Annealing effect on the evolution of adiabatic shear band under dynamic shear loading in ultra-fine-grained iron

In the present study, evolution of adiabatic shear band (ASB) and annealing effect on the evolution of ASB in ultra-fine-grained (UFG) Fe under dynamic shear loading were investigated. The UFG Fe was processed by equal-channel angular pressing (ECAP) via route BC. After 6 passes, the grain size of UFG Fe reaches ∼500 nm, as confirmed by means of Transmission Electron Microscopy (TEM). Examination of micro-hardness and grain size of UFG Fe shows a transition from recovery to grain growth at annealing temperature of approximately 400 °C. The high-strain-rate response of UFG Fe was characterized by hat-shaped specimen set-up in Hopkinson bar experiments. With increased shear deformation, the evolution of ASB was found to be a two-stage process, namely a nucleation stage followed by a thickening stage. In the thickening stage, ASB evolution is accompanied by increasing in both thickness and micro-hardness of ASB. The increased micro-hardness in ASB and TEM observations inside ASB indicate that grains in the shear band are further refined. Once the shear band is initiated, cracks also nucleate and propagate within ASB, which produces failure in the material. After 450 °C post-ECAP annealing, development of ASB and crack propagation were inhibited under dynamic shear loading. The results indicated that the ductility under dynamic shear loading can be significantly improved by appropriate post-ECAP annealing without losing the strength of the UFG Fe.