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URN etd-0529108-152744 Statistics This thesis had been viewed 2683 times. Download 874 times. Author Jiun-hung Pan Author's Email Address No Public. Department Materials Engineering Year 2007 Semester 2 Degree Ph.D. Type of Document Doctoral Dissertation Language English Page Count 128 Title Superplasticity in a Zn-22 wt.% Al Alloy Keyword High strai Superplasticity Zn-22 wt.% Al alloy Zn-22 wt.% Al alloy Superplasticity High strai Abstract In this study thermomechanical treatments designated as AR (as-rolled), 1A (1-step post-annealed) and 2A (2-step post-annealed) conditions were applied to a Zn-22 wt.% Al alloy to produce equiaxed grain structures for low temperature low strain rate (LTLS), low temperature high strain rate (LTHS), high temperature low strain rate (HTLS) and high temperature high strain rate (HTHS) superplasticity studies. The microstructure, annealing characteristics, superplastic properties and activity energy of the AR, 1A and 2A Zn-22 wt.% Al alloy specimens were studied by using scanning electron microscope (SEM), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD), microhardness measurements, tensile test and scratch marker line test.
The results showed that the AR, 1A and 2A specimens exhibited different types of deformation characteristics in the LTLS, LTHS, HTLS and HTHS superplasticity according to their microstructural features, especially the grain size and the grain boundary properties. A LTHS superplasticity is found in the sub-micron grain size AR and 1A Zn-Al alloy, but not the 2A Zn-Al alloy. The maximum tensile elongations and the m values in a LTHS condition (at 25oC and 5 × 10-3 ~ 1 × 10-1 s-1) are 209% and 0.2 for AR specimens, and 310% and 0.31 for 1A specimens. The HTLS superplasticity governed by grain boundary sliding (GBS) mechanism found in the AR, 1A and 2A Zn-Al alloys at 260oC under strain rates range from 1 × 10-4 to 2 × 10-3 s-1, exhibited maximum elongations of 633%, 721% and 998% and m values of 0.81, 0.73 and 0.95, respectively. In addition, a large portion (29%) of misorientation of β grains in 1A Zn-Al alloy specimen falls in the range of 1 ~ 15 degree and the average grain boundary angle was found to 40 degree. After prolonged elevated temperature annealing, it was found that the amount of LAGBs (1 ~ 15 degree) in the Zn-Al alloy specimen (2A) decreases to 16% and the average grain boundary angle of 2A specimen increases to 52 degree.
A model based on the absorption of dislocation pile-up by grain boundary is proposed in this study to explain the ”work softening” behavior and the LTHS superplasticity in fine-grained Zn-Al alloy by the grain boundary-sensitive dynamic recovery (GB-DRV). On the other hand, an ”anneal hardening” phenomenon is found in the sub-micron grained Zn-Al alloy when subjected to a post-annealing treatment at 240oC. From results of the activation energy calculation, the offsets in marker line examination and the superplasticity properties, it was concluded that the deformation mechanisms governing the LTLS, LTHS, HTLS and HTHS superplasticity in AR and 1A Zn-Al alloys are GB-DRV, GB-DRV, GBS and dislocation slip (DS), respectively. The deformation mechanisms governing the HTLS and HTHS superplasticity in the coarsened 2A Zn-Al alloy are exactly the same as those in the AR and 1A Zn-Al alloys (i.e. GBS and DS, respectively). However, the deformation mechanism found in the LTLS superplasticity of the 2A Zn-Al alloy is a mixed type mechanism consisted of GBS and DS, and that in the LTHS condition with poor tensile ductility is the DS mechanism.
Advisor Committee Chih-fu Yang - advisor
Files Date of Defense 2007-05-20 Date of Submission 2008-05-29