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URN etd-0822104-155155 Statistics This thesis had been viewed 2764 times. Download 959 times. Author Ming-Chieh Chuang Author's Email Address No Public. Department Materials Engineering Year 2003 Semester 2 Degree Master Type of Document Master's Thesis Language English Page Count 141 Title Formation and Properties of High Strain Rate Superplastic Zn-22 wt.% Al Alloy by Equal Channel Angular Extrusion Keyword Zn-Al alloy superplasticity elongation ECAE ECAE elongation superplasticity Zn-Al alloy Abstract In this study thermomechanical processings were applied to a Zn-22 wt.% Al alloy to produce fine equiaxed grain structure for the high strain rate superplasticity studies. The microstructure, hardness and superplastic properties of Zn-22 wt.% Al alloy was studied by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), microhardness measurements and tensile tester. The results showed that the microduplex Zn-22 wt.% Al alloy specimens exhibited low temperature high strain rate (LTHS) and high temperature high strain rate (HTHS) superplasticity when they were solution treated, quenched and properly thermomechanical processed through low temperature aging and equal channel angular extrusion (ECAE).
A “work softening” phenomenon has been observed in the ECAE processing of the Zn-Al alloy at the extrusion temperatures investigated from -10℃ to 50℃. A boundary related DRV model based on the absorption of dislocation pile-up by grain boundary is proposed to explain the “work softening” behavior. The ECAE not only work softened the Zn-Al alloy but also converted the low angle boundaries of precipitates to high angle boundaries in the process of repeatedly DRV, which eventually leads to the high strain rate superplasticity in the ECAEed Zn-Al alloy. According to the microstructure examinations on deformed tensile specimens and the analysis of the superplasticity data, the mechanisms governing the HTLS and HTHS superplasticity were proven to be GBS and boundary related DRV, respectively. For the LTHS superplasticity occurred in the Zn-Al alloy, the governing mechanism is the boundary related DRV.
Advisor Committee Chih-fu Yang - advisor
Liu-ho Chiu - co-chair
Tung-han Chuang - co-chair
W. H. Wang - co-chair
Files Date of Defense 2004-07-26 Date of Submission 2004-08-22