Effect of high strain rate on plastic deformation of a low alloy steel subjected to ballistic impact

Abstract

The formation of multiple adiabatic shear bands

Impacting hardenable steel such as 4340, results in the formation of adiabatic shear bands (ASBs). Previous studies have shown that the presence of carbides/second-phase particles in the pre-deformation microstructures of 4340 steel increases their susceptibility to the formation of ASBs. The current study examines the role of carbides on the microstructure and properties within evolved ASBs in 4340 steel after impact. Geometric phase analysis was used to map local deformation fields within the evolved ASBs. It was observed that carbide fragmentation due to plastic deformation of carbides produces both residual carbides and residual carbide particles in regions away from the shear bands. Extensive carbide fragmentation produces fine residual carbide particles which are redistributed within the ASBs. This is attributed to strain localization within the ASBs which result in higher local strain and strain rates within the shear bands than in regions outside the bands. In addition, it is observed that the residual carbide particles trap and pin dislocations within the shear bands and contribute to an increase in local hardening. A more homogenous distribution of narrower and shorter rotational and shear-strain fields were revealed by the local deformation maps within the evolved ASBs. Lattice deformation mapping revealed that the ferrite matrix, prior to impact, had broader and longer rotational and shear-strain fields perpendicular to the direction of impact. This is attributed to lattice-invariant deformation and shape deformation processes that occur on specific crystallographic planes during martensitic transformation. It is concluded that strain localization during high strain rate deformations does not occur on specific crystallographic planes. This results in a more regular distribution of internal lattice rotational and strain fields within the evolved ASBs.

Adiabatic shear bands in uranium alloy projectiles/penetrators, during penetration, allow them to “self-sharpen,” a process that is absent in most tungsten alloy projectiles/penetrators. U-0.75 wt % Ti alloy samples have been accelerated to impact steel targets, and the distribution of adiabatic shear bands in residual samples has been studied in detail to understand the effect of self-sharpening on penetration. In our study, self-sharpening was evidenced by the distribution of the shear bands in the recovered sample. The shear bands formed during impact were observed to change direction when they crossed grain boundaries, which indicated that the grain boundaries had an influence on the adiabatic shear bands of U-0.75 wt % Ti. Micro-hardness test results showed that the Vickers micro-hardness in the adiabatic shear zone was 18% lower than that in the matrix. In the split-Hopkinson pressure bar (SHPB) experiment, a strain rate of around 2891 s−1 was the threshold strain rate that triggered the formation of adiabatic shear bands in the U-0.75 wt % Ti alloy.

Investigation of the twin-induced adiabatic shear bands evolution in a Mg-Al-Mn alloy under ballistic impact

In the present work, the adiabatic shear characteristics of our recently designed α + β dual-phase Ti alloy at different strain rates have been investigated by hat shaped specimen. The deformation process is divided into three stages: work hardening stage, steady stage, and unstable thermal softening stage. Along or near the shear deformation paths, the microvoids and the cracks can be captured at the strain rate of 1.8 × 104 s−1, 2.0 × 104 s−1, and 2.3 × 104 s−1, both of which contribute to the stable and unstable softening. It is found that dynamic stored energy of cold work will be significantly improved by the enhanced high strain rate. In the view of coupling analysis of inverse pole figure and grain boundary map, it seems that low angle grain boundaries present a good resistance to the formation of cracks and thermal softening. On the contrary, high angles grain boundaries are typically located in ASBs and their affecting regions, which is in line with the reported results. While the geometrical necessary dislocation (GND) density of adiabatic shear band (ASB) and its surroundings increased significantly, the width of the ASB becomes wider as the strain rate increases, which is consistent with the theory of sub-grain rotation dynamic recrystallization model. The formation of multiple ASBs in the corner position is schematically illustrated and the average elastic modulus and hardness of the ASB region are lower than the α and β phases, combined with the GND analysis, which proves that the ASB is a thermal softening zone in this experiment.

