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研究生： 李士瑋
Shih-Wei Lee
論文名稱： 以往復式擠型法改善鋁合金/鎂合金機械性質及超塑性行為之研究
Study on the improved mechanical properties and high-strain-rate superplasticity of Al and Mg alloys processed by reciprocating extrusion
指導教授： 葉均蔚
Jien-Wei Yeh
口試委員:
學位類別： 博士
Doctor
系所名稱： 工學院 - 材料科學工程學系
Materials Science and Engineering
論文出版年： 2006
畢業學年度： 94
語文別： 中文
論文頁數： 318
中文關鍵詞： 鋁合金 、鎂合金 、往復式擠型 、高速超塑性 、細晶 、共同晶界滑移 、破裂應變 、沿晶破斷 、動態再結晶
外文關鍵詞： aluminum alloys, magnesium alloys, reciprocating extrusion, high-strain-rate superplasticity, fine grain structure, cooperative grain boundary sliding, fracture strain, transgranular, dynamic recrystallization
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    • 本研究為改善鋁合金及鎂合金之性質以擴展其應用性，針對非析出硬化型之高強度鋁合金5056與5083及析出硬化型之高強度鋁合金7075及低密度双相鎂合金Mg-15Al-1Zn與Mg-20Al-1Zn施以往復式擠型以提升性質，首先觀察不同次數擠製後微結構的改變與演進，包括介在物、析出相、晶粒尺寸與分佈狀態。隨後測試其室溫機械性質與高溫超塑性的表現，研究中包含探討微結構細化及性質提升的原因，並提出一些模型與理論計算來驗證其現象。

    5056鋁合金經往復式擠型10次後介在物可以被細化至2微米以下並均勻散佈，而晶粒尺寸則由樹枝晶細化至4.6微米，晶粒內更發現尺寸0.2微米的次晶粒，其存在證明往復式擠型過程中確實發生動態再結晶現象。往復擠型產生的細晶結構在靜態退火中穩定性可以高達500℃；而在500℃、應變速率5x10^-3 s^-1下，5056合金往復擠型10次後的最大伸長量為460%，超塑性發生區域的m值約為0.45。經由計算，此合金的超塑性行為可以利用晶界滑移公式來描述。而往復擠型10次後室溫的強度與5456-H34相當，但伸長率更高，可歸因於細晶結構與介在物尺寸縮小的效應。

    5083合金經合金設計添加較多晶粒穩定元素Zr、Cr及Mn，以5083ZM為代號，經由往復式擠型後發現介在物與晶粒的細化程度與5056合金相似，尺寸分別為2微米及4.5微米。但5083ZM合金藉由細微散佈相阻礙效果可使得靜態退火的穩定化極限溫度比5056合金提高25℃。將擠型10次後的試片在500℃、應變速率5x10^-2 s^-1時可達最大伸長量1013%，m值約為0.45，可見5083ZM合金具有高速超塑性。另在固定應變速率1x10^-2 s^-1下，亦發現400℃至500℃具有約600-800%伸長率，顯示在400℃即具有高速超塑性。由於400℃以上的流變應力值都小於15MPa，更顯示超塑性成型所須應力甚低。經由起始應力與活化能的計算，證明5083ZM合金在500℃下的超塑性是屬晶界滑移機制，散佈相增加會提高超塑活化能。本研究提出富Zr契合相提升超塑性能的解釋模型，可成功地解釋細晶契合相何以優於非契合相。

