Microwave
processing
of
high
entropy
alloys:
A
powder
metallurgy
approach
Paolo
Veronesi
a,*,
Elena
Colombini
a,
Roberto
Rosa
a,
Cristina
Leonelli
a,
Marco
Garuti
ba
DeptofEngineering“EnzoFerrari”,UniversityofModenaandReggioEmilia,Italy
b
MKS,AlterProducts,ViaP.eM.Curie,8,42122ReggioEmilia,Italy
ARTICLE INFO Articlehistory: Received13July2016 Accepted18February2017 Availableonlinexxx Keywords: Microwaveignition Combustionsynthesis Powdermetallurgy ABSTRACT
MicrowavesattheISMfrequencyof2450and5800MHzhavebeenexploitedtoprepare FeCoNiCrAl-familyhighentropyalloysbydirectheatingofpressedmixturesofmetalpowders.Theaimofthisworkis toexploreanewmicrowaveassistednear-net-shapetechnology,usingpowdermetallurgyapproachfor thepreparationofhighentropyalloys,abletoovercomethelimitsofcurrentmeltingtechnologies (defectsformation)orsolidstateones(timedemanding).Resultsshowthatdirectmicrowaveheatingof thepowderprecursorsoccurs,andfurtherheatinggenerationisfavoredbytheignitionofexothermal reactionsinthecompound.Microwaveprocessing,exploitedbothfortheignitionandsustainingofsuch reactions,hasbeencomparedtoreactivesinteringinlaboratoryfurnaceandmechanicalalloyingina planetaryballmilling.Resultsdemonstratethatmicrowaverequiredtheshortesttimeandlowestenergy consumption,thusitispromisingtime-andcost-savingsyntheticroute.
©2017ElsevierB.V.Allrightsreserved.
1.Introduction
High-entropy alloys (HEA) are a class of multi-component alloyscomposedof5ormoreprincipalconstituentelementsand eachwithaconcentrationbetween5and35atomic%[1].These alloyshaveatendencytoformsimplestructures,likeFCCandBCC, insteadof intermetalliccompounds [2]. Regardless of the HEA composition, this family of alloys shows several interesting features; in particular they tend toform simple solid solution phases with the possible presence of nanostructures or even amorphous structures [3], presenting Vickers hardness ranging from100to1100HV30,accompaniedbyagoodthermalstability andexcellentresistancetoannealsoftening[4].Duetothisbroad spectrumofproperties,theHEAhavemanypotentialapplications
[5],includingmechanicalpartsandfurnacepartsrequiringhigh strength, thermal stability and wear and oxidation resistance. Sincetheintroductionofhighentropyalloyconceptinliterature, severalproductiontechniqueshavebeentriedtosynthesizethese materials.Accordingtotheliteratureresearch,uptonow,mostof the production attempts followed one of the four following techniques:fromtheliquidstate(arcmelting,inductionmelting), from thesolid state(mechanical alloying, powder metallurgy), fromthegasstate(sputteringtechniques,mainlyforcoatings)and
fromelectrochemicalprocess(againmainlyforcoatings)[5].The syntheticrouteshouldguaranteeshortalloyingtime(highenergy densityontheload,rapidmeltingandhencereduced contamina-tion by the surrounding environment), efficient cooling and capability to operate in controlledatmosphere. Such conditions canbeachievedusinghighfrequencyelectromagneticfields,likein microwaveheating,providedtheloadiscapabletocouplewiththe incidentelectricandmagneticfield.Scientificliteratureregarding the use of microwaves to prepare HEA is limited to only one contributionbyTengetal.[6]whosynthesizedFeCoNiCuAlHEAby microwave-assisted combustion synthesis, starting from oxidic precursors as raw materials (and hence having alumina as byproduct). Microwave assisted combustion synthesis of pure metal powders as reactants has already been used during last decade by some of the authors to prepare intermetallics [7], functionallygradedmaterials[8],orasajoiningtechniquebetween dissimilarmaterials[9].Theadvantageofapplyingmicrowavesto combustion synthesis reactions resulted in high purity of the products [10], rapid ignition of the reaction [11], possibility to control the products microstructure [12] and cooling rate after synthesis,especiallyinpresenceofferromagneticreactants[13].
