Abstract
Pre-deformation and post-deformation microstructure characterization was conducted on tempered 4340 steel and commercial pure copper specimens under impact to determine the microstructural changes and the mechanism of grain refinement that occur during the evolution of ASBs. It was observed that the movement and multiplication of dislocations, elongation of grains, breaking of elongated grains, rotation, carbide fragmentation and boundary refinement of broken grains occur simultaneously during the evolution of ASBs in the impacted 4340 steel specimens. The extent of these mechanisms depends on the imposed local strain and strain rate. Extensive grain refinement coupled with high density of dislocations results in the shear band structures being more susceptible to crack nucleation and propagation. In copper, it was observed that sequential occurrence of emergence of dislocations, dislocation cell formations with varying cell boundaries and cell interiors, dynamic recovery and extensive micro-twinning results in the formation of the shear bands. The structure within the evolved shear bands becomes less brittle after the onset of dynamic recovery and micro-twinning. The differences in the mechanism of grain refinement and evolution of the shear bands in both materials is attributed to the differences in the mobility of dislocations, the rate of strain hardening and strain hardening exponents in both materials studied.
Highlights
Metals and other materials exhibit the formation of narrow bands of extreme strains known as Adiabatic Shear Bands (ASBs) when deformed at high strain rates and large strains [1]
The current study shows that the mechanism of evolution of adiabatic shear bands in tempered steel specimens are different from those observed in pure copper specimens
Pre-impact and post-impact microstructure characterization was conducted on tempered 4340 steel (BCC) and pure copper (FCC) specimens to determine the mechanism of evolution of ASBs. This was done in order to determine the effect of crystal structure and microstructure on the mechanisms and appearance of ASBs in both materials
Summary
Metals and other materials exhibit the formation of narrow bands of extreme strains known as Adiabatic Shear Bands (ASBs) when deformed at high strain rates and large strains [1]. It has been reported that BCC metals exhibit an increased propensity to ASB when their grain sizes are reduced into the ultrafine-grain and nano-crystalline regime [2]. This is attributed to the associated changes in the strength, strain hardening behaviour and strain rate sensitivity when the grain size changes [2]. It has been observed that the ASBs associated with FCC metals, such as aluminium and copper, are diffused and exhibit a plastic flow phenomenon [3] The influence of these factors on the formation of ASBs during dynamic loading makes the phenomenon of ASBs very complex
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