TEM study of the effect of high-temperature thermal cycles on the stability of the Y-Al-O oxides in MA956 ODS steel

j materres technol.2019;8(5):3719–3725

w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

TEM

study

of

the

effect

of

high-temperature

thermal

cycles

on

the

stability

of

the

Y-Al-O

oxides

in

MA956

ODS

steel

J.R.O.

Leo

_,

_S.R.

_Moturu

_,

_M.E.

_Fitzpatrick

a_{MaterialsEngineering,TheOpenUniversity,WaltonHall,MiltonKeynesMK76AA,UK}

b_{FacultyofEngineering,EnvironmentandComputing,CoventryUniversity,PrioryStreet,CoventryCV15FB,UK}

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Articlehistory:

Received8January2019 Accepted18June2019 Availableonline11July2019

Keywords:

MA956 ODS

Characterization Oxides

Heattreatment TEM

a

b

s

t

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t

MA956, a commercialferritic gradeof Oxide-Dispersion-Strengthened (ODS) steel,was investigatedusingtransmissionelectronmicroscopy(TEM)toevaluatetheinfluenceof high-temperaturethermalcyclesonthenanometricdispersedoxides.Analysesoftheoxidesize distributionwerecarriedoutonfoilsfromtheas-receivedMA956andfollowingtreatment at1285◦Cforonehour,andshowthatgrowthoftheoxideshasoccurredunderthethermal cycle.Implicationsfortheoxidestabilityinthesteelmatrixarediscussed,inthelightofthe oxidechemicalcomposition.

©2019TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Incoloy alloy MA956 is a commercial oxide-dispersion strengthened (ODS)steelwith aferritic matrix [1]. Itis an FeCrAlalloysystem,whosecompositionaimsathigh oxida-tionresistance.Thehighcontentofchromium(20%wt.)and aluminium(5%wt.)inthematrixcausestheformationofan adherent,slow-growingaluminascalethatprovidessuperior resistancetoaggressiveenvironmentsandhigh-temperature

∗ _{Correspondingauthor.}

E-mail:[email protected](J.Leo).

1 _{PresentlyatCoventryUniversity.} 2 _{PresentlyatStrathclydeUniversity.}

oxidation[2].Thealloyexhibitsoutstandingstrengthat tem-peraturesover1000◦C,owingtothefinedispersoidsresulting fromtheformationofcomplexY-Al-Ooxides,homogeneously distributedthroughthematrix[3].

Previous investigations on a variety of ODS steels that were concerned with the size of the nanodispersoids for strengthening revealed higher stability than age-hardening precipitates,alongwithinsolubilityinthematrixat tempera-turesnearthemeltingpointofthebasealloy[4,5].Whilethe chemicalnatureoftheoxidehasnotbeenofprimary con-cern,anumberofstudiesinvestigatedtheeffectofalloying elementsontherefinementandontheformationofcomplex oxideparticles[6–8],duringprocessingofthealloys.However, nodirectinvestigationhasbeencarriedoutonthestability oftheoxidesaftermanufacturing.Thisworkconcentrateson

https://doi.org/10.1016/j.jmrt.2019.06.027

Fig.1–(a)EBSDmapsofMA956grainsinthelongitudinaldirection,and(b)thecorrespondingIPFmapsshowingthe predominantfibres,especiallyintherollingdirection(X0).

Table1–ChemicalcompositionoftheMA956steel.

Cr Al Ti Yttria C N Mn Si

19.4 4.8 0.38 0.51 0.015 0.022 0.10 0.04

Ni Cu S P Co O Fe

0.05 0.02 0.008 0.005 <0.01 0.23 Bal.

theevaluationofthestabilityoftheY-Al-Ocomplexoxidesof theMA956subjecttothermalprocessingfollowing manufac-turing.

2.

Experimental

Approach

MA956materialwassuppliedbyIncotest(aDivisionof Spe-cialMetalsWigginLTD),UK.Cylindricalsampleswith16mm diameterand100mmlengthwerecutfromtheas-received hot-extrudedbars.Chemicalcompositionwasprovidedbythe manufacturer,asshowninTable1.

Characterizationofthematerialintheas-received condi-tion wascarriedout byscanning electronmicroscopy with electronbackscatterdiffraction(SEM/EBSD)andtransmission electronmicroscopy.Sampleswere extractedfrom boththe cross-section(perpendiculartothebaraxis)andlongitudinal (alongthebaraxis)directions.

