Chapter 4: Solidification Processes

Solidification Processes begins by establishing the thermodynamic and kinetic principles of nucleation, distinguishing between homogeneous nucleation, which requires significant undercooling to overcome the energy barrier of creating a new interface, and the practically dominant heterogeneous nucleation, where impurities or mould walls drastically reduce the activation energy through wetting angles. The text examines growth mechanisms, contrasting the continuous, diffusion-controlled growth of atomically rough metallic interfaces with the lateral growth steps, such as surface nucleation or screw dislocation spirals, required for atomically smooth or faceted interfaces. A central theme is the stability of the solid-liquid interface; in pure metals, heat flow and latent heat removal dictate whether the front remains planar or evolves into thermal dendrites, particularly in supercooled liquids. The chapter expands into alloy solidification, defining the partition coefficient and deriving the Scheil equations to model solute redistribution and coring under non-equilibrium conditions where solid-state diffusion is negligible. The phenomenon of constitutional supercooling is introduced as the governing criterion for interface breakdown, explaining the morphological transition from planar fronts to cellular and dendritic substructures based on the ratio of the temperature gradient to the growth velocity. Furthermore, the text details cooperative growth in eutectic systems, where interlamellar spacing is inversely related to growth rate, and peritectic transformations, which often result in incomplete reactions and multiphase structures due to diffusion barriers. Finally, these concepts are applied to industrial processing, describing the zonal structure of ingots—characterized by chill, columnar, and equiaxed zones—and the specific dynamics of continuous casting and fusion welding, where moving heat sources and epitaxial growth influence segregation patterns and grain orientation.