Single-Mechanism Rate Theory for Dynamic Strain Aging in fcc Metals

A full thermal-activation rate theory for dynamic strain aging is developed for the case where a single rate-dependent strengthening mechanism controls dislocation motion in a material and where that mechanism is modified by additional time-dependent strengthening by, e.g., solute diffusion. The analysis shows that negative strain rate sensitivity cannot be obtained within such a framework, a conclusion previously reached by Hähner [1]. However, the strain rate sensitivity can be greatly reduced over a range of strain rates, making negative strain rate sensitivity more accessible by other mechanisms. In addition, the aging mechanism naturally gives rise to an instantaneous positive strain rate sensitivity and stress relaxation behavior under strain-rate jump conditions, putting the concepts advanced by McCormick [2, 3] on a quantitative footing. Finally, the model shows that non-steady state strain histories can also give rise to apparent material softening. The results here set the stage for subsequent work wherein consideration of multiple strengthening mechanisms (solute and forest hardening) operating together and with the same underlying aging mechanism can predict negative strain rate sensitivity, and its strain-rate, strain, temperature, and concentration-dependence, in quantitative agreement with data in Al-Mg solid solution alloys