ThepreparationforEBSDinvolvedgrindingwithSiCpaper, polishingwithdiamondparticlessuspension(1␮mand6␮m); andafinalpolishingstagewithcolloidalsilicasuspension OP-S,whichconsistsofsiliconoxideparticlesofsub-micronsize range(<0.1␮m)inanalkalinemedium,inordertoachievethe polishedsurfacefinish.Cross-sectionalandlongitudinal sam-pleswereEBSD-scannedusingstepsizesof5␮mand20␮m, respectively.

For the TEM survey, three foils were prepared by man-ual grindingwithSiCpapersof40,15 and5␮m,cutintoa 3-mm-diameter diskand,then,furtherthinnedbyjet elec-tropolishing in a solution of 5% perchloric acid and 95% ethanolat–50◦C.Imagingwasconductedinbright-fieldmode (BF)inaJEOLJEM2100TEMtostudythesizedistributionofthe oxides.Atotalof449oxideparticleswerecountedand mea-suredfromeightdifferentmicrographs,obtainedbyvarying theareaofthefoilunderinvestigationaswellasthetiltangle ofthespecimenholder,inordertobestatistically representa-tive.

Initsas-receivedcondition,theMA956containshigh resid-ual stresses, and mechanical tests showed poor ductility. Based on the recrystallization temperatures ofthe MA956 foundinpreviousworks[1,9],aheattreatmentwasdevisedat 1285◦Cforonehourfollowedbyafurnacecool,atemperature suitableforasubcriticalannealing,sinceitissituatedabove 0.9Tm.Thepurposeofthetreatmentwasstressrelief.Previous

studiesontheMA956suggestedthattheonlyexpectedphase changeatthistemperatureistheformationoftheCr-rich˛’ -phase,whichtakesmorethan100hofisothermaltreatment tobeformed[10,11].Therefore,nophasetransformationwas expected forsuchashort timeinterval ofexposureto the thermalcycle.

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3.

Results

3.1. Microstructureoftheas-receivedMA956

Themicroscopysurveyrevealedelongatedgrains,columnar inshapeandstronglyorientedalongthelongitudinalaxisof thebars,centimetresinlengthandseveralmillimetreswide. Thisisinagreementwiththemicrostructurereportedinthe literature[12–14].

SEM/EBSD analysis provided further insight into the microstructure. As seen from Fig. 1, the columnar grains, elongatedalongtheextrusiondirection,presentastrong pref-erentialorientation,mainlycharacterisedbythe{100}<110> system,alsoknownas˛-fibre;andthe{111}<110>system,the

-fibre.InFig.2a,threeEBSDmapsofadjacentareasofthe MA956microstructureareseenandcolour-codedbytheIPF contouring,highlightingthe fibres,whichare confirmedby theInversePoleFigures(IPF)showninFig.1b.

Fig.2showsthemicrostructureoftheas-received mate-rialobservedfromthecross-section.Adistributionfunction, based on the fraction ofhigh-angle grain boundaries, was obtainedfrom the EBSDand usedto estimatethe average grainsizeoverfourareas of3750␮m2_{, whichwasfoundto}

be376±48␮m.

3.2. Oxidesizedistribution–as-receivedMA956

TEM images show a relatively homogeneous distribu-tion of nanoscale oxides throughout the matrix. In some

micrographs,theoxidesseemtoalignwiththegrain bound-aries,reflectingadegreeofpreferentialorientationwiththe extrusion direction of the material. Fig. 3 illustrates these findings.Someinteractionsbetweendislocationsandoxide particleswereobserved.AsFig.3dportrays,therearesome matrixdislocationsthatseemtohavebeeneffectivelypinned bythenanometricoxides.

Thehistogramforsizedistributionoftheoxidesisshown in Fig. 4. The average size determined was calculated as 34±7nm,avaluepertainingtothedesiredrange,according tosomeresearchers,forensuringeffectivenessinpinning dis-locations[15–17].

