High strain-rate deformation of T8-tempered, cryo-rolled and ultrafine grained AA 2099 aluminum alloy

Abstract

The mechanical response of as-received coarse-grained (CG) AA 2099-T8 aluminum alloy under high strain-rate loading was compared with those of cryo-rolled and ultrafine grain (UFG) specimens. The UFG alloy was developed using thermomechanical processing that involved solution heat treatment of the as-received AA 2099-T8 alloy, followed by cryo-rolling and subsequent annealing. High strain-rate testing of cylindrical specimens of the UFG, cryo-rolled and CG alloy was carried out using the split Hopkinson pressure bar system, while scanning and transmission electron microscopic techniques were used for microstructural characterization of the specimens before and after the test. The true strain and true strain rate of the specimens increased as the impact momentum (IM) was increased. Grain refinement via thermomechanical processing did not improve the dynamic yield strength of AA 2099 aluminum alloy, which is attributed to the development of more non-equilibrium UFG boundaries during the dynamic deformation of UFG AA 2099 specimens. However, the formation of adiabatic shear bands (ASBs) which trigger dynamic fracture in alloys during high strain-rate deformation was highly suppressed by grain refinement. Whereas only deformed shear band (DSB) developed in cryo-rolled and UFG specimens, the as-received CG AA 2099-T8 alloy developed transformed shear band (TSB), along which crack nucleated and propagated. The dissolution of precipitates due to adiabatic heating increased towards the TSB. Of the possible strengthening sources in AA 2099 alloy, precipitation strengthening contributed mostly to the yield strength under dynamic impact loading at IM≤22 kg. ms−1, while strengthening by dislocation-dislocation interactions dominated at IM≥22 kg. ms−1.