Dynamic deformation and adiabatic shear microstructures associated with ballistic plug formation and fracture in Ti–6Al–4V targets

Abstract

Cylindrical, flat-nose, 4340 steel projectiles (2.0 cm height) were impacted onto Ti–6Al–4V targets (2.5 thick) at velocities ranging from 633 to 1027 m/s. Plug formation was observable at 633 m/s and exited the target between 1006 and 1027 m/s. Adiabatic shear bands (ASBs) composed of dynamically recrystallized (DRX) grains and cracks formed both vertically and horizontally (parallel or perpendicular) to the impact axis. The ASB characteristically white-band thickness varied from 5 to 40 μm, with the thicker bands occurring at the highest impact velocity. ASBs and cracks increased in frequency with increasing impact velocity, forming a cylindrical flow regime characterizing the plug. TEM analysis showed a greater than order of magnitude decrease in the DRX regime composing the ASBs, while EBSD analysis showed a residual DRX, grain-size regime varying from 50 to 900 nm. There was no evidence for an α → β transformation occurring within the ASB/DRX regime.

Introduction

A projectile striking a target effectively distributes its initial kinetic energy between itself as a deforming and eroding body, and the deforming and eroding target, as well as the energy required to separate a plug from the rear of a target with finite thickness [1]. For small projectiles impacting semi-infinite or infinite targets (with thicknesses orders of magnitude greater than the projectile dimensions) at high velocity, the penetration into the target may be only a small crater which scales with the projectile diameter; while the projectile is fragmented or eroded, leaving some fractional mass in the cratered target. The crater is formed by target material flowing along a narrow zone at the crater wall, usually in the solid state. As the projectile geometry and density changes, the target responds accordingly by accommodating deeper penetration for larger aspect ratios of the penetrator (length-to-diameter), and higher penetrator densities relative to the target density (ρ_p/ρ_t) [2], [3], [4], [5]. However, as the projectile velocity exceeds hypervelocity, penetration is diminished by projectile fragmentation and erosion, and even vaporization at very high impact velocities and very small (or correspondingly small) projectiles. Many of these variations in projectile–target interactions and target penetration responses are illustrated conceptually in Fig. 1.

The flow of target material along the penetration channel wall or crater wall forming a rim at the target surface occurs by the creation of a zone of dynamically recrystallizing material which accommodates solid-state flow by allowing the extremely small recrystallized grains to slide or glide relative to one another. This superplastic flow, accommodating strains well beyond the conventional deformation accommodated by dislocation or other shear mechanisms such as deformation twinning [5], is localized at or near the projectile/target interface regime forming adiabatic shear bands (ASBs).

This localized shearing can occur in any material, and is the preferential mode of accommodating severe plastic deformation, especially in low strain hardening materials which are prone to thermal softening. Adiabatic shear phenomena characterized by dynamic recrystallization (DRX) accommodating shear instabilities is especially favorable at high strain rates or both high strains (ɛ) and high strain rates ($\dot{\epsilon }$), which can give rise to large, localized temperature differences (or heat): ΔT ∝ (ɛ)($\dot{\epsilon }$). The competition between strain hardening and thermal softening was originally proposed by Zener and Hollomon [6] while Recht [7] proposed a critical strain rate criterion for shear band formation. Shear bands or shear band regimes characterized by DRX in fact accommodate the severe plastic deformation in ballistic penetration, explosive welding, wire drawing and friction welding, particularly friction-stir welding; as well as mechanical alloying [8].

For massive projectiles impacting finite targets at relatively high impact velocities (>600 m/s) the target can form a plug of material beneath the projectile which can separate from the target as shown in Fig. 1(a). The energy required for plug separation or break-out from the target is proportional to the plug diameter, the target material shear strength, and the square of the target thickness [1]. This energy derives from the projectile kinetic energy and is localized at the projectile circumference where the shear bands ultimately form a cylindrical, solid-state flow zone allowing the plug volume to be pushed out of the target. At the back surface of the target where the plug emerges, the shear band cylinder fractures or separates catastrophically.

The formation of the shear band cylinder accommodating the target plug separation involves deformation-induced microstructure evolution which ideally begins with dislocation sub-structure development and densification, DRX accommodating shear localization and forming shear bands (ASBs) which grow and coalesce, forming major flow regimes parallel to the projectile impact axis [9], [10].

Plug formation and evolution of associated microstructures and microstructural regimes in Ti–6Al–4V targets is of interest because this alloy has been demonstrated to be particularly attractive as an armor material since it provides better ballistic resistance than steel or aluminum [11], [12], [13], [14], [15]. While microstructural studies involving plugging in Ti–6Al–4V have been performed (see [10]), they lack a systematic and comprehensive overview. This paper examines deformation microstructures contributing to, and composing, adiabatic shear bands in thick Ti–6Al–4V targets (nominally 2.5 cm) impacted by 2 cm diameter, 4340 steel projectiles at impact velocities ranging from 0.633 to 1.027 km/s. Optical metallography (OM) scanning electron microscopy (SEM), especially electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) are utilized to characterize the associated target microstructures and especially the ASBs. The microstructural observations are correlated with residual target and plug-related microhardness maps which together provide structure-property-related validation for computer simulations.