**2.2 Directional solidification**

The alloy samples were melted and solidied directionally upwards in an experimental setup described elsewhere (Ares et. al., 2007). It was designed in such a way that the heat was extracted only through the bottom promoting upward directional solidication to obtain the columnar-to-equiaxed transition (CET), see Figure 1 (a).

In order to reveal the macrostructure, after solidication the samples were cut in the axial direction, polished, and etched using concentrated hydrochloric acid for 3 seconds at room temperature for the zinc-aluminum alloys, followed by rinsing and wiping off the resulting black deposit. The microstructures were etched with a mixture containing chromic acid (50 g Cr2O3; 4 g Na2SO4 in 100 ml of water) for 10 seconds at room temperature (Vander Voort, 2007). Typical longitudinal macrostructure of different areas of the sample are shown in Figure 1 (b) to (d).

The position of the transition was located by visual observation and optical microscopy. The distance from the chill zone of the sample was measured with a ruler. It is noted in Figure 1 that the CET is not sharp, showing an area where some equiaxed grains co-exist with columnar grains. As was reported before, the size of the transition area is in the order of up to 10 mm (Ares et al., 2007, 2010). The grain structure was inspected by visual observation under Arcano® optical microscopy.
