摘 要
輕金屬近年來發展日趨成熟，鎂合金的應用範圍越來越廣泛。輕量化所需材料強度可以藉由晶粒細化的方式來提升。本研究嘗試在熔煉鎂合金中照射超音波以期達到晶粒細化之目的。本實驗使用的材料為純鎂、AZ91和AM60。利用光學顯微鏡與掃描式電子顯微鏡進行鎂合金的微觀組織分析，並配合X光能量散佈光譜儀進行介在物組成鑑定，最後探討其細化機制。實驗結果顯示，純鎂的晶粒尺寸由500μm減至200μm，隨著震盪時間的增長而減小。但是AZ91和AM60細化效果不如純鎂顯著。原因為鋁錳顆粒與氣泡結合後降低異質成核的能力。另外，本研究亦嘗試在純鎂中添加氧化鎂和奈米碳管等細化劑，其晶粒尺寸亦可細化到100μm之效果。晶粒細化主要受熔液中的空泡和介在物所影響。音波空泡會產生過冷度而促進成核點的發生，最終凝固的空泡亦會阻礙晶粒的成長。介在物有效地分散也可以達到細化組織之效果。以上乃依照本實驗結果將超音波對熔煉鎂合金產生晶粒細化所隱涵之機制探討出來。

Abstract
The development of light metals grows up completely in recent years. Applications of magnesium alloys are getting popular. Light weight must conform to the mechanical properties obtained by grain refinement. An aim of our study was to determine the effects of grain refinement of magnesium alloys by ultrasonic treatment. The materials used in this experimental were pure Mg, AZ91 and AM60. Analysis of the microstructure of the magnesium alloys were observed by optical microscope and scanning electrical microscope, the constitution of inclusion in the matrix identified by EDS. Finally, we tried to find out the mechanism for grain refinement of magnesium alloys by ultrasonic treatment. The grain size of pure magnesium reduced from 500 μm to 200 μm. However, the effects of grain refinement of AZ91 and AM60 were not apparent due to Al-Mn compound trapped in the bubble, which restricted the ability/ potency of heterogeneous nucleation. Moreover, both of the refiners, MgO particles and Carbon nanotubes, were available in this study. Its grain size of magnesium was about 100 μm. The grain refinement is affected by bubbles and inclusions in the melt. The results suggest that the bubbles generated from ultrasonic vibration induce surface cooling to nucleate. In solidification, bubbles can prevent grains from growth. The inclusions dispersed effectively in the melt will be also a good method for refining microstructure. The implications of these results for understanding the mechanism of grain refinement of magnesium by ultrasonic vibration treatment are discussed.

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