Modeling the flow stress and work hardening behavior of an Al-Cu-Li alloy using a microstructure sensitive superposition law

A comprehensive framework for analyzing the flow stress and work hardening behavior of an Al-Cu-Li alloy as a function of precipitation state is developed in the current work. The backbone of the flow stress model is the usage of microstructure sensitive superposition exponents for quantifying the overall strengthening contribution from the different obstacles (precipitates, solutes, forest dislocations) in the microstructure. Analytical calculations based on microstructure-based inputs reveal that superposition exponents have a complex dependence on the precipitation state, which needs to be accounted for in the model for a consistent prediction of the flow curves. The simplifying practice of linear superposition of strengthening mechanisms leads to a large discrepancy between experiments and simulations. The usage of microstructure sensitive flow stress model also has implications on the macroscopic work hardening rate in the sense that precipitates have a direct effect on the global work hardening rate apart from the conventional wisdom of precipitates affecting the global work hardening rate indirectly by altering the dislocation recovery rate. With respect to the indirect effect on work hardening, a strong increase in dynamic recovery propensity was observed with ageing time, which is manifested in terms of decrease in uniform elongation with ageing time. The insights developed from the current work reveal that when the ratio of dynamic recovery rate and dislocation storage rate reaches a threshold value of ∼0.8, the material is highly susceptible to necking. The threshold value was validated for different formable grade Al alloys.