Low temperature superplasticity in a friction stir processed ultrafine grained Al Zn Mg Sc alloy.pdf

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1、ActaMaterialia53(2005)4211–4223www.actamat-journals.comLowtemperaturesuperplasticityinafriction-stir-processedultrafinegrainedAl–Zn–Mg–ScalloyI.Charit1,R.S.Mishra*CenterforFrictionStirProcessingandDepartmentofMetallurgicalEngineering,UniversityofMissouri,Rolla,MO65409,USAReceived28May2004;rece

2、ivedinrevisedform9May2005;accepted13May2005Availableonline5July2005AbstractFrictionstirprocessing(FSP)wasusedtocreateamicrostructurewithultrafinegrains(0.68lmgrainsize)inanas-castAl–8.9Zn–2.6Mg–0.09Sc(wt.%)alloy.Theultrafinegrainedalloyexhibitedsuperplasticityatrelativelylowtemperaturesandhighe

3、r
2
1strainrates.Optimumductilityof1165%atastrainrateof3·10sand310°Cwasobtained.Enhancedsuperplasticitywasalsoachievedatatemperatureaslowas220°C.ExperimentallyobservedparametricdependenciesandmicrostructuralexaminationsindicatedthattheoperatingdeformationmechanismmightbetheRachingergrainbound

4、aryslidingaccommodatedbyintragran-ularslip.TheFSPmicrostructurebecamehighlyunstableat390°Conwards,thus,affectingductilityadversely.Insitutransmissionelectronmicroscopyheatingwasusedtounderstandtheinstabilityphenomenon,whichhasbeenattributedtothedropinparticlepinningforcesduetothedissolutionofm

5、etastableprecipitatesandmicrostructuralheterogeneity.Ó2005ActaMaterialiaInc.PublishedbyElsevierLtd.Allrightsreserved.Keywords:Aluminumalloy;Frictionstirprocessing;Ultrafinegrainedmicrostructure;Superplasticity1.Introductionatureand/orincreasingstrainrate.Conventionalsuper-
4plasticityistypical

6、lyachievedatslowstrainrates(10–
3
1Inrecentyears,therehasbeengrowinginterestinthe10s)andhigherhomologoustemperatures(0.80Tm,preparationofultrafinegrained(UFG)microstructureswhereTmisthemeltingpointofthematrixalloy).Pre-(grainsize<1lm)inaluminum(Al)alloysforimprov-viousworks[1–7]haveshownthatth

7、eoptimumsuper-ingtheirmechanicalproperties,suchas,strength,ductil-plasticitycouldnotbeachievedinFSPAlalloysity,fatigue,fracturetoughnessandsuperplasticity.below400°Cduetolargergrainsizes(>1lm).Hence,Severalprocessingtechniqueshavebeendevelop