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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.Themetals在人类社会的发展中，金属的应用起着关键性的作用。构成物质的大约100种基本元素中，大约有一半为金属。金属和非金属之间的区别不是特别明显。最基本的定义集中在元素原子间存在的连接形式和与这些原子相关联的电子的某些特性。然而，在实际应用中，可以将具有某些特性集合金属定义为某种元素。除了少数例外金属在常温下是固态的。它们是热和电的良导体，不透光。它们往往具有较高的密度。许多金属具有延展性，也就是说，在不被破坏的情况下它们的形状在外力的作用下可以发生变化。引起永久变形所需的力和最终使金属断裂所需的力相当大，尽管发生断裂所需的力远没有像所预期的撕开金属原子所需的力那么大。从我们的观点来看，在所有的特性中结晶性是最重要的。结晶体是这样一种结构，组成它的原子定位在规则的三维排列中，仿佛位于三维棋盘的方格的角上。原子间距随着原子大小呈规律性变化，原子间距是金属的一种特性。三维排列的轴线决定了晶体在空间中的方向。在工程实践中应用的金属由大量的晶体组成，这些晶体称之为晶粒。在大多数情况下，晶粒在空间中是自由排列的。在原子范围内，晶粒之间相互接触紧密结合。晶粒之间连接区域被称为晶界。commonlyusedinengineeringpracticearecomposedofalargenumberofsuchcrystals,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,onlyafewareproducedandusedinlargequantitiesin绝对纯净的金属从来也没有被生产出来过。即使绝对纯净的金属可以生产出来，工程师们对它们也并不会特别感兴趣，因为它们很柔软、脆弱。实际应用中的金属往往都包含着一定数量的一种或多种外来金属或非金属元素，这些外来元素可能是有害的也可能是有益的或者它们对某种特定的属性没有影响。如果是有害的，这些外来元素被认为是杂质。如果是有益的，它们被认为是合金元素。在工程材料中往往被特意地加入一定数量的合金元素。得到的物质被叫做合金。金属和合金区别不大。金属这个词可以包括工业用纯金属和它的合金。也许可以这样说，合金元素越故意的被添加，被添加的合金元素的量越大，那么生产出来的产品越倾向于被称之为合金。不管怎样，如果想使一种金属或合金在使用中表现出稳定一致的特性，在其中添加何种化学成分，它的量多大都应该在控制范围之内。因此，当想了解决定金属和合金性质的因素时，应充分考虑它们的化学组成。在50种左右的金属元素里，工程实践中只有少数金属被大量生产和使用。到目前为engineeringpractice.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,havetob