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1、工程材料及其成形技术基础（Engineeringmaterialsandtheirformingtechnologyfoundation）Chapter1Structureandpropertiesofengineeringmaterials1-1explainthedifferenceandrelationofmetalbond,ionicbondandcovalentbondAnswer:thedifference:(1).Themetallicbondiselectrostaticattractionbetweenthemetalion
2、sandfreeelectronsbetweentheformationofacombinationofmetal,itsroleistomake[1].metalexhibitedgoodconductivity;themetal[2].hasgoodthermalconductivity;[3].metalhasatemperaturecoefficientofresistanceis;[4].metalshowsgoodplasticity;[5].metalluster.(2).Theionicbond:whennonmetalato
3、mcontactelectropositivemetalatomandelectronegativity,theformerlosttheoutermostelectronsintopositiveions,whichhavebecomeelectronicnegativeions,positiveandnegativeionsduetoelectrostaticattractionandinteractionbetweensyntheticcompounds,thisinteractioniscalledionicbond.Theionic
4、bondbindingforcestrong,therefore,theioniccompoundmeltingpoint,boilingpoint,highhardness,thermalexpansioncoefficientisverysmall.(3).Whenthecovalentinteractionoftwoatomswiththesameortwodifferentkindsofatoms,atomstoformcommonvalenceelectronstothecombination,thiscombinationisca
5、lledacovalentbond.Thecovalentbondisverystrongcovalentcrystalhasahighmeltingpoint,hardness,nottheconductor.Contact:theionicbondandcovalentbondstrengtharestrongest,andthemetalbondisnext.Infact,onlyonebondmaterialisrare.Mostmaterialsareoftenacombinationofseveralbonds,andabondi
6、ngbondisthemaincomponentWhatisthedifferencebetween1-4singlecrystalandmulticrystal?Whysinglecrystalhasanisotropy,whilepolycrystalhasnoanisotropy?Answer:thedifference:thecrystallographicorientationofidenticalcrystalcalled"singlecrystal",inasinglecrystal,allcellsshowedthesameo
7、rientation,sothesinglecrystalisanisotropic.ThecrystalstructurecomposedofmultiplegrainsiscalledpolycrystalReason:inthesinglecrystallattice,allshowedthesameorientation,sothesinglecrystalisanisotropic.Butbecauseeachinthecrystalgrainorientationdisorder,theisotropic,thisphenomen
8、oniscalled"pseudoastatism".1-5whatisthepointdefect?What'stheeffectontheperformance
