Why carbon fits in octahedral void even though tetrahedral void is larger in BCC? Why solubility of carbon is more in FCC than in BCC even though FCC is more closed packed

α – Fe (BCC) 

The centers of tetrahedral voids are located on the faces of the cube; (½ ¼ 0) and other equivalent positions. 6 faces have total 24 tetrahedral voids, but each face of cube is shared by 2 unit cells. Hence 12 tetrahedral voids. Radius of tetrahedral void (r) = 0.29 R (radius of atom).

The centers of octahedral voids at midpoint of edges; (0 0 ½) and equivalent positions and at the center of each phase; (½ ½ 0) and other equivalent positions. 

Unsymmetrical nature of octahedral void (2 atoms are nearer to the octahedral center as compared other 4 atoms) makes it smaller than tetrahedral void. Radius of Octahedral void (r) = 0.15 R (radius of atom).

γ – Fe (FCC) 

The centers of tetrahedral voids lies on the body diagonals; (¼ ¼ ¼) (¾ ¾ ¾) and equivalent positions. total 8 tetrahedral voids (two voids on each diagonal). Radius of tetrahedral void (r) = 0.225 R (radius of atom).

The centre of the octahedral voids at body centre (½ ½ ½) and midpoints of unit cell edges; (½ 0 0) (0 ½ 0) (0 0 ½) and other equivalent positions, 4 octahedral voids. Radius of Octahedral void (r) = 0.414 R (radius of atom).
 
In α – Fe tetrahedral void can accommodate an atom of 0.36 A° radius, where as octahedral void can accommodate an atom of 0.19 A° radius.
A carbon atom in tetrahedral void displace all 4 atoms of the Iron at 4 corners of the tetrahedron cause more distortion where as if it present in octahedral void displaces only 2 nearest atoms resulting in less distortion. This makes the interstitial solubility of ‘C’ in α – Fe more difficult than its solubility in γ – Fe.

In γ – Fe octahedral void can accommodate a spherical atom of 0.52 A°, where as tetrahedral void can accommodate an atom of 0.28 A° radius, thus C (0.77 A°) or N (0.71 A°) are sits in larger octahedral void with an expansion of lattice.

Ferrite: it is an interstitial solid solution of carbon in α - iron (BCC). The maximum solubility of carbon in ferrite is 0.02 wt% at 727°C and the minimum is 0.00005 wt% at 20°C. the size of the largest atom that can fit in octahedral void is 0.19 A°, which is much smaller than carbon atom (0.71 A°). so the solubility is extremely limited. It is soft and ductile. Ferrite is ferromagnetic upto 768°C becomes paramagnetic above this temperature.

Austenite: it is an interstitial solid solution of carbon in γ - iron (FCC). The maximum solubility of carbon is 2.1 wt% at 1146°C which decreases to 0.77 wt% at 727°C. the size of the largest atom that can fit in octahedral void is 0.52 A°. correspondingly the solubility is larger here compared to ferrite. It is soft, ductile, malleable, tough and non-magnetic. It is stable above 727°C in plain carbon steels but can be obtained even at room temperature by adding elements like Ni or Mn in steels.