3.3. MicrostructureoftheannealedMA956

Althoughsubjectedtoaveryhightemperature,thematerial didnotpresentanychangeinitsmicrostructure,whichstill presentsthesamefeaturesastheoriginalstateoftheMA956: thatis,the˛-and-fibretexturesofthecolumn-likegrains, earlierdepicted inFigs.1and 2.Themicrostructureofthe annealedmaterialisfeaturedinFigs.5and6,inthe longitu-dinalandtransverse(cross-sectional)directions,respectively. While it is known that ferritic grains may grow easily tomillimetresizeunderheattreatmentsatrecrystallization temperatures[18],theresultsseemtoindicatemicrostructural stability, which is reasonable, considering that the manu-factured MA956 already presents columnar grains several centimetreslongandhundredsofmicrometreswide.There mayalsobeabeneficialpinningeffecttothegrainboundaries fromtheoxidedispersion.Estimatesofthegrainsizeinthe

Fig.3–TEMmicrographsoftheas-receivedMA956.In(a),particlesandclustersalignwiththeboundary.(b)and(c)areEDS scansoftheindicatedtargets.(d)showsparticlespinningdislocations(highlighted).

Fig.4–Oxidesizedistributionoftheas-receivedMA956.

cross-sectionaldirectionmadefromthedistributionfunction averaged362±39␮mforthegrainsize.Thematerial,observed fromthetransversedirection,isseeninFig.6.

The TEM investigation on the foils extracted from the annealedmaterialshowedanaveragediameterof41±8nm fortheoxideparticles.Theresultantdistributionhistogramis

showninFig.7,whereanalmostidenticaldistributiontothat ofFig.4isnoticeable.

Given the similarities of the results from the measure-mentsoftheoxideparticlesize,aheteroscedasticstudent-t test(thatis,atestdesignedwiththeassumptionofunequal variances)wascarriedout withasignificanceprobabilityof 5%, having the null hypothesis stating that no significant difference existsbetweentheoxide sizedistributioninthe as-receivedmaterialandintheheat-treatedone.Forthe num-berofdegreesoffreedomassociatedwiththesamplesizeof the oxides, thecritical t-value oftheprobability for accep-tance ofthe null hypothesis was determined as 1.96.The t-value calculatedforthe testreached5.08,amuchhigher thanthemaximumvalueforaffirming,with95%confidence, thatthetwosizedistributionsareequivalent.Therejection ofthenullhypothesis,then,meansthat,statistically, differ-encesbetweentheoxidedistributionsintheas-receivedand theannealedMA956existandaremeasurable.Summarised detailsoftheconductedhypothesistestarefoundinTable2.

4.

Discussion

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Fig.5–EBSDmapoftheannealedMA956inthelongitudinaldirection(a);theIPFfiguresindicatethepredominanceofthe samefibreorientationsasobservedfortheas-receivedmaterial(b).

material to much higher temperatures than conventional materials,reachingtemperaturesnearthemeltingpoint,but stillretainingtheconfigurationofthedispersoids,hasbeen verifiedasafundamentalasset.Nohandco-workersreported stabilityofthe oxides indiffusionbonding cycles reaching temperaturesashighas1200◦C[19,20];Sitteletal.[21]found nodifferenceintheoxidesidedistributionofPM2000ferritic steelsubjecttodiffusionbondingcyclesat1100◦Cand1300◦C.

Otherstudiesprovidesupporttothenotionofstableoxides inharshenvironments[22,23].However,thecommonpointin alltheseinvestigationsisthefactthattheoxidesstudiedwere complexY-Ti-Ooxides.Titaniumhasbeendefinedasan effec-tiverefineroftheoxidesinthematrix[8,24,25].Aluminium, althoughalsousedforrefiningtheprimaryY2O3oxide,does

nothavethesameeffectasTi,andheattreatmentsathigh temperatures(above1100◦C)duringfabricationofthealloy

Table2–Hypothesistestsummaryanddetailsforthemeansoftheoxidesizedistributions—as-receivedandannealed MA956.

