The direct observation of solidification as a function of gravity level

The solidification of a metal-model material, NH₄Cl−H₂O, was directly observed on Earth at 1 g and at 10⁻⁵ g on a suborbital rocket flight. In the 1 g experiments, nucleation started at the cold walls and then dendrites and dendritic debris were swept into the central region by fluid flow. The numerous crystals in the central zone created an equiaxed zone. Secondary dendrite arms were oriented toward the cold wall with suppressed arm growth in the direction of the flow pattern. The necking and fragmentation of secondary arms were observed. The variation in secondary and tertiary arm spacing ranged from 27 to 38 pct. Individual dendrites grew at similar rates to interface fronts. When solidified in low g, only four nuclei grew to form the complete casting. There were no free floating crystals or visible dendrite remelting. Symmetrical dendrite growth into the fluid and some necking of secondary arms occurred but no coarsening or fragmentation resulted. The growth rate of interfaces was less than that of individual dendrites. Total growth was columnar with no equiaxed zone being formed.