Formation process, microstructure characterization, and evolution mechanism of adiabatic shear bands in U-5.7Nb alloy under dynamic deformation

Uranium-niobium (U-Nb) alloys as structural materials play an important role in nuclear industry. During their utilizing, U-Nb alloys tend to dynamically fracture along the adiabatic shear bands, which affects their performances. In order to understand the formation process and mechanisms, we systematically investigate the adiabatic shear bands formed in U-5.7Nb alloys, where systematical characterization methods were applied, such as optical microscope, scanning electron microscope, and high-resolution transmission electron microscope. Using stopper rings, the U-5.7Nb alloy was impacted by a split Hopkinson pressure bar at a strain rate of 8 000 s-1 to perform seven strain levels. The formation process of adiabatic shear bands in U-5.7Nb alloys was characterized, and we found two types of adiabatic shear bands: the deformed ones together with the transformed ones. They are formed at different stage of severe shear localization. The deformed adiabatic shear bands mainly included highly deformed and elongated grains, and the transformed adiabatic shear bands were basically composed of ultrafine equiaxed grains with size of about 150 nm. High density of dislocations or dislocation cells were found in these ultrafine equiaxed grains. The occurrence of ultrafine equiaxed grains in the adiabatic shear bands of U-5.7Nb alloys was deduced to be rotational dynamic recrystallization mechanism.