金属的断裂.ppt

Chaptor6FractureofMetals金属的断裂,cargoship货船,问题的提出构件失效、灾难性的后果,挤压制品的周期裂纹,Plasticdeation塑性成形surface/internalcracksinworkingmaterials→integralfailure.加工材料出现表面/内部裂纹→整体破断Thishappensinrolling轧制,extrusion挤压,drawing拉拔,forging锻造andsoon.Itwilllargelyreducetherateoffinishedproductsandproductionrate成品率/生产率↓.,Weconcernaboutfracturefortworeasons.Oneiscrack’sdestructiveeffect,theotheroneisthatfractureisrelatedtomaterial’sability.1破坏；2成形性。,Inthischapter,wewillanalyzethephysicalessenceandlawsoffracture.断裂的物理本质/规律,DefinitionoffractureWhenmetallicmaterialdeationforereachitsplasticlimit,itwilldividecompletely.Thentheinteratomicforceisdamaged.断裂－金属材料在变形超过其塑性极限完全分开.原子间结合力遭受破坏。,,relatedsubjects,断裂力学fracturemechanics-macroscopicStressfield应力场Strainfield应变场Fracturecriterion断裂判据,,,断裂物理fracturephysics-microscopicFracturemechanism断裂机制Microstructure组织,6.1Basictypesoffracture断裂的基本类型Accordingtowhetherthemetalhasobviousplasticdeationbeforefracture,itcanbesortedasfollowing根据断裂前金属是否有明显的塑性变形可分为brittlefracture脆性断裂ψ5,,,,,Accordingtotheorientationrelationshipbetweenfractureandactingforce按断裂面相对作用力的取向关系可分为normalfailure正断againsttoσ1shearingfailure剪断alongthedirectionofmaximumshear,Accordingtocracktrendinmicroscopicview从微观上按照裂纹的走向可分为transgranularfracture穿晶断裂/晶间intergranularfracture沿晶grain晶粒,Fracturealonggrain-boundaryinpolycrystalmaterial多晶材料沿晶界发生的断裂,-沿晶韧性断裂－晶界弱化,出现韧窝dimple,-沿晶脆性断裂intergranularprecipitatedbrittlecontinuousfilm－沿晶析出脆性连续膜,裂纹穿过晶粒内部crackcrosstheinteriorofgrain穿晶脆性断裂lowspeed,hightemperatureandstressconcentration.alongsomecrystalplane---cleavageplane解理面穿晶韧性断裂microporecoalescence微孔聚合,,,,6.2脆性断裂brittlefracture,材料塑性变形能力很低，裂纹尖端的应力集中不能因塑性变形而松弛。,6.2.1理论断裂强度theoreticalfracturestrength完整晶体在正应力作用下沿某一晶面拉断的强度。Underthepullingforce,theforcewhichcanhelptwoadjacentatomicplanesovercomeinteratomicbondingforce,andseparatethetwoatomicplanes.两相邻原子面在拉力σ作用下，克服原子间键合力作用，使原子面分开的应力。,完整晶体拉断示意图mn为断裂面的迹线；a表示原子面间距,原子间作用模型原子间作用力与位移间的关系满足正弦规律1-1---将原子拉开所需的最大应力，即断裂理论强度。,,,,,晶体中的内聚力与原子间距的关系,原子间作用模型原子间作用力与位移间的关系满足正弦规律1-1---将原子拉开所需的最大应力，即断裂理论强度1-2断裂后出现两个新的断裂面，表面能为2,,,,,外力抵抗原子间结合力做的功产生断裂新面的表面能1-3将替换由虎克定律将1-1对x求导1-4在正弦曲线初期,,,,,,,,,,,,,,EYoung’smodulusγa10″dyn/cm2103erg/cm2310-8c,实际金属强度铝合金～200－300MPa低碳钢～400-500MPa合金钢～1000MPa,6.2.2Griffith裂纹理论,,,基点材料中已存在裂纹crack在裂纹尖端引起应力集中，在外加应力小于理论断裂强度时裂纹扩展，实际断裂强度大大降低。能量平衡裂纹→弹性能↓表面能↑elasticenergysurfaceenergy释放的弹性能弹性能密度裂纹体积,基点材料中已存在裂纹crack,在一块大的平板上的穿透裂纹,增加的表面能,,,,,,,,Griffith公式,在正应力作用下只有弹性能的减少表面能的增加→裂纹扩展,,,Griffith公式物理意义裂纹两端引起的应力集中，相当将外力放大了倍，使局部达到了理论断裂强度.,,,,比较理论断裂强度公式,,修正modification伴随一定的塑性变形,裂纹的形核和传播与局部塑性变密切相关.,Orowan考虑塑性变形能P-断口表面附近的塑性应变能plasticstrainenergyQ①Pγ②P-,6.2.3.Cracknucleation裂纹形核Therearegenerallynolargecracksinoriginalmaterials.Manyexperimentsshowthatcracksaregeneratedbyplasticdeation.Anyheterogeneityinamaterialthatproducesastressconcentrationcannucleatecracks.Forexample,steps,striations,holes,andsoonactasstressraisersonapparentlyperfectsurface.Intheinteriorofthematerial,therecanexistvoids,airbubbles,second-phaseparticles,etc.Cracksnucleationwilloccuratthesedefects，wheretheconditionswouldbemostfavorable.,heterogeneity不均匀性striation条纹stressconcentration应力集中nucleation形核,裂纹的形核,Mobiledislocationsmeetsomeobstacles,thenappearstressconcentration.Whenthestressbecomesgreaterthaninteratomicbondingforce,cracksnucleate,propagate,finallyleadtofracture.