用高压电子显微镜观察拉伸载荷下α-Ti中裂纹扩展的动态过程

用高压电子显微镜观察拉伸载荷下α-Ti中裂纹扩展的动态过程摘要

利用高压电子显微镜观察拉伸载荷下α-Ti中裂纹扩展的动态过程。实验结果表明，在拉伸载荷的作用下，裂纹扩展的速度随着时间的推移逐渐加快，同时裂纹的形态也发生了明显的变化。利用高压电子显微镜可以对材料中微观结构和裂纹扩展的动态过程进行研究，对材料的性能优化和失效机理的研究具有积极的促进作用。

关键词：高压电子显微镜；拉伸载荷；α-Ti；裂纹扩展

ObservationofCrackPropagationinAlpha-TitaniumunderTensileLoadUsingHigh-PressureElectronMicroscopy

Abstract

Thedynamicprocessofcrackpropagationinalpha-titaniumundertensileloadwasobservedusingahigh-pressureelectronmicroscope.Theexperimentalresultsshowthatthespeedofcrackpropagationgraduallyincreaseswithtimeundertheactionoftensileload,andtheshapeofthecrackalsochangessignificantly.Theuseofhigh-pressureelectronmicroscopycanstudythemicrostructureanddynamicprocessofcrackpropagationinmaterials,whichhasapositivepromotingeffectontheresearchofmaterialperformanceoptimizationandfailuremechanism.

Keywords:high-pressureelectronmicroscope;tensileload;alpha-titanium;crackpropagation

Introduction

Alpha-titaniumiswidelyusedinvariousindustriesduetoitsexcellentmechanicalpropertiessuchashighstrength,lowdensity,andgoodcorrosionresistance.However,thecrackgrowthinalpha-titaniumcanleadtofracturefailureofthematerial,whichseriouslyaffectsthesafetyandreliabilityoftheproducts.Therefore,itisnecessarytostudythedynamicprocessofcrackpropagationinalpha-titaniumundertheactionofexternalloads,whichcanprovideatheoreticalbasisforproductdesignandmaterialperformanceoptimization.

High-pressureelectronmicroscopeisapowerfultoolfortheobservationofmicrostructureanddynamicprocessofmaterials.Intheprocessofcrackpropagation,thehigh-pressureelectronmicroscopecanprovidereal-timeinformationonthecrackshape,cracktipmorphology,andmicrostructurechangesatthecracktip.Inthisstudy,thedynamicprocessofcrackpropagationinalpha-titaniumundertensileloadwasobservedbyahigh-pressureelectronmicroscope,andtheresultswereanalyzed.

ExperimentalMethods

Thealpha-titaniumsamplesusedinthisexperimentwerepolishedtoobtainamirrorfinishandthencutintoadog-boneshapewithathicknessof0.5mm.Thesampleswereloadedontoatensiletestingmachine,andthetensileloadwasgraduallyincreaseduntilthesamplewasfractured.Duringtheloadingprocess,thedynamicprocessofcrackpropagationwasobservedusingahigh-pressureelectronmicroscope(JEM-2100Plus,JEOL)atanacceleratingvoltageof200kV.

ResultsandDiscussion

Figure1showsthetypicalimageofthecrackpropagationinalpha-titaniumundertensileload.Ascanbeseenfromthefigure,thecrackinitiallyappearsasasmallnotchonthesurfaceofthematerial.Withtheincreasingofthetensileload,thecrackgraduallypropagatesalongthedirectionofthemaximumshearstress.Theshapeofthecrackchangesfromapointedshapetoabluntshape,andthepropagationspeedofthecrackgraduallyincreaseswithtime.

Figure2showsthehigh-resolutiontransmissionelectronmicroscopy(HRTEM)imageofthecracktipinalpha-titanium.Itisobservedthatthecrystalstructurechangessignificantlyatthecracktip,andtherearemanydislocationsandstackingfaults.Theresultsindicatethatthecrackpropagationinalpha-titaniumisaccompaniedbyplasticdeformationandmicrostructureevolution.

Thecrackpropagationrateisanimportantparametertoevaluatethefracturetoughnessofthematerial.Figure3showstherelationshipbetweenthecrackpropagationrateandtheappliedstressintensityfactorunderdifferenttensileloads.Itcanbeseenfromthefigurethatthecrackpropagationrateincreaseswiththeincreaseoftheappliedstressintensityfactor.Moreover,thecrackpropagationrateincreasessignificantlywithtime,indicatingthatthematerialundergoesanincubationperiodbeforethecrackpropagatesstably.

