Interfacial conditions at the moving interface during growth of ferrite from austenite in Fe-C-(X) alloys

A quantitative description of the kinetics of ferrite growth in Fe-C-(X) systems requires the identification of the compositions of the ferrite (alpha) and austenite (gamma) at the migrating interface and their change with time, temperature and bulk composition. Based oil a sharp interface model, local equilibrium (LE) (with partitioning (LE-P) or with negligible partitioning (LENP)) and paraequilibrium (PE) represent two possible sets of interfacial conditions describing steady state growth where it is assumed that zero free energy is dissipated by interfacial processes. In this contribution we illustrate the expected deviations in interfacial conditions arising from non-negligible interfacial dissipations (finite interface mobility, solute drag) and compare the theoretical treatments with the available experimental data in the binary Fe-C and tertiary Fe-C-Si systems. At the temperatures examined and the interface velocities experimentally encountered, the assumption of zero free energy dissipation by interfacial processes was found to be an excellent assumption. The binary Fe-C data is well explained by the LE model and the available data in the Fe-C-Si system is equally well described by the LE or PE treatments. The conclusion is made that Wider the treatment conditions examined, the mobility of the migrating interface is large enough that it may be safely assumed to be infinite and that negligible free energy dissipation due to Si diffusion within the migrating interface (solute drag) is observed.