Microstructures of Al-3%Mg alloy and commercially pure aluminum uni-directionally solidified at various speeds

Abstract

Al-3%Mg alloy and commercially pure aluminum were uni-directionally solidified at various speeds (0.530.008cm/sec) and their microstructures, especially subboundary structures and secondary phase decomposition, were studied by electron microscopic examination.
Many dislocations were observed in the both materials solidified at high speeds. They were considered to have resulted from the constitutional stress due to the solid segregations. Subboundary structures were more frequently observed with the decrease in the speed of solidification. They consisted of relatively regular networks of dislocations in Al-3%Mg alloy, and of densely tangled dislocations in commercially pure aluminum. These boundaries were arranged along dendritic cell boundaries with the secondary phase particles or along the secondary dendritic arms boundaries with high solute segregations.
In commercially pure aluminum, the secondary phase decomposes chiefly consisted of Al₆Fe when solidified at a high speed. However, the amount of Al₆Fe decreased and that of Al₃Fe increased with the decrease in solidification speed. At the speed of 0.011cm/sec in commercially pure aluminum, no Al₆Fe phase was observed, but there coexisted Al₃Fe and an eutectic decompose of unknown structure (perhaps, AlFeSi ternary compound).