- 學位論文
時效處理對預應變Al-Cu-Li合金的影響及T1 析出物的原子尺度研究
Effect of Aging Treatments on Pre-Strain Al-Cu-Li Alloy and Atomic-Scale Investigation of T1 Precipitates
摘要
本實驗使用高強度且輕量化的AA2050 (Al-Cu-Li) 鋁合金，對其施加預應變，使材料內部充滿差排以促使析出物成核，再以不同的時效方法（如人工時效和潛變時效）促使大量奈米析出物T1生成，以強化材料基地。隨後測量材料的機械性能，包括硬度及拉伸試驗，並使用穿透式電子顯微鏡（TEM）和小角度X光散射（SAXS）分析不同時效階段的析出物形貌，探討材料顯微結構與機械性能之間的關係。 在材料預應變後，利用明場像及高角度環形暗場影像（HAADF）技術，觀察到T1析出物在差排、二次相邊界及晶界上的異質成核。發現基地中的差排和二次相邊界上析出許多單層T1，而晶界上則出現特殊的八層粗大T1析出物。這樣的晶界偏析也會造成析出空乏區。使用潛變時效能減少析出空乏區的大小，進而減少其帶來的負面影響。相比人工時效，潛變時效不僅可以提高材料的降伏強度（增加21MPa），還能提高材料的延展性，提供了一種有效的方法。 由於雙球差穿透式電子顯微鏡Spectra 300的引入，能夠深入了解析出物的原子尺度結構，本文還分析了T1的前驅物GP(T1)及T1之間的差異與它們的in-situ轉變機制，並發現T1在兩種增厚機制互相糾結的特殊現象。此外，拍攝了拉伸試驗後的試片，展示了在不同晶帶軸下T1析出物被差排切過的影像。最後，通過積分差分相位對比（iDPC）技術，首次實現了T1析出物中鋰原子的直接成像，為深入解析析出物的各項機制提供了新的研究方法。
並列摘要
This study utilizes the high-strength and lightweight AA2050 (Al-Cu-Li) aluminum alloy, which is pre-strain to introduce dislocations within the material, facilitating the nucleation of precipitates. Subsequent aging treatments, such as artificial aging and creep aging, are applied to generate a significant amount of nano-scale T1 precipitates, thereby strengthening the matrix. The mechanical properties, including hardness and tensile tests, are then measured. Additionally, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) are used to analyze the morphology of the precipitates at different aging stages, exploring the relationship between the microstructure and mechanical properties of the material. After pre-strain, bright-field (BF) and high-angle annular dark-field (HAADF) imaging techniques reveal the heterogeneous nucleation of T1 precipitates at dislocations, second-phase boundaries, and grain boundaries. Numerous single-layer T1 precipitates are observed at dislocations and second-phase boundaries, while unique eight-layer thick T1 precipitates appear at grain boundaries. This elemental segregation at grain boundaries also results in precipitate-free zones. Creep aging reduces the size of these precipitate-free zones, thereby decreasing their adverse effects. Compared to artificial aging, creep aging not only increases the yield strength of the material (by 21 MPa) but also enhances its ductility, providing an effective treatment method. With the introduction of the double spherical aberration-corrected transmission electron microscope Spectra 300, an in-depth understanding of the atomic-scale structure of precipitates is achieved. This study also examines the differences between GP(T1) (also known as T1p) and T1, along with their in-situ transformation mechanisms, revealing a unique phenomenon where T1 thickening mechanisms are intertwined. Additionally, post-tensile test samples are imaged, showing dislocation shearing of T1 precipitates along different crystallographic orientations. Finally, by employing integrated differential phase contrast (iDPC) techniques, the direct imaging of lithium atoms within T1 precipitates is achieved for the first time, offering a novel method for detailed analysis of precipitate mechanisms.