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Title: Modelling the Yield Strength and Strain Hardening of Ultrafine Grained Aluminium (Modelleren van de vloeigrens en de vervormingsversteviging van submicron Al-legeringen)
Other Titles: Modelling the Yield Strength and Strain Hardening of Ultrafine Grained Aluminium
Authors: Chen, Enze; S0178133
Issue Date: 3-May-2012
Abstract: Equal channel angular pressing (ECAP) is a severe plastic deformation method which can produce ultrafine-grain material. Due to the characteristics of ECAP, those processed materials will exhibit asymmetric textures close to the simple-shear texture and a complex microstructure containing ultrafine grains. The texture and microstructure will significantly influence the subsequent mechanical properties of ECAP’ed materials. A tension-compression asymmetry, showing different yield strengths during tension and compression tests, is one example of resulting mechanical behaviour.The aim of this study is to develop a method to model the yield strength and strain hardening behaviour of the ultrafine-grained material, to predict the tension-compression asymmetry, and to eventually develop an understanding of the relationship between the macroscopic mechanical behaviour and the microstructural development.
Table of Contents: Foreword i
Abstract iii
Samenvatting v
List of Symbols vii
Symbols vii
Subscripts and superscripts x
Mathematical notations x
Abbreviations xi
Chapter 1 Introduction 1
1.1 Aim of the research 1
1.2 Outline of the text 1
Chapter 2 Literature review 3
2.1 Equal-channel angular pressing 3
2.1.1 Brief History and advantage of ECAP 4
2.1.2 Equipment of ECAP 4
2.1.3 Geometry of the ECAP die and corresponding imposed strain 5
2.1.4 The ECAP processing routes and corresponding shear plane 7
2.1.5 Parameters in ECAP and their corresponding influence 9
2.2 Microstructure evolution of FCC material during ECAP 10
2.2.1 Definition of the dislocation structures 10
2.2.2 Microstructures of ECAP’ed aluminium 13
2.3 Dislocation-based modelling of work hardening 15
2.3.1 Activation criterion for the slip systems 15
2.3.2 General framework of dislocation-based work hardening models 19
2.3.3 Overview of some important dislocation-based models 20
2.4 Texture evolution during ECAP (FCC case) 23
2.4.1 Reference frame 23
2.4.2 Ideal simple shear texture vs. the single pass ECAP texture 24
2.4.3 Influence of the multiple-pass ECAP on texture 26
2.5 Modelling the texture evolution in ECAP 26
2.5.1 Texture prediction: from single crystal to polycrystal 27
2.5.2 Existing polycrystal models 27
2.5.3 Modelling the texture evolution in ECAP 31
2.6 Back-stress and corresponding models 33
2.6.1 Strain-path change effect 33
2.6.2 Kinematic hardening and the sub-layer model 34
2.6.3 Phenomenological models including back-stress 35
2.6.4 Existing dislocation-based models about back-stress 39
2.7 Summary 43
Chapter 3 Experimental 45
3.1 Equipments 45
3.1.1 Mechanical test 45
3.1.2 Texture measurement 47
3.1.3 Equal-channel angular pressing 47
3.2 ECAP processing of AA1050 48
3.2.1 Investigated material AA1050 48
3.2.2 ECAP sample and processing 49
3.3 Texture of the ECAP processed AA1050 49
3.3.1 Texture samples preparation and measurement 49
3.3.2 Texture of the single pass ECAP’ed AA1050 52
3.3.3 Texture of the ECAP’ed AA1050 via route C 54
3.3.4 Texture of the ECAP’ed AA1050 via route Bc 56
3.3.5 The evolution of the ECAP texture components 58
3.4 Mechanical tests of the ECAP processed AA1050 59
3.4.1 Tensile tests 59
