What is a Cubic Crystal Structure?
The cubic crystal structure is one of the seven known crystal structures in crystallography. Its unit cells are shaped in the form of cubics, which is the most common and simple type found in minerals, crystals, and of course, metals. Named after the French physicist Auguste Bravais, the cubic crystalline structure can take three different forms, also known as Bravais lattices:
- Primitive cubic
- Body-centered cubic
- Face-centered cubic
Their common features include identical edge length (a=b=c) and identical, right angles (α=β=γ=90°).
Primitive Lattice
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 ).
