||Effects of current density and strain rate on the dynamic impact response and microstructural evolution of 7075-T6 aluminum alloy
||Department of Mechanical Engineering
7075-T6 aluminum alloy
Hopkinson pressure bar
This study investigates the deformation behavior and dislocation substructure of 7075-T6 aluminum alloy under different strain rate and electric current by using split–Hopkinson pressure bar equipped with the electrically-assisted manufacturing system at strain rates ranging from 2200s-1 to 3000s-1and at constant current of 0A and 5.05A, respectively. The effects of strain rate and electric current on the mechanical properties and dislocations substructure were evaluated, and the relationships between mechanical properties and dislocations substructure were also discussed.
The experimental results show that the strain rate and current play a key role on the mechanical response. For a given strain rate, the flow stress increases with increasing current, except for the specimen deformed at strain rate of 3000s-1. Under a constant current, the increase of the flow stress with strain rate was observed. An obvious variation of the strain rate sensitivity and thermal activation volume with strain rate and current was also found。The Vickers hardness measurement show that the micro-hardness was affected strongly by the strain rate and current.
SEM observation results show that the fracture of the specimens occurs only for the specimen deformed at 3000s-1 and 0A and was dominated by a ductile mode. Moreover, the TEM observations results show that for a given current, the dislocation density increases with increasing strain rate. Under the constant strain rate, the dislocation density increases with increasing current for the specimen deformed at 2200s-1, but decreases with increasing current at 3000s-1. The relationship between work-hardening stress and square root of the dislocation density can be described by the Bailey-Hirsch equation.
第一章 緒論 1
第二章 理論探討與文獻回顧 4
2-1 鋁合金簡介 4
2-1-1 鋁與鋁合金 4
2-1-2 鋁合金分類與熱處理製程 5
2-1-3 鋁合金7075-T6介紹與應用 8
2-2 電流輔助成形(Electrically-Assisted Manufacturing) 9
2-2-1 電流輔助成形簡介 9
2-2-2 焦耳熱效應 10
2-3 塑性變形機械測試類別 11
2-3-1 靜態或極低之應變速率(〖10〗^(-8)<ε ̇<〖10〗^(-5) s^(-1)) 12
2-3-2 低速之應變速率(〖10〗^(-5)<ε ̇<〖10〗^0 s^(-1)) 12
2-3-3 中速之應變速率(〖10〗^0<ε ̇<〖10〗^2 s^(-1)) 12
2-3-4 高速之應變速率(〖10〗^2<ε ̇<〖10〗^4 s^(-1)) 12
2-3-5 極高速之應變速率(〖10〗^4<ε ̇<〖10〗^7 s^(-1)) 12
2-4 一維波傳理論 13
2-5 霍普金森高速撞擊試驗機之原理 15
2-6 材料塑性變形機制 18
2-6-1 恆溫機制 19
2-6-2 熱活化機制 19
2-6-3 差排黏滯機制 20
2-7 電塑效應對於差排之影響 21
第三章 實驗方法及步驟 35
3-1 實驗流程 35
3-2 實驗儀器與設備 35
3-2-1 霍普金森撞擊試驗機 35
3-2-2 電流輔助成型裝置 37
3-2-3 慢速切割機 37
3-2-4 研磨拋光機 38
3-2-5 震動拋光機 38
3-2-6 維氏硬度機(Vickers hardness test) 38
3-2-7 高階三束型聚焦離子束顯微鏡 39
3-2-8 掃描式電子顯微鏡(SEM) 39
3-2-9 穿透式電子顯微鏡(TEM) 39
3-3 實驗步驟 40
3-3-1 實驗試件製備 40
3-3-2 動態衝擊試驗 40
3-3-3 硬度測試試片製備 41
3-3-4 掃描式電子顯微鏡(SEM)試片製備 41
3-3-5 穿透式電子顯微鏡(TEM)試片製備 42
第四章 實驗結果與討論 45
4-1 應力-應變曲線 45
4-2 應力、應變與應變速率之關係 46
4-3 應變速率敏感性係數 47
4-4 熱活化體積 48
4-5 維氏硬度觀察 49
4-6 掃描式電子顯微鏡(SEM)斷面觀察 50
4-7 穿透式電子顯微鏡(TEM)結構觀察 51
第五章 結論 80
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