3.4.2 Compression tests 66
3.4.3 Torsion tests 71
3.4.4 Conclusion of the mechanical tests 72
3.5 Investigation of the transient yielding phenomenon during the compression tests 75
3.5.1 Design of the experiments 75
3.5.2 Results 75
3.5.3 Conclusion of the transient yielding 79
3.6 Summary 80
Chapter 4 Development of a multi-scale modelling strategy 83
4.1 Introduction 83
4.2 Hypothesis 84
4.3 Framework 85
4.3.1 Structure and interface 85
4.3.2 Length scale and corresponding transfers 86
4.3.3 Main flow chart of the framework 87
4.4 Modelling texture evolution using Taylor-type model 88
4.4.1 Introduction 88
4.4.2 Current version vs. original version 88
4.5 Microstructure development 90
4.5.1 Reconsideration of the Estrin-Tóth model 90
4.5.2 Strain rate insensitivity 91
4.5.3 The algorithm of the Estrin- Tóth model 92
4.6 Developing a dislocation-based back-stress model 93
4.6.1 Introduction 93
4.6.2 Reconsideration of the Sauzay model 94
4.6.3 Designing of the back-stress model 96
4.6.4 Co-operation with the Estrin-Tóth hardening model 97
4.6.5 A new dislocation-based back-stress model 97
4.6.6 Micro-Macro transfer 98
4.7 Implementation details about programming 99
4.8 Conclusion 99
Chapter 5 Assessment of the multiscale modelling strategy 101
5.1 Introduction 101
5.2 Settings 101
5.2.1 Virtual ECAP dies and reference frames 101
5.2.2 Setting for the texture model 105
5.2.3 Material parameters 105
5.3 Texture prediction 106
5.3.1 Validation of the current ALAMEL implementation 106
5.3.2 Initial texture 110
5.3.3 Single ECAP pass 110
5.3.4 Via route C 116
5.3.5 Via route Bc 116
5.3.6 The evolution of the average Taylor factor 118
5.4 Microstructure evolution 119
5.4.1 The dislocation density 119
5.4.2 Cell size and its distribution 120
5.4.3 Microscopic hardening behaviour 122
5.5 Simulated mechanical behaviour during ECAP 123
5.5.1 Via route C 123
5.5.2 Via route Bc 124
5.6 Tension/compression asymmetry of the ECAP’ed AA1050 126
5.6.1 Tension/compression simulation for route C samples 127
5.6.2 Via route Bc 129
5.6.3 Predicted tension/compression asymmetry 130
5.7 Conclusion 131
Chapter 6 Applications 133
6.1 Prediction of the Tension/Compression asymmetry of ECAP processed FCC material using an integrated model based on dislocation and back-stress 133
6.1.1 Introduction 133
6.1.2 Modelling ECAP process in route A 134
6.1.3 Boundary condition of uniaxial tension and compression 134
6.1.4 Simulation and results 135
6.1.5 Discussion 139
6.1.6 Conclusion 139
6.2 Modelling the free-end torsion test of the ECAP’ed pure aluminium 140
6.2.1 Strain and stress status during torsion 140
6.2.2 Algorithm 144
6.2.3 Simulation setting 144
6.2.4 Results 145
6.2.5 Discussion 146
6.2.6 Conclusion 147
Chapter 7 Conclusions 149
7.1 Main achievements and conclusions of this thesis 149
7.2 Suggestion for future work 153
Appendix 155
Appendix A 155
A.1 The algorithm of the full-constraint Taylor model 155
A.2 The algorithm of the Advanced LAMEL model 158
Appendix B 163
B.1 The TAME software 163
B.2 installing the software 163
B.3 Commands 164
B.4 Setting 164
B.5 Default deformation 165
Reference List 167
Curriculum vitae 177
Working Experience 177
Academic background 177
List of publications 178
Publications in international journals 178
ISBN: 978-94-6018-503-8
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Physical Metallurgy and Materials Engineering Section (-)
Structural Composites and Alloys, Integrity and Nondestructive Testing

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