AA7050 (Al-Zn-Mg) 和 AA2050 (Al-Cu-Li) 鋁合金 原子級析出物之演化

Abstract

常用於航太與國防產業之鋁合金擁有高強度與輕量化特性。對於時效強化AA7050 (Al-Zn-Mg-(Cu))鋁合金與AA2050 (Al-Cu-Li)鋁合金而言，高強度特性，取決於基地內添加不同溶質原子，如：鋅(Zn)、鎂(Mg)、銅(Cu)和鋰(Li)等原子；並搭配不同時效熱處理，如：二階段時效熱處理(two-step ageing treatment)和潛變時效熱處理(creep age forming treatment)，而促使奈米尺度-析出物大量生成而強化基地。近年來，電子顯微鏡應用於鋁合金-析出物形貌與顯微結構分析，逐漸由常見高解析晶格影像技術(即：lattice imaging)，轉變為搭配球面像差校正原子級影像技術(即：Z-contrast imaging)。本研究結合穿透式電子顯微鏡-高解析影像(HR-TEM)和球面像差校正-高解度掃描式電子顯微鏡-原子級影像(Cs-corrected HAADF-STEM)，建立Al-Zn-Mg-(Cu)和Al-Cu-Li鋁合金其:(1)納米等級析出物顯微結構、(2)析出物之間成核成長機制與(3)析出物和差排之間交互作用。 在鑑定不同析出物種類與結構時，高解析穿透式電子顯微鏡對應不同析出物之晶格影像，方可轉換成相對應快速傅立葉轉換衍射圖譜(FFT diffractograms)，再結合模擬之衍射圖譜方可辨別不同析出物之結構，如：本研究所探討AA7050 (Al-Zn-Mg-(Cu))鋁合金中GP zones、η'析出物和η析出物與AA2050 (Al-Cu-Li)鋁合金中T1析出物和θꞌ析出物。 此外，球面像差校正高解掃描式電子顯微鏡-原子級影像搭配第一原理(Vienna Ab-initio Simulation Package，VASP)原子布局模擬，方能更深入探討析出物之原子結構。本研究提出AA7050 (Al-Zn-Mg-(Cu))鋁合金中含有13種η析出物種類，有別於以往文獻所報導的11種η析出物。藉由不同析出物之晶軸觀察方向，如:[10'1' ̅0]η，[1'2' ̅10]η，和[0001]η，歸類主要三種原子排列結構分別為：sandwiched stacking、zig-zag stacking和hexagonal stacking結構。而溶質原子團聚至析出物/鋁基地界面，改變其周圍應變能而造成逐層之間部分原子堆疊錯誤(stacking fault)產生，方可視為層與層之成長機制(layer-by-layer growth mechanism)所造成。此外，伴隨析出物成長過程中，成核成長於相同或不同habit plane上的析出物，相互碰撞產生結合(coalescence)現象，而誘發複雜的晶界面(grain boundary)或高對稱性的雙晶界面(twin boundary)，進一步顛覆鋁合金析出物顯微結構演化之概念。並再藉由析出物內部真實原子排列條紋(lattice fringes)相對應於鋁基地之方位關係，證實另外兩類η析出物，並命名為:η4'和η12。而AA2050 (Al-Cu-Li)鋁合金中，片狀分布於{11'1' ̅}Al、{100}Al和{120}Al面之T1析出物(4 variants)、θꞌ析出物(3 variants)和S析出物(12 variants)與相對應單一片狀前驅物GP(T1) zones、GP(θꞌꞌ) zones和GPB zones，亦由球面像差校正高解掃描式電子顯微鏡，主要探討T1析出物和θꞌ析出物其內原子排列結構與析出物之間交互作用。對豎立於{11'1' ̅}Al面之T1析出物而言，Li原子層鑲埋於擁有高原子序對比的兩側Al-Cu/Al-Li原子層；相反地，對豎立於{100}Al面之θꞌ析出物而言，擁有高原子序對比的數層Cu原子卻被富含Li原子之δꞌ相所包覆。而T1析出物和θꞌ析出物成長過程中，其不同variants之間常有hard impingement和soft impingement現象發生。甚至θꞌ析出物不同variants之間有相互阻撓其成長現象。 於探討不同析出物的成核機制時，利用即時穿透式電子顯微鏡-高解析影像分析AA7050 (Al-Zn-Mg-(Cu))鋁合金析出物的相轉化過程，即GP (I and II) zones → η'析出物→ η析出物。於GP zones → η'析出物過程中，基地內，部分GPII zones完全溶解消失，而於基地不同位置隨之成核成長出η'析出物，此成核成長方式被稱為：separated成核機制。除此之外，由穿透式電子顯微鏡-高解析影像，觀察單一顆析出物同時含有相近六方最密堆積之η'析出物與η2析出物結構，且結合奈米尺度-能量散射X-ray光譜儀解析其相對應位置之η'析出物與η2析出物不同組成成分。由結構與組成成分的變化則可推測η'析出物隨時效熱處理過程逐漸被η2析出物，此成核成長方式被稱為：in-situ成核機制。除此之外，亦可觀察到有富含Mg含量之sandwiched stacking和富含Zn含量之zig-zag stacking結構同時存在於單一析出物內。 於AA2050 (Al-Cu-Li)鋁合金，觀察θꞌ析出物成核與成長過程。成長於基地{100}Al面之單層富含Cu原子層，成核於θꞌ析出物與S析出物之相交處或S析出物與基地之界面處，此成核成長方式被稱為：sympathetic成核機制。且藉由不同時間點之高解度掃描式電子顯微鏡-原子級影像觀察發現單層Cu原子層GP(θꞌꞌ) zones → θꞌ析出物過程亦為in-situ成核機制。而清楚可見的滑移面存在於豎立在{11'1' ̅}Al面之T1析出物和{100}Al面之θꞌ析出物，可視為cutting機制下，差排與攜出物交互作用之結果，而顛覆以往文獻上T1析出物和θꞌ析出物對析出強化之貢獻。

