最新机械设计制造及其自动化专业英语翻译超级大全.docx

最新机械设计制造及其自动化专业英语翻译超级大全.docx

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最新机械设计制造及其自动化专业英语翻译超级大全

Unit1Metals

金属

Theuseofmetalshasalwaysbeenakeyfactorinthedevelopmentofthesocialsystemsofman.Oftheroughly100basicelementsofwhichallmatteriscomposed,abouthalfareclassifiedasmetals.Thedistinctionbetweenametalandanonmetalisnotalwaysclear-cut.Themostbasicdefinitioncentersaroundthetypeofbondingexistingbetweentheatomsoftheelement,andaroundthecharacteristicsofcertainoftheelectronsassociatedwiththeseatoms.Inamorepracticalway,however,ametalcanbedefinedasanelementwhichhasaparticularpackageofproperties.

Metalsarecrystallinewheninthesolidstateand,withfewexceptions（e.g.mercury）,aresolidatambienttemperatures.Theyaregoodconductorsofheatandelectricityandareopaquetolight.Theyusuallyhaveacomparativelyhighdensity.Manymetalsareductile-thatis,theirshapecanbechangedpermanentlybytheapplicationofaforcewithoutbreaking.Theforcesrequiredtocausethisdeformationandthoserequiredtobreakorfractureametalarecomparativelyhigh,although,thefractureforcesisnotnearlyashighaswouldbeexpectedfromsimpleconsiderationoftheforcesrequiredtotearaparttheatomsofthemetal.

Oneofthemoresignificantofthesecharacteristicsfromourpointofviewisthatofcrystallinity.Acrystallinesolidisoneinwhichtheconstituentatomsarelocatedinaregularthree-dimensionalarrayasiftheywerelocatedatthecornersofthesquaresofathree-dimensionalchessboard.Thespacingoftheatomsinthearrayisofthesameorderasthesizeoftheatoms,theactualspacingbeingacharacteristicoftheparticularmetal.Thedirectionsoftheaxesofthearraydefinetheorientationofthecrystalinspace.Themetalscommonlyusedinengineeringpracticearecomposedofalargenumberofsuchcrystals,calledgrains.Inthemostgeneralcase,thecrystalsofthevariousgrainsarerandomlyorientedinspace.Thegrainsareeverywhereinintimatecontactwithoneanotherandjoinedtogetheronanatomicscale.Theregionatwhichtheyjoinisknownasagrainboundary.

Anabsolutelypuremetal（i.e.onecomposedofonlyonetypeofatom）hasneverbeenproduced.Engineerswouldnotbeparticularlyinterestedinsuchametalevenifitweretobeproduced,becauseitwouldbesoftandweak.Themetalsusedcommerciallyinevitablycontainsmallamountsofoneormoreforeignelements,eithermetallicornonmetallic.Theseforeignelementsmaybedetrimental,theymaybebeneficial,ortheymayhavenoinfluenceatallonaparticularproperty.Ifdisadvantageous,theforeignelementstendtobeknownasimpurities.Ifadvantageous,theytendtobeknownasalloyingelements.Alloyingelementsarecommonlyaddeddeliberatelyinsubstantialamountsinengineeringmaterials.Theresultisknownasanalloy.

Thedistinctionbetweenthedescriptors“metal”and“alloy”isnotclear-cut.Theterm“metal”maybeusedtoencompassbothacommerciallypuremetalanditsalloys.Perhapsitcanbesaidthatthemoredeliberatelyanalloyingadditionhasbeenmadeandthelargertheamountoftheaddition,themorelikelyitisthattheproductwillspecificallybecalledanalloy.Inanyevent,thechemicalcompositionofametaloranalloymustbeknownandcontrolledwithincertainlimitsifconsistentperformanceistobeachievedinservice.Thuschemicalcompositionhastobetakenintoaccountwhendevelopinganunderstandingofthefactorswhichdeterminethepropertiesofmetalsandtheiralloys.

Ofthe50orsometallicelements,onlyafewareproducedandusedinlargequantitiesinengineeringpractice.Themostimportantbyfarisiron,onwhicharebasedtheubiquitoussteelsandcastirons（basicallyalloysofironandcarbon）.Theyaccountforabout98%byweightofallmetalsproduced.Nextinimportanceforstructuraluses（thatis,forstructuresthatareexpectedtocarryloads）arealuminum,copper,nickel,andtitanium.Aluminumaccountsforabout0.8%byweightofallmetalsproduced,andcopperabout0.7%,leavingonly0.5%forallothermetals.Asmightbeexpected,theremaindersareallusedinratherspecialapplications.Forexample,nickelalloysareusedprincipallyincorrosion-andheat-resistantapplications,whiletitaniumisusedextensivelyintheaerospaceindustrybecauseitsalloyshavegoodcombinationsofhighstrengthandlowdensity.Bothnickelandtitaniumareusedinhigh-cost,high-qualityapplications,and,indeed,itistheirhighcostthattendstorestricttheirapplication.

Wecannotdiscussthesemoreesotericpropertieshere.Sufficeittosaythatawholecomplexofpropertiesinadditiontostructuralstrengthisrequiredofanalloybeforeitwillbeacceptedinto,andsurvivein,engineeringpractice.Itmay,forexample,havetobestrongandyethavereasonablecorrosionresistanc