机械设计制造及其自动化专业英语翻译(最新整理).docx

机械设计制造及其自动化专业英语翻译(最新整理).docx

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Unit1  Metals

金属

       1 Theuseofmetalshasalwaysbeenakeyfactorinthedevelopmentofthesocialsystemsofman.Oftheroughly100basicelementsofwhichallmatteriscomposed,abouthalfareclassifiedasmetals.The distinction betweenametalandanonmetalisnotalways clear-cut.Themostbasicdefinition centersaround thetypeofbondingexistingbetweentheatomsoftheelement,andaroundthe characteristics of certainoftheelectronsassociatedwiththeseatoms.Inamorepracticalway,however, a metal canbedefinedasan elementwhich hasaparticularpackageofproperties.

       2 Metalsare crystalline whenin the solidstateand,withfew exceptions （e.g.mercury）,aresolidat ambient temperatures.Theyaregoodconductorsofheatandelectricityandare opaque tolight.Theyusuallyhaveacomparativelyhighdensity.Manymetalsare ductile-that is,their shapecanbechangedpermanently bytheapplicationofaforcewithoutbreaking. Theforcesrequiredtocausethisdeformationandthoserequiredtobreakorfractureametalarecomparativelyhigh,although,thefractureforcesisnotnearly ashighaswouldbeexpectedfromsimpleconsiderationoftheforcesrequiredtotearaparttheatomsofthemetal.

       3 Oneofthemoresignificantofthesecharacteristicsfromourpointofviewisthatofcrystallinity. Acrystallinesolidisoneinwhichtheconstituentatomsarelocatedinaregularthree-dimensionalarrayasiftheywerelocatedatthecornersofthesquaresofathree-dimensionalchessboard. Thespacingoftheatomsinthearrayisofthesameorderasthesizeoftheatoms,theactualspacingbeingacharacteristicoftheparticularmetal.Thedirectionsoftheaxesofthearraydefinetheorientationofthecrystalinspace.Themetalscommonlyusedinengineeringpracticearecomposedofalargenumberofsuchcrystals,calledgrains.Inthemostgeneralcase,thecrystalsofthe various grainsarerandomlyorientedinspace.Thegrainsareeverywhereinintimatecontactwithoneanotherandjoinedtogetheronanatomicscale.Theregionatwhichtheyjoinisknownasagrainboundary.

       4 Anabsolutelypure metal（i.e. onecomposedofonlyonetypeofatom）hasneverbeenproduced.Engineerswouldnotbeparticularlyinterestedinsuchametalevenifitweretobeproduced,becauseitwouldbesoftandweak.Themetalsusedcommercially inevitably containsmallamountsofoneormoreforeignelements,either metallic ornonmetallic.Theseforeignelementsmaybe detrimental,theymaybebeneficial,ortheymayhavenoinfluenceatallonaparticularproperty.Ifdisadvantageous,theforeignelementstendtobeknownas impurities.Ifadvantageous,theytendtobeknownas alloyingelements.Alloyingelementsarecommonlyadded deliberately in substantial amountsinengineeringmaterials.Theresultisknownasanalloy.

       5 Thedistinctionbetweenthedescriptors “metal” and “alloy” isnotclear-cut.Theterm “metal” maybeusedtoencompassbothacommerciallypuremetalanditsalloys. Perhapsitcanbesaidthatthemoredeliberatelyanalloyingadditionhasbeenmadeandthelargertheamountoftheaddition,themorelikelyitisthattheproductwillspecificallybecalledanalloy. Inanyevent,the chemical compositionofametaloranalloymustbeknownandcontrolledwithincertainlimitsif consistent performanceistobeachievedinservice. Thuschemicalcompositionhastobetakenintoaccountwhendevelopinganunderstandingofthefactorswhichdeterminethepropertiesofmetalsandtheiralloys.

       6 Ofthe50orsometallicelements,onlyafewareproducedandusedinlargequantitiesinengineeringpractice.Themostimportantbyfaris iron,on which arebasedthe ubiquitous steelsandcastirons（basicallyalloysofironandcarbon）.Theyaccountforabout98%byweightofallmetalsproduced.Nextinimportanceforstructuraluses（thatis,forstructuresthatareexpectedtocarryloads）are aluminum,copper,nickel,andtitanium.Aluminumaccountsforabout0.8%byweightofallmetalsproduced,andcopperabout0.7%,leavingonly0.5%forallothermetals.Asmightbeexpected,the remaindersare allusedin ratherspecialapplications.Forexample,nickelalloysareused principally incorrosion-andheat-resistantapplications,whiletitaniumisused extensively intheaerospaceindustrybecauseitsalloyshavegoodcombinationsofhighstrengthandlowdensity.Bothnickelandtitaniumareusedinhigh-cost,high-qualityapplications,and,indeed,itistheirhighcostthattendstorestricttheirapplication.

       7 Wecannotdiscussthesemore esoteric propertieshere. Sufficeittosay thatawholecomplexofpropertiesinadditiontostructuralstrengthisrequiredofanalloybeforeitwillbeacceptedinto,andsurvivein,engineeringpractice. Itmay,forexample,havetobestrongandyethaver