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Vol.31, No.7, 859 ~ 867, 1993

Title

Stress Dependence of Deformation Behavior in Superplastic 7475 Al Alloy

신동혁Dong H . Shin, 주연준Yeon J . Joo, 조종원Chong W . Cho, 이동헌Dong H . Lee, 맹선재Sun C . Maeng

Abstract

Superplastic properties of 7475 Al alloy were evaluated at 516℃ over the strain-rate range of 6.3×10^(-5) to 2.1×10^(-2)s^(-1). The strain-rate sensitivity(m) was determined either from the true stresses and the true strain rates at the points of maximum load, or from change in strain rates. The latter method shows clear distinction between Region I(low stress region) and Region II (superplastic region). Observation of 7475 Al alloy after superplastic deformation was done by optical, scanning electron and transmission electron microscopy. Optical micrographs revealed that at low stress($lt;2 MPa) dispersoidfree zones(DFZ) formed at grain boundaries, primarily normal to the tensile direction. This result demonstrates the importance of diffusional flow at low stress. Also, scanning electron micrographs revealed that cavitation was reduced in the DFZ. Finally, transmission electron micrographs showed that dispersoid particles were accumulated at grain boundaries during the superplastic deformation. This result suggested that the deformation behavior at low stress was most likely a consequence of the existence of a thershold stress.

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