Strain Rate Dependence of Dynamic Flow Stress Considering Viscous Drag for 6061 Aluminum Alloy at Very High Strain Rates

In the previous paper, in order to evaluate the strain rate dependence of the dynamic flow stress of aluminum alloys, 6061-O and -T6, high strain rate tests were performed at strain rates ranging from 1×10^3_> to 3×10^4_>, and strain rate reduction tests were also conducted in the strain rate range from about 1×10^4_> to 2×10^4_>, which is the strain rate range before reduction. A steep increase in the flow stress was observed for 6061-0 at the strain rate of about 5×10^3_>. The above phenomenon, however, was not observed for 6061-T6 in the strain rate range where the strain rate reduction tests were conducted and also the strain rate dependence of the flow stress was negligibly small. In this paper, to clarify the difference between the above-mentioned phenomenon for 6061-0 and -T6, a simplified model for dislocation kinetics under dynamic plastic deformation is used which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag. The equation derived from the kinetic model reveals that the steep increase in the flow stress of 6061-0 observed at the strain rate of about 5×10^3_> is attributed to the rate dependence of the viscous drag on the dislocation motion and furthermore, the increase in the mobile dislocation density lowers a velocity of moving dislocations and shifts the transition region, or the strain rates in which the steep increase in the flow stress becomes to appear, to the higher strain rate side. It is expected that the flow stress of 6061-T6 increases abruptly in the strain rate region above about 2×10^4_>