We conducted this study to clarify the relationship between manganese solubility and grain sizes of Mg-Al alloys. Mg-5%Al, Mg-9%Al, and Mg-11%Al alloys were prepared using high-purity magnesium (>99.99%) and aluminum (99.99%). These alloys were melted at 1123 K by adding electrolytic manganese (99.99%) and solidified as alloy ingots after continuous stirring for 21.6 ks. The alloy ingots were then remelted at 933, 998, and 1073 K, and held statically at each of these temperatures for 21.6 ks. By this process, the excess Mn that existed over its liquid solubility precipitated and sedimented to the bottom of the melt. Thus, the solubility of Mn in liquid Mg-Al alloys was determined by analyzing the upper surface of the ingots quenched from each holding temperature. Consequently, the following equations were derived as experimental formulae for determining solubility of Mn in liquid Mg-Al alloys at different temperatures: & Y=1.79-6.22×10^-2X (1073 K),
& Y=1.28-4.41×10^-2X (998 K),
& and Y=0.90-3.35×10^-2X (933 K), where X is the concentration of Al (mass%) and Y is solubility of Mn (mass%).
Mg-9% Al alloys containing 0 to 3% Mn were prepared in order to investigate the influence of Mn on grain sizes, and microscopic observations of these alloys were carried out with and without superheating of the specimens. The grain diameter of high-purity Mg-9% Al alloys (0% Mn) is approximately 40 μm, which is finer than that of the commercial AZ91E magnesium alloy with superheating. Therefore, high-purity Mg-9% Al alloys have an essentially fine-grained structure. An increase in the Mn content tended to coarsen the grain structure of Mg-9% Al alloys that contain 0.02 to 2.27% Mn; however, the coarse-grained structure can be refined by superheating. Superheating plays a role in resetting the coarse-grained structure to a high-purity structure (initially fine-grained) when Mn is added. Further, it is clarified that the solubility of Mn in liquid Mg-Al alloys exhibits no relationship with either the grain size or the superheating effect.