A systematic study of grain refinement during impact of 4340 steel

Inhomogeneous plastic deformation at high strain rates and large strains results in the concentration of deformation and strain localization in adiabatic shear bands (ASBs). In the current study, AISI 4340 steel specimens were impacted at increasing strain rates/strains (impact momentums) to capture the formation of adiabatic shear bands. Advanced specimen preparation techniques using Focused Ion Beam (FIB) and extensive electron microscopy were used to identify the sequence of evolution of the shear bands. It was observed that the structure of the shear bands that evolve after strain localization starts out with elongation of the grains due to grain reorientation in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. For higher strain rates/strains during impact, the elongated grains break along the initiated dislocation boundaries resulting in the creation of smaller elongated broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the shear band structures demonstrates that the local deformation is inhomogeneous and that these mechanisms occur concurrently. It is concluded that the evolution of the shear band structure can be considered as a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands.