Magnesium and Aluminum

Class: Elemental Structure

Related Structures: Ferrite, Polonium

Notable Properties: Simple Solid, Structural Material

The face centered cubic unit cell of aluminum
The face centered cubic unit cell of aluminum

Description:

Aluminum is a common structural material and the parent structure type of many advanced aerospace superalloys. Owing to its low density (small atomic mass) and stiffness it has become a choice material for structures under dynamic load and transportation (i.e. bicycles and airplanes). The crystal lattice is face centered cubic and its packing can be thought of as alternating layers of Al atoms with six nearest neighbors with an ABCABC layering scheme.

ccp2
Packing scheme found in the crystal structure of aluminum. Atoms are colored to draw attention to the ABCABC layering pattern.

Aluminum adopts one of the two most common structure types among the elements, that is the cubic and hexagonal close packed structure types. These two structure types have similar packing densities and stabilities and it is not uncommon for elements to interconvert between the two structures with changes in pressure and temperature.

ptable-cc

As is apparent from the periodic table above, of the 99 elements with known crystal structures:

     32 are hexagonal close-packed (hcp, dhcp)

     21 are  cubic close-packed structure type (ccp)

And of the simple solids:

     12 are body center cubic (bcc)

     1 is primitive cubic (pc)

Hexagonal close-packed is the other close packing scheme of the simple elemental solids and the most common. It is typified by the particularly abundant element magnesium. The hcp structure type is of lower symmetry than the ccp structure type (i.e. hexagonal instead of cubic) but has a simpler layering scheme of ABABAB instead of ccp’s ABCABC. Magnesium-aluminum alloys are central materials in the modern aerospace industry

Packing structure for gold. Gold adopts a hexagonal close-packed structure type that can be described as an ABAB layer packing where each Au atom has six nearest neighbors. Atoms are colored to illustrated this alternating pattern.
Packing structure for magnesium. Magnesium adopts a hexagonal close-packed structure type that can be described as an ABAB layer packing where each Au atom has six nearest neighbors. Atoms are colored to illustrated this alternating pattern.

 

 

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