2.Materialsandmethods
In this worknon-equiatomic high entropyalloys have been prepared,startingfrommetallicpowdersmixtures,whichinclude
* Correspondingauthor.
E-mailaddress:paolov@unimore.it(P.Veronesi). http://dx.doi.org/10.1016/j.cep.2017.02.016 0255-2701/©2017ElsevierB.V.Allrightsreserved.
xxx–xxx
ContentslistsavailableatScienceDirect
Chemical
Engineering
and
Processing:
Process
Intensi
fication
atleast oneferromagnetic element(Fe, Co, Ni) andone highly reactiveelementcouple,likeAl-Ni,Al-Fe,Al-Coinordertoimprove heatgenerationduetobothmagneticfieldcontribution (micro-waveheating)and exothermal reactioncontribution. Moreover, theuseof Alis expectedtohavethesynthesis initiated below 700C [14], roughly corresponding to the melting point of aluminum.Elementalpowdershavebeenusedasreactants: Fe97,0%purity,particlesizelessthan44
m
m–Sigma-Aldrich Co99.8%purityparticlesizelessthan2m
m–AlfaAesar Ni99.7%purityparticlesizelessthan5m
m–Sigma-Aldrich Cr99.0%purityparticlesizelessthan44m
m–AlfaAesar Al99.0%purityparticlesizelessthan75m
m–Sigma-AldrichTheproperamountofpowderswasweighed,topreparea non-equiatomicmixtureof:
1.FeCoNiCrAl2.5
2.FeCoNiCr0.5Al2
andmixedinanagatejarforapproximately30min.Theexcess Al,accordingtopreviousfindingsbytheauthors[14],allowsto minimizesegregationphenomena,duetotheformationofan Al-rich liquid phase during heating, able to wet the remaining powdersandpromotereactionsand diffusion.Uniaxialpressing wasusedat300MPatoformreactivedisc-shapedspecimensof6g asweightand20mmdiameterand5mmheight.
Twodominantfactorsestablishtheformationofasolidsolution (SS), theenthalpy of mixing Hmix and atomic size mismatch
d
accordingtoHume-Rotheryrules.Therefore,Hume-Rotheryrules aretheearliestguidetotheformationofSSalloys[15].Theseare quantifiablebycomposition-weightedtermsaccountingfor differ-encesinatomradii(
d
r)andelectronegativity(x
),aswellasoftheaverageValenceElectronConcentration(VEC).Froma thermody-namicpointofview[16],thebalancebetweentheentropyand enthalpy contributions to the formation of a solid solution is capturedbytheparameter
V
,asdefinedinEq.(1)V
¼TmD
SmixD
Hmix ð1Þwhere
D
Smix is the entropy of mixing and Tm the meltingtemperature.The most critical factor that decides whether an
alloycrystallizesintoBCCorFCCstructureappearstobeitsVEC,as shown in Eq. (2) (the VEC of an alloy is calculated from the weightedaverageVECoftheconstituentcomponents)[17].
VEC¼X
N
i¼1
xiVECi ð2Þ
AccordingtopreviousstudiesonHEA[17–19],thecharacteristic parameters to achieve solid solutions are reported in Table 1 [20,21].Inthisstudy,FeNiCoCrAlfamilyofHEAhavebeenselected, extendingtheAlcontentuptothelimitsofformationofthesolid solutions(SSregioninFig.1),andpreciselytheFeCoNiCr0.5Al2and
FeCoNiCrAl2.5HEAs,whoseparametersare:
forFeCoNiCrAl2.5:
d
=6.8%,V
=1.2,D
Hmix=16.1[kJ/mol][20],D
Smix=12.6[J/molK],VEC=6.2,Tm=1510[K],Dx
=0.13%.forFeCoNiCr0.5Al2:
d
=6.7%,V
=1.2,D
Hmix=16.0[kJ/mol][20],D
Smix=12.7[J/molK],VEC=6.6,Tm=1515[K],Dx
=0.13%.HEAsynthesisexperimentshavebeenperformedindifferent ways,inordertocomparealternativesyntheticroutes:
conventionalheating in a muffle furnace, at1100C, undera constantArflux(20NmL/min),OptolabMod.furnace,1200W power,forlessthan2minholdingtimeatmaximum tempera-ture.6gdiscshapedload.
