Strain rate dependence of dynamic flow stress of 7075 aluminium alloy at very high strain rates

In order to evaluate the strain rate dependence of the dynamic flow stress of aluminium alloys, 7075-O and -T6, high strain rate tests are performed at strain rates ranging from about 1000/s to 30000/s, and strain rate reduction tests are also conducted in the strain rate range from about 10000/s to 28000/s. A steep increase in the flow stress is observed for both 7075-O and -T6. There is a difference in the strain rate at which the flow stress increases steeply for 7075-O and -T6. A simplified model for dislocation kinetics under a dynamic plastic deformation is proposed to explain the difference, which represents the rate dependence of the viscous drag on the dislocation motion. The model predicts that a mean velocity of a moving dislocation of 7075-T6 is lower than that of 7075-O at a given strain rate and further, 7075-T6 is higher in the moving dislocation density than 7075-O. The increase in the moving dislocation density causes the transition region, or the strain rates at which the steep increase in the flow stress becomes to appear, to shift to the higher strain rate side.