Effect of initial properties on the flow strength of aluminum during quasi-isentropic compression

A magnetic loading technique was used to ramp load pure aluminum and 6061 aluminum alloy to peak stresses of approximately 29 GPa. The peak loading rate was approximately 106 s, followed by unloading from peak stress at a rate of about 105 s. The pure aluminum samples had impurity levels ranging from about 10 ppm to 0.5 wt % and average grain sizes in the range of 144-454 μm. The 6061 alloy was prepared in either the T6 condition with grain sizes of 5-50 μm, or in the T0 or T6 heat treatment condition with a grain size of about 40 μm. A wave profile technique was used to estimate the compressive strength during unloading. It was found that the compressive strength estimated during unloading increased with peak stress for all materials and that the change in strength was insensitive to initial material properties. This observation is in agreement with previous results obtained from shock loading of the same materials [H. Huang and J. R. Asay, J. Appl. Phys. 98, 033524 (2005)] and suggests that the deformation mechanisms, which govern strength properties at high strain rates, are similar for the two cases. It was also found that the quasielastic strain occurring during unloading saturated at about 2% in 6061-T6 alloy for peak stresses in the range of 10-50 GPa. The change in quasielastic strain was found to be similar for pure aluminum and 6061 alloys.