臺灣博碩士論文加值系統

The formation of shear bands and the true stress - true strain curves were studied by plane strain compression of 7050 Al alloy and pure Al single crystal. Two types of plane strain compression specimens were studied. One is channel die specimens and the other is thin sheet specimens. The plane strain compression test was conducted at room temperature by compression in S direction to different amount of reduction in thickness with T or L direction constrained.
Macroscopic shear band was observed in compression of 7050 Al alloy. Assuming that the formation of shear bands was determined by the plane of maximum shear stress in continuum mechanics, the fluctuation of true stress - true strain curves of channel die and thin sheet specimens can be simulated by the ideal σ/2τy vs. ε curve. Most of the angles of shear bands of channel die specimens are between 40˚ to 50˚. Most angles of shear bands of thin sheet specimens are between 45˚ to 57˚. This result can be explained by the shape of specimens and the characteristics of the dies.
Pure Al single crystal specimen in Cubic {100}<001>, Goss {110}<001> and Brass{110}<1-12> orientation were plane strain compressed in a channel die. In contrast to 7050 Al alloys, no macroscopic shear bands were observed in Al single crystals specimens. For all single crystal specimens, the true flow strength increase monotonically with true strain. The Cubic and Brass oriented single crystals have nearly the same initial flow strength. They both show linear work hardening behavior but the work hardening rate of Cubic oriented crystal is higher than that of Brass oriented crystal. In contrast, the Goss oriented crystal shows a higher initial flow strength and a parabolic work hardening behavior with a decreasing work hardening rate. The flow strength of Cubic oriented crystal goes above that of Goss oriented crystal when the true strain is larger than 0.5.
In the compression test, the initially cubic shaped Al single crystal in Brass orientation gradually turns into a parallelepiped. The acute angle of the parallelepiped as a function of compressive strain can be predicted by the slip of the active systems of Brass orientation.

The formation of shear bands and the true stress - true strain curves were studied by plane strain compression of 7050 Al alloy and pure Al single crystal. Two types of plane strain compression specimens were studied. One is channel die specimens and the other is thin sheet specimens. The plane strain compression test was conducted at room temperature by compression in S direction to different amount of reduction in thickness with T or L direction constrained.
Macroscopic shear band was observed in compression of 7050 Al alloy. Assuming that the formation of shear bands was determined by the plane of maximum shear stress in continuum mechanics, the fluctuation of true stress - true strain curves of channel die and thin sheet specimens can be simulated by the ideal σ/2τy vs. ε curve. Most of the angles of shear bands of channel die specimens are between 40˚ to 50˚. Most angles of shear bands of thin sheet specimens are between 45˚ to 57˚. This result can be explained by the shape of specimens and the characteristics of the dies.
Pure Al single crystal specimen in Cubic {100}<001>, Goss {110}<001> and Brass{110}<1-12> orientation were plane strain compressed in a channel die. In contrast to 7050 Al alloys, no macroscopic shear bands were observed in Al single crystals specimens. For all single crystal specimens, the true flow strength increase monotonically with true strain. The Cubic and Brass oriented single crystals have nearly the same initial flow strength. They both show linear work hardening behavior but the work hardening rate of Cubic oriented crystal is higher than that of Brass oriented crystal. In contrast, the Goss oriented crystal shows a higher initial flow strength and a parabolic work hardening behavior with a decreasing work hardening rate. The flow strength of Cubic oriented crystal goes above that of Goss oriented crystal when the true strain is larger than 0.5.
In the compression test, the initially cubic shaped Al single crystal in Brass orientation gradually turns into a parallelepiped. The acute angle of the parallelepiped as a function of compressive strain can be predicted by the slip of the active systems of Brass orientation.

Abstract i
Acknowledgement iii
Contents iv
I. Introduction 1
II. Literature review 3
II-1. Heat treatment of 7050 Al alloy 3
II-2. Formation of shear bands 3
II-3. Microstructure of Al single crystal 5
III. Experimental procedures 7
III-1. AA7050 aluminum alloy 7
III-1-1. Material 7
III-1-2. Plane strain compression tests 8
III-2. Aluminum single crystal 10
III-2-1. The growth of aluminum single crystal 10
III-2-2. Orientation determination of the aluminum
single crystal 11
III-2-3. Compression specimens 14
IV. Results 15
IV-1. Channel die compression of AA7050 aluminum alloy 15
IV-1-1. The stress-strain curve 15
IV-1-2. Macroscopic shear bands 16
IV-2. Narrow platen compression of AA7050 aluminum alloy 17
IV-2-1. The stress-strain curve 17
IV-2-2. Macroscopic shear bands 18
IV-3. Aluminum single crystal 19
IV-3-1. Laue back-reflected pattern 19
IV-3-2. The stress - strain curves 19
IV-3-3. Microscopic observation 20
V. Discussion 22
V-1. Channel die specimens of AA7050 aluminum alloy 22
V-2. Thin sheet specimens of AA7050 aluminum alloy 24
V-3. Aluminum single crystal 25
V-3-1. Flow strengths of Al single crystals 26
V-3-2. The shape of Al single crystal after plane
strain compression 28
VI. Conclusions 30
VII. Reference 32

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