ON THE MASSIVE TRANSFORMATION

The massive transformation γ → α in binary FeC alloys is explored by means of a model taking into account the thermodynamic properties of the α and γ phases. In addition the model takes into account the finite mobility of the phase interface and the solute drag, i.e. the dissipation of the driving force caused by a finite solute gradient over the interphase. It is found that inside a region bounded by the α/α + γ phase boundary of the FeC phase diagram and a critical line below the T0 line there are two solutions to the growth equations. One solution corresponds to a high growth rate controlled mainly by the finite mobility of the phase interface and corresponding to a weak solute drag. The other solution represents a lower growth rate controlled mainly by a strong solute drag. As the carbon content of the austenite is increased at a constant temperature the two solutions move closer to each other and at a critical composition before the T0 line is reached they coincide and beyond that no massive transformation is possible. The position of the critical composition depends on L/M, the ratio of the solute diffusivity, inside the interphase, and the phase interface mobility. Inside the α one-phase region there is only one solution corresponding to a high growth rate and a weak solute drag. © 1990.