Grain refinement of cast Mg-Al alloys

Abstract

To understand the grain refinement mechanisms by C addition and superheating of Mg-Al system alloys, two kinds of experiments were performed. In the first experiment, a commercial AZ91E magnesium alloy was melted and C2Cl6 was added as a carbon nucleant at 1023 K. In the second experiment, normal-purity and high-purity Mg-Al alloys containing 5.6 to 14.1%Al made of 99.9 and 99.99%Mg and 99.99 and 99.999%Al were superheated at 1123 K for 900 s before casting. Metallographic examinations were performed to clarify the solidified structures. In the addition of carbon, rod-like subsance as nucleants could be observed in each grain. EPMA analysis indicated that rod-like subatance was constituted of Al, C and O. The grain refinement of high-purity Mg-Al alloys occur without the superheating. Therefore it is difficult to evaluate the effect of impurities on superheating. In high-purity Mg-Al alloys, however, it seems that minute amount of C is responsible for grain refinement.