Non-stoichiometric cementite by rapid solidification of cast iron

This paper deals with rapidly solidified binary Fe-C alloys containing 3.8 wt% and 4.3 wt% C. Their microstructure reveals that, as an effect of undercooling, the conventional eutectics have been suppressed and there is occurrence of the ferrite-cementite eutectic. Both phases are metastable: ferrite is supersaturated in C and cementite has a C deficiency. This is demonstrated by means of determination of phase fractions in metallographic sections and of lattice parameters via Rietveld refinement of X-ray diffraction patterns. A major consequence of non-stoichiometricity of cementite is the reduced value of the Curie temperature (up to 17 °C) with respect to that of the equilibrium compound. The maximum C deficiency in Fe3C1-x is estimated as x = 0.02. The free energy of defective cementite has been computed by means of the two sublattice model and compared with that of ferrite and austenite obtained from an assessed phase diagram. It is shown that the nonstoichiometric phase is close in energy to the equilibrium one for a composition range of several atomic per cent. It is suggested that this helps in explaining the mechanism of cementite dissolution by heavy deformation of ferrous alloys, and the ease of cementite nucleation in castings.