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 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).
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.
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.