最后机械设计制造及其自动化专业英语翻译.docx

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1、最后机械设计制造及其自动化专业英语翻译Unit1Metals在人类社会的发展i起着关键性的作用「构成物丿基本元素中，大约有一半为金属之间的区别不是特别明义集中在元素原子间存在的些原子相关联的电子的某些实际应用中.可以将具有某左义为某种元素C除了少数例外金属的=它们是热和电的良导体往往具有较高的密度。许多彳也就是说.在不被破坏的情在外力的作用卜町以发生变形所需的力和最终使金属断大，尽管发生断裂所需的力的撕开金属原子所需的力那f从我们的观点来看.在晶性是最重要的。结晶体是药成&的原子宗Q亦捋VI的Theuseofmetalshasalway

2、sbeenakeyfactorinthedevelopmentofthesocialsystemsofman.Oftheroughly100basicelementsofwhichallmatteriscomposed,abouthalfareclassifiedasmetals.Thedistinctionbetweenametalandanonmetalisnotalwaysclear-cut.Themostbasicdefinitioncentersaroundthet)peofbondingexistingbetweenthe

3、atomsoftheelement,andaroundthecharacteristicsofcertainoftheelectronsassociatedwiththeseatoms.Inamorepracticalway,however,amelalcanbedefinedasanelementwhichhasaparticularpackageofproperties.Metalsarecrystallinewheninthesolidstateand.withfewexceptions(e.g.mercury),aresolid

4、atambienttemperatures.Theyaregoodconductorsofheatandelectricityandareopaquetolight.TheyusuallyhaveacomparativelyhighdensityManymetalsareductile-hatis.theirshapecanbechangedpermanentlybytheapplicationofaforcewithoutbreaking.Theforcesrequiredtocausethisdefonnaiionandthose

5、requiredlobwakfractuream^talarecomparativelyhigh,although,thefractureforcesisnotnearlyashighaswouldbe巳xpectEdfromsimpleconsiderationoftheforcesrequiredtotearapartthealomsofthemelaLOneofthemoresignificantofthesecharacteristicsfromourpointofviewisthatofcrvstallinitv.Acryst

6、allinesolidisoneinwhichcalledgrains.Inthemostgeneralcase・thecrystalsofthevariousgrainsarerandomlyorientedinspace・Thegrainsareeverj^hereinintinoatecontactwithoneanotherandjoinedtogetheronanatomicscale・Theregionatwhichtheyjoinisknownasagrainboundary.Anabsolutelypuremetal(i

7、.e・onecomposedofonlyonetypeofatom)hasneverbeenproduced・Engineerswouldnotbeparticularlyinterestedinsuchametalevenifitweretobeproduced,becauseitwouldbesoftandweak・Themetalsusedcommerciallyinevitablycontainsmallamountsofoneormoreforeignelements・eithermetallicornonmetallie・T

8、heseforeignelementsmaybedetrimentaltheymaybebeneficial,ortheymayhavenoinfluenceatallonaparticularproper