A crystal structure (FCC) with atoms at the corners of the cube and at the center of each face. The unit cell contains 4 atoms; the coordination number is 12 and the packing factor about 0.74. Examples include Cu, Al and Ni.
Japanese name: 面心立方格子
Face-centered cubic appears in 1 practice question across 1 topic, including Crystal structures and Miller indices.
FCC is the structure obtained by stacking the most densely packed {111} planes in the sequence ABCABC…, and it is also called cubic close-packed. The atoms touch along the face diagonals, so the nearest-neighbor distance is a/√2; for copper (a = 0.3615 nm) it is about 0.256 nm. Besides Cu, Al, Ni, Au, Ag and Pb, γ-iron and austenitic stainless steels also have this structure. A major reason to study it is that most FCC metals are soft, highly ductile and easy to form.
FCC has 12 slip systems of the {111}<110> type, 4 planes × 3 directions, which easily satisfy the five independent slip systems a polycrystal needs to change shape freely. In addition, the stress needed to move dislocations (the Peierls stress) is small and depends little on temperature. FCC metals therefore stay ductile even at liquid-nitrogen temperature and show no distinct ductile-to-brittle transition like BCC metals. This is why aluminum alloys and austenitic stainless steels are chosen for LNG tanks and cryogenic vessels.
An FCC unit cell contains a net 4 octahedral sites, at the body center and the midpoints of the 12 edges, and 8 tetrahedral sites, at the centers of the eight small cubes into which the cell can be divided. In the hard-sphere model, the largest sphere that fits has a radius about 0.414 times (octahedral) or 0.225 times (tetrahedral) that of the host atom. Carbon in γ-iron sits in octahedral sites. The NaCl structure can be understood as an FCC array of anions with all octahedral sites filled by cations, and zinc blende as one with half the tetrahedral sites filled.
A perfect FCC dislocation (a/2)<110> splits into two Shockley partial dislocations (a/6)<112>, with a stacking fault between them where the stacking sequence departs from ABC. The lower the stacking fault energy of a metal, the wider the separation of the partials and the harder cross-slip becomes, so the metal work-hardens more readily and forms twins more easily on annealing. In general, aluminum is a typical example of high stacking fault energy, while silver and austenitic stainless steels have low values; this helps explain differences in their work hardening and recrystallization behavior.
Answer: B. 12
In FCC, each atom touches 6 atoms within its own {111} plane and 3 atoms in each of the {111} planes above and below, so the coordination number is 12. This is the maximum possible for packing spheres of equal size, and HCP also has 12. Eight is the coordination number of BCC (the body-center atom touches the 8 corner atoms of the cube), and 6 is that of simple cubic (SC). Structures with larger coordination numbers also have higher packing factors: about 0.52 for SC, about 0.68 for BCC, and about 0.74 for FCC and HCP. Cu, Al, Ni, Au and γ-iron are typical FCC examples.
The close-packed planes are stacked ABC in FCC and AB in HCP, so the number of slip systems differs. FCC deforms easily even as a polycrystal with its 12 slip systems, while HCP has few basal slip systems and tends to have lower ductility at room temperature.
Yes, they are the same structure. The {111} planes perpendicular to the body diagonal of the unit cell are the close-packed planes, and the structure made by stacking them in ABC order is face-centered cubic when described with a cubic unit cell.
The atoms touch along the face diagonal, so the nearest-neighbor distance is half the face diagonal, a/√2, equal to twice the atomic radius. For aluminum with a lattice parameter of 0.405 nm, it is about 0.286 nm.
Atomic packing factor Body-centered cubic Hexagonal close-packed Miller indices