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研究生:李信委
研究生(外文):Shinn-Wei Li
論文名稱:AZ31B鎂合金室溫至500℃之拉伸性質與其變形組織探討
論文名稱(外文):A Study on the Tensile Properties andDeformation Microstructures of AZ31B Mg Alloy from Room Temperature to 500℃
指導教授:陳立輝陳立輝引用關係呂傳盛呂傳盛引用關係
指導教授(外文):Li-Hui ChenTruan-Sheng Lui
學位類別:碩士
校院名稱:國立成功大學
系所名稱:材料科學及工程學系碩博士班
學門:工程學門
學類:材料工程學類
論文種類:學術論文
畢業學年度:90
語文別:中文
論文頁數:64
中文關鍵詞:晶界滑移  動態再結晶高溫拉伸鎂合金
外文關鍵詞:grain boundary slidingtensile propertiesdynamic recrystallizationMg alloys
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中文摘要

本研究採用AZ31B鎂合金之完全退火材於室溫至500℃之間,在固定初始應變速率為8.0×10-4s-1的條件下進行拉伸試驗,並觀察其變形組織特徵,以瞭解其變形組織與拉伸特性的溫度依存性。

拉伸試驗結果顯示,在200℃以上的應力-應變曲線可以觀察到有抖動的情形產生,這是由於材料在拉伸過程產生動態再結晶所導致。材料的降伏以及抗拉強度僅隨溫度上升而下降,總延伸率則隨溫度上升而增加,並於450℃達到最大值;而均勻延伸率隨著溫度上升而呈現先降後升的趨勢。

經由次表面的觀察顯示,AZ31B鎂合金的變形組織會隨溫度改變而呈現不同形貌。首先,室溫及100℃可觀察到形變雙晶產生於高應變量的晶粒內,且產生雙晶所需的晶粒應變會隨著溫度升高而增加。在200~500℃的次表面觀察不到形變雙晶,但是會有動態再結晶的產生,而且再結晶的晶粒徑會隨溫度上升而增加。此外,在350~500℃則可以觀察到因晶界滑移行為所產生的晶粒偏移及晶界遷移現象。

綜合探討AZ31B鎂合金的變形組織與均勻塑性變形能力的關係;根據n值的量測結果,動態再結晶的產生會降低材料的加工硬化率,因此會促進頸縮行為的發生並降低材料的均勻延伸率。而晶界滑移所導致的應變速率硬化效果,則會降低動態再結晶對於材料的軟化作用,進而增加AZ31B鎂合金在400~500℃的均勻延伸率,並因抑制頸縮現象的進一步發生,而增加高溫延性。

在500℃會由於γ相Mg17Al12的完全消失而產生異常粗大晶粒,減少其晶界面積;此一現象不利於動態再結晶及晶界滑移的產生,因而導致500℃的總延伸率會略低於450℃的試片。
ABSTRACT

The tensile tests of fully-annealed AZ31B Mg alloy were performed at a constant initial strain rate of 8.0×10-4s-1 from room temperature to 500℃, and the characteristics of the deformed microstructure was examined to investigate the features of deformation structure and tensile properties against the tensile temperature.

Experimental results indicate that the serrations of stress-strain curve could be observed above 200℃ which resulted from dynamic recrystallization (DRX). Also, the flow stress of the specimens decreased with increasing temperature. Total elongation increased with a raising temperature and then reached a maximum value at 450℃. Notably, there was a decrease in the uniform elongation with a higher tensile temperature, but this tendency reverses when the temperature exceeded 400℃.

There were some differences in the deformation microstructure when the tensile was performing at different temperatures. Twinning was observed in heavily deformed grains at room temperature and 100℃, and a higher plastic strain was needed for obtaining twins when the tensile temperature was raised. No deformation twins can be observed when the tensile temperature reached 200℃. Meanwhile, DRX took place and the recrystallized grain size was getting more lager with a higher temperature. In addition, grain rotation and grain boundary migration due to grain boundary sliding (GBS) were observed from 350℃ to 500℃.

