An extended mechanical threshold stress plasticity model: Modeling 6061-T9 aluminum alloy

Abstract

The mechanical threshold stress plasticity model of Follansbee and Kocks was designed to predict the flow stress of metals and alloys in the regime where thermally activated mechanisms are dominant and high temperature diffusion effects are negligible. In this paper we present a model that extends the original mechanical threshold stress to the high strain-rate regime (strain rates higher than 104 s-1) and attempts to allow for high temperature effects. We use a phonon drag model for moderate strain rates and an overdriven shock model for extremely high strain rates. A temperature dependent model for the evolution of dislocation density is also presented. In addition, we present a thermodynamically-based model for the evolution of temperature with plastic strain. Parameters for 6061-T6 aluminum alloy are determined and compared with experimental data. The strain-rate dependence of the flow stress of 6061-T6 aluminum is found to be in excellent agreement with experimental data. The amount of thermal softening is underestimated at high temperatures (greater than 500 K) but still is an improvement over the original model. We also find that the pressure dependence of the shear modulus does not completely explain the pressure dependence of the flow stress of 6061-T6 aluminum alloy.