    Mg-Al合金經合金設計以双相結構來改善鎂合金的機械性質與高溫超塑性，鋁含量分別為9、15及20 wt%，第二相b相的體積分率由10%增加至45%，經325℃往復擠型10次後平均晶粒尺寸細化至9.2微米、2.8微米、2.5微米。隨著鋁含量的增加，Mg-Al合金的降伏強度明顯的增加，但是伸長率卻因而降低。Mg-20Al-1Zn幾乎不具伸長率，不適用於結構材料。Mg-15Al-1Zn合金降伏強度為303MPa、最大抗拉強度為361MPa，伸長率尚有3.6%；此合金若再經由T6處理，降伏強度更提高為363MPa、最大抗拉強度432MPa，伸長率為2.8％，其比強度因而超過7075-T6合金，可用於更高比強度但韌性較不要求的應用。在275℃、300℃及325℃下，Mg-15Al-1Zn與Mg-20Al-1Zn合金超塑性質遠優於AZ91D，且具有高速超塑性。此高速超塑性將使Mg-15Al-1Zn合金的超塑成行具有成本上的競爭力。利用拉伸試片表面與剖面觀察可以發現超塑性變形後，微結構中的b相具有微觀超塑性的現象，證明低熔點的b相具有調適合金高速超塑性變形的特性，並且隨其體積分率的增加可以增加Mg-Al合金系統的高速超塑性表現。本研究提出一双相Mg-Al合金的共同晶界滑移模型解釋其高速超塑性行為。

    7075合金利用往復式擠型法擠型1、5、10及20次後發現平均晶粒尺寸在擠型5次即達到細化極限，為3.5微米；而介在物顆粒在擠型10次後得到最佳的散佈與最小的尺寸0.6微米。往復擠型20次後7075合金降伏與最大抗拉強度比起始材分別降低了9%與11%，但延展性、斷面縮減率、破裂應變與KIC卻增加78%、270%、390%與210%。強度損失的現象與柔軟的無析出帶體積分率增加有關。隨擠型次數增加，拉伸試片的頸縮變形現象越來越明顯；破斷面由沿晶破斷逐漸變為100%的穿晶破斷，此現象與晶粒及介在物尺寸的細化有關，本研究對前述之相關性皆作深入探討。由於韌性的大幅改善，擠型20次可使7075合金的設計強度高達550MPa，比傳統7075合金提高200MPa。

    The aim of this study is to improve the properties of aluminum and magnesium alloys and expand their applications. The unique method, named “reciprocating extrusion”, was applied on conventional aluminum alloys, such as 5056, 5083 and 7075, and new dual-phase magnesium alloys, such as Mg-15Al-Zn and Mg-20Al-Zn. The evolution of microstructures and mechanical properties at room temperature and superplasticity at high temperatures were examined. The relationship between refined microstructures and enhanced properties were built up, and theoretical calculations and models were proposed to prove some outstanding phenomena.

    The grain size in 5056 aluminum alloy was reduced to 4.6 microm and the coarse inclusions refined to 2 microm after ten passes of reciprocating extrusion. A subgrain structure was formed in the interior of the fine grains, indicating that dynamic recrystallization occurred during extrusion. Dynamic recrystallization in the billet is proposed to be repeatedly induced with the number of extrusion passes until a limiting grain size was obtained. Thereafter, dynamic recrystallization was no longer activated because grain boundary sliding, instead of dislocation gliding, accommodated the deformation strain required for extrusion. The fine-grained structure and its substructure were stable up to 500℃. The maximum elongation of the alloys after ten passes extrusion is 460% at 5x10^-3 s^-1 and 500℃ with a m value of 0.45. After calculation, the dominant of superplasticity in the alloys is grain boundary sliding. The alloys extruded with ten extrusion passes exhibited a superior combination of strength and ductility over commercial 5456-H34. The enhancement of mechanical properties is related to the fine grain structure and refined inclusions.