Relationship between the ability of uniformly plastic deformation (which can be presented by the strain hardening exponent (n)) and deformation microstructural features indicate that DRX softened the structure and induced local necking and thus the uniform elongation was reduced when the tensile temperature ranging from 200℃ to 400℃. Strain rate hardening due to GBS suppressed the DRX softening and local necking so that the uniform and total elongations tended to increase at the temperatures of 400℃ and 500℃.

In addition, when the specimen was tensiled at 500℃, dissolution of γphase Mg17Al12 resulted in abnormal grain growth and less total grain boundary area, which was disadvantageous to the DRX and GBS, and hence the total elongation at 500℃ was lower than that at 450℃.
目錄

中文摘要 …………………………………………………………… Ⅰ
英文摘要 …………………………………………………………… Ⅲ
目 錄 …………………………………………………………… Ⅴ
圖表目錄 …………………………………………………………… Ⅷ

第一章 前言………………………………………………………… 1

第二章 文獻回顧…………………………………………………… 3
2-1 AZ31B鎂合金…………………………………………… 3
2-1-1 鎂合金之分類規範及意義………………………… 3
2-1-2 合金元素添加及其效應…………………………… 3
2-2 鎂合金之變形行為……………………………………… 4
2-2-1 溫度效應…………………………………………… 4
2-2-2 動態再結晶………………………………………… 5
2-2-3 晶界滑移現象……………………………………… 5
2-3 晶粒成長………………………………………………… 6

第三章 實驗方法…………………………………………………… 13
3-1 材料準備………………………………………………… 13
3-2 微觀組織觀察…………………………………………… 13
3-3 拉伸試驗………………………………………………… 13

第四章 實驗結果…………………………………………………… 19
4-1 H材與O材微觀組織之異同…………………………… 19
4-2 拉伸試驗結果…………………………………………… 20
4-2-1 拉伸破斷試片之外觀形貌………………………… 20
4-2-2 高溫拉伸曲線特徵………………………………… 20
4-2-3 拉伸機械性質……………………………………… 21
4-3 拉伸破斷面觀察………………………………………… 21
4-4 拉伸試片表面晶粒觀察………………………………… 22
4-5 拉伸試片次表面觀察…………………………………… 22
4-5-1 晶粒間距與拉伸溫度之關係……………………… 22
4-5-2 變形組織…………………………………………… 23

第五章 討論………………………………………………………… 48
5-1 變形微觀組織探討……………………………………… 48
5-1-1 頸縮現象與次表面之組織………………………… 48
5-1-2 形變雙晶組織……………………………………… 48
5-1-3 動態再結晶………………………………………… 50
5-1-4 晶粒成長…………………………………………… 51
5-2 n值與拉伸溫度之關係………………………………… 52
5-3 晶界滑移現象…………………………………………… 53
5-4 動態再結晶與均勻塑性變形…………………………… 54
5-5 動態再結晶與晶界滑移………………………………… 56
5-6 晶粒粗大化與塑性變形………………………………… 56

