesphase,orthattherearemetastablephasesthatformasaresultoftheslowdiffusionofNbduringheattreatments below the monotectoid for β-Zrdecomposition, or that the errors in theanalyses are inordinately large. A detailedbody of work is required that concernscommercial-type alloys and the phases thatare present above and below the ZrNbmonotectoid, with correlative quantitativechemical and structural analyses to assesscomposition, crystallography and, ideally,local Fe environments with parallelMössbauerandmodellingapproaches.AcknowledgementsAllanHarteandMichaelPreusswould like toacknowledge funding of their time fromEPSRC[EP/I005420/1andEP/M000737/1].References[1] F.Garzarolli,R.Schumann,E.Steinberg,

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