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Branaganetal.DevelopingBMNintoindustrialproducts

1Successfulstrategiestoprocessglassformingliquidmelts(1)into(2a)surfacetechnologyand(2b)monolithictechnology

propagation.Inordertodeveloptensileductility,devi-tri cationmustbeavoided,andtheenablingstructuretypeisfoundtobeaspinodalglassmatrixmicrocon-stituent(SGMM).Inthesubsequentsections,thesuccessfulcommercialapproachesillustratedinFig.1willbedescribedinadditionaldetail.

Surfacetechnology

Forsurfacetechnology,theglassformingalloyisutilisedasacoating,whichbyitsnatureisalwaysappliedontoasubstrateandnotutilisedinapplicationswherethecoatingisgoingtocarryastructuralload.Throughcarefulmanipulationoftheapplicationprocessandsubstrateselection,highsystemtoughnesscanbedevel-opedwithouttheneedforintrinsicmaterialtoughnessofthecoating.8,9Inthermalspraycoatings,hightoughnesscoatingscanbedevelopedduetothenatureofthethermalsprayprocesswherethecoatingcanbedepositedinacompressivestressstateduetotheshotpeeningeffectofthecontinuousbuildingupofsemimoltenparticlesintoindividuallayersathighvelocity.Inweldoverlayhardfacing,ductilehightoughnessbackingmaterialsincludingplaincarbon(i.e.A36)andlowalloyhightoughnesssteels(i.e.4140)canbeutilisedwithfullmetallurgicalbondingachievedtotheductilesubstratematerial.

Owingtothedevelopmentofsystemtoughness,bothmetallicglassanddevitri ednanocompositestructurescanbeutilisedcommerciallythroughthesolidi cationpathwaysshownbythemodelcontinuouscoolingtransformation(CCT)diagraminFig.2.Asshownbycoolingrate1(CR1),thegoalistosolidifyatahighenoughcoolingrate,whichmissesthenoseoftheglasstocrystallineCcurve,representingtheglasstocrystal-linetransformation.Bythisroute,auniformmetallicglassstructurecanbedeveloped,which,dependingonthecriticalcoolingrateformetallicglassformation,canprovideawideoperationalwindow.Atcoolingrate2(CR2),itispossibletoundercoolsuf ciently,followedbyrapidnucleationfromthesupercooledliquidmeltto

formthecompletelycrystallinedevitri ednanocom-positestructure.Theprocesswindowtoproduceadevitri ednanocompositestructuredirectlyfromthemeltissmallanddif culttoachieveonanindustrialscale.Thus,amorescalableapproachistooverquenchintothemetallicglassstateandthenheattreattocompletelydevitrifyasshownbythehorizontalarrow.Thiselevatedtemperatureexposurecanbeaccomplishedbyasinglestageheattreatmentasindicatedorcanoccurinsituinelevatedtemperatureapplications,forexamplecoatingsappliedforerosion/corrosioninanoperatingcoilorbiomassboilers.10

Forsurfacetechnologyapplications,dependingontheenvironmentandtherequiredproperties,thecoatingcanbeutilisedinametallicglassstate,apartiallydevitri edorafullydevitri edcondition.Paramountpropertiesofperformancethatcanbedevelopedarehardnesslevelsintherangeofmanyceramics,11–13wearanderosionresistanceperformancelikehardmetals(i.e.WC)8,14,15andcorrosionresistancelikenickelbasedsuperalloys16–18inselectedenvironments.

InFig.3,thetechnologicaldevelopmentofmetallicglassesintoaplatformsurfacetechnologyisshown.Thedevelopmentbeganwithinitiallyverythin(,1mm)physicalvapourdepositioncoatingsappliedthroughlaserablationorsputteringandwithonlyanarrowprocesswindowrequiringextremelyhighcoolingratesaty109Ks21.Throughcontinuousalloydevelopment,thecriticalcoolingrateformetallicglassformationwasreducedordersofmagnitudetothe,104Ks21range(Fig.3b),whichenabledthermalsprayapplicationtechniques.Typicalthermalspraycoatingthicknessesarefrom0?1to1mmandappliedthroughtechniquessuchasplasmaspray,highvelocityoxyfuelsprayandtwinrollwirearcspray.Furtherreductionincriticalcoolingratestothey102Ks21range(Fig.3c)enabledthick(typicallyfrom3to10mm)weldoverlayhard-facingapplicationtechniquesincludinggasmetalarcwelding,plasmatransferredarcwelding,openarcweldingandsubmergedarcweldingforweld

overlay

1194MaterialsScienceandTechnology2013

VOL

29

NO

10