Equiaxed dendrites are a common solidification growth form in metal alloy castings. The growth process is mainly governed by heat and solute diffusion and at a later growth stage particularly by the interaction of adjacent grains. Modern dendrite growth models take into account the overlap of solutal fields surrounding each dendrite and are frequently used to predict resulting microstructures (Souhar et al. 2016; Chen et al. 2014; Guo et al. 2013; McFadden & Browne 2012). However, these complex models require precise experimental data for reliable comparison and validation.
X-ray imaging is the method of choice as it provides in-situ and real-time monitoring of the solidification process. With our laboratory X-ray facility we are able to observe the solidification of a 200 µm thin horizontally aligned Al-Ge sample. Our main objective is to analyze the interactive relationship between structure formation and solutal field evolution. The thin sample geometry and especially the horizontal sample alignment result in preferentially near diffusive solidification conditions since buoyancy- and gravity-driven melt flows are minimized. Well defined temperature conditions inside the sample are realized by our near isothermal furnace developed explicitly for equiaxed dendrite growth (Becker et al. 2015).
Although we use a polychromatic X-ray source, the conversion of measured intensities to absolute concentrations in the liquid is possible. A function that converts intensities to concentrations has been determined by performing calibration measurements of samples of known compositions and thickness independent from the actual experiment. We will present this conversion approach and show the solute diffusion profiles developing in front of dendrite tips.
Our measurements of dendrite tip velocities in combination with local concentrations provide novel data for model comparison. Special attention must be payed to the moment when solutal fields from adjacent grains overlap, as this juncture represents the transition from free dendritic growth to interactive growth. By applying measured concentrations to the McFadden-Browne (McFadden & Browne 2012) growth model (an extension of the Lipton-Glicksman-Kurz sharp interface model (Lipton et al. 1987) which allows for increased solute levels) agreement between predicted and measured tip velocities has been found.