Sample No.ofparticles Meanparticlesize StandardDeviation

As-received 449 34.2 18.1

Heat-treated 436 41.1 22.3

t-testassumingunequalvariances(heteroscedastic) two-taileddistribution

DegreesofFreedom 838 CriticalValue(5%) 1.96

HypothesizedMeanDifference 0

PooledVariance 411.68 Statusofthenull

hypothesis

Rejected

TestStatistics 5.08

Fig.7–Oxidesizedistributionfortheheat-treated (1285◦C/1h)MA956.

renderthecomplexY-Al-Ooxidescoarserthanthoseofthe Y-Ti-Ogroup[24].Moreover,whenaluminiumispartofasteel compositioninhigheramounts,comparedtotitanium,Y-Al-O oxidesareformedoverthemorestableY-Ti-Ocomplexoxides

[26].Whilethepresentworkcannotdeterminewhetherthe oxidesintheMA956belong exclusivelytothe Y-Al-O com-plex,anumberofauthorsindicatethepredominanceofsuch oxidesinthematerial,morespecifically, yttrium-aluminium-perovskite,YAP(YAlO3)[2], yttrium-aluminium-garnet,YAG

(Y3Al5O12) [3,14]; and yttrium-aluminium-monoclinic, YAM

(Y4Al2O9)[27],owingtothehighaffinitybetweenaluminium

andoxygen.

AvisualcomparisonofFigs.4and7allowstheconclusion thattheparticlesgrewslightly.Thehypothesistestprovideda statisticalbasisforthisobservation.Theresultsofthis inves-tigation,alongwiththeconsiderationsfoundintheliterature, indicate that,evenif afurtherdegreeofrefinementofthe yttriacanbeachievedinAl-addedalloys,asaresultof,for example,amorecontrolledmanufacturingprocessinwhich particlesofthepowdersareexposedtomoreviolent fragmen-tation,theY-Al-Ooxideswillnotretaintheirsize,ifexposedto hightemperaturetreatmentsandprocesses.Thisfinds agree-mentwitharecentinvestigationbyZhangandPantleon[28], whoreportedsignificantgrowthoftheYAPoxidesfrom14nm toapproximately19nm,25nmand28nmafterannealingthe ODSFeCrAlsteelPM2000at1300◦Cfor1,10and100h, respec-tively.

Thestabilityoftheoxideparticlesunderharshconditions orhigh temperaturethermalcycles seemstobeultimately

relatedtothechemicalcompositionoftheoxides.Therefore, investigationsonthestabilityofoxidesshouldnotonlylook into the transformations duringthe manufacturing stages, but,also,duringlaterexposuretohightemperatureprocesses. Giventhatmanyelementsareaddedinanefforttoproduce fineroxidesinthematrix[6,29],furtherstudiesarerequired tocheckforstabledispersoidsathightemperaturesin post-manufacturingprocesses.

Aquestionthatmayarisefromthefindingsofthework pre-sentedregardsthemechanismsforgrowthoftheoxides.This hasnotbeeninvestigated,but,giventheveryhigh tempera-tureoftheheattreatment,thehighcontentsofaluminium andthepresenceofoxygendissolvedinthematrix,diffusion oftheseelements totheoxide, changingitsstoichiometric proportionsisplausible.Another possiblewayisattributed toOstwaldripening,thatis,adecreaseofthetotalenergyof thesystembyeliminationofinterfaces,asgrowthtakesplace atthe expenseofthevolume fractionsofsmaller particles

[30].Suchaninvestigation,however,wouldalsohavetorely onmeanparticlespacingandparticledensitymeasureinthe twoconditions.

Finally, it may well be the case that the thermal cycle affected the grain structure. A hypothesis test would be required to check the equivalence between the averages determinedfromthepopulationofgrainsineachcondition. However,thefocusoftheanalysishasbeensetontheoxides, whichconstitutethemainstrengtheningfeaturesoftheODS alloys.

5.

Conclusions

1 Heattreatmentat1285◦ContheMA956ferriticODSsteel causedthenanometricdispersedoxidestogrowslightly. Ashiftinthesizedistributionwasobserved,when com-paringthe as-receivedmaterialto the annealedversion. Ahypothesistestprovidedsupporttotheobserved differ-encesbetweentheparticles,whichgrewfromanaverageof 34nmtoanaverageof41nmwiththeheattreatment. 2 TheY-Al-Ocomplexoxidesarefoundnottobeasstableas

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Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgements

JROLwasfundedthroughtheRCUKEnergyProgramme,and we are grateful to EPSRC for funding via the PROMINENT NuclearFissionconsortiumgrant.MEFisgratefulforfunding fromtheLloyd’sRegisterFoundation,acharitablefoundation helpingtoprotectlifeandpropertybysupporting engineering-relatededucation,publicengagementandtheapplicationof research.

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