Grain refining mechanism of high-purity Mg-9 mass percent Al alloy ingot and influence of Fe or Mn addition on cast grain size

摘要

Grain refining mechanisms of Fe - and Mn - free high-purity Mg-Al alloy ingots and the influences of a small addition of Fe or Mn on their cast structures were investigated. In order to prepare the alloy, a distilled pure magnesium (< 99.99 percent) and a commercial high-purity aluminum (99.99 percent) were melted, and given amounts of Fe and Mn were added into the molten alloys. Microscopic observation shows that the grain structure of high-purity alloys refined naturally without the use of a grain-refiner or superheat treatment, and that the addition of Fe and Mn elements coarsened the grains. In the range of Fe or Mn where magnesium will never crystallize from the melt as a primary crystal, both the grain-coarsening and the effect of superheat treatment is more remarkable. EPMA and AES analyses made clear a dispersion of fine particles composed of Mg, O, Al and C elements in the high-purity alloy. Some combination of these elements seems to cause an effective nucleation substance for magnesium crystal, which is similar to those whose presences have been confirmed in AZ91E magnesium alloys to which a proper amount of carbide was introduced. In coarse-grained alloys, Fe and Mn elements coexist with the above-mentioned nucleation substances, and thus disturb the grain refinement.