Growth of equiaxed dendritic crystals settling in an undercooled melt, Part 2: Internal solid fraction

Experiments are conducted to measure the internal solid fraction evolution of equiaxed dendritic crystals that are freely growing and settling in an undercooled melt using the transparent model alloy succinonitrile–acetone. The internal solid fraction is determined from the measured settling speed and crystal envelope shape and size. Depending on the melt undercooling and acetone concentration, the internal solid fraction is found to vary between 0.55 and 0.1. In all experiments, the internal solid fraction decreases continually during settling. Based on heat and solute balances, a model is developed for predicting the internal solid fraction evolution under convective conditions. Nusselt and Sherwood number correlations are obtained that allow for the calculation of the thermal and solutal boundary layer thicknesses at the crystal envelope. The measured and predicted internal solid fraction evolutions are found to be in good agreement.