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ClassificationofBulkMetallicGlassesbyAtomicSizeDifference,HeatofMixingandPeriodofConstituentElementsandItsApplicationtoCharacterizationoftheMainAlloyingElementAkiraTakeuchiandAkihisaInoueInstituteforMaterialsResearch,TohokuUniversity,Sendai980-8577,JapanBulkmetallicglasses(BMGs)havebeenclassifiedaccordingtotheatomicsizedifference,heatofmixing(Hmix)andperiodoftheconstituentelementsintheperiodictable.TheBMGsdiscoveredtodateareclassifiedintosevengroupsonthebasisofapreviousresultbyInoue.Thesevengroupsareasfollows:(G-I)ETM/Ln-LTM/BM-Al/Ga,(G-II)ETM/Ln-LTM/BM-Metalloid,(G-III)Al/Ga-LTM/BM-Metalloid,(G-IV)IIA-ETM/Ln-LTM/BM,(G-V)LTM/BM-Metalloid,(G-VI)ETM/Ln-LTM/BMand(G-VII)IIA-LTM/BM,whereETM,Ln,LTM,BMandIIArefertoearlytransition,lanthanide,latetransition,groupIIIB–IVBandgroupIIA-groupmetals,respectively.ThemainalloyingelementofternaryG-I,G-VandG-VII,ternaryG-IIandG-IV,andternaryG-VIBMGsisthelargest,intermediateandsmallestatomicradiuscomparedtotheotheralloyingelements,respectively.ThemainalloyingelementofternaryBMGsbelongingtoG-I,G-V,G-VIandG-VIIisanelementintheatomicpairwiththelargestandnegativevalueofHmix(HmixL.N.),whilethemainelementofternaryBMGsbelongingtoG-IIandG-IVisindependentoftheatomicpairwithHmixL.N..ThecharacteristicsofthemainelementderivedfortheternaryBMGsaredirectlyapplicabletomulticomponentBMGsbelongingtoG-I,G-II,G-IV(Mg-basedBMGs),G-VandG-VII.Themainelementcanbethelarger-sizedelementintheatomicpairwithHmixL.N.orinthesamegroupastheotherelementsformulticomponentBMGsbelongingtoG-III,G-IV(Be-containingZr-basedBMG)andG-VI.ThemainelementofBMGsbelongingtoG-VItendstochangefromtheelementwiththesmallestatomicradiusinaternarysystemtoanelementwitharelativelylargeatomicsizeinamulticomponentsystem.Thechangeisduetoanincreaseinglass-formingabilitythroughmulticomponentalloyingofBMGsbelongingtoG-VI.TheresultsoftheclassificationofBMGsobtainedinthepresentstudyareimportantforfurtherdevelopmentofBMGs,withtheresultsprovidingaroadmapforthedevelopmentofnewBMGcompositions.(ReceivedJune16,2005;AcceptedAugust8,2005;PublishedDecember15,2005)Keywords:bulkmetallicglass,mixingenthalpy,atomicradius,periodictable,mainelement,ternarysystem,multicomponentsystem,glass-formingability,atomicconfiguration,interchangeability1.IntroductionItiswidelyknownthatmetallicglassesaresolidalloysexhibitingmanysuperiorpropertiestocrystallinealloys.Theuniquepropertiesoriginatefromtherandomatomicarrange-mentofmetallicglassesthatcontrastswiththeregularatomiclatticearrangementfoundincrystallinealloys.Rapidquenchingfromthemeltforthefabricationofmetallicglassesisrequiredsincetherandomatomicarrangementisanon-equilibriumstate.However,thesizeofmetallicglasssamplesfabricatedinthismannerisconstrainedtolessthanonemillimeterduetothehighcriticalcoolingraterequiredfortheformationofconventionalmetallicglasses.Onlyafewmetallicglasseshavebeenreportedtohavebeenformedasbulkmaterialsupuntiltheearly1980s1)duetotheconstraintsonthesamplesize.Since1988,anumberofalloyswithahighglass-formingabilityandabletobefabricatedasbulkmetallicglasses(BMGs)havebeendiscoveredinmulticomponentMg-,2–4)La-,5)Zr-,6,7)Fe-,8,9)andPd-based10,11)alloysystems.Sub-sequently,Inoue12)succeededinclassifyingtheBMGsintofivegroupsbyfocusingonthecharacteristicsoftheconstituentelementswithrespecttothechemicalspeciesandtheiratomicsizedifferences.ThisclassificationisusefulforunderstandingthecharacteristicsofBMGs.However,thisclassificationisinadequateforthedevelopmentofnewBMGsforanumberofreasons.First,anupdatedreviewoftheclassificationsystemofBMGsisrequiredasmanyBMGshavebeendiscoveredsincetheclassificationsystembyInouewasproposedbackin2000.Second,theclassificationsystemdescribesthealloysystembutnottherangeofglass-formingcompositionsinthatsystem.Forinstance,itiswidelyacceptedthattheZr-basedZr–Al–NialloyisatypicalBMG,formingBMGswhenthecompositionisZr-rich.ThepreviousclassificationsystemorganizestheZr–Al–NisystemintoasinglegroupofBMGs,eventhoughBMGsfabricatedtodateinthesystemcomprisingZr,NiandAlareneitherAl-basednorNi-basedBMGs.Furthermore,theZr–Al–Nisystemhasawidecompositionrangefortheformationofmetallicglasses6)overtheNi-richsideandaseparatecompositionrangeforformationoftheamorphousphase6)intheAl-richside.Third,thisclassificationofBMGsfailstoincludePd-andPt-basedalloysystems,althoughthesealloysarewidelyacceptedastypicalBMGs.TheinadequaciesmentionedaboveillustratethattheclassificationsystemforBMGsisincompleteduetoalackofdefiningproperties,asidefromchemicalspeciesandatomicsizedifferences.Inotherwords,amorepracticalclassifica-tionsystemforBMGscanbedevelopedbytakingintoaccountimportantquantities,suchasheatofmixingoftheconstituentelements.12)Inaddition,itisreasonabletoassumethattheperiodicityoftheelementsintheperiodictableisalsolikelytobeanimportantquantitysincetheatomicsizeandheatofmixingshouldberelatedtotheperiodoftheelement.ThepurposesofthepresentpaperaretosummarizethecharacteristicsoftheknownBMGsintermsoftheatomicsizedifference,heatofmixingandperiodoftheconstituentelements,andtoproposeaclassificationsystemforBMGsthatcouldbeusedtoa