Face-Centered Cubic (FCC): The Close-Packed Crystal Structure

Face-centered cubic (FCC) is one of the three crystal structures that dominate metals, alongside body-centered cubic (BCC) and hexagonal close-packed (HCP). In an FCC unit cell, atoms sit at all eight cube corners and at the center of each of the six faces. Counting the shared atoms — one-eighth of each corner atom and one-half of each face atom — gives a net of four atoms per unit cell. FCC is a close-packing arrangement: it stacks close-packed atomic planes in an ABC-ABC sequence (the cubic close-packed form), giving each atom a coordination number of 12 — it touches twelve nearest neighbors. The result is an atomic packing factor of about 0.74, meaning roughly 74% of the volume is filled by atoms. This is the densest possible packing of equal spheres, which is why FCC and HCP, which also reaches 0.74, are both called close-packed. BCC, by contrast, fills only about 68% of its volume. The structure's defining mechanical trait is ductility. FCC crystals contain four families of close-packed {111} planes, each with three close-packed slip directions, yielding twelve slip systems along which dislocations can glide under stress. Because these planes are so densely packed, the shear stress needed to move dislocations is low, so FCC metals deform plastically rather than fracturing. This is the atomic reason gold and copper can be drawn into wire and hammered into foil. BCC metals have more slip planes in principle but no truly close-packed ones, so they tend to be stronger but more brittle, especially at low temperature; HCP metals have only a few easy slip systems and are often the least ductile. FCC metals include aluminum, copper, gold, silver, nickel, platinum, and lead. Iron is a notable shape-shifter: it is BCC at room temperature (ferrite) but transforms to FCC austenite (gamma-iron) above about 912 degrees Celsius, a transition central to steel heat treatment. See Steel Crystal Structures: Austenite, Ferrite, Martensite for how this phase change underpins hardening, and Ductility Versus Stiffness: Why Gold Doesn't Make Beams Bend Better for why high ductility does not imply stiffness.