摘要 A356(Al-7Si-0.35Mg)合金屬於亞共晶型Al-Si合金，其微結構為初晶鋁、共晶矽及富鐵相。未改良之共晶矽會以粗針狀型態出現，而此種型態將對合金之機械性質造成相當不良之影響；現今常以Na、Sr及Pb作為改良劑改良共晶矽型態。本研究目的在於探討添加不同含量稀土元素La、Ce對A356合金之微結構及其機械性質之影響。 實驗設計添加各種不同含量(0.2、0.4、0.6、0.8及1.0wt.%)La、Ce之A356合金，以金相觀察(OM)、破斷面觀察、電子微探儀(EPMA)、影像分析及熱分析法觀察合金微結構並評估其共晶矽改良與否及其改良之程度；並以硬度試驗、拉伸試驗及品質指標來評估A356合金之機械性質。 實驗結果顯示，稀土元素La的添加量達至0.6wt.%以上時，其共晶矽開始有較明顯的改良效果，延伸率及品質指標也開始有較明顯提升的現象；當La添加至1.0wt.%時，其共晶矽已完全改良至球狀纖維化共晶矽，延伸率及品質指標也有相當大幅度的提升，其改良效果類似於工業上常用之改良劑Sr。當稀土元素Ce的添加量達至0.6wt.%時，其共晶矽開始產生改良效果，延伸率及品質指標也開始有較明顯提升的現象；當添加1.0wt.% Ce時，其共晶矽呈現出層狀結構，類似於以Sb改良後之微結構，其延伸率及品質指標相較於未經改良之A356合金亦有大幅度的改善。 Abstract Modification of the Al-Si eutectic from a flake-like to a fine fibrous silicon structure can be achieved in two ways ; by addition of certain elements(chemical modification) or with a rapid cooling rate(quench modification). Several elements are known to cause chemical modification. The most common elements used in industry today are Sr and Na, which change the silicon morphology from coarse and plate-like to fine and fibrous, and Sb which causes a refinement in the plate-like silicon structure. The aim of the present paper was to investigate the individual effects of rare earth element La and Ce on the eutectic microstructure and mechanical properties in A356 (Al-7Si-0.35Mg) alloys. In this study, eutectic microstructures and mechanical properties in samples modified by minor rare earth La and Ce in an A356 alloy have been determined by optical microscopy(OM), scanning electron microscopy(SEM),thermal analysis, image analysis, hardness test and tensile test. The fine fibrous eutectic structure that was obtained with La additions is typical of that obtained with conventional modifiers such as Sr and Na. Modification on the eutectic microstructure and mechanical properties improves with increasing additions of La until a fully modified eutectic structure is obtained at 1.0 wt.% La. Large additions of Ce did not modify the eutectic silicon and only resulted in a small refinement of the plate-like morphology. The behavior of the majority of rare earth element Ce is therefore more similar to that of Sb than Sr or Na.
