Flow behavior and microstructure evolution of 7055 aluminum alloy impacted at high strain rates

The flow behavior and microstructure evolution of 7055 aluminum alloy impacted at the strain rates from 1000 s−1 to 6000 s−1 were investigated by a compressive split-Hopkinson pressure bar, optical microscopy, transmission electron microscopy and scanning transmission electron microscopy. The maximum stress tends to increase first with the increase of strain rate from 1000 s−1 to 4000 s−1, and then decrease at higher strain rate. The strain rate sensitivity is positive in the strain rate range of 1000-4000 s−1, but became negative above 4000 s−1 because of the presence of shear bands and shear band induced cracks due to intensive strain localization. Impact deformation prompts transformation of η′ phase into η2 phase, and changes the morphology of precipitates from a platelet-like shape to a globe-like shape.