fcc: face-centred cubic
Definition:The fcc system refers to a unit cell with atoms on its eight corners and on each side.
Explanation:According to the point lattice model in crystallography, the atomic nuclei of the fcc lattice are located at the corners and in the center of each face of the unit cell. The diagonals along the plane of the cell, <110>, correspond to the closest packed direction of the fcc lattice. Since the volume of the unit cell is not completely filled by the atomic spheres, interstitial sites exist between the atoms. These interstitial sites can be classified as either octahedral or tetrahedral depending on their geometry (Fig. 2).
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Fig. 1: (a) Unit cell of a fcc point lattice. (b) Unit cell according to the ball model. (c) Geometric relationships in the fcc lattice

Fig. 2: Interstitial sites in the fcc lattice.
SFB-Link:The knowledge of the crystal structures appearing in steels is of major importance for the fundamental understanding of the mechanisms ruling especially the sophisticated high Mn- steels’ material behavior.
References:Materials Science of Steel – Textbook for Students at RWTH Aachen, W. Bleck (Ed.) 3rd edition 2013