Degradationbehaviorof Mg-based biomaterials containing different long-period stacking ordered phases(12)

distributionoftensilestress.Inaddition,asdegradationprocessispreceded,bothsurfaceoxidationfilmsarebrokendown.However,incontrasttoas-castalloycontaining18R-LPSOphase,thealloywith14H-LPSOphaseexhibitsrapideroxidationfilmremediationability,whicheffectivelyinhibitsthecontactbetweensolutionandthematrix.

Methods

Samplepreparation.AnormalcompositionMg-2Dy-0.5Zn(at.%)alloyhasbeenpreparedbychillcastingtechnology.Thealloyhadbeenpreparedinatantalum-crucibleunderacovergasmixtureofCO2andSF6.Aftermixingat720uCfor1h,thealloywascasttothemouldpreheatedat600uC.Thefilledmouldwasheldat670uCfor1hundertheprotectivegas.Then,thewholetantalumcruciblewiththemeltingalloywasimmersedtotheflowingwaterat600ml/s.Whenthebottomoftantalumcrucibletouchedthewater,itstoppedfor2second.Assoonastheliquidleveloftheinsidemeltwasinalignmentwiththeheightofoutsidewater,thesolidificationprocesswasfinished.Thediameterandlengthoftheingotwere70mmand80mm,respectively.ThechemicalcompositionofMg-1.91Dy-0.32Zn(indexedasMDZ-C)wasmeasuredbyX-rayfluorescencespectroscopy.Theelementcompositionwaslowerthanthenominalcompositionowingtotheelementburningloss.Moreover,thetotalcontentofmainimpurities(Cu,FeandNi)islowerthan0.015wt.%.Microstructureandphaseidentification.Themicrostructuralinvestigationswereperformedusingscanningelectronmicroscopywithafieldemissiongun(SEM-FEG,JEOLJSM-7001F).Thestandardmetallographicprocedureswereapplied,includinggrinding,polishingandetching.Thesampleswereetchedinapicralsolutiontorevealgrainboundaries.Theelementalconcentrationinthematrixandphaseswere

investigatedbySEMequippedwithenergydispersiveX-rayanalysis(EDX)(INCAfromOxford).Theaveragevalueswereobtainedbasedonatleastfiverandomspots.Calorimetricresponseoftheas-castalloywasmeasuredusingdifferentialscanningcalorimetry(DSC,STA449C).Aheatingrateof6uC/minwasemployedunderargonpurgeat35ml/min.Thephaseshavebeenidentifiedbytransmissionelectronmicroscopy(TEM,JEM-2010).ThinfoilsamplesfortheTEMobservationwerepreparedusinganIonPolishingSystem(RES101).

Electrochemicaltest.Electrochemicalbehaviorsweretestedusingapotentiostat/frequencyresponseanalysissystem(Bio-logic,VSP).Experimentswerecarriedoutinathree-electrodeelectrochemicalcell,inwhichasaturatedcalomelelectrode(SCE)asthereferenceelectrode,aplatinummeshascounterelectrodeandthespecimenunderinvestigationastheworkingelectrode.Theexperimentswerecarriedoutin0.9wt.%NaClaqueoussolutions.InordertoreducethefluctuationofpH,theratioofsolutiontosurfaceis80mlto1mm2.Duringthewholetest,thetrayswereplacedinanincubator.Thetemperatureintheincubatorwasmaintainedat25uC.Opencircuitpotential(OCP)measurementsweremadebetweentheworkingelectrodeandthereferenceelectrodewithoutcurrentbeingpassedtothecounterelectrodes.This

measurementshowedthepotentialatwhichtheanodicandcathodicreactioncurrentsattheworkingelectrode/solutioninterfacewerebalanced.TheOCPtestswerebegunimmediatelyafterthespecimenwasimmersedinthesolutionandweremeasuredfor6hduration.

Thecyclicpolarizationswereperformedafterimmersingfor10min,2hand4h.Theforwardscannedpolarizationcurvesstartedfromacathodicpotentialof

21800mV,wherethesurfacesofthealloyswerenotyetcorroded,andextendedtothevertexpotentialof250mV.Theforwardscanwasfollowedbyareversescanbacktothefinalpotential21950mV.Thescanratewas2.5mV/s,thestepheightwas1mV.Thesurfacemorphologyaftercyclicpolarizationtestafterimmersingfor10minwasdriedandobservedbySEMimmediately.

Theelectrochemicalimpedancespectroscope(EIS)wascarriedoutatopen

potentialwithanappliedsignalof10mVrms.Thescanningfrequencyrangedfrom100kHzdownto0.1Hz.Thesampleswereimmersedin0.9wt.%NaClsolutionsfordifferenttimes,viz.2h,4h,6h,8hand12h,toinvestigatethecorrosionprocess.AfterEIStests,theMDZ-CandMDZ-545samplesweredriedandobservedbySEMimmediately.

Immersiontestandproductanalysis.Theimmersiontestsweretoinvestigatethecorrosionmechanismandtorevealthedevelopmentandgrowthofcorrosion

productsofMDZalloysinNaClaqueoussolutions.Tothispurpose,threecylinder(Ø1035)specimenscontaining18R-LPSOor14HLPSOphaseswereimmersioninNaClaqueoussolutionsatdifferenttimesviz.10min,20min,60minand120min.Theywerecleanedbyrinsingwithpurewater,andthenrinsedbyethanolanddriedinairpriortoSEM-FEGobservation.

Thephasecompositionofcorrosionproductsonthesamplesimmersedupto12hinthetestsolutionswereconfirmedbyx-rayelectrondiffractiondirectly(XRD,Rigaku).TheXRDanalysiswasperformedbyscanningfrom20uto80uwithastepsizeof0.02uwithCuKaradiation.Thefilamentcurrentandaccelerationvoltagewere30mAand40kV,respectively.

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