Let’s have a look at how metals arrange themselves. This is important because it allows us to know exactly what is happening during processes such as heat-treating.
Metals are crystalline in nature. That is, they are arranged in an organized, structured way. The structure of a crystal is a simple shape that is repeated over and over again in three dimensions. Like these crates. The simplest repeating shape is called a “unit cell.” Different metals have different unit cell shapes or “lattice structures.” Some of the most common crystal lattice structures seen in metals are these: body-centered cubic, face centered cubic, and hexagonal-close packed. Let’s have a look at each of these.
The body-centered cubic lattice has a simple box, or cube shape. There is an atom at each of the eight corners of the cube. In the middle, or body of the cube, there is an extra atom. We like to abbreviate this lattice structure as BCC. Pure Iron at room temperature has a BCC structure.
Another common lattice structure is the face-centered cubic or FCC lattice. This structure also looks like a cube, but has additional atoms on each of the six faces of the cube. When iron is heated up, it will change from a BCC to an FCC structure. If you think about the spaces between the atoms, you might think that it would change if the structure changed — and you would be right. We’ll look at that later on when we start talking about heat treating metals.
Let’s look at one final lattice structure… This is the Hexagonal-close packed lattice. See how the atoms arrange themselves in a hexagonal shape? You might see this lattice in metals like room temperature zinc, cobalt, and titanium.