Aluminium alloys are widely used in the aerospace and national defense industries owing to their high strength and light weight properties. For the age-hardenable AA7050 (Al-Zn-Mg-(Cu)) aluminium and AA2050 (Al-Cu-Li) aluminium alloys, the high strength depends on the nano-scale precipitates, of which the formation is governed by the variable additions of solute atoms, such as Zn, Mg, Cu, and Li in the Al matrix, accompanying with the different ageing treatments, such as two-step ageing and creep age forming (CAF) treatments. Recently, the electron microscope has evolved from a general instrument for analysing the lattice images of nano-scale precipitates in Al-based alloys to the Cs-corrected microscope of choice, with which the Z-contrast configurations of precipitates can be observed with atomic-scale microstructure evolution. In this study, it is straightforward to establish the atomic-scale microstructural evolutions of precipitates (i.e., the phase identifications and nucleation mechanisms) by employing high resolution transmission electron microscopy (HR-TEM) and Cs-corrected high-angle-annular-dark-field scanning-transmission-electron microscopy (Cs-corrected HAADF-STEM). For identification of the microstructures of precipitates, HR-TEM was employed to investigate the lattice images of precipitates in combination with the corresponding FFT diffractograms and simulated patterns according to the orientation relationships between precipitates and the Al matrix, such as GP zones, η' precipitates, and η precipitates in the AA7050 aluminium alloy, and T1, and θꞌ precipitates in the AA2050 aluminium alloy. In addition, Cs-corrected HAADF-STEM micrographs combined with first principles calculation (Vienna Ab-initio Simulation Package, VASP) clearly showed precipitates with different atomic Z-contrast configurations and atomic arrangements. Here, we have proposed that 13 types of η precipitates existing in the AA7050 aluminium alloy, instead of 11 types as reported previously. The HAADF STEM images taken along [10'1' ̅0]η, [1'2' ̅10]η, and [0001]η illustrated three main kinds of the projected atomic-scale configurations of η-MgZn2 crystal, i.e., the sandwiched stacking, zig-zag stacking, and hexagonal stacking structures. The precipitates developed in the layer-by-layer growth, supplied with precursors such as Zn, Cu, and Mg which solute atoms that were segregated around the MgZn2/Al interface due to its higher lattice strain energy. Furthermore, coalescence occurred between two precipitates grown with the similar or different habit planes, yielded a complicated grain boundary or a symmetric twin boundary. Based on the difference in orientation relationships and atomic lattice-fringes between η-types and the Al matrix reported so far, two new types of η precipitates have been recognized and named η4' and η12. On the other hand, the microstructures of T1 precipitates (4 variants), θꞌ precipitates (3 variants), S precipitates (12 variants), grown on {11'1' ̅}Al, {100}Al and {120}Al habit planes, respectively, with their mono-layer precursors, GP(T1) zones, GP(θꞌꞌ) zones, and GPB zones also can be analyzed by Cs-corrected HAADF-STEM. For the atomic configuration of T1 precipitates, two combined Al-Cu/Al-Li layers with the higher Z contrast surround the dark center Li-layer. For the atomic configuration of θꞌ precipitates, several Cu-rich layers are coated with the Cu-solute depleted zones, which can be recognized as δꞌ phases. The hard impingement or soft impingement would occur, wherein the one part of different T1 and θꞌ variants would be block out by the other variant. To investigate the nucleation mechanisms of precipitates, the transformation sequence of GP (I and II) zones → η' precipitates → η precipitates in the AA7050 aluminium alloy was investigated with serial in-situ HRTEM frames. The investigation found that separated nucleation of η' precipitates occurred elsewhere as the adjacent GPII zone dissolved. Furthermore, evidence from HR-TEM images coupled with nanometer-scale energy-dispersive X-ray (EDX) revealed in-situ nucleation of a new η2 precipitate (one form of η), wherein it gradually developed from the original η' precipitate via a similar hexagonal structure with different compositions. The in-situ transition product was composed of two distinctive regions; one was identified as η', and the other, as η. In the AA2050 (Al-Cu-Li) aluminium alloy, it was focused on the phenomena of θꞌ precipitate formation. The newly-formed nucleus of GP(θꞌꞌ) with a mono-Cu layer on the {100}Al habit planes appeared in the area adjacent to the joint between θꞌ and S precipitates or at the edge of an individual S precipitate, which was recognized as probably sympathetic nucleation. Moreover, from ex-situ HR-STEM observation, the transition of a single Cu-layer GP(θꞌꞌ) → θꞌ was presumed to be transformation via in-situ nucleation. In addition, the observation along the respective [110]Al and [001] Al zone axes, the clear slip planes showing the shearing on T1 and θꞌ precipitates indicated the cutting mechanisms between precipitates and dislocations.