李克

新东方在线推荐：2018年考研一次顺利提分课程！！一科不过，全科免费Theintensiveworkofmaterialsscientistsandsolid-statephysicistshasgivenrisetoaclassofsolidsknownasamorphousmetallicalloysorglassymetals.Thereisagrowinginterestamongtheoreticalandappliedresearchersalikeinthestructuralpropertiesofthesematerials.Whenamoltenmetalormetallicalloyiscooledtoasolid,acrystallinestructureisformedthatdependsontheparticularalloycomposition.Incontrast,moltennonmetallicglass-formingmaterialswhencooleddonotassumeacrystallinestructure,butinsteadretainastructuresomewhatlikethatoftheliquid—anamorphousstructure.Atroomtemperaturethenaturallong-termtendencyforbothtypesofmaterialsistoassumethecrystallinestructure.Thedifferencebetweenthetwoisinthekineticsorrateofformationofthecrystallinestructurewhichiscontrolledbyfactorssuchasthenatureofthechemicalbondingandtheeasewithwhichatomsmoverelativetoeachother.Thus,inmetals,thekineticsfavorsrapidformationofacrystallinesstructurewhereasinnonmetallicglassestherateofformationissoslowthatalmostanycoolingrateissufficienttoresultinanamorphousstructure.Forglassymetalstobeformed,themoltenmetalmustbecooledextremelyrapidlysothatcrystallizationissuppressed.Thestructureofglassymetalsisthoughttobesimilartothatofliquidmetals.OneofthefirstattemptstomodelthestructureofaliquidwasthatbythelateJ.D.BernaloftheUniversityofLondon,whopackedhardspheresintoarubbervesselinsuchawayastoobtainthemaximumpossibledensity.Theresultingdense,random-packedstructurewasthebasisformanyattemptstomodelthestructureofglassymetals.CalculationsofthedensityofalloysbasedonBernal-typemodelsofthealloysmetalcomponentagreefairlywellwiththeexperimentallydeterminedvaluesfrommeasurementsonalloysconsistingofanoblemetaltogetherwithametalloidsuchasalloysofpalladiumandsiliconoralloysconsistingofironphosphors,andcarbon,althoughsmalldiscrepanciesremained.OnedifferencebetweenrealalloysandthehardspheresareainBernalmodelsisthatthecomponentsofanalloyhavedifferentsize,sothatmodelsbasedontwosizesofspheresaremoreappropriateforabinaryalloyforexample.Thesmallermetalloidatomsofthealloysmightfitintoholesinthedenserandom-packedstructureofthelargermetalatoms.Oneofthemostpromisingpropertiesofglassymetalsistheirhighstrengthcombinedwithhighmalleability.Inusualmaterials,onefindsaninverserelationbetweenthetwoproperties,whereasformanypracticalapplicationssimultaneouspresenceofbothpropertiesisdesirable.Oneresialobstacletopracticalapplicationsthatislikelytobeovercomeisthefactthatglassymetalswillcrystallizeatrelativelylowtemperatureswhenheatedslightly.1.Theauthorisprimarilyconcernedwithdiscussing[A]crystallinesolidsandtheirbehavioratdifferenttemperatures.[B]moltenmaterialsandthekineticsoftheformationoftheircrystallinestructure.[C]glassymetalsandtheirstructuralcharacteristics.[D]metallicalloysandproblemsindeterminingtheirdensity.2.Theauthor’sattitudetowardtheprospectsfortheeconomicutilizationofglassymetalsisoneof[A]disinterest.[B]impatience.[C]optimism.[D]apprehension.3.Accordingtothetext,whichofthefollowingdeterminesthecrystallinestructureofametallicalloy?[A]Atwhatratethemoltenalloyiscooled.[B]Howrapidtherateofformationofthecrystallinephaseis.[C]Howthedifferent-sizedatomsfitintoadenserandom-packedstructure.[D]Whatthealloyconsistsofandinwhatratios.4.Whichofthefollowingbestdescribestherelationshipbetweenthestructureofliquidmetalsandthestructureofglassymetals,asitispresentedinthetext?[A]Thelatterisanillustrativeexampleoftheformer.[B]Thelatterisalarge-scaleversionoftheformer.[C]Theformerisastructuralelaborationofthelatter.[D]Theformerisafairapproximationofthelatter.5.Itcanbeinferredfromthetextthat,theoretically,moltennonmetallicglassesassumeacrystallinestructureratherthananamorphousstructureonlyiftheyarecooled[A]veryevenly,regardlessoftherate.[B]rapidly,followedbygentleheating.[C]veryslowly.[D]toroomtemperature. 更多

考研网快讯，据广西大学研究生院消息，2014年广西大学农业机械化工程考研报录比还未公布。为方便网友及时了解广西大学农业机械化工程的报录比信息，故开启了关于广西大学农业机械化工程考研报录比的帖子，网友可以进入原帖参与讨论。        同时也可以收藏2014年各院校考研报录比统计，考研网会第一时间及时发布关于广西大学报录比的信息。      更多