The primitive lattice, also known as simple cubic (SC), contains 8*1/8 atoms per unit cell at each corner of the cubic. The unit cell is called a cubic unit cell. While simple cubics do not appear in metals directly, they can be present in alloying elements. The only exception to this rule is α-Polonium, a radioactive metal whose polonium atoms are situated at each corner of the lattice. Primitive lattices can also be found in high pressure modifications of antimony and phosphorus.
Body-Centered Cubic Structure (BCC)
Besides the 8*1/8 atoms at the corners of the cube, a body-centered cubic also contains one metal atom in the middle of the cubic. This arrangement of atoms means that body centered cubic crystal structures consist of 8*1/8+1 atoms per unit cell. Metals, that have a typical bcc-structure are chromium, molybdenum, and α-iron, which is also known as ferrite. A body centered cubic can be formed by one primitive cubic falling into another. One corner atom then acts as the middle atom that is distinctive for the BCC-structure.
Face-Centered Cubic (FCC)
As the name implies, the face-centered cubic contains an additional 1/2 atom at each face of the cube. This means that there are 8*1/8+1+6*1/2 atoms in total in one cell. Aluminium, gold, nickel, copper, platinum, and γ-iron (also known as austenite) are the most common metals that display a face-centered cubic structure. The FCC-structure can also be formed by primitive lattices. Three of the simple cubics need to attach themselves to another simple cubic so that their corner atoms can function as face-atoms in the fourth primitive lattice.
Cubic Crystal Structures of Steel
When it comes to steel, the three most common structures are:
- Body-Centered Cubic Ferrite: The α-ferrite can be found in carbon-iron compounds, and is one of the most common and simple forms of steel. At temperatures up to 912°C, steel typically displays this type of crystal structure.
- Face-Centered Cubic Austenite: The α-ferrite transforms into γ-austenite at 912 °C. Stainless steel grades, such as 316L and 304 are austenites and possess a face-centered cubic structure.
- Body-Centered Tetragonal Martensite: Steel types with a martensitic structure display a body centered tetragonal structure. This is typically achieved by quenching an austenite extremely quickly, making it impossible for excess carbon to be released. However, this system is not a cubic structure, due to the fact that one edge length is not the same length as the other two (a=b≠c ).