Effect of high strain rate on adiabatic shear susceptibility and microstructures in Al0.4CoCrFeNi high-entropy alloy

An examination of adiabatic shearing behavior in ZK60 alloy with different states of heat treatment

Effect of prior heat treatment on the dynamic impact behavior of 4340 steel and formation of adiabatic shear bands

Aluminium-scandium (Al-Sc) alloy is subjected to shear deformation at high strain rates ranging from 3.0×105 s−1 to 6.2×105 s−1 using a compressive-type split-Hopkinson pressure bar (SHPB). The effects of the strain rate on the shear stress, adiabatic shear band characteristics, and fracture features of the Al-Sc alloy are systematically examined. The results show that both the shear stress and the strain rate sensitivity increase with an increasing strain rate. In addition, it is shown that an adiabatic shear band is formed within the deformed specimens for all values of the strain rate. As the strain rate is increased, the width of the shear band decreases, but the microhardness increases. Moreover, the distortion angle and the magnitude of the local shear strain near the shear band both increase with an increasing strain rate. At a strain rate of 3.0×105 s−1, the fracture surface is characterised by multiple transgranular clearage fractures. However, for strain rates greater than 4.4×105 s−1, the fracture surface has a transgranular dimple-like characteristic, and thus it is inferred that the ductility of the Al-Sc alloy improves with an increasing strain rate.

The current study presents the effects of strain and temperature on the mechanical response and microstructure evolution in AA7075-T651 at high strain rates. Compression tests have been performed at room temperature (RT), 200, 300 and 400 °C using a Split-Hopkinson pressure bar (SHPB) setup with strain rates ranging between 1400 and 5300 s−1. For deformation at RT, the flow stress increases with increase in strain rate. Whereas deformation at elevated temperatures show a non-monotonous behavior of the flow stress with respect to the strain rate. This trait is attributed to the pronounced effects from the adiabatic shear bands (ASBs); namely, distorted shear bands (DSBs) and transformed shear bands (TSBs); and cracks resulting from the plastic deformation instability during hot deformation. The sequence of microstructure evolution is: inhomogeneity in the initial microstructure – DSB – TSB – crack –fracture. The feasibility of formation and growth of ASBs and cracks increases with increase in strain and temperature, neglecting any significant effect from the strain rate. During the compression tests, temperature of the material rises due to adiabatic heating. Considering a certain strain developed in the material, this adiabatic temperature rise decreases as the deformation temperature is increased. Furthermore, during individual deformation processes, the temperature rise increases with increasing strain. The adiabatic temperature leading to the formation of TSB is approximated to be 0.7 times of the melting temperature of the alloy. These results from the current study are to be used in developing a physics-based material model for the alloy.Article HighlightsAt elevated temperatures, compression with Split-Hopkinson bars produce two types of shear bands and cracks.Evolution of shear bands and cracks is promoted by increase in strain and temperature irrespective of strain rate.Adiabatic temperature approximating to 70% of the melting point forms refined grain structure of transformed band.

The dynamic response of a β titanium alloy to high strain rates and elevated temperatures

High strain-rate deformation of T8-tempered, cryo-rolled and ultrafine grained AA 2099 aluminum alloy

As a model material, commercial pure titanium was rolled to plates with different dislocation boundaries. The dynamic mechanical response of Ti specimen was analyzed during impacted with Split Hopkinson Pressure Bar (SHPB) at different strain rates, and microstructure evolution was investigated using optical microscopy and transmission electron microscopy. It was found that adiabatic shear sensitivity was decreased with increasing strain rates for all as-annealed, 25% and 50% cold rolled states. To the contrary, for 70% cold rolled state the adiabatic shear sensitivity was increased with increasing strain rates. The microstructure of adiabatic shear bands (ASBs) were developed from elongation morphology to fine equiaxed grains in the specimens of 25% cold rolled state, and ASBs became broader with increasing strain rate.

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M3C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350 °C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.