材料科学基础英文版课件_(11)

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1、ChapterThreeStructuresofMetalsandCeramicsLecturer:ShenhuaSONGUnitcellsMetalliccrystalstructuresCeramiccrystalstructuresCrystalsystemsCrystallographicdirectionsandplanesUnitcellsSmallrepeatatomgroupsinacrystalstructureRepresentationofaunitcell:(a)hard-spheremodel;(b)r

2、educedhard-spheremodel;(c)anaggregateofmanyatomsMetalliccrystalstructures(1)Threemajortypes:1)Face-centeredcubic(FCC)crystalstructures2)Body-centeredcubic(BCC)crystalstructures3)Hexagonalclose-packed(HCP)crystalstructuresMetalliccrystalstructures(2)1)Face-centeredcub

3、ic(FCC)crystalstructure2RaEachunitcellcontains4atomsMetalliccrystalstructures(3)Twootherfeatures:coordinationnumber(CN)andatomicpackingfactor(APF)CN:thenumberofnearest-neighbouratoms–12forFCCAPF:thefractionofatom-occupiedvolumeinaunitcellAPF=(atom-occupiedvolumeinaun

4、itcell)/(unitcellvolume)ForFCCcrystals:4atomsinaunitcellatom-occupiedvolume=4(4/3)(R3)=4(4/3)(3.14(a2/4)3=0.74a3unitcellvolume=a3APF=atom-occupiedvolume/unitcellvolume=0.74Somecommonmetals:Al,Ni,Au,Cu,andMnMetalliccrystalstructures(4)IntersticesinFCCstructures:

5、octahedralintersticesandtetrahedralintersticesRo=(a–2R)/2=(22R-2R)/2=0.414R(1/2,1/2,1/2)Rt=3a/4–R=(3/4)22R)–R=0.224R(1/4,1/4,1/4)Diagonallength:3aaMetalliccrystalstructures(5)2)Body-centeredcubic(BCC)crystalstructureEachunitcellcontains2atomsCN=8APF=2(4/3)(R3

6、)/a3=2(4/3)(3.14(a3/4)3/a3=0.68Metalliccrystalstructures(6)IntersticesinBCCstructures:octahedralintersticesandtetrahedralintersticesRoz=(a–2R)/2=(4R/3-2R)/2=0.155RRoxy=(a2–2R)/2=((4R/3)2-2R)/2=0.633RRt=a5/4-R=(4R/3)5/4–R=(5/3–1)R=0.291Ra=4R/3Metalliccry

7、stalstructures(7)3)Hexagonalclose-packed(HCP)crystalstructureEachunitcellcontains6atoms(12(1/6)+2(1/2)+3=6)2R=aR=a/2CN=12c/a=?APF=?Metalliccrystalstructures(8)c/2xfgex=a3/3c/2aegfa2=(c/2)2+(a3/3)2c/a=(8/3)1/2=1.633(idealratio,actually1.58~1.89)Equilateraltriangle

8、ah=asin60o=a3/2x=2h/3feMetalliccrystalstructures(9)APF=atom-occupiedvolume/unitcellvolumeunitcellvolume=baseheight=(6(1/2)a2sin6