Substructure Evolution and Deformation Modes in Shock-loaded Niobium

An electron microscopy study has been conducted of the substructure evolution of polycrystalline niobium shock loaded to 6 and 37 GPa. It was found that dense dislocation tangles and loops dominate the substructure of samples shocked at 6 GPa, whereas slip bands and microbands on {110} and twins on {112} in addition to tangles and loops are observed in samples shocked at 37 GPa. The twins observed possess the characteristic lenticular shape similar to those seen in shock-loaded molybdenum and are consistent with the shape predicted by existing twin models. The twin elements and characters do not appear to be different from those induced by conventional deformation at low strain rates. Slip bands on {110} are considered to result from an independent deformation mode which is not a consequence of the termination of twin growth inside the crystal. Microbands, which are the most predominant feature, were developed within 10° of {110} planes with a slight misorientation (1° or less) across the bands. Macroscopic shear bands were not found in either of the shock-loaded samples. Comparison between the microstructures in shock-loaded and slowly deformed samples shows no drastic difference. It is argued that the prevalent deformation modes in shock-loaded niobium are not extraordinarily distinct from conventional deformation.