臺灣博碩士論文加值系統

本研究主要探討AZ80鎂合金之析出硬化反應以及經不同熱軋製程處理後之超塑性特性。析出硬化實驗主要針對125-300℃之間進行研究，透過硬度測試(Microhardness test)及拉伸試驗(Tensile test)探討析出物對機械性質之影響。另外利用XRD、OM、SEM及TEM對析出物做一系列仔細的分析。超塑性實驗主要針對三種不同熱軋方向的試片進行超塑性實驗，溫度選擇在200-400℃，拉伸速率則選擇3×10-3、1×10-3及3×10-4(s-1)共三個應變速率。實驗結果則利用OM及SEM作進一步分析，了解溫度及應變速率對顯微組織演變之影響。
析出硬化實驗結果顯示，150-300℃各時效溫度材料均有明顯之硬化。其中以175℃/256 h時效處理之試片有最佳之硬度，約增加38%。顯示AZ80經時效處理硬度增加有限，不同於一般可析出硬化之鋁合金。而這主要的原因則歸咎於其析出物(Mg17Al12)之特性。經觀察，此析出物依其析出類型可分為連續及不連續析出物，按其形貌則可再細部加以區分為層狀、橢球狀、晶界間析出物、費德曼組織(Widmanstätten structure)以及不規則板狀結構。由與這些析出物均無法有效的阻擋差排移動，因此硬化效果不佳。而不同形貌之析出物在不同時效溫度其大小及密度也會有所不同，此即造成不同硬度之主因。
超塑性實驗結果顯示，材料在300℃/3×10-4s-1時有最佳之伸長率，約350%。在此溫度下伸長率激增的原因可分為兩部分：一是由於300℃接近材料再結晶溫度(Recrystallization temperature)，材料產生動態再結晶(Dynamic recrystallization)提供足夠之晶粒，使其依靠晶界滑移(Grain boundary sliding)產生大量變形；另一原因是由於300℃材料很快便析出Mg17Al12，而此溫度下大部份的析出物都在晶界處生成，因此可以有效的阻礙晶粒的成長，維持細晶組織。在這兩者同時作用之下材料得以獲得高的伸長率。

This study mainly focuses on precipitation hardening behavior and superplasticity of AZ80 magnesium alloy. The precipitation hardening experiment was conducted between 125-300℃. The influence of the precipitate on mechanical properties was measured by microhardness test and tensile test. The precipitate was investigated in detail by XRD, OM, SEM and TEM. The superplastic experiment focuses on three different rolling directions. The temperature ranges from 200-400℃ and the strain rates were 3×10-3、1×10-3 and 3×10-4(s-1). The results were analyzed by OM and SEM to further understand the influence of temperature and strain rate on microstructural evolution.
The result of the precipitation hardening experiment indicates that each aging temperature between 150-300℃ shows hardening effect. The highest hardness is occurred at 175℃/256 h and the hardness increment is about 38%. The result shows that the hardness increment is low compared with precipitation-hardenable aluminum alloys. This is influenced by the nature of Mg17Al12 precipitates. The precipitates can be divided into continuous and discontinuous precipitates by their forming mechanism. They can be further divided into lamellar, elliptical, intergranular, Widmanstätten structure and irregular slab by their morphologies. Because the precipitate can not effectively resist dislocation moves, the hardening effect is poor. At different aging temperature the morphology, the size, and the distribution density of Mg17Al12 precipitates are also different. These reasons then lead to different hardness.
The result of superplastic experiment shows that the material has best elongation (about 350%) at 300℃/3×10-4s-1. The rapid elongation increase at this temperature has two reasons: one is because 300℃ is close to the recrystallization temperature of the material, dynamic recrystallization then occurs and provides enough grains for grain boundary sliding which produces large deformation; the other reason is because Mg17Al12 precipitates rapidly at 300℃ and most of them are formed on grain boundaries, they can effectively impede grain growth to remain the fine grain structure. These two effects appear at the same time and contribute to high elongation.

