SUPERPLASTIC DEFORMATION MECHANISMS OF MECHANICALLY ALLOYED ALUMINUM

The superplastic deformation of mechanically alloyed aluminum has been shown to occur at unusually high strain rates. Although superplastic elongations of up to 570% have been obtained, the strain rate sensitivity, m, is low, between 0.25 and 0.3. An analysis of the threshold stresses present due to the dispersed particles indicates that the rate limiting deformation mechanism is not grain boundary sliding (which was observed), but solute atom drag of dislocations in the bulk of the grains. Deformation in the superplastic regime occurs without cavitation, due to rapid diffusion and easy intragranular slip to accommodate stress concentrations in the boundary. Two phenomena that reduce potential elongation are serrations in the flow curve (from solute atom breakaway), and the effects of thermal flow instabilities arising from conduction of adiabatically generated heat into specimen ends. The effect of alloying composition on deformation mechanisms is described. Modifications to the alloy composition and the mechanical alloying process that may improve the superplastic forming potential of mechanically alloyed aluminum are suggested.