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Branaganetal.DevelopingBMNintoindustrialproducts

aboilertubecoatingappliedinpowerplant;bweldoverlayhardfacing/hardbandingontooljoint;cweldoverlayhardfa-cingappliedtolargeshoveldipper;dwearplateinhaultruckbedliners

4ExamplesofindustrialproductsbasedonsurfacetechnologyapplicationofBMN

glassesdoexhibitaroomtemperaturedeformationmechanism;however,whenexposedtoatensileload,itexperiencescatastrophicfailureduetoshearbandfor-mation,freevolumecreation,shearsofteningandrunawayshearpropagation.5Recently,wehavereportedaspeci cstructuretypecalledaSGMMstructure,whichhastheabilitytodeformwithoutrunawayshearpro-pagationresultingintheachievementofsigni cantlevelsofglobalplasticityandusableductility.21–24

ThestructuralformationmodelfortheSGMMstructurerequiredformonolithictechnologyisshowninFig.5.Thekeytoformingthisenablingstructureisasolidi cationpathwaywherebyglassdevitri cationisavoided,whichisshownbythecoolingcurvesmissingthenoseoftheglassdevitri cationtransformation.Thisisbecauseglassdevitri cation,whetherornotitisper-formedinsingleormultiplestages,orwhetherinvolvingrecovery,relaxationorrecrystallisation,resultsinthecreationofbrittlestructures.Theglassformingnatureofthealloyisparamounttoformasupersaturatedglassmatrixduetothehighsolubilityofelementsinthemetallicglassstructure.Keytosubsequenttransformationistheabilityofthemulticomponentalloytoexhibiteitherastableormetastablemiscibilitygapatasuf cientlylowtemperaturerangetoallowtransformationinametallicglassmatrixthroughspinodaldecomposition.Unlikedevitri cation,spinodaltransformationisnotnucleationcontrolledbutinsteadinvolvesphaseseparationfromfreeenergydrivencompositionalgradientsduetothepresenceofeitherastableormetastablemiscibilitygap.Duringcooling,afterametallicglassisformed,thissupersaturatedsolutionthenundergoesspinodaldecompositiontocreateadistributionofvery nenanoscale(typically1–10nm)precipitatesintheglassmatrix.AsshowninFig.5,dependingonthecoolingrate,thespinodalstructurecanbeobservedtobehaveasexpectedforaspinodaldecompositiontransformation,andearlystage(CR4),middlestage(CR5)andlatestage(CR6)spinodalstructurescanbeobserved.Notethattheearlystagespinodalphasesareverysmallat,2nmandwheninitiallyformsappeartobesemicrystalline,butinlaterstages,oncetheycoarsengreaterthany6nmbecomecompletelycrystalline.

OncetheSGMMstructureisformed,ithastheuniqueabilitytodeformunderatensileload.InFig.6a,ahighdensityofshearbandscanbeobservedinasampleproducedwiththeSGMMstructureandthentensiletesteduntilfailure.Twotypesofinteractionsareobserved,whicharecalledinducedshearbandblunting(smallcircles)andshearbandarrestinginteractions(SBAI)(largecircles).21–24InFig.6b,additionaldetailsoftheinducedshearbandbluntingprocessareshownasapropagatingshearbandisinteractingwiththeSGMMstructureandisbluntedduetocomplexmultifoldinteractionsoftheshearbandwiththeSGMMstructurethroughlocaliseddeformationinducedchangesinclud-ingphasetransformation,phasegrowthandinsitunanocrystallisation.21–24InFig.6c,additionaldetailsoftheSBAIprocessareshown,whichshowthataftershearbandsarecreated,theyinteractwithexistingshearbandsandareoftensplitandbluntedafterashortdistance.Theresultistheformationofhighdensitiesofshearbandsupto105–106linespervolume,analogoustodislocationsincrystallinemetals,leadingtosigni -cantlevelsofglobalplasticity.

ThetargetedSGMMstructurehasbeensuccessfullyproducedinmanyproductformsincludingmicrowiresthroughTaylor–Ulitovskywireproduction, bresthroughvariationsofindustrialscalemeltspinningprocessesandfoilsthroughplanar owcasting.In

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1196MaterialsScienceandTechnology2013

VOL

29

NO

10