microwaveheatingat2450MHzor5800MHzfrequency,undera constantArflux(20NmL/min),inTE10nsinglemodeapplicator, whosegeometryhasbeendescribedindetailsinpreviouswork
[13].6gdiscshapedload.
mechanicalalloyinginplanetarymillingwithsteelballs,usinga PlanetaryBallMillPM100byRetschGmbH,fortimesfrom1to 60hat250rpminanargonatmosphere.Aball-to-powdermass ratioof15:1 wasused.Stearicacidwasadded aswell,asthe processingcontrollingagent(PCA)toavoidcoldwelding.30g powderload.
Table1
Suitablerangetoformsolidsolution.
DSmix DHmix d
SolidSolutionPhase 10<DSmix<19.5 22<DHmix<7 0<d<8.5
BMGs 7<DSmix<14 35<DHmix<8.5 d<9
Fig.1.PhaseformationaccordingtoVanddparametersandpositionoftheHEAssubjectofthiswork. xxx–xxx
Thechoiceofthesinglemodeapplicatorliesinitspossibilityto exposetheloadtoregionsof predominantelectricormagnetic field [22], even if both contributions have tobe considered to heating,duetotheperturbationoftheelectromagneticfieldinthe cavityprovokedbythepresenceoftheload[23].
Thermalsynthesis,occurringwithastrongexothermaleventin case of excess Al content, was stopped immediately after the ignitionofthereaction,inordertoavoidpossibleannealingeffects due toextended exposure to microwaves or hightemperature. Mechanical alloying,instead, was conductedat differenttimes, beingimpossibletoexternallydetecteventsabletoindicatethe occurrenceofthewantedreactions.
The crystal structure of mixed powders and as-synthesized alloys was characterized by X-ray diffractometer(X'Pert PRO – PANAlytical) with Cu-Ka radiation. The microstructure of the powderswasobservedusingscanningelectronmicroscopy(SEM, ESEM–Quanta200–FEI),aftercuttingandpolishing.
3.Resultsanddiscussion
Thedifferentsynthetic routesinvestigated tosynthesizethe FeCoNiCr0.5Al2HEArequireddifferentminimumaveragetimesand
powertoachieve a propersolid solution,as shown inTable 2, togetherwithliteratureresultsreferredtoarcmeltingofsimilar alloys.
Mechanicalalloyingconductedforlessthan15hdidnotallow toachievethedesiredsolidsolution,asshowninFig.2,andhencea timeof60hwaschosenforthecomparison'ssake.However,15h willbeusedtoassesstheminimumspecificenergyconsumption forthesynthesis,asaconservativehypothesisthataftersuchtime thesynthesisoccurs.
TheformationofthedesiredHEAinthesystemFeCoNiCr0.5Al2:
hasbeenconfirmedbyX-raydiffraction,asshowninFig.3,where allthesyntheticroutesinvestigatedaresummarized.Thepeaks patterns are in agreement with literature results [24–26]. All patterns exhibit the solid solution formation, characterized by
typicalBCC structures,aspredicted byVECnumber<6.87[17]. XRDanalyses,supportedalsobyEDSanalyses,provedtheabsence ofintermetallicphases,despitetheexcessaluminumusedinsome compositions.
Samples,afterreactivesinteringbymicrowaveorconventional heating, retained their shape, indicating that the powder metallurgy approach is a suitable near-net shape technology. However,apronouncedporosityisleft,asdiscussedlater.
