Lattice Structures

Atoms are the building blocks of all materials. They are put together in a great variety of ways and bonded or "held together" by cohesive forces in a manner characteristic of a particular material. In a liquid state the atoms of metal are said to be in somewhat random arrangement, having short-range order. At times several unlike atoms will arrange themselves in the characteristic pattern of a particular metal. However, this is a probability event. Since the forces are weak and there is much activity taking place, they soon separate and re-form again. This phenomenon of random grouping, scattering, and regrouping for short periods of time is characteristic of the liquid state. As the random grouping mechanism becomes less frequent and the atomic movement of unlike atoms become more agitated, the material may become a gas.

As the energy input decreases, the random movement of the unlike atoms becomes less frequent, the bonding becomes stronger, and ordered arrays of atoms form lattices.

A crystal is a repeating array. In describing this structure we must distinguish between the pattern of repetition (the lattice type) and what is repeated (the unit cell). The most fundamental property of a crystal lattice is its symmetry. In three-dimensions, unit cells stack like boxes, filling the space, making the crystal. The different colors are just to show the separate boxes - each unit cell is identical.

Figure 1. Cubic Lattice Structure

Figure 2. Hexagonal Lattice Structure

 

If we take a unit cell and stack it, we produce a lattice. Notice that once we begin stacking the unit cells, we never change the orientation of any subsequent unit cells as they stack. In other words, once the orientation of a unit cell is determined, all unit cells within that lattice have the same orientation.

Figure 3. Orientation of the unit cells in a lattice.

Unit Cell: When a solid has a crystalline structure, the atoms are arranged in repeating structures called unit cells, which are the smallest units that show the full symmetry of a crystal.

Lattice: The three dimensional array formed by the unit cells of a crystal is called lattice.

When a crystalline solid starts to form from the molten or gaseous state, these unit cells will tend to stack in a three-dimensional array, with each cell perfectly aligned, and they will form a crystal. If crystals are growing in a melt at the same time, the crystals will eventually meet and form grains. The junction of the grains is called grain boundaries.

The majority of metals have one of three well-packed crystal structures:

Face-centered cubic (F.C.C.)

Body-centered cubic (B.C.C.

Hexagonal-close-packed (H.C.P.)

Figure 4. Face Centered Cubic (F.C.C) Lattice Structure

To view the FCC crystal structure visit the following links:

FCC-(I)

FCC-(II)

FCC-(III)

FCC-(IV)

 

Figure 5. Body Centered Cubic (B.C.C) Lattice Structure

To view the BCC crystal structure visit the following links:

BCC-(I)

BCC-(II)

BCC-(III)

BCC-(IV)

 

Figure 6. Closed Packed Hexagonal (C.P.H) Lattice Structure

To view the CPH lattice structure visit the following links:

CPH (II)

 

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Last update: September 10, 1999

By: Serdar Z. Elgun