Grain Refining Mechanisms of Magnesium Alloy by Superheat-treating

C_2 - CI_6晶粒细化剂在工业上用于含铝镁合金的晶粒细化。然而，它有一个很大的问题，因为它是由氯化物分解产生的二氧胺物质。目前，它根本不能使用。另一方面，众所周知，过热处理对Mg-AI系合金的晶粒细化是有效的，但晶粒细化的机理尚未明确。该方法的优点是不使用C_2、CI_6等有害的谷物精制剂，保持了环境的清洁，是一项有用的技术，但由于温度高，消耗了较大的热能。本研究的目的是阐明az91镁合金的晶粒细化机理。将合金液在不同温度下淬火，经过过热处理后浇注到模具中。每个样品显示每个高温状态作为淬火。每个样品都经过光学显微镜和EPMA检查。初生镁枝晶是由AI_4 C_3的非均相核生成的。镁合金中Mn、Fe、Si、Cu等杂质元素在镁合金熔液中熔化，而C则留在过热处理后的合金熔液中。在此过程中，C与AI反应生成AI_4 C_3。并对高纯度Mg-AI合金进行了过热处理和不过热处理。由于AI_4 - C_3化合物无需过热处理就能很容易地形成，因此两种样品的晶粒都得到了细化。结果表明：工业AZ91合金中AI_4 C_3化合物是在过热处理过程中形成的，并形成了镁的初晶形核。

The most popular grain refiner consisting of C_2 CI_6 was used in industry for grain refining of magnesium alloy containing an aluminum element. However, it has a great problem, because it generate the dyoxine matter by the decomposition of chloride. At present, it cannot use at all. On the other hand, it is well known that the superheat treatment is effective for grain refining of Mg-AI system alloys, however, the grain refining mechanism is not clarified yet. The advantage of this method keeps clean the environment, because harmful grain refiners such as C_2 CI_6 were not used so that it is useful technology, except the larger thermal energy is consumed because of the high temperature.The purpose of this investigation is to clarify the grain refining mechanisms of the superheat-treated AZ 91 magnesium alloy. The molten alloys were quenched at various temperatures and cast into the mold after superheat treatment. Each sample reveals each high temperature state as quenched. Each sample is examined by an optical microscope and EPMA. The primary magnesium dendrites were generated from the heterogeneous nuclei of AI_4 C_3. The impurity elements in the magnesium alloy such as Mn, Fe, Si, and Cu were melted in the molten magnesium alloy, however, C was remained in the superheat treated molten alloy. In this process, C and AI reacted and formed AI_4 C_3. High purity Mg-AI alloy was also cast with and without superheat treatment. Both samples were refined for the grains because the AI_4 C_3 compounds were able to form easily without the superheat treatment. It is concluded that the AI_4 C_3 compounds in the commercial AZ91 alloy were formed during the superheat-treatment and they nucleated the primary crystals of magnesium.