Structural materials such as steel are easily susceptible to strain localization which results in the formation of Adiabatic Shear Bands (ASBs) during impact. It is generally agreed that the initiation and development of ASBs are manifestations of damage in metallic materials subjected to high strain rates and large strains of deformation and may lead to catastrophic failure instantaneously. There have been several experimental and theoretical investigations on ASBs since it was first reported by Zener and Hollomon in 1944. However, because of the complexity of the problem of formation of ASBs, their very narrow nature in the microstructure (~1 to 350μm), and the rapid rates of deformation, it is virtually impossible to observe their evolution and mechanism of formation during impact. In this study, it is hypothesized that a systematic study of the microstructure of a material prior-to, and after impact can be used to track the microstructural changes that occur during the evolution of ASBs. This research, is initiated to systematically study the microstructure of AISI 4340 steel prior to impact, after impact and after post-impact annealing to determine the effect of the pre-deformation microstructure on the nucleation and initiation of ASBs, and the mechanism of evolution of ASBs during impact.

This study used state-of-the-art microstructural characterization techniques such as the FIB and STEM/HRTEM to reveal that initial microstructural inhomogeneity produces nucleation sites for the initiation of ASBs during impact. It was observed that double misfit interfaces and boundary layers, formed around precipitated carbides (interface between reinforcements and matrix), increased the volume fraction of dislocation sources within the pre-impact specimens. It is demonstrated that the intersection of an activated dislocation source with the direction of maximum shear (regions of stress concentrations) within the specimens during impact, is a necessary condition for the points of intersection to act as possible sites for the nucleation and initiation of ASB depending on the rate of dislocation generation, local strain and strain rate. In addition, the structure that evolves after strain localization starts out with elongation of the grains in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. The elongated grains break along the initiated dislocation boundaries as strain/strain rate increases 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 evolved shear band structures demonstrate that the local deformation is dependent on the imposed local strain and strain rate and that these mechanisms occur concurrently during impact. Also, plastic deformation of precipitated carbides results in carbide fragmentation creating residual carbide particles which are redistributed within the ASBs. The residual carbide particles trap and pin dislocations and contribute to increase in local hardening in the shear bands. The results obtained, which are specific to the behavior of BCC ferritic Pearlitic hardenable steels, lead to the conclusion that the evolution of ASBs is a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands at the end of passage of the stress wave.