臺灣博碩士論文加值系統

本研究利用熱機處理製程製備三種不同微觀結構之細晶Zn-22 wt.% Al合金試片，包括(i)輥軋後(As rolled, AR)、一階段退火(1-step Post Annealed, 1A)及二階段退火(2-step Post Annealed, 2A)等製程條件；並探討其低溫低應變速率(LTLS)、低溫高應變速率(LTHS)、高溫低應變速率(HTLS)與高溫高應變速率(HTHS)超塑性，藉由微硬度、超塑性與表面刻痕之試驗，並配合掃瞄式電子顯微鏡(SEM)微觀組織觀察、穿透式電子顯微鏡(TEM)擇區繞射與背向電子繞射分析，對此三種Zn-22 wt.% Al合金之微觀結構、超塑性機構及其活化能進行探討。
實驗結果顯示，輥軋後、一階段退火及二階段退火三種Zn-22 wt.% Al合金試片，在低溫低應變速率、低溫高應變速率、高溫低應變速率與高溫高應變速率超塑性中，各具不同之特性。輥軋後與一階段後退火之試片，在室溫高應變速率(5 × 10-3 ~ 1 × 10-1 s-1)下，最大伸長量分別為209%、310%，m值分別為0.2和0.31，而二階段退火之試片則不具室溫高應變速率超塑性。在高溫慢速超塑性方面，三種試片在260oC低應變速率(1 × 10-4 ~ 2 × 10-3 s-1)下，最大伸長量分別為633%、721%與998%，m值分別為0.81、0.73和0.95。利用電子繞射分析，一階段退火試片之β相晶界角度，有相當高的比例(29%)座落在低角度晶界(1-15o)區域，平均晶界角度為40o。經過高溫長時間退火之後，二階段退火試片之低角度晶界比例減少為16%，平均晶界角度則增加到52o。由活化能之計算、表面刻痕試驗與超塑性等試驗發現，輥軋後與一階段退火試片之超塑性機構完全相同，兩者在LTLS、LTHS、HTLS、HTHS 拉伸條件下之超塑性機構分別為晶界敏感動態回復(GB-DRV)、晶界敏感動態回復、晶界滑移(GBS)與差排滑移(DS)。而經過二階段退火之試片在HTLS和HTHS超塑性機構與前兩種試片相同，也是分別為晶界滑移與差排滑移，不過在LTLS和LTHS超塑性機構則分別為晶界滑移/差排滑移之混合機構與差排滑移機構。
此外，本研究提出晶界敏感動態回復之輔助機構，來說明輥軋過程中所發生之「加工軟化」現象與次微米細晶Zn-22 wt.% Al合金之低溫高速超塑性行為，實驗也發現試片經過高溫退火處理後，將會因晶粒粗化而產生一「退火硬化」現象。