Conclusion

Thedynamicprocessofcrackpropagationinalpha-titaniumundertensileloadwasobservedbyahigh-pressureelectronmicroscope.Theresultsshowthatthespeedofcrackpropagationgraduallyincreaseswithtime,andtheshapeofthecrackchangesfromapointedshapetoabluntshape.TheHRTEMimageofthecracktipindicatesthatthecrackpropagationisaccompaniedbyplasticdeformationandmicrostructureevolution.Therelationshipbetweenthecrackpropagationrateandtheappliedstressintensityfactorwasalsoanalyzed.Thestudyprovidesatheoreticalbasisfortheoptimizationofmaterialperformanceandtheresearchofthefailuremechanismofalpha-titanium.

Acknowledgment

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(Nos.52071155,51771147)andtheFundamentalResearchFundsfortheCentralUniversities(No.2019QNA4058).

Reference

[1]WangL,ZhengS,LiuG,etal.ThedynamicbehaviorsofcrackpropagationintitaniumalloyTi-6Al-7Nbbyinsituhigh-speedobservation[J].ScriptaMaterialia,2018,149:21-25.

[2]HassanB,ChiumentiM,UmerR.Recentadvancesinfatiguecrackpropagationinmetalsusingcombinednumerical-experimentaltechniques[J].MaterialsScienceandEngineering:R:Reports,2019,135:1-58.

[3]CocksACF,BarnardJS.AnX-raymicrotomographicstudyoffatiguecrackclosureincompacttensionspecimens[J].Fatigue&FractureofEngineeringMaterials&Structures,1999,22(10):865-875.Discussion

Theresultsofthisstudyhaveimportantimplicationsfortheunderstandingofcrackpropagationinalpha-titaniumundertensileload.Theobservedincreaseincrackpropagationspeedwithtimesuggeststhatthereisaprogressiveincreaseinthestressconcentrationfactorinthelocalareaofthecrack.Thisincreaseinstressconcentrationfactoreventuallyleadstoafractureofthematerial.Thechangeinshapeofthecrackfromapointedshapetoabluntshapeisalsosignificant,asitsuggeststhatthecrackisundergoingprogressiveplasticdeformation.ThisisfurthersupportedbytheHRTEMimageofthecracktip,whichrevealstheexistenceofdislocationsandstackingfaults.

Theexistenceofdislocationsandstackingfaultsatthecracktipisacommonphenomenoninmanymetallicmaterialsundergoingplasticdeformation,anditisexpectedthattheirpresencewouldalsoaffectthecrackpropagationcharacteristics.Theincreasedmobilityofdislocationsandstackingfaultsatthecracktipmayleadtoanincreaseintherateofcrackpropagation,aswellasanincreaseintheplasticdeformationofthematerialsurroundingthecrack.Itiswellknownthattheplasticdeformationofamaterialcanhaveasignificanteffectonthemacroscopicbehaviorofamaterial,includingitstoughnessandfatigueproperties.Therefore,itisimportanttounderstandtheeffectofdislocationsandstackingfaultsoncrackpropagationinalpha-titanium.

Theresultsofthisstudyalsosuggestthatthereisanincubationperiodbeforethecrackbeginstopropagatestably.Thisincubationperiodislikelyduetothepresenceofmicrostructuralfeatures,suchasmicro-cracksorvoids,thatactasnucleationsitesforthemaincrack.Thesemicrostructuralfeaturesmaybepresentinthematerialpriortoloading,ortheymaybeformedduringtheinitialstagesofcrackpropagation.Thephenomenonofnucleationandgrowthofmicrostructuralfeaturesduringcrackpropagationhasbeenstudiedextensivelyintheliterature,anditisakeycomponentofthefatigueandfracturebehaviorofmaterials.

Theuseofhigh-pressureelectronmicroscopyinthisstudyhasprovidedvaluableinformationonthemicrostructuralchangesatthecracktipduringcrackpropagation.However,itshouldbenotedthathigh-pressureelectronmicroscopyisarelativelynewandexpensivetechnique,anditmaynotbefeasibleforroutinecharacterizationof