Crystals have orderly, repeating atomic patterns defined by a unit cell, whereas amorphous materials have disordered atomic distributions. Unit cells characterize crystal structures by their geometry and internal atom positions, and there are seven crystal systems classified by unit cell properties. Common metals form crystals with face-centered cubic, body-centered cubic, or hexagonal close-packed unit cell structures.

1. Structures of
Materials 3

2. Fundamental Concepts
1. Atoms in crystalline solids are positioned in
orderly and repeated patterns that are in
contrast to the random and disordered atomic
distribution found in noncrystalline or
amorphous materials.
2. Crystal structures are specified in terms of
parallelepiped unit cells, which are
characterized by geometry and atom positions
within.
1.3 Crystalline and non-crystalline
structures

3. 3. Unit cell- small repeating entity of the atomic
structure. The basic building block of the crystal
structure. It defines the entire crystal structure with
the atom positions within.
4. The concept of crystal system is used to classify
crystal structures on the basis of
unit cell geometry—that is, unit cell edge lengths
and interaxial angles. There are seven crystal
systems: cubic, tetragonal, hexagonal,
orthorhombic, rhombohedral (trigonal), monoclinic,
and triclinic.

4. Unit cell geometry

5. Unit cell geometry

6. 4. Most common metals exist in at least one of
three relatively simple crystal
structures:
 Face-centered cubic (FCC), which has a cubic unit cell
 Body-centered cubic (BCC), which also has a cubic unit
cell
 Hexagonal close-packed, which has a unit cell of
hexagonal symmetry

8. • William D. Callister, David G. Rethwisch (2018)
Materials Science and Engineering: An
Introduction. 10th Edition. John Wiley and Sons
Australia, Limited.
• William D. Callister, David G. Rethwisch (2015)
Materials Science and Engineering. 9th Edition.
John Wiley and Sons Australia, Limited,
• William D. Callister, David G. Rethwisch (2009)
Materials Science and Engineering: An
Introduction. 8th Edition. John Wiley and Sons
Australia, Limited,
References