Study of the formation of adiabatic shear bands in steel during impact

Both metals and other materials may exhibit the formation of narrow bands of extreme strains when impacted at high strain rates and large strains. These are known as Adiabatic Shear Bands (ASBs). They are observed during material processing such as forging and machining as well as wear and in armor plates during impact by projectiles. The prevailing theory for their formation is that they form in narrow bands because of two competing mechanisms occurring sequentially: strain hardening followed by thermal softening from the retained heat due to the impact. However, recent studies suggest that the formation of ASBs may be a simultaneous occurrence of different mechanisms which starts with the emergence of dislocations depending on the imposed local strain and strain rate. This study uses different methodologies to explore the microstructure of ASBs in a hardened low alloy steel. The study includes the effect of the initial microstructure on the formation of ASBs. The Focused Ion Beam technique was used to prepare transmission electron microscopy samples from regions within the shear bands to eliminate the induced further deformation which could be produced by conventional approaches of electropolishing. The present study reveals that each of these methodologies complements each other. Also more complex series of mechanisms including dislocation cell formation, texture development, dynamic recrystallization and carbide dissolution accommodate the excessive strains that occur during the evolution of the shear bands.