材料的结构和变形材料工程外文文献翻译中英文翻译外文翻译.docx
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材料的结构和变形材料工程外文文献翻译中英文翻译外文翻译.docx
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材料的结构和变形材料工程外文文献翻译中英文翻译外文翻译
Chapter2StructureandDeformationinMaterials
2.1INTRODUCTION
2.2BONDINGINSOLIDS
2.3STRUCTUREINCRYSTALLINEMATERIALS
2.4ELASTICDEFORMATIONANDTHEORETICALSTRENGTH
2.5INELASTICDEFORMATION
2.6SUMARRY
OBJECTIVES
Reviewchemicalbondingcrystalstructureinsolidmaterialsatabasiclevel,andrelatethesetodifferencesinmechanicalbehavioramongvariousclassesofmaterials.
Understandthephysicalbasisofelasticdeformation,andemploythisestimatethetheoreticalstrengthofsolidsduetotheirchemicalbonding.Understandthebasicmechanismsofinelasticdeformationduetoplasticityandcreep.
Learnwhyactualstrengthsofmaterialsfallfarbelowthetheoreticalstrengthtobreakchemicalbonds.
2.1INTRODUTION
Awidevarietyofmaterialsareusedinapplicationswhereresistancetomechanicalloadingisnecessary.Thesearecollectivelycalledengineeringmaterialsandcanbebroadlyclassifiedasmetalsalloys,polymers,ceramicsandglasses,andcomposites.SometypicalmembersofeachclassaregiveninTable2.1.
Differencesamongtheclassesofmaterialsastochemicalbondingandmicrostructureaffectmechanicalbehavior,givingrisetorelativeadvantagesanddisadvantagesamongtheclasses.ThesituationissummarizedbyFig.2.1.Forexample.thestrongchemicalbondinginceramicsandglassesimpartsmechanicalstrengthandstiffness(highE),andalsotemperatureandcorrosionresistance,butcausebrittlebehavior.Incontrast,manypolymersarerelativelyweaklybondedbetweenthechainmolecules,inwhichcasethematerialhaslowstrengthandstiffnessandissusceptiblecreepdeformation.
Startingfromthesizesaleofprimaryinterestinengineering,roughonemeter,thereisaspanof10ordersofmagnitudeinsize,downtothesaleoftheatom,whichisaround10-10m.ThissituationandvariousintermediatesizescalesofinterestareindicatedinFig.2.2.Atanygivensizescale,anunderstandingofthebehaviorcanbesoughtbylookingatwhathappensatasmallerscale;Thebehaviorofamachine,vehicle,orstructureisexplainedbythebehaviorofitscomponentparts,andthebehaviorofthesecaninturnbeexplainedbytheuseofsmall(10-1to10-2m)testspecimens,andthematerials.Similarly,themacroscopicbehaviorofthematerialisexplainedbythebehaviorofcrystalgrains,defectsincrystals,polymerchains,andothermicrostructurefeaturesthatexistinsizerangeof10-3to10-9m.Thus,knowledgeofbehaviorovertheentirerangeofsizefrom1mdownto10-10mcontributestounderstandingandpredictingtheperformanceofmachines,vehicles,andstructures.
Thischapterreviewsomeofthefundamentalsneededtounderstandmechanicalbehaviorofmaterials.WewillstartatthelowerendofthesizescaleinFig.2.2andprogressupward.Theindividualtopicsincludechemicalbonding,crystalstructures,defectsincrystals,andthephysicalcausesofelastic,plastic,andcreepdeformation.Thenextchapterwillthenapplytheseconceptsindiscussingeachoftheclassesofengineeringmaterialsinmoredetails.
2.2BONDINGINSOLIDS
Theseareseveraltypesofchemicalbondsthatholdatomsandmoleculestogetherinsolids.Threetypesofbonds-ionic,covalent,andmetallic-arecollectivelytermedprimarybonds,Primarybondsarestrongandstiffanddonoteasilymeltwithincreasingtemperature.Theyareresponsibleforthebondingofmetalsandceramics,andtheyprovidetherelaxinghighelasticmodules(E)inthesematerials.VanderWaalsandhydrogenbonds,whicharerelativelyweak,arecalledsecondarybonds.Theseareimportantindeterminingthebehaviorofliquidsandasbondsbetweenthecarbon-chainmoleculesinpolymers.
2.2.1PrimaryChemicalBonds
ThethreetypesofprimarybondsareillustratedinFig.2.3.Ionicbondinginvolvesthetransferofoneormoreelectionsbetweenatomsofdifferenttypes.Notesthattheoutershellofelectronssurroundinganatomisstableifitcontainseightelectrons(exceptthatthestablenumberistwoorthesingleshellofhydrogenorhelium),Hence,anatomofthemetalsodium,withonlyoneelectroninitsoutershell,candonateanelectrontoanatomofchlorine,whichhasanoutershellwithsevenelectrons.Afterthereaction,thesodiumatomhasanemptyoutershellandthechlorineatomhasastableoutershellofeightelections.Theatomsbecomechargedions,suchasMa+andCl-,whichattractoneanotherandformachemicalbondduetotheiroppositeelectrostaticcharges.Acollectionofsuchchargedions,equalnumbersofeachinthiscase,formsanelectricallyneutralsolidarrangementintoaregularcrystallinearray,asshowninFig.2.4.