In this study thermomechanical treatments designated as AR (as-rolled), 1A (1-step post-annealed) and 2A (2-step post-annealed) conditions were applied to a Zn-22 wt.% Al alloy to produce equiaxed grain structures for low temperature low strain rate (LTLS), low temperature high strain rate (LTHS), high temperature low strain rate (HTLS) and high temperature high strain rate (HTHS) superplasticity studies. The microstructure, annealing characteristics, superplastic properties and activity energy of the AR, 1A and 2A Zn-22 wt.% Al alloy specimens were studied by using scanning electron microscope (SEM), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), microhardness measurements, tensile test and scratch marker line test.
The results showed that the AR, 1A and 2A specimens exhibited different types of deformation characteristics in the LTLS, LTHS, HTLS and HTHS superplasticity according to their microstructural features, especially the grain size and the grain boundary properties. A LTHS superplasticity is found in the sub-micron grain size AR and 1A Zn-Al alloy, but not the 2A Zn-Al alloy. The maximum tensile elongations and the m values in a LTHS condition (at 25oC and 5 × 10-3 ~ 1 × 10-1 s-1) are 209% and 0.2 for AR specimens, and 310% and 0.31 for 1A specimens. The HTLS superplasticity governed by grain boundary sliding (GBS) mechanism found in the AR, 1A and 2A Zn-Al alloys at 260oC under strain rates range from 1 × 10-4 to 2 × 10-3 s-1, exhibited maximum elongations of 633%, 721% and 998% and m values of 0.81, 0.73 and 0.95, respectively. In addition, a large portion (29%) of misorientation of β grains in 1A Zn-Al alloy specimen falls in the range of 1 ~ 15 degree and the average grain boundary angle was found to 40 degree. After prolonged elevated temperature annealing, it was found that the amount of LAGBs (1 ~ 15 degree) in the Zn-Al alloy specimen (2A) decreases to 16% and the average grain boundary angle of 2A specimen increases to 52 degree.
A model based on the absorption of dislocation pile-up by grain boundary is proposed in this study to explain the ”work softening” behavior and the LTHS superplasticity in fine-grained Zn-Al alloy by the grain boundary-sensitive dynamic recovery (GB-DRV). On the other hand, an ”anneal hardening” phenomenon is found in the sub-micron grained Zn-Al alloy when subjected to a post-annealing treatment at 240oC. From results of the activation energy calculation, the offsets in marker line examination and the superplasticity properties, it was concluded that the deformation mechanisms governing the LTLS, LTHS, HTLS and HTHS superplasticity in AR and 1A Zn-Al alloys are GB-DRV, GB-DRV, GBS and dislocation slip (DS), respectively. The deformation mechanisms governing the HTLS and HTHS superplasticity in the coarsened 2A Zn-Al alloy are exactly the same as those in the AR and 1A Zn-Al alloys (i.e. GBS and DS, respectively). However, the deformation mechanism found in the LTLS superplasticity of the 2A Zn-Al alloy is a mixed type mechanism consisted of GBS and DS, and that in the LTHS condition with poor tensile ductility is the DS mechanism.

Chinese Abstract i
English Abstract iii
Table of Content v
List of Tables vii
List of Figures viii
List of Abbreviation xii


I. Introduction 1
II. Literature Survey 3
2.1 Superplasticity 3
2.1.1 Introduction of Superplasticity 3
2.1.2 Fine-Grained Superplasticity 4
2.1.3 High Strain Rate Superplasticity, HSRSP 6
2.1.4 Low Temperature Superplasticity, LTSP 7
2.2 Microstructure Aspects of Superplasticity 8
2.2.1 Monotectoid Transformation of a Zn-22 wt.% Al Alloy 8
2.2.2 The Store and Release of Strain Energy 9
2.2.3 Recrystallization and Grain Growth 10
2.3 Mechanical Aspects of Superplasticity 11
2.4 Mechanisms of Superplasticity 13
2.4.1 Grain Boundary Sliding 13
2.4.2 Grain Boundary-Sensitive Dynamic Recovery 14
2.4.3 Activation Energy Associated with Superplasticity 15
2.5 Analysis of Misorientation 16

III. Experimental Procedures 18
3.1 Materials Preparation 18
3.2 Thermomechanical Treatment 18
3.3 Microstructure Observation 19
3.4 DSC Measurement 20
3.5 Microhardness Measurement 20
3.6 Superplasticity Test 21
3.7 Determination of Strain Rate Sensitivity 21
3.8 Determination of Activation Energy for Superplasticity 22
3.9 Scratch Marker Line Test 23
3.10 TEM Analysis 23
3.11 EBSD Analysis 23

IV. Results and Discussion 25
4.1 Microstructure Observation .25
4.1.1 The General and Discontinuous Precipitation 25
4.1.2 Morphology of the Zn-22 wt.% Al Alloy 26
4.2 DSC Measurement 27
4.3 Microhardness Measurement 28
4.4 Superplasticity Test 32
4.4.1 Superplasticity in the AR Zn-22 wt.% Al Alloy 32
4.4.2 Superplasticity in the 1A Zn-22 wt.% Al Alloy .34
4.4.3 Superplasticity in the 2A Zn-22 wt.% Al Alloy 36
4.5 Determination of Strain Rate Sensitivity 37
4.6 Activation Energy Calculation 39
4.7 Fracture Observation 42
4.8 Scratch Marker Line Test 45
4.9 TEM Analysis 48
4.10 EBSD Analysis 49

V. Conclusions 52
References 55

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