Body-Centered Cubic Unit Cells - draft

body-centeredcubic unitcell

Presented by: Group #3

what is a body-centered cubic unit cell?

body-centered cubic unit cell

• Body-centered cubic (BCC) crystal structure is one of the most common ways for atoms to arrange themselves in metals. This unit cell is based on the Bravais lattice of the same name, which indicates that the lattice points appear not only at the corners, but in the center of the unit cell as well.

Body-centered cubic unit cell

• This unit cells contains two atoms and has a coordination number of 8.

(8 Corners of a given atom x 1/8 of the given atom’s unit cell) + 1 additional lattice point/body center atom = 2 atoms

BODY-CENTERED CUBIC UNIT CELL

• It is more common than that of the simple cubic unit cell due to tightly packed atoms.
• It has a packing efficiency of 68% compared to the simple cubic unit cells' 52%.
• It is harder and less malleable than closer packed materials such as gold.

BODY-CENTERED CUBIC UNIT CELL

• BCC metals also have a ductile-to-brittle transformation temperature (DBTT), which means that they are brittle at lower temperatures and ductile at higher temperatures.

BODY-CENTERED CUBIC UNIT CELL

• BCC metals also tend to have a high melting point because they have a large number of nearest neighbors and next-nearest neighbors.

body-centered cubic unit cell

Examples of metals with BCC crystal structure:

• lithium
• sodium
• potassium
• vanadium
• chromium
• iron
• rubidium
• niobium
• molybdenum
• cesium
• barium
• europium
• tantalum
• tungsten
• alpha iron

Interstitial Sites

Interstitial sites are the spaces inside a crystal where another kind of atom could fit. BCC has two types of interstitial sites: octahedral and tetrahedral.

Octahedral Sites

Tetrahedral Sites

• BCC has 6 octahedral sites, which means that a small interstitial atom could fit in 6 positions such that it is equally surrounded by 6 FCC lattice atoms. These octahedral interstitial atoms can be r = 0.155R size.

• BCC also has 12 tetrahedral sites, which means that a small interstitial atom could fit in 12 positions such that it is equally surrounded by 4 FCC lattice atoms. These tetrahedral atoms can be r = 0.291R size.

Interstitial sites

Summary