結合預時效與預應變處理對Al-Cu-Li合金的影響及T1 析出物的銀鎂偏析現象

Abstract

本研究所使用的AA2050(Al-Cu-Li)合金，隸屬於 2xxx系列，由於鋰（Li）元素的添加，使其具有高強度且輕量化的特性，在航空航太領域備受重視。為了獲得與工業原材料相當的機械性能，本研究重點探討預時效（Pre-aging）的有無對材料機械性質及微結構的影響。預時效的主要目標是在材料內部初步生成高密度的 GP區，這些區域能夠有效耗散差排滑移所產生的能量，進而減少預應變過程中的局部應力集中，使差排分佈更加均勻。因此，這種機制有助於促進 T1與θ’的析出均勻化，提升材料的強度，同時改善其延性。隨後透過拉伸試驗與穿透式電子顯微鏡（TEM）分析，比較不同熱處理條件下析出物的形貌與分佈，最終探討析出物與機械性質之間的關聯。 當材料經過預應變處理（未經預時效）後，在穿透式電子顯微鏡的明場像（BF）及高角度環形暗場影像（HAADF）中，可以觀察到靠近晶界處易形成大面積的T1析出物空乏區（PFA）。此外，由於差排易發生糾纏，使得T1析出物傾向聚集，最終形成類似次晶界（sub-grain boundary）的形貌。這種析出物分布不均的現象嚴重限制了材料的降伏強度與延性。當額外施加2h@120°C的預時效後，由於減緩了晶界附近預應變所導致的差排糾纏，延展性明顯提升。然而，降伏強度並未有顯著變化。透TEM觀察發現，這是因為能顯著提升強度的 θ'析出物仍處於早期的GP(θ’) 階段，其對差排移動的阻礙效果仍然有限。當預應變從 5%提高至8%（CPT8%），由於更高的差排密度，促進了θ'析出物的成核與成長，提升析出強化效果，使材料的降伏強度大幅提升。 本研究透過雙球差校正穿透式電子顯微鏡（Spectra 300），在原子尺度下觀察 GP(T1)--> T1'-->T1以及 GP(θ')-->θ'的析出物轉變機制，並進一步探討 T1與θ'之間的交互作用。研究結果間接證實，θ'對於T1在影響材料降伏強度方面具有更顯著的作用。 此外，透過EDS分析，對T1析出物進行定性與半定量分析，藉此解析各元素在析出物內的分佈。進一步探討 T1析出過程中關鍵合金元素的濃度變化，以揭示其在析出行為中的角色與影響。 最後，透過積分差分相位對比(iDPC)技術，實現了GP(T1)--> T1'-->T1中原子交換過程的可視化（鋁原子逐漸被鋰原子取代），進而揭示T1'相在GP(T1)向T1成長途徑中所扮演的關鍵角色。並且在較高指數的[112]Al晶軸下將T1析出物中的鋰原子清楚成像，為析出物的原子排列行為提供更進一步的驗證。

The AA2050 (Al-Cu-Li) alloy used in this study belongs to the 2xxx series. Due to the addition of lithium (Li), it exhibits high strength and lightweight characteristics, making it highly valued in the aerospace industry. This research focuses on investigating the effects of pre-aging on the mechanical properties and microstructure of the alloy to achieve mechanical performance comparable to industrial-grade materials. The primary objective of pre-aging is to initially generate a high density of GP zones within the material. These zones effectively dissipate the energy generated by dislocation slip, reducing local stress concentration during pre-straining and promoting a more uniform dislocation distribution. Consequently, this mechanism facilitates the uniform precipitation of T1 and θ' phases, enhancing the material's strength while improving its ductility. The study further employs tensile testing and transmission electron microscopy (TEM) to compare the morphology and distribution of precipitates under different heat treatment conditions, ultimately exploring the relationship between precipitates and mechanical propertiesWhen the material undergoes pre-straining without pre-aging, bright-field (BF) and high-angle annular dark-field (HAADF) images obtained from TEM reveal that large precipitation-free areas (PFA) of T1 often form near grain boundaries. Moreover, due to the tendency of dislocations to entangle, T1 precipitates are prone to clustering, eventually forming sub-grain boundary-like structures. This uneven distribution of precipitates severely limits the material's yield strength and ductility. However, when an additional 2 hours of pre-aging at 120°C is applied, the reduction in dislocation entanglement near grain boundaries during pre-straining significantly enhances ductility. Nonetheless, the yield strength shows no substantial improvement. TEM observations indicate that this is because the θ' precipitates, which significantly enhance strength, remain in the early GP(θ') stage, providing limited resistance to dislocation motion. When the pre-strain level is increased from 5% to 8% (CPT8%), the higher dislocation density further promotes the nucleation and growth of θ' precipitates, enhancing the precipitation strengthening effect and significantly improving the material's yield strength. This study employs a double-spherical aberration-corrected transmission electron microscope (Spectra 300) to observe the transformation mechanisms of GP(T1) -->T1'--> T1 and GP(θ') --> θ' at the atomic scale, further exploring the interaction between T1 and θ' precipitates. The results indirectly confirm that θ' exerts a more significant influence on yield strength compared to T1. Additionally, through EDS analysis, the distribution of key elements within the T1 precipitates is qualitatively and semi-quantitatively analyzed, providing insights into the concentration changes of critical alloying elements during the T1 precipitation process, thus revealing their roles and effects in the precipitation behavior. Finally, using the integrated differential phase contrast (iDPC) technique, the atomic exchange process within GP(T1) --> T1' --> T1 is visualized, showing the gradual replacement of aluminum atoms by lithium atoms. This visualization reveals the critical role of the T1' phase in the growth pathway from GP(T1) to T1. Moreover, the IDPC-STEM imaging at the higher-index [112]Al crystal axis successfully visualizes lithium atoms within T1 precipitates, providing further evidence of atomic arrangement behavior in precipitates.