Mechanical threshold stress model for 6061-T6 aluminum

Our paper on the Mechanical threshold stress (MTS) model for 6061-T6 aluminum has been accepted by JoMMS.  There are several things of interest in the paper:

1) The use of a phonon drag model to predict the sharp increase in flow stress at strain rates above 10,000 /s.  This behavior is seen in a  number of materials and is hard to fit using standard power law plasticity models.  Our model does a good job in this regard.

2) The sharp drop in flow stress at high temperatures.  A slight modification to the MTS model allows us to do better than previous versions of the model.  But there is still some way to go.

3) There is a strong pressure dependence of the flow stress in many metals and alloys.  We try to predict the pressure dependence by incorporating a pressure-dependent shear modulus in the model.  However, we find that just the pressure dependence of the shear modulus cannot account for the entire effect seen in experiments.  A more fundamental look at the mechanisms involved is needed.  That's an area of further research for those of you who are into dislocation dynamics. 

In addition, we also present models for

1) The Taylor-Quinney coeffcient for converting plastic work into heat.  Our model is based both on thermodynamical constraints and experimental data.

2) Dislocation density change based on nucleation and annihilation of  mobile and forest dislocations.  We suggest that some of the numbers used by previous authors as an estimate for mobile dislocation density might be off by an order of magnitude.

The abstract is given below and a preprint of the paper is attached.

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 10$^4$ 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.