改變銅添加量以及熱處理製程對AA7075鋁合金之奈米級析出物以及機械性質的影響

Abstract

本研究聚焦於Al-Zn-Mg-Cu鋁合金之顯微結構對於機械性質的影響。實驗比較不同的固溶熱處理，以及不同溫度的人工時效下，合金顯微結構與機械性質之改變。並以晶體結構學為基礎，建立電子顯微鏡影像之奈米級析出物量化方法，連結材料之顯微結構與機械性質。研究之兩種材料皆被歸類為AA7075鋁合金，分別為Al-5.98Zn-2.86Mg-1.61Cu (7075-LCu)以及Al-5.98Zn-2.83Mg-1.98Cu (7075-HCu)鋁合金，兩者主要差異為Cu含量。 本研究包含合金材料在取得階段，以及熱處理前後的顯微組織以及機械性質。熱處理研究範圍包含固溶熱處理，自然時效以及人工時效，即工業上所稱之T4以及T6製程。 於熱處理前階段（即材料取得階段），本研究以微差式掃描量熱法及場發射電子微探儀發現7075-LCu與7075-HCu之二次相組成有明顯差異，並據此設計多段式固溶熱處理以防止初熔(incipient melting) 現象的發生，實驗結果發現多段式持溫的熱處理相較於單一溫度熱處理，可在大幅降低初熔的情況下，有效將二次相固溶回到基地，同時提升合金之延展性約20%。 自然時效階段，本研究發現兩種合金皆可在一週之內硬度大幅提升75%，根據穿透式電子顯微鏡觀察，並結合拉伸結果以及小角度X-ray散射實驗分析，得知此強化應為固溶原子團的析出所貢獻。 在人工時效階段，本研究分別就100、120、140以及160 °C之0 – 24 小時內找出尖峰時效，並針對所有尖峰時效的奈米級析出物，進行電子顯微影像進行量化統計，以及機械性質之量測，建立析出物與機械性質之連結。研究結果發現機械形質與析出物的兩個因子相關，分別是尺寸與密度。研究發現，在490 °C固溶熱處理30分鐘後，後對兩種材料而言，最佳的人工時效條件皆為120 °C持溫 24 小時。此時破斷強度可分別達 623 MPa (7075-LCu)以及632 MPa (7075-HCu)。η' 相析出物的厚度則分別為2.15奈米 (7075-LCu)以及2.04奈米 (7075-HCu)，而密度分別為3.27×1017 cm-3以及3.52×1017 cm-3。實驗結果可與材料科學的cutting through 與Orowan looping理論機制相互呼應。 針對Al-Zn-Mg-Cu鋁合金之主要強化相之一η' 相，本研究發現其粒徑可成長到10 nm，厚度可達到5 nm而尚未轉變成η (MgZn2)相，並透過STEM-EDX結果分析推測Cu元素在成長過程逐漸擴散進入η' 相中。在相同熱處理溫度以及時間下，平均而言，η'以及η析出物在7075-HCu鋁合金較7075-LCu鋁合金中之成長行為緩和，機械性質也較佳。

This research focused on the relation between the microstructure and the mechanical properties in the Al-Zn-Mg-Cu aluminum alloys. Different solid solution treatments and artificial ageing treatments were designed, and the consequent microstructures and mechanical properties were further analyzed. The mechanical properties were found to be related to the size and the number densities of the precipitates. This finding corresponds to the cutting through-Orowan looping theorem. Based on the crystal structure, a method for quantifying nanoscale precipitates in electron microscopy images was established in this study. This method was applied to require the information of size and number density of the major precipitates, η', in the AA7075 alloys. The two studied aluminum alloys were both AA7075 alloys. The main difference between the two was the Cu content. They were Al-5.98Zn-2.86Mg-1.61Cu (7075-LCu) and Al-5.98Zn-2.83Mg-1.98Cu (7075-HCu) aluminum alloys, respectively. The heat treatments for the alloys included solution heat treatment, natural ageing (NA) and artificial ageing, known as the T4 and T6 processes in the industry. In the stages before and after solution heat treatment, differential scanning calorimetry (DSC) and an electron probe micro analyzer (EPMA) were used to characterize the secondary phases of 7075-LCu and 7075-HCu. This study found a significant difference in the major secondary phases of the two alloys. Solid solution treatments with special steps were designed based on this finding. The stepped solid solution treatments greatly reduced the phenomenon of incipient melting and effectively dissolved the secondary phases back into the Al matrix. Compared with the single-step solution treatment, the designed stepped solution treatments improved the ductility of the alloy by 20%. In the NA stage, this study found that the hardnesses of both alloys could be increased by 75% within one week. According to observations by transmission electron microscope (TEM), analysis of tensile tests, and analysis of small angle X-ray scattering (SAXS), this strengthening effect is contributed by solutes smaller than 2 nm in radius, which form during quenching and NA. In the artificial ageing stage, in this study, peak ageing conditions were determined within 24 hours at 100, 120, 140, and 160 °C, respectively, and electron microscopy images of nano-scale precipitates were analyzed for all peak ageing conditions. Measurements were performed to establish a link between the precipitates and mechanical properties. In the present work, the average size and the number density of the η' precipitates were found to correlate with the mechanical properties of the Al-Zn-Mg-Cu alloys. The best ageing treatment for both studied alloys is artificial ageing at 120 °C for 24 h, which produces strengths of 623 MPa for 7075-LCu and 632 MPa for 7075-HCu, respectively. Correspondingly, the average thicknesses of η' precipitates are 2.15 nm (7075-LCu) and 2.04 nm (7075-HCu), and the average number densities of η' precipitates are 3.27×1017 cm-3 and 3.52×1017 cm-3, respectively. Regarding the η' phase, the major strengthening phase of Al-Zn-Mg-Cu aluminum alloys, this study found that the diameter could grow to 10 nm and the thickness could reach 5 nm without the particle transforming into the η (MgZn2) phase. STEM-EDX results suggested that Cu gradually diffused into the η' phase during the growth process, leading to Cu-depleted regions around the η'/η precipitates. In addition, it was also found that, with the same heat treatment temperature and time, the growth behavior of η' in 7075-HCu aluminum alloys was milder than that in 7075-LCu aluminum alloys, and the mechanical properties of the samples were also better.