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Superplasticity and interfaces in coarse grained metallic systems

PhD student: ZhengGuo Chen
Visiting scientist: Dr. Tony Kazantzis
The mechanical properties, the anisotropy and the dislocation microstructures of two coarse-grained aluminum alloys were investigated by a combination of uniaxial tension and transmission electron microscopy. They exhibited optimum superplasticity at 10-2 s-1 and at T equal to 425°C and above 475°C with maximum elongations close to or above 300% depending on the specimen orientation. We will analyze the secondary necking instabilities that are associated with a large volume fractions of soft grains. They produce microstructures that exhibit maxima in the cube and Goss component of the deformed grains and a slight grain refinement. The deformation lies between the five power law and the power law breakdown. At the secondary necking instabilities, the average dislocation velocity increases, most dislocations break away form their solute atmospheres and thermally activated deformation occurs with high activation energies. Grain boundary sliding is prohibited due to the presence of coherent precipitates that pin effectively the grain boundary motion.

superplasticity

Figure: The deformed, (a), the recovered (RV), (c) and the recrystallized (RX) grain partitions (e). The insets (b), (d) and (f) show the sub-grain (SGB), low-angle (LAGB) and the high-angle grain boundaries (HAGB) in the partitions of (a), (c) and (e), respectively. The lighter gray grains have better Average Image Quality i.e. less distortions.