    The 5083 aluminum alloys modified with grain refiner, 0.25% Zr and 0.46% Mn, were processed by reciprocating extrusion to yield high-strain-rate superplasticity above 400 ℃ and superior room-temperature mechanical properties. Without any prior homogenization treatment, ten extrusion passes could give the cast billets an equiaxed grain structure with a grain size of about 4.5 microm and a subgrain size about 0.2 microm, and a uniform distribution of fine inclusions and dispersoids in the matrix. The fine-grained structure was stable up to 525 ℃, giving the alloy a high-strain-rate and low-stress superplasticity over a wide operating temperature of 400-500 ℃. In the tensile test at 500 ℃, a maximum elongation of 1013% and a low flow stress of 7.7 MPa at 5x10^-2 s^-1 were achieved. The apparent and true activation energies for low temperatures (300-400 ℃) without high-strain-rate superplasticity were 220.6 and 208 kJ/mol, respectively, whereas those at high temperatures (400-500 ℃) were 88.4 and 98.7 kJ/mol, respectively. Further analysis confirms that grain boundary sliding is the dominant mechanism over the high-strain-rate region from 1x10^-2 s^-1 to 5x10^-1 s^-1 at 500 ℃, and power-law breakdown mechanism occurs over the strain rate from 5x10^−4 s^−1 to 1x10^−2 s^−1 at 300 ℃. The high-strain-rate superplasticity was more strongly enhanced by Zr addition than addition of Cr and Mn. Two enhancing mechanisms for the maximum superplastic elongation and the optimum strain rate are proposed. This study concludes that the effectiveness of Zr is caused by the fineness and the coherency of Zr-rich dispersoids in the matrix.

    A superior combination of specific strength and low-temperature high-strain-rate superplasticity of two-phase Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be achieved with reciprocating extrusion directly from as-cast billets. The volume fraction of b phase increases from 10% to 45% with increasing Al content from 9 wt% to 20 wt%. The average grain size of Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be refined to 2.8 and 2.5 microm, respectively, after ten passes at 325℃. The yield strength increases but elongation reduces with increasing aluminum content in Mg-Al alloy. In the as-extruded state, the Mg-20Al-1Zn alloy has less ductility. However, the elongation, yield strength and ultimate tensile strength of Mg-15Al-1Zn were 3.6%, 306 MPa and 376 MPa, respectively. After T6 heat treatment, Mg-15Al-1Zn alloy has an excellent improvement on yield and ultimate tensile strength, which are 363 MPa and 432 MPa, respectively. It is noticed that the specific strength of Mg-15Al-1Zn is larger than 7075-T6. The pronounced strengthening mechanism was attributed to the high volume fraction of fine-grained hard Mg17Al12 phase in the refined a-Mg matrix. At 275, 300 and 325℃, both the Mg-15Al-1Zn and Mg-20Al-1Zn have better high-strain-rate superplasticity than AZ91D. The optimum elongation of Mg-15Al-1Zn at least 1610% in company with a high m value of 0.7 was obtained at the strain rate of 1x10^-2 s^-1 when tested at 325 ℃. The excellent superplasticity at high strain rates was also mainly contributed by the high volume fraction of Mg17Al12 phase, which displays easier grain boundary sliding than alpha-Mg phase. The apparent activation energy for superplastic flow is thus smaller than that of the boundary diffusion in Mg. In addition, the mechanisms for the filament formation and cooperative grain boundary sliding are discussed. The increase on volume fraction of beta phase could improve the performance of high-strain-rate superplasticity in Mg-Al alloys. A cooperative grain boundary sliding mechanism is proposed to explain how beta phase could enhance the superplasticity in two-phase Mg-Al alloys.

    The reciprocating extrusion method was also applied on 7075 Al alloy to refine the grains and inclusions with the aim of improving mechanical properties. Various extrusion passes, that is 1, 5, 10, 20, were chosen to investigate the microstructure evolution and property variation. Experimental results indicate that grain refining largely ceases after five extrusion passes whereas significant inclusion refining continues up to the 20th pass. Comparing the properties of the 20-pass extrudates with those of the starting material with zero pass, yield strength and tensile strength decrease by 9% and 11%, respectively, while elongation, reduction of area, fracture strain and KIC increase by 78%, 270%, 390%, and 210%, respectively. However, compared to the typical properties of 7075 alloys, yield strength, tensile strength, elongation and KIC are all improved and increase by 11%, 3 %, 68%, and 200%, respectively. The ductility and toughness improvement is attributable to the refinement of both grains and inclusions, while the strength loss results from the increased volume fraction of soft PFZ associated with grain refinement. Investigation of tensile fracture surfaces demonstrates the transition of the fracture mode from predominantly intergranular to completely transgranular. This phenomenon in company with the extensive necking behavior is consistent with the significant increasing of ductility and toughness. The reciprocatingly extruded 7075-T6 alloys exhibit superior strength-ductility and strength-toughness combinations to conventional ones. This characteristic could largely increase the designed stress standard for the airframe structure based on the fail-safe design concept.

