Phase Transformation Kinetics

The mechanisms of solidification during cooling of a metal from an atomic point of view are analyzed using the liquid-solid surface free energy and wetting angles for different solid and liquid materials. The process for homogeneous nucleation and critical nucleus size calculations are explained from a thermodynamic (free energies) point of view as a function of undercooling and nucleation rate final amount of nucleus forming crystals are presented. A comparison between homogeneous and heterogeneous processes is described and the resulting microstructures including formation of dendrites due to growth in preferential directions is explained, while its relation to temperature, time, and transformation curves is shown (Johnson-Mehl-Avrami model). Also continuous cooling curves are obtained. Furthermore, unidirectional nuclei growth is also presented considering different heat removal mechanisms and examples of dendritic structures depending on the thermal characteristics of the mold, are presented.