In situ synchrotron tomographic investigation of the solidification of an AlMg4.7Si8 alloy

The solidification sequence of an AlMg4.7Si8 alloy is imaged in situ by synchrotron microtomography. Tomograms with (1.4 mu m)(3)/voxel have been recorded every minute while cooling the melt from 600 degrees C at a cooling rate of 5 K min(-1) to 540 degrees C in the solid state. The solidification process starts with the three-dimensional evolution of the alpha-Al dendritic structure at 590 degrees C. The growth of the a-Al dendrites is described by curvature parameters that represent the coarsening quantitatively, and ends in droplet-like shapes of the secondary dendrite arms at 577 degrees C. There, the eutectic valley of alpha-Al/Mg2Si is reached, forming initially octahedral Mg2Si particles preferentially at the bases of the secondary dendrite arms. The eutectic grows with seaweed-like Mg2Si structures, with increasing connectivity. During this solidification stage Fe-aluminides form and expand as thin objects within the interdendritic liquid. Finally, the remaining liquid freezes as ternary alpha-Al/Mg2Si/Si eutectic at 558 degrees C, increasing further the connectivity of the intermetallic phases. The frozen alloy consists of four phases exhibiting morphologies characteristic of their mode of solidification: alpha-Al dendrites, eutectic alpha-Al/Mg2Si Chinese script with Fe-aluminides, and interpenetrating alpha-Al/Mg2Si/Si ternary eutectic. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.