Formation of double-diffusive layers in the directional solidification of binary solution

Abstract

The convective flows in the directional solidification of ammonium chloride solution cooling from below, categorized as the salt-finger convection, the plume convection and the bulk convection, have been found to be influential on the structure of resultant casting. Under some circumstances, as will be shown in the present study, the bulk convection will be replaced by the double diffusive convection in horizontally stratified layers; which will also cause a deleterious effect on the mechanical properties of the casting. The formation of the horizontal double diffusive layers (DDLs) turns out to be a result of the interaction between the vertical concentration gradient and the horizontal temperature gradient, both induced by the plume convection. The vertical concentration gradient is established due to the less concentrated fluid brought by the plumes to the top boundary and the horizontal temperature gradient is built-up due to the local cooling resulting from the plumes. Consequently, the DDL generates in the upper half of the bulk fluid region and then moves steadily downwards. As it reaches the melt/mush interface, the DDL structure breaks down, causing a slight remelting of the crystal of the mush. Results also show that the critical condition for the onset of DDL determined previously applies well for the present system. The DDL configuration can be closely related to the zonation in the magma chamber and the segregation of alloy casting.