無中文摘要

Abstract

Equal channel angular extrusion (ECAE) has been used to investigate the formation of submicron grain structures in copper deformed to ultra-high plastic strains by different die angles, deformation routes, and deformation temperatures. The result was characterized by the use of transmission election microscopy (TEM), and the evolution of the deformed Cu depended on several parameters such as die angle, deformation route, and deformation temperature. It has been demonstrated that the most effective method of forming high angle boundaries and recrystallized grains by severe plastic deformation is to rotate billets with a constant clockwise 90o between each pass (route BC) via a 90o die angle. Besides, the temperature effect on the microstructural evolution is studied. With increasing deformation temperature, the microstructure becomes more homogeneous because the climb and the cross-slip of dislocations are easier at higher temperatures, and the fraction of high angle boundaries, recrystallized grains and size of them are increased significantly with the deformation temperature. In addition, the thermomechanical process was also investigated in the present work. It is suggested that a uniform submicron grained structure could be obtained by increasing the deformation temperature and decreasing the intermediate annealing temperature to promote dynamic recovery and to inhibit discontinuous recrystallization.

Table of Content

Table of Content………………………………………………………………………i
List of Tables…………………………………………………………………..……...iii
List of Figures………………………………………………………………………...iv
Acknowledgements………………………………………………………………….viii
Abstract……………………………………………………………………………….ix
I. Introduction………………………………………………………………………….1
II. Literature Review…………………………………………………………………..3
2.1 The principle of ECAE………………………………………..………………...3
2.2 Microstructures developed by ECAE………………………………..………...10
2.2-1 Deformation route………………………………………………………...10
2.2-2 Die angle………………………………………………………………….12
2.2-3 Microstructure developed in different materials………………………….12
2.3 Properties of submicrometer-grained (SMG) materials…………………….....15
2.4 The deformation structure of FCC metals………………………………..……17
2.5 Dynamic recovery and dynamic recrystallization…………………………..…23
2.5-1 Continuous dynamic recrystallization…………………………………….25
III. Experimental Procedure………………………………………………………….29
IV. Results……………………………………………………………………………31
4.1 Characteristics of ECAE……………………………………………..………..31
4.2 Effect of die angle……………………………………………………………..48
4.3 Effect of deformation route……………………………………………………68
4.4 Effect of deformation temperature…………………………………………….78
4.5 Effect of static recovery……………………………………………………….92
V. Discussion………………………………………………………………………101
5.1 General features………………………………………………………………101
5.2 Formation of HABs and recrystallized grains………………………………101
5.3 Effect of die angle……………………………………………………………104
5.4 Effect of deformation temperature…………………………………………105
5.5 Suggestion for future study…………………………………………………107
VI. Conclusion……………………………………………………………………...108
Reference……………………………………………………………………………109
Appendix………………………………………………………………………….113

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