动力学和莫表面废除Fe-Mo合金

ELSEVIERSurfaceScience352-354(1996)206-210~#N::I~°~~-~i~isurfacescience~.................~i~..~t;g~=~i~,~...KineticsofSandMosurfacecosegregationinFe-MoalloysA.Rolland,M.M.Montagono,J.Caban6*ED1FIS(URA443-CNRS)-LaboratoiredeM~tallurgie,Case511,Facult£desSciencesetTechniquesdeSaintJ£rgme,13397MarseilleCedex20,FranceReceived5September1995;acceptedforpublication31October1995AbstractThisworkconcemsanexperimentalstudyofsulphursurfacesegregationinFeMo(0.5,2and3.5at%)alloyswithnitrogenasresidualimpurity.Augerelectronspectroscopyandaradiochemicaltechniqueareused.Athightemperature(T560°C),anincreaseofthemaximumofsegregatedsulphurwithbulkMocontentisexplainedbyacosegregationeffectprobablywiththeformationof2D(Fe,Mo)sulphides.Atlowtemperature,NandMocosegregatewhichcanbeexplainedbythetendencytoform2DMonitridesandsulphursegregationtakesplaceforverylongsegregationtreatments.Keywords:Alloys;Augerelectronspectroscopy;Radioactivetracermethods;Surfacesegregation1.Introduction2.MaterialsandtechniquesSurfacesegregationinironbasealloyshasbeeninvestigatedforalongtime[1,2];recently,isothermsofsulphursegregationonpureironsurfaceshavebeendetermineddirectlywitharadioactivetracermethod[3]previouslyusedtodrawadsorptionisothermsofsulphuronvariousmetals[4].ThepresentworkoncosegregationeffectsinFe(Mo)alloyscontainingsmallamountsofsulphurandni-trogenisperformedbyassociatingtheAugerelec-tronspectroscopy(AES)andtheradiochemicaltech-niquementionedabove.*Correspondingauthor.Fax:[phone];e-mail:[e-mail]vely0.5,2and3.5at%Moareprepared.Puremetals(99.998%Feand99.98%Mo)aremeltedinalevitationhighfre-quencyfurnaceequippedwithawatercooledcru-cible.Homogeneousalloyswitharegulargrainsize(0.1-1mmdiameter)areobtainedafterseveralsuc-cessivemechanicalandheattreatments(800-895°C).Finallythesurfaceiscarefullypolishedmechanicallyandchemically[5].AESspectraobtainedafterioncleaning,onlyshowFe(47,598,651and703eV)peaksandasmallC(272eV)peak.However,whenasampleisheated,smallMo(186eV),S(152eV)andN(379eV)Augersignalsaredetected;theydisappeareasilyafterashortioncleaning.Aspreviouslyshown[3,6],acontrolledamountofsulphurcanbedissolvedinFeandFe(Mo)samples0039-6028/96/$15.00©1996ElsevierScienceB.V.AllfightsreservedSSDI0039-6028(95)01132-3A.Rollandetal./SurfaceScience352-354(1996)206-210207byadiffusiontreatmentinaH2-H2SmixturewheretheratioH2S/H2remainsconstant.Inthepresentcase,samplesareheatedat800°Cduring30handH2S/H2isabout10-3.Augerelectronspectroscopy(AES)isperformedwithanusualequipment:anUHVchamberequippedwitha3keVelectrongun,aniongunforcleaningthesurfaceandacylindricalmirroranalyzer.AESisusedtotesttheinitialsurfacecompositionandtofollowsegregationkineticswhenthesampleisheated.Inmostcases,afirstspectrumistakenatroomtemperatureafteramildioncleaningofthesurface.ThesampleisthenrapidlyheatedtothesegregationtemperatureandthevariationswithtimeofseveralAugersignals(usuallyFe(47eV)and(703eV),Mo(186eV),S(152eV))arerecorded.Finallyaspectrumistakenagainattheendofthesegregationtreatment.TheradioactivetracermethodhasbeenpreviouslydescribedindetailinRef.[3].Briefly,aH2-H2Smixturelabelledwiththeisotope35S(softt-rayemitter)isusedtoobtainequilibriumbetweensul-phursegregatedtothesurface,sulphurdissolvednearthesurfaceandthegasmixture;theamountofsulphurafterheattreatmentisdeducedfromsurfaceradioactivitymeasurements.Thesurfaceradioactiv-ityAandtheremainingradioactivityA~afterchem-icalremovingofthesuperficialsulphuraremea-sured.ThedifferenceAs=A-A~,duetothesur-faceexcessofsulphur,isproportionaltothesegre-gatedamountFsinatomscm-a.BycomparingAstoAe,theactivityofastandard[3],thesegregatedamountFs(atomscm-a)isobtained.Inaddition,thepeaktopeakheightofthesulphurAugersignal(152eV)isproportionaltothesurfaceradioactivityAs(i.e.toFs)[7]sothatthesulphurAugersignaliscalibratedbyreferringtotheamount,i.e.Fs=7.2×1014atomscm-z,determinedbytheradiochemicaltechniqueonastandardmadeofonesulphurmonolayerona(100)coppersurface[3].3.Experimentalresults3.1.MaximumquantityofsegregatedsulphurExperimentalconditionscorrespondingtothemaximumquantityofsegregatedsulphurinFeMoalloyscanbedefinedfromthesegregationdatapreviouslyobtainedoniron[3;5]:H2S/H2ratios(2x10-4-10-3),temperature(650-800°C).ResultsobtainedinthisdomainbyusingtheradioactivetracermethodandtheAEStechniquearerespec-tivelygiveninTables1and2.Itisclearthatthemaximumamountofsegregatedsulphur,Fsatomscm-2,deducedfromradioactivitymeasurements,in-creasesregularlywithMoconcentrationinthealloy;ThiseffectisconfirmedbyAESanalysisofthesamesamples(Table2):theratios/S///Fe(47)and/S//Fe(703)betweentheintensitiesofsulphur(Is)andiron(IFe(703)andIFe(47))signals,increasewithMobulkconcentration.LetusmentionthattheseratiosarecalculatedwithoutaccountingforadecreaseoftheFesignalwhichislikelytooccurasthesulphuramountincreases;asaconsequence,AESdatatendtoenhancetheMoeffect.OnAESspectraobtainedaftersegregationtreatments,themainAugersignalscorrespondtoSandFewhileMosignalsarenotdetectedeasilyespeciallyonaFeMo(0.5at%)sam-ple(Fig.1):3.2.Segregati