Basedontheseresults,themicrowaveassistedsynthesisofthe FeCoNiCrAl2.5 has been performed by microwave heating, to
confirmthepositiveeffectsoftheexcessaluminum.Inthiscase, microwave heating in predominant E or H field has been investigatedaswellandtheresultingX-Raypatternsareshown inFig.4.
MicrowaveassistedsynthesisinpredominantHfieldregionsof the applicator required even shorter times, i.e. 19s, with no relevantdifferenceinthefinalproducts,comparedtopredominant E-fieldedregion.Ahighermagnificationobservationofthe45.5 peakinFig.4showsthatitsFullWidthatHalfMaximumisslightly largerincaseofE-fieldprocessing,possiblyindicatingthataless homogenousalloy,withstrainedlattice,hasbeenachieved.This assumptionhasbeenconfirmedbySEM-EDSobservations,where regions withdeviation fromthetheoretical stoichiometryhave
Table2
SynthesisconditionsIwouldavoidthe.
Method P[W] Time[s]
Microwave5.8GHz 180 25
Microwave2.45GHz 300 38
LabFurnace@1100C 1200 72
LabFurnaceincludingheating 1200 3072
BallMilling 750 216000
ArcFurnace[21] 46000 80
Fig.2. XRDpatternsatdifferenttime:0–3–8–15–60h.
Fig.3.XRDpatternsoftheFeCoNiCr0.5Al2HEA,fromtoptodown:conventional
furnace,microwaveheatingat5.8GHz,microwaveheatingat2.45GHz,mechanical alloyingafter60hmillingtime, starting powdermixture.The peakat31 is ascribabletothemountingresinused.
Fig. 4.XRD patterns of the FeCoNiCrAl2.5 HEA, microwave synthesized in
predominantEorHfield,2.45GHz. xxx–xxx
beenobserved.Suchdeviations regardmainlyFeandCr,whose particlesizeishigherthattheremainingpowders(exceptforAl, whichisnotaccountedfor,sinceitisintheliquidphaseduringthe synthesis).Fig.5showsthebackscatteredelectronmicrographsof thesamplesprocessedinpredominantE-ofH-field.
Microstructuralinvestigationsshowedalsothatapronounced porosityisachievedaftersynthesis,whileretainingoftheinitial shape imparted during forming. This can be ascribed to the presence of the liquid phase (initially molten aluminum, then solutionswithAl),whichwetsthesolidparticles,leavingporosity and generating also shrinkage porosity after solidification. However, not being predominant, the liquid phase does not deform the sample, which retains its shape. However, such porosity could be reduced by means of a modification of experimental set-up, allowing to impart a moderate pressure during synthesis, as already performed in previous works on aluminidessynthesis[11,27].
The microstructure of FeCoNiCr0.5Al2 analyzed by SEM-EDS
elementalmapsisshowninFig.6.Microstructuralanalysisshows thatalltechniquesleadtosimilarelementaldistribution homoge-neity.
According to previous work involving microwave assisted aluminidessynthesis [28], it is possible to suppose that these
microstructuresaretheresultoftheinitialformationofmoltenAl, followedbyexothermalreactionstakingplacebetweenFe-Al, Co-Al and Ni-Al, which provide a further heat contribution and promotetheexistenceoftheliquidphase.
ThechoiceofoperatingatexcessAlcomparedtoequiatomic compositions proved successful in improving homogeneity of samplespreparedbymicrowaveheating:thepresenceofAlfavors the liquid-solid reactions, highly exothermal, with consequent increaseofthekineticofreactionandsynthesistemperature.Such exothermal contribution adds to the microwave heating one, which,alonewouldleadtomuchlesshomogenousstructures,as showninFig.7,referredtoequiatomicFeCoNiCrAl,processedat 2.45GHzat300W.
