Application of the cyclic phase transformation concept for determining the effective austenite/ferrite interface mobility

• Effective interface mobilities have been determined.
• The austenite-to-ferrite interface mobility strongly depends on composition.
• The ferrite-to-austenite interface mobility is mainly a function of temperature.
• The model has a great potential for multi-scale simulations.

A series of cyclic partial austenite–ferrite phase transformation computer simulation experiments have been performed to elucidate the rate controlling dissipative processes during austenite-to-ferrite and the ferrite-to-austenite transformation in lean C–Mn steels. The transformation kinetics is analyzed by comparing the results of two complementary sharp interface models – one is based on the assumption of local equilibrium at the migrating interface − in the other model diffusion in the interface and the interfacial reaction is implemented by an effective interface mobility but substitutional diffusion in the bulk phases is neglected.Values for effective interface mobilities have been obtained for both the austenite-to-ferrite transformation and vice versa. By means of effective mobilities which depend only on initial composition and temperature, the transformation kinetics has been studied for other heat treatments than used to determine the effective interfacial mobility values. Although substitutional diffusion in the bulk is not taken into account, for the low Mn alloys it is possible to obtain similar trends by the effective mobility model as provided by the local equilibrium model. At modest to high interface velocities long range diffusion of the substitutional alloying elements can be ignored but then the effects of local diffusion processes near the interface need to be taken into account via an effective interface mobility. The effective mobility for the austenite-to-ferrite transformation differs from the effective mobility during the ferrite-to-austenite transformation in a rather essential manner.