致謝......................................................i
中文摘要.................................................ii
Abstract................................................iii
Chapter 1 Introduction....................................1
Chapter 2 Literature review...............................3
2.1 Preface...............................................3
2.2 Compositions and designations of magnesium alloys.....3
2.3 Effects of added elements on mechanical properties....3
2.3.1 Common alloying elements............................4
2.3.2 Impurity elements...................................6
2.4 The phase diagrams of Mg-Al, Mg-Zn and Mg-Al-Zn.......6
2.5 Microstructures and properties of AZ series magnesium alloys....................................................6
2.6 Precipitation hardening...............................6
2.6.1 Precipitation hardening mechanism...................7
2.6.2 Strengthening mechanism.............................7
2.6.2.1 Orowan looping mechanism..........................7
2.6.2.2 Cutting mechanism.................................8
2.6.2.3 Precipitation mechanism of AZ series magnesium alloys....................................................9
2.7 Introduction to superplasticity......................10
2.7.1 Deformation theory of superplasticity..............10
2.7.2 Deformation mechanism of superplasticity...........12
2.7.3 Necessary conditions of superplasticity ...........12
Chapter 3 Experimental Procedure.........................26
3.1 Precipitation hardening experimental procedure.......26
3.1.1 Experimental material..............................26
3.1.2 Hot rolling........................................26
3.1.3 Solution treatment.................................26
3.1.4 Aging treatment....................................27
3.1.5 HV hardness test...................................27
3.1.6 Tensile test at room temperature...................27
3.1.7 OM and SEM microstructure observation..............27
3.1.8 XRD analysis.......................................27
3.1.9 TEM analysis.......................................28
3.1.10 Flowchart of precipitation hardening experiment...28
3.2 Superplasticity experiment...........................28
3.2.1 Experimental material..............................28
3.2.2 Annealing treatment................................28
3.2.3 Hot rolling........................................28
3.2.4 Uniaxial superplasticity experiment................29
3.2.5 Dual axial superplasticity experiment..............29
3.2.6 OM microstructure observation......................29
3.2.7 SEM Fractography observation.......................29
3.2.8 Flowchart of superplasticity experiment............29
Chapter 4 Results and discussion.........................38
4.1 Result and discussion of precipitation hardening experiment...............................................38
4.1.1 Mechanical properties..............................38
4.1.1.1 Hardness.........................................38
4.1.1.2 Yield strength, tensile strength and elongation..39
4.1.2 Effects of the hot rolling directions on mechanical properties...............................................39
4.1.3 XRD analysis.......................................39
4.1.3.1 XRD analysis of the as-extruded and the solution treated material.........................................39
4.1.3.2 XRD analysis of the aged specimens...............40
4.1.4 Microstructure observation.........................40
4.1.4.1 Precipitate compositions and morphologies........40
4.1.4.1.1 Mg17Al12 discontinuous precipitation...........40
4.1.4.1.2 Mg17Al12 continuous precipitation..............41
4.1.4.1.3 AlMnFe precipitation...........................41
4.1.4.2 OM microstructure observation....................42
4.1.4.3 Observation of the specimens aged at 200℃.......43
4.1.4.4 Microstructure observation of other aging temperatures.............................................44
4.1.4.5 Variation of Discontinuous Precipitate Morphology...............................................44
4.1.4.6 Variation of Continuous Precipitate Morphology...45
4.2 Result and discussion of superplasticity experiment..47
4.2.1 Effects of annealing temperature on microstructure and hardness.............................................47
4.2.2 Micorstructure observation of as-rolled materials..48
4.2.3 Result of superplasticity experiment...............48
4.2.4 The effect of temperature on elongation............48
4.2.5 Effect of strain rate on elongation................49
4.2.6 Fractography observation...........................49
4.2.7 Microstructure observation.........................50
4.2.8 Strain rate sensitivity coefficient (m)............52
4.2.9 Gas forming........................................52
Chapter 5 Conclusions...................................102
References..............................................104
Table 2.1. Designations of elements in magnesium alloys..15
Table 2.2. Temper designations for magnesium alloys......15
