材料科學(xué)基礎(chǔ)第02章2

1、材料科學(xué)基礎(chǔ)Fundamental of MaterialsLesson four.Crystal structure is the real arrangement of atom in crystals Crystal structure Space lattice + Basis or structure unit2.3 Crystal Structure and Complex Lattice+=Fe : Al = 1 : 1FeAlThe difference between space lattice and crystal structure 23 atoms / cell1.

2、BCC Example: -Fe , V, Nb, Ta, Cr, Mo, W, alkali metals n = 2 atoms/cell CN=8 The number of nearest neighbours around each atom is called Coordination Number.Typical crystal structures of metals Packing fraction To determine, The atom is looked as a hard sphere, and the nearest neighbours touch each

3、other. For BCC, Volume of atoms / cell Volume of unit cell2. FCC Example: -Fe , Al , Ni , Pb , Cu , Ag , Au , stainless steal n= 81/8+61/2=4 atoms/cell CN=12 3. HCP Example: Be, Mg, Zn, Cd, Zr, Hf Ti( low temperature) n CN12 0.74Structurea0 vs. rAtoms per cellCoordination NumberPacking factorExample

4、sSC160.52Polonium (Po),-MnBCC280.68Fe,Ti,W,Mo,Nb,Ta,K,Na,V,Zr,CrFCC4120.74Fe,Cu,Au,Pt,Ag,Pb,NiHCP2120.74Ti,Mg,Zn,Be,Co,Zr,Cd4. Summary2.4 Interstices in typical crystals of metals.Two types of Interstitials in typical crystals Octahedral interstitial Tetrahedral interstitialDefinition: In any of the

5、 crystal structures, there are small holes between the usual atoms into which smaller atoms may be placed. These locations are called interstitial sites.1. Octahedral interstitialBCCFCCHCP2. Tetrahedral interstitialBCCFCCHCP.Determination of the sizes of interstitialsDefinition: By size of an inters

6、titial we mean diameter of the maximum hard sphere which can be accommodated in the interstitial without distorting the lattice.didadiameter of interstitial atomdiameter of atom in lattice pointOctahedral interstitial condition for touching For BCCFor FCCTetrahedral interstitialHLADCBinterstitialhos

7、t atomFor BCCFor FCCSummarynCNintersticesdi/daoct.tete.oct.tete.BCC280.6866/2=31212/2=60.150.29FCC4120.7444/4=188/4=20.410.22HCP6120.7466/6=11212/6=20.410.22Examples and DiscussionsBoth FCC and BCC are close-packed structures while BCC is more open?The interstitial atoms most likely occupy the oct.

8、interstitial position in FCC and HCP, while in BCC two types of interstitial can be occupied equally.3. The solid solubility in BCC is much lower than in FCC.4. Diffusion of interstitial atoms in BCC diffusion is much faster than in FCC or HCP at same temperature.5. Determine the relationship betwee

9、n the atomic radius and the lattice parameter in SC, BCC, and FCC structures when one atom is located at each lattice point.6. Determine the density of BCC iron, which has a lattice parameter of 0.2866nm.Solution: For a BCC cell, Atoms/cell = 2 a0 = 0.2866nm = 2.866108cm Atomic mass = 55.847g/mol Volume of unit cell = a03 = 23.5410 24cm3/cell De