机械设计制造及其自动化专业英语翻译(最新整理)Word格式文档下载.docx
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机械设计制造及其自动化专业英语翻译(最新整理)Word格式文档下载.docx
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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
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
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