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研究生:余育陞
研究生(外文):Yu-Sheng Yu
論文名稱:鋅-22wt.﹪鋁合金之加工軟化與退火硬化研究
論文名稱(外文):A Study on the Strain Softening and Anneal Hardening in Zn-22 wt% Al Alloy
指導教授:楊智富楊智富引用關係
指導教授(外文):Chih-Fu Yang
學位類別:碩士
校院名稱:大同大學
系所名稱:材料工程學系(所)
學門:工程學門
學類:綜合工程學類
論文種類:學術論文
論文出版年:2005
畢業學年度:93
語文別:英文
論文頁數:150
中文關鍵詞:鋅-22 wt.﹪鋁合金加工軟化退火硬化
外文關鍵詞:anneal hardeningstrain sofeningZn-22 wt.﹪Al
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本研究利用熱機處理來製備細晶(d = 0.3 μm)與粗晶(d = 2 μm)結構之鋅-22 wt.% 鋁合金,並探討其加工行為與退火硬化行為。藉由示差掃瞄熱卡計 (DSC) 、微硬度、及壓縮測試,並且配合掃瞄式電子顯微鏡 (SEM)微觀組織之觀察,探討鋅-22 wt.% 鋁合金之微觀結構與加工軟化及退火硬化的關係。研究結果顯示,在-10℃至250℃之溫度區間鋅鋁合金會出現加工軟化之現象,而軟化後之鋅鋁合金在後續高溫退火時會出現退火硬化之現象。由實驗結果分析得知,加工軟化之機構乃是熱加工時之動態再結晶(DRX)所導致之軟化行為,而具備高角度晶界之極細β相晶粒可促進動態再結晶,而加強加工軟化之效果。另一方面,退火硬化之機構則是藉由退火消除(粗化)這些極細β相晶粒,阻礙動態再結晶所導致之軟化,回復材料正常強度。
為進一步驗證加工軟化之機構,本研究針對壓縮實驗所得結果提出一「複合應力-應變圖形」模型來解析鋅鋁合金內所發生之結構變化,對應於此「複合應力-應變圖形」,鋅鋁合金內個別α相與β相所發生之塑性變形行為與動態再結晶(應變軟化)行為均得以清楚辨別。
In this study thermomechanical processings were applied to a Zn-22 wt.% Al alloy to produce fine- and coarse-grained structures for the strain softening and anneal hardening studies. The microstructure, hardness and compression curve of Zn-22 wt.% Al alloy were studied by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), microhardness measurements and compression tester. The results showed the occurrence of a strain softening phenomenon in the Zn-Al alloy in the temperature range from -10℃ to 250℃ and an anneal hardening behavior in the softened Zn-Al alloy upon high temperature annealing. The mechanism of the strain softening behavior was found to be the DRX-induced softening during hot working, which can be facilitated by the formation of ultra-fine β grains with high-angle-boundaries; the mechanism of the anneal hardening behavior, on the other hand, was the annihilation of the ultra-fine β grains by a high temperature grain coarsening treatment to retard the occurrence of the DRX-induced softening and to restore the “normal strength” of the alloy.
A composite stress-strain curve model was proposed in this study to resolve the mechanism of strain softening in the Zn-22 wt.% Al alloy. By using this model detailed informations regarding to the deformation and the DRX (i.e. the strain softening behavior) in individual α and β phases were disclosed from the experimental compressive stress-strain curve.
