Structure of MetalsDOE-HDBK-1017/1-93COMMON LATTICE TYPESIn a face-centered cubic(FCC) arrangement of atoms, the unit cell consists of eight atomsat the corners of a cube and one atom at the center of each of the faces of the cube.In a hexagonal close-packed (HCP) arrangement of atoms, the unit cell consists of threelayers of atoms. The top and bottom layers contain six atoms at the corners of a hexagonand one atom at the center of each hexagon. The middle layer contains three atomsnestled between the atoms of the top and bottom layers, hence, the name close-packed.Figure 2 Common Lattice TypesMost diagrams ofthe structural cellsfor the BCC andFCC forms of ironare drawn asthough they are ofthe same size, asshown in Figure 2,but they are not.I n t h e B C Carrangement, thestructural cell,which uses onlynine atoms, ismuch smaller.Rev. 0Page 7MS-01
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