Strain hardening rate in relation to microstructure in precipitation hardening materials

The influence of microstructure on strain hardening is studied through Kocks-Mecking plots in a number of systems showing precipitation hardening : Al-Zn-Mg, Al-Mg-Si-Cu, and Fe-Cu. The presence of a supersaturated solid solution is shown to result in an extremely high work hardening rate, due to dynamic precipitation during the straining. When precipitation occurs, a drastic change in the work hardening capability is observed, which can be related to the type of precipitate-dislocations interactions and to the residual solute content. Shearable precipitates do not seem to influence greatly the work hardening behavior, which is then mostly controlled by the solute content. Non-shearable precipitates induce a high initial hardening rate. However this high initial value cannot be sustained to high strains due to extensive dynamic recovery in the solute-depleted matrix. From the analysis of the work hardening rate, it seems that precipitates remain shearable up to very large sizes and to very overaged states in the AI-Mg-Si-Cu and the Fe-Cu alloys, which has important consequences on the modeling of the hardening curve of these alloys.