Solidification-of-metal-复习题.ppt

Solidificationofmetal金属的凝固 REVIEW 1BasicSolidificationConcepts GlossaryHomogeneous h m d i ni s 自发的Heterogeneous het r u d i nj s 非自发的Equiaxed i kwi kst 等轴的Columnar k l mn 柱状的Nucleus晶核Nuclei晶核复数Nucleation形核Nucleationagent形核剂Embryo embri u 晶芽Insoluble不溶解的Planar平面的Cellular seljul 胞状的Dentritic树枝状的Morphology m f l d i 形态Radii半径Substrate基材 母材Temperaturegradient温度梯度Freeenergy自由能Epitaxial epi t ksi l 联生的Epitaxialgrowth联生结晶Competitive k m petitiv 竞争的Competitivegrowth竞争生长Constitutionalundercooling成分过冷 Thetwomainmechanismsbywhichthenucleationofsoledparticlesinliquidmetaloccursarehomogeneousnucleationandheterogeneousnucleation HomogeneousNucleationHomogeneousnucleationisconsideredfirstsinceitisthesimplestcaseofnucleation Homogeneousnucleationinaliquidmeltoccurswhenthemetalitselfprovidestheatomsneededtoformanuclei 1 2Nucleation HomogeneousNucleation Homogeneousnucleationrequiresaconsiderableamountofundercooling whichmaybeasmuchasseveralhundreddegreesCelsiusforsomemetals seeTable1 1 Table1 1Valuesforthefreezingtemperature heatoffusion surfaceenergy andmaximumundercoolingforselectedmetal 1 2Nucleation HomogeneousNucleation Heterogeneousnucleationisnucleationthatoccursinaliquidonthesurfacesofitscontainer insolubleimpurities andotherstructuralmaterialthatlowerthecriticalfreeenergyrequiredtoformastablenucleus Sincelargeamountsofundercoolingdonotoccurduringindustrialcastingoperationsandusuallyrangebetween0 1and10 thenucleationmustbeheterogeneousandnothomogeneous 1 2Nucleation HeterogeneousNucleation 2 2EPITAXIALGROWTHATFUSIONBOUNDARY 2 2 1NucleationTheory Figure2 6Epitaxialgrowthofweldmetalnearfusionline Suchagrowthinitiationprocess shownschematicallyinFigure2 6 iscalledepitaxialgrowthorepitaxialnucleation Thearrowineachgrainindicatesitsdirection 2 3COMPETITIVEGROWTHINBULKFUSIONZONE Figure2 8Competitivegrowthinbulkfusionzone Therefore duringsolidificationgrainswiththeireasy growthdirectionessentiallyperpendiculartothepoolboundarywillgrowmoreeasilyandcrowdoutthoselessfavorablyorientedgrains ass