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 μm)³/voxel have been recorded every minute while cooling the melt from 600 °C at a cooling rate of 5 K min^-1 to 540 °C in the solid state. The solidification process starts with the three-dimensional evolution of the α-Al dendritic structure at 590 °C. The growth of the α-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 °C. There, the eutectic valley of α-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 α-Al/Mg₂Si/Si eutectic at 558 °C, increasing further the connectivity of the intermetallic phases. The frozen alloy consists of four phases exhibiting morphologies characteristic of their mode of solidification: α-Al dendrites, eutectic α-Al/Mg₂Si "Chinese script" with Fe-aluminides, and interpenetrating α-Al/Mg₂Si/Si ternary eutectic.