FRAGMENTATION OF DENDRITIC CRYSTALS DURING SOLIDIFICATION OF AQUEOUS AMMONIUM CHLORIDE

Abstract

To obtain an improved understanding of the production, development, and transport of crystal fragments during directional solidification of binary alloys, experiments have been performed for an aqueous ammonium chloride solution. For vertical directional solidification (VDS) induced by cooling from below, fragmentation occurred in two stages during the solidification process, each requiring conditions conducive to remelting of dendrite arms. Production increased with increasing and decreasing values of the initial NH4Cl composition and chill wall temperature, respectively. For horizontal directional solidification (HDS) induced by cooling from the side, fragmentation occurred in three stages, the first depending exclusively on mechanical fracture and the others involving remelting as a required or dominant production mechanism. Production increased with decreasing values of both the initial NH4Cl composition and chill wall temperature. While solid volume fractions of approximately 5% are representative of slurries created by HDS, solid volume fractions are less than 0.5% for VDS. Hence, although fragmentation can have a significant influence on flow and species redistribution in HDS, its effect is likely to be negligible for VDS.