Superplasticity observed at high strainrates around 100/s in mechanicalloyed (MA) Alalloys containing Mg and/or Cu was studied byusing TEM and SEM, in contrast to MA pure Al whichshowed only a small elongation. Microstructure ofthe former consisted of thermally-recoveredsubmicron grains with smooth boundaries, whilethat of the latter was highly distorted. Graindeformation estimated from the grain shaperemained within a several 10% even aftersuperplastic elongation. Further, when elongationwas large, grain was concluded to originate mainlyfrom the sliding of grain boundaries under the condition of less cavitation.
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