Nucleation under Non-Equilibrium Conditions

Steel of peritectic composition is widely used globally in construction, automobile and energy generation, to name just a few, with production by continuous casting techniques, exceeding 1.3 billion t/a. However, production rates and quality in steel of near-peritectic composition are hampered by the occurrence of surface cracks. The root cause of these defects has been attributed to the occurrence of a massive-type of phase transition followed by a sudden volume contraction in the meniscus region of a continuous caster. This sudden volume contraction leads to detachment of the thin solidifying shell from the mould, which leads to cracks and in the extreme, to breakouts. At the high heat extraction rates and hence, cooling rates, in the meniscus region of the caster, nucleation of austenite can be significantly constrained in Fe-C alloys of near-peritectic composition, leading to the observed massive-like phase transformation. These findings cannot be explained by the classical nucleation theory and Griesser et al proposed that the diffusional solute flux initially generated by the solid gradient in the parent δ-phase, increases the Gibbs free energy barrier to nucleation of an austenite nucleus by variations in statistical fluctuations of atoms attaching onto an embryonic nucleus. The free energy is increased as if particles are added to an open thermodynamic system. By building on this broader concept, the present study was undertaken to study at a fundamental level by experimental and theoretical analysis, constrained nucleation of austenite in peritectic alloy systems. Following an extensive critical analysis of the pertaining literature, the first part of the study was devoted to re-assessing earlier experimental observations and extending it to unsteady conditions of phase transformations. Fe-C and Fe-Ni alloys of peritectic composition were examined in-situ, in a high-temperature laser-scanning confocal microscope (HTLSCM) by using a concentric solidification technique. In order to map the temperature distribution in thin cylindrical specimens, the radial temperature distribution including the temperature at the solid/liquid interface, was experimentally measured under near-to isothermal conditions. Based on the experimental data, a two-dimensional transient heat transfer model was developed by finite element analysis techniques. This model enabled calculation of the dynamic temperature distribution in the radial direction of the sample under rapid cooling conditions, up to 100 K/min cooling rates. MICRESS©, a commercial phase-field simulation package was then used to determine the solute distribution in the radial direction of the concentric samples. The two-dimensional simulation domain was redesigned and improvised in order to avoid the geometrical mismatch with the experimental set-up and the temperature history of the solidifying interface, calculated from the heat transfer model was then imported into the phase-field model. By these techniques, it was possible to closely simulate the experimental investigations under higher cooling rates. The calculations of the δ/liquid interface velocity, the peritectic reaction rates and the peritectic transformation rates were then compared to the respective experimental measurements and good agreement was found