| Summary: | 碩士 === 大同大學 === 材料工程研究所 === 88 === In this study thermal and thermomechanical processing were applied to a Zn-22%Al alloy to produce fine grain microstructures for high strain rate superplasticity. The microstructure, aging characteristics, grain boundary information and the superplastic properties of the Zn-Al alloy were studied by using SEM, microhardness test, electron back-scattering diffraction (EBSD) and high temperature tensile test. The results showed that an ultra-fine equiaxed grain mixture can be obtained by the general precipitation when 1-step aged at 0℃ for a sufficient time period. These ultra-fine grain structures, however, bears only a small percentage of high angle boundaries and showed relatively poor superplastic behaviors. By adding a high temperature aging process the 2-step and 3-step aging treatments produced fine equiaxed grain structures with proper amount of high-angle boundaries and resulted in excellent high strain rate superplastic behaviors at 250℃.
The high strain rate superplasticity achieved in the thermal treated Zn-22%Al alloy specimens are better than those obtained in the thermomechanical processed specimens. The reasons for the relatively poor performance in tensile elongation in the thermomechanical processed specimens are most likely due to the presence of a great deal of low-angle boundaries and the elongated grains, which hampered the grain boundary sliding needed for proper superplasticity.
A {0 0 0 1} texture of β grains of the Zn-Al alloy was found lying parallel to the rolling plane in all of the thermal and thermomechanical processed Zn-22%Al alloy sheet specimens. High temperature aging or annealing treatments were found unable to weaken the texture.
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