Observation of the microstructure in the adiabatic shear band of 7075 aluminum alloy

A considerable amount of adiabatic shear bands (ASBs) were obtained by means of the thick-walled cylinder (TWC) external explosive collapse technique. Two types of shear bands with different morphologies are distinguished on the cross-section of the tube, which are called deformed band and transformed band, respectively. Cracks are confirmed to develop from the transformed bands rather than the deformed band. Transmission electron microscopy (TEM) investigation indicates that ultrafine grains, with average size less than 100 nm, are produced at the center of the transformed band. At the edge of the transformed band the grains are elongated in the shearing direction. The grains in the matrix are two orders of magnitude larger than those in the transformed band. The precipitation within the shear band and the matrix is quite different. Calculation estimates that the temperature in the shear band exceeds the recrystallization temperature of the 7075 aluminum alloy. It is proposed that dynamic recrystallization occurs in the transformed band and produces the ultrafine grains. Microhardness test results show that the transformed band is much “harder” than the matrix.