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1、NanoStructuredMaterials,Vol.11,No.7,pp.925±934,1999ElsevierScienceLtdPergamonCopyright©1999ActaMetallurgicaInc.PrintedintheUSA.Allrightsreserved.0965-9773/99/$±seefrontmatterPIIS0965-9773(99)00392-XFATIGUEPROPERTIESOF5056Al-MgALLOYPRODUCEDBYEQUAL-CHANNELANGUL
2、ARPRESSING1*1112A.Vinogradov,S.Nagasaki,V.Patlan,K.KitagawaandM.Kawazoe1DepartmentofMechanicalSystemsEngineering,KanazawaUniversity,Kanazawa920-5667,Japan2SendaiInstituteofMaterialScienceandTechnology,YKKCorporation,Miyagi981-3341,Japan(ReceivedJune15,1999)(A
3、cceptedSeptember29,1999)AbstractÐThefatiguebehaviourofthefine-grain5056Al-alloyprocessedbyequal-channelangularpressing(ECAP)isexplored.Thismaterialexhibitsaslightlyenhancedfatiguelifeatlowstressamplitudes.However,noimprovementinthefatiguelimitisobserved.Fatig
4、ueperformanceisdiscussedintermsoffatiguelife,cracknucleationandpropagation.Structuralchangesduringfatigueareinvestigatedbytransmissionelectronmicroscopy.Itisshownthatthefinestructureachievedduringprocessingisunstableandtendstorelaxwithcycling,resultinginlocal
5、recoveryofthepre-deformedmaterial.Structurerelaxationduringfatigueissupposedtoprovokenotablecyclicsofteningwhichisparticularlypronouncedathigherappliedstresses.Itisfoundthatthecrackgrowthrateisgreaterinthefine-grainECAPmaterialthaninitscoarse-graincounterpart
6、.Thelatterisattributedtotheroughness-inducedcrackclosureandcrackdeflectionswhichismoresignificantinconventionalalloy.Theimprovementoffatiguepropertiesatlow-cyclicregimeisbelievedtobeduetoahigherresistancetocracknucleationinthefine-grainedmaterialhavingalarger
7、yieldstressvalue.©1999ActaMetallurgicaInc.1.IntroductionNanocrystallineandultra-fine(,1mm)grainmaterialsattractedmuchattentioninrecentyearssincetheyexhibitratherhighmechanicalpropertiessuchasyieldstress[1]andhardness[2]incombinationwithlargeenoughplasticityan
8、devensuperplasticityatrelativelylowtemperaturesandhighstrainrates[e.g.3,4].Mostpublicationsemphasizeenhancementofpropertiesoffine-grainedmaterialswhencomparedtotheirannealedcounterpartsha
