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Crystaline Structures
Crystaline Structures
12
Mechanical Engineering
Undergraduate 3
05/07/2014

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Term
Cubic Axial lengths and angles
Definition
a=b=c, alpha=beta=gamma=90 degrees
Term
Tetragonal Axial lengths and angles
Definition
a=b not=c, alpha=beta=gamma=90 degrees
Term
Orthorhombic Axial lengths and angles
Definition
a NE b NE c, alpha=beta=gamma=90 degrees
Term
Rhombohedral Axial lengths and angles
Definition
a=b=c, alpha=beta=gamma NE 90 degrees
Term
Hexagonal Axial lengths and angles
Definition
a=b NE c, alpha=beta=90 degrees, gamma=120 degrees
Term
Monoclinic Axial lengths and angles
Definition
a NE b NE c, alpha=gamma=90 degrees NE beta
Term
Triclinic Axial lengths and angles
Definition
a NE b NE c, alpha NE beta NE gamma NE 90 degrees
Term
Three crystal structures of most elemental metals at room temperature
Definition
Body-centered cubic (bcc)
Face-centered cubic (fcc)
Hexagonal close packed (hcp)
Term
Atomic packing factor (APF)
equation
Definition
Fraction of the unit cell volume occupied by the atoms
APF=N(atoms)V(atoms)/V(crystal)
Term
Body-centered cubic (bcc)
Definition
One atom at the center of the unit cell
One-eighth atom at each corner
Two atoms in each bcc unit cell
Bcc metals: -Fe, V, Cr, Mo, W
Unit cell face lenght a=4r/sqrt(3)
Term
Face-centered cubic (fcc)
Definition
One-half atom in the center of each face
One-eight atom at each corner
Four atoms in each fcc unit cell
Fcc metals: -Fe, Al, Ni, Cu, Ag, Pt, Au
a=4r/sqrt(2)
Term
Hexagonal close packed (hcp)
Definition
Based on hexagonal Bavais lattice
Two atoms associated with Bravais lattice point
One atom centered within the unit cell, various fraction atoms at corners (four 1/6, four 1/12 atoms)
Total of 2 atoms in unit cell
APF=0.74
Hcp material: Be, Mg, -Ti, Zn, Zr
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