Transition of dominant diffusion process during superplastic deformation in AZ61 magnesium alloys
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1/8/04
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Transition of Dominant Diffusion Process during Superplastic Deformation in AZ61 Magnesium Alloys Y.N. WANG and J.C. HUANG The superplastic behavior of the AZ61 magnesium alloy sheet, processed by one-step hot extrusion and possessing medium grain sizes of 12 m, has been investigated over the temperature range of 523 to 673 K. The highest superplastic elongation of 920 pct was obtained at 623 K and a deformation rate of 1 104 s1. In the lower and higher strain rate regimes, with apparent m values of 0.45 and 0.25, respectively, grain-boundary sliding (GBS) and dislocation creep appeared to dominate the deformation, consistent with the scanning electron microscopy (SEM) examination. The SEM examination also revealed that individual GBS started to operate from the very initial deformation stage in the strain rate range with m 0.45, which was attributed to the relatively high fraction (88 pct) of high-angle boundaries. The analyses of the superplastic data over 523 to 673 K and 5 105 to 1 103 s1 revealed a true stress exponent of 2, and the activation energy was close to that for grain-boundary and lattice diffusion of magnesium at 523 to 573 K and 573 to 673 K, respectively. The transition temperature of activation energy is 573 K, which is attributed to the change in the dominant diffusion process from grain-boundary diffusion to lattice diffusion. It is demonstrated that the effective diffusion coefficient is a valid parameter to characterize the superplastic behavior and the dominant diffusion process.
I. INTRODUCTION
MAGNESIUM alloys have high potential as lightweight structure materials owing to their low density. However, they usually exhibit low formability attributed to their hexagonal close-packed crystal structure with a limited number of operative slip systems near room temperature, and thus cold forming of magnesium alloys is restricted to mild deformation. As a result, most magnesium products have been fabricated by casting, in particular, by die casting. Magnesium alloys are found to be much more workable at elevated temperatures as additional slip systems become available. Thus, hot rolling, extrusion, and forging have been used commercially to produce magnesium plates, rods, and tubes. Recently, applications of the superplastic pressing (or forming or forging) technique to magnesium alloys have attracted much attention,[1–15] and this may effectively produce complex engineering components directly from the wrought products. It is expected that superplastic pressing for commercial low-priced magnesium alloys could be developed into one important future fabrication means for the automobile, architecture, and electronic appliance industries. It is well known that superplasticity is associated with a small grain size. Smaller grain size is desirable in enhancing ductility and promoting high strain rate superplasticity (HSRSP) and low-temperature superplasticity (LTSP). Recently, equal channel angular press has been used to produce microstructure with fine gr
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