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하태권,장영원,손중락 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.8
The superplastic deformation behavior of a Zn-0.3wt.%Al alloy has been investigated in this study, by employing a new thermomechanical treatment (TMT) process. The TMT process employed appears to produce a fine and equiaxed grain structure that is very stable exhibiting no further grain growth. A remarkable tensile elongation of nearly 1400% has been obtained at room temperature for a Zn-0.3 wt.%Al alloy with the average grain size of 1 ㎛. This elongation seems to be the largest reported so far in the open literatures for dilute Zn-Al alloys. Room-temperature superplasticity is very benificial for the direct investigation of superplastic deformation mechanisms by using in-situ TEM and SEM studies without severe experimental constraints. While the room-temperature superplasticity has been observed in several microduplex superplastic materials, only a few investigations have been reported for single phase alloys so far. Using the superplastic material obtained in this study, a more systematic study on the physicsal mechanisms of superplastic deformation in single-phase alloys appear to be possible through an in-situ TEM or SEM observation. The major deformation mechanism was found to be the grain boundary sliding for this alloy as reported for other single phase alloys. The strain rate sensitivity parameter of a specimen with 1 ㎛ grain size showed a somewhat lower value of 0.41 considering the large elongation of 1400%.
Zn-0.3wt.% Al 합금의 초소성 변형에 따른 미세조직의 변화
이원절,박찬경,하태권,손중락,장영원 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.9
The superplastic deformation of Zn-0.3wt.%Al alloys were investigated at room temperature under various strain rate. These alloys were highly elongated up to 1400% at the initial strain rate of 2×10^(-4)/s, revealing the rate sensitivity parameter of about 0.4. In order to investigate the microstructural change during superplastic deformation, the transmission electron microscopy(TEM) studies were performed on the specimen elongated up to the 100% tensile strain at various strain rate. The microstructure was composed of zinc rich matrix phase, with 1㎛ in grain size, and Al rich second phases precipitated generally at grain boundaries and/or triple point. In region III(5×10^(-2)/s), the high density of dislocations were observed within the grains and deformation twins were also formed within the grains. In region II(2^*10^(-4)/s), however, the dislocations were highly localized along the grain boundaries and deformation twins were not observed. Observation of severe deformation along the grain boundaries and grain boundary migration(GBM) suggest that grain boundary sliding(GBS) is a dominant mechanism of superplastic deformation in this alloy.