The ‘Solute-Drag’ effect on migrating interfaces during solid-state phase transformations

Steel is the most important alloy for engineering applications. The mechanical properties of steels depend sensitively on their microstructures. These microstructures are usually tailored through a series of thermal and/or mechanical treatments and invariably involve one or more solid-state phase transformations. Control of microstructural evolution requires a detailed understanding of thermodynamics and kinetics of phase transformations. The kinetics is particularly important since it can be strongly affected by small additions of alloying elements and this effect is often qualitatively interpreted in terms of the ‘solute-drag’ effect, well known in the field of grain growth and recrystallization where the migrating interface is a grain boundary. The current understanding of the ‘solute-drag’ effects of alloying elements on phase boundaries is not yet sufficient to quantitatively describe phase transformation kinetics. This thesis uses controlled decarburization to monitor the phase interface migration between α growing into γ in a range of specially designed high purity steels. The reason for using steels is that the thermodynamics of the ferrite (α) and austenite (γ) phases, as a function of composition and temperature have already been well characterized and can be routinely calculated using computational thermodynamics. Moreover, among different solid-state phase transformations in steels, the γ to α transformation is usually the first solid-state phase change encountered during processing of steels and an understanding of this transformation is central to controlling microstructure development of steels. The controlled decarburization approach allows the kinetics of planar interface migration to be followed with high accuracy (avoiding stereological problems associated with traditional approaches) as a function of treatment temperature and alloy composition. The experimental results obtained on Fe-C-Si and Fe-C-Co systems are used to quantitatively test the temperature and composition dependence of the newly developed Zurob et al. model for interface migration that includes dissipation of Gibbs free energy due to ‘solute-drag’. Following this evaluation, this new α growth model is extended to quaternary (Fe-C-X-Y) systems to take into account the possibility of the interaction between solutes in the migrating boundary (Coupled Solute Drag Effect) and is tested with growth kinetic data obtained for the Fe-C-Mn-Si system.