机械制造工程开卷习题答案

QUESTIONS1.1Explainthedifferencebetweenaunitcellandasinglecrystal.Answer(frompage#40):TheSmallestgroupofatomsshowingthecharacteristiclatticestructureofaparticularmetalisknownasaUnitCell.Itisthebuildingblockofacrystal,andasinglecrystalcanhavemanyunitcells.1.2Intablesoncrystalstructures,ironislistedashavingbothabccandafeestructure.Why?Answer(frompage#41):Samemetalmayformdifferentstructures,becauseofalowerenergyrequirementatthattemperature.Forexample,ironformsabccstructure(alphairon)below912°C(1674°F)andabove1394°C(2541°F),butifformsanfccstructure(gammairon)between912°Cand1394°C.1.3a.Defineanisotropy,andwhatmaterialscanyouthinkofotherthanmetalsthatexhibitanisotropicbehavior?Answer(frompage#42):Becausetheb/aratioisdifferentfordifferentdirectionswithinthecrystal,asinglecrystalhasdifferentpropertieswhentestedindifferentdirections.Wesaythatasinglecrystalisanisotropic.Where“a”isthespacingoftheatomicplanesand“b”isinverselyproportionaltotheatomicdensityintheatomicplanes.Acommonexampleofanisotropyiswovencloth,whichstretchesdifferentlywhenwepullitindifferentdirections,orplywood,whichismuchstrongerintheplanardirectionthanalongitsthicknessdirection(itsplitseasily).1.4Whateffectsdoesrecrystallizationhaveonpropertiesofmetals?Answer(frompage#50):Recrystallizationdecreasesthedensityofdislocations,lowersthestrength,andraisestheductilityofthemetal.1.5Whatisstrainhardening?Andwhateffectsdoesithaveonthepropertiesofmetals?Answer(frompage#45):Theeffecto9fanincreaseinshearstressthatcausesanincreaseintheoverallstrengthofthemetalisknownastheworkhardeningorstrainhardening.Theeffectonthepropertiesofmetalsisthegreaterthedeformation,thegreaterthenumberofentanglements,henceanincreaseinthemetal’sstrength.1.6Explainwhatismeantbystructure-sensitiveandstructure-insensitivepropertiesofmetals.Answer(frompage#43,44):Mechanicalandelectricalpropertiesofmetals,suchasyield,fracturestrength,andelectricalconductivity,areadverselyaffectedbythesedefects;theseareknownasstructure-sensitiveproperties.Ontheotherhand,theirphysicalandchemicalpropertiessuchasmeltingpoint,specificheat,coefficientofthermalexpansion,andelasticconstants(e.g.,modulusofelasticityandmodulusofrigidity)arenotsensitivetothesedefects;theseareknownasstructure-insensitiveproperties.1.7Makealistofeachofthemajorkindsofimperfectioninthecrystalstructureofmetals,anddescribethem.Answer(frompage#4):Linedefects,calleddislocationsPointdefects,suchasvacancy(missingatom),andinterstitialatom(extraatominthelattice),oranimpurity(foreignatom)thathasreplacedtheatomofthepuremetal.Volumeorbulkimperfections,suchasvoidsorinclusions(nonmetallicelementssuchasoxides,sulfides,andsilicates);Planarimperfection,suchasgrainboundaries.1.8Whatinfluencedoesgrainsizehaveonthemechanicalpropertiesofmetals?Answer(frompage#46):Grainsizesignificantlyinfluencesthemechanicalpropertiesofmetals.Atroomtemperature,alargegrainsizeisgenerallyassociatedwithlowstrength,lowhardness,andlowductility.Largegrains,particularlyinsheetmetals,alsocausearoughsurfaceappearanceafterthematerialhasbeenstretched.1.9Whatistherelationshipbetweenthenucleationrateandthenumberofgrainsperunitvolumeofametal?Answer(frompage#45):Thenumberandsizeofthegrainsdevelopedinaunitvolumeofthemetaldependsontherateatwhichnucleation(theinitialstageofformationofcrystals)takeplace.1.10Whatisaslipsystem,andwhatisitssignificance?Answer(frompage#42):Thecombinationofaslipplaneanditsdirectionofslipareknownasaslipsystem.Ingeneral,metalswithslipsystemsof5oraboveareductile,whereasthosewithslipsystemsbelow5arenot.ItalsodeterminestheprobabilityofShearstress.1.11Explainthedifferencebetweenrecoveryandrecrystallization.Answer(frompage#49):Recovery,whichoccursatacertaintemperaturerangebelowtherecrystallizationtemperatureofthemetal,thestressesinthehighlydeformedregionsarerelived.Subgrainboundariesbegintoform(aprocesscalledpolygonization),withnoappreciablechangeinmechanicalpropertiessuchashardnessandstrength.Recrystallizationistheprocessinwhich,atacertaintemperaturerange,newequiaxedandstrain-freegrainsareformed,replacingtheoldergrains,incalledrecrystallization.1.12Whatishotshortness,andwhatisitssignificance?Answer(frompage#47):Hotshortnessiscausedbyelocalmeltingofaconstituentoranimpurityinthegrainboundaryatatemperaturebelowthemeltingpointofthemetalitself.Whensubjectedtoplasticdeformationatelevatedtemperatures(hot-working),thepieceofmetalcrumblesanddisintegratesalongthegrainboundaries.Examplesareantimonyincopper,leadedsteels,andleadedbrass.1.13Explainthedifferencesbetweencold-,warm-,andhot-workingofmetals.Answer(frompage#51):Cold-workingreferstoplasticdeformationthatisusually,butnotnecessarily,carriedoutatroomtemperature.Whenthedeformationiscarriedoutabovetherecrystallizationtemperature,itiscalledhot-working.Asthenameimplies,warm-workingiscarriedoutatintermediatetemperatures.Thuswarmworkingisacompromisebetweencold-andhot-working.1.14Describewhattheorangepeeleffectis.Answer(frompage#50,51):Largegrainsproducesaroughsurfaceappearanceonthesheetmetals,calledorangepeel,whentheyarestretchedtof