X-ray radiography observations of columnar dendritic growth and constitutional undercooling in an Al-30wt%Cu alloy

In situ synchrotron X-ray radiography of columnar dendritic and eutectic growth under directional solidification of Al-30wt%Cu has been carried out with nominal spatial and temporal resolutions of 1.4 μm and 150 ms, respectively. The images can be processed to reveal spatiotemporal co-ordinates of the solid-liquid phase fronts for both α-dendrites and the proceeding eutectic. Also estimates for local solute boundary layers ahead of and in the mushy zone can be extracted from image processing. Steady-state growth could not be realized in the experiments. Presumably this relates to a combination of: (a) insufficient time available for initial instabilities to evolve into a steady state; (b) primary α-branches may interact through solute diffusion in the confined sample volume; (c) the inevitable presence of thermo-solutal flow; and (d) a long-term sample position dependent drift in the furnace to sample heat transfers. Nevertheless, the data collected contain information that may be valuable for microstructure modelling of dendritic growth.