    中文摘要………………………………………………………………I 英文摘要………………………………………………………………IV 謝誌……………………………………………………………………IX 總目錄…………………………………………………………………X 圖目錄…………………………………………………………………XVI 表目錄…………………………………………………………………XXVI 第壹章 前言…………………………………………………………1 第貳章 概論 2-1材料的高溫變形行為及機制公式………………………………7 2-1-1高溫變形的類別………………………………………………7 2-1-2高溫變形的機制地圖…………………………………………12 2-2超塑性行為………………………………………………………24 2-2-1超塑性的發展歷史與介紹……………………………………24 2-2-2超塑性的種類…………………………………………………25 2-2-3輕金屬及其合金的超塑性表現………………………………30 2-2-4超塑變形機制、變因與基本公式……………………………40 2-2-4-1 微結構因素-晶粒尺寸指數(p)及平均晶粒尺寸(d)……46 2-2-4-2與溫度有關的因素-擴散係數(D)及剪應力模數(G)……47 2-2-4-3顆粒效應-起始應力………………………………………49 2-2-4-4双相結構效應…………………………………………… 50 2-2-4-5次晶粒效應……………………………………………… 63 2-2-5超塑性基本公式中重要參數的求法…………………………68 2-2-5-1活化能的求法………………………………………………67 2-2-5-2晶粒指數的求法………………………………………… 69 2-2-5-3 n值的推算及起始應力(threshold stress)的方法… 74 2-2-6超塑性在工業上的應用及瓶頸………………………………74 2-2-6-1超塑性成型的特色…………………………………………74 2-2-6-2超塑性成型的工業應用………………………………… 78 2-2-6-3超塑性成型的瓶頸……………………………………… 83 2-3高速超塑性行為…………………………………………………86 2-3-1高速超塑性現象及定義………………………………………86 2-3-1-1鋁基複合材料的高速超塑性現象……………………… 86 2-3-1-2鋁基機械合金的高速超塑性…………………………… 89 2-3-1-3鋁合金中的高速超塑性現象…………………………… 93 2-3-1-4鎂合金的高速超塑性現象……………………………… 94 2-3-2合金如何達到高速超塑性的要求……………………………96 2-4製作超塑性合金的方法………………………………………100 2-4-1液相製程……………………………………………………100 2-4-2固相製程……………………………………………………103 2-4-2-1熱機處理法………………………………………………103 2-4-2-2機械合金法………………………………………………103 2-4-2-3激烈塑性應變法…………………………………………105 2-4-3固/液相製程-攪拌摩擦製程………………………………116 2-4-4電沉積製程…………………………………………………117 2-5往復式擠型法…………………………………………………117 2-5-1往復式擠型的基本原理……………………………………117 2-5-2往復式擠型的研究成果……………………………………123 2-5-3與其他細晶法之比較………………………………………128 參考文獻……………………………………………………………132 第叁章 往復式擠型法改善5056合金性質 3-1 前言……………………………………………………………141 3-2實驗方法………………………………………………………143 3-2-1 5056鋁合金擠錠的製備……………………………………143 3-2-2往復式擠型…………………………………………………143 3-2-1-1往復式擠型機構造與動作……………………………… 143 3-2-1-2 5056鋁合金之往復式擠型條件……………………… 147 3-2-3微結構觀察…………………………………………………148 3-2-1-1光學顯微鏡觀察………………………………………… 148 3-2-1-2 穿透式電子顯微鏡觀察……………………………… 149 3-2-4靜態退火處理………………………………………………149 3-2-5常溫機械性質測試…………………………………………150 3-2-6超塑性測試…………………………………………………150 3-3結果與討論……………………………………………………154 3-3-1往復式擠型後微結構特性…………………………………154 3-3-1-1介在物的散佈…………………………………………… 154 3-3-1-2晶粒尺寸………………………………………………… 159 3-3-1-3次結構…………………………………………………… 167 3-3-2靜態退火……………………………………………………169 3-3-3超塑性………………………………………………………169 