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1、.Crystal structure is the real arrangement of atom in crystals Crystal structure Space lattice+Basis or structure unit,2.3 Crystal Structure and Complex Lattice,第1页/共27页,第2页/共27页,Fe:Al=1:1,The difference between space lattice and crystal structure,第3页/共27页,23 atoms/cell,第4页/共27页,1.BCC Example:-Fe,V,
2、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,第5页/共27页,Packing fraction To determine,The atom is looked as a hard sphere,and the nearest neighbours touch each other.For BCC,第6页/共2
3、7页,2.FCC Example:-Fe,Al,Ni,Pb,Cu,Ag,Au,stainless steal n=81/8+61/2=4 atoms/cell CN=12,第7页/共27页,3.HCP,Example:Be,Mg,Zn,Cd,Zr,Hf Ti(low temperature)n CN12 0.74,第8页/共27页,4.Summary,第9页/共27页,2.4 Interstices in typical crystals of metals,.Two types of Interstitials in typical crystals Octahedral interstit
4、ial Tetrahedral interstitial,Definition:In any of the crystal structures,there are small holes between the usual atoms into which smaller atoms may be placed.These locations are called interstitial sites.,第10页/共27页,1.Octahedral interstitial,BCC,第11页/共27页,FCC,第12页/共27页,HCP,第13页/共27页,2.Tetrahedral int
5、erstitial,BCC,第14页/共27页,FCC,第15页/共27页,HCP,第16页/共27页,.Determination of the sizes of interstitials,Definition:By size of an interstitial we mean diameter of the maximum hard sphere which can be accommodated in the interstitial without distorting the lattice.,第17页/共27页,Octahedral interstitial condition
6、 for touching,第18页/共27页,For BCCFor FCC,第19页/共27页,Tetrahedral interstitial,H,L,A,D,C,B,interstitial,host atom,第20页/共27页,For BCC,For FCC,第21页/共27页,Summary,第22页/共27页,Examples and Discussions,Both FCC and BCC are close-packed structures while BCC is more open?The interstitial atoms most likely occupy th
7、e oct.interstitial position in FCC and HCP,while in BCC two types of interstitial can be occupied equally.,第23页/共27页,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 relati
8、onship between the atomic radius and the lattice parameter in SC,BCC,and FCC structures when one atom is located at each lattice point.,第24页/共27页,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 Density,第25页/共27页,Exercise,Determine the coordinates of centers of both the octahedral and the tetrahedral interstitials in HCP referred to a,b and c.,第26页/共27页,Thank you!,4,第27页/共27页,
