On the transition from local equilibrium to paraequilibrium during the growth of ferrite in Fe-Mn-C austenite

The concept of paraequilibrium, introduced a long time ago by Hultgren and analyzed thermodynamically by Hillert, has been analyzed in detail by means of a kinetic model. The concept applies to alloy systems where there is a large difference in mobility of the different components, e.g. in steels where substitutional elements diffuse many orders of magnitude slower than interstitial solutes like H, C and N. By definition paraequilibrium means that only the mobile elements are equilibrated while the sluggish ones behave as a single element. In the present work we have applied a phenomenological model that takes into account the diffusion in the matrix in front of a moving phase interface, a finite interface mobility, the effect of surface tension and the sluggishness of diffusion across the phase interface, i.e. the solute drag. We have studied theoretically the edgewise growth of a ferrite platelet in Fe-Mn-C alloys. In particular we have studied how the growth process depends on the Mn content. We have investigated under what conditions paraequilibrium or full thermodynamic equilibrium is established locally at a moving phase interface.