CHINESE ABSTRACT i
ENGLISH ABSTRACT ii
TABLE OF CONTENT iv
LIST OF TABLE vi
LIST OF FIGURES vii

CHAPTER
Ⅰ. INTRODUCTION 1

Ⅱ. LITERATURE REVIEW. 3
2.1 Hot working of metals 3
2.2 Dynamic recovery and dynamic recrystallization under hot working conditions …………………………………………………………..6
2.2.1 Dynamic recovery………...………….………………………..7
2.2.2 Dynamic recrystallization……………….………………….....9
2.3 Factors affecting the elevated temperature mechanical behaviors….
…………….…………………….…………………..…………....13
2.3.1 Zener-Hollomon parameter………………………………….16
2.3.2 Initial grain size……………………………………………...18
2.4 Introduction of superplasticity…...……………….……………...19
2.4.1 Mechanical Aspects of superplasticity……………………....20
2.4.2 Requirements for superplasticity…………………………….22
2.4.2.1 Fine grain size………………………………………..….23
2.4.2.2 Presence of second phase ……………………………....24
2.4.2.3 Nature of grain boundary structure……………………..25
2.4.2.4 Shape of grains…………………….…………………....26
2.4.2.5 Mobility of grain boundaries………………...…………26
2.4.2.6 Grain boundaries and their resistance to tensile separation……………….…………………………….26
2.4.3 Superplasticity in Zn-22 wt.% Al...………………………….27
2.4.4 Grain growth of Zn-Al alloys during superplastic deformation…………………….…………………………..28
2.4.5 Monotectoid transformation of Zn-22 wt.% Al alloy………..29

Ⅲ. EXPERIMENTAL PROCEDURES 31
3.1 Materials preparation 31
3.2 Thermomechanical treatment 32
3.3 Microstructure and properties examination………………….…...33
3.3.1 Microstructural examination…………………………………33
3.3.2 Differential scanning calorimeter measurement 34
3.3.3 Microhardness……………………………………………….35
3.3.4 Superplasticity test 35
3.3.5 Compression test …………………………………………… 36

Ⅳ. RESULTS AND DISCUSSION 37
4.1 DSC measurement. 37
4.2 Microstructure observation 38
4.2.1 The precipitation and coarsening 39
4.2.2 Deformation morphology of dual-phase Zn-Al alloy…………………………...………………………...40
4.2.3 Morphology evolution upon post-annealing treatment…….44
4.3 Hardness test……………………………………………………..46
4.4 Mechanisms of strain softening and anneal hardening…………..51
4.5 Tension/ surperplasticity test……………………………………..52
4.6 Compression…………………...……………………..…………..54
Ⅴ. CONCLUTIONS 62
REFERENCES 64





LIST OF TABLE

Table 2.1 Typical superplastic materials 71
Table 2.2 Summary of proposed models of grain boundary sliding…....72
Table 3.1 Chemical compositions of the Zn-22 wt.% Al alloy 73
Table 3.2 Pre-strain conditions….. 74
Table 4.1 Tensile properties of standard pre-strained fine-grained Zn-Al alloy….. 75
Table 4.2 Tensile properties of standard pre-strained coarse-grained Zn-Al alloy………………………………………………………….76
Table 4.3 Illustration of the influences of temperature, strain rate and grain size on the elevated temperature S-S curve…………...77
Table 4.4 Analysis of deformation and DRX characteristics from the compressive S-S curve of an un-pre-strained coarse-grained Zn-22 wt.% Al alloy at 250℃ and 1×10-3 strain rate 78






















LIST OF FIGURES

Fig. 2.1 Schematic of two deformation process extensively used for hot working of metals and their alloys. (a) In extrusion, the reduction in area is accomplished by a ram forcing the material through the die. (b) In rolling, this is accomplished by drawing the material between two rotating rolls that supply the necessary force for the reduction in thickness………………………………….………79