Table 2.3. Effects of each element in magnesium alloys...16
Table 3.1. The composition (wt%) of the as-received AZ80 alloy....................................................31
Table 4.1. Mechanical properties of the as-extruded materials rolled in different directions.................54
Table 4.2. Mechanical properties of the aged specimens rolled parallel to the extrusion direction...............54
Table 4.3. Mechanical properties of the aged specimens rolled perpendicular to the extrusion direction..........55
Table 4.4. Mechanical properties of the aged specimens rolled parallel and perpendicular to the extrusion direction alternatively..................................55
Table 4.5. Dimensions (μm) of the precipitates in AZ80 aged at various temperatures (℃) and times (h)..........56
Table 4.6. The temperature ranges (℃) of various precipitates.............................................56
Table 4.7. Results of the gas forming experiment.........57

Fig. 2.1. Mg-Al phase diagram............................18
Fig. 2.2. Mg-Zn phase diagram............................18
Fig. 2.3. Mg-Al-Zn ternary phase diagram.................19
Fig. 2.4. The production of dislocation loops by interaction between dislocateions and precipitates.......19
Fig. 2.5. The strengthening mechanism of interaction between dislocations and precipitates by cutting.........19
Fig. 2.6. The relationship between the particle radius and the shear stress.........................................20
Fig. 2.7. TEM dark field images of the AZ91 precipitate aged at 200℃ for (a) 2 h, (b) 16 h and (c) 49 days......20
Fig. 2.8. The aging curve of AZ91........................21
Fig. 2.9. The β phase continuous precipitate lying parallel to (0001) basal plane. The OR is (0001)m // (110)p and 〈1-210〉m //〈1-11〉p. (a) Bright image and (b) diffraction pattern..................................................22
Fig. 2.10. The β phase continuous precipitate lying perpendicular to (0001) basal plane. The OR is (0001)m // (1-11)p and〈10-10〉m //〈110〉p. (a) Bright image and (b) diffraction pattern......................................22
Fig. 2.11. The β phase continuous precipitate having an angle to (0001) basal plane. The OR is (0001)m // (1-15)p and〈10-10〉m //〈110〉p. (a) Bright image and (b) diffraction pattern..................................................23
Fig. 2.12. Typical tensile test curve and deformation character of (a) superplastic materials and (b) general materials................................................23
Fig. 2.13. (a) The stress-strain curve and (b) the log scaled strain rate sensitivity coefficient (m)-strain rate curve of superplastic materials..........................24
Fig. 2.14. The effects of the grain size on flow stress and strain rate sensitivity coefficient at constant temperature..............................................25
Fig. 3.1. The appearance of the as-rolled samples rolled at 400℃: (a) rolled parallel to the extrusion direction, (b) rolled perpendicular to the extrusion direction and (c) rolled parallel and perpendicular to the extrusion direction alternatively..................................32
Fig. 3.2. The edge of the as-rolled samples rolled at 400℃: (a) rolled parallel to the extrusion direction, (b) rolled perpendicular to the extrusion direction and (c) rolled parallel and perpendicular to the extrusion direction alternatively..................................33
Fig. 3.3. Specification of (a) room temperature tensile specimen and (b) superplastic specimen...................34
Fig. 3.4. The flowchart of precipitation hardening experiment...............................................35
Fig. 3.5. Slip bands on the edge of the specimen.........36
Fig. 3.6. The flowchart of superplasticity experiment....37
Fig. 4.1. The indentation of (a) 50 g and (b) 1 kg.......58
Fig. 4.2. The aging curve of AZ80........................58
Fig. 4.3. The yield strength of the aged specimens rolled in different directions. Aged at (a) 150℃, (b) 175℃ and (c) 200℃................................................59
Fig. 4.4. The tensile strength of the aged specimens rolled in different directions. Aged at (a) 150℃, (b) 175℃ and (c) 200℃................................................60
Fig. 4.5. The elongation of the aged specimens rolled in different directions. Aged at (a) 150℃, (b) 175℃ and (c) 200℃....................................................61
Fig. 4.6. The XRD spectrum of the as-extruded AZ80.......62
Fig. 4.7. The XRD spectrum of the specimen rolled parallel to the extrusion direction...............................62
Fig. 4.8. The XRD spectrum of the specimen solution treated at 420℃ for 1 h.........................................62
Fig. 4.9. The XRD spectrum of the specimen aged at 175℃ for (a) 32 h, (b) 64 h and (c) 128 h.....................63
Fig. 4.10. The XRD spectrum of the specimen aged at 200℃ for (a) 32 h, (b) 64 h and (c) 128 h.....................64
