文档介绍：Chapter Three Structures of Metals and Ceramics Lecturer: Shenhua SONG ? Unit cells ? Metallic crystal structures ? Ceramic crystal structures ? Crystal systems ? Crystallographic directions and planes Unit cells Small repeat atom groups in a crystal structure Representation of a unit cell: (a) hard-sphere model; (b) reduced hard-sphere model; (c) an aggregate of many atoms Metallic crystal structures (1) Three major types: 1) Face-centered cubic (FCC) crystal structures 2) Body-centered cubic (BCC) crystal structures 3) Hexagonal close-packed (HCP) crystal structures Metallic crystal structures (2) 1) Face-centered cubic (FCC) crystal structure 2R a 24aR?4 2aR? Each unit cell contains 4 atoms Metallic crystal structures (3) Two other features: coordination number (CN) and atomic packing factor (APF) CN: the number of nearest- neighbour atoms – 12 for FCC APF: the fraction of atom-occupied volume in a unit cell APF = (atom-occupied volume in a unit cell)/(unit cell volume) For FCC crystals: 4 atoms in a unit cell atom-occupied volume = 4 ?(4/3) (?R 3 ) = 4 ?(4/3) ?( ?(a? 2/4) 3 = a 3 unit cell volume = a 3 APF = atom-occupied volume/unit cell volume = mon metals: Al, Ni, Au, Cu, and Mn Metallic crystal structures (4) Interstices in FCC structures: octahedral interstices and tetrahedral interstices R o = ( a– 2R)/2 = (2 ? 2R - 2R)/2 = (1/2,1/2,1/2) R t = ?3a /4 – R = ( ? 3/4) ?2 ? 2R) – R = (1/4,1/4,1/4) Diagonal length: ?3a a Metallic crystal structures (5) 2) Body-centered cubic (BCC) crystal structure aR34?4 3aR? Each unit cell contains 2 = 8 APF = 2?(4/3) (?R 3 )/a 3 = 2?(4/3) ?( ?(a? 3/4) 3 /a 3 = Metallic crystal structures (6) Interstices in BCC structures: octahedral interstices and tetrahedral interstices R oz = ( a– 2R)/2 = (4R/ ? 3 - 2R)/2 = R oxy = ( a ?2– 2R)/2 = ((4R/ ? 3) ? 2 - 2R)/2 = R t = a ? 5/4 - R = (4R