Thenumberofelectronstransferredmaydifferfromone.Forexample,inthesaltMgCl2andinthatintheoxideMgO,twoelectronsaretransferredfromanMg2+ion.Electronsinthenext-to-lastshellmayalsobetransferred.Forexample,ironhastwooutershellelectrons,butmayfromeitherFe2+orFe3+ions.Manycommonsalts,oxides,andothersolidshavebondsthataremostlyorpartiallyionic.Thesematerialstendtobehardandbrittle.Covalentbondinginvolvesthesharingofelectronsandoccurswheretheoutershellarehalffullormorethanhalffull.Thesharedelectronscanbethoughtofasallowingbothatomsinvolvedtohavestableoutershellsofeight(ortwo)electrons.Forexample,twohydrogenatomseachshareanelectronwithanoxygenatomtomakewater,H2O,ortwochlorineatomsshareoneelectrontoformthediatomicmoleculesCl2.Thetightcovalentbondsmakesuchsimplemoleculesrelativelyindependentofoneanother,sothatcollectionsofthemtendtoformliquidsorgasesatambienttemperatures.
Metallicbondingisresponsiblefortheusuallysolidformofmetalsandalloys.Formetals,theoutershellofelectronsisinmostcaseslessthanhalffulleachatomdonatesitsouterelectronstoa"cloud"ofelectrons.Theseelectronsaresharedincommonbyallofthemetalatoms,whichhavebecomepositivelychargedionsasaresultofgivingupelectrons.Themetalionsarethusheldtogetherbytheirmutualattractiontotheelectroncloud.
2.2.2DiscussionofPrimaryBonds
Covalentbondshavetheproperty-notsharedbytheotherprimarybondsofbeingstronglydirectional.Thisarisesfromcovalentbondsbeingdependedonthesharingelectronswithspecificneighboringatoms,whereasionicandmetallicsolidsareheldtogetherbyelectrostaticattractioninvolvingallneighboringions.
Acontinuesarrangementofcovalentbondscanformathree-dimensionaltomakeasold.Anexampleiscarbonintheformofdiamond,inwhicheachcarbonatomssharesanelectronwithfouradjacentones,Theseatomsarearrangedatequalanglestooneantherinthree-dimensionalspace,asillustratedinFig2.5.Asaresultofthestrongdirectionalbonds,thecrystalisveryhardandstiff.Anotherimportantcontinuousarrangementofcovalentbondsisthecarbonchain.Forexample,inthegasethylene,C2H4,eachmoleculeisformedbycovalentbondsasshowninFig2.6.However,ifthedoublebondbetweenthecarbonatomsisreplacedbyasinglebondtoeachoftwoadjacentcarbonatoms,thenalongchain,moleculecanform.Theresultisthepolymercalledpolyethylene.
Manysolids,suchasSiO2andotherceramicshavechemicalbondsthathaveamixedionic-covalentcharacter.TheexamplesgivenpreviouslyofNaClforionicbondinganddiamondforcovalentbondingdorepresentcasesofnearlypurebondingofthesetypes,butmixedbondingismorecommon.
Metalsofmorethanonetypemaybemeltedtogethertoformanalloy.Metallicbondingisthedominanttypeinsuchcases.However,intermetallic,compoundsmayfromwithalloys,oftenashardparticles.Thesecompoundshaveadefinechemicalformula,suchasTiAl3orMg2Ni,andtheirbondingisgenerallyacombinationofthemetallicandionicorcovalenttypes.
2.2.3SecondaryBonds
Secondarybondsoccurduetothepresenceofanelectrostaticdipole,whichcanbeinducedbyaprimarybond.Forexample,inwater,thesideofahydrogenatomawayfromthecovalentbondtotheoxygenatomhasapositivecharge,duetothesoleelectronbeingpredominantlyonthesidetowardtheoxygenatom.Conservationofchargeovertheentiremoleculethenrequiresanegativechargemolecules,asillustratedinFig.2.7.
Suchbonds,termedpermanentdipolebonds,occurbetweenvariousmolecules.Theyarerelativelyweak,butareneverthelesssometimessufficienttobindmaterialsintosolids,watericebeinganexample.Wherethesecondarybondinvolveshydrogenasinthecaseofwater,itisstrongerthanotherdipolebondsandiscalledahydrogenbond.
VanderWaalsbondsarisefromthefluctuatingpositionsofelectronsrelativetoanatom'snucleus.Theunevendistributionofelectricchargethatthusoccurscausesaweakattractionbetweenatomsormolecules,Thistypeofbondcanalsobecalledafluctuatingdipole-distinguishedfromapermanentdipolebondbecausethedipoleisnotfixedindirectionasitisinawatermolecule.Bondsofthistypeallowtheinertgasestoformsolidsatlowtemperature.
Inpolymers,covalentbondsformthechainmoleculesandattachhydrogenandotheratomstothecarbonbackbone.Hydrogenbondsandothersecondarybondsoccurbetweenthechainmoleculesandtendtopreventthemfromslidingpastoneanother.ThisisillustratedinFig.2.8forpolyvinylchlorine.Therelativeweaknessofthesecondarybondsaccountsforthelowmeltingtemperatures,andthelowstrengthsandstiffnessofthesematerials.
第2章材料的结构和变形
2.1说明
2.2晶体的结合能
2.3晶体材料的结构
2.4弹性形变和理论强度
2.5非弹性形变
2.6总结
目标
回顾固体材料间晶体结构的结合能在同一基本能级,并与这些差异力作用在不同类型的材料。
理解弹性形变的物理基础,由于其化学成键所以使用这种方法估计
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