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1、Chapter3MagnesiumandBerylliumManufacturingTechnologyforAerospaceStructuralMaterialsConsideringtheirlocationontheperiodictableofelements,onewouldthinkthatbothmagnesiumandberylliumwouldplaymajorrolesasaerospacestruc-turalmaterials.However,bothoftheseextremelylightweightmetalshaveseriousl
2、imitationsthatmakethemonlyminorplayers.Magnesiumalloysusu-allycompetewithaluminumalloysforstructuralapplications.Comparedtohighstrengthaluminumalloys,magnesiumalloysarenotasstrong(tensilestrengthof20–50ksivs.40–80ksi)andhavealowermodulusofelasticity(6.5msivs.10–11msi).However,magnesium
3、issignificantlylighter(0.063vs.0100lb/in3)andthereforeitsalloysarecompetitiveonaspecificstrengthandmodulusbasis.Magnesiumalloys,withtheirhexagonalclose-packedstructure,mustusuallybeformedatelevatedtemperatures,whilealuminumcanbereadilyformedatroomtemperature.Inaddition,magnesiumalloy
4、sarenormallymoreexpensivethancomparablealuminumalloys.However,thebiggestobstacletotheuseofmagnesiumalloysistheirextremelypoorcorrosionresistance.Magnesiumoccupiesthehighestanodicpositiononthegalvanicseries,and,assuch,thereisalwaysthestrongpotentialforcorrosionasshownintheexampleofFig.3
5、.1.However,someoftheneweralloyshavemuchbettercorrosionresistancethantheolderalloys.AsshowninFig.3.2,someofthenewercastalloysapproachthecorrosionresistanceofcompetingaluminumcastingalloys.Magnesiumalloysdohaveverygooddampingcapacityandcastingshavefoundapplicationinhighvibrationenvironme
6、nts,suchashelicoptergearboxes.1Berylliumisalsoaverylightweightmetal0068lb/in3thathasgoodstrengthpropertiesandaveryhightensilemodulus(44msi).Theseproperties,combinedwithitsattractiveelectricalandthermalproperties,haveledtoitsuseinhighvalueaerospaceelectronicandguidancesystemapplicat
7、ions.However,berylliummustbeprocessedusingpowdermetallurgythatmakesitcostly.Likemagnesium,itshexagonalclose-packed(HCP)crystallinestructuregreatlyimpairsitsformability.Finally,berylliumpowderanddustaretoxic,whichFig.3.1.SeverelyCorrodedMagnesiumPart94MagnesiumandBerylliumAZ91C1100100