Fig. 4.11. Discontinuous lamellar structure in AZ80 aged at 175℃ for 256 h..........................................65
Fig. 4.12. Discontinuous elliptical structure in AZ80 aged at 150℃ for 64 h........................................65
Fig. 4.13. Discontinuous intergranular structure in AZ80 aged at 200℃ for 64 h...................................65
Fig. 4.14. Dark field image of continuous Widmanstätten structure in AZ80 aged at 200℃ for 8 h..................66
Fig. 4.15. Continuous irregular slab structure in AZ80 aged at 250℃ for 32 h........................................66
Fig. 4.16. Irregular slab and Widmanstätten structure intermix with each other. Specimens aged at 250℃ for 32 h........................................................66
Fig. 4.17. SEM image of the chemical polished specimen...67
Fig. 4.18. The morphology of AlMnFe precipitate..........67
Fig. 4.19. EDX analysis of the AlMnFe precipitate........67
Fig. 4.20. The microstructure of the as-extruded AZ80....68
Fig. 4.21. The microstructure of the specimen rolled parallel to the extrusion direction......................68
Fig. 4.22. The microstructure of the specimen rolled perpendicular to the extrusion direction.................69
Fig. 4.23. The microstructure of the specimen rolled parallel and perpendicular to the extrusion direction alternatively............................................69
Fig. 4.24. The SEM microstructure of the specimen aged at 200℃ for 30min. (a) Lower magnification and (b) higher magnification............................................70
Fig. 4.25. The SEM microstructure of the specimen aged at 200℃ for 1 h. (a) Lower magnification and (b) higher magnification............................................70
Fig. 4.26. The SEM microstructure of the specimen aged at 200℃ for 2 h. (a) Lower magnification and (b) higher magnification............................................70
Fig. 4.27. The SEM microstructure of the specimen aged at 200℃ for 4 h. (a) Lower magnification and (b) higher magnification............................................71
Fig. 4.28. The SEM microstructure of the specimen aged at 200℃ for 8 h. (a) Lower magnification and (b) higher magnification............................................71
Fig. 4.29. The SEM microstructure of the specimen aged at 200℃ for 16 h. (a) Lower magnification and (b) higher magnification............................................71
Fig. 4.30. The SEM microstructure of the specimen aged at 200℃ for 32 h. (a) Lower magnification and (b) higher magnification............................................72
Fig. 4.31. The SEM microstructure of the specimen aged at 200℃ for 64 h. (a) Lower magnification and (b) higher magnification............................................72
Fig. 4.32. The SEM microstructure of the specimen aged at 200℃ for 128 h. (a) Lower magnification and (b) higher magnification............................................72
Fig. 4.33. The SEM microstructure of the specimen aged at 200℃ for 256 h. (a) Lower magnification, (b) higher magnification and (c) the magnified image of the area in (b)......................................................73
Fig. 4.34. The continuous precipitate in the specimen aged at 200℃ for 4 h.........................................73
Fig. 4.35. The continuous precipitate in the specimen aged at 200℃ for 8 h.........................................74
Fig. 4.36. The continuous precipitate in the specimen aged at 200℃ for 16 h........................................74
Fig. 4.37. The continuous precipitate in the specimen aged at 200℃ for 32 h........................................74
Fig. 4.38. The continuous precipitate in the specimen aged at 200℃ for 64 h........................................75
Fig. 4.39. The continuous precipitate in the specimen aged at 200℃ for 256 h.......................................75
Fig. 4.40. The interface between the continuous and discontinuous precipitates in the specimen aged at 200℃ for 256 h................................................75
Fig. 4.41. The SEM microstructure of the specimen aged at 125℃ for 128 h. (a) Lower magnification and (b) higher magnification............................................76
Fig. 4.42. The SEM microstructure of the specimen aged at 150℃ for 16 h. (a) Lower magnification and (b) higher magnification............................................76
Fig. 4.43. The SEM microstructure of the specimen aged at 150℃ for 64 h. (a) Lower magnification and (b) higher magnification............................................76
Fig. 4.44. The SEM microstructure of the specimen aged at 150℃ for 256 h. (a) Lower magnification and (b) higher magnification............................................77
Fig. 4.45. The SEM microstructure of the specimen aged at 175℃ for 128 h. (a) Lower magnification and (b) higher magnification............................................77
Fig. 4.46. The SEM microstructure of the specimen aged at 175℃ for 256 h. (a) Lower magnification and (b) higher magnification............................................77
