Concentration Stratification and Crystal Dynamics in Solidifying Ammonium Chloride Solutions

Many alloys reveal an upwardly directed solutal buoyancy in the interdendritic mushy region as they solidify. Such a situation leads to a solute enrichment at the top of the melt pool and is prone to form segregation channels. Based on both experimental observations and numerical simulations applying an aqueous ammonium chloride solution subject to side cooling, this contribution discusses the complex interplay of different phenomena that leads to a sedimentation bed that is far from being homogenous. Concentration and flow stratification at the top part of the test cell, interdendritic flow channels transporting both segregated liquid and also crystal fragments into the bulk melt, crystal agglomerations that form at indentations observed at the exit of flow channels, and collapsing of crystal agglomerations and sliding down of mushy zone segments, all these phenomena lead to a highly unsteady solidification dynamic. These observations are relevant in fields such as metallurgy, materials science, geology, and environmental science and could enhance the design of industrial processes and contribute to natural system modeling.