7075鋁合金之奈米級析出物與機械性質關聯性研究

Abstract

本篇論文主要研究7075鋁合金的析出強化，透過不同熱處理製程影響析出物的發展，可分為無自然時效(NA0d)及自然時效7天(NA7d)兩大類，並深入探討析出物大小、分佈及體積分率與機械性質之間的關係。 對於主要強化相η’析出物的研究，主要透過穿透式電子顯微鏡(Transmission Electron Microscope, TEM)進行觀察及統計，透過投影修正的方式還原圓盤狀η’析出物之圓形面積，再轉換為等效直徑進行比較，另外透過將η’析出物豎立的方式，得到η’析出物之厚度資訊。而小角度X-ray散射實驗(Small Angle X-ray Scattering, SAXS)則是用以彌補穿透式電子顯微鏡對於析出物定量上的不足，輔助確認η’析出物之大小以及體積分率，進而與硬度試驗及拉伸試驗結果作比對。對於析出物種類的鑑定，則仰賴高解析穿透式電子顯微鏡(High Resolution TEM, HRTEM)拍攝之高解析晶格影像，經過快速傅立葉轉換(Fast Fourier Transform, FFT)所得之繞射圖型，比對由CaRine Crystallography模擬之理論繞射圖型 於自然時效階段之TEM影像並無觀察到球狀的GPI Zone以及片狀的GPII Zone存在， 只能觀察到團狀的原子，且無法確定GPI Zone生成的數量，透過快速傅立葉轉換並無發現明顯的特殊繞射點， 但這些前期析出物確實提供了大量的強化效果。 η’析出物的大小，大致上隨著人工時效溫度升高而增大，將不同熱處理條件之硬度試驗結果與η’析出物的大小進行比對，隨著析出物由小到大，硬度由低到高再回到低，於NA0d120℃-24h的條件下達到最高硬度，與析出強化理論中的析出物臨界大小與強度的趨勢符合。

This study is mainly about precipitation hardening mechanism in 7075 aluminum alloy, by using different heat treatment to affect the evolution of nano-scale precipitates, combined with the information of nano-precipitates, such as size, distribution, volume fraction, etc., find out the relationship between nano-precipitates and the mechanical properties in 7075 aluminum alloy. The research about the main strengthening phase, η’ precipitates, is done by Transmission Electron Microscope and Small Angle X-ray Scattering. The size, morphology and distribution of η’ precipitates is observed by Transmission Electron Microscope image, from [110]Al zone axis, there are two types of η’ precipitates in edge-on condition, while others are projected on (110) Al . After some revise and statistic, the equivalent diameter and thickness of η’ precipitates will be well measured. To identify the type of precipitates, the image from High-Resolution Transmission Electron Microscope (HRTEM) is important, by Fast Fourier Transform, the diffraction pattern of precipitates will be acquired. Furthermore, Small Angle X-ray Scattering can collect the precipitates’ signal and get the information of nano-precipitates, such as size, morphology and volume fraction. Compared with the result from TEM, the size and morphology is determined. Thus, the relationship between nano-scale precipitates and mechanical property can be analyze, base on the theorem of precipitation hardening mechanism, when the particle size equal to the critical size, the hardening effect will be the strongest. In this research, 7075 aluminum alloy after NA0d120℃-24h, has the strongest hardening effect.