ThebackscatteredelectronsSEMimagesofFig.7stillpresent regionswithdifferentgreyscales,indicatingamicrostructurethat isnotperfectlyhomogenous,wherepartsofFeandCrpowdersare notreacted,asalreadydemonstratedbyauthorsinapreviouswork
[21]. There is no evidence of differences between the micro-structuresof HEAsachieved by2.45GHzand 5,8GHz, probably indicatingthatthereactionoccursinsucharapidandexothermal manner that it is dominated by the heat generated during synthesis, ratherthan ontheinputmicrowavepower.The only relevant difference is in terms of reaction ignition time and
Fig.5.BSEmicrographsandEDSspectraofFeCoNiCrAl2.5HEA,microwavesynthesizedinpredominantE(top)orHfield(bottom),2.45GHz.
duration, which is faster in case of 5.8GHz. At this higher frequenciesthepowerpenetrationdepthinthepressedpowders resultshalvedwithrespecttothe2.45GHzcase,hencepractically the same amount of power is dissipated into a much smaller volume.Theresultofthisconditionisthatwithinthepenetration depththetemperatureisrapidlyincreasedandhencetheignition conditionsarereachedearlier.
Given the similarity of results of the different techniques investigated,and usingliteraturedata,a tentativecalculationof
the specific energy consumption of each synthetic route was performed, and theresults arecompared and shown in Fig.8. Among the thermal methods, microwaves are a fast and cost-savingtechnique,comparedtotraditionalfurnace(includingthe pre-heatingtime,whichistimenecessarytoreachthe tempera-tureofignition,atleast700C [14])or arcmelting However,it mustbepointedoutthatincaseofconventionalfurnace,theload isonlypartiallyfillingthefurnace,forapproximatively15%ofthe availablevolume.Hence,incaseoflargersample,itisexpected
Fig.6. SEM/EDSelementalmapsoftheFeCoNiCr0.5Al2HEAsynthesizedwithdifferentroutes.
thatthespecificenergyconsumptioncoulddecreasebyanorderof magnitude.Asamatteroffact,energyconsumptionisdominated bytheheatingofthefurnaceparts(heatingelements,refractory lining),andonlyaminorfractionoftheinputpowerisusedtoheat heload.Nevertheless,undertheseconditions,microwaveheating stillresultthemostenergysavingapproach,especiallywhenusing the5.8GHzfrequency.However,theuseofthisfrequencycanpose problemswhen scalinguptheprocess, due tothe lackof high powergenerators operating at this frequency. The ball milling techniquesresultedthelongestandthemoreenergy intensive; however,ithastheadvantageofproducingHEAsinthepowder form,and withsmall particlesize, a conditionwhich makes it suitableforfurtherprocessingbyconventional(orfield-assisted) powdermetallurgytechniques.
4.Conclusions
HighentropyalloysbelongingtotheFeCoNiCrAlsystemshave been successfully prepared by microwaveheating at 2450and 5800MHz of metallic powders compacts. In this study, high entropy alloys have been synthesized exploiting microwave heatingandthesupplementaryheatcontributionofexothermal reactionsoccurringduringsynthesis,in caseofexcessAlinthe startingmixture.Comparisontootherthermal(heatinginfurnace) andnotthermaltechniques(mechanicalalloying)andtoliterature results (arc melting) allowed to evaluate the specific energy consumptionofthedifferentsyntheticroute.Microwaveheating resultedthelessenergyintensive,intheexperimentalconditions investigated.Thesmalldimensionsofthemicrowaveapplicator used and the sort synthesis time makes this synthetic route interestingfromaprocessintensificationpointofview,andthe benefits becomeparticularly relevantin case manufacturingof near net shape parts is required. The useof excessaluminum improvedhomogeneityofthefinalalloy,duetothepresenceofa liquidphase(initiallymadeofmoltenaluminum)whichimproves thereactionkinetic.Samplescharacterizationconfirmedthatthe investigatedthermalmethodsofpowdermetallurgyapproachare suitabletoretaintheshapeoftheloadimpartedduringformingby uniaxialpressing,despitethegenerationofanextendedporosity. Theminoritarianpresenceoftheliquidphaseandtheextremely rapidand pressure-lessmicrowavesynthesis conditionsdidnot leadtodensificationofthestartingpowders.
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