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
10 s)
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.
Keywords
mechanical threshold stress model, 6061-T6 aluminum, high
strain rate, high temperature, high pressure
