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There are two simple regular lattices that achieve this highest average density. They are called face-centered cubic (FCC) (also called cubic close packed) and hexagonal close-packed (HCP), based on their symmetry. Both are based upon sheets of spheres arranged at the vertices of a triangular tiling; they differ in how the sheets are stacked ...
Slip in face centered cubic (fcc) crystals occurs along the close packed plane. Specifically, the slip plane is of type , and the direction is of type < 1 10>. In the diagram on the right, the specific plane and direction are (111) and [1 10], respectively.
This type of structural arrangement is known as cubic close packing (ccp). The unit cell of a ccp arrangement of atoms is the face-centered cubic (fcc) unit cell. This is not immediately obvious as the closely packed layers are parallel to the {111} planes of the fcc unit cell. There are four different orientations of the close-packed layers.
The face-centered cubic lattice (cF) has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of four lattice points per unit cell (1 ⁄ 8 × 8 from the corners plus 1 ⁄ 2 × 6 from the faces).
Indices in curly brackets or braces such as {100} denote a family of plane normals which are equivalent due to symmetry operations, much the way angle brackets denote a family of directions. For face-centered cubic and body-centered cubic lattices, the primitive lattice vectors are not
The possible cross-slip planes are determined by the crystal system. In body centered cubic (BCC) metals, a screw dislocation with b=0.5< 1 11> can glide on {110} planes or {211} planes. In face centered cubic (FCC) metals, screw dislocations can cross-slip from one {111} type plane to another.
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In crystalline metals, slip occurs in specific directions on crystallographic planes, and each combination of slip direction and slip plane will have its own Schmid factor. As an example, for a face-centered cubic (FCC) system the primary slip plane is {111} and primary slip directions exist within the <110> permutation families.