Aluminum and lead – one is a lightweight, corrosion-resistant material commonly used in the aerospace industry; the other is dense, used to block x-rays, toxic to humans and a terrible material choice for constructing airplane bodies. What could these two incredibly different metals possibly have in common? They both have the same atomic crystalline structure at room temperature.
Materials are made up of a wide variety of atomic structures. However, metals in particular almost always have their atoms organized in a crystalline lattice structure. This means that the atoms of metals are arranged in a patterned, three-dimensional way that repeats itself throughout large portions of the metal. Within the crystalline structure group there are a range of subgroups that organize the shape of these crystalline structures. The three most common crystalline structures in metal are face-centered cubic, body-centered cubic and hexagonal close-packed.
Face-centered Cubic
One of the most common crystalline structures is face-centered cubic (FCC). The FCC crystalline structure gets its name from its cube shape and the locations of the atoms within that cube. There are eight atoms that are distributed among the eight corners of the crystalline structure. Each of those eight atoms are part of other adjacent cubic structures as well. In addition to the atoms located on the corners of the FCC structure, there are also six atoms located on the center of each cube face, hence the name face-centered cubic.
There are many different types of metal with the FCC crystalline structure. The two examples in the introduction, aluminum and lead, are two metals that have the FCC structure at room temperature. Nickel and precious metals such as gold, platinum and silver have the FCC crystalline structure as well. Iron does not have the FCC crystalline structure at room temperature, but when heated to a certain temperature, the typical ferrite body-centered cubic found in iron begins to transform to austenite, which does have an FCC crystalline structure. Adding certain alloying elements (e.g., nickel) to steel allows for steel to be austenitic, and therefore FCC, at room temperature. An example of this is austenitic stainless steel. (Learn more in the article An Introduction to Stainless Steels.)
Body-centered Cubic
The body-centered cubic (BCC) crystalline structure is another abundant type of atomic structure found in metals. Like the FCC crystalline structure, the BCC crystalline structure gets its name from its shape. The BCC crystalline structure is in the form of a cube with eight atoms distributed among the eight corners similar to the FCC crystalline structure. What is different about the BCC crystalline structure is that rather than having an atom at each of the six faces, it has only one atom that is inside the cube. This atom is centered in the body of the cube, which is the reason for the name body-centered cubic.
Many metals are comprised of the BCC crystalline structure. As previously mentioned, iron in its ferrite form is a member of the BCC family of metals. Also falling under the BCC crystalline structure umbrella at room temperature are niobium, chromium and vanadium. Potassium, sodium, lithium and other alkaline metals are also typically constructed by the BCC crystalline structure.
Metals with the BCC crystalline structure typically have less strength than metals with the FCC and HCP crystalline structures at room temperature.
Hexagonal Close-Packed
Hexagonal close-packed (HCP) is a crystalline structure that is somewhat more complex than the FCC and BCC crystalline structures. If one hexagonal close-packed structure were separated from other hexagonal close-packed structures adjacent to it, it would be comprised of 17 atoms. There are six atoms spread evenly among each vertex of a hexagon. An additional six atoms are distributed equally across the vertices of another hexagon. Additionally, there is an atom in the center of each of these hexagons. Both groups of atoms in the hexagons are aligned with one another. Sandwiched, or packed, in between these two hexagons is a group of three atoms that are not in line with the atoms in either of the hexagons. The atoms in the hexagons are shared with adjacent HCP structures.
The HCP crystalline structure is found in several different metals. Titanium and cadmium are two of the most commonly used metals that are comprised of the HCP crystalline structure at room temperature. Cobalt, zinc and zirconium are a few other well-known examples.
The HCP crystalline structure has few ways that slipping can occur, giving these materials a high strength but typically a brittle failure mode.