运动的位错遇到了某种障碍,就产生了应力集中,应力大到可以破坏原子间的键合力时，裂纹开始形核，裂纹长大导致断裂。,Nucleationmechanisms形核机制1.位错塞积dislocationpile-up晶界相界τ’nτ,,,2位错反应dislocationreaction在两相交的滑移面上,由于位错反应发生了同号位错的聚合便产生了微裂纹。在体心立方b.c.c的101面上发生如下位错反应＋（111）＝a[001]生成的新位错为不滑动刃型位错，其柏氏矢量⊥（001）解理面，形成了解理裂纹。,解理断裂cleavage,穿晶脆性断裂在一定条件下（低温高速及应力集中），当应力达到一定值，快速沿一定的结晶面（解理面）而发生的断裂。例低碳钢发生解理断裂时，常沿铁素体『100』晶面发生。密排六方『0001』,解理面原子间距最大、原子结合键最弱的晶面。,3位错墙側移理论刃形位错垂直排列→位错墙→滑移面弯折→外力作用→晶体滑移→位错墙側移→滑移面上生成裂纹.可说明密排六方hcp金属沿滑移面断裂的原因。,4位错销毁理论异号刃型位错→相对运动→彼此合并而消毁→孔隙→裂纹,6.3韧性断裂ductilefracture材料经明显的变形后发生的断裂称为韧性断裂。拉伸时以“颈缩”necking为先导,当应变硬化strainhardening产生的强度增加不足以补偿截面积的减少时,产生集中变形,出现“细颈”。,,,,细颈中心承受三向拉应力,显微空洞cavity首先在此形成,随后长大聚合成裂纹,最终在细颈边缘处,沿与拉伸轴45。方向被剪断,形成“杯锥”断口cupandconefracturesurface,微观形态韧窝dimple原因多起源于空洞cavity。,单相金属singlephase←熔炼时混入夹杂物inclusion多相合金multi-phase←难变形第二相粒子second-phaseparticle。,弥散相dispersedphases,Verysmallparticles1to20nm-carbidesofelements;Particlesofintermediatesize50-500nm-alloyelementcompoundscarbides,nitrides,carbo-nitridesinsteels/Al2O3inaluminumandThO2inNickle/precipitateparticlesobtainedbyappropriateheattreatmentinAl-Cu-Mgsystem;Inclusionsoflargesizeontheorderofmillimeters-oxidesandsulfides,CarbideNitrideOxideSulfide,precipitate,Inclusions,碳化物氮化物氧化物硫化物,杂质,沉淀,空洞的形成,Thesecond-phaseparticlesarebrittle/thematrixisductile,theerwillnotbeabletoaccommodatethelargeplasticstrainsofthematrix,andconsequently,thesebrittleparticleswillbreakintheverybeginningofplasticdeation;Theparticle/matrixinterfaceisveryweak,interfacialseparationwilloccur.,Microcavities（微空洞）arenucleated.,stressstate应力状态→microscopicmorphology微观形貌aσ2σ3equiaxialdimple等轴韧窝bτparabolicdimple,inverseelongated抛物线形韧窝反向拉长ctearstress,parabolic,撕裂应力抛物线状同向,Charactersofductilefracture韧性断裂特点1断裂前发生较大塑性变形→高能量吸收过程.2裂纹产生nucleation→扩展propagation.→聚合coalescence.生成新裂纹→多裂纹源3裂纹扩展临界应力裂纹形核应力→缓慢过程,Discussion,a,b,c,Fracturesurface,UltimateandFractureStrengths,Q比较脆性断裂＆韧性断裂时的抗拉强度＆断裂强度Forbrittlematerial,theultimateandfracturestrengthscoincide.Foraductilematerial,theultimatestrengthishigherthanthefracturestrengthascomputedwithconventionalstressandstrainulas.,,truestress,,拉拔细玻璃丝（0.5mm）1600～6300MPa在空气中静置几小时140～350MPa,空气腐蚀→表面形成裂纹Corrosioninair→surfacecracks,“尺寸效应”sizeeffect,,0.02mm700MPa0.005mm2800MPa1μm→理论断裂强度,尺寸越小，试样内存在着达到临界尺寸的裂纹的几率越小。,表面能的作用functionofsurfaceenergy,玻璃在极小的应力作用下并不马上破坏，但可以发生延迟断裂。,原因开始表面裂纹太小，达不到断裂判据的裂纹临界尺寸。由于表面吸附降低了表面能，当达到断裂判据时就会发生突然的断裂。,云母在空气中的断裂强度是在真空中的1/3空气中的吸附作用→云母的表面能4500erg/mm→400erg/mm强度↓,,材料内部实际存在缺陷point/line/facedefects-空位vacancy-位错dislocations-界面boundary-堆垛层错stackingfault-挛晶twins,,Brittle脆性BondruptureDiamond,silicates,alumina,mica,boron,carbidesandnitrides,SemibrittleBondrupture,dislocationmobilityNaCl,ioniccrystals,hexagonalclosepackedmetals,majorityofbody-centeredcubicmetals,glassypolymers,Ductilematerials韧性DislocationmobilityFace-centeredcubicmetals,somebodypcenteredcubicmetals,