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1、机械制造基础考试重点（Keypointsofmechanicalmanufacturingfoundationexamination）Crystalstructureofcommonmetals:BCClattice,closepackedcubiclattice,sixsquarelattice.Theconceptofsinglecrystal:thephysical,chemicalandmechanicalpropertiesofasinglecrystalindifferentdirectionsarediff
2、erent,thatis,anisotropy.Theactualmetalispolycrystallinestructure,soitshowsisotropicpropertiesmacroscopically.Defectsincrystals:thesecrystaldefectscanbeclassifiedintothreecategories:pointdefects,linedefectsandsurfacedefects.Thecoolingcurveandcoolingphenomenonofpur
3、emetals:theprocessthatmetalschangefromliquidtosolidcrystalsiscalledcrystallization.Thecoolingprocessofpuremetalfromliquidtosolidcanbeexpressedbythecurveofcoolingandtemperaturemeasuredintheprocessofcooling,whichiscalledthermalanalysis.Thedifferencebetweentheoretic
4、alcrystaltemperatureT0andactualcrystallizationtemperatureT1=T=T0-T1iscalledundercooling".Theundercoolingdegreeisnotaconstantvalue.Thegreaterthecoolingrateofliquidmetalis,thelowertheactualcrystallizationtemperatureTis,andthegreatertheundercoolingdegreeis,thegreate
5、rtheTvalue.Theactualmetalisalwayscrystallizedinsupercooledconditions,sosupercoolingisanecessaryconditionformetalcrystallization.CrystallizationprocessofmetalsWhentheliquidmetaliscooledtothecrystallizationtemperature,itscrystallizationprocessis:first,alotofcrystal
6、nucleus(nucleation)isproducedintheliquid,andthecrystalnucleushasbeengrowingup.Therelationshipbetweengrainsizeandmechanicalpropertiesofmetal:atroomtemperature,finegrainmetalhashigherstrength,hardness,plasticityandtoughnessthancoarsegrainmetal.Inproduction,themetho
7、dofrefininggrainisasfollows:(1)increasesupercoolingdegreeIncreasingundercoolingdegreeduringcrystallizationwillreducegrainsizeaftercrystallizationT.Modification(3)additionalvibrationAlloy:analloyconsistingoftwoormorethantwometallicelementsormetalsandnonmetals.Phas
8、e:thecompositionandstructureofalloysarethesame,andthereisadistinctinterfacebetweenphaseandalloy.Solidsolution:homogeneousphaseformedbydissolutionofalloyinsolid