第六章 結論……………………………………………………… 59
參考文獻…………………………………………………………… 61

圖表目錄

表2-1 ASTM標準鎂合金分類規範四部份之記號。………… 8
圖2-1 Al-Mg二元相圖。﹝10﹞…………………… 9
圖2-2 HCP結構的滑移系統﹝11﹞:(a)底面滑移;(b)柱面滑移;(c)錐面滑移;(d)錐面滑移。(滑移面以灰色區域表示,箭頭為滑移方向)…………… 10
圖2-3 鎂單晶滑移系統與其臨界剪斷應力之關係。(根據文獻﹝12﹞重繪)………………… 11
圖2-4 晶界滑移與差排滑移所造成之應變差異:(a)ε=0之晶粒;(b)ε=0.5,且應變完全由差排滑移所產生;(c)ε=0.5,且應變完全由晶界滑移所產生。(根據文獻﹝26﹞重繪)……… 12
表3-1 AZ31B鎂合金薄板化學組成。( wt% )…………… 15
圖3-1 AZ31B鎂合金H材之金相組織。…………………… 16
圖3-2 345℃退火之退火時間與硬度之關係,其硬度值在8小時以後趨於一定值。……………………………………… 17
圖3-3 板狀拉伸試片示意圖。…………………………… 18
圖4-1 AZ31B鎂合金O材之金相組織。…………………… 25
圖4-2 (a)晶界析出物Mg17Al12形態(H材,部分析出物如箭頭所示);(b)於400℃退火之顯微組織,幾無晶界析出物存在。…26
圖4-3 O材晶粒內析出物:(a) SEM相片;(b)TEM明視野像;(c)晶粒內析出物之EDS成分分析。…………………… 27
圖4-4 不同溫度下O材拉伸試片外觀,可以觀察到頸縮程度之差異。… 28
圖4-5 O材拉伸曲線:(a)室溫拉伸工程應力-應變曲線圖。(後頁續)……………………………… 29
圖4-5 O材拉伸曲線:(b)高溫拉伸工程應力-應變曲線圖;(c)部分曲線之放大。(續前頁)………………………… 30
圖4-6 溫度與拉伸機械性質之關係:(a)U.T.S.;(b)Y.S.。(後頁續)…………………………… 31
圖4-6 溫度與拉伸機械性質之關係:(c)T.E;(d)U.E.。(續前頁)……………………………… 32
圖4-7 拉伸破斷面特徵:(a)R.T.;(b)100℃;(c)200℃。(後頁續)…………………………… 33
圖4-7 拉伸破斷面特徵:(d)250℃試片的巨觀組織,中央為一孔洞;(e)圖d孔洞的內部特徵;(f)圖d試片的邊緣。(後頁續)…… 34
圖4-7 拉伸破斷面特徵:(g)300℃;(h)350℃;(i)400℃。(後頁續)……………………………… 35
圖4-7 拉伸破斷面特徵:(j)450℃;(k)500℃。(續前頁) 36
圖4-8 拉伸試片表面:(a)未拉伸;(b)圓圈部份可以看到晶粒偏移現象,350℃;(c)晶粒1與2的刮痕方向呈現較大角度的差異,450℃。………………… 37
圖4-9 拉伸破斷面之次表面組織:(a)R.T.;(b)100℃;(c)200℃。(後頁續)…………………………… 38
圖4-9 拉伸破斷面之次表面組織:(d)250;(e)300℃;(f)350℃。(後頁續)………………………… 39
圖4-9 拉伸破斷面之次表面組織:(g)400;(h)450℃;(i)500℃。(續前頁)……………………………… 40
圖4-10 頸縮區域晶粒徑的測量結果。(沿拉伸方向)…… 41
圖4-11 距破斷面約5mm之次表面組織:(a)R.T.;(b)100℃;(c)200℃。(後頁續)……………………… 42
圖4-11 距破斷面約5mm之次表面組織:(d)250℃;(e)300℃;(f)350℃。(續前頁)……………………… 43
圖4-11 距破斷面約5mm之次表面組織:(g)400℃;(h)450℃;(i)500℃。(續前頁)……………… 44
圖4-12 形變雙晶特徵:(a)R.T.;(b)100℃,如箭頭所示。……… 45
圖4-13 未腐蝕之次表面組織,可看到空孔(黑色箭頭所示)及析出物顆粒(白色箭頭所示):(a)400℃;(b)450℃。…… 46
圖4-14 晶界遷移現象:(a)400℃;(b)500℃,圖中箭頭標示其晶界位置。……………………………………………… 47
圖5-1 n值與溫度之關係圖。…………………………… 57
圖5-2 AZ31B-H24鎂合金m值與應變速率之關係﹝28﹞。… 58
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