3-3-4常溫機械性質………………………………………………180 3-4 結論……………………………………………………………182 參考文獻……………………………………………………………183 第肆章 往復式擠型法改善5083ZM合金性質之研究 4-1 前言……………………………………………………………187 4-2實驗方法………………………………………………………189 4-2-1 5083ZM鋁合金擠錠的製備…………………………………189 4-2-2超塑性測試…………………………………………………190 4-3結果與討論……………………………………………………190 4-3-1鑄造狀態與擠型狀態微結構觀察…………………………190 4-3-2靜態退火處理觀察晶粒穩定性……………………………196 4-3-3超塑性行為…………………………………………………196 4-3-4活化能計算..………………………………………………203 4-3-4-1外觀活化能……………………………………………… 203 4-3-4-2真實活化能……………………………………………… 205 4-3-5細晶5083ZM不同溫度下的超塑變形機制…………………209 4-3-5-1測試溫度300至400℃…………………………………… 209 4-3-5-2測試溫度400℃至500℃………………………………… 210 4-3-6超塑性外觀活化能與真實擴散活化能差值來源…………210 4-3-7超塑性伸長量與應變速率增加的機制……………………212 4-3-7-1改善合金伸長量的機制………………………………… 214 4-3-7-2最佳超塑應變速率增加的機制………………………… 217 4-4 結論……………………………………………………………221 參考文獻……………………………………………………………222 第伍章 Mg-Al-Zn合金室溫機械性質與高速超塑性行為之研究 5-1 前言……………………………………………………………227 5-2實驗方法…………………………………………………………232 5-2-1鎂合金擠錠的製備……………………………………………232 5-2-2微結構觀察……………………………………………………233 5-2-2-1光學顯微鏡觀察………………………………………… 233 5-2-2-2 掃描式電子顯微鏡觀察………………………………… 233 5-2-3時效處理與硬度測試…………………………………………234 5-2-4常溫機械性質測試……………………………………………234 5-2-5超塑性測試……………………………………………………234 5-3結果與討論………………………………………………………236 5-3-1往復式擠型後微結構…………………………………………236 5-3-2機械性質………………………………………………………243 5-3-2-1擠型狀態的機械性質………………………………………243 5-3-2-2 Mg-15Al-1Zn合金的固溶處理特性與時效硬化行為… 247 5-3-3往復式擠型後Mg-Al-Zn合金超塑性…………………………250 5-3-3-1 Mg-15Al-1Zn及Mg-20Al-1Zn合金的超塑性行為………250 5-3-3-2 Mg-15Al-1Zn及Mg-20Al-1Zn合金超塑性變形外觀活化 能計算 ………………………………………………………………255 5-3-4超塑拉伸測試後試片表面及縱剖面觀察與微觀超塑性現象 ………………………………………………………………………257 5-3-5□相增加Mg-15Al-1Zn合金超塑性的機制…………………265 5-4結論………………………………………………………………270 參考文獻……………………………………………………………272 第陸章 利用往復式擠型法改善7075鋁合金之研究 6-1 前言……………………………………………………………277 6-2實驗方法…..……………………………………………………278 6-2-1 7075鋁擠錠的取得…………………………………………278 6-2-2往復式擠型條件………………………………………………280 6-2-3時效處理與常溫機械性質影響………………………………280 6-3結果與討論………………………………………………………281 6-3-1擠型次數對於晶粒結構與介在物分布的影響………………281 6-3-2擠型次數對於機械性質的影響………………………………289 6-3-3擠型次數對於破裂模式的影響………………………………295 6-3-4強度與韌性結合的優勢………………………………………301 6-3結論………………………………………………………………304 參考文獻……………………………………………………………305 第柒章 總結 7-1 總結……………………………………………………………309 7-1-1鋁鎂合金添加錳鉻鋯元素後經往復式擠型後之特性………309 7-1-2鎂鋁合金多量添加鋁元素經往復式擠型後之特性…………313 7-1-3鋁鋅鎂合金經往復式擠型後之特性…………………………315 7-2 未來研究方向…………………………………………………316

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