鎂合金相對於其它結構金屬重量輕、比強度高，因此具有很高的發展性。但其鑄造成品在常溫下的力學性能不佳，因此一般得須透過塑性加工或是熱處理方能改善，而凝固組織的形態是決定材料力學性能的關鍵因素。方向性凝固鑄造為透過周邊熱束縛及單一方向的冷卻的方式使得金屬材料的凝固組織形成柱狀組織而非傳統鑄造的等軸晶粒組織，而透過其所製備的成品在機械性質有較優異的表現。本研究將利用方向性凝固熔煉爐製備AZ31、AZ91鎂合金、AZ31/Al2O3p、AZ91/Al2O3p鎂合金複材，並探討溶質元素含量及冷卻速率對於微觀組織及機械性質的影響。
　　本研究結果顯示，方向性凝固鑄件中段會形成柱狀晶粒組織，隨著冷卻速率的提升β相Mg17Al12分布越細緻且排列越整齊。添加Al2O3p會抑制柱狀晶的成長，AZ91/Al2O3p方向性成長範圍明顯縮短，而AZ31/Al2O3p鑄件各段顯微組織則皆為等軸晶粒。AZ31、AZ91平均硬度值皆會隨著冷卻速率提高而上升，在添加陶瓷顆粒後AZ31/Al2O3p平均硬度值會上升但AZ91/Al2O3p會下降。改以方向性凝固製備後兩種合金的降伏強度、極限強度皆會提升，AZ31分別提升了161.6%、48.9%；AZ91則分別提升71%、69%。AZ91改以方向性凝固後鑄件之延展性能有效提升，但AZ31在高冷卻速率條件下延展性下降。

Magnesium alloy possesses great potential because it has relatively lower weight and better specific strength than other constructional alloys. However, the mechanical properties of its cast product are poor at room temperature, usually it requires further plastic manufacturing or heat treatment to enhance, and the key factor affecting the mechanical properties is the microstructure after solidification. Directional solidification casting is a method to make the metallic material form columnar structure instead of equiaxed structure, which is commonly observed in conventional casting. And the product made by directional solidification exhibit superior mechanical properties. This study fabricated AZ31, AZ91, AZ31/Al2O3p and AZ91/Al2O3p from the directional solidification furnace, and the effect of solute element and cooling rate on the microstructure and mechanical properties was investigated.
The results showed that columnar grains were formed inside the middle part of casting, the β phase (Mg17Al12) became finer and distributed orderly as the cooling rate increased. The addition of Al2O3p will restrict the growth of columnar grains, there was significantly decreasing on the range of directional growth for AZ91/Al2O3p, while AZ31/Al2O3p exhibited full equiaxed structure. After transferring to directional solidification process, both of two alloys show improvement on the yield strength and ultimate tensile strength, which is 161.6%, 48.9% for AZ31 and 71%, 69% for AZ91. The ductility of AZ91 were improved, but the ductility of AZ31 decreased in high cooling rate condition.

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