Introduction
Variouselementsinteracttoformasubstancewithmetalliccharacteristicscalledmetalcompounds.Thecompositionofthemetalcompoundcangenerallyberepresentedbyachemicalformula.Thelatticetypeofmetalcompoundsisdifferentfromanycomponent,andgenerallyhasacomplexlatticestructure.Itsperformancecharacteristicsarehighmeltingpoint,highhardnessandhighbrittleness.Whenmetalcompoundsappearinthealloy,thehardnessandwearresistanceofthealloycanusuallybeincreased,buttheplasticityandtoughnesswillbereduced.Metalcompoundsareimportantphasesofmanyalloys.
Organicmetalcompoundsrefertoaclassofcompoundsthathaveadirectbondbetweencarbonandmetal.Duetothecharacteristicsofdifferentmetals,thestabilityofthisbondisdifferent.Forexample,themagnesiumatomintheGrignardreagentisdirectlyconnectedtothecarbonchain,andcarbonismoreelectronegativethanmagnesium;therefore,thecarbonatomadjacenttothemagnesiumatomaccumulatesmorenegativecharges,resultinginthiscarbon-magnesiumbondVeryreactive.
Theoreticalbasisofmetalcompounds
Structure
Whentheelectroniclayerstructure,atomicradiusandcrystaltypeoftheelementsformingthealloyarequitedifferent,itiseasytoformmetalcompounds(Alsoknownasintermetalliccompounds).Thecrystaltypeofametalcompoundisdifferentfromitsgroupedmetal,forminganewphaseofitsown.Thestructuretypesofmetalcompoundalloysarerichanddiverse.Therearemorethan20,000kinds,andthelistisendless.Somestructurescanfindrelatedtypesofioniccrystalsorcovalentcrystals,andsomeareuniquestructuretypes.Forexample,theNaTlunitcellisaCsClunitcell.8timesthesuperstructure;MgCu2isanexampleoftheso-calledLavesphase;CaCu5isanexampleofalayeredstructure;Nb3Snstructureisanimportantalloysuperconductor.ThehomologouscompoundNb3Geispracticallyusedinhigh-resolutionNMR;MoAl12isanexamplewithacomplexcoordinationstructure.Therearemanytypesofmetalcompounds,andtheirlatticetypesaresimpleandcomplex.Accordingtothecharacteristicsofthecompoundstructure,theycanbedividedintothefollowingthreecategories:(1)Normalvalencecompounds:(2)Electroniccompounds;(3)Interstitialcompounds.
Compositionlaw
Thecompositionofmetalcompoundsisverycomplex,andtherearestillmanylawsthatareunknown.Twotypesoflawshavebeensummarized:oneisequivalenttometalandnon-metalThevalencecompositionofthecompound,suchasMg2SnandMg2Pb,canbeunderstoodaccordingtothe"groupvalence"oftheperiodicsystem,thatis,Mgisadivalentelement,andSnandPbaretetravalentelements.Theotheristheso-calledelectroncompoundswhosecompositionisdeterminedbytheratioofthenumberofelectronsandthenumberofatomsofthetwometals.However,thecountofthe"numberofelectrons"oftheconstituentelementsoftheelectroniccompoundisunusualandtherearedisputes,anditisgenerallyaccepted.Theruleis:the"numberofelectrons"oftheperiodicsystemVIIIelementsFe,Co,Ni,Ru,Rh,Pd,Os,IrandPtiszero,theIBgroupCu,Ag,Auis1,andtheIIBgroupZn,Cd,HgandⅡAgroupBeandMgare2,ⅢAgroupAl,In,Gaare3,IVAgroupSi,Ge,Sn,Pbare4,etc.Therearethreebasictypesoftheratioofthenumberofelectronstothenumberofatoms:3:2,21:13and7:4,whichcanbeunderstoodasCuZn,Ag3Al,Cu9Al4,Cu3Snandothermetalcompounds.Eachoftheabovethreetypesofelectroniccompoundshasaspecificstructureandbecomesβ,γ,andεphases,respectively.Forexample,Cu5Zn8isa21:13typeelectroniccompound,whichisaverylargecubicunitcellcontaining52atomsandiscalledγ-brass.Typestructure.Manyelectroniccompoundswithachemicalformulaofamultipleof13atomshavethisstructure,suchasFe5Zn21,Cu31Sn8andsoon.Theγandεphasestructuresareomitted.
Heattreatmentofmetalcompounds
Thepurposeofheattreatmentistoobtainanorderedstructuretoimproveitsplasticityandtoughness.Therearemainlythefollowingprocessingmethods.
