中文摘要
本研究分別採用噴覆凝固與金屬模重力鑄造兩種製程製備AC9A高矽鋁合金鑄錠，以得到不同粗細程度之凝固組織，並將此二試料經擠型後進行反覆熱衝擊試驗，以探討組織細化效應對該材料抗熱衝擊疲勞特性的影響。
金相觀察結果顯示，經擠型之噴覆凝固試料，其初晶矽及基地α-Al相晶相當微細，兩者均約為4μm左右，且其矽晶(包括初晶及共晶矽)呈圓鈍顆粒狀。相較之下，重力鑄造擠型材組織粗大許多(初晶矽以及α-Al相晶粒徑分別約70μm與32μm)，其初晶矽呈不規則板片狀，其共晶矽則為針棒狀。
為理解兩組試料機械性質上的差異，於熱衝擊疲勞試驗前，本研究先進行室溫至500℃拉伸性質探討，得知在室溫至500℃的溫度區間內，噴覆擠形材的拉伸強度與延性皆大於鑄造擠形材，加工硬化指數的比較結果亦顯示噴覆擠形材具有較佳的均勻塑變能力，此差異性尤其以室溫與100℃兩種拉伸溫度環境最為明顯。此外，各測試溫度下之噴覆擠形材拉伸破斷面均呈靨渦狀，而鑄造擠形材破斷面則可全面性觀察到斷裂的粗大初晶矽。
經由固定次數熱衝擊後進行室溫拉伸之實驗結果發現，於不同熱衝擊溫度下(本研究選定300℃、 350℃及400℃)，鑄造擠形材的拉伸強度均隨熱衝擊次數的增加而降低，而噴覆擠形材之熱衝擊誘發強度劣化現象並不明顯，甚至在較低的測試溫度條件下材料之降伏強度不降反升。由破斷面與次表面組織觀察結果推測，導致鑄造擠形材強度明顯劣化的原因應與熱衝擊過程中表面附近因反覆熱應力導致該區域粗大初晶矽顆粒破裂以及其與基地α-Al相間介面剝離有關。上述現象並未在噴覆擠形材經熱衝擊組織中被觀察到，應是該試料組織中矽晶微細、圓鈍且分佈均勻，不易因熱衝擊而造成破裂，加上其與基地間因膨脹係數差異所導致的熱應力集中效應較小，未導致介面剝離，因而具有良好的抗熱衝擊疲勞阻抗。

Abstract
To examine the microstructural refining effect on thermal shock fatigue of high-Si aluminum alloys, billets of the AC9A alloys have been fabricated by two processes in this study, namely, spray forming and permanent casting (metal mold casting). Both the spray-formed and metal mold-cast billets were extruded into rod-shaped specimens and then designated as “SFE” and “MME” respectively.
Microstructures of SFE specimens were extremely fine. Both the primary Si particles andα-Al grains were about 4 μm and with an equi-axed appearance. On the other hand, the MME specimens possessed a coarser structure. The massive primary Si particles with an irregular blade-like pattern and dendritic α-Al grains were about 70μm and 32μm respectively. In addition, eutectic Si particles were acicular in MME specimens but nodular in SFE specimens.
For realizing the differences in mechanical properties between the MME and SFE materials, tensile tests were performed from room temperature to 500℃ before thermal shocking testing. Experimental results indicate that the tensile strength and ductility of the SFE samples were superior than those of the MME samples. The SFE samples also exhibited a higher strain hardening exponent and better workability. Also, the tensile fracture surface of SFE samples showed a dimple pattern, while large amounts of broken primary Si particles could be observed on the fracture surface of the MME specimens.
Thermal shock fatigue was performed between the testing temperature (300℃, 350℃, and 400℃ were chosen in this study) and room temperature. After a fixed number of thermal shocking cycles, tensile properties of thermal-shocked specimens were examined. The results show that the tensile strength of the MME decreased with a higher cyclic numbers in all testing conditions. On the other hand, the thermal shock induced deterioration in tensile strength was not significant in the case of the SFE samples. Notably, the yield stress of the SFE specimens slightly increased after thermal shock cycling at lower testing temperature.
According to the observation results of fracture surface and subsurface microstructure, the thermal shock induced deterioration in tensile strength of the MME specimens could be directly related to the rupture of primary Si particles and interface separation between the primary Si and the Al matrix close to specimen surface which mainly resulted from the differences in coefficient of thermal expansion during the thermal shock cycling. Similar phenomena were not observed in thermal-shocked SFE specimens. In brief, the fair thermal shock fatigue resistance of the SFE specimens can be attributed to the fine, equi-axed, uniformly distributed primary Si particles which caused less thermal stress concentration and thus prevented from particle breaking and interface separating.

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