说明材料类毕业论文文献翻译中英文对照.docx

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1、Unit1MetalsTheuseofmetalshasalwaysbeenakeyfactorinthedevelopmentofthesocialsystemsofman.Oftheroughly100basicelementsofwhichallmatteriscomposed,abouthalfareclassifiedasmetals.Thedistinctionbetweenametalandanonmetalisnotalwaysclear-cut.Themostbasicdefinitioncentersaroundth

2、etypeofbondingexistingbetweentheatomsoftheelement,andaroundthecharacteristicsofcertainoftheelectronsassociatedwiththeseatoms.Inamorepracticalway,however,ametalcanbedefinedasanelementwhichhasaparticularpackageofproperties.Metalsarecrystallinewheninthesolidstateand,withfew

3、exceptions(e.g.mercury),aresolidatambienttemperatures.Theyaregoodconductorsofheatandelectricityandareopaquetolight.Theyusuallyhaveacomparativelyhighdensity.Manymetalsareductile-thatis,theirshapecanbechangedpermanentlybytheapplicationofaforcewithoutbreaking.Theforcesrequi

4、redtocausethisdeformationandthoserequiredtobreakorfractureametalarecomparativelyhigh,although,thefractureforcesisnotnearlyashighaswouldbeexpectedfromsimpleconsiderationoftheforcesrequiredtotearaparttheatomsofthemetal.Oneofthemoresignificantofthesecharacteristicsfromourpo

5、intofviewisthatofcrystallinity.Acrystallinesolidisoneinwhichtheconstituentatomsarelocatedinaregularthree-dimensionalarrayasiftheywerelocatedatthecornersofthesquaresofathree-dimensionalchessboard.Thespacingoftheatomsinthearrayisofthesameorderasthesizeoftheatoms,theactuals

6、pacingbeingacharacteristicoftheparticularmetal.Thedirectionsoftheaxesofthearraydefinetheorientationofthecrystalinspace.Themetalscommonlyusedinengineeringpracticearecomposedofalargenumberofsuchcrystals,calledgrains.Inthemostgeneralcase,thecrystalsofthevariousgrainsarerand

7、omlyorientedinspace.Thegrainsareeverywhereinintimatecontactwithoneanotherandjoinedtogetheronanatomicscale.Theregionatwhichtheyjoinisknownasagrainboundary.Anabsolutelypuremetal(i.e.onecomposedofonlyonetypeofatom)hasneverbeenproduced.Engineerswouldnotbeparticularlyinterest

8、edinsuchametalevenifitweretobeproduced,becauseitwouldbesoftandweak.Themetalsusedcommerciallyinevitablyc