(1)Theintermetalliccompoundintheas-caststateofhightemperaturehomogenizationannealinggenerallyhascompositionsegregationandcastingstress.Thetreatmentlaysagoodfoundation,andthistreatmentgenerallylastsformorethantenhoursatatemperatureabove1000°C.
(2)Inordertoincreasetheroomtemperaturetoughnessofmetalcompounds,oilquenchingisoftenheatedtothecrystalformtransitionorphasetransitiontemperature,andthenplacedinoilforquenchingtreatment,suchasforFe-AlintermetalliccompoundsThetypicaltreatmentprocessis:heatingto1000°C,holdingfor5hours,andthenplacingitin700°Coilforcooling,seereferencefordetails.
(3)Thermomechanicalheattreatmentiscurrentlythemosteffectivetreatmentmethodforincreasingthetoughnessofmetalcompounds.Itismainlythroughforging,rolling,extrusionandotherthermaldeformationtreatmentstomakethestructureConducivetothedirectionchangeofincreasingtoughness,seetheliteraturefortypicalprocesses.
Theroomtemperaturebrittlenessofmetalcompoundshasalwaysbeenaproblemthatplaguestheapplicationofsuchmaterials.Foralloysofthesamecomposition,duetodifferentprocessingmethodsandchangesinprocessparameters,thefinalmicrostructureandmechanicalpropertiesmaydiffergreatly.Thermomechanicaltreatmentprocessesarewidelyusedinthepreparationofintermetalliccompounds.ThismethodcanbeusedtoobtaingeneralAproductwithhighstrengthandhighplasticitythatcannotbeachievedbyprocessing.
DevelopmentandApplication
Inmetalmaterials,metalcompoundshavebeenusedasthestrengtheningphaseofthemetalmatrix.Peopleachievethepurposeofcontrollingtheperformanceofthematrixmaterialbychangingthetype,distribution,precipitationstateandrelativecontentofintermetalliccompounds.Duetoitsmanyuniqueproperties,intermetalliccompoundsthemselves,asanewtypeofmaterial,areincreasinglybeingresearchedanddeveloped.Becauseofitshightemperatureresistanceandcorrosionresistance,intermetalliccompoundshavebecomeimportantstructuralmaterialsinmanyindustrialsectorssuchasaviation,aerospace,transportation,chemicalindustry,machinery,etc.;duetotheirspecialphysicalpropertiessuchassound,light,electricity,andmagnetism,theycanbeusedassemiconductors,Magnetic,hydrogenstorage,superconductivityandotherfunctionalmaterials.Especiallyorderedintermetalliccompoundsusedashigh-temperaturestructuralmaterialshavemanygoodmechanicalproperties,oxidationresistance,corrosionresistance,andhighspecificstrength.Duetothelong-rangeorderlyarrangementoftheiratomsandtheinter-atomicmetalbondandcovalentbondCoexistencemakesitpossibletohaveboththeplasticityofmetalandthehigh-temperaturestrengthofceramics,soithasgreatapplicationprospects.
However,thebrittlenessofmetalcompoundshindersitsapplication.Untilthebeginningofthe1980s,twobreakthroughsweremadeintheresearchonthetougheningofintermetalliccompounds.0.02%～0.05%ofBtoughensthematerial,andtheroomtemperaturetensileelongationisincreasedfromnearly0to40%～50%;thesecondisthattheUnitedStatesOakRidgeNationalLaboratoryfoundanon-plastichexagonalD019structureofCo3V,NiandFeareusedtoreplacepartofCo,whichcanbetransformedintoaface-centeredcubicL12structure,andbrittlematerialsbecomematerialswithgoodplasticity.Thesedevelopmentshaveenabledpeopletoseethehopeandprospectsofintermetallichigh-temperaturestructuralmaterials,andsetoffaresearchupsurgeintheworld.
Currently,asanorderedintermetalliccompoundofhigh-temperaturestructuralmaterials,themajorresearchesathomeandabroadthathavemadesignificantprogressaremainlyNi-Al,Ti-AlandFe-AlthreesystemsA3BandABtypealuminides.
Thepropertiesofmetalcompoundalloysandthemetalsthatcomposeitareoftenquitedifferent.Withthedevelopmentofnewtechnologiesandnewprocesses,avarietyofnewfunctionalmaterialsandstructuralmaterialshavebeendeveloped.Amongthem,themosttypicalmetallicfunctionalmaterialsareamorphousmetals,shapememoryalloys,dampingalloys,superconductingmaterials,andhydrogenstorage.Alloys,ultra-finepowders,etc.;newstructuralmaterialsincludesuperplasticalloys,ultra-hightemperaturealloys,etc.Thesemetalmaterialshaveexcellentproperties,awiderangeofuses,andhavebroadapplicationprospects.