Fig. 4.47. The SEM microstructure of the specimen aged at 250℃ for 2 h. (a) Lower magnification and (b) higher magnification............................................78
Fig. 4.48. The SEM microstructure of the specimen aged at 250℃ for 32 h. (a) Lower magnification. (b) Widmanstätten structure and (c) irregular slab strucuture..............78
Fig. 4.49. The SEM microstructure of the specimen aged at 300℃ for 1 h. (a) Lower magnification and (b) higher magnification............................................79
Fig. 4.50. The SEM microstructure of the specimen aged at 300℃ for 16 h. (a) Lower magnification and (b) higher magnification............................................79
Fig. 4.51. The morphologies of elliptical precipitates, (a)150℃-256 h, (b)200℃-64 h (T6), and (c)250℃-32 h (T6)..80
Fig. 4.52. Widmanstätten structures of (a)150℃-256 h, (b)200℃-64 h (T6), and (c)250℃-32 h (T6) specimens........81
Fig. 4.53. Specimen aged at 200℃ for 256 h shows an asymmetric hexagon structure.............................82
Fig. 4.54. TEM image and the corresponding diffraction pattern of Widmanstätten structure of the alloy aged at 250℃ for 32 h. (a)Bright field image, (b) corresponding diffraction pattern of the Widmanstätten precipitate lying in (0001) zone axis, (c) the reconstructed pattern, (d) the computed symmetric pattern, and (e) the computed six variants of the precipitate..............................83
Fig. 4.55. The grain size variation after annealing......84
Fig. 4.56. The hardness variation after annealing........84
Fig. 4.57. The microstructure of the specimen rolled parallel to the extrusion direction to 1 mm thickness at 400℃....................................................85
Fig. 4.58. Results of superplastic experiment............85
Fig. 4.59. (a) The appearance of the tested specimens, (b) nominal stress- strain curves and (c) true stress-strain curves at 3×10-3s-1......................................86
Fig. 4.60. (a) The appearance of the tested specimens, (b) nominal stress- strain curves and (c) true stress-strain curves at 1×10-3s-1......................................87
Fig. 4.61. (a) The appearance of the tested specimens, (b) nominal stress- strain curves and (c) true stress-strain curves at 3×10-4s-1......................................88
Fig. 4.62. SEM fractography of the specimen tested at 200℃. (a), (b) 3×10-3s-1, (c), (d) 1×10-3s-1(e), (f) 3×10-4s-1 with different magnifications.......................89
Fig. 4.63. SEM fractography of the specimen tested at 250℃. (a), (b) 3×10-3s-1, (c), (d) 1×10-3s-1(e), (f) 3×10-4s-1 with different magnifications.......................90
Fig. 4.64. SEM fractography of the specimen tested at 300℃. (a), (b) 3×10-3s-1, (c), (d) 1×10-3s-1(e), (f) 3×10-4s-1 with different magnifications.......................91
Fig. 4.65. SEM fractography of the specimen tested at 350℃. (a), (b) 3×10-3s-1, (c), (d) 1×10-3s-1(e), (f) 3×10-4s-1 with different magnifications.......................92
Fig. 4.66. SEM fractography of the specimen tested at 400℃. (a), (b) 3×10-3s-1, (c), (d) 1×10-3s-1(e), (f) 3×10-4s-1 with different magnifications.......................93
Fig. 4.67. The filament structure of the specimen tested at 400℃, 3×10-4s-1.........................................94
Fig. 4.68. The growth mechanism of the filament structure.94
Fig. 4.69. The microstructure of the specimen tested at (a), (b) 250℃, (c), (d) 300℃, (e), (f) 350℃,(g), (h) 400℃ and a strain rate of 3×10-4s-1. (a), (c), (e), (g) The gauge and (b), (d), (f), (h) the grip region.........95
Fig. 4.70. The microstructure of the specimen tested at 300℃, 3×10-4s-1 and stopped testing at (a), (b) 0.497 mm, (c), (d) 10 mm and (e), (f) 25 mm. (a), (c), (e) the gauge and (b), (d), (g) the grip region........................96
Fig. 4.71. The microstructure of the gauge region of the specimen tested at (a) 300℃, (b) 350℃ and (c) 400℃, and a strain rate of 3×10-4s-1...............................97
Fig. 4.72. The flow stress-strain rate curves............98
Fig. 4.73. The test results of the gas forming experiment at 300℃-150psi..........................................98
Fig. 4.74. The test results of the gas forming experiment at 350℃-150psi..........................................99
Fig. 4.75. The test results of the gas forming experiment at 400℃-100psi..........................................99
Fig. 4.76. The test results of the gas forming experiment at 400℃-150psi.........................................100
Fig. 4.77. The fracture site image of the specimen rolled parallel to the extrusion direction.....................100
Fig. 4.78. The fracture site image of the specimen rolled perpendicular to the extrusion direction................101
Fig. 4.79. The fracture site image of the specimen rolled parallel and perpendicular to the extrusion direction alternatively...........................................101

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