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 mum and 150 ins, respectively. The images can be processed to reveal spatiotemporal co-ordinates of the solid-liquid phase fronts for both alpha-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 alpha-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. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.