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1、蘭州理工大學碩士生畢業(yè)論文摘要本文通過拉伸卸載實驗及原位拉伸SEM觀察對全層和雙態(tài)TiAI基合金損傷機理進行了研究。研究結果表明:在平板拉伸卸載試驗中,如果采用位移控制的方式,材料則產生微裂紋從而導致材料發(fā)生損傷;隨著載荷的增加,原先產生的微裂紋繼續(xù)擴展,而微裂紋擴展的同時又產生大量新的微裂紋:預損傷加快裂紋產生、擴展,使得損傷程度進一步加強,裂紋面密度增大。通過統(tǒng)計分析發(fā)現(xiàn)裂紋面密度可以作為衡量損傷程度的損傷參量:不同程度的預損傷使得材料的彈性模量降低,使材料抵抗裂紋產生、擴展的能力下降,但是對這種材料的最終斷裂性能沒有影響。在拉伸的過程中如果采用載荷控制的方式,材料則不易產生
2、微裂紋。微裂紋即使產生了,也來不及擴展,這樣的結果使得材料的彈性模量、斷裂應力、斷裂應變、屈服應力等幾乎沒有變化。另外通過原位拉伸卸載試驗發(fā)現(xiàn):帶缺口試樣和平板試樣在拉伸卸載過程中的損傷機理不同,斷裂過程也完全不一樣。對于缺口(預裂紋)試樣,由于缺口根部應力集中的作用,其斷裂過程主要是主裂紋首先起裂、擴展并最后斷裂的過程。而對于平板拉伸試樣,先產生很多微裂紋,隨著外加應力的進一步增加,這些微裂紋擴展,當微裂紋和外加應力共同作用足以引起微裂紋進一步貫穿的瞬間,試樣整體發(fā)生解理斷裂。關鍵詞:TtAl基合金;損傷機理;拉伸卸載;原位拉伸卸載:載荷控制;位移控制蘭州理工大學碩士生畢業(yè)論文
3、AbstractThispaperfocusesonthestⅢ腳gofdamagemechanismsofTiAl-basedalloysoffullylamellarandduplex.ThedamagemechanismoflamellarandduplexTiAlalloyshasbeeninvestigatedintensile-unloadexperimentsandsituSEMexperiments.Thestudiesindicatethat,ifthedisplacement-controlledisapplied,theflattensileprocessp
4、roducesmicmcrackswhichresultindamageofmaterials.Withloadincreasing,microerackswillpropagateandatthesametime,alotofnewmicrocmckswillcomeintobeing.P∞-damgewillincreasetheinitiationandpropagationofcracks觴wellasthedegreeofdamage.Pre-damageatdifferentdegreeswilldecreasethematerialelasticmoduleandt
5、heabilityagainstcrackinitiationandpropagation;butthepre—damagehasnoeffectontheultimatefi-actureproperties.Iftheload-controlledisapplied,themicrocrackswillnotcomeintobeingeasily,eveniftheyhave,theywillhavenotimetopropagate.Sothedamagehasnoinfluenceontheelasticmodde,thestressoffracture,thestrai
6、noffxacmmandyieldingstress.Inaddition,theresultofin-situtensile-anloadexperimentindicates:duringthetensile-unloadpmcess,the丘a咖reprocessiscompletelydifferentbecausethedamagemechanismisdifferenttospecimenswithnotchandflatspecimens.Tothespecimenswithnotch(pre·fracture),theprocessofcausingfractur
7、eistheonethatisinitiatedbythemain6鼉眥ewhichwillpropagateandfracturebecauseofthecentralizationofstressintherootofnotch;while,tothefiatspecimens,itismanymicrocracksthatwillfistcomeintobeing,andtheywillpropagatewimtheincreasingofstress.Atthemomem