An interpretation of the true stress–true stain behavior for a mechanically milled, superplastic Al(Mg(Cu alloy from a dislocation dynamics viewpoint

Stress–strain curve in tension for a mechanically milled Al-1.5Mg-4.0Cu (mass%) alloy [IN9021] under a high strain rate superplastic condition (temperature:748K, nominal strain rate:10° s−1) was analyzed from a viewpoint of dislocation dynamics by computer-simulation, taking into consideration that the actual true strain rate decreases with straining. The simulation showed a decrease in both the immobilization rate of dislocations U and the re-mobilization probability of immobile dislocations Ω but an increase in the ratio U/Ω with increasing plastic strain, indicating the loss of ability for sustaining further plastic deformation. The onset of local necking, that is the local stain concentration, is suggested to be due to the absorption and annihilation of dislocations into grain boundary cavities that were formed by grain boundary sliding. The strain rate sensitivity m is formulated from a dislocation dynamics viewpoint, and the simulation indicates that it does not remain constant during deformation but decreases with increasing plastic strain.