Fig. 2.2 Schematic of microstructural changes possible during hot working. In (a), dynamic recovery occurs during forming, and this is followed by static recovery subsequent to the shape change. If only recovery takes place, the grains change shape commensurately with the macroscopic shape change. In (b) the grains recrystallize after deformation is effected (static recrystallization). In (c) dynamic recrystallization (i.e., recrystallization during deformation ) take place and this is followed by static recrystallization 80
Fig. 2.3 Schematic of the true stress-true strain behavior of a material undergoing dynamic recovery. The flow stress rises to a steady-state value-which increases with increases in temperature-when a steady-state substructure is developed …...81
Fig. 2.4 (a) Schematic of the true stress-true strain-behavior of a material
undergoing dynamic recrstallization. (b) True stress-true strain
relationships for an Fe-0.25 wt.% C steel during hot working at
1373 K…………………………………….....…………..…….82
Fig. 2.5 Strain rate-strain behavior for a materials creeping at a low (lower curve) and a high (upper curve) stress. At low stresses, the strain rate decreases with time (strain) to the steady-state value dictated by the substructure developed. At the high stress level, the substructure developed initiates recrystalization and an oscillatory strain rate-strain behavior results; the strain rate is a maximum just after recrystallization and a minimum subsequent to the initiation of recrystallization 83
Fig. 2.6 (a) An oscillatory stress-strain curve is obtained during dynamic recrystallization when the strain (εc) required to initiate recrystallization is greater than the strain required to completely recrystallize the material. The strain cycle for the “first” wave, εs, is the sum of εc and εx. (b) Nonoscillatory behavior is observed when εx> εc and the steady-state flow stress is obtained at εs (= εx+ εc) 84
Fig. 2.7 The variation of εc and εx with stress. At low stresses, εc> εx and periodic dynamic recrystallization is found, and vice versa. The data are appropriate for Ni at 1210 K 85
Fig. 2.8 Influence of temperature on the compression flow curves
determined at a strain rate of 2×10-3 s-1 on OFHC copper.…………………………………………………..……...86
Fig. 2.9 Effect of initial grain size on hot compression flow curves for OFHC copper, deformed at 775 K and a strain rate of 2×10-3 s-1…………………………………………………...…………87
Fig. 2.10 Effect of strain rate on hot compression flow curves for Cu- 1.91 wt. % Be alloy at a temperature of 1073 K 88
Fig. 2.11 Schematic illustration of the logarithmic plot of stress
versus strain rate of superplastic materials………...………....84
Fig. 2.12 The tensile ductility of natural aged Zn-Al alloys at various Zn content……………………………………………………….90
Fig. 2.13 The phase diagram of Al-Zn system………………………….91
Fig. 2.14 TTT diagram of Zn-22 wt.%Al alloy……………………..…..92
Fig. 3.1 Flow chart showing the experimental procedures.…………….93
Fig. 3.2 Variable-speed roller…………………………………………...94
Fig. 3.3 JEOL 5600 scanning electron microscope……………………..95
Fig 3.4 TA 2910 DSC……………...……………………………………96
Fig. 3.5 Future-Tech FM-7 Vickers microhardness tester………………97
Fig. 3.6 Crank press for manufacture of sheet-form tensile specimens...98
Fig. 3.7 Geometries of tensile specimen………………………………..99
Fig. 3.8 Hung-Ta HT 8150 materials test system with a 5-heat zone
electrical furnace……………………………………………..100
Fig. 3.9 Hung-Ta HT 8150 materials test system with an reverse-tension type compression apparatus.…………………..……………..101
Fig. 4.1 DSC curve of a cold worked fine-grained Zn-22 wt.% Al alloy.
………………………………………………………………..102
Fig. 4.2 DSC curve of a cold worked coarse-grained Zn-22 wt.% Al alloy………………………………………………………….103
Fig. 4.3 Microstructures of Zn-22 wt.% Al; (a) as-cast, (b) homogenized at 380℃ for 48 hours, (c) hot rolled at 380 ℃ and air cooled, (d) solution treated, quenched and aged in -10℃ coolant for 24 hours, (e) grain coarsened at 200℃ for 24 hours.. 104
Fig. 4.4 Microstructures of Zn-Al alloy specimens showing the morphology changes by pre-straining﹔ (a) fine-grained specimen, prior to pre-straining, (b)fine-grained specimen, after standard pre-straining, (c) coarse-grained specimen, prior to pre-straining, (d) coarse-grained specimen, after standard pre-straining, (e) coarse-grained specimen, pre-strained at a slower rolling speed condition, (f) coarse-grained specimen, pre-strained at a higher temperature condition…………….…105
Fig. 4.5 Microstructure evolution in a standard pre-strained fine-grained Zn-22 wt.% Al alloy upon post-annealing at 250℃ for (a) 0 minute, (b) 10 minutes, (c) 30 minutes, (d) 90 minutes, (e) 180 minutes, (f) 360 minutes and (g) 1440 minutes……………….106
Fig. 4.6 Microstructure evolution in a standard pre-strained coarse-grained Zn-22 wt.% Al alloy upon post-annealing at 250℃ for (a) 0 minute, (b) 10 minutes, (c) 30 minutes, (d) 90 minutes, (e) 180 minutes, (f) 360 minutes and (g) 1440 minutes…
………….…………………………………………………...108
Fig. 4.7 Microstructure evolution in a slower speed pre-strained coarse-grained Zn-22 wt.% Al alloy upon post-annealing at 250℃ for (a) 0 minute, (b) 10 minutes, (c) 30 minutes, (d) 90 minutes, (e) 180 minutes, (f) 360 minutes and (g) 1440 minutes.
…………………………………..…………………………...110
Fig. 4.8 Microstructure evolution in a higher temperature pre-strained coarse-grained Zn-22 wt.% Al alloy upon post-annealing at 250℃ for (a) 0 minute, (b) 10 minutes, (c) 30 minutes, (d) 90 minutes, (e) 180 minutes, (f) 360 minutes and (g) 1440 minutes..……………………………………...………………112
Fig. 4.9 Strain softening of fine- and coarse-grained Zn-22 wt.% Al alloys by mechanical rolling at selected sets of rolling conditions…..114
Fig. 4.10 Anneal hardening of various pre-strained fine- and coarse-grained Zn-22 wt.% Al alloys at (a) room temperature and (b) 250℃…….……………………………………………115
Fig. 4.11 Effect of post-annealing at (a) room temperature and (b) 250℃on the hardness of coarse-grained Zn-22 wt.%Al alloy specimens pre-strained at standard rolling temperature and rolling speed to strains of 0, 30% and 60%. …………………116
Fig. 4.12 Schematic diagrams showing (a) the formation low angle boundary dual-phase precipitates and (b) the evolution of them to high angle boundary dual-phase particles.…………….…117
Fig. 4.13 Morphology of a failed tensile specimen of standard pre-strained fine-grained Zn-22 wt.% Al alloy tested at room temperature under an initial strain rate of 1 × 10-2 s-1.……………………………………………………..……..118
Fig. 4.14 Morphology of a failed tensile specimen of standard pre-strained coarse-grained Zn-22 wt.% Al alloy tested at room temperature under an initial strain rate of 1 × 10-2 s-1.............119
Fig. 4.15 Morphology of a failed tensile specimen of standard pre-strained fine-grained Zn-22 wt.% Al alloy tested at 250℃ under an initial strain rate of 1 × 10-3 s-1……………………...120
Fig. 4.16 Morphology of a failed tensile specimen of standard pre-strained coarse-grained Zn-22 wt.% Al alloy tested at 250℃ under an initial strain rate of 1 × 10-3 s-1……………………….………….…………………………121
Fig. 4.17 Tensile flow curve of a standard pre-strained fine-grained Zn-22 wt.% Al alloy at -20℃, 25℃ and 250℃ under initial strain rates of 1 × 10-2 s-1 and 1 × 10-3 s-1. 122
Fig. 4.18 Tensile flow curve of a standard pre-strained coarse-grained Zn-22 wt.% Al alloy at -20℃, 25℃ and 250℃ under initial strain rates of 1 × 10-2 s-1 and 1 × 10-3 s-1.……………………..……………………………………...123
Fig. 4.19 Compressive flow curves of Zn-22 wt.% Al alloy specimens at room temperature and 1 × 10-2 s-1 strain rate﹔ (a) fine-grained Zn-Al alloy standard pre-strained, (b) coarse-grained Zn-Al alloy standard pre-strained, (c) coarse-grained Zn-Al alloy pre-strained at a slower rolling speed, (d) coarse-grained Zn-Al alloy pre-strained at a higher temperature...………………………………………………...124
Fig. 4.20 Microstructures of Zn-Al alloy specimens compressive strained at room temperature and 1 × 10-2 s-1 strain rate to 65% strain; (a) fine-grained Zn-Al alloy, standard pre-strained, (b) coarse-grained Zn-Al alloy, standard pre-strained, (c) coarse-grained Zn-Al alloy, pre-strained at a slower rolling speed of 41mm/s, (d) coarse-grained Zn-Al alloy, pre-strained at a higher temperature of 100℃……………………………. ..125
Fig. 4.21 Effect of the amount of pre-strain on the compressive flow curve of coarse-grained Zn-22 wt.% Al alloy, tested at RT under strain rates of (a) 1 × 10-2 s-1 and (b) 1 × 10-3 s-1 .……………………………………………………...…...126
Fig. 4.22 Effect of the amount of pre-strain in coarse-grained Zn-Al alloy on the maximum compressive stress tested at room temperature and strain rates of 1 × 10-2 s-1 and 1 × 10-3 s-1……………….127
Fig. 4.23 Compressive flow curves of Zn-22 wt.% Al alloy specimens at 250℃ and 1 × 10-3 s-1 strain rate﹔ (a) fine-grained Zn-Al alloy standard pre-strained, (b) coarse-grained Zn-Al alloy standard pre-strained, (c) coarse-grained Zn-Al alloy pre-strained at a slower speed, (d) coarse-grained Zn-Al alloy pre-strained at a higher temperature. 128
Fig. 4.24 Microstructures of Zn-Al alloy specimens compressive strained at 250℃ and 1 × 10-3 s-1 strain rate to 65% strain﹔ (a) fine-grained Zn-Al alloy, standard pre-strained, (b) coarse-grained Zn-Al alloy, standard pre-strained, (c) coarse-grained Zn-Al alloy, pre-strained at a slower rolling speed of 41mm/s, (d) coarse-grained Zn-Al alloy, pre-strained at an elevated temperature of 100℃.. 129
Fig. 4.25 Microstructures of hot rolled Al-5 wt.% Zn (pseudo single α phase) and Zn-1.5 wt.% Al (pseudo single β phase) alloys .130
Fig. 4.26 Compressive flow curves of Al-5 wt.% Zn and Zn-1.5 wt.% Al alloys at 250℃ and 1 × 10-3 s-1 strain rate…………………. 131
Fig. 4.27 Schematic diagrams showing the construction of various types of S-S curve in Zn-22 wt.% Al alloy by the overlap of individual S-S curve of α and β phases.……………………………………………..………..132
Fig. 4.28 The compressive flow curve of un-pre-strained coarse-grained Zn-22 wt.% Al alloy, tested at 250℃ under a strain rate of 1×10-3 s-1.. 133
Fig. 4.29 Effect of the amount of pre-strain on the compressive flow curve of coarse-grained Zn-22 wt.% Al alloy, tested at 250℃ under strain rates of (a) 1×10-2 s-1 and (b) 1×10-3 s-1……………………...…………………………………....134
Fig. 4.30 Microstructural evolution in un-pre-strained coarse-grained Zn-22 wt. % Al alloy﹔compression test at 250℃ under an initial strain rate of 1×10-3 s-1 to true strain of (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4 and (f) 0.5.………………………………………………….……...135
Fig. 4.31 Microstructural evolution in 45% pre-strained coarse-grained Zn-22 wt. % Al alloy﹔compression test at 250℃ under an initial strain rate of 1×10-3 s-1 to true strain of (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4 and (f) 0.5..……………………………………………………….....136
Fig. 4.32 Microstructural evolution in 60% pre-strained coarse-grained Zn-22 wt. % Al alloy﹔compression test at 250℃ under an initial strain rate of 1×10-3 s-1 to true strain of (a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e) 0.4 and (f) 0.5.……………………………………………………..…….137
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