變形對AA7075鋁合金顯微結構的影響及 η相析出物中相變誘導雙晶之原子級研究

Abstract

本研究探討材料微結構與機械性質之關聯，以7系列鋁合金作為研究對象。7系列鋁合金透過奈米析出物η、η’ 提高機械強度，本研究透過掃描/穿透式電子顯微鏡（Scanning/Transmission Electron Microscope, S/TEM）觀察析出物分布、尺寸與形貌，並藉由高解析穿透式影像（HR-TEM）解析η的複雜方位關係，與高解析掃描式電子顯微鏡（HR-STEM）拍攝η原子級影像，深入探索其中的缺陷及雙晶關係。 本研究的第一部分探討施加應變對於AA7075鋁合金中析出物的影響，分析其對於機械性質的影響，並提出改良製程以有效提高降伏強度。研究中發現，時效前的預變形（Pre-stretching）所導入的差排，會在後續時效析出過程中，導致析出物在差排上的粗化，從而對析出強化產生負面效果。為了解決這一問題，本研究提出了時效中期變形（Interrupted-stretching）製程，能有效減少析出物的粗化程度，在相同應變量6%的情況下，提高AA7075合金降伏強度43 MPa。 本研究的第二部分針對η原子級影像進行深入分析，在析出物η中辨認了三種雙晶關係，並且因為雙晶關係產生新的析出物種類η17。此外，研究還發現η（C14結構）具有C15結構的前驅物，並且通過原位相轉變（In-situ transformation）機制進行轉變。前驅物的揭示解釋了η多種方位關係及其中缺陷（包括疊差Stacking Faults和雙晶Twins）的起源。

This study investigates the relationship between microstructure and mechanical properties of AA7xxx series aluminum alloys. These alloys enhance mechanical strength through nano-precipitates η and η’. Using Scanning/Transmission Electron Microscopy (S/TEM), the distribution, size, and morphology of the precipitates were observed. High-Resolution Transmission Electron Microscopy (HR-TEM) was used to analyze the complex orientation relationships of η, and High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) captured atomic-level images of η, exploring defects and twinning relationships in detail. The first part of this study examines the effect of applied strain on the precipitation evolution in AA7075 aluminum alloy and its impact on mechanical properties. A process improvement is proposed to effectively increase the yield strength. It was found that dislocations introduced by pre-stretching before aging lead to coarsening of precipitates along dislocations during subsequent aging, negatively affecting precipitation strengthening. An interrupted-stretching process during mid-aging was proposed to mitigate precipitate coarsening. With the same 6% strain, this process can improve the yield strength by 43 MPa in the present AA7075 alloy. The second part of this study provides an in-depth analysis of atomic-level images of η precipitates, identifying three types of twinning relationships. These relationships resulted in a new precipitate orientation relationship, η17. Additionally, it was discovered that η (C14 structure) has a precursor with C15 structure, which transforms via an in-situ transformation mechanism. The revelation of this precursor explains the various orientation relationships of η and the origins of defects, including stacking faults and twins.