Carbon in cubic and tetragonal ferrite

It is generally assumed that the phase diagrams and equilibrium thermodynamic data that apply to the conventional Fe-C system, also are relevant to the case where supersaturated ferrite is in contact with austenite. It seems that this may not be correct, since a change in the symmetry of the ferrite unit cell in the presence of excess carbon has the potential to alter the nature of the phase equilibrium. The implications of these discoveries are presented in the context of the early recognition by Cottrell and co-workers, of the importance that should be attached to the tetragonal symmetry of the octahedral interstices in the ferritic allotrope of iron.