On the Combined Role of Strain Hardening and Strain Rate on the Degree of Heterogeneity of Plastic Flow in an Al-2.5%Mg Alloy

Jerky flow due to the Portevin-Le Chatelier (PLC) effect is investigated in an Al-2.5%Mg alloy loaded in tension under constant driving velocity at room temperature. We analyzed the strain and strain rate dependences of the characteristics of the PLC instabilities, such as the type of instability, the magnitude of the stress drops and the critical plastic strain for the onset of unstable plastic flow. The study shows the combined role of strain hardening and applied strain rate on instability which can lead to transitions between types of instability. Work hardening and applied strain rate reduction promote Dynamic Strain Aging (DSA) and consequently lead to intensification of strain localizations. The results suggest that pipe diffusion mechanism of solute Mg atoms, through forest dislocation cores, is more relevant than bulk diffusion mechanism in our experiments conditions. It is shown that, when all mobile dislocations perform a successful activation event, the resulting elementary incremental strain also depends on the applied strain rate. Only the strain dependence of this concept is taken into account by most DSA models to describe plastic instabilities.