兩面擠製對A800鋁合金之顯微結構及析出物之影響

The Effects of Two-Side Extrusion on Microstructure and Precipitation of A800 Aluminum Alloy

廖宥瑄(Yow-Shiuan Liaw)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202101939

摘要

本實驗使用由傑出材料所提供高強度Al-Zn-Mg-Cu系列之鑄材A800。採用兩面擠製之製程，透過嚴重塑性變形的方式，希望能得到類似於超細晶的晶粒細化功效。同時，經過擠型的材料，晶粒會產生特定方向的分布取向稱為織構。織構的產生會帶來織構強化的效應。而由高變型量所帶來的內部缺陷，被認為會促進析出物的生成以及相變，增加其析出強化的效果。因此，以此製程為中心，探討此製程在各階段熱處理，對材料機械性質的影響，以及造成此機械性質的顯微結構與析出物。研究以拉伸試驗及硬度來分析材料之機械性質。以電子背向散射繞射 (Electron Back-Scattered Diffraction, EBSD)來進行晶粒大小以及織構強度的分析，分別討論細晶強化以及織構強化對機械性質的影響。再以穿透式電子顯微鏡 (Transmission Electron Microscope, TEM)對材料內部析出物的種類進行判斷。最後，搭配小角度X光散射儀(Small-angle X-ray scattering, SAXS)來計算析出物的大小，以及其相對體積分率和相對表面積，討論其對機械性質的影響。
研究分為三部分進行討論。第一部分討論熱擠製對機械性質的影響。隨著擠製量的增加，晶粒能有效細化，並產生<111>//ED及<100>//ED纖維織構。且變形時所帶來的大量缺陷能促使析出物析出，且擠製使析出物尺寸較小，增加析出強化的效果。第二部分討論熱擠製後經過T4X熱處理對機械性質的影響。從EBSD結果可發現晶粒細化效果仍然存在，且<111>//ED纖維織構達到飽和。從TEM及SAXS結果可以發現，經過熱處理，原本明顯的擠製促進析出效果大幅降低，可能是源自於缺陷的消失，然而還是能觀察到些微擠製刺激析出相變的現象，因此析出強化效果經擠製後表現較好。同時，一種近期才發現的Y相析出物可以於擠製後被觀察到，此析出物具有高長寬比因而被認為是很好的析出強化析出物。第三部分討論熱擠製後經過T6熱處理對機械性質的影響。結果可以發現，擠製後晶粒仍保有細化效果，而織構相比與T4X熱處理已無變化。此階段基地內皆佈滿η'與η析出物，η種類主要為η1與η2。從析出物的平均尺寸及相對體積分率仍能發現擠製促進析出物析出成長相變的現象。然而η析出物的轉變會使析出強化效果較差。

關鍵字

鋁合金 ； 嚴重塑性變形 ； 反覆鐓粗擠製 ； 晶粒細化 ； 纖維織構 ； T4X熱處理 ； T6熱處理 ； 析出強化 ； 穿透式電子顯微鏡 ； 小角度散射

並列摘要

An Al-Zn-Mg-Cu-Sc-Zr alloy named A800 provided by Advanced Material Specialty INC. was used in this study. A newly developed severe plastic deformation (SPD) manufacturing process which is similar to repetitive-upsetting extrusion was used. The aim of this study is to know whether this newly developed manufacturing process can obtain similar mechanical properties as other often used SPD techniques and the microstructures after this process. The main strengthening effects for aluminum alloys are grain-boundary strengthening, texture strengthening and precipitate strengthening; therefore, this study focused on the difference between grain size, texture and precipitates after the process and how they influenced the mechanical properties. The investigations of grain size distribution and texture were conducted by Electron Back-Scattered Diffraction (EBSD). Transmission Electron Microscopy (TEM) and High Resolution-TEM (HRTEM) were used to observe the microstruces and precipitates evolution. Furthermore, Small-angle X-ray scattering (SAXS) was used to analyze the relative volume fraction and size of the precipitates.
This study was divided into three parts. The first part investigated the difference between processed and non-processed materials’ mechanical properties and the microstructures that results in the final mechanical properties. The average grain size decreased as the deformation rate increase. <111>//ED and <100>//ED fiber textures could be seen in processed material which could lead to a texture strengthening. GP zones were found in all conditions and according to SAXS results, precipitation in processed material was faster.
The second part investigated the difference between processed and non-processed materials’ mechanical properties and the microstructures that results in the final mechanical properties after they obtained T4X solution heat treatment. Average grain size in all conditions remained the same and also the trend, processed material still obtained small average grain size which led to a better grain-boundary strengthening effect. Textures in processed material was still combined with both <111>//ED and <100>//ED fiber textures. GP zones could be found in all conditions; however, a newly investigated Y phase could be seen in processed material which might lead to a better precipitate strengthening effect. Precipitation was still faster in processed material, but the effect was not as obvious as it was before T4X heat treatment.
The third part investigated the difference between processed and non-processed materials’ mechanical properties and the microstructures that results in the final mechanical properties after they obtained T6 aging heat treatment. The results in this part showed a slight increase in average grain size, but processed material still held a smaller average grain size. Texture remains the same as in T4X treatment, indicating that no recrystallized grain was formed nor subgrain rotation occurred. Preicipites from both processed and non-processed material were η' and η precipitates. An acceleration in precipitation of processed material still could be seen.

並列關鍵字

Aluminium alloys ； Severe Plastic Deformation ； Grain-boundary strengthening ； Texture strengthening ； Precipitate strengthening ； T4X heat treatment ； T6 heat treatment ； Transmission Electron Microscope (TEM) ； Small Angle X-ray Scattering (SAXS)

參考文獻

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