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Experimental data for the validation of numerical models coupling solidification and hydrodynamics are very rare. Many experiments made in the field of solidifications are performed with pure metals or alloys (Al-Cu, Pb-Sn, etc) which are opaque and do not allow direct observation of the hydrodynamic.

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It is well known that the growth and motion of equiaxed crystals govern important microstructural features, especially in larger castings such as heavy ingots. To determine the origin of the equiaxed crystals, heterogeneous nucleation, and/or fragmentation of dendrite arms from columnar regions are often discussed. In the present study, we demonstrate that under certain conditions relatively large areas of mushy regions slide downward and form spectacular crystal avalanches. These avalanches crumble into thousands of dendritic fragments, whereby the larger fragments immediately sediment and the smaller proceed to behave as equiaxed crystals. Traces of such crystal avalanches can be seen by conspicuous equiaxed layers in the lower part of the casting. From the arguments in the discussion, it is believed that such a phenomenon may occur in alloys which reveal an upward solutal buoyancy in the interdendritic mush. This would include certain steels and other alloys such as Cu-Al, Pb-Sn, or Ni-Al-alloys. Moreover, the occurrence of crystal avalanches contribute to the formation of V-segregations.

Ammonium chloride is commonly used as a buffer solution to control pH levels in a wide variety of chemical and medical applications and is also used as a fertilizer because it acts as a sufficient source of nitrogen for the soil. More recently it is used to create an experimental benchmark, useful to model/simulate metal solidification. In electronics and metallurgy it is also used for cleaning, to prevent the formation of oxides during welding or smelting of metals. In the literature different values are available for the thermo-physical parameters and in the current paper an overview of measured or calculated values of the most important properties is presented. For an ammonium chloride−water solution different phase diagrams are accessible, and the calculation of the liquidus and solidus line is completed. A comparison of calculated heat capacity values for ammonium chloride is made with the literature values. Measured data for the ammonium chloride density are available in the literature, and the values for different temperatures and concentrations are presented here. Thermal conductivity values are gathered in the present work. The viscosity can be estimated in between 283 and 333 K and for mass fraction up to 0.324 kg•kg −1 , with a model for the calculation of the aqueous solutions viscosity, based on the viscosity of solute and water. The variation curve of diffusivity values with the concentration, exists only for 293 and 298 K. For this reason an approximation with NH 3 diffusivity values, which are measured for different temperatures and concentrations, can be recommended. Additional analysis of two experimental measurements, performed in order to estimate the ammonium chloride diffusivity in water and further extract the Gibbs−Thomson coefficient, is done.

The influence of the melt flow on the solidification structure is bilateral. The flow plays an important role in the solidification pattern, via the heat transfer, grain distribution, and segregations. On the other hand, the crystal structure, columnar or equiaxed, impacts the flow, via the thermosolutal convection, the drag force applied by the crystals on the melt flow, etc. As the aim of this research was to further explore the solidification-flow interaction, experiments were conducted in a cast cell (95 * 95 * 30 mm 3 ), in which an ammonium chloride-water solution (between 27 and 31 wt pct NH 4 Cl) was observed as it solidified. The kinetic energy (KE) of the flow and the average flow velocity were calculated throughout the process. Measurements of the volume extension of the mush in the cell and the velocity of the solid front were also taken during the solidification experiment. During the mainly columnar experiments (8 cm liquid height) the flow KE continuously decreased over time. However, during the later series of experiments at higher liquid height (9.5 cm), the flow KE evolution presented a strong peak shortly after the start of solidification. This increase in the total flow KE correlated with the presence of falling equiaxed crystals. Generally, a clear correlation between the strength of the flow and the occurrence of equiaxed crystals was evident. The analysis of the results strongly suggests a fragmentation origin of equiaxed crystals appearing in the melt. The transition from purely columnar growth to a strongly equiaxed rain (CET) was found to be triggered by (a) the magnitude of the coupling between the flow intensity driven by the equiaxed crystals, and (b) the release and transport of the fragments by the same flow recirculating within the mushy zone. Graphical AbstractMIHAELA STEFAN-KHARICHA, ABDELLAH KHARICHA, MENGHUAI